Interdependence of virtual electrode polarization and conduction velocity during premature stimulation

Fiber orientation in the heart plays a crucial role in the anisotropic nature of impulse propagation and the formation of virtual electrode polarization (VEP). The relationship between anisotropic conduction velocity and VEP is not fully understood.

We recorded transmembrane potential from the surface of isolated rabbit hearts (N = 7) in the presence of 15 mmol/L of diacetylmonoxime using a video imaging system. We paced each heart at a constant cycle length of 250 milliseconds and delivered a premature stimulus (S2) from the same unipolar electrode. The S2 pulses of either polarity were applied at various coupling intervals (CIs); S2 duration was 10 milliseconds and stimulus amplitude was 40 mA. Impulse propagation at 250 milliseconds occurred via elliptical waves with an anisotropy ratio (AR) of approximately 2.5. Owing to VEP, AR varied dramatically as a function of S2 polarity and CI. At intermediate CIs (approximately 150 milliseconds), AR was decreased by a factor of 3 for anodal stimulation and increased by a factor of 2 for cathodal stimulation. At slightly shorter CIs (approximately 115 milliseconds), impulse propagation was blocked, leading to unidirectional block and the initiation of reentrant arrhythmias. Conduction block always occurred along fibers for anodal stimulation and across fibers for cathodal stimulation.

Fiber orientation plays a prominent role in impulse propagation during premature stimulation such as that which occurs during pacing of ventricular tachycardia and ventricular fibrillation. The interdependence of VEP and conduction velocity acts to dramatically alter AR and arrhythmogenesis.