Central body temperature as a guide to optimal heart rate.

Studies in man suggest that fixed-rate artificial pacemakers do not return hemodynamic function to normal, since the principal mechanism for the increase in cardiac output with exercise, increased heart rate, is not restored. Special pacemakers are available that can detect atrial activity and pace the ventricles in coordination, but nearly half of the patients receiving artificial pacemakers have abnormal atrial function (atrial fibrillation, sick sinus syndrome). This study examined the effects of exercise on the temperature of blood returning to the right atrium. Precision thermistors, placed in the right hearts of conscious dogs, recorded temperature increases of 1 degree C (range 0.4-1.5 degrees C) during submaximal treadmill exercise. Temperature change correlated well with work load and changes in heart rate.