In the course of a series of studies on neuromuscular function it became desirable to investigate in man the metabolism of skeletal muscle in situ.For this purpose it was decided to apply the Zuntz-Fick principle to determine the exchange of metabolites between muscle and blood in the forearm with circulation occluded at the wrist.The selection of this segment was guided by the following considerations: (a) Convenience; (b) the large muscular volume relative to the volume of skin (1); (c) the predominance of blood flow through muscle relative to skin (2, 3); and (d) the relative avascularity of other structures suggesting that no less than 80 per cent of total forearm blood flow is muscular in distribution.In addition to the estimation of arterio-venous concentration differences in substances of interest, the principle requires a measurement of blood flow through the segment under study. Of methods in current use, only venous-occlusion plethysmography provides an absolute numerical value for blood flow through a human extremity. Despite the wealth of observations recorded by this method it was deemed desirable to develop another approach which might yield continuous recording of blood flow as well as serial samples of arterial and venous blood. Furthermore, the sources of error of the plethysmographic method have been studied carefully but not overcome completely (4), and, to our knowledge, no verification of the method by an independent technic has been made in man.

Measurements of CO and SV were moderately strong in correlation and agreement which may suggest PAC measurements overestimate actual values. The significance of tricuspid regurgitation and mitral regurgitation during early resuscitation is unknown.