The decision to start a weaning trial in a critically ill patient often implies that patients have correct arterial blood gases. The fact that capnography could provide a non-invasive tool to assess alveolar ventilation, and thereby PaCO2, induced some investigators to evaluate the utility of capnography during weaning periods. However, these studies have yielded controversial results. Although some authors reported that patients monitored with pulse oximetry and capnography required less blood gas sampling, others found that variations in PetCO2 did not correctly indicate changes in PaCO2 in patients with parenchymal lung disease, particularly emphysema. To analyze this clinical problem, we evaluated the relationship between PaCO2 and PetCO2 before weaning and during a weaning trial to determine the ability of PetCO2 to identify clinically relevant episodes of hypercapnia. Interestingly, we found that monitoring PetCO2 and pulse oximetry provided good assessment of hypercapnic episodes and oxygenation impairment during weaning from mechanical ventilation. Although capnography does not detect minor changes in PaCO2 with 100 per cent accuracy, continuous PetCO2 monitoring reduces the need for arterial blood sampling. Accordingly, although not replacing invasive arterial determinations entirely, capnography seems reliable for use as a non-invasive tool for assessing alveolar ventilation during weaning in general intensive care patients
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