Pulmonary Gas Exchange in the Intensive Care Unit
Acute respiratory failure is an acute insufficiency of the gas exchange function of the lungs. Respiratory failure is always associated with abnormal arterial blood gases, in the sense that the gradient between alveolar PO2 (PAO2) and arterial PO2 (PaO2) (AaPO2) is increased, while PaCO2 is increased, normal or decreased. Respiratory failure therefore is classically defined using arterial blood gases criteria, generally by a PaO2 lower than 60 mmHg and a PaCO2 higher than 50 mmHg. Accordingly, pulmonary gas exchange in patients admitted to the intensive care unit are evaluated with repetitive arterial blood gas analysis. However, PaO2 and PaCO2 are determined not only by the gas exchange function of the lungs, but also by extrapulmonary factors including inspired PO2 (PIO2) and mixed venous PO2 (PvO2) which in turn depends on cardiac output (Q), O2 extraction, haemoglobin, and P50. Also, the tolerance to abnormal PaO2 and PaCO2 is largely dependent on the patient’s past history and current illness.
KeywordsChronic Obstructive Pulmonary Disease Interstitial Lung Disease Adult Respiratory Distress Syndrome Hypoxic Pulmonary Vasoconstriction Mixed Venous Blood
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