Multivariate prediction of spontaneous repetitive responses in ventricular myocardium exposed in vitro to simulated ischemic conditions.

Guinea-pig ventricular myocardium was partly exposed to normal Tyrode's superfusion and partly to altered conditions (using modified Tyrode's solution) set to simulate acute myocardial ischemia (PO2 80 +/- 10 mmHg; no glucose; pH 7.00 +/- 0.05; K+ 12 mM). Using a double-chamber tissue bath and standard microelectrode technique, the occurrence of spontaneous repetitive responses was investigated during simulated ischemia (occlusion) and after reperfusing the previously ischemic superfused tissue with normal Tyrode's solution (reperfusion). In 62 experiments (42 animals) the effects of: (1) duration of simulated ischemia (1321 +/- 435 s), (2) stimulation rate (1002 +/- 549 ms) and (3) number of successive simulated ischemic periods (occlusions) (1.58 +/- 0.92) on: (1) resting membrane potential, (2) action potential amplitude, (3) duration of 50 and 90\% action potentials and (4) maximal upstroke velocity of action potential were studied. All variables were considered as gradients (delta) between normal and ischemic tissue. Both during occlusion and upon reperfusion, spontaneous repetitive responses were coded as single, couplets, salvos (three to nine and > 10) or total spontaneous repetitive responses (coded present when at least one of the above-mentioned types was seen). The incidence of total spontaneous repetitive responses was 31\% (19/62) on occlusion and 85\% (53/62) upon reperfusion. Cox's models (forced and stepwise) were used to predict multivariately the occurrence of arrhythmic events considered as both total spontaneous repetitive responses and as separate entities. These models were applicable since continuous monitoring of the experiments enabled exact timing of spontaneous repetitive response onset during both occlusion and reperfusion. In predicting reperfusion spontaneous repetitive responses, total spontaneous repetitive responses and blocks observed during the occlusion period were also considered. Total occlusion spontaneous repetitive responses were predicted by: (1) longer delta 50\% action potential duration (t = 2.68), (2) shorter delta 90\% action potential duration (t = -2.17) and (3) fewer occlusive periods (t = -2.46). Total reperfusion spontaneous repetitive responses were predicted by a longer delta action potential amplitude (t = 2.18). Due to few events during occlusion, prediction of individual arrhythmic entities was not possible. Upon reperfusion single spontaneous repetitive responses were predicted by longer delta maximal upstroke velocity of action potential (t = 2.59) and shorter delta 90\% action potential duration (t = -2.55); couplets were predicted by longer delta 50\% action potential duration (t = 3.26); longer delta action potential amplitude predicted salvos (> 10) (t = 3.26).(ABSTRACT TRUNCATED AT 400 WORDS)