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Evaluation of a new mask and scavenging system for nitrous oxide used in labour and delivery Chessor, Ed 2008

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       Evaluation of a new mask and scavenging system for nitrous oxide used in labour and delivery       Authors: Ed Chessor a Marieke Verhoeven b Kay Teschke a,c      a  School of Occupational & Environmental Hygiene, University of British Columbia, 3rd Floor, 2206 East Mall, Vancouver, BC V6T 1Z3, e-mail: echessor@interchange.ubc.ca  b  Universeteit Utrecht, Institute for Risk Assessment Sciences  c  Department of Health Care and Epidemiology, University of British Columbia, Mather Buildings, 5804 Fairview Avenue, Vancouver, BC V6T 1Z3, e-mail: teschke@interchange.ubc.ca   Page 1 1. Background  Entonox, a 50/50 mixture of nitrous oxide and oxygen is used by women in labour to ease pain. The mother-to-be controls administration of nitrous oxide by placing a mask over her nose and mouth and inhaling. This opens an LSP Elder CPR/ Demand valve which delivers gas to the mask. When the desired dose has been received, the mask is removed from the face. There is a scavenging (local exhaust ventilation) system connected to the masks in some labour/delivery rooms at Richmond Hospital. When the mask is sealed to the face, the force of the mother’s exhaled breath opens a valve and the exhaled breath flows through a hose to the outdoors. The system provides no exhaust air flow when the mask is not sealed to the face. The scavenging system could be considered an intermittent flow local exhaust ventilation (LEV) system.  Breath exhaled when the mask is not in contact with the mother’s face may contain up to 4 percent nitrous oxide. As a result, the nitrous oxide concentration in a labour/delivery room may often exceed 200 ppm. Nurses and others may experience severe headaches when working in a room where Entonox is being used. Exposure to nitrous oxide anaesthetic gas in dental offices and various medical settings has been linked to increased risk of spontaneous abortion and congenital birth defects.(1,2) For these reasons agencies responsible for occupational health, e.g., the Workers’ Compensation Board of British Columbia, and the Australian National Occupational Health and Safety Commission, have set an 8 hour exposure limit of 25 ppm.  The US National Institute for Occupational Safety and Health recommends a limit of 25 ppm during the period of exposure.  A previous attempt to control nurses’ exposure to nitrous oxide in labour rooms utilized an enclosing hood placed on the head of the bed, covering the mother’s head and shoulders when she was in the bed.(3) Based on evidence from a report, Richmond Hospital staff decided the hood would interfere with the movement of the mother and nurse, and was unacceptable for that reason.  Women in labour at Richmond Hospital, and most hospitals in British Columbia are offered a range of injectable anaesthetics, in addition to Entonox. Entonox is usually offered first, as it is less likely to cause complications for the mother and child.(4)  2. Development of a New Mask  In October 1999, a team of Mechanical Engineering students at the University of British Columbia took on the task of developing a more effective scavenging system for exhaled nitrous oxide in labour/delivery rooms.  They assessed the performance of the scavenging and general ventilation systems in a labour/delivery room at Richmond hospital, and were given a sample mask, demand valve and scavenging hose from the hospital. Information on the respiratory capacity – forced expiratory volume in one second (FEV1) – of healthy females of child bearing age was obtained from Dr. Susan Kennedy of the School of Occupational & Environmental Hygiene. A design goal for the air flow of a continuous scavenging system was based on the FEV1 information. An attempt to draw that air flow through the sample scavenging hose provided by the hospital resulted in an intolerable noise.   Page 2 A suitable duct (or hose) diameter was calculated for the desired flow at a velocity of 10 meters/second (2000 fpm). The velocity was chosen to avoid sound levels greater than the background in the occupied room, and keep the hose light and flexible. The hose of a spirometer in the SOEH laboratory had the desired diameter. It also had the other properties we sought, except for length. A 2.7 meter (9 foot) length was found, which appeared adequate. A sample was purchased.  The students considered the possibility of drawing exhaust air through the existing facepiece, whenever the patient was not inhaling. No means could be devised to avoid the need for the patient to compete with the exhaust system for the Entonox. The possibility that the exhaust system might open the demand valve and draw Entonox when the patient was not inhaling was also raised. That would increase gas consumption, costs and emissions in the facility.  2.1 The First Prototype  The system design that was first proposed had the exhaust hose connected to a 150 mm (6 inch) diameter hemisphere. The previous half face mask was mostly contained in the hemisphere, and the demand valve was connected through a hole in the hemisphere.  The search for a suitable plastic hemisphere produced a polyethylene bowl that was the desired diameter and very nearly hemispheric. Openings were made for the exhaust hose connection and the demand valve connection to the mask. An adjustable speed fan was connected to the hose, but no gas was connected to the demand valve.  The first prototype was tested for its ability to control exhaled carbon dioxide. A continuous reading and datalogging IAQ monitor was used to measure the carbon dioxide concentration in a transparent chamber while a student researcher breathed normally with the mask at various distances from his face. Exhaust flow rates of 10, 15 and 20 cubic feet per minute were evaluated, with the mask held 50 to 150 mm in front of the researcher’s face. With a flow of 15 cfm, the carbon dioxide concentration went from 590 to 1190 ppm in three minutes. Increasing the flow to 20 cfm caused an obvious increase in noise, and very little improvement in CO2 levels. The control of carbon dioxide was deemed unsatisfactory. The researcher pointed out that the inner mask seemed to be deflecting exhaled breath back past his ears, beyond the capture range of the outer mask. He suggested putting a one-way valve in the inner mask so some exhaled breath could go directly to the exhaust hose. This valve would need to open at the velocity pressure of exhaled breath. An inhalation valve from a negative pressure respirator was found to meet the need.  2.2 The Second Prototype  The second prototype, with a new exhalation valve installed in the inner mask, was tested at the same exhaust flow rates. With flow rates of 15 and 20 cfm, carbon dioxide concentrations in the chamber went from 580 to 650 ppm in the first 90 seconds, and stayed at 650 ppm for the rest of the 3 minute test. Significantly higher carbon dioxide concentrations were measured at 10 cfm.   Page 3 The next phase of development of the mask produced plastic fittings that connected the scavenging hose to the outer mask, and a filter that was installed between the hose and fitting. This mask (shown in the photo) was then tested in a field trial at the hospital.   3. Field Testing  Field testing was done in Labour/Delivery Rooms 6 and 7 at Richmond Hospital.  