INTRAPULMONARY GAS MIXING AND PULMONARY GAS-EXCHANGE IN ARTIFICIALLY VENTILATED DOGS

To investigate the effect of incomplete gas mixing between tidal air and residual gas on pulmonary gas exchange, anaesthetized dogs were ventilated artificially with breathing patterns with different durations of the post-inspiratory apnoea (t(a) = 0, 0.5, 1.0 and 2.0 s), where tidal volume, breathing frequency, inspiratory and expiratory flow patterns were kept constant. We determined the alveolar ventilations (V-A) of He and SF6 from the product of end-expiratory lung volume (V-L,V-E') and specific ventilation (V-A/V-L,V-E'). V-L,V-E' was determined by the dilution technique and the specific ventilations of the two gases were obtained from their multiple-breath washout. Further, tracer amounts of acetone, ether and enflurane were infused continuously into a peripheral vein and a bolus of a gas mixture of krypton, Freon12 and SF6 was introduced into the peritoneal cavity. We determined the Excretion (E) and Retention (R) of these six gases according to the multiple-inert-gas-elimination technique (MIGET). V-A increased with increasing t(a), where V-A,V-He was about 14% larger than V-A,V-SF6 For both gases, however, the increase in V-A relative to control (V-A for t(a) = 0) was virtually the same: 9, 11 and 19% (mean values) for t(a) = 0.5, 1.0 and 2.0 s respectively. For all dogs the E/R curve shifted to larger E values with increasing t(a). E for the most soluble tracer gas (acetone) increased by 11, 21 and 25% for t(a) = 0.5, 1.0 and 2.0 s respectively. V-A, determined with MIGET from the ventilation/perfusion distribution, increased by almost the same percentages. These results are interpreted to indicate that pulmonary gas exchange is substantially impaired by incomplete intra-acinar gas mixing.