1. The blood flow in the forearm and the calf of six healthy volunteers was measured at rest and after exercise by impedance plethysmography using pulsatile (QZp) and venous occlusion (QZocc) methods, and by venous occlusion strain gauge plethysmography (Qsg).

2. At rest, the impedance QZp method gave values slightly higher than those of Qsg. In the forearm, the ratio QZp to Qsg was 1.26 in the supine position and 1.97 in the upright sitting position. For the calf muscle, the ratios were 1.08 in the supine position and 1.23 in the upright position.

3. Immediately after exercise, Qsg increased from resting values of approximately 2–4 ml min−1 100 ml−1 to mean values of 16–25 ml min−1 100 ml−1 in upright and supine arm or leg exercise. In contrast, the QZp values after exercise increased to only 3.1–4.6 ml min−1 100 ml−1. QZocc likewise failed to show increases in flow except in the supine leg exercise, where flow increased to 8.7 ml min−1 100 ml−1.

4. In an additional subject, it was shown that electrode position had no significant effect on the QZp blood flow measurement after exercise.

5. The failure of QZp to accurately follow the change in Qsg with exercise was probably due in part to pulsatile venous outflow. In addition, changes in microvessel packed cell volume and shear rate may influence the observed QZp. It is concluded that impedance plethysmography is not valid for estimation of limb blood flow during reactive hyperaemia after exercise.

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