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The occurrence of adverse events in relation to time after registration for coronary artery bypass surgery:… Sobolev, Boris G; Fradet, Guy; Kuramoto, Lisa; Rogula, Basia Apr 11, 2013

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RESEARCH ARTICLE Open AccessThe occurrence of adverse events in relation totime after registration for coronary artery bypasssurgery: a population-based observational studyBoris G Sobolev1*, Guy Fradet2, Lisa Kuramoto3 and Basia Rogula3AbstractBackground: Our objective was to evaluate the effect of delays on adverse events while waiting for coronary arterybypass grafting (CABG).Methods: An observational study that prospectively followed patients from registration on a wait list to removal forplanned surgery, death while waiting, or unplanned emergency surgery. The population-based registry provideddata on 12,030 patients with a record of registration on a wait list for first-time isolated CABG surgery between1992 and 2005.Results: In total, 104 patients died and 382 patients underwent an emergency surgery before planned CABG. Thedeath rate was 0.5 per 1000 patient-weeks in the semiurgent group and 0.6 per 1000 patient-weeks the nonurgentgroup, adjusted OR = 1.07 (95% confidence interval [CI] 0.69—1.65). The emergency surgery rate of 1.2 per 1000patient-weeks in the nonurgent group was lower compared to 2.1 per 1000 patient-weeks in the semiurgent group(adjusted OR = 0.72, 95% CI 0.54–0.97). However, the nonurgent group had a greater cumulative incidence ofpreoperative death than the semiurgent group for almost all weeks on the wait list, adjusted OR = 1.92 (95% CI1.25–2.95). The surgery rate was 1.2 per 1000 patient-weeks in the nonurgent group and 2.1 per 1000 patient-weeksin the semiurgent group, adjusted OR = 0.72 (95% CI 0.54–0.97). The cumulative incidence of emergency surgerybefore planned CABG was similar in the semiurgent and nonurgent groups, adjusted OR = 0.88, (95% CI 0.64–1.20).Conclusion: Despite similar death rates in the semiurgent and nonurgent groups, the longer waiting times in thenonurgent group result in a greater cumulative incidence of death on the wait list compared to that in thesemiurgent group. These longer waiting times also offset the lower rate of emergency surgery before plannedadmission in the nonurgent group so that the cumulative incidence of the emergency surgery was similar in bothgroups.Keywords: Coronary artery bypass surgery, Hospital wait list, Prioritization, Preoperative death, Emergency surgeryBackgroundDelaying access to surgical procedures is a common alter-native to having surplus capacity available at all times [1].As argued elsewhere, surgical wait lists have beenaccepted on the ground that they provide efficient use ofresources in health systems that budget the number ofsurgical procedures [2]. For example, cardiac servicesacross Canada use wait lists to manage access to coronaryartery bypass surgery (CABG) in periods when demandexceeds funded capacity [3-5]. Explicitly queuing patientsaccording to urgency of required treatment is used tofacilitate access to care within a clinically appropriate time.However, despite the concern that delays in necessarytreatment could lead to poor clinical outcomes, the pointat which the delay for CABG becomes too long has notbeen established [6].Our objective was to evaluate the effect of delays onthe occurrence of adverse events while waiting forCABG. In particular, we conducted an observationalstudy to achieve a better understanding of whether* Correspondence: boris.sobolev@ubc.ca1The University of British Columbia, 828 West 10th Avenue, Vancouver, BCV5Z 1M9, CanadaFull list of author information is available at the end of the article© 2013 Sobolev et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the CreativeCommons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andreproduction in any medium, provided the original work is properly cited.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74http://www.cardiothoracicsurgery.org/content/8/1/74longer delays for coronary artery bypass grafting contrib-ute to worsening of the condition in less urgent patientswaiting for planned CABG, and to estimate the risk ofunplanned emergency surgery among these patients. Weprospectively followed patients from registration on await list for first-time CABG to removal for plannedsurgery, death while waiting, or unplanned emergencysurgery. We used all relevant records from thepopulation-based registry of patients with angiographicallyproven coronary artery disease identified as needing by-pass surgery on a non-emergency basis between 1992 and2005. Primary comparisons have been done across syn-thetic cohorts of patients defined by the urgency at thedecision to proceed with surgery.MethodsData sourcesData from the British Columbia Cardiac Registries (BCCR)were used to identify the study participants and theirdemographic, clinical and treatment characteristics. Thispopulation-based patient registry prospectively capturesthe date of booking request for operating room time, andthe date of and reason for removal from the wait list, forall adult patients accepted for CABG in any of the fourcardiac centers in the province [7]. To identify cardiaccatheterization dates and coexisting medical conditions,we used each patient’s provincial health number to deter-ministically link BCCR records to the Canadian Institutefor Health Information (CIHI) Discharge Abstract Data-base (DAD) [8]. To identify coexisting conditions, we useddiagnoses reported in the DAD within one year prior tothe booking request. Census data on the decile of medianincome in enumeration area