Carbon dioxide and large volume ventilation in the management of patients undergoing cardiac surgery.

Abstract

Respiratory alkalosis produced by ippb in patients undergoing cardiothoracic surgery may reduce cardiac output, facilitate the induction of cardiac arrhythmias, induce digitalis toxitity, decrease cerebral blood flow and shift the oxyhaemoglobin dissociation curve to the left. These effects are most threatening in patients undergoing cardiac surgery whose cardiac reserve is poor and whose myoeardial irritability is enhanced. This study was carried out in HO patients who underwent either open or closed heart surgery for acquired cardiac disease. Three groups of patients were each managed in a different way. Group a (48 patients) was ventilated with the carbon dioxide absorber in place and no carbon dioxide added to the breathing mixture; Group b (27 patients) had the carbon dioxide absorber eliminated from the circuit; and Group c (35 patients) had the carbon dioxide absorber eliminated from the circuit and 2.5 per cent carbon dioxide added to the breathing mixture. All patients were ventilated with tidal volumes which were 1,5 to 3.4 times greater than those recommended by Radford. All had satisfactory Pco2. The Paco2 and pH were significantly different for the three groups. The mean Paco2 for Groups a, b, and c were 30.6, 36.7, and 44.7 mm Hg respectively. An arterial pH greater than 7.44 was observed in 88 per cent of Group a and 33 per cent of Group b, but in only 6 per cent of Group c patients. A greater latitude in ventilatory volume was permissible in Group c subjects to attain a homogeneous result within a desired range of Paco2 levels. Severe acidaemia was not a problem. This study shows that respiratory alkalosis can be avoided while maintaining suitable oxygenation without risk of respiratory acidaemia in patients undergoing cardiac surgery. Empirically, this can be predictably obtained by the use of at least 6 liters of fresh gas, tidal volumes 2.0 to 2.5 times that calculated from the Radford nomogram, removal of the carbon dioxide absorber and addition of 2.5 per cent of carbon dioxide and 50 per cent of oxygen to the inspired mixture.