Study of acid-base balance in open-heart surgical patients.

ACID-BASE DERANGEMENTS are frequently encountered in surgical patients, preoperatively, during the surgical procedure, and postoperatively. For some time surgeons and anaesthetists have concerned themselves with the control of these changes, and more recently the postoperative period has come under close scrutiny as regards acidosis and decreased arterial saturation. 1,~ The advent of open-heart surgery by means of a total cardiopulmonary by-pass, on patients with abnormal cardiovascular physiology subjected to artificial t~aemodynamic control, has resulted in tremendous experimental and clinical activity towards a better understanding and control of acid-base status. 3,4.5 During the past five years nearly 500 patients have undergone cardiopulmonary by-pass at the Vancouver General Hospital using the Lillehei-DeWall bubble-type oxygenator 6 and more recently a modification of the Gross disk-type oxygenator. 7 Control of these patients has been maintained by monitoring the direct arterial and venous pressures, the electrocardiogram, the electroencephalogram, arterial and venous oxygen saturations, and the arterial and venous pH. By these means a reasonably satisfactory knowledge of the patient's condition has been available at all times. Extracorporeal circulation methods in the future will undoubtedly improve. However, at this stage we feel that the maintenance of high standard perfusion is aided by the continuance of our monitoring control. Recently, our interest has centred around more particular control of acid-base balance from both the respiratory and metabolic aspect. Although of some value in our earlier cases the pH of the blood alone was found to be inadequate in assessing the respiratory and metabolic components of the operative and postoperative acid-base balance. During the past one and one-half years we have undertaken a comprehensive study of the over-all acid-base changes in 150 clinical open-heart cases. For this study we have chosen to use the Astrup micro-apparatus, s'9,1~ This instrument was selected because of the rapidity with which samples could be analysed and also because of its temperature-compensating mechanism; all readings were taken at 38 ~ C. The apparatus also has the additional advantage of requiring a minute quantity of blood, such as can be obtained from a finger-prick; yet it provides values comparable with those from an arterial lblood sample in the maiority of circumstances, n We have found that the freq\/ent monitoring of the individual patient as regards pCO2, arterial pH, base excess, and standard bicarbonate according to the Astrup nomogram t2 has considerably improved our handling of these cases.