THE INFLUENCE OF VASCULAR FACTORS ON MEAN PRESSURE, PULSE PRESSURE AND PHASIC PERIPHERAL FLOW

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Abstract
In view of contrary opinions as to whether changes in peripheral resistance or concurrent alterations in the modulus of elasticity with rising pressures dominate changes in mean pressure and pulse pressure, more physical tests were made on a circulation model. In this way, tubes with volume elasticity coefficients resembling those detd. post mortem in man could be employed, and the variations exhibited by optically recorded pressure pulses could be analyzed. It was found that the diminishing effect that augmented resistance has on pulse pressure in tubes with linear volume elasticity relations persists until the distensi-bility decreases to a degree found only late in life or during pronounced arteriosclerosis. With lesser degree of rigidity the curves show characteristic changes in form, and while pulse pressure shows a tendency to become larger than in perfectly elastic tubes it is rarely greater at very high levels than at low levels of pressure. The importance of diminishing elasticity in early and middle adult life as a factor determining the directional change in pulse pressure with increasing peripheral resistance has been over-emphasized by those guided chiefly by theoretical considerations. No evidence could be obtained that diminishing elasticity with increasing pressure plays a dominant role in the dog''s aorta in situ when it was perfused with pulsating pressures and the peripheral resistance was increased. As long as the capacity continues to increase to a reasonable extent with rising pressures, the pulse pressure continues to decrease. When the expansion of the aorta at any pressure is reduced below a minimum the effects of diminished distensibility exhibit themselves. They consist in a marked elevation of systolic pressure, a reduction of diastolic pressure and an increase in pulse pressure. The mean pressure is not affected significantly. Integration of pressure pulses suggests that with lessened arterial extensibility the efflux from the aorta is increased considerably during systole and reduced greatly during diastole. Analysis of recorded variations of venous flow indicate, however, that unless the peripheral distributing vessels share in the reduction of elasticity they continue to buffer the flow through arterioles into capillaries with the result that the relative systolic and diastolic flows remain approximately the same. However, the buffering action is unable to maintain flow during the latter portion of diastole, hence capillary and venous flows become pulsatile. It is stressed that such pulsations may occur despite marked constriction of arterioles and that it differs from the capillary and venous pulses due to vasodilatation in that the former are due to diminishing flow late in diastole, the latter to augmented flow during systole. When pressure pulses are recorded from anesthetized dogs while peripheral resistance is increased by stimulation of pressor fibers, the pulse pressure does not decrease, it increases. In view of the observations that such stimulation is accompanied by an immediate and progressive increase in cardiac output, such expts. do not prove that the augmenting effect of diminishing elasticity is more pronounced during life or that it is brought into action by such stimulations.

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