Gas exchange in tidally ventilated and non-steadily perfused lung model.

We studied the effect of cyclic lung perfusion - fast cycle in synchrony with heart beats and slow cycle in synchrony with ventilation - on gas exchange in a lung model. There was almost no effect in the fast cycle. In a homogeneous single-lung unit, arterial PO2 increased, and the (A - a)DO2 decreased (by approximately 0.5 Torr), as the amplitude of the slow cyclic lung perfusion (TIP) increased. The calculated (A - a)DO2 and (a - A)DCO2 were negative. Maximal PaO2 was found when peak lung perfusion was delayed with respect to ventilation by 0.2 of a cycle. In a non-homogeneous nine-unit lung, cyclic lung perfusion caused an increase in PaO2 and a decrease in (A - a)DO2 by 2 Torr as compared to steady perfusion. No apparent negative (A - a)DO2 was found, but apparent negative (a - A)DCO2 was calculated at no pulmonary shunt and also with 5% shunt. The correlation of cyclic lung perfusion to the reduced (A - a)DO2 in dense-gas breathing - where large swings of pleural pressure are expected - and its effect on the diffusion capacity of the lung are discussed. Non-steady perfusion of the lung as caused by ventilatory movements expanded our understanding of gas exchange and shed some light on a few controversial experimental findings, such as the negative (a - A)DCO2, the decreased (A - a)DO2 while breathing dense gas, and the effects of gas density on diffusion capacity of the lung.