Vertical distribution of perfusion and inspired gas in supine man.

In order to specify if a closing volume greater than expiratory reserve volume could influence the distribution of perfusion (Qr), we measured vertical Qr in 6 supine subjects before and after inhalation of an oxygen enriched gas mixture. In addition, we studied Qr and inhaled gas (V alv.) at different lung volumes. We observed a preferential perfusion of non-dependent zones of the supine lung during tidal breathing from FRC correlated to the amount of airway closure represented by the difference between FRC and closing capacity. After oxygen breathing, the distribution of perfusion is reversed and flow is preferentially distributed in the dependent zones of the supine lung. This O2 sensitive effect was time and/or volume history dependent since Qr distribution at FRC after a breath to total lung capacity was similar to that after 3 min of tidal breathing with oxygen mixture. At FRC, the ventilation was preferentially distributed to the non-dependent lung zones. The vertical gradient of both Qr and V alv. increased progressively at higher lung volumes and Va/Q ratios were usually greater in non-dependent zones.