were based on the postalcode of the patient’s residence.PatientsWe studied patients who had a record of registration ona wait list for first-time isolated CABG surgery fromJanuary 1, 1992 to December 31, 2005, and who had arecord of catheterization procedure in the DAD. Theinception cohort had 14,049 records of registration forCABG from January 1, 1991 to December 31, 2005. Weexcluded 567 records of patients for various reasons:procedure at registration was not isolated CABG (312),procedure at registration or at surgery was not first-timeCABG (62), emergency cases at the time of registration(34), missing operating room reports (4), removed onthe registration date (101), registration was on a week-end and admission was day after (14), or the patient hadmultiple episodes (40). We also excluded 1,452 recordsof patients who were registered in 1991 (797) or did nothave a catheterization date (655). The remaining 12,030records had either the surgery date or the date andreason of removal from the list without surgery.Primary study variableThe study variable was urgency group at registration cate-gorized as urgent, semiurgent, and nonurgent. When pla-cing patients on wait lists in British Columbia, Canada, allcardiac surgeons indicate the urgency of CABG accordingto angiographic findings, symptom severity, and left ven-tricular dysfunction (ejection fraction less than 50%) toensure timing of revascularization according to the pro-vincial guidelines: within one week for urgent procedures,within six weeks for semiurgent procedures, and within 26weeks for nonurgent procedures [9].OutcomesThe primary outcomes were (1) preoperative death fromall causes and (2) unplanned emergency surgery whileawaiting a planned CABG. Surgeons on call made the deci-sion to operate on patients who presented to the emer-gency or admitting department. All admissions from theemergency department and admissions from other loca-tions bearing an emergency code were classified asunplanned emergency surgery. The date at which asurgeon’s office submits the operating room bookingrequest for surgery serves as the date of registrationon the list. Because scheduling is done weekly, wait-list time for each patient was computed as the number ofcalendar weeks from registration to removal from waitlists or end of study period. We restricted the analysis tothe first 52 weeks following registration because of thelack of information to identify periods when patients werenot ready for surgery, which might have contributedto extended waits.Potential confoundersThe existing literature suggests that elderly patients aremore likely to undergo revascularization as an urgentprocedure [10], that smaller diameter of the coronaryvessels may account for the higher risk of adverse car-diovascular events among women [11], that co-existingconditions may delay open heart surgery [12], that insti-tutional constraints and individual care providers mayaffect clinical outcomes [13], that patients with a lowersocioeconomic status may wait longer for cardiac sur-gery [14], and that changes in practice or the availabilityof supplementary funds may reduce the waiting timeuntil surgery [15]. To identify comorbidities at the timeof registration, we used diagnoses reported in the DADwithin one year prior to registration. The reference cat-egory was defined as no coexisting conditions. The firstcomparison category was defined as patients with anyof the following conditions at presentation: congestiveheart failure, diabetes mellitus, chronic obstructive pul-monary disease, cancer, or rheumatoid arthritis [16]. Thesecond comparison category was defined as patientsSobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 2 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Table 1 Characteristics of 12,030 patients, registered for bypass surgery in British Columbia 1992–2005, overall and byurgency group at registrationCharacteristic All patients* (n = 12,030) Urgent (n = 739) Semiurgent (n = 8,769) Nonurgent (n = 2,304)SexMen 9981 (83.0) 599 (81.1) 7327 (83.6) 1878 (81.5)Women 2049 (17.0) 140 (18.9) 1442 (16.4) 426 (18.5)Age group (years)<50 851 (7.1) 49 (6.6) 606 (6.9) 187 (8.1)50–59 2665 (22.2) 140 (18.9) 1946 (22.2) 541 (23.5)60–69 4510 (37.5) 266 (36.0) 3313 (37.8) 858 (37.2)70–79 3648 (30.3) 247 (33.4) 2652 (30.2) 657 (28.5)≥80 356 (3.0) 37 (5.0) 252 (2.9) 61 (2.6)Coronary anatomy at registrationLeft main 1780 (14.8) 493 (66.7) 1253 (14.3) 27 (1.2)Multivessel† 8715 (72.4) 195 (26.4) 6673 (76.1) 1792 (77.8)Limited‡ 1535 (12.8) 51 (6.9) 843 (9.6) 485 (21.1)Comorbidity at registrationMajor conditions§ 2901 (24.1) 184 (24.9) 2084 (23.8) 556 (24.1)Other conditions|| 2856 (23.7) 217 (29.4) 2139 (24.4) 462 (20.1)None 6273 (52.1) 338 (45.7) 4546 (51.8) 1286 (55.8)Calendar period at registration1992–1996 4489 (37.3) 390 (52.8) 3239 (36.9) 822 (35.7)1997–2001 4293 (35.7) 200 (27.1) 3049 (34.8) 1013 (44.0)2002–2005 3248 (27.0) 149 (20.2) 2481 (28.3) 469 (20.4)Institution at registration1 2668 (22.2) 137 (18.5) 1987 (22.7) 523 (22.7)2 2873 (23.9) 258 (34.9) 2380 (27.1) 202 (8.8)3 2914 (24.2) 62 (8.4) 1455 (16.6) 1249 (54.2)4 3575 (29.7) 282 (38.2) 2947 (33.6) 330 (14.3)Institution booking catheterization1 2759 (22.9) 152 (20.6) 2079 (23.7) 500 (21.7)2 2775 (23.1) 249 (33.7) 2296 (26.2) 201 (8.7)3 2037 (16.9) 36 (4.9) 1048 (12.0) 857 (37.2)4 2798 (23.3) 211 (28.6) 2235 (25.5) 331 (14.4)Other 1661 (13.8) 91 (12.3) 1111 (12.7) 415 (18.0)Mode of admission for catheterizationEmergency department 862 (7.2) 100 (13.5) 644 (7.3) 107 (4.6)Otherwise¶ 11168 (92.8) 639 (86.5) 8125 (92.7) 2197 (95.4)Urgency at admission for catheterizationElective 9600 (79.8) 496 (67.1) 6920 (78.9) 2007 (87.1)Emergency