The direct effect of leg position on calf blood flow measured by venous occlusion plethysmography

SummaryBackgroundVenous occlusion plethysmography is commonly used to assess changes in calf blood flow (CBF). Although the leg is often positioned above the level of the heart to aid venous emptying during periods of cuff deflation, its direct effect on measured CBF is not known. We therefore planned to determine if CBF is affected by raising the calf region at the same body position during constant vasoconstrictor sympathetic nerve drive and haemodynamic variables.MethodsWe measured concomitant heart rate, arterial pressure, muscle sympathetic nerve activity (MSNA), calf blood flow (CBF) and calf vascular resistance (CVR) in the semi-supine position with the leg supported by the heel at various elevations above the horizontal level.ResultsIn 26 subjects we found that raising the leg to 40 ± 0.65° significantly increased CBF by about 23 ± 4.7% at constant haemodynamic variables and MSNA levels. Furthermore, in 10 of the 26 subjects this effect was graded within the same constant conditions. When the calf region was elevated to two positions at 22 ± 3.5° and 40 ± 1.5° from the horizontal level the increase in CBF, respectively, amounted to 13 ± 5.9% and 37 ± 5.9%.ConclusionsIt was shown that measurement of CBF by strain gauge venous occlusion plethysmography is directly affected by the position of the calf region above the horizontal level. It is suggested that this could confound measurement of calf blood flow in longitudinal and interventional studies.