Fig. 1

Diaphragm motion and ventilation/perfusion distribution in the awake and in the anesthetized subject. The lung ventilation-to-perfusion ratio (V/Q) is color-coded from white (high V/Q), to green (V/Q ≈ 1), to red (low V/Q). Diaphragm shape at end expiration (continuous line) and end inspiration (dashed line) in the supine position is shown. Intra-abdominal pressure increases in the ventro-dorsal direction due to gravity (blue arrows) and displaces the dorsal part of the diaphragm more cephalad than the ventral part at end expiration. During mechanical ventilation the pressure applied by the mechanical ventilator displaces the ventral part of the diaphragm, which faces less intra-abdominal pressure, more than the dorsal part (passive movement). Ventilation will thus be distributed preferentially to the ventral lung regions, increasing the ventilation-to-perfusion ratio (V/Q) of these areas. In contrast, dorsal lung regions will receive less ventilation and their V/Q will be lower (a). During spontaneous breathing (either assisted or unassisted), both the ventral and the dorsal part of the diaphragm move (active contraction). Ventilation will distribute more homogeneously along the ventro-dorsal axis of the lung and will more closely match perfusion (V/Q ≈ 1) (b)