or urgent 2430 (20.2) 243 (32.9) 1849 (21.1) 297 (12.9)Weeks between catheterization and registration0–1 6651 (55.3) 519 (70.2) 4743 (54.1) 1268 (55.0)2–3 2066 (17.2) 120 (16.2) 1564 (17.8) 357 (15.5)4–5 1041 (8.7) 40 (5.4) 811 (9.2) 174 (7.6)6–7 642 (5.3) 17 (2.3) 483 (5.5) 131 (5.7)Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 3 of 14http://www.cardiothoracicsurgery.org/content/8/1/74presenting with other coexisting chronic conditions, asdefined elsewhere [17].Other confounders include hospital booking cath-eterization to address variation in standards and calen-dar year of surgery decision as a proxy of changes inpractice and available funding. We also included thetime between catheterization and surgery, the mode ofadmission for catheterization, urgency at admission forcatheterization, which may differ substantially among hos-pitals affecting estimates of the total of delays in undergo-ing the operation [18]. The time between catheterizationand registration was computed as the number of calendarweeks. The catheterization dates were obtained from theCIHI DAD and defined as the most recent diagnostic(Canadian Classification of Procedure (CCP) codes 4892–4898, 4996, 4997) or therapeutic (CCP codes 4802, 4803,4809) catheterization performed within one year preced-ing and including the date of booking. We used the dateof most recent catheterization procedures (diagnostic ortherapeutic) because the results of this procedure are mostlikely linked to decision to operate [19].Probability of remaining on the list and weekly eventratesThe probability of remaining on the list within a certaintime of registration was estimated using the product-limit method [20]. Time to removal from the lists wascompared across urgency groups using the log-rank test[21]. Average weekly event rates were calculated as thenumber of events divided by the sum of observedwaiting times measured in weeks.Cumulative incidence of eventThe cumulative incidence function (CIF) of an event isthe proportion of CABG candidates experiencing theevent of interest (e.g. death) instead of competing events(e.g. planned surgery) by a certain time on the wait list[22,23]. Both the event rate and the probability ofremaining on the list influence the CIF. Therefore, if theCIF of an event differs between two groups when theevent rates are the same, then it is the probabilities ofremaining on the list that contribute to this difference.Using Gray’s test, the CIF was compared across urgencygroups [24]. Further details on the cumulative incidenceof event may be found in Additional file 1.Regression modelsThe effect size of urgency group on weekly rates ofdeath and unplanned emergency surgery were esti-mated using discrete-time survival regression models,which naturally gives rise to the odds ratio (OR) [25].To estimate the effect of urgency group on the cumu-lative incidence of death and unplanned emergencysurgery, regression methods for CIF were used [26].Further details on regression of CIF may be found inAdditional file 1.In these regression models, we adjusted for potentialconfounders allowing for at least 10 events per variable[27]. In the regression models for preoperative death, weadjusted for sex, age decade, comorbidities at registra-tion, calendar period of registration, and time betweencatheterization and registration. In the regression modelsfor unplanned emergency surgery, we adjusted for sex, ageTable 1 Characteristics of 12,030 patients, registered for bypass surgery in British Columbia 1992–2005, overall and byurgency group at registration (Continued)≥8 1630 (13.5) 43 (5.8) 1168 (13.3) 374 (16.2)Socioeconomic decile1 1160 (9.6) 77 (10.4) 818 (9.3) 246 (10.7)2 1208 (10.0) 68 (9.2) 888 (10.1) 236 (10.2)3 1172 (9.7) 85 (11.5) 832 (9.5) 241 (10.5)4 1182 (9.8) 46 (6.2) 916 (10.4) 202 (8.8)5 1122 (9.3) 82 (11.1) 825 (9.4) 194 (8.4)6 1103 (9.2) 67 (9.1) 799 (9.1) 211 (9.2)7 1119 (9.3) 65 (8.8) 805 (9.2) 214 (9.3)8 1167 (9.7) 65 (8.8) 862 (9.8) 222 (9.6)9 1138 (9.5) 75 (10.1) 819 (9.3) 233 (10.1)10 1124 (9.3) 76 (10.3) 818 (9.3) 203 (8.8)Unknown or missing 535 (4.4) 33 (4.5) 387 (4.4) 102 (4.4)*Includes 218 patients for whom urgency was not provided.†Two or three-vessel disease with stenosis of the proximal left anterior descending (PLAD) artery.‡Two-vessel disease with no stenosis of the PLAD artery or one-vessel disease with stenosis of the PLAD artery.§Congestive heart failure, diabetes mellitus, chronic obstructive pulmonary disease, rheumatoid arthritis, or cancer.||Peripheral vascular disease, cerebrovascular disease, dementia, peptic ulcer disease, hemiplegia, renal disease, or liver disease.¶Clinics or day surgery from reporting hospital, or direct patients from admitting department.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 4 of 14http://www.cardiothoracicsurgery.org/content/8/1/74group, coronary anatomy at registration as a proxy forseverity of coronary disease, comorbidities at registration,calendar period at registration, institution at registra-tion, institution at catheterization, mode of admission atcatheterization, urgency at admission for catheterization,and time between catheterization and registration. Weperformed additional analyses, in which we adjusted forsocioeconomic decile in these models.The Behavioural Research Ethics Board of the Universityof British Columbia approved the study protocol, Certifi-cate of Approval H06-80651.ResultsPatient characteristicsOverall, this population-based study included 12,030patients who were registered on a wait list for first-time isolated CABG surgery from January 1, 1992 toDecember 31, 2005. Among these patients, the majorityhad semiurgent status (73%), were men (83%), and