The significance of a change in Pa02 occuring in a patient with cardiovascular instability cannot be ascertained unless the values for PV0 and CV02 are known. A fall in Pa02 could 2 reflect worsening of pulmonary function or deterioration of cardiac output (with resultant increase in systemic oxygen extraction). PEEP/CPAP would be an appropriate therapy in the former case but frankly deleterious in the latter if applied prior to cardiovascular stabiliza- tion. References 1. Douglas ME, Downs JB, Dannemiller FJ et al (1976) Change in pulmonary venous admixture with varying inspired oxygen. Anesthesia and Analgesia 55:688-695 2. Kelman GR, Nunn JF, Prys-Roberts C et al (1967) The influence of cardiac output on arterial oxygena- tion. A theoretical study. Br J Anaesth 39:450-458 3. Kirby RR (1986) Respiratory vs cardiovascular dysfunction - How can we differentiate? ASA Refresher Course Lectures, 102 4. Nunn JF (1977) Applied respiratory physiology (2nd ed). Butterworth et Co 5. Van Aken H, Lawin P (1981) Der EinfluB des Herzminutenvolumens auf die arterielle Oxygenation.