Venous occlusion plethysmography and Doppler ultrasound in the assessment of the dynamic response of blood flow at the onset of exercise in the lower limb
Quantification of the dynamic characteristics of muscle blood flow during exercise can reveal a significant amount about the physiological state, which cannot be measured under resting conditions. Venous occlusion plethysmography (VOP) is a non-invasive blood flow measurement technique suitable to measuring whole limb blood flow, however, conflicting evidence exits pertaining to its accuracy in an exercise setting. The aim of this study was to test the validity of VOP against the more established blood flow measurement technique of Doppler ultrasound (DU) to quantify the dynamic response of leg blood flow (LBF) during exercise. Ten healthy young male participants performed 6 bouts of 6 min intermittent (3-s duty cycle: 1-s contraction, 2-s relaxation) plantar-flexion exercise of the right calf muscle in the prone position on a custom-built calf ergometer at 30, 50 and 70% of maximum voluntary contractions (MVC). Simultaneous VOP and DU measurements of LBF were recorded and repeated on two separate days so that in total, 4 bouts were performed at 30 and 50% MVC and 2 bouts at 70% MVC. Two empirical models (triphasic or quadraphasic)1 were fitted to averaged LBF data of all the individual time series of LBF using a weighted least-squares non-linear regression procedure2. A quadraphasic model was fitted in participants in whom a second or slow ‘decay' in blood flow was apparent (50% of the cases), but only the first and second ‘growth' phases and the first ‘decay' phase (which followed the first growth phase and was apparent in most cases) were compared across measurement techniques. Responses (shown as means ± SD) were compared using a Student's T-test or Wilcoxon Signed Rank test. The time constants (s) of the second growth phase of the LBF kinetic response were not different between measurement techniques (30% MVC: 20.4 ± 10.5 vs. 27.2 ± 19.7, 50% MVC: 19.0 ± 11.9 vs. 16.1 ± 7.3, 70% MVC: 9.1 ± 4.9 vs. 11.0 ± 3.9). Similarly, the end-amplitudes (mL.min-1) at 30 and 50% MVC were not different between VOP and DU (30% MVC: 353 ± 109 vs. 359 ± 95; 50% MVC: 535 ± 171 vs. 588 ± 182) but they were significantly lower for VOP than DU at 70% MVC (667 ± 230 vs. 798 ± 261). In addition, resting LBF estimates were also significantly lower for VOP than DU. The rest of the kinetic parameters were not affected by measuring technique. The current study supports the validity of using VOP as an accurate technique to measure the dynamic response of lower limb blood flow during exercise. However, VOP tends to underestimate the total amplitude of the LBF response at high intensities (i.e. 70% MVC) most likely given that arterial inflow into the limb is progressively reduced as the venous volume and pressure rises, with the possibility of venous outflow.
