The response of arterial blood pressure to an increase in intrathoracic pressure has been shown to be predictive of pulmonary capillary wedge pressure. We devised a new method, which we termed the arterial pressure ratio. We defined arterial pressure ratio as the ratio of systolic blood pressure of the final beat during the strain phase of the Valsalva manoeuvre to that during apnoea before the manoeuvre, and tested the accuracy of arterial pressure ratio in predicting pulmonary capillary wedge pressure. In 30 patients scheduled for elective abdominal aortic reconstruction, following induction of general anaesthesia and tracheal intubation, a 20-G catheter and pulmonary artery catheter were inserted through the radial artery and right internal jugular vein, respectively. Pulmonary capillary wedge pressure was then measured during a brief period of apnoea and the Valsalva manoeuvre was performed by application of pressure to the reservoir bag. Airway pressure was maintained at 30 cmH2O for 10 s and then released. Radial arterial pressure and airway pressure were recorded simultaneously, and arterial pressure ratio was calculated. There was a close linear correlation between arterial pressure ratio and pulmonary capillary wedge pressure (r = 0.88, p < 0.0001).