THE EFFECT OF MECHANICAL VENTILATION ON OXYGEN-CONSUMPTION IN CRITICALLY ILL PATIENTS

We measured oxygen consumption (VO2) during spontaneous breathing with continuous positive airway pressure (CPAP), assist control ventilatio n (AC), and control ventilation during muscle relaxation (AC-MR) in ei ght patients undergoing resuscitation from cardiopulmonary failure. VO 2 decreased in all eight patients between CRAP and AC-MR; mean VO2 (25 5 +/- 92 ml/min) on CPAP exceeded that on AC-MR (209 +/- 79 ml/min) (p < 0.005). Compared with CPAP, AC without MR reduced VO2 in five of ei ght patients and mean VO2 (227 +/- 59 ml/min) tended to decrease (p = 0.14); clinical examination did not distinguish patients requiring MR to reduce VO2 further. If VO2 on CPAP approximates VO2 during spontane ous breathing, the difference between CPAP and AC-MR (VO2resp) represe nts the decrement of VO2 that can be obtained during muscle rest. Both VO2resp and the mechanical work performed by the ventilator on the re spiratory system were increased to about five times the efficiencies r eported for normal patients, but VO2resp did not correlate with the me chanical work because of a wide range of respiratory muscle efficienci es. 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 VO2 by more than 20%; beyond stabilizin g pulmonary gas exchange, these interventions preserve limited O-2 del ivery (QO(2)) for other vital organs.