3.1  Objectives  The main objective of this field trial was to compare the concentrations of nitrous oxide found in the nurses’ breathing zones when using the new and existing scavenging systems. In addition, there were three related objectives: • to determine whether the prototype exhaust system could reduce nurses’ exposures to nitrous oxide to levels below the Exposure Limit in the Occupational Health and Safety Regulation of the Workers’ Compensation Board of BC; • to learn what factors related to the rooms, the nurses’ activities, the patients’ activities and the ventilation systems contributed to increased or reduced nitrous oxide levels in the labour/delivery rooms; and • to measure the acceptability of the new system to patients and staff in labour/delivery rooms.  3.2 The Facility  The Maternity Ward at Richmond Hospital is on the third floor of a four storey building. There are seven Labour/Delivery rooms, all accessed from a 2 meter wide corridor. A Caesarian Section room, the nursery and post-delivery rooms for mothers are part of this ward.  Rooms 6 and 7 are equipped with bathtubs and piped in nitrous oxide and oxygen. Rooms 6, 7 and the Caesarian Section room are also the only ones equipped with waste anaesthetic gas scavenging systems. All three rooms are served by the same scavenging fan.  The dimensions of Room 6 and its bathroom are shown in Figure 1. The general ventilation system supplies air near the door, just below ceiling level. The air supply grilles have two layers of adjustable blades, so the air could be aimed in several directions. We found the outer blades set to block air flow.  Air is exhausted through a grille near the floor, in the corner opposite the door. The design flow rate is 315 litres per second. The exhaust grille is about 4.7 meters from the head of the bed.  Richmond hospital installed ducts from the mothers’ bedside and a point near the shower in the bathroom, that exhaust to the scavenging system duct in the adjacent corridor. These ducts, and the fittings in the walls were made for use in a domestic built- in vacuum cleaner system. They fit the hose well, and were ideal for our purpose. Photo 2 shows the new mask and it’s exhaust hose connected to the fitting in the wall.  The  Page 4 connection point for the old scavenging system hose was in the cabinet at the left side of the bed. Photo 3 shows the new mask in more detail.  Photo 4 shows the existing mask with the scavenging fitting and hose.  Figure 1:  Room 6, Maternity Ward, Richmond Hospital   Photo 1 Room 6, Maternity Ward, Richmond Hospital This shows Min seated at the researcher’s note table, the Miran on the table, and the patient’s bed on the right.  The sampling hose from the bed would be connected to the Miran before a patient started using Entonox.  The blue cylinder just to the left of the table is the Entonox source for the next patient.   Photo 2  Room 6, Maternity Ward, Richmond Hospital  This shows the new mask in Marieke’s right hand, and the 38 mm scavenging hose from it to the fitting in the cabinet, on her right.  The smaller diameter blue hose goes from the demand valve on the mask to the regulator on the Entonox tank, in the lower left corner of the photo.  Marieke is holding the sampling hose for the Miran in her left hand.   Photo 3  The new mask, without the Entonox supply hose                    Page 5    Photo 4:  The existing mask and scavenging hose  The fan in the scavenging system did not develop enough static pressure to give the 15 cfm flow we needed through the hose, so a supplemental inline centrifugal fan was installed.  Early in this project we measured the air velocity at the exhaust grilles in the rooms being studied. Staff responsible for ventilation system maintenance indicated that the flow was constant. A few weeks into the sampling program we checked the velocity and found it had changed significantly. We began measuring the velocity at the grille each time we sampled, and found significant day to day change. No explanation for this change was provided by hospital staff.  3.3 Concentration Measurement Methods  The mask and our research plan were introduced to supervisors and some nurses at informal meetings. Consent letters and recruitment forms were circulated to all nurses on the ward. Researchers were stationed on site during the day, from Monday to Friday and met staff on duty several times a day.  All mothers who attended Richmond Hospital for labour and delivery in 2001 during times when the study team was present were asked to participate in the study, either in prenatal classes or when they arrived at the hospital. The study was designed to recruit approximately equal numbers of deliveries using the new scavenging system (room 6 only) and the existing system (rooms 6 or 7). Sampling using the existing system started shortly before the supplementary fan was installed in the room 6. Participation in the study required that the patient use nitrous oxide for a minimum period of 5 minutes.  When a patient was enrolled in the study, the concentration of nitrous oxide in air about 500 mm above the head of her bed was measured with a Miran 1A Infrared Gas Analyzer throughout the labour and delivery period, starting at the beginning of use of nitrous oxide. When the patient was in the bathroom, measurements of the nitrous oxide concentrations in the bathroom were made via a hose fastened to the bathroom wall 1.5 meters above the floor, and 0.5 meters from the tub/shower. The other end of the hose was connected to the direct reading Miran when the patient went into the bathroom The measurements of this direct-reading monitor were recorded every 30 to 60 seconds.  A second Miran 1A monitored nitrous oxide concentrations at the nurses’ station.  The nurse assigned to each patient wore a Nitrous Oxide Monitor (Item Number X575, AT Labs). This was shipped to AT Labs for analysis a few hours after the measurement session ended.  The concentrations of nitrous oxide in the nurse’s breathing zone might be influenced by a number of factors under the control of the patient or the nurse, and by the general ventilation system. Research staff recorded the time of day, and nurse and patient  Page 6 actions each time a change was observed, in the patient’s room and at the nursing station. The following variables were recorded as often as they changed: • whether the patient had the mask pressed to her face; • the distance from the patient’s face to the mask; • whether the nurse was in the room; • the distance between the nurse and the patient; • whether the patient was in the bed, sitting in the chair, in the bathroom or walking; and • whether nitrous oxide was being used in other rooms on the ward.  The cylinder of Entonox was weighed before the patient started to use it, and just after the baby was born, to determine the mass of gas consumed. Air velocity 20 mm in front of the exhaust grille was measured during each session.  3.2 Questionnaires Soliciting Feedback about the Masks  Questionnaires were designed to solicit patient and nurse feed back about the acceptability, reliability, and ease of use of the new and existing masks, and to solicit any other comments. Patients were interviewed approximately 60 minutes after delivery, and nurses were interviewed when their monitoring badge was removed. The interviews lasted around 5 minutes each.  3.5 Data Analysis  All analyses were conducted using SAS and S-Plus.  Two  datasets were created to get the most information from the measurements:  • One file (Full Period Data) was based on the nurses’ personal and room nitrous oxide concentrations averaged over the full duration of the labour and delivery period. These data were used to examine the fundamental question of whether the room and nurses’ long-term exposures differed when attending patients using the new versus the existing scavenging system, and whether their exposures met the WCB Exposure Limit for nitrous oxide when their patients were using the new mask.  • The other file (Minute-to-Minute Data) included all the minute-to-minute area nitrous oxide measurements from the Miran Infrared Analyzer and the corresponding minute-to-minute patient and nurse activities. It was used to examine, with more statistical power, the factors related to increased or decreased nitrous oxide concentration.  Descriptive statistics (counts for categorical data; means, ranges, standard deviations, and frequency distributions for continuous data) were calculated for all available variables.  3.5.1 Full Period Data  The nitrous oxide concentrations (both room area and nurses’ personal) when using the new and existing masks were compared using t-tests.  Page 7  The proportion of the nurses’ personal exposures to nitrous oxide exceeding the WCB Exposure Limit was calculated for four scenarios: • An 8-hour average worst-case concentration was estimated by assuming the average concentration measured during the measurement period remained the same for a full 8-hour shift • An 8-hour average best-case concentration was estimated by assuming that during the unmeasured period the nurse had no exposure • A 12-hour average worst-case concentration was estimated by assuming the average concentration measured during the measurement period remained the same for a full 12-hour shift • A 12-hour average best-case concentration was estimated by assuming that during the unmeasured period the nurse had no exposure. The estimated 8- and 12-hour average exposures were compared to the WCB Exposure Limits for 8- and 12-hour shift lengths.  In order to examine which factors, in addition to the mask type, might be influencing the full-period room air concentrations, exposure modeling was done. The distribution of the full-period room concentration data was not clearly normal or log-normal, so the data were not transformed prior to analysis. To start, the correlations between all independent variables (Pearson r) were examined. Among pairs with r > 0.70, only one was selected for further inclusion in the determinants of exposure model (the one more logically explained as associated with exposure or the one more strongly associated with exposure in initial analyses). Initially, the “simple” association between each independent variable, controlling for mask type, and the log-transformed exposure concentration was examined.  These initial analyses indicated non-linear relationships between certain of the patient activities and nitrous oxide concentrations. The activity “patient at the bed not using nitrous oxide” was split into three different categories: “patient at the bed before ever using nitrous oxide”, “patient not using nitrous oxide for a period up to 15 minutes post- use”; and “patient not using nitrous oxide for a period longer than 15 minutes post-use”. The categories were determined by examining the decay in room nitrous oxide concentrations after a period of use. The concentration data indicated that after 15 minutes the patient was no longer adding nitrous oxide to the room air.  Two other variables were collapsed into one. Since it was impossible to know whether a patient was using nitrous oxide in the bathroom, “patient in the bathroom using” and “ patient in the bathroom not using” were collapsed into one variable.  All variables with p < 0.10 in the simple modeling were offered in a manual backwards stepwise multiple regression model (Proc Reg, SAS). Variables with the highest p-values ≤ 0.10 were eliminated one at a time, then the model was refitted until all included variables had p < 0.10.  3.5.2 Minute-to-Minute Data  The minute-to-minute data was used to model, with more power, the factors which influenced the concentrations of nitrous oxide in the room. The distribution of the minute- to-minute room concentration data was positively skewed and approximately log-normal,  Page 8 so exposures were log-transformed (base 10) prior to analysis to improve the efficiency of the models and to ensure that predicted concentrations were greater than zero.  For the analysis, only the nitrous oxide measurements recorded every minute were included (i.e., measurements made at more frequent intervals were excluded from the file) to give equal periods between measurements.  A test for autocorrelation was done to determine whether measurements recorded close in time to each other were correlated (S-Plus). This showed that there was autocorrelation greater than 0.6 for measurements taken within 5 minutes of each other. However, because the tasks had unpredictable cycles, a time correction was probably not going to solve the problem. A different approach was chosen: a mixed statistical model with patient and time as random effects (Proc Mixed, SAS). One of the possibilities for Proc Mixed is an individual growth model, designed for exploring longitudinal data over time.   Table 1: Results of test for autocorrelation, for  up to 11 time lags Time between measurements (minutes) Correlation coefficient 0 1.00 1 0.78 2 0.71 3 0.68 4 0.65 5 0.62 6 0.55 7 0.53 8 0.49 9 0.47 10 0.42 11 0.39   Examinations of correlations between independent variables, and of simple associations between independent variables and nitrous oxide concentration, controlling for mask type, were conducted as for the full-period modeling. All variables with p < 0.10 in the simple modeling were offered in a manual backwards stepwise multiple regression model. Variables with the highest p-values ≤ 0.10 were eliminated one at a time, then the model was refitted until all included variables had p < 0.10. In order to derive an R- squared for the final Proc Mixed model, the model’s predicted values were calculated and compared with the log-transformed nitrous oxide concentrations using linear regression (Proc Reg, SAS).  3.5 Results: Full Period Data  Thirty-three patients were recruited into the study and used nitrous oxide for at least 5 minutes during their labour and delivery period. Sixteen used the existing mask and  Page 9 scavenging system and 17 the new mask and scavenging system. The room air concentration and the attending nurse’s personal exposure was measured for all 33 patients. In three cases, the personal monitoring badge of the attending nurse fell on the ground during the work shift, so the personal exposure data was excluded.  The results are divided into four parts: room air concentration; personal exposure; nurse and patient activities and room characteristics; and exposure modeling. 3.5.1 Room Air Concentration  Table 2 summarizes average, minimum and maximum concentrations of nitrous oxide in the labour and delivery room, stratified by whether the patient was using the new or existing mask. Table 3 provides similar data for the nursing station. The average concentration of nitrous oxide in the labour and delivery room using the new mask system was less than half that in the rooms using the existing scavenging system, a statistically significant difference. The air concentrations at the nursing station were much lower than those in the delivery room, and although the new system did seem to result in lower concentrations in this area as well, the difference was not significant.  