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.