Temperature sensitivity of the human cardiac pacemaker during exercise.

The temperature sensitivity of the human cardiac pacemaker was investigated during exhaustive exercise. From graded runs to exhaustion, we established the relationship between maximum exercise heart rate (HRmax) and rectal temperature (Tr). After warm-up periods of varying intensity and duration, four male subjects completed 4 to 6 runs each, each run performed on a separate day. For every subject there was a strong linear correlation between HRmax and Tr (r = 0.79 to 0.96). Various measures of the temperature sensitivity were: linear sensitivity, 8.8 +/- 4.3 beats min-1.degrees C-1; Q10, 1.6 +/- 0.4 and the Arrhenius constant, mu, 35.9 +/- 16.6 kJ.mol-1. At HRmax the value for linear temperature sensitivity was similar to, but the values for Q10 and mu lower than, those observed previously for intrinsic heart rate. Sympathetic influence on the cardiac pacemaker during exercise may cause this reduction, by shifting the pacemaker location to cells with a lower temperature sensitivity, or by altering a rate-limiting step determining the diastolic pacemaker potential.