Table 2: Labour and delivery room air concentrations of nitrous oxide (in ppm) using the new and existing N2O scavenging systems  New Existing Number of measurements 17 16 Arithmetic mean  39.7* 82.2* Standard deviation 21.7 48.9 Geometric mean  34.1 69.1 Geometric standard deviation 1.80 1.89 Minimum concentration 11.5 16.6 Maximum concentration 89.0 211 *T-test for differences in means, p = 0.0027  Table 3: Nursing station air concentrations of nitrous oxide (in ppm) using the new and existing N2O scavenging systems  New Existing Number of measurements 17 16 Arithmetic mean  6.04 9.16 Standard deviation 2.95 6.73 Geometric mean  4.83* 5.92* Geometric standard deviation 2.36 2.50 Minimum concentration 0.51 1.12 Maximum concentration 9.99 43.4 *T-test for differences in means, p = 0.25   3.5.1 Nurses’ Personal Exposures  Table 4 summarizes the nurses’ personal exposures while caring for patients using the new and existing mask, as measured during the monitoring period. Nurses’ average exposure with the new scavenging system was almost half that with the old system.  Page 10  Table 5 uses the same data, but extrapolates the time period to a full 8- or 12-hour shift length. The estimated worst case assumes the nurses were exposed to the same average concentration throughout the shift as during the monitoring period; the best case assumes the nurses had no exposure in the unmonitored period. This table shows the number and proportion of measurements that would be above the WCB Exposure Limits for nitrous oxide. Note that for the 12-hour shift length, the Exposure Limit  is adjusted for the extended work shift. In all scenarios (8- or 12-hour shift, best case or worst case assumptions, new mask or existing mask), some measurements exceeded the WCB limits, however, there were fewer exceedances using the new mask.  Table 4: Nurses’ personal exposures to nitrous oxide (in ppm) during measurement period  New Existing Number of measurements 16 14 Average time period of measurement, in minutes (range) 152 (70 – 251) 148 (43 – 290) Arithmetic mean  40.2 69.3 Standard deviation 43.9 33.2 Geometric mean  25.3* 60.9* Geometric standard deviation 2.66 1.74 Minimum concentration 8.2 21 Maximum concentration 170 120 *T-test for differences in means, p = 0.053   Table 5: Nurses’ personal exposures to nitrous oxide (in ppm), extrapolated to 8-hour and 12-hour shift lengths  New Existing Number of measurements 16 14 12-hour concentrations, based on worst case assumption* # (%) Measurements exceeding WCB 12-hour Limit (12.5 ppm) 12 (75%) 14  (100%) 12-hour concentrations, based on best case assumption† Arithmetic mean  13.5 18.8 Standard deviation 15.2 11.8 Geometric mean  9.10 16.0 Geometric standard deviation 2.41 1.79 Minimum concentration 3.94 6.74 Maximum concentration 63.6 51.0 # (%) Measurements exceeding WCB 12-hour Limit (12.5 ppm) 5 (31%) 9 (64%) 8-hour concentrations, based on worst case assumption* # (%) Measurements exceeding WCB 8-hour Limit (25 ppm) 7 (44%) 13 (93%) 8-hour concentrations, based on best case assumption† Arithmetic mean  19.0 25.9 Standard deviation 22.4 8.59 Geometric mean  12.3 22.3 Geometric standard deviation 2.48 1.76 Minimum concentration 2.86 9.23 Maximum concentration 93.3 66.5 # (%) Measurements exceeding WCB 8-hour Limit (25 ppm) 4 (25%) 6 (43%)  Page 11 * Worst case assumption: during time period not measured, nurse exposed to same levels of nitrous oxide as during the measurement period (therefore summary data same as that shown in Table 4) † Best case assumption: during time period not measured, nurse not exposed to nitrous oxide     Page 12 3.5.3 Patient/Nurse Activities and Characteristics of Mask Use  Tables 6 and 7 summarize information about patient activities, nurse activities, and mask use characteristics, for the existing and new masks, respectively. These data provide an overview of what happened during the measurement periods.  It is particularly interesting for instance that, for both mask types, the patients on average spent about 35% of the measurement period using nitrous oxide and another approximately 50% of the time was within 15 minutes of a period of nitrous oxide use (when exhalation of the gas is expected to be high). Yet during only 30-40% of the time was the mask close enough to the face to effectively scavenge exhaled breath (within 15 cm of the face). It is also interesting to note that on average the nurses spent about half the measurement period within 3 meters of the patient, and most of that time was spent within arm’s length of the patient.  Table 8 indicates the rooms in which the measurements were made, and with which mask. Only room 6 had the new scavenging system, but on occasion the old system was used in that room.  Table 6: Activities and characteristics during existing mask use measurements  N Mean Std Min       Max Proportion of time that patient was: at the bed before using nitrous oxide 16 0.04 0.1 0 0.42 at the bed and using nitrous oxide 16 0.33 0.21 0.01 0.95 at the bed and not using for less than15 min 16 0.46 0.18 0.01 0.75 at the bed and not using for more than 15 min 16 0.07 0.1 0 0.33 at the chair and not using  16 0.03 0.09 0 0.3 at the chair and using  16 0.02 0.05 0 0.19 walking and not using 16 0 0 0 0 walking and using 16 0 0 0 0 in the bathroom  16 0.04 0.07 0 0.24 Total time patient breathed through mask (min) 16 52.4 27.5 11.1 104 Total patient activity time (minutes) 16 150 73.7 45.6 283 Proportion of time that the mask was: closer than 15 cm from the patient’s face 16 0.33 0.1 0.21 0.5 between 15 and 30 cm from the patient’s face 15 0.2 0.15 0.04 0.5 further than 30 cm from the patient’s face 16 0.48 0.22 0.15 0.76 Total mask to face activity time (minutes) 16 144 76.6 45.6 280 Proportion of time that nurse was: within arm’s length of the patient 16 0.36 0.25 0.02 0.94 between arm’s length and 3 meters of the patient 16 0.15 0.15 0 0.52 more than 3 meters from the patient 16 0.06 0.06 0 0.19 in the bathroom with the patient 16 0.01 0.04 0 0.17 outside the study room 16 0.43 0.26 0.06 0.89 inside the studied room 16 0.62 0.25 0.11 0.98 Total nurse activity time (minutes) 16 151 74.3 45.6 285 Total time that the nurse wore the dosimeter (min) 14 148 70.0 43.0 290 Amount of gas used during the measurement (kg) 15 0.75 0.71 0.02 2.67 Air velocity through exhaust grille (fpm) 4 66.5 13.2 54.0 83    Page 13 Table 7: Activities and characteristics during new mask use measurements  N Mean Std Min        Max Proportion of time that patient was: at the bed before using nitrous oxide 17 0.01 0.02 0 0.07 at the bed and using nitrous oxide 17 0.35 0.2 0.07 0.69 at the bed and not using for less than15 min 17 0.54 0.39 0.08 1.8 at the bed and not using for more than 15 min 17 0.12 0.14 0 0.58 at the chair and not using  17 0.01 0.02 0 0.08 at the chair and using  17 0.04 0.18 0 0.74 walking and not using 17 0 0.01 0 0.03 walking and using 17 0 0.01 0 0.05 in the bathroom  17 0.02 0.04 0 0.15 Total time patient breathed through mask (min) 17 51.87 36.09 8.33 157.09 Total patient activity time (minutes) 17 132.22 50.93 50.96 236.50 Proportion of time that the mask was: closer than 15 cm from the patient’s face 17 0.40 0.26 0.08 0.90 between 15 and 30 cm from the patient’s face 17 0.16 0.15 0.1 0.55 further than 30 cm from the patient’s face 17 0.43 0.25 0 0.92 Total mask to face activity time (minutes) 17 134.39 51.67 50.95 246.50 Proportion of time that nurse was: within arm’s length of the patient 17 0.49 0.21 0.23 0.87 between arm’s length and 3 meters of the patient 17 0.12 0.17 0 0.63 more than 3 meters from the patient 17 0.01 0.02 0 0.08 in the bathroom with the patient 17 0 0 0 0.02 outside the study room 17 0.37 0.21 0.04 0.70 inside the studied room 17 0.64 0.20 0.30 0.96 Total nurse activity time (minutes) 17 132.08 51.86 50.96 241.50 Total time that the nurse wore the dosimeter (min) 17 152.0 58.43 70.00 251.00 Amount of gas used during the measurement (kg) 17 0.57 0.4 0.1 1.32 Air velocity through exhaust grille (fpm) 12 83.42 19.86 58.00 125.00 Air flow through exhaust grille (cfm) 12 222.7 53.0 154.9 333.8 Air flow through mask at the start of the day (cfm) 17 14.09 3.20 9.50 20.90 Air flow through mask at end of measurement (cfm) 5 13.10 1.63 10.50 14.40   Table 8: Percent, number of measurements in each room, stratified by new and existing mask types  New mask Existing mask Room 6 (23 measurements in total) 74%, 17 26%, 6 Room 7 (10 measurements in total) 0%, 0 100%,10   3.5.4 Exposure Modeling  In order to consider which factors most influenced the room air nitrous oxide concentrations, an empirical exposure model was built. In addition to the fundamental issue in question, mask type, other factors considered potentially related to exposure included those outlined in Tables 5, 6 and 7 above.  