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Comparison of intramuscular absorption above and below the level of a spinal cord injury Spurrell, Valerie Jean


This study was designed to determine the difference in drug absorption between intramuscular injections given above and below the lesion in persons with a spinal cord injury. The theoretical framework identified three categories of factors known to influence serum drug absorption following intramuscular injections: drug action, diffusion area, and blood flow, with a focus on muscle innervation as one factor in the blood flow category. Hypotheses tested in this study were that serum trough and serum peak levels are greater and absorption time is shorter when I.M. injections of gentamicin are given above the level of a spinal cord injury than when given below. The study used an experimental, repeated measures design with counterbalancing and subjects acted as their own controls by receiving injections both above and below the level of injury. A total of five serum samples were drawn before and after a series of injections in each site to determine any differences in serum drug absorption. When it was not possible to obtain the sample size intended, case analysis was used to study the three subjects who enrolled in the study. Factors which may have influenced the difficulty in obtaining a larger sample are identified and discussed. The results of the study indicated that of the three cases, only in the first case were the serum trough and peak levels higher following deltoid injections than following gluteal injections. The contrary results for the second and third case may have been related to differences in the number of injections and differences in the dosing intervals. The time between serum samples was too great to accurately differentiate the absorption times, but pharmacokinetic values associated with the elimination phase suggested that absorption was delayed following injections below the level of injury. In all three cases the elimination rate constant was smaller, the half-life longer, and the volume of distribution larger following injections below the level of injury. In addition, the logarithmic plotting of serum levels following injections in paralyzed muscle was non-linear, adding further support to the supposition that absorption was delayed. With a sample size of three, conclusions are tentative. The findings suggest that peak and trough serum levels may be greater following injections above the level of injury as compared to below if the dosing interval is every 12 hours and steady state is achieved prior to serum samples being drawn. Given possible delayed absorption from paralyzed muscle, shortened dosing intervals may result in drug accumulation. The results of this study have implications for practice related to I.M. site selection, dosing levels, and serum levels. Education for health professionals must address innervation as a possible influence on I.M. drug absorption. This study provides insight into areas for further refinement of the theoretical framework as well as areas for further research.

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