Human calf precapillary resistance decreases in response to small cumulative increases in venous congestion pressure

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Abstract
We studied human lower limbs to test the hypothesis that the application of small cumulative venous congestion pressure steps is associated with a reduction in precapillary resistance.Strain gauge plethysmography was performed on twenty-one young subjects (22.7 ± 0.6 years). At each of the small cumulative pressure steps, limb blood flow was estimated from the initial slope of the volume response to transient (10 s duration) elevations of venous congestion pressure to 90 mmHg, after which the congestion pressure was returned to the previous value. The blood flow at each pressure was also expressed as a percentage of the initial control value. Peak tibial arterial blood flux was assessed, in four of the subjects, using colour duplex ultrasonography and the same congestion pressure protocol.We used Darcy's Law to predict the limb arterial blood flow and blood flux at each venous congestion pressure, assuming that both mean arterial blood pressure and precapillary resistance remained constant.The mean ± s.e.m. control arterial blood flow at the lowest venous congestion pressure, 4.8 ± 0.1 mmHg, was 2.77 ± 0.18 ml min−1 (100 ml)−1. At the highest venous congestion pressure, 59.2 ± 0.2 mmHg, arterial blood flow was 2.45 ± 0.35 ml min−1 (100 ml)−1 (121.6 ± 16.9 % of the initial value). This did not differ significantly from the initial control value, but was significantly greater than the predicted value of 0.77 ± 0.13 ml min−1 (100 ml)−1 (28.6 ± 2.1 % of the initial value) calculated assuming constant resistance and sustained mean arterial pressure. The tibial arterial peak blood flux at 58.3 mmHg venous congestion pressure was 102.2 ± 2.3 % of the control value, which was significantly greater than the predicted 17.2 ± 1.3 % of control, calculated for this pressure, assuming constant resistance and sustained mean arterial pressure.Our data show that lower limb arterial blood flow is sustained when venous congestion pressure is raised using small cumulative steps, even at congestion pressures approaching mean arterial blood pressure. These data support the notion that precapillary resistance is influenced by signals generated at the microvascular and post microvascular levels and transmitted via the endothelium.