The role of physiological deadspace and shunt in the gas exchange of patients with pulmonary hypertension: a study of exercise and prostacyclin infusion.

Haemodynamic and gas exchange measurements were made at rest, on supine exercise and on acute vasodilatation with intravenous prostacyclin in eight patients with pulmonary hypertension. This enabled an assessment of the contribution of V/Q imbalance to the abnormal gas exchange on the condition. At rest arterial oxygen tension, PaO2 (mean 8.1 +/- 1.7 kPa) and mixed venous oxygen tension, PVO2 (3.6 +/- 0.4) were reduced. The physiological shunt, Qs/Qt (15 +/- 17%) and the dead space, Vd/Vt (0.47 +/- 0.11) were elevated above normal. Exercise produced an increase in cardiac index, a fall in PVO2, no significant change in PaO2 and also no appreciable changes in the Vd/Vt and Qs/Qt. Intravenous prostacyclin increased the cardiac index and raised the PVO2 and the PaO2 but again with no significant changes in Vd/Vt and Qs/Qt. We conclude that ventilation-perfusion imbalance as shown by increased Vd/Vt and Qs/Qt, contributes significantly to the abnormal gas exchange in pulmonary hypertension. But neither index was altered by exercise or vasodilation; the latter improves the hypoxemia by increasing the PVO2 from an increase in the cardiac output.