The first step in the modeling was to consider which independent variables were highly correlated with each other. The following had strong positive Pearson correlations (> 0.70): • Percentage of time the mask was closer than 15 cm to the patient’s face and the nurse was within arm’s length of the patient • Percentage of time the patient and the nurse were in the bathroom • Percentage of time the mask was closer than 15 cm to the face and the patient was at the bed using nitrous oxide • Percentage of time that the patient had not started using nitrous oxide and percentage of time the mask was between 15-30 cm of the patient’s face  Page 14 The following had strong negative Pearson correlations (< -0.70): • Percentage of time the mask was within 15 cm of the patient’s face and percentage of time the mask was further away • Percentage of time the mask was further from the patient’s face and the time spent in the bed using nitrous oxide • Percentage of time the nurse was inside vs. outside the patient’s room Variables selected for further modeling were those that were more directly related to increases in exposure (e.g., time spent in the room by the nurse, rather than time spent away from the room).  After considering the relationship of the independent variables to nitrous oxide concentrations in simple linear regression, the following variables were offered in the multiple linear regression model: • mask type • weight entonox used during the measurement period • room Because there were only 33 full period measurements, the multiple regression model was of necessity parsimonious (i.e., it could include only a small number of independent variables).  Table 9 summarizes the coefficients, standard errors and p values of the independent variables included in the final determinants of exposure model for room concentrations of nitrous oxide. The new mask reduced exposures on average by 41 ppm, increasing weights of nitrous oxide used increased concentrations, and exposures were higher when the nurse spent more time in the room, perhaps because her help was needed to use the nitrous oxide. Table 10 summarizes the fit of the model; it explained 44% of the variance in room nitrous oxide concentrations.  Table 9: Coefficients (β), standard errors (SE), and p-values for independent variables in the multiple regression modela of factors influencing full-period room air concentrations of nitrous oxide.  β SE p-value Mask type (new = 1, existing = 0)  -40.6 12.3 0.0026 Weight entonox per measurement (in kg)  22.4 11.0 0.051 Time nurse spent in labour and delivery room (in min)  48.2 27.1 0.085 Intercept 36.6 20.5 0.084 a Proc Reg procedure in SAS  Table 10: Fit of multiple regression model for full-period room air concentration nitrous oxide Model characteristics Number of observations 33 Degrees of freedom 3 F value 7.22 p-value 0.0010 R2  0.44    Page 15 3.6 Results: Minute-to-Minute Data  Within the thirty-three labour and deliveries during which measurements were taken, minute-to-minute data were collected at 4,719 one-minute intervals. The data include not only the nitrous oxide measurements near the head of the patient’s bed and in the nursing station, but also patient activities and mask location in relation to the patient’s face, providing a much richer data set for modeling the determinants of room air concentration. In total 2,314 data points were collected with the new mask and 2,405 were collected with the existing mask.  The following sections provide first a descriptive summary of the patients’ activities and mask locations, then details of the determinants of exposure modeling.  3.6.1 Patient Activities and Mask Locations  Table 11 shows the proportions of the study measurement periods during which the nitrous oxide delivery and scavenging masks were various distances from the patients’ faces. It is interesting to note that, for both mask types, when the mask was not next to the face (i.e., within 15 cm), the tendency was for the patients to hold it too far away to scavenge exhaled breath (> 30 cm).  Table 11: Percentage of measurement period that the nitrous oxide mask was the following distances from the patient’s face Distance mask to face: Both Masks Existing Mask New Mask <15 cm 36.9 33.3 40.3 Between 15 and 30 cm 17.8 19.5 16.3 > 30 cm 45.8 48.2 43.5  Tables 12 and 13 summarize the patients’ activities, cross-tabulated with the mask distance from the patients’ faces, Table 12 for the existing mask, and Table 13 for the new mask. Data for both mask types indicate that patients do not always have the mask close to the face when they are using the nitrous oxide, and once they stop using the nitrous oxide, the mask is usually more than 30 cm from the face.  Table 12: Existing Mask – percentage of measurement period that patients spent in certain activity and percentage of activity with mask various distances to the face Percentage of activity with mask: Patient activity: Percentage of measurement period in activity 0-15 cm from the face 16-30 cm from the face > 30 cm from the face at the bed never used before  3.49 0 4.76 95.24 at the bed and using nitrous oxide  33.2 89.36 7.51 3.13 at the bed ≤ 15 min after use  45.0 4.16 32.96 62.88 at the bed > 15 min after use  8.77 0 0 100.00 at the chair and not using  0 0 0 0 at the chair and using 0 0 0 0 in the bathroom  4.74 21.93 16.67 61.40 walking and not using  3.66 17.05 23.86 59.09 walking and using  1.08 88.46 0 11.54  Page 16  Table 13: New Mask – percentage of measurement period that patients spent in certain activity and percentage of activity with mask various distances to the face Percentage of activity with mask: Patient activity: Percentage of measurement period in activity 0-15 cm from the face 16-30 cm from the face > 30 cm from the face at the bed never used before  0.56 0 92.31 7.69 at the bed and using nitrous oxide  35.3 92.52 1.59 5.88 at the bed ≤ 15 min after use  43.8 13.03 32.74 54.23 at the bed > 15 min after use  11.3 0 3.05 96.95 at the chair and not using  0.26 0 0 100.00 at the chair and using 0.35 0 0 0 in the bathroom  8.08 21.93 15.51 62.57 walking and not using  0.39 0 0 100.00 walking and using  0 100.00 0 0   3.6.2 Exposure Modeling  In order to consider which factors most influenced the minute-to-minute room air nitrous oxide concentrations, a second empirical exposure model was built. Because of the much greater size of the data set, it provided the opportunity to consider many more potential exposure determinants. It also posed some complications, since the measurements taken within a single labour and delivery could be correlated, in particular those measurements closest in time to each other. This model therefore controlled for within-day correlation by designating patient as a random variable and controlling for autocorrelation by time of measurement within the day.  Because of strong evidence from frequency distributions that the room air concentrations were positively skewed and symmetrically distributed when transformed, the concentration data were log-transformed (base 10) prior to modeling.  