Am. J. Respir. Crit. Care Med., Vol 151, No. 1, 01 1995, 210-214.
The effect of mechanical ventilation on oxygen consumption in critically ill patients
CA Manthous, JB Hall, R Kushner, GA Schmidt, G Russo and LD Wood
Department of Medicine, University of Chicago, Illinois.
We measured oxygen consumption (VO2) 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. VO2
decreased in all eight patients between CPAP and AC-MR; mean VO2 (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 VO2 in five of eight
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 spontaneous breathing,
the difference between CPAP and AC-MR (VO2resp) represents the decrement of
VO2 that can be obtained during muscle rest. Both VO2resp 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 VO2resp 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 VO2 by
more than 20%; beyond stabilizing pulmonary gas exchange, these
interventions preserve limited O2 delivery (QO2) for other vital organs.
