The Cost of Respiratory Effort in Postoperative Cardiac Patients.

THE incidenee of postoperative respiratory insufficiency following major tho-racie or abdominal surgery is significant.' 3 We have previously emphasized the difficulties involved in the early recognition of respiratory insufficiency,4 5 and have pointed out that even a minimal degree will fre-(luently produce a lowered arterial oxygen tension, with an associated serious arrhyth-mia and a decreased eardiac reserve, and result in a vicious cycle that ends in unnecessary death. Furthermore, assisted ventilation may be necessary to avoid respiratory acido-sis, which would further impair myocardial function. Untilwe can determine the patient's margin of safety, we advocate the prophy-lactic use of a volume-controlled respirator following open-heart surgery to tide the patient over the immediate and critical postop-erative period. The rospirator must have sufficient power to meet the patient's needs, must be safe, and in no way productive of deleterious changes in the eardiovascular system or the parenchyma of the lung. The Engstrom volume-controlled respirator6 is the only one we have found that meets these criteria in the severely ill patient. Routine postoperative use of the respirator with careful monitoring of hemodynamic and respiratory functions has emphasized an important aspect of postoperative physiology. The need for adequate oxygenation and carbon dioxide elimination is well understood. 552 patient to maintain adequate respiratory function has not been sufficiently appreciated, despite the fact that Bjork and Engstrom implicated respiratory work load as a major causative factor of respiratory insufficieney.7 8 The magnitude of this work load was first recognized by us when the clinical courses of patients who were carried on the respirator for a period of a few hours to several days were compared to other cardiac patients whose ventilation was not assisted. Those with respiratory assistance had a smooth recovery after anesthesia and required very little sedation , despite the continued use of an endo-tracheal tube. Exhaustion, restlessness, and apprehension were not present unless respiratory insufficiency was severe and intractable. The patients were alert and had an ample reserve of strength. Their appearance contrasted sharply with that of patients who were not assisted. This difference raised the possibility that the mere physical work of breathing contributed significantly to postoperative morbidity, to delayed congestive failure, and to a progressive respiratory insufficieney secondary to exhaustion. Therefore, a study was undertaken to evaluate the work and thus the energy cost of breathing in the postopera-tive patient.