Methods Catheterization of the right heart was explained to the patient in detail 24 h before the procedure and was conducted in the routine manner at the Instituto National de Torax, La Paz. Patients were fasting on the morning of the test and considered to be in a basal state having also received sodium seconal 50 mg 30min prior to the procedure. Pressures from the right heart and pulmonary artery and wedge position as well as from a systemic artery were measured by means of Sanborn physiological pressure transducers and inscribed on a direct writing Sanborn recorder. Mean pressure was registered by direct electronic integration. All pressures were referred to atmospheric zero-level at 10 cm above the X-ray table top.

Resting minute ventilation (VE), respiratory frequency (f/min), tidal volume (VT), minute oxygen consumption (V02), and carbon dioxide elimination (VCC⅛) were obtained from a four minute expired air collection in a Douglas bag. All gas volumes are expressed as V or V–(BTPS) 2 ml, or 1/min/m2, except for VC· 2 and VC02 which are given at STPD3. Oxygen and carbon dioxide contents (FEC⅛ and