Comparison of an impedance device to a displacement plethysmograph for study of finger blood flow

Empirical values of finger blood flow, determined with a commercial electrical impedance device, were compared with blood flow measurements made with a pulsatile air displacement plethysmograph. The measurements were made on the medial phalanges of the first, second, and third fingers of the left and right hands. Blood flow indices were calculated from extrapolated slopes of the recorded impedance and mechanical changes occurring during systole. Two series of observations were made on seven healthy male subjects. The first series consisted of 33 simultaneous measurements with the impedance rheograph and the air displacement plethysmograph on the same finger segment. In the second series of experiments, 16 measurements were made on comparable segments with the rheograph on one side of the body and the air displacement plethysmograph on the other side. The positions of the mechanical and electrical transducers were then exchanged and the procedure repeated. The right and left results were then averaged for each of the two flow measurement systems. Blood flow indices, determined with the impedance rheograph, were consistently 30% lower than those obtained with the air displacement system. The overall mean (+/- S.E.) for the impedance to air displacement blood flow ratio for the first series of measurements was 0.69 +/- 0.03 (range = 0.46 to 1.02). The mean (+/- S.E.) impedance to air displacement ratio for the second series of experiments was 0.68 +/- 0.06 (range = 0.30 to 1.14).