ABSTRACTAblation of atrioventricular conduction is now widely accepted in the management of supraventricular arrhythmias. Reports of high temperatures, high pressures, and gas production suggest that there may be adverse effects on the blood, the electrode, and the cardiovascular system. In this investigation, using samples of fresh, heparinized pig blood, we measured hemolytic damage, the liberated gas volume and composition, and electrode erosion associated with high-energy electrical ablation. The blood was tested in a 10 liter tank at room temperature. Impulses of 10 to 400 J were applied to new USCI No. 6F bipolar pacing electrodes using both positive and negative polarities. Voltage and current waveforms were recorded. The volume of gas liberated with cathodal electrodes was 0.50,ul/J up to 50 J and 0.29 gl/J above 100 J. It was composed predominantly of hydrogen and nitrogen, with carbon dioxide and oxygen. With positive electrodes, the gas volume was linearly related to energy at 4.34 gVIJ up to 200 J and also contained carbon monoxide. The hemolysis was directly proportional to impulse energy for both cathodal and anodal electrodes, being 1.37 gl/J and 4.48 ul/J, respectively. Electrode erosion was substantial but clinically acceptable. We conclude that there are marked differences in the energy conversion processes and, where the same energy can achieve a comparable clinical effectiveness, there are advantages in using a cathodal electrode polarity. It is also advisable to use lower energies.