Variables not offered in the multiple linear regression models were excluded either because there was no reasonable a priori support for the hypothesis that there could be a relationship between the factor and the exposure, the factor was not associated with the exposure in “simple” linear regression while controlling for mask type (p>0.10), or it was strongly correlated with another selected variable more reasonably considered directly related to exposure. Independent variables which were correlated were expected and similar to those in the full period model, e.g., positive correlation between use of nitrous oxide and close to face mask location.  The following variables were offered to the multiple linear regression model: • Mask type; • Patient activities: at the bed never used before, at the bed > 15 min after use, at the bed and using nitrous oxide, at the chair and using, in the bathroom using, walking and using; • An interaction term for “new mask” and “ patient in the bathroom”, and • An interaction term for “new mask” and “patient at the bed > 15 min after use”. The interaction terms were used for characteristics which varied considerably for the different mask types.  Page 17  Tables 14 summarizes the coefficients, standard errors and p values of the independent variables included in the final determinants of exposure model for minute-to-minute room air concentrations of nitrous oxide. Lower exposures were associated with use of the new mask, the period of time before the patient used nitrous oxide, periods more than 15 minutes after use of the nitrous oxide (especially with the new mask), and periods when the patient was walking and using the nitrous oxide. The latter decrease may be a chance finding given that this patient activity was rare. It may also be due to the patient being farther from the point at which samples were drawn into the direct reading monitor. Higher exposures were observed during nitrous oxide use at the bed or chair, during periods when the patient was in the bathroom, particularly in the “new mask” deliveries (use of nitrous oxide during periods in the bathroom was unknown, since the researchers could not make observations). When using the existing mask, there was no scavenging available in the bathroom. Flow through the new scavenging system, for the new mask, dropped dramatically during use in the bathroom. This was attributed to water mist blocking the air filter in the scavenging hose.  Table 15 summarizes the fit of the model; it explained 63% of the variance in room nitrous oxide concentrations.  Table 14: Coefficients (β), standard errors (SE), and p-values for independent variables in the multiple regression modela of factors influencing minute-to-minute room air concentrations of nitrous oxide (log-transformed). All variables are dichotomous.  β SE p-value New mask (N = 2314)  -0.27 0.23 0.0183 Patient at the bed never used before (N = 97) -2.28 0.08 <0.0001 Patient at the bed > 15 min after use (N = 473) -0.73 0.05 <0.0001 Patient at the bed and using (N = 1615) 0.15 0.023 <0.0001 Patient at the chair and using (N = 26) 0.34 0.13 0.0122 Patient in the bathroom (N = 301) 0.53 0.07 <0.0001 Patient walking and using (N = 8) -1.17 0.24 <0.0001 Interaction term: “New mask” and “Patient in the bathroom”  0.56 0.10 <0.0001 Interaction term: “New mask” and “Patient at the bed > 15 min after use” -1.00 0.07 <0.0001 Intercept 0.09 0.34 <0.0001 N = number of measurements in dataset with this characteristic a Proc mixed procedure in SAS, with day as random variable and with autocorrelation between and within the day.    Page 18 Table 15: Fit of multiple regression model for minute-to-minute room air concentration of nitrous oxide Model characteristics Log of Room air concentration nitrous oxide (lppm) Number of observations 4719 Degrees of freedom 9 Chi2 2155.38 p-value <0.0001 -2 Res Log Likelihood 9748.3 R2 a 0.63 GMb of predicted values 3.63 GSDb of predicted values 0.87 Minimum predicted value 0.78 Maximum predicted value 6.43 a the proportion of variance explained (R2) could not be calculated via the proc mixed in SAS, therefore the predicted values were taken and a simple linear regression was done with measured minute-to-minute room air concentrations. b GM = geometric mean, GSD = geometric standard deviation   Real-time Area Concentrations of Nitrous Oxide Existing mask, N = 1069 Time- intervals less than 1 minute 1069N = pp m 400 350 300 250 200 150 100 50 0 -50      Page 19  Real-time Area Concentrations of Nitrous Oxide New mask, N = 1192 Time- intervals less than 1 minute   1192N = pp m 400 350 300 250 200 150 100 50 0 -50          Page 20   3.7  Results: Questionnaire Soliciting Feedback about the Masks  3.7.1 Patients Thirteen patients who used the new system were interviewed and responded to up to 8 questions.    Four had previous experience with Entonox in labour, one found the old mask easier to hold, two found the new mask easier to hold. Twelve expressed a willingness to use the system again. One patient liked the feeling of air from the scavenging system flowing over her face. The outer mask made uncomfortable contact with the face of two patients.  Several suggestions were offered to designers of the next version of the system. These were: Make the (outer) mask a little smaller Make the outer mask clear instead of green. Make the handle (fitting that connects the 38mm hose?) longer. Make mask so patient can use it without holding it. Longer hose. (two patients mentioned this)  All of the patients said that the new system was demonstrated adequately and explained properly before they used it.  3.7.2 Nurses  In general, the nurses were curious about the new system and were interested in the results of the study. Two nurses found the new mask harder to use because the whole system was too big and because the hose was too short and too big too handle. The nurses also thought that a clear bowl would be better. Two nurses said future versions of the mask should be smaller, to make it easier to use.  One nurse said the outer mask should be softer. Nurses found that assembling the hose, filter and mask took too much time.  The hose fell off the outer mask several times. All of the nurses said that the mask was demonstrated and explained clearly before they had to work with it.  3.7.3 Research Staff  Although the existing system had been used for years, there were problems with it. The mask accidentally disconnected from the hose twice and one time the demand valve did not work properly.  The researchers also found the hose on the new system too short. When the patient moved to the side of the bed opposite the system, the hose would not reach. The inlet in the wall for the scavenging system was on the right side of the patient. For left-handed patients it was a little bit harder to work with the new system because the hose was too short to reach where they wanted to have their left hand. The whole construction seemed a bit big, because sometimes the whole face of the patient was hidden by the bowl so the patient was not able to see what was happening in the room. When the patient was in the bathroom the scavenging system had to be disconnected and reconnected in the bathroom, adding to workload and care of the system.  Page 21  The researchers noted that the inner mask fell off the fitting from the demand valve several times with some of the later participants.  This may have been caused by excessive movement of the demand valve in the outer mask, or deterioration of the inner mask.  A more rigid connection between the demand valve and the outer mask was suggested.  