between60 to 79 years of age (68%) (Table 1). As expected, urgentpatients were sicker than semiurgent or nonurgent pa-tients because they had a higher prevalence of left maincoronary artery disease (p < 0.001) and more comorbidities(p < 0.001). The institution at registration and the institu-tion booking catheterization also differed across urgencygroups (p < 0.001). Fewer nonurgent patients were admittedfor catheterization through the emergency department(p < 0.001) and more tended to be elective admissionsfor catheterization (p < 0.001). More urgent patients wereregistered on a wait list within a week of catheterization(p < 0.001). Socioeconomic decile also differed across ur-gency groups (p = 0.04).Table 2 Outcomes of registration for bypass surgery in British Columbia 1992–2005, by urgency group at registrationOutcome Urgent (n = 739) Semiurgent (n = 8,769) Nonurgent (n = 2,304) All patients (n = 12,030*)Death before surgery, no. (%) 4 (0.5) 63 (0.7) 32 (1.4) 104 (0.9)Unplanned emergency surgery, no. (%) 48 (6.5) 264 (3.0) 65 (2.8) 382 (3.2)Planned surgery, no. (%) 655 (88.6) 7,512 (85.7) 1,627 (70.6) 9,957 (82.8)Mean waiting time (STD), weeks 6 (7) 12 (10) 19 (12) 13 (11)Median waiting time (IQR), weeks 3 (1–7) 10 (5–17) 16 (9–26) 10 (5–18)Abbreviations: CABG = coronary artery bypass graft; STD = standard deviation; IQR = interquartile range.*Includes 218 patients for whom urgency was not provided.1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 5200.10.20.30.40.50.60.70.80.91UrgentSemiurgentNonurgentTime  since  registration  (weeks)Probability  of  remaining  on  the  listFigure 1 Estimated probability of remaining on wait list, by time since registration and urgency group.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 5 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Among these patients 9,957 (82.8%) underwent plannedsurgery within 1 year of registration and the remainingwere removed from the list for various reasons: 104 (0.9%)died, 382 (3.2%) had unplanned emergency surgery, 257(2.1%) continued to receive medical treatment, 231 (1.9%)declined surgery, 86 (0.7%) were transferred to anothersurgeon or hospital, 321 (2.7%) were removed for otherreasons, and 692 (5.8%) remained on the list after 52weeks or at the end of the study period. In total, almost500 (4%) patients had an adverse event while waiting for a1 2 3 4 5 6 7 8 9 101112050100150Calendar  monthInstitution 1SemiurgentNonurgentWait  list  size1 2 3 4 5 6 7 8 9 1011120255075100Calendar  monthInstitution 2Wait  list  size1 2 3 4 5 6 7 8 9 101112050100150200Calendar  monthInstitution 3Wait  list  size1 2 3 4 5 6 7 8 9 101112050100150Calendar  monthInstitution 4Wait  list  sizeFigure 2 Distribution of weekly wait-list size by calendar month for semiurgent and nonurgent groups in each institution.Table 3 Weekly rate of all-cause preoperative death, unplanned emergency surgery, and planned surgery in relation tourgency group, for patients registered for bypass surgery in 1992–2005, as measured by odds ratios derived fromdiscrete-time survival regression models*Preoperative deaths Emergency surgeries Planned surgeriesGroup Totalwait†No. ofeventsEvent rate‡(95% CI)OR§(95% CI)No. ofeventsEvent rate‡(95% CI)OR||(95% CI)No. ofeventsEvent rate‡(95% CI)OR||(95% CI)Urgent 4,676 4 0.9 (0.0–1.7) – 48 10.3 (7.4–13.2) 4.9 (3.4–7.2) 655 140.1 (129.3–150.8) 2.2 (2.0–2.5)Semiurgent 123,138 63 0.5 (0.4–0.6) 1.0 264 2.1 (1.9–2.4) 1.0 7512 61.0 (59.6–62.4) 1.0Nonurgent 53,232 32 0.6 (0.4–0.8) 1.1 (0.7–1.7) 65 1.2 (0.9–1.5) 0.7 (0.5–1.0) 1627 30.6 (29.1–32.0) 0.7 (0.6–0.7)Abbreviations: CI = confidence interval, OR = odds ratio.*Did not include 218 patients for whom urgency was not provided: 5 died, 5 had unplanned emergency surgery, 163 underwent planned surgery, 45 removed forother reasons.†Waiting time measured in patient-weeks.‡Weekly event rate was calculated as the number events divided by the sum of waiting times (per 1000 patient-weeks).§Adjusted for sex, age decade, comorbidities at registration, calendar period of registration, and time between catheterization and registration.||Adjusted for sex, age group, coronary anatomy, comorbidities at registration, calendar period at registration, institution at registration, institution atcatheterization, mode of admission at catheterization, urgency at admission for catheterization, and time between catheterization and registration.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 6 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Table 4 Odds ratios of preoperative death, unplanned emergency surgery, and planned surgery for patient and centerfactors, for patients registered for bypass surgery in 1992–2005, derived from discrete-time survival regression models*Factor Preoperative deathsOR (95% CI)Emergency surgeriesOR (95% CI)Planned surgeriesOR (95% CI)Urgency group at registrationUrgent NA1 4.93 (3.38–7.18) 2.22 (2.02–2.45)Semiurgent 1.00 1.00 1.00Nonurgent 1.07 (0.69–1.65) 0.72 (0.54–0.97) 0.67 (0.63–0.72)SexMen 1.00 1.00 1.00Women 0.48 (0.25–0.93) 1.06 (0.82–1.38) 0.89 (0.84–0.94)Age decade 1.36 (1.08–1.71) NA2 NA2Age group (years)<50 NA2 1.32 (0.87–1.99) 1.10 (1.01–1.20)50–59 NA2 1.00 1.0060–69 NA2 1.04 (0.78–1.37) 1.05 (1.00–1.11)70–79 NA2 1.20 (0.90–1.60) 1.07 (1.01–1.13)≥80 NA2 0.99 (0.52–1.88) 0.84 (0.73–0.96)Coronary anatomy at registrationLeft main NA3 1.00 1.00Multivessel† NA3 1.38 (0.96–1.97) 0.83 (0.78–0.89)Limited‡ NA3 1.93 (1.23–3.02) 0.95 (0.86–1.03)Comorbidity at registrationMajor conditions§ 1.73 (0.97–3.09) 1.01 (0.77–1.32) 0.91 (0.85–0.96)Other conditions|| 1.00 1.00 1.00None 0.96 (0.55–1.66) 0.88 (0.67–1.16) 0.87 (0.82–0.92)Calendar Period at registration1992–1996 