Estimating blood flow with two plethysmographs: 'whitney' strain gauges vs 'dohn' air-filled cuffs

Two different forms of plethysmograph are commonly used to estimate limb blood flow noninvasively in humans: 1) the 'Whitney' mercuryin-silastic strain gauge, and 2) the 'Dohn' air-filled, double-walled, latex cuff. We tested the hypothesis that both techniques provide equal estimates of both absolute and relative blood flow. Bilateral calf blood flows were measured at rest and during reactive hyperemia (immediately after 10 min bilateral femoral occlusion) in 8 subjects (ages 21-40) in two sessions on the same day. Whitney and Dohn cuffs were (Figure Presented) placed on opposite legs for one session, and were g . reversed for the second session. Simultaneously | measured blood flow estimates were compared by 8 linear regression analysis, as shown in the " .. example figure from one subject Blood flow estimates correlated highly between techniques in i all subjects (r2 range, 0.96-0.99), demonstrating ,'identical tracking of relative changes in blood flow. ° However, the Whitney estimates were consistently °'D°""' m°od>70 lower than Dohn estimates by - 40% (slope of regression was 0.47 to 0.77 Whitney/Dohn), indicating a bias toward lower absolute blood flow values that was most notable at higher flow rates. These data indicate that both techniques provide similar estimates of relative limb blood flow changes, but lower absolute values are seen using the mercury-in-silastic strain gauge.