Printed from acutecaretesting.org
July 2010
Pseudo-hypoxemia
Summarized from Chen K, Puana R, Price K et al . The role of point of care testing in the early diagnosis of pseudo-hypoxemia in myeloproliferative disorders. Respiratory Care 2010; 55: 777-79
Arterial blood gas analysis includes measurement of partial pressure of oxygen (pO2(a)) and calculation of oxygen saturation (sO2(a)), both useful for assessing patient oxygenation status. Hypoxemia is diagnosed if pO2(a) is below the lower limit of the reference range (10.6 kPa, 88 mmHg). Pseudo-hypoxemia is diagnosed if the measured (in vitro) pO2(a) indicates hypoxemia, but true (in vivo) pO2(a) is normal; in essence falsely reduced pO2(a).
If not recognized, pseudo-hypoxemia can result in needless patient investigation and intervention. Pseudo-hypoxemia is highlighted in a recently published case history. The case concerns a 61-year-old man with a 3-year history of the bone marrow disease, myelofibrosis, who was admitted to the emergency department after being found unresponsive at home.
The indication was that he was suffering adverse effects of the methadone he had been prescribed to alleviate the severe bone pain associated with worsening myelofibrosis. Full blood count on admission to intensive care revealed reduced Hb (7.1 g/dL) along with a massive increase in white cell count (348,000/mL) and platelet numbers (294,000/mL), all consistent with worsening myelofibrosis.
Arterial blood was sampled and sent for analysis in the laboratory. This revealed acidosis and severe hypoxemia (pO2(a) 6.5 kPa, 49 mmHg). Chest X-ray, clinical history and physical examination provided no explanation for the severe hypoxemia, but oxygen therapy was continued. A second sample sent to the laboratory revealed continuing hypoxemia (pO2(a), 6.6 kPa, 50 mmHg) and acidosis despite oxygen therapy.
To check the validity of laboratory-derived results, a hand-held "point-of-care" blood gas analyzer available in the unit was employed. This returned a quite different pO2(a) (30 kPa, 225 mmHg) consistent with his clinical condition and receiving oxygen therapy.
The falsely reduced pO2(a) (pseudo-hypoxemia) was attributed to oxygen consumption by the extreme number of white cells in the sample during transit to the laboratory. Hyperleucocytosis is a well-documented cause of pseudo-hypoxemia, which can, as in this case, be avoided by analyzing samples immediately they are collected. The paper includes useful discussion of issues surrounding pseudo-hypoxemia.
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