Table 1.

Summary of physiological changes between supine and prone positions in spontaneous breathing and mechanical ventilation. The physiological effects of prone position on ventilation (V̇) and perfusion (Q̇) with IPPV are greater in injured lungs when lung density is increased. Vt, tidal volume

	Spontaneous breathing		IPPV
	Supine	Prone	Supine	Prone
FRC	Dependent on body position Minimal when supine, greater when upright.	Midway between upright and supine	Usually reduced, more so if neuromuscular blocking agent used. Dependent on PEEP level	As for supine
Respiratory muscles	Supine: two-thirds of Vt from intercostal activity Upright: two-thirds of Vt from diaphragm	As for supine	Muscles inactive	Muscles inactive Chest wall compliance reduced
Regional ventilation	Slightly greater in dependent region (supine = dorsal; upright = bases)	More uniform than supine	Predominantly non-dependent (ventral)	Greater in non-dependent (dorsal) Less heterogeneity
Regional perfusion	Greater in dependent region (supine = dorsal; upright = bases)	Greater in dependent (ventral)	Predominantly dependent (dorsal)	Greater in dependent (ventral)
V̇/Q̇ ratio	Lower in dependent regions	Lower in dependent regions (ventral). Less heterogeneity	High degree of mismatch	Better matching than supine