1.33 (0.85–2.11) 1.13 (0.90–1.43) 1.16 (1.10–1.22)1997–2001 1.00 1.00 1.002002–2005 0.84 (0.48–1.47) 0.74 (0.55–0.98) 1.05 (0.99–1.10)Institution at registration1 NA3 1.71 (0.98–3.01) 0.78 (0.70–0.87)2 NA3 1.00 1.003 NA3 0.79 (0.47–1.33) 0.54 (0.49–0.59)4 NA3 1.23 (0.29–5.22) 1.98 (1.59–2.46)Institution from where catheterization was booked1 NA3 0.70 (0.40–1.23) 1.24 (1.11–1.39)2 NA3 1.00 1.003 NA3 1.00 (0.58–1.74) 0.98 (0.89–1.08)4 NA3 0.82 (0.19–3.50) 0.73 (0.59–0.92)Other NA3 0.31 (0.18–0.52) 1.16 (1.07–1.25)Mode of admission for catheterizationEmergency department NA3 5.66 (3.86–8.28) 0.96 (0.87–1.07)Otherwise¶ NA3 1.00 1.00Urgency at admission for catheterizationElective NA3 1.00 (0.70–1.43) 0.85 (0.79–0.90)Emergency or urgent NA3 1.00 1.00Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 7 of 14http://www.cardiothoracicsurgery.org/content/8/1/74planned CABG. Table 2 shows outcomes of registrationfor CABG across urgency groups. Among patients whounderwent surgery after unplanned emergency admission,the distributions of age (p = 0.80), sex (p = 0.46), time be-tween catheterization and registration (p = 0.29), and so-cioeconomic status (p = 0.28) did not seem to differ acrossurgency group at registration. Other characteristics dif-fered across the groups (p < 0.001).Distribution of wait-list timesThere were differences in the probabilities of remaining onthe list by a certain week across the three urgency groups,with shorter times in higher urgency groups (log-ranktest = 1329.2, df = 2, p < 0.001, Figure 1). In the nonurgentgroup, 75% of patients were remaining on the list after 9weeks, 50% after 19 weeks, and 25% after 34 weeks,whereas 50% and 25% were remaining after 10 and 19weeks in the semiurgent group and 3 and 7 weeks in theurgent group, respectively. There did not appear to be sea-sonality in the wait-list size for semiurgent and nonurgentgroups (Figure 2). As well, there was no variation in wait-list size in the urgent group over calendar months (medianwait-list size = 1; interquartile range = 0 to 2.Weekly preoperative event ratesIn total, there were 104 deaths for 184,820 patient-weeksof remaining on the list: 4 over 4,676 patient-weeks inurgent, 63 over 123,138 patient-weeks in semiurgent,and 32 over 53,232 patient-weeks in nonurgent (Table 3).The weekly death rate varied from 0.9 per 1000 patient-weeks in the urgent group to 0.5 per 1000 patient-weeksin the semiurgent group and 0.6 per 1000 patient-weeksin the nonurgent group. After adjustment, the weeklydeath rate in the nonurgent group was similar to thesemiurgent group (OR = 1.07, 95% confidence interval[CI] 0.69—1.65) (Table 3).Table 4 Odds ratios of preoperative death, unplanned emergency surgery, and planned surgery for patient and centerfactors, for patients registered for bypass surgery in 1992–2005, derived from discrete-time survival regression models*(Continued)Time between catheterization and registrationPer week 1.01 (0.98–1.03) NA2 NA20–1 weeks NA2 1.14 (0.85–1.53) 1.11 (1.04–1.17)2–3 NA2 1.00 1.004–5 NA2 1.17 (0.76–1.78) 1.09 (1.00–1.18)6–7 NA2 1.02 (0.61–1.69) 0.87 (0.79–0.97)≥8 NA2 0.87 (0.60–1.28) 0.87 (0.81–0.94)Abbreviations: OR = odds ratio, CI = confidence interval, NA1 = urgent patients were excluded from this analysis, NA2 = age was entered with alternative coding(continuous versus categorical), NA3 = not enough events per regression variable.*Did not include 218 patients for whom urgency was not provided: 5 died, 5 had unplanned emergency surgery, 163 underwent planned surgery, 45 removed forother reasons.†Two or three-vessel disease with stenosis of the proximal left anterior descending (PLAD) artery.‡Two-vessel disease with no stenosis of the PLAD artery or one-vessel disease with stenosis of the PLAD artery.§Congestive heart failure, diabetes mellitus, chronic obstructive pulmonary disease, rheumatoid arthritis, or cancer.||Peripheral vascular disease, cerebrovascular disease, dementia, peptic ulcer disease, hemiplegia, renal disease, or liver disease.¶Clinics or day surgery from reporting hospital, or direct patients from admitting department.Table 5 Cumulative incidence of all-cause preoperative mortality, unplanned emergency surgery, and planned surgeryin relation to urgency group, for patients registered for bypass surgery in 1992–2005, as measured by odds ratiosderived from regression models for pseudovalues of cumulative incidence functions*Preoperative deaths Emergency surgeries Planned surgeriesUrgency No. ofpatientsNo. ofevents% events†(95% CI)OR‡(95% CI)No. ofevents% events†(95% CI)OR§(95% CI)No. ofevents% events†(95% CI)OR§(95% CI)Urgent 739 4 0.5 (0.0–1.1) – 48 6.5 (4.7–8.3) 2.5 (1.7–3.6) 655 88.6 (86.3–90.9) 3.9 (3.4–4.6)Semiurgent 8,769 63 0.7 (0.5–0.9) 1.0 264 3.0 (2.7–3.4) 1.0 7512 85.7 (84.9–86.4) 1.0Nonurgent 2,304 32 1.4 (0.9–1.9) 1.9 (1.3–3.0) 65 2.8 (2.1–3.5) 0.9 (0.6–1.2) 1627 70.6 (68.8–72.5) 0.5 (0.5–0.6)Abbreviations: CI = confidence interval, OR = odds ratio.*Did not include 218 patients for whom urgency was not provided: 5 died, 5 had unplanned emergency surgery, 163 underwent planned surgery, 45 removed forother reasons.†At 52 weeks.‡Odds ratio of death adjusted for sex, age decade, comorbidities at registration, calendar period of registration and time between catheterization and registration.§Odds ratio of admission adjusted for sex, age group, coronary anatomy, comorbidities at registration, calendar period of registration, institution at registration,institution at catheterization, mode of admission at catheterization, urgency at admission for catheterization, and time between catheterization and registration.