Permissive hypercapnia impairs pulmonary gas exchange in the acute respiratory distress syndrome

Current recommendations for mechanical ventilation in the acute respiratory distress syndrome (ARDS) include the use of small tidal volumes (V-T), eve n at the cost of respiratory acidosis. We evaluated the effects of this per missive hypercapnia on pulmonary gas exchange with the multiple inert gas e limination technique (MIGET) in eight patients with ARDS. After making base line measurements, we induced permissive hypercapnia by reducing V-T from 1 0 +/- 2 ml/kg to 6 +/- 1 ml/kg (mean +/- SEM) at constant positive end-expi ratory pressure. After restoration of initial V-T, We infused dobutamine to increase cardiac output ((Q) over dot) by the same amount as with hypercap nia. Permissive hypercapnia increased (Q) over dot by an average of 1.4 L(. )min(-1.)m(2), decreased arterial oxygen tension from 109 +/- 10 mm Hg to 9 2 +/- 11 mm Hg (p < 0.05), markedly increased true shunt (Q) over dot(S)/(Q ) over dot(T)), from 32 +/- 6% to 48 +/- 5% (p < 0.0001), and had no effect on the dispersion of (V) over dot(A)/(Q) over dot. On reinstatement of bas eline V-T with maintenance of a high (Q) over dot, (Q) over dot(S)/(Q) over dot(T) remained increased, to 38 +/- 6% (p < 0.05), and Pao,remained decre ased, to 93 +/- 4 mm Hg (p < 0.05). These results agreed with effects of ch anges in V-T and (Q) over dot predicted by the mathematical lung model of t he MIGET. We conclude that permissive hypercapnia increases pulmonary shunt , and that deterioration in gas exchange is explained by the combined effec ts of increased (Q) over dot and decreased alveolar ventilation.