VENTILATION HETEROGENEITY IS INCREASED IN HYPOCAPNIC DOGS BUT NOT PIGS

Hypocapnia increases ventilation/perfusion ((V) over dot A/(Q) over do t)) heterogeneity in dogs, possibly by adversely affecting distributio n of ventilation through its effects on collateral ventilation. Becaus e pigs lack collateral ventilation, we compared the effects of hypocap nia on ventilation heterogeneity in pentobarbital-anesthetized, mechan ically-ventilated dogs and pigs. Simultaneous multiple breath washouts of helium and nitrogen were used to assess the uniformity of the vent ilation distribution by the phase III (Sn-III) method. Ventilation het erogeneity was partitioned into two components, e.g. convective-depend ent inhomogeneity (cdi) and diffusive-convective-dependent inhomogenei ty (dcdi). Pulmonary gas exchange was also measured in pigs by the mul tiple inert gas elimination technique. Ventilation heterogeneity was i ncreased (P < 0.01) in hypocapnic dogs. Inspiration of CO2 decreased v entilation heterogeneity by decreasing dcdi (P < 0.01). In contrast, V entilation heterogeneity was not increased in hypocapnic pigs. However , hypocapnia increased (V) over dotA/(Q) over dot heterogeneity by 18% (P<0.05) in pigs. We conclude that hypocapnia increases ventilation h eterogeneity in dogs but not in pigs, most likely related to an inters pecies difference in collateral ventilation. (C) 1998 Elsevier Science B.V.