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 8 of 14http://www.cardiothoracicsurgery.org/content/8/1/74In total, there were 382 unplanned emergency surger-ies for 184,820 patient-weeks of remaining on the list: 48over 4,676 patient-weeks in urgent, 264 over 123,138patient-weeks in semiurgent, and 65 over 53,232 patient-weeks in nonurgent (Table 3). The surgery rate variedfrom 10.3 per 1000 patient-weeks in the urgent group to2.1 per 1000 patient-weeks in the semiurgent group and1.2 per 1000 patient-weeks in the nonurgent group. Afteradjustment, the weekly surgery rate was almost five timeshigher in the urgent group (OR = 4.93, 95% CI 3.38–7.18)and 28% lower in the nonurgent group (OR = 0.72, 95% CI0.54–0.97), compared to the semiurgent group (Table 3).After additional adjustment for socioeconomic decile, theeffects were similar in the urgent group (OR = 4.89, 95% CI3.33–7.16) and in the nonurgent group (OR = 0.72, 95% CI0.53–0.98).Table 4 shows the ORs of the all-cause preoperativedeath, unplanned emergency surgery, and planned sur-gery for the potential confounders that include patient-and center-specific factors.Cumulative incidence of eventIn total, 0.9% (95% CI 0.7–1.0) of patients registeredfor CABG died before planned surgery: 4 urgent, 63semiurgent, 32 nonurgent, and 5 with unknown urgency(Table 5). The nonurgent group had a greater cumu-lative incidence of preoperative death than the semiurgentgroup for most weeks on the wait list (Gray’s teststatistic = 9.4, df = 1, p = 0.002, Figure 3). Afteradjustment, the odds of death before planned surgerywere 1.9 times higher in the nonurgent group com-pared to the semiurgent group (OR = 1.92, 95% CI1.25–2.95) (Table 5). We attribute the higher cumulativeincidence of preoperative deaths in the nonurgent groupto the longer waiting times, because the death rates weresimilar in the semiurgent and nonurgent groups.In total, 3.2% (95% CI 2.9–3.5) of patients registeredfor a planned CABG had an unplanned emergency sur-gery: 48 urgent, 264 semiurgent, and 65 nonurgent(Table 5). The urgent group had the highest cumulativeincidence of unplanned emergency surgery for all weekson the wait list compared to the other two groups(Gray’s test statistic = 29.2, df = 2, p < 0.001, Figure 4).However, the cumulative incidences were not differentbetween the semiurgent and nonurgent groups (Gray’stest statistic = 0.28, df = 1, p = 0.60). After adjustment, theodds of unplanned emergency surgery were 2.5 timeshigher in the urgent group (OR = 2.49, 95% CI 1.71–3.61)but not different in the nonurgent group (OR = 0.88, 95%CI 0.64–1.20) as compared to the semiurgent group(Table 5). After additional adjustment for socioeconomicdecile, the effect did not change in the urgent group(OR = 2.45, 95% CI 1.67–3.59) and in the nonurgentgroup (OR = 0.87, 95% CI 0.63–1.20). The similar cu-mulative incidence of emergency surgery suggests thatthe longer waiting times in the nonurgent group offsetthe lower rate of emergency surgery in this group com-pared to the semiurgent group.1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 5200.0050.010.0150.02SemiurgentNonurgentGray’s test: p = 0.0020.7%1.4%Time  since  registration  (weeks)Cumulative  incidence  of  preoperative  deathFigure 3 Estimated cumulative incidence of all-cause preoperative death by urgency group.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 9 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Table 6 shows the ORs of all-cause preoperative death,unplanned emergency surgery, and planned surgery forpatient- and center-specific factors derived from theregression model for CIF.Analysis of competing eventsIn total, 9,957 patients underwent planned surgery over184,820 patient-weeks: 655 over 4,676 patient-weeks(140.1 per 1000 patient-weeks) in the urgent group,7,512 over 123,138 patient-weeks (61.0 per 1000 patient-weeks) in the semiurgent group, and 1,627 over 53,232patient-weeks (30.6 per 1000 patient-weeks) in thenonurgent group (Table 3). After adjustment, the weeklysurgery rate was over two times higher in the urgentgroup (OR = 2.22, 95% CI 2.02–2.45) and 33% lower inthe nonurgent group (OR = 0.67, 95% CI 0.63–0.72), com-pared to the semiurgent group. After additional adjust-ment for socioeconomic decile, the effects were similar inthe urgent group (OR = 2.24, 95% CI 2.03–2.48) and inthe nonurgent group (OR = 0.68, 95% CI 0.64–0.72).Overall, 82.8% (95% CI 82.1–83.4) of patients regis-tered for CABG underwent planned surgery: 88.6% (95%CI 86.3–90.9) in the urgent group, 85.7% (95% CI84.9%–86.4%) in the semiurgent group, and 70.6% (95%CI 68.8–72.5) in the nonurgent group (Table 5). The ur-gent group had the highest cumulative incidence ofplanned surgery for all weeks on the wait list, followed bythe semiurgent group and the nonurgent group had thelowest cumulative incidence (Gray’s test statistic = 539.6,df = 2, p < 0.001, Figure 5). After adjustment, the odds ofplanned surgery were about four times higher in the ur-gent group (OR = 3.94, 95% CI 3.36–4.62) and 48% lowerin the nonurgent group (OR = 0.52, 95% CI 0.48–0.57) ascompared to the semiurgent group (Table 5). After add-itional adjustment for socioeconomic decile, the effect didnot change in the urgent group (OR = 3.95, 95% CI 3.35–4.64) and in the nonurgent group (OR = 0.52, 95% CI0.48–0.57).DiscussionOur results confirm that queuing patients according tourgency of treatment contributes to a higher proportionof preoperative death among CABG candidates in theless urgent category. Even though the death rate wassimilar in the nonurgent and semiurgent groups, 0.5versus 0.6 per 1000 patient-weeks, patients in the non-urgent group were remaining on the list longer, whichresulted in a doubled cumulative incidence of all-causedeath compared to the semiurgent group (OR = 1.92,95% CI 1.25–2.95). Our results also suggest that longerwaiting times offset the lower rate of emergency surgerybefore planned admission in the nonurgent group thanin the semiurgent group, 1.2 versus 2.1 per 1000 patient-weeks, so that the cumulative incidence of the emer-gency surgery is similar (OR = 0.9, 95% CI 0.6–1.2).We studied patients who were registered on a wait listfor first-time isolated CABG surgery. Patients who1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 5200.020.040.060.08UrgentSemiurgentNonurgentGray’s test: p < 0.001 6.5%3.0%2.8%Time  since  registration  (weeks)Cumulative  incidence  of  emergency  surgeryFigure 4 Estimated cumulative incidence of unplanned emergency surgery by urgency group.