British Journal of Anaesthesia, Vol 77, Issue 5 678-683, Copyright © 1996 by The Board of Management and Trustees of the British Journal of Anaesthesia
C. J. Joyce and K. G. Hickling
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.
COMMENTARY
Permissive hypercapnia and gas exchange in lungs with high Qs/Qt: a mathematical model
Department of Intensive Care, Christchurch Public Hospital, Christchurch, New Zealand
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