Gravity, the belly, and the diaphragm: you can't ignore physics.

Using a radiologic technique, the position and pattern of movement of the diaphragm have been evaluated in three adult volunteers, both awake and anesthetized, during spontaneous ventilation and with muscle paralysis and mechanical ventilation. Studies were made with the subjects in supine and left lateral decubitus positions with tidal and large-volume breaths. Positive end-expiratory pressure (PEEP) was added in studies of two subjects. During spontaneous ventilation awake or anesthetized, because of regional mechanical advantages, the dependent part of the diaphragm had the greatest displacement despite the higher intraabdominal pressure in this region. Paralysis, awake or anesthetized, caused a cephalad shift of the end-expiratory position of the diaphragm that was disproportionately large in dependent regions. It also reversed the pattern of diaphragmatic displacement. The passive diaphragm was displaced preferentially in nondependent zones where abdominal pressure is least. Consequently, PEEP could not restore the diaphragm to its awake functional residual capacity position, and large breaths also could not duplicate the pattern of displacement achieved spontaneously.