3.7.4 Others’ concerns  The lawyer representing Richmond Hospital, and the Medical Chief of Staff wanted to monitor a volunteer’s exhaled carbon dioxide concentration while using the new and the present entonox system. Exhaled carbon dioxide is measured during surgical anesthesia as a patient safety measure. Some exhaled breath is contained in the facepiece of a mask, and rebreathed. If a patient breathes a large portion of her exhaled breath, her exhaled carbon dioxide will increase, and serious harm may result. One of the researchers breathed entonox through the present system and the new mask every 3 minutes for 40 minutes, while exhaled carbon dioxide was monitored. There was no significant difference in his exhaled carbon dioxide concentration.  4.0 Conclusions  The nitrous oxide mask and scavenging systems and the general ventilation tested in this study both failed to protect nurses and other occupants of the labour and delivery rooms from concentrations of nitrous oxide in excess of the WCB Exposure Limits. The new mask and scavenging system did reduce average concentrations in the room and nurses’ average personal exposures to about half those measured with the existing mask and scavenging system.  Factors which increased concentrations of nitrous oxide in the labour and delivery room included use of Entonox at the bed or chair, and time spent in the bathroom. The new mask was associated with lower concentrations in general, and with additional reductions in the periods more than 15 minutes post Entonox use.  Nurses, research staff and some patients found the outer mask of the new system too large and difficult to hold, and the hose too short to allow optimum mobility. The data showed that patients did not hold the mask close to the face after using nitrous oxide, minimizing gas scavenging in the 15-minute period post use when exhaled concentrations were high. A design that addresses these concerns needs to be tested in the laboratory prior to another field trial.  The filter of the new scavenging system got wet and blocked air flow when the system was used in the shower. The next field trial should be done without a filter in the scavenging hose.    Page 22 5.0 Recommendations to Health Care Facilities  5.1 Scavenging in Labour and Delivery Rooms  The results of this study indicate that improvements can be made to the existing nitrous oxide mask and scavenging system to reduce exposures of nurses and other personnel in the labour and delivery room.  The new system itself should be reconfigured based on data from this trial. In particular, the system should be designed to improve its mobility, its ease of use (i.e., size, weight, ease of holding), and the likelihood that it will be held close to the face post-use during the nitrous oxide exhalation period. Consultations with patients and nursing staff during this redesign would be very helpful.  5.2 General Ventilation in Labour and Delivery Rooms  In rooms where there is no scavenging system (5 of 7 rooms at Richmond Hospital), dilution by general ventilation is the only exposure control currently in place.  General ventilation also supplements the scavenging exhaust systems used in room 6 and 7.  While waiting for optimization of the scavenging mask system and its introduction in all labour and delivery rooms, the general ventilation systems in these rooms should provide as much air movement as possible at the head of the patient bed. What is possible will be limited by the need to avoid overcooling the newborn infant, when it is placed on the mother’s chest. The air supply diffusers should be of a type that is not readily adjusted by nurses or other occupants of the room. Low velocity displacement- style diffusers are suggested.  An example of a better general exhaust system placement was observed by the authors at labour and delivery rooms in Langley Hospital. Key features of the system are two perforated faceplate style diffusers located over the foot of the bed, and an exhaust grill in the wall about 1.2 meters from the head of the bed. The diffusers send supply air straight down, and it spreads in all directions when it hits the bed.  Based on the amounts of Entonox used in the deliveries during this study, we calculated the general ventilation rates required to dilute nitrous oxide concentrations to less than the WCB Exposure Limits. The average mass of Entonox used per patient was 0.668 kg, and the average time over which it was used was 89 minutes. To keep the average concentration in the room below the 8-hour Exposure Limit, the general ventilation flow rate would need to be about 800 litres/second.  If the general ventilation system were designed to control the 90th percentile nitrous oxide emission, the flow rate would need to be about 1,600 litres/second.  The current design flow rates for the labour and delivery rooms at both Richmond and Langley Hospitals are in the range of 250 to 350 litres/second. If the only control is general ventilation at 325 litres per second, the median Entonox use rate will produce an average concentration of 63 ppm, 2.5 times the 8 hour Exposure Limit. In addition, we observed that when the nurse is in the room she has her face within 600 mm of the patient's face much of the time. The concentration there is likely to be several times the room average. Better selection and location of supply and exhaust grills should reduce the difference between the concentration near the patient and the average concentration in the room. Our measurements of nurses’  Page 23 exposures confirm overexposures on a regular basis, even with scavenging systems in place.  5.3  Overall Guidance  The following guidance is offered to the designers of any new facility where patients will self-administer anaesthetic gas. All labour and delivery rooms should be equipped with a scavenging system for exhaled anaesthetic. The duct leading to the hose fitting must be sized for a flow of at least 7 liters per second (15 cfm) for each room (50 mm diameter suggested).  The general ventilation system should provide 800 liters per second of supply and exhaust air flow. At least 65% of the air flow must be delivered through displacement type diffusers with perforated face plates. These should be located over the foot of the bed. Diffusers that can be adjusted from the room, without tools, should not be used. At least 50% of the exhaust flow must be removed through a grille or grilles located in the wall, within 1.2 meters of the head of the bed.  If Entonox is used in the shower or bath, the bathroom should be large enough to allow a general exhaust flow of at least 200 litres per second while maintaining comfortable air velocities for a wet person. The bathroom should be equipped with a scavenging system for exhaled anaesthetic. The duct leading to the hose fitting must be sized for a flow of at least 7 liters per second (15 cfm) for each room. (50 mm diameter suggested).  Air exhausted from rooms where anaesthetic gas is routinely used should not be recirculated or exhausted where it might contaminate air intakes.  5.4  Other Recommendations  Another method to control exposures is to reduce the nurses’ shift length from 12 to 8 hours. This will reduce the total mass of nitrous oxide breathed during a shift, but it may not change the amount breathed over a month or a year. The WCB Exposure Limit for 12-hour shifts is one-half the limit for 8-hour shifts, because of both the increased exposure time and decreased excretion time away from the work setting.  To improve use of the scavenging system in future trials, it’s introduction to staff should be carefully planned. Consider sending all staff in the facility a brochure describing the mask system in detail and the research program.  