Left ventricular energetics. Heat loss and temperature distribution of canine myocardium.

The sum of total left ventricular heat loss and left ventricular mean total external power was compared with the product of oxygen consumption and its energy equivalent. Myocardial blood flow, measured with 15 +/- 3 micrometers radioactive microspheres, was multiplied by the transcoronary arteriovenous temperature difference and by oxygen content difference to obtain coronary heat loss and oxygen consumption, respectively. Since only part of the heat is carried away by the coronary system a thermodilution technique was used to obtain the ratio between the heat removed by the coronary system and the external heat loss. A correction was made for the endothermic reactions of hemoglobin deoxygenation and carbon dioxide reactions with blood. Left ventricular oxygen consumption corresponded to 2.26 +/- 0.66 W/100 g, and for the sum of total left, ventricular heat loss and external power, 2.09 +/- 0.51 W/100 g was found (n = 14). In a second series, the measured transmyocardial temperature distribution was compared with the calculated temperature distribution, assuming that heat production in the myocardium is uniform and that heat is lost by coronary flow and diffusion. When thoracic and luminal myocardial surface temperatures were about equal, blood flow was found to be about the same in the various layers of the heart, whereas myocardial temperature was found to be highest near the middle of the wall (0.36 +/ 0.07 degrees C warmer than luminal temperature (n = 6). When thoracic surface temperature was increased or decreased (by + 1.56 +/- 0.99 degrees and -1.10 +/- 0.59 degrees C, respectively), consistent changes were seen for the temperature distribution in the myocardium, but not for the local flow (endo/epi ratio: 1.06 +/- 0.29 and 0.96 +/- 0.21, respectively). These data suggest that myocardial blood flow is independent of tissue temperature.