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Combined effect of beating rate and hydrostatic pressure on excitation in cardiac muscle.

Canine cardiac Purkinje fibers stimulated at frequencies from 1-4 Hz were exposed to hydrostatic pressures of 1 and 150 ATA. Both increases in stimulation frequency and pressure slowed conduction and decreased maximum upstroke velocity of the action potential. Depression of these variables was greatest when rapid rate and elevated pressure were combined. Membrane potential decreased at 150 ATA at all frequencies. Post-overdrive hyperpolarization was reduced 45% at 150 ATA. Action potential duration increased at 150 ATA at all frequencies. Elevated pressure had no effect on the interval dependence of repolarization time to -60 mV but had a slight effect on time to full repolarization. Pressure effect on resting potential was reduced in elevated extracellular potassium. Arrhythmias developed in more than 50% of the tissues at 150 ATA and a frequency of 4 Hz. The results are discussed with respect to ionic mechanisms underlying action potential propagation, to electrogenic pump activity, and to the arrhythmogenic nature of pressure-frequency interactions.