Mechanism for slowing of heart rate and associated changes in pulmonary circulation elicited by cold injectate during thermodilution cardiac output determination in dogs.

The authors investigated the mechanism for slowing of heart rate (HR) and associated changes in the pulmonary hemodynamics caused by cold injectate during thermodilution method. To clarify whether the slowing of HR after cold injectate is due to the cooling of the sinoatrial (SA) node or a reflex mediated by the autonomic nerves, we directly measured the myocardial temperature of the SA node region by a thermistor probe, and evaluated the HR responses to iced injectate after autonomic blockade in anesthetized dogs. Additionally, pulmonary blood flow (PBF) was continuously measured by an electromagnetic flowmeter during the delivery of injectate. The direction and magnitude of changes in HR after injectate were significantly dependent upon the injectate temperature (P less than 0.01). Thereby, the changes in HR correlated well with those in myocardial temperature of the SA node region (r = 0.987, P less than 0.01). However, the HR responses to injectate were unaffected by cervical vagotomy or stellate ganglionectomy. A significant decrease in PBF was noted in most cases during the slowing of HR. It was concluded that the slowing of HR after cold injectate during thermodilution in dogs is primarily due to the direct cooling of the SA node.