Oxygen cost of breathing. Changes dependent upon mode of mechanical ventilation

We describe a patient with respiratory failure who demonstrated marked increases in O2 consumption (VO2) when breathing with synchronized intermittent mandatory mechanical ventilation (SIMV). When the mode of ventilation was changed to facilitate inspiratory gas flow (pressure-support) during spontaneous breathing, O2 consumption decreased 27 percent. Several important factors contributing to the increased O2 cost of breathing in patients requiring mechanical ventilation are reviewed, including the high internal resistance of demand-flow SIMV systems.