Standardization of skeletal muscle ischemic injury

Abstract

Observations on skeletal muscle function after a timed ischemic interval suggests significant interanimal variability. The purpose of this study is to compare the use of function versus time as a method for standardizing the degree of ischemic injury. Muscle function was measured by recording the isometric contraction to direct supramaximal tetanic stimulation of the anterior tibialis muscle (AT). Muscle cell viability was determined by the reduction of triphenyltetrazolium chloride (TTC)/g tissue measured by a spectrophotometric assay. Twenty-four New Zealand White rabbits underwent an interval of ischemia to one anterior tibialis muscle produced by collateral ligation and unilateral inflow control, using the contralateral AT as a control. The duration of ischemia was determined by one of three methods: Group I, n = 8, underwent ischemia for 3 hr (3 hr); Group II, n = 8, underwent ischemia until AT muscle function decreased to 20% of control (20%); and Group III, n = 8, underwent ischemia until AT muscle function decreased to <5% of control (<5%). Following the ischemic injury, both physiologic function and cellular viability were measured and expressed as a percentage of control. After 3 hr of ischemia, the mean function was 15.5 with a standard deviation of 20.2, and the TTC reduced/g tissue was 24.7 with a standard deviation of 24.8. When the ischemic interval was determined by a decline of muscle function to 20% of control, the mean TTC reduced was 33.4 ± 7.6. The mean TTC reduced, when the ischemic interval was terminated when muscle function reached <5% of control, was 13.5 ± 7.2. There is a high degree of correlation between the percentage function and the percentage TTC reduced for each animal ( r = 0.933). When time is used to standardize an ischemic interval there is a large degree of variability in the resulting cellular viability (SD = 24.8). This difference was significantly reduced ( P < 0.01) in both the 20 and <5% groups by use of function as a physiologic endpoint to mark the duration of the ischemic insult (SD = 7.6 and 7.2). We conclude that because of interanimal variability, use of function is superior to a predetermined timed ischemic interval as a method of standardizing the degree of an ischemic insult in skeletal muscle.