Shunting of Heat in Canine Myocardium is Considerable

Abstract

Shunting of heat in the heart was studied by sinusoidal infusion (0.005–0.1 Hz) of cold saline into the left canine coronary bed. The resulting temperature change in the coronary sinus was measured. A model of heat transport in the heart containing a mixing chamber, a countercurrent exchanger and heat conduction across endocardium to the ventricular lumen and across epicardium to the thoracic cavity was developed. The model showed that shunting in the countercurrent exchanger alone does not affect the mean transit time of heat, but heat loss by conduction shortens this characteristic time. In the presence of heat loss across endocardium and epicardium, shunting results in an enhanced reduction of mean transit time. The measured mean transit time for heat (14.3 ± 4.5 s, mean ±SD) was only 22% of the theoretical mean transit time (65 ± 1 s) predicted under the assumption that no heat conduction to the environment takes place, and which is the inverse of flow rate (87.4 ± 1.5 ml min⁻¹ per 100 gram). The amount of heat leaving via the coronary sinus (heat recovery) was 76 ± 11% of that injected in the coronary artery. Using the model it is estimated from these data that 62% of the injected heat is shunted. Injection of cold saline in an epicardial vein gave no temperature change distally (peripherally) in the adjacent artery, so that shunting between large epicardial vessels is small. The maximum temperature change in the myocardium following an atrial injection of cold indicator was only 25% of that in the coronary sinus, indicating that heat shunts before reaching the capillaries. We conclude that shunting of heat is of considerable quantitative importance, and occurs at the arteriolar level.