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 10 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Table 6 Odds ratios of preoperative death, unplanned emergency surgery, and planned surgery for patient and centerfactors, for patients registered for bypass surgery in 1992–2005, derived from regression models for cumulativeincidence functions*Factor Preoperative deathsOR (95% CI)Emergency surgeriesOR (95% CI)Planned surgeriesOR (95% CI)Urgency group at registrationUrgent NA1 2.49 (1.71–3.61) 3.94 (3.36–4.62)Semiurgent 1.00 1.00 1.00Nonurgent 1.92 (1.25–2.95) 0.88 (0.64–1.20) 0.52 (0.48–0.57)SexMen 1.00 1.00 1.00Women 0.51 (0.27–0.98) 1.14 (0.87–1.49) 0.86 (0.78–0.93)Age decade 1.26 (1.00–1.59) NA2 NA2Age group (years)<50 NA2 1.20 (0.79–1.83) 1.12 (0.98–1.28)50–59 NA2 1.00 1.0060–69 NA2 0.99 (0.75–1.32) 1.05 (0.97–1.15)70–79 NA2 1.14 (0.85–1.54) 1.04 (0.95–1.13)≥80 NA2 1.09 (0.57–2.10) 0.69 (0.56–0.85)Coronary anatomy at registrationLeft main NA3 0.73 (0.50–1.05) 1.36 (1.23–1.50)Multivessel† NA3 1.00 1.00Limited‡ NA3 1.24 (0.89–1.73) 1.30 (1.17–1.45)Comorbidity at registrationMajor conditions§ 1.90 (1.06–3.41) 1.15 (0.87–1.51) 0.81 (0.74–0.89)Other conditions|| 1.00 1.00 1.00None 1.08 (0.62–1.89) 1.05 (0.79–1.39) 0.77 (0.71–0.84)Calendar Period at registration1992–1996 1.31 (0.83–2.07) 1.03 (0.81–1.30) 1.40 (1.30–1.51)1997–2001 1.00 1.00 1.002002–2005 0.83 (0.48–1.44) 0.76 (0.57–1.03) 1.05 (0.97–1.14)Institution at registration1 NA3 1.84 (1.16–2.91) 0.50 (0.42–0.58)2 NA3 1.00 1.003 NA3 1.16 (0.69–1.93) 0.38 (0.33–0.44)4 NA3 0.92 (0.33–2.62) 1.97 (1.45–2.68)Institution from where catheterization was booked1 NA3 0.63 (0.40–1.01) 1.28 (1.09–1.51)2 NA3 1.00 1.003 NA3 0.97 (0.57–1.66) 0.99 (0.85–1.15)4 NA3 0.86 (0.30–2.47) 0.73 (0.53–1.00)Other NA3 0.26 (0.15–0.45) 1.36 (1.20–1.54)Mode of admission for catheterizationEmergency department NA3 5.94 (4.05–8.71) 0.83 (0.70–0.98)Otherwise¶ NA3 1.00 1.00Urgency at admission for catheterizationElective NA3 1.13 (0.79–1.62) 0.70 (0.63–0.77)Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 11 of 14http://www.cardiothoracicsurgery.org/content/8/1/74underwent the procedure by direct admission to hospitalon a non-emergency basis were not included in theanalysis. Considering that cardiac surgeons in BritishColumbia have discretion for direct admission of theirpatients, these two groups may be incomparable interms of their clinical presentation and waiting time[28]. Our analysis focused on preoperative events,such as death on wait list. Therefore, we did not re-port on postoperative events.Our study had several limitations. First, because of itsobservational nature, patient and clinical factors that in-fluence the risk of preoperative events might have differ-ent distributions across urgency groups. To address thisissue, we used regression adjustment for measured factors.We also attempted to capture unmeasured factors byusing calendar period as a proxy for changes in the popu-lation of CABG patients. Even so, these techniques maynot fully address the issue of potential confounding due tounmeasured factors. The time to surgery may differ be-tween patients treated by surgeons with high volume ofCABG procedures and surgeons who perform a diverserange of cardiac procedures. Second, a potential concernis misclassification of the recorded urgency for treatment,because surgeons may select patients from the wait listsbased on various considerations, such as best use of oper-ating time or the availability of hospital resources. There-fore, the occurrence of preoperative events might havebeen influenced by the individual surgeon’s threshold foraccepting a patient for non-urgent treatment. Third, thetime to surgery may reflect patient and clinician decisionsfollowing the registration for surgery, in addition to sys-tem and clinical factors. We did not have this information.Therefore, our results can be interpreted only as the neteffect of timing of surgery in a possibly self-selectedpatient population. Fourth, care received outside of theCanadian health care system (e.g., paid for separately) mayimpact waiting times and outcomes. In our study, therewas no mechanism to ascertain such cases, if any,separately from the registry data. Finally, several studieshave shown that coexisting conditions are underreportedin administrative databases for patients discharged aftercardiovascular procedures [29-31]. As such, the effect oftiming of surgery may be attributable to unmeasured clin-ical factors, which might result in an upward bias insurvival effect for those unfit for the operation.ConclusionIn conclusion, the contribution of this article is two-fold.First, we present the perspective of health service researchon studying the risk of adverse events while waiting forrecommended treatment. The estimates of cumulative in-cidence of adverse events on CABG wait lists, which is afunction of both the event rate and the