Effect of varying pacing waveform shapes on propagation and hemodynamics in the rabbit heart

The propagation characteristics of myocardium stimulated with anodal, cathodal, and equiphasic biphasic pacing pulses were examined in Langendorff-perfused rabbit hearts. Conduction velocity measurements were made using an array of bipolar extracellular electrodes, transmembrane potentials recorded using floating intracellular microelectrodes, and hemodynamics measured by fluid-filled catheter transducer systems. Anodal (A) stimulation pulses improved the electrical conduction at all the stimulus amplitudes tested in both longitudinal (e.g., 5 V 2-msec pulse: [A]54.9 ± 0.7 cm/sec; cathodal [C]49.7 ± 1.5 cm/sec) and transverse (e.g., 5 V 2 msec pulse: [A]31.3 ± 1.7 cm/sec; [C]23.3 ± 2.9 cm/sec) directions. Microelectrode recordings verified that increased conduction velocities of the anodal pulses were associated with faster upstrokes of the action potentials. The increased threshold associated with anodal pulses may be overcome by using a biphasic (B) waveform, in effect adding a second phase (e.g., 2-msec pulse: [A] 2.03 ± 1.3 V; [C]3.85 ± 1.5 V; [B]2.15 ± 0.9 V). The conduction speeds achieved by the biphasic pulses were found to be comparable to the equivalent anodal pulses (e.g., 5 V 2-msec pulse: [B]55.2 ± 1.7 cm/sec longitudinal and 32.4 ± 2.1 cm/sec tranverse). It is postulated that the enhanced conduction by anodal and biphasic pulses may be due to preconditioning of the myocardium before stimulation, resulting in more vigorous action potential upstrokes. In preliminary experiments, it was observed that improved conduction elicited by these pulses also resulted in enhanced contractility as measured by shortened electromechanical delays and faster rate of rise of pressure development (dP/dt max : [A] 25.4 ± 0.4 mm Hg/sec; [C]19.4 ± 0.8 mm Hg/sec; [B] 25.7 ± 1.2 mm Hg/sec, respectively). Use of novel hybrid pulses involving an anodal component may offer a way for implanted pacemakers to enhance the electromechanical response of the heart.