Follow up with meetings with all staff to provide more detailed explanations and to answer questions. A video that explains how and why nitrous oxide should be exhaled into the mask should be made and shown to as many patients as possible, perhaps when they make a pre-labour visit to the maternity ward, or at pre-natal classes.  5.5  Nitrous Oxide Use in Other Hospital Departments  Patients in some emergency wards are given the option of inhaling Entonox for pain control. Patients using Entonox should be in a room and near an exhaust grill that has a flow of 800 litres/second. The mask should be connected to a scavenging system.   Page 24 5.6  Program Monitoring  Exhaust systems serving areas where anaesthetic gas is routinely used should be continuously monitored for air velocity or static pressure.  The personal exposure of hospital staff working near patients who are using Entonox, e.g. nurses, should be checked at least yearly. Where exposures exceed legislated or locally adopted levels, corrective action will be required.  6.0 Acknowledgements   The evaluation of the new scavenging system was funded in part by the Occupational Health and Safety Agency for Healthcare in BC (OHSAH).  Anna Matheson, Manager of Health & Safety at Richmond Hospital brought the need for a better scavenging system to our attention, and provided crucial support and assistance throughout the process. Writing the grant application was a notable part of her contribution.  Prof. Anthony Hodgson, of the Mechanical Engineering Department at UBC opened the door for us to recruit students Jennifer Oh, Samira Barakat, Eric Hung and Ciaran McMahon for the 1999 to 2000 school year.  Jennifer and Ciaran brought Kasra Asrar Haghighhi into the project for 2000 – 2001.  Prof. Don McAdam was their supervisor.  Many staff at Richmond Hospital provided support and assistance during the field evaluation.  Special thanks to Barb Stoddard, Program Manager, Women's Health, Lyn Jones, Clinical Resource Nurse and all the other nurses in Maternity.  The Biomedical Engineering Department, and especially Helen Robbins provided the Miran we used in Rooms 6 and 7, and other instruments.  Joe Kasstan, Manager Engineering, and Richard Jordsvar, Maintenance Foreman got the fan and ducts installed, and helped make the outer mask more comfortable for patients.  Our field researchers became part of the maternity ward for most of a year.  Marieke Verhoeven, Samira Barakat, Kathryn Toews, Min Xu, and Yun Tang each contributed several months.  Two of them were in the hospital, sampling, putting data into Excel or just waiting for a participant, 5 days a week.  Marieke returned to do the data analysis.  Our heartfelt thanks to all of you for having moved us closer to the goal of making nurses’ nitrous oxide headaches much less common.  References  1.  Rowland, A.S. et al. Nitrous oxide and spontaneous abortion in female dental assistants. American Journal of Epidemiology 1995; 141(6):531-8 2.  Guirguis, S.S. et al. Health effects associated with exposure to anaesthetic gases in Ontario hospital personnel. British Journal of Industrial Medicine 1990;47:490-497 3. Bernow, J. et al. Pollution of delivery ward air by nitrous oxide, Effects of various modes of room ventilation, excess and close scavenging Acta Anaesthesiol Scand 1984;28:119-123 4.  British Columbia Reproductive Care Program. Obstetric Guideline 4, Pain Management During Labour May 2000  Page 25 5.   Workers’ Compensation Board of British Columbia. Occupational Health and Safety Regulation. Part 5: Chemical and Biological Substances. 1998   Page 26  School of Occupational & Environmental Hygiene Eval uat i on of  a new mask and scavengi ng syst em f or  ni t r ous oxi de used i n l abour  and del i ver y Emission Control for Humans  School of Occupational & Environmental Hygiene Wha t  i s  t he  Pr obl e m?  Second-hand nitrous oxide unhealthy for nurses • spontaneous abortions • birth defects • headaches • drowsiness  School of Occupational & Environmental Hygiene Wha t  i s  t he  s our c e  of Ni t r ous  Oxi de  A Cylinder of mixed N2O and O2  Through the Mom • leaks from valve, hose, regulator a possibility  School of Occupational & Environmental Hygiene Pr e s e nt  Cont r ol s  General Ventilation  Supply near ceiling  Exhaust near floor  315 litres/second  School of Occupational & Environmental Hygiene Fl oor  Pl a n   School of Occupational & Environmental Hygiene Room 6   School of Occupational & Environmental Hygiene The  Pr e s e nt  Ma s k   School of Occupational & Environmental Hygiene The  Ol d Sc a ve nge r  Intermittent Flow Scavenging  Positive Pressure in Mask Req’d  School of Occupational & Environmental Hygiene Ne w Cont r ol  Continuous Scavenging at 15 cfm  General Ventilation as before  School of Occupational & Environmental Hygiene Ne w Ma s k Up Cl os e   School of Occupational & Environmental Hygiene Ne w Ma s k & Sa mpl e  Hos e   School of Occupational & Environmental Hygiene Expe r i me nt a l  Me t hod  Measure Parameters  Size of room  Room ventilation air flow  Time of day for all actions  Weight of gas cylinder before and after  With new mask, exhaust air flow before and after  Personal exposure from dosimeter analysis  School of Occupational & Environmental Hygiene Me t hod -  c ont i nue d  and observe  Location and mask related actions of patient  location of nurse  concentration of nitrous oxide reported by Miran in room  use of N2O in other rooms nearby  concentration of N20 at nursing station.  School of Occupational & Environmental Hygiene Me t hod -  La s t  pa r t  Request feedback from nurses  Ease of use  problems with mask or patient  Request feedback from Patients  School of Occupational & Environmental Hygiene Re s ul t s  Nurses’ exposures were reduced about 50%  Rate of Entonox used varied from 10 to 0.15 grams/minute  Patients held mask more than 300 mm from their face  There was an apparently random variation in the room* ventilation rate.  After the exhaust system was used in the shower flow rate dropped 80%  School of Occupational & Environmental Hygiene Fe e dba c k  Most patients were willing to use new mask again  Some patients wanted a longer exhaust hose Some patients found mask uncomfortable to hold  Hose connection point should  have been closer to the bed  School of Occupational & Environmental Hygiene Mor e  f e e dba c k  Outer mask made uncomfortable contact with some patient’s faces  Hose fell out of mask fitting for some patients.  Filter between mask and hose was difficult to install   School of Occupational & Environmental Hygiene Conc l us i ons  Continuous scavenging flow of 15 cfm results in significantly lower exposure than intermittent flow scavenging  Mask design needs improvement  Keeping mask closer to patient’s face would reduce exposures  School of Occupational & Environmental Hygiene Mor e  Conc l us i ons  Better General Ventilation is needed in Richmond, and is possible.  More hospitals should have exposures & control systems checked  School of Occupational & Environmental Hygiene Re c omme nda t i ons  –  Ge ne r a l Ve nt .   School of Occupational & Environmental Hygiene Cha nge  Suppl y & Exha us t  Displacement Supply over Bed  Exhaust near head of bed  School of Occupational & Environmental Hygiene Suppl y Di f f us e r s  & Exha us t   School of Occupational & Environmental Hygiene Be d,  Exha us t  Gr i l l e s   School of Occupational & Environmental Hygiene Ot he r  Appl i c a t i ons  Ambulances  Emergency ward  Other wards?

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