probability ofremaining on the list, may be useful to hospital managers.Our results provide evidence for capacity planning inmanaging access to CABG that would minimize the pre-operative adverse events associated with treatment delay,if unavoidable. For example, the point at which the waitfor CABG becomes too long can be established as theperiod by the end of which, for a given surgical capacity,the proportion of preoperative deaths exceeds a safetystandard accepted in the health system, e.g. postoperativein-hospital mortality in this patient population. Second,we provide data on risks associated with the anticipateddelays in undergoing the recommended coronary revascu-larization. In deciding on the duration of time that non-Table 6 Odds ratios of preoperative death, unplanned emergency surgery, and planned surgery for patient and centerfactors, for patients registered for bypass surgery in 1992–2005, derived from regression models for cumulativeincidence functions* (Continued)Emergency or urgent NA3 1.00 1.00Time between catheterization and registrationPer week 1.02 (0.99–1.04) NA2 NA20–1 weeks NA2 1.07 (0.79–1.45) 1.17 (1.07–1.29)2–3 NA2 1.00 1.004–5 NA2 1.14 (0.74–1.77) 1.05 (0.92–1.19)6–7 NA2 1.08 (0.64–1.82) 0.83 (0.71–0.97)≥8 NA2 0.96 (0.65–1.43) 0.82 (0.73–0.92)Abbreviations: OR = odds ratio, CI = confidence interval, NA1 = urgent patients were excluded from this analysis, NA2 = age was entered with alternative coding(continuous versus categorical), NA3 = not enough events per regression variable.*Did not include 218 patients for whom urgency was not provided: 5 died, 5 had unplanned emergency surgery, 163 underwent planned surgery, 45 removed forother reasons.†Two or three-vessel disease with stenosis of the proximal left anterior descending (PLAD) artery.‡Two-vessel disease with no stenosis of the PLAD artery or one-vessel disease with stenosis of the PLAD artery.§Congestive heart failure, diabetes mellitus, chronic obstructive pulmonary disease, rheumatoid arthritis, or cancer.||Peripheral vascular disease, cerebrovascular disease, dementia, peptic ulcer disease, hemiplegia, renal disease, or liver disease.¶Clinics or day surgery from reporting hospital, or direct patients from admitting department.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 12 of 14http://www.cardiothoracicsurgery.org/content/8/1/74emergency treatment can be delayed safely, policy-makersmay find it useful to measure the risk of preoperativedeath among those who remain untreated by a certaintime after registration on a wait list. For example, Figure 6shows that, conditional on not having undergone CABG bythe time recommended by the provincial guidelines, therisk of preoperative death reaches 0.3% for the semiurgentgroup and 2.1% for the nonurgent group.1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 5200.050.10.150.3%2.1%SemiurgentNonurgent 13.0%7.9%Time  since  registration  (weeks)Conditional  probability  of  preoperative  deathFigure 6 Estimated conditional probability of preoperative death by urgency group.1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 5200.250.50.751UrgentSemiurgentNonurgentGray’s test: p < 0.001 88.6%85.7%70.6%Time  since  registration  (weeks)Cumulative  incidence  of  planned  surgeryFigure 5 Estimated cumulative incidence of planned surgery by urgency group.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 13 of 14http://www.cardiothoracicsurgery.org/content/8/1/74Additional fileAdditional file 1: Methods for cumulative incidence of event.Competing interestsThe authors declare that they have no competing interests.Authors’ contributionsBS conceived the study concept and design, participated in analysis andinterpretation, and drafted the manuscript. GF participated in dataacquisition and critically revised the manuscript. LK participated in analysisand interpretation, and drafted the manuscript. BR performed statisticalanalysis and drafted the manuscript. All authors read and approved the finalmanuscript.AcknowledgementsThis study received financial support from the Canada Research ChairsProgram (BS), the Canada Foundation for Innovation (BS). None of thesponsors had a role in the study design; in the collection, analysis, andinterpretation of data; in the writing of the report; or in the decision tosubmit the paper for publication. We are indebted to nurses, cardiacsurgeons and cardiologists in the participating hospitals for their efforts toensure the completeness and accuracy of the registry data.Author details1The University of British Columbia, 828 West 10th Avenue, Vancouver, BCV5Z 1M9, Canada. 2The University of British Columbia, 2251 Pandosy Street,Kelowna, BC V1Y 1T1, Canada. 3Centre for Clinical Epidemiology andEvaluation, Vancouver Coastal Health Research Institute, 828 West 10thAvenue, Vancouver, Canada.Received: 19 November 2012 Accepted: 19 February 2013Published: 11 April 2013References1. Thomas SJ, Williams MV, Burnet NG, Baker CR: How much surplus capacityis required to maintain low waiting times? Clin Oncol (R Coll Radiol) 2001,13:24–28.2. Fierlbeck K: Health care in Canada: A Citizen’s guide to policy and politics.Toronto: University of Toronto Press; 2011.3. 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Med Care 2002, 40:675–685.doi:10.1186/1749-8090-8-74Cite this article as: Sobolev et al.: The occurrence of adverse events inrelation to time after registration for coronary artery bypass surgery: apopulation-based observational study. Journal of Cardiothoracic Surgery2013 8:74.Sobolev et al. Journal of Cardiothoracic Surgery 2013, 8:74 Page 14 of 14http://www.cardiothoracicsurgery.org/content/8/1/74

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