A COMPARISON BETWEEN FREQUENCY POTENTIATION AND POSTEXTRASYSTOLIC POTENTIATION IN VITRO AND IN SITU

Until now, investigations of the force-frequency relationship and on postextrasystolic potentiation have mainly been performed in isolated heart muscle preparations. The present paper deals with similar studies carried out in situ. 1. Using an especially developed, electronically directed stimulation device the compensatory effects of haemodynamics in intact animals could largely be avoided. Accordingly, the validity of the laws, governing the contractile behaviour of isolated muscle preparations could also be demonstrated in the dog heart in situ.-The experimental arrangement allows the separation of the effects of postextrasystolic potentiation from those provoked by the Starling mechanism. 2. In vitro and in situ, the occurrence of extrasystoles does not only potentiate the contraction following the estrasystole but also the subsequent beats. The earlier the extrasystole occurs, the stronger the potentiating effect is. 3. In contrast to the studies in vitro, frequency potentiation could not be demonstrated satisfactorily in the dog heart in situ, probably as a result of the compensatory influence of the circulation. 4. On the other hand upon transition from a higher to a lower frequency in situ a clear cut potentiation was observed in the poststimulation phase. This potentiation could still be demonstrated when ventricular filling pressure and enddiastolic pressure in the aorta did not change any more. As in the isolated muscle preparation, a larger difference between the frequencies of stimulation provoked a larger degree of increased contractility. 5. The possibility to use frequency and postextrasystolic potentiation for the diagnostic evaluation of myocardial insufficiency in situ is discussed. © 1969 Springer-Verlag.