The difference between PaCO2 and PetCO2 can be used to determine the appropiate level of positive end-expiratory pressure (PEEP) in acute respiratory failure. In both experimental animals and patients, the smallest PaCO2 - PetCO2 gradient coincided with the level of PEEP providing the best arterial oxygenation and the least intrapulmonary shunt. When lung overdistension was caused by higher levels of PEEP, the PaCO2 - PetCO2 gradient increased due to an increase in alveolar dead-space with no further improvement in arterial oxygenation or intrapulmonary shunt. Nevertheless, the utility of the PaCO2 - PetCO2 gradient for setting optimal PEEP levels seems to be limited to those patients who are able to recruit collapsed lung units with PEEP ( B.!a.0ch...®L§l 1987).
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