Today in Morning Report, we discussed a variety of cases. Among them was an interesting case of a patient who experienced a syncopal event and on initial assessment, was found to be hypoxic with a normal chest radiograph. Causes of hypoxia can include:
1)Respiratory Hypoxia - This refers a situation when respiratory failure leads to hypoxemia.
Most commonly, this is caused by ventilation-perfusion mismatch (ventilation to areas of the lung that are not perfused) as can occur with a PE. Hypoventilation can also be a cause of hypoxia, but this is classically associated with increases to the PaCO2. A third cause is shunting of blood away from parts of the lung that are oxygen rich (perfusion to diseased lung) as can occur in pneumonia or atelectasis.
2) Hypoxia Secondary to High Altitude - The available oxygen for respiration is a consequence of the atmospheric pressure. Recall that the pAO2 = FI02(Patm-PH2O) - (PaCO2/RQ)*. As the atmospheric pressure drops, so does the quantity of oxygen available at the alveolus for inspiration.
3)Hypoxia Secondary to Right-to-Left Extrapulmonary Shunting - A portion of arterial blood bypasses the lung and, as such, is not oxygenated.
4) Anemic Hypoxia - The bulk of oxygen is carried in the blood by hemoglobin. If the concentration of hemoglobin is too low, the ability to carry oxygen in the blood is compromised.
5) Carbon Monoxide (CO) Intoxication - Carboxyhemoglobin (COHb) does not readily dissociate oxygen and this leads to tissue hypoxia.
6) Circulatory Hypoxia - With decreases in effective circulation, more oxygen content is extracted at the tissue level. This leads to poorer oxygen content in the venous return to the heart and subsequent hypoxia.
* PAtm = Atmospheric Pressure, PH2O= Water vapour Pressure, PaCO2 = Arterial Carbon Dioxide pressure, FIO2 = fractional inspired O2 content, RQ=respiratory quotient.
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