Potentiation of cardiac vagal action by cold

The effect of cold on vagal action at the heart was studied in sheep, dogs and an isolated guinea pig atrial preparation. During cardiac output measurements in unanaesthetized sheep, by the thermodilution method, bradycardia was evoked on injection of cold indicator in eight of 12 sheep studied. This bradycardia was consistently evoked when blood pressure was increased, but not at normal blood pressure levels. In the guinea pig atrial preparation, which has one vagus nerve attached, bradycardia was evoked by electrical stimulation of the vagus nerve. When the preparation was cooled this bradycardia was potentiated. In anaesthetized dogs, the cut peripheral end of one vagus was stimulated electrically at different frequencies. The linear relationship between pulse interval and vagal frequency was then compared at deep body temperatures of 35, 37, 39 and 41 degrees C. This comparison showed that the vagus prolonged pulse interval more effectively when the animal was cool (35 degrees C) than when it was warm (41 degrees C).