THE EFFECT OF CARBON DIOXIDE ON CARDIAC OUTPUT IN PATIENTS UNDERGOING MECHANICAL VENTILATION FOLLOWING OPEN HEART SURGERY

Measurements of cardiac output were made on twelve patients undergoing intermittent positive pressure ventilation following cardiopulmonary bypass surgery. Initial control measurements were made during mild hyperventilation, repeated after addition of carbon dioxide to the inspired gases and again following return to the control situation. The ventilatory pattern was not changed during the course of the studies. There was a mean increase of 17.4 mm Hg in the arterial carbon dioxide tension (Paco2) following the addition of carbon dioxide and this was accompanied by a statistically significant rise of cardiac output of 14%. This occurred despite the fact that more than 50% of the patients required increased sedation to control ventilatory efforts. After withdrawal of carbon dioxide the Paco2 and cardiac output fell to values which were not significantly different from the control situation. Heart rate, stroke volume and right atrial pressure all increased with the increase in Paco2 and decreased after the withdrawal of carbon dioxide. There were no significant changes in airway or oesophageal pressures. These results suggest that the cardiovascular status of patients following open heart surgery is better maintained when control of ventilation is achieved by sedation at normocapnia than by hypocapnia alone.