Tolerance of Isolated Cardiac Muscle to Hypoxia: Force-Frequency Interrelationships 1

Abstract

Summary

The performance of isolated left ventricular muscle preparations during 60 min of hypoxia (95 % N₂, 5 % CO₂) and 30 min of reoxygenation (95 % O₂, 5 % CO₂) was examined at differing stimulation rates (1, 12 and 60 stimuli/min). Since a change in inotropic state accompanies a change in cardiac rate, studies were carried out in two species with different force-frequency relationships over the heart rate range examined the rat and the guinea pig. Preparations were studied at the apices of their length tension curves while contracting iso-metrically 12 times a min at 28°.

At increasing stimulation rates, under oxygenated conditions, isometric tension fell in rat ventricular muscle while the reverse is true in the guinea pig. Despite the differing force-frequency responses, both species demonstrated progressively poor performance during hypoxia at increasing stimulation rates. This was manifested by a more rapid and greater decline in mechanical activity, more severe contracture during hypoxia and and slower recovery upon reoxygenation.