Cardiac cryolesions: factors affecting their size and a means of monitoring their formation.

Twenty-seven endocardial cryolesions were created in mongrel dogs and analyzed to determine the effects on cryolesion size of both the initial myocardial temperature (37 degrees C versus 12 degrees C) and the pressure within the nitrous oxide delivery line (tank pressure of more than 700 pounds per square inch [psi] versus tank pressure of less than 700 psi). In addition, local myocardial temperatures were monitored to determine their utility in the intraoperative determination of the extent of cryothermic cell death. Cryolesion volume was significantly affected by both the initial myocardial temperature (p less than 0.001) and the line pressure (p = 0.014). In a 37 degrees C myocardium, the mean lesion volume ranged from 0.501 +/- 0.183 cc at line pressures lower than 700 psi to 0.839 +/- 0.258 cc at line pressures greater than 700 psi. In a 12 degrees C myocardium, the mean volume was 1.151 +/- 0.436 cc at line pressures lower than 700 psi and 1.361 +/- 0.288 cc at line pressures higher than 700 psi. A myocardial temperature of 0 degrees C occurs at the edge of the area of cell death. When analyzing the range from -5 degrees to +5 degrees C, the probability of a point at or lower than 0 degrees C falling inside the cryolesion is 84.2%. Monitoring intramyocardial temperature will predict the border of a cryolesion.