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    IM Core Excerpt: Hypoxemia

    This blog is meant to be a resource for physicians treating patients with Hypoxemia during the COVID-19 pandemic. This information has been pulled from our 18th Edition Internal Medicine Core. 

    Hypoxemia (low oxygen tension) has 6 causes

    1. Ventilation/Perfusion (V/Q) mismatch is the main cause of hypoxemia in chronic lung diseases and responds well to 100% O2. It is due to inadequately perfused airspaces or perfused areas that are inadequately ventilated. Examples include asthma, COPD, alveolar disease (e.g., pneumonia), interstitial disease, and pulmonary vascular disease (e.g., pulmonary hypertension, pulmonary embolism). The hypoxemia improves after oxygen administration. 
    2. Right-to-left (R-to-L) shunting is seen in acute respiratory distress syndrome (ARDS), where hypoxemia is due to perfusion of nonventilated alveoli. ARDS does not respond well to 100% O2 but responds better to positive end-expiratory pressure (PEEP). See Ventilator Support for ARDS in MedStudy Core. Other causes of R-to-L shunting, besides alveolar collapse, include intraalveolar filling (e.g., pneumonia, pulmonary edema), atelectasis, intracardiac shunt, and vascular shunt. PEEP can worsen a R-to-L intracardiac shunt by increasing the shunt fraction as a result of increased right-sided pressures.
    3. Decreased alveolar ventilation is seen with decreased tidal volumes, low respiratory rates, and apnea. By definition, decreased alveolar ventilation always has a high PaCO2 with hypoxemia. The A-a gradient (DA-aO2, discussed next) is normal. Common causes are drug overdose, neuromuscular diseases, and CNS disorders.
    4.  Decreased diffusion actually has little effect on hypoxemia at rest but it can play a role in exerciseinduced desaturations. It takes a tremendous amount of membrane thickening or loss of the alveolar-capillary interface to markedly decrease the diffusion of O2. The DLCO test measures how well inspired CO diffuses from the alveoli to the RBC hemoglobin and acts as a surrogate marker for CO2 and oxygen diffusion. Low DLCO occurs with ILDs and emphysema, in which symptoms improve with supplemental O2. Other causes include pulmonary vascular disease and significant anemia. Hypoxemia at rest occurs when the DLCO is approximately ≤ 30% of predicted. It can occur at higher DLCO if there is an increased cardiac output, as with a rapid heart rate or if there is additional lung pathology. With an increased cardiac output, the time for diffusion is limited, so decreased O2 transfer occurs. Increased DLCO is seen with alveolar hemorrhage, polycythemia, and acute asthma attack.
    5.  High altitude decreases atmospheric pressure and results in a reduced PAO2. The A-a gradient is normal unless lung disease is present.
    6.  Low mixed venous O2 (i.e., venous oxygen pressure [PῡO2]) can decrease the PaO2 during resting conditions, secondary to the normal shunt that exists (~ 5%). It can also exaggerate all other causes of low PaO2. A low mixed venous O2 is caused by low cardiac output or an increased tissue extraction of oxygen. A lower PῡO2 can induce hypoxic vasoconstriction, increasing R-to-L baseline anatomical shunting. 

    Again: Supplemental O2 does not cause a significant increase in PaO2 when there is R-to-L shunting or shunt physiology. The A-a gradient is normal with hypoventilation and with high altitudes.

    Check out the next blog in this series: IM Core Excerpt: Interstitial Pneumonia

    internal medicine medstudy core free trial pulmonary medicine

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