STARLING RESISTOR VERSUS COMPLIANCE - WHICH EXPLAINS THE ZERO-FLOW PRESSURE OF A DYNAMIC ARTERIAL PRESSURE-FLOW RELATION

Arterial pressure at zero flow (Pz = 0) that is higher than venous pressure (Pv) in dynamic pressure-flow relations has been explained by the presence of an arteriolar Starling resistor (SR) mechanism (i.e., vascular waterfall) or the discharge of vascular capacitance. To determine which was predominant, I studied in vivo hind limbs of 18 anesthetized dogs in which femoral arteries were cannulated with in-line electromagnetic flow probes to measure inflow (Q(in)), Pv was controlled, and collateral flow was eliminated with a tourniquet. Pz=0 was obtained by turning flow to zero. Three tests were applied: 1) Pv was raised in steps with either constant Q(in) or constant arterial pressure (Pa) to determine the pressure at which upstream vascular characteristics were affected by a change in Pv, 2) the time to reach Pz = 0 was varied to determine compliance effects, and 3) an equation was developed to determine if experimentally derived parameters could explain Pz = 0 without invoking an SR. With constant Q(in) and a Pz = 0 of 56.9 ± 11.7 mm Hg (time to Pz = 0,3 seconds), Pv could be raised by 9.6 ± 6.2 to 16.3 ± 6.0 mm Hg before Pa increased, and with constant Pa, Pv could be raised by 6.8 ± 7.3 to 14.0 ± 8.0 mm Hg before Q(in) decreased. With increasing times to reach Pz = 0, Pz = 0 initially dropped precipitously, but then decreased by only a small amount over the next 5-10 seconds even though arterial pressure was much above Pv. This could be explained by an SR mechanism with a critical pressure of 42.3 ± 11.4 mm Hg and an arterial complaince of 0.0104 ± 0.0023 ml·mm Hg-1 (n = 6). There was no value for the compliance that described the results when the arterial outflow pressure was Pv. Thus, this study supports the hypothesis that an SR mechanism is present in the vascular system. It is most likely precapillary, and in the resting limb, it has a value of 40-50 mm Hg. However, Pz = 0 in dynamic pressure-flow studies of less than 4 seconds is also greatly influenced by capacitance effects and the initial Pa.