Postural Changes in Respiratory Function

Changes in body position alters the functional residual capacity (FRC). Most anesthetics reduce FRC in the recumbent but not sitting position. Inspired gas distribution in anesthesia-paralyzed subjects whose lungs are mechanically ventilated, is different from that in the awake state in all but the prone position. The function of the diaphragm is altered by postural changes. The pattern of motion of the diaphragm is different during mechanical ventilation than during spontaneous breathing. Also the end-expiratory shape is affected by induction of anesthesia, but this shape change contributes little to the reduction of FRC. The distribution of pulmonary blood flow is determined not only by gravity, but also by an intrinsic non-gravity dependent factor. These two factors can be additive in some positions but opposing in others.