Volume-pressure relations of the human pulmonary "venous" system studied by radionuclide angiocardiography and passive leg elevation, with special reference to the effect of nitroglycerin.

In an attempt to clarify the effect of nitroglycerin (NTG) on the human pulmonary "venous" (P"V") system, short segments of volume-pressure (V-P) curves in the P"V" system were estimated in 50 patients before and after the sublingual administration of NTG. The pulmonary "venous" volume (P"V"V) was estimated with our own method, using radionuclide (RN) angiocardiography. The mean pulmonary artery wedge pressure (PAW) was measured using a Swan-Ganz catheter. The increments of P"V"V and PAW that occurred during passive elevation of the legs were clues to the estimation of V-P "curves" in this system before and after NTG. In order to calculate a short segment of a V-P curve, the P"V"V was plotted on the ordinate and the PAW on the abscissa. In addition, we presented the effective unstressed volume graphically by extrapolating each short segment of V-P curves towards the volume axis (ordinate). We obtained the following results from the present study; The sublingual administration of NTG caused the P"V"V-PAW-plots to shift to the left (-5.2 +/- 3.6 mmHg, mean +/- SD) and downwards (-19 +/- 11 ml, mean +/- SD). The V-P curves appeared to shift to the left and become steeper after NTG. The average slope (delta V/delta P) was steeper after NTG (19.2 +/- 11.3 ml/mmHg, mean +/- SD) than before NTG (12.4 +/- 7.9 ml/mmHg). The graphically-obtained effective unstressed volume was unchanged with NTG, at least within the operating range of PAW. It is highly probable that NTG causes the tone of the P"V" system to be decreased by way of a relaxation of the vascular beds of the P"V" system.