Evaluation of gas exchange

Hypercapnia in the presence of normal alveolar ventilation is a hallmark of (micro)vascular occlusion. Therefore arterial and mixed venous PCO2 should be analyzed and the physiological dead-space quantified:

where PaCO2 and are arterial and mean expiratory PCO2 respectively. A VJVT ratio of 0.3 to 0.45 is commonly observed in ventilated patients, while values above

0.5 to 0.6 indicate severe impairment of the VjQ ratio. The diagnostic value of evaluating arterial oxygenation is limited as there is no close relationship between the degree of microvascular occlusion and the severity of hypoxemia. Also, hypoxemia is not specific as mechanisms other than microvascular occlusion may decrease PaO2. Interestingly, the relative extent of impairment of oxygenation and CO 2 elimination may help identify the size of vessels involved in vascular occlusion. Occlusion of small arteries (< 100 ^m) mainly increases intrapulmonary shunt, causing severe hypoxemia, while occlusion of larger vessels (> 50-1000 ^m) predominantly increases VD/VT, causing severe hypercapnia.

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