Influence of different cuff inflation protocols on capillary filtration capacity in human calves -- a congestion plethysmography study

Abstract

It has been suggested that venous congestion plethysmography (VCP) substantially underestimates microvascular permeability by activation of a veni-arteriolar constrictor mechanism, even when using small (< 25 mmHg) congestion pressure steps. We studied human lower limbs of 18 young healthy volunteers to test whether the congestion pressure step size of the VCP protocol has an influence on the values of the capillary filtration capacity (CFC) and isovolumetric venous pressure (P(vi)). Two different dual stage VCP pressure step protocols, with 3 and 10 mmHg steps, were used in randomised order and separated by a transient reduction in congestion pressure. Since lymph flow is known to increase after venous congestion, we also looked to see if changes in the estimated lymph flow (J(v)L) occur as a result of these VCP protocols. The measured CFC (median [25th; 75th percentile]) was 2.6 [2.5; 3.2] x 10(-3) ml (100 ml)(-1) min(-1) mmHg(-1) with the 3 mmHg pressure step protocol, which was not different from the value of 2.9 [2.7; 3.4] x 10(-3) ml (100 ml)(-1) min(-1) mmHg(-1) obtained with 10 mmHg pressure steps. However, when either of these step sizes was applied after a transient venous decongestion, significantly higher values of CFC, 4.0 [3.4; 4.1] x 10(-3) and 3.5 [3.1; 4.5] x 10(-3) ml (100 ml)(-1) min(-1) mmHg(-1), respectively, were obtained (P < 0.05). The assessment of P(vi) was also independent of the pressure protocol (10 mmHg: 8.0 [5.7; 13.2] mmHg and 3 mmHg: 15.7 [12.5; 18.5] mmHg), but when P(vi) was measured after the transient deflation, significantly higher values were found with both 10 and 3 mmHg steps (24.1 [20.9; 27.3] and 30.4 [28.9; 30.9] mmHg, respectively; P < 0.01). The transient pressure reduction was associated with a rise in estimated J(v)L from 0.04 [0.03; 0.05] to 0.12 [0.08; 0.18] and 0.04 [0.04; 0.05] to 0.09 [0.07; 0.10] ml (100 ml)(-1) min(-1), respectively (P < 0.01). The first stage data from these protocols shows that the value of CFC is not influenced by the size of the cumulative venous pressure steps, providing they are of 10 mmHg or less. The data also show that J(v)L can be estimated with small step VCP protocols. We hypothesise that the sudden reduction in cuff pressure after venous congestion is associated with a temporary upregulation of lymph flow. As the congestion pressure is raised again, there is a modulation of the enhanced lymph flow, such that the resulting CFC slope appears greater than that obtained in the first stage of the protocol.

Figure 1. The relationship between the values of cuff pressure (P_cuff) and corresponding net fluid filtration rate (J_v), that were found in one study

The regression line is based on the data obtained above the pressure at which the first J_v component was observed (filled circles only). The slope of the linear regression represents the capillary filtration capacity (CFC), the intercept with the x-axis gives the value of the isovolumetric venous pressure (P_vi).

Figure 2. The different pressure protocols used in this VCP study

The bottom trace, of these original records, depicts the pressure in the congestion cuffs (P_cuff), whereas the top trace represents the volume change. A, in the pressure protocol of Study I first a series of 10 mmHg pressure steps was applied. After intermediate reduction of P_cuff to 30 mmHg the congestion cuff was then re-inflated using cumulative pressure steps of 3 mmHg. B, in Study II the order of the pressure protocols was inverted and the 3 mmHg pressure steps were applied in the first phase of the VCP protocol.

Figure 3. Changes of the microcirculatory parameters during different VCP protocols

Individual as well as median [10th, 25th, 75th and 90th percentiles] values of isovolumetric venous pressure and predicted lymph flow are given. A, isovolumetric venous pressure (P_vi) obtained with the different pressure step protocols, before and after intermediate deflation (*P < 0.01; Wilcoxon signed rank test). B, predicted lymph flow (J_vL) assessed subsequently with either 3 mmHg or 10 mmHg pressure steps (*P < 0.01; Wilcoxon signed rank test).

Figure 4. The changes of net fluid filtration values (J_v) before (1^stInf₁₀, circles) and after (2^ndInf₃, triangles) the transient deflation of congestion cuffs to 30 mmHg

After deflation of the cuff, lower net fluid filtration values were found during the second increase in cuff pressure, giving rise to higher P_vi and CFC values.