Permissive hypercapnia and gas exchange in lungs with high Qs/Qt: a mathematical model

British Journal of Anaesthesia
C J Joyce, K G Hickling

Abstract

Low volume ventilation with permissive hypercapnia is becoming widely used in the treatment of acute respiratory distress syndrome. A mathematical model was developed to examine the effects of hypoventilation on pulmonary gas exchange in lungs with a range of shunt fractions. Hypoventilation did not worsen gas exchange, provided the inspired oxygen concentration was high enough to maintain PAO2 at an adequate level. In lungs with a high shunt fraction, some improvement in gas exchange may result, but these effects are small. A rightwards shift of the oxygen-haemoglobin dissociation curve induced by hypercapnia, is likely to be beneficial rather than detrimental in patients with acute respiratory distress syndrome. This analysis was limited to the direct effects of hypoventilation in lungs with constant shunt fractions, and did not encompass a number of possible secondary effects such as changes in cardiac output with PaCO2, changes in shunt fraction associated with a reduction in mean airway pressure and possible direct effects of hypercapnia on the pulmonary vasculature or airways.

Citations

Oct 23, 1998·Resuscitation·M Engoren, M Plewa
Oct 13, 2006·British Journal of Anaesthesia·J G Hardman, J J Ross
May 13, 2006·American Journal of Respiratory and Critical Care Medicine·João B BorgesMarcelo B P Amato
Jun 5, 2007·Journal of Cardiothoracic and Vascular Anesthesia·Michael S AvidanMichael Pasque

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