Reduced oxygen consumption in catabolic states with mechanical ventilation

Most critically ill patients have a high catabolic rate to which sepsis, restlessness, and the hormonal stress response may be contributing factors. This has been confirmed by measurement of oxygen consumption. (Vo2) as a reflection of energy expenditure. While acutely ill patients may adapt to this situation, one of their adaptative mechanisms is to increase their minute ventilation. Increased minute ventilation increases the work of breathing and the oxygen cost of respiratory work. The higher the minute ventilation, the higher the percentage of oxygen consumed for respiratory work. In order to evaluate the magnitude of these factors, 20 acutely ill patients (severe burns, multiple injuries, and comatose states) were studied by measurements of Vo2 during mechanical ventilation and after disconnection from the ventilator for short periods of time; i.e., 20 min. Paired tests, on and off ventilator, were performed successively three to five times on each patient ventilated for various reasons, but not those in respiratory distress. The tests were mainly performed during the period of weaning from mechanical ventilation. A reproducible increase in Vo2 was observed in most patients when they were disconnected from mechanical ventilation and allowed to breathe spontaneously. The observed reduction of Vo2 with mechanical ventilation was interpreted as a direct result of mechanical ventilation. The possibility of extending the indications for mechanical ventilation in hypercatabolic state patients without acute respiratory failure is discussed.