| Abstract: | Impulse Propagation in Ischemia. The disturbances in impulse propagation in the heart with regional ischemia and the heart with chronic infarction, which underlie the initiation and perpetuation of arrhythmias, are briefly reviewed. Evidence is presented that reentry is responsible for ventricular tachycardia in both conditions. During acute ischemia, reentrant excitation is characterized by unstable functional reentrant circuits, the properties of which are determined by the changes in transmembrane action potential. Tachycardia is caused by a single unstable reentrant circuit. Fibrillation ensues when a single circuit is broken up into many independent reentrant wavelets. In contrast, ventricular excitation during sustained monomorphic tachycardia in hearts with a healed infarct is characterized by stable reentrant circuits determined by the architecture of surviving myocardial cell bundles within the infarct. In acutely ischemic myocardium, conduction velocity is reduced to about half of the value in normal myocardium, due to the reduced action potential amplitude and upstroke velocity. After about 5–10 minutes ischemic cells become inexcitable at resting membrane potentials of -50 to -60 mV. An important characteristic of ischemic cells is the dependence of the action potential upstroke on cycle length mainly because of the markedly prolonged recovery from inactivation of the fast Na+ current in partially depolarized cells. Changes in transmembrane potentials do not occur homogeneously throughout the ischemic zone, and small differences in resting potential (associated with small differences in local K+ accumulation) of depolarized cells result in large local differences in refractory periods. The time dependence of refractoriness explains why an increase in sinus rate, or a single premature beat, may produce conduction block at certain sites, and allow conduction in adjacent areas, thus setting the stage for reentry. A decrease in electrical cell-to-cell coupling, which also is a factor in decreasing conduction velocity, only occurs after 15–20 minutes of ischemia. Its role in the type 1B arrhythmias, which coincide with the period when uncoupling begins, remains to be elucidated. The cellular electrophysiology of myocardial cells surviving in the subendocardium of infarcts in human hearts is close to normal. Conduction slowing may be caused by “zigzag” conduction along small bundles that are separated by fibrous tissue, and that merge and divide over small distances. (J Cardiovasc Electrophysiol, Vol. 3, pp. 77–87, February 1992) |
