Transmural pressure as a determinant of basic intrinsic heart rate.

It was observed that the heart rate was minimum at zero transmural pressure. The mean heart rate at zero transmural pressure was 23+/-5/min. This mean heart rate increased from 23+/-5/min to a peak value of 40+/-6/min (74% acceleration) when the transmural pressure was raised from 0 to +4 mm Hg and to a similar peak value of 36+/-8/min (56% acceleration) when the transmural pressure was lowered from 0 to -4 mm Hg. The peak values attained at +/-4 mm Hg were highly significant (p less than 0.001). It is concluded that the heart rate at zero transmural pressure represents the basic intrinsic pacemaker frequency independent of neural, humoral, thermal and haemodynamically induced mechanical influences.