To determine the effects of posture on the venodilatory response to nitroglycerin (TNG), the change in forearm venous volume after inflation of an upper arm cuff to 30 mmHg above cuff zero (VV) was measured during control conditions and after TNG (0.8 mg spray) in 18 healthy young volunteers in the supine position and the sitting position. VV was 3.24 ± 0.98 ml/100 ml arm in the supine position and 2.46 ± 1.32 ml/100 ml arm in the sitting position. TNG increased VV by 0.56 ± 0.19 ml/100 ml arm in supine subjects, but by only 0.38 ± 0.17 ml/100 ml arm in sitting subjects (P = 0.013). When limb volume was measured in the forearm and calf without using a cuff to produce venous congestion, the increase in limb volume with TNG was significantly greater in the sitting than in the supine position. Because the fall in both systolic and diastolic pressure and the rise in heart rate were significantly greater after TNG was administered in the sitting position, it is suggested that a greater reflex venoconstriction occurred in this posture, which antagonized the TNG-induced increase in venous distensibility. In the seated position, the effect of gravity more than compensated for the impaired venodilatory response to TNG. These results suggest that TNG causes a greater reduction in venous return to the heart when administered in the sitting position than in the supine position.
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