Fixed Obstacle in Anisotropic Myocardium

Background. The aim of this study was to test the hypothesis that electrical uncoupling and depression of the fast sodium channels have differential effects on propagation of the electrical impulse relative to the fiber orientation. Methods and Results. In a model of reentrant ventricular tachycardia (VT) (mean cycle length, 144±13 msec) around a ring of anisotropic myocardium in 10 Langendorff-perfused rabbit hearts, the effects of extracellular K+ concentration ([K'],) and heptanol were studied. [K']. and heptanol each had a dose-dependent effect on VT cycle length. However, high [K+]. slowed the VT mainly by depressing longitudinal conduction, whereas heptanol preferentially depressed transverse conduction. The ratio between longitudinal and transverse conduction velocities progressively decreased with high [K']. and progressively increased with heptanol. Heptanol terminated VT at a mean concentration of 3.5±0.5 mM. The cycle length before termination was 446+120 msec (p<0.001). In eight of 10 experiments, termination occurred by failure of conduction during transverse propagation. VT terminated at a mean [K']. of 11.6± 1.8 mM. The cycle length before termination was 493+341 msec (p<0.01). In seven of 10 cases, termination occurred by failure of conduction during longitudinal propagation. In the remaining five episodes (two with heptanol and three with high [K'].), termination occurred by collision of the reentrant beat with an antidromic impulse being reflected within the ring. In a separate series of six hearts, tetrodotoxin was administered during VT. Like high [K']., tetrodotoxin prolonged the cycle length of the VT by preferentially slowing longitudinal conduction, and VT was terminated by longitudinal block. Conclusions. During reentrant VT, electrical uncoupling of cells by heptanol or modification of active membrane properties by high [K'],, or tetrodotoxin has a differential depressing effect on propagation of the impulse relative to the fiber orientation. (Circulation 1991;84:1307-1318)