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The Engstrom volume-controlled respirator is used routinely in patients after open-heart surgery as a short-term prophylaxis to avoid hypoxia, respiratory acidosis, and excessive fatigue. The respirator should be discontinued when pulmonary function is improved sufficiently to the point where the work of breathing is no longer excessive. With a spirometer designed for the Engstrom respirator, we have determined oxygen consumption and cardiac output in the spontaneously breathing patient and while assisted by the respirator. The difference between the two measurements reflects the oxygen cost of breathing and thus the work that can be spared the heart, by proper mechanical ventilation, during the period of acute postoperative stress. We have demonstrated that the work of breathing, even in good risk patients, is appreciably increased postoperatively, that it is maximal immediately and then falls. These data lend objective support for the prophylactic use of respiratory assistance in patients after cardiac surgery. A similar application to other debilitated surgical and medical patients should be investigated.

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