The effect of mechanical ventilation on oxygen consumption in critically ill patients

We measured oxygen consumption (V̇O₂) during spontaneous breathing with continuous positive airway pressure (CPAP), assist control ventilation (AC), and control ventilation during muscle relaxation (AC-MR) in eight patients undergoing resuscitation from cardiopulmonary failure. V̇O₂ decreased in all eight patients between CPAP and AC-MR; mean V̇O₂ (255 ± 92 ml/min) on CPAP exceeded that on AC-MR (209 ± 79 ml/min) (p < 0.005). Compared with CPAP, AC without MR reduced V̇O₂ in five of eight patients and mean V̇O₂ (227 ± 59 ml/min) tended to decrease (p = 0.14); clinical examination did not distinguish patients requiring MR to reduce V̇O₂ further. If V̇O₂ on CPAP approximates V̇O₂ during spontaneous breathing, the difference between CPAP and AC-MR (V̇O(2resp)) represents the decrement of V̇O₂ that can be obtained during muscle rest. Both V̇O(2resp) and the mechanical work performed by the ventilator on the respiratory system were increased to about five times the efficiencies reported for normal patients, but V̇O(2resp) did not correlate with the mechanical work because of a wide range of respiratory muscle efficiencies. These efficiencies are less than those reported in normal patients, which may reflect the effect of sepsis, acidemia, hypoxia, or other conditions in these patients. We conclude that mechanical ventilation with muscle relaxation reduces V̇O₂ by more than 20%; beyond stabilizing pulmonary gas exchange, these interventions preserve limited O₂ delivery (Q̇O₂) for other vital organs.