Hypoxemia due to increased venous admixture: influence of cardiac output on oxygenation
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In the hypoxemic patient under mechanical ventilation, changes in cardiac output may influence the level of arterial oxygenation, with several and sometimes opposite effects. The purpose of this Physiological Note is to give the reader the physiological background to understand these effects, which are often highly relevant for the bedside management of patients with acute lung injury.
In the healthy lung, the venous blood returning to the right heart and flowing through the pulmonary artery to the pulmonary capillaries will be fully saturated by oxygen during the passage through the alveolar part of the capillary. The prerequisite for complete saturation of hemoglobin with oxygen is sufficiently high oxygen partial pressure in the alveoli. Complete saturation is approached if the alveolar partial pressure of oxygen exceeds 13.3 kPa (100 mmHg). This prerequisite is achieved during normoventilation of ambient air with normal lungs at the sea level. Hypoventilation...
KeywordsCardiac Output Arterial Oxygenation Hypoxic Pulmonary Vasoconstriction Mixed Venous Blood Venous Admixture
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