Outline the effect of MO on functional residual capacity.
MO reduces functional residual capacity (FRC). During the course of normal tidal ventilation, the lung inflates and deflates. During exhalation of a normal tidal volume the lung deflates partially, but much of it remains inflated. The amount of gas remaining in the lung after expiration of a normal tidal volume is the FRC. Normally, the FRC maintains lung inflation above the closing volume. Closing volume is that lung volume at which terminal air units collapse. Reduced FRC associated with MO allows the lungs to deflate and reach closing volume during exhala-tion. The result is alveolar collapse, ventilation/perfusion mismatching, and hypoxemia.
FRC is further reduced in the supine position and by paralysis. Consequently, MO patients are at increased risk for hypoxemia during anesthesia and surgery.