Rationale of venous occlusion plethysmography.

The experimental evidence reported here supports the validity of venous occlusion plethysmog-raphy. 1 modified the technic of Brodie, 2 venous occlusion plethysmography has been widely employed to estimate blood flow in fingers, toes, and limbs. Several excellent comprehensive reviews of the technical pitfalls of the method are available. 3-4 The validity of the venous occlusion technic of plethysmography rests upon four basic assumptions: (1) that the application of the pneumatic collecting cuff pressure does not affect arterial pressure or inflow, (2) that complete venous tamponade is effected for a finite period, (3) that the resulting increase in venous pressure does not initially reduce the rate of arterial inflow, and (4) that the impounded blood causes the segment under study to swell proportionally to the rate of arterial inflow without significant displacement of tissue or body fluids from the plethys-mograph. Adequate evidence exists concerning the first point. 1 ' 5 The studies to be described were designed to test the other three points. METHODS A water displacement plethysmograph with a strain gage sensing unit was used for the measurement of segmental forearm blood flow in human subjects, all of whom were adult males without evidence of cardiovascular disease. A PE50 catheter was inserted through a thin-walled no. 18 needle into the brachial artery of the forearm under study. Another PE50 catheter was similarly introduced into a superficial vein of the forearm segment. Simultaneously with the plethysmogram brachial artery and venous pressures were measured with Statham strain gages and recorded on a Sanborn multichannel recorder. For the purpose of testing the competence of the collecting cuff, the leakage of an indicator substance past the cuff was correlated with simultaneously recorded venous pressure in the obstructed area of forearm. For this purpose 20-40 >xc. of I 131 human serum albumin were injected into a dorsal hand vein following which the collecting cuff was suddenly inflated. On one occasion the I 131 injection was made into the radial artery. The forearm below the collecting cuff was carefully screened by means of several layers of %" lead sheeting. Preliminary phlebograms using Diodrast and the Fairchild camera demonstrated that injection into a superficial hand vein results in deep vein filling and that the earliest evidence of gross leakage is in the basilic vein. Accordingly, a scintillation counter was placed over the confluence of the cubital and basilic veins in such a position that it would monitor …