RESUMO
BACKGROUND: The physiological effects of biphasic pacing have not been studied in compromised hearts. METHODS: Myocardial infarction was induced in 12 sheep by high coronary artery ligation. Perioperative mortality was 33%. The surviving eight animals exhibited increased left ventricular volume and reduced percent fractional shortening. Two weeks after the infarction, sheep were implanted with atrial-triggered, right ventricular pacemaker systems capable of pacing with cathodal (cathodal pulse) and biphasic (anodal pulse followed by cathodal pulse) waveforms, and randomly assigned to an initial mode. At 3-month intervals, or whenever pacing was lost for any reason, the pacing system was switched to the alternative mode. Cardiac function was assessed at 2- to 3-week intervals through the use of echocardiograms. Successful pacing was confirmed over an average of 8 weeks in each mode. RESULTS: Cathodal pulsing had neither beneficial nor deleterious effect on the diminished cardiac performance induced by myocardial infarction. When compared to the cathodal mode, biphasic pulsing improved cardiac performance as reflected by decrease of diastolic and systolic ventricular volumes, reduction in left ventricular systolic diameter, and increases in percent fractional shortening. When compared to the unpaced state after the myocardial infarction, the percent fractional shortening was significantly increased by biphasic pacing. Concordant trends in improvement in the other cardiac parameters were also observed for the biphasic mode. No ventricular tachyarrhythmias or mortality was associated with biphasic stimulation. CONCLUSION: When compared to cathodal pacing, myocardial biphasic pacing has no safety issues in sheep that have undergone a large myocardial infarction. Importantly, biphasic pulsing elicited significant benefits in cardiac performance.