Am. J. Respir. Crit. Care Med., Volume 162, Number 1, July 2000, 209-215

Permissive Hypercapnia Impairs Pulmonary Gas Exchange in the Acute Respiratory Distress Syndrome

FRANÇOIS FEIHL, PHILIPPE ECKERT, SERGE BRIMIOULLE, OLIVIER JACOBS, MARIE-DENISE SCHALLER, CHRISTIAN MÉLOT, and ROBERT NAEIJE

Department of Internal Medicine, Lausanne University Hospital, Lausanne, Switzerland; and Department of Intensive Care, Erasmus University Hospital, Brussels, Belgium

Current recommendations for mechanical ventilation in the acute respiratory distress syndrome (ARDS) include the use of small tidal volumes (VT), even at the cost of respiratory acidosis. We evaluated the effects of this permissive hypercapnia on pulmonary gas exchange with the multiple inert gas elimination technique (MIGET) in eight patients with ARDS. After making baseline measurements, we induced permissive hypercapnia by reducing VT from 10 ± 2 ml/kg to 6 ± 1 ml/kg (mean ± SEM) at constant positive end-expiratory pressure. After restoration of initial VT, we infused dobutamine to increase cardiac output () by the same amount as with hypercapnia. Permissive hypercapnia increased by an average of 1.4 L · min¹ · m², decreased arterial oxygen tension from 109 ± 10 mm Hg to 92 ± 11 mm Hg (p < 0.05), markedly increased true shunt ( S/ T), from 32 ± 6% to 48 ± 5% (p < 0.0001), and had no effect on the dispersion of A/ .VA/. On reinstatement of baseline V T with maintenance of a high , S/ T remained increased, to 38 ± 6% (p < 0.05), and Pa_O2 remained decreased, to 93 ± 4 mm Hg (p < 0.05). These results agreed with effects of changes in VT and predicted by the mathematical lung model of the MIGET. We conclude that permissive hypercapnia increases pulmonary shunt, and that deterioration in gas exchange is explained by the combined effects of increased and decreased alveolar ventilation.

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