PHASIC VARIATIONS IN PERIPHERAL CORONARY RESISTANCE AND THEIR DETERMINANTS

Abstract

To study the phasic changes in peripheral coronary resistance qualitatively and quantitatively, pressure changes in a peripheral coronary branch were recorded optically by 2 procedures. Such records indicate that our current conceptions regarding the time relations, character and magnitude of peripheral coronary resistance requires some revision: Normally, the peripheral coronary pressure (PCP) increases quickly during isometric contraction and the first moments of ejection, rises more gradually to a summit during the shortening phase, decreases abruptly during isometric relaxation and is influenced but little by subsequent lengthening of ventricular muscle. Such time relations together with the demonstrations (a) that PCP curves are not materially affected when the regions involved extend instead of shorten (ischemia), and (b) that at constant diastolic pressures, systolic coronary pressure increases proportionately to systolic aortic pressure, when the latter rises indicate that intramural and intraventricular tension rather than muscle fiber length predominantly detns. the resistance. The fact that the systolic max. pressure is reduced somewhat when the involved muscle-area extends instead of shortens can be interpreted to mean either that muscle shortening is normally of supplementary assistance or that stretching increases the capacity of the coronary branches sufficiently to prevent full development of systolic pressure. Peripheral coronary resistance is not affected to any discoverable extent by transmission of pressure from collateral branches because (a) the magnitude of flow from an open peripheral ramus is very small; (b) the steep and major rise of PCP occurs prior to development of max. aortic pressure, and (c) clamping of the right or/and left circumflex rami produce no phasic changes in resistance and only such deviations in magnitude as can be better explained by concurrent changes in the dynamics of ventricular contraction. In beats equivalent as regards contractile force, the systolic pressure max. reached depends upon the diastolic pressure level from which a beat starts, i.e., on the degree of coronary filling. Volume-elasticity studies of the coronary system, interpreted in conjunction with pressure changes and flow detns., strongly suggest that the systolic backflow is of the order of 0.03 cc. which is considerably less than usually stated. Since this back-flow is less than that required for development of the total pressure of which the myocardium is capable, pressure curves recorded from a peripheral ramus do not allow an appraisal of the max. resistance developed under natural conditions of coronary distention. The facts presented are of fundamental importance in understanding phasic changes of coronary flow and in interpreting the actions of drugs on the intact coronary circulation.