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

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.

Evaluation of the Time Course of Vascular Responses to Venous Congestion in the Human Lower Limb

“…The lower vascular resistance for the whole limb after minutes of venous congestion is analogous to the findings of Gamble et al [13] who increased venous congestion pressure in small (<10 mm Hg) increments each lasting 5 min, so that cuff pressure of 50 mm Hg was not achieved for approximately 45 min. In this protocol, calf blood flow determined by brief transient further cuff inflations, similar to those used in the present study, showed no decrease at all, and this was confirmed by measurements of flow velocity in the popliteal artery using ultrasound.…”

“…Subjects rested supine with the legs supported so that the widest part of the calf lay horizontal with the right atrium [13, 20]. Arterial blood flow to the non-dominant calf was estimated using a non-mercury strain gauge incorporated into the Filtrass 2001 system (DOMED Medizintechnik, Krailling, Germany) during brief (<12 s) venous occlusion thigh cuff inflations.…”

“…A constrictor response is thus evoked when venous congestion is sustained for minutes, as shown for cutaneous tissues [5, 8, 12], but not when held only for seconds, e.g. for the plethysmographic determination of limb arterial blood flow [13]. …”

The insulin sensitiser pioglitazone does not influence skin microcirculatory function in patients with type 2 diabetes treated with insulin

“…A biphasic response is observed consisting of an initial rapid increase in calf circumference attributable to venous filling when cuff pressure exceeds venous pressure, followed by a gradual increase in volume caused by fluid filtration which occurs when cuff pressure exceeds filtration pressure at the microvascular interface. The slope of the linear relationship between cuff pressure and limb volume (and thus fluid filtration) is termed Kf [26]. The xaxis intercept of filtration capacity slope with the pressure axis is PVi, which is therefore the pressure at which there is neither net filtration nor reabsorption.…”

Microvascular Filtration Is Increased in Postural Tachycardia Syndrome

“…32 Assuming tissue pressure, reflection coefficient, and oncotic pressures are unchanged during cuff occlusion, the rate of change in limb volume, dVol t /dt, is a function of P vasc and lymphatic drainage, where P vasc is determined by venous occlusion pressure. Most generally, one expects lymphatic drainage to change as P vasc changes.…”