Reduced pO₂ - an unusual case history

In health the partial pressure of oxygen (pO₂) in arterial blood is maintained between 10.6 and 13.3 kPa (80-100 mmHg). Hypoxemia (reduced arterial pO₂) is usually a signal of significant respiratory or cardiac disease and respiratory failure is defined as arterial blood pO₂ < 8 kPa (<60 mmHg). 

So when arterial blood gas analysis revealed a pO₂ of 6.2 kPa (47 mmHg) in a 72 year old lady complaining of fatigue, the full panoply of diagnostic testing that modern medicine has to offer were deployed in an attempt to find a respiratory/cardiac explanation for the patients severe, life threatening hypoxemia. 

However, as outlined in the lady’s recently published case-history, chest radiography, computed tomography (CT) scan of chest, pulmonary function tests and transthoracic bubble contrast echocardiography all failed to identify an abnormality. 

Moreover the lady showed no clinical signs of respiratory distress and oxygen saturation measured by pulse oximetry was normal (98 %), suggesting - in sharp contradiction to arterial blood gas testing - adequate oxygenation status. It was eventually confirmed that the patient was not in fact hypoxemic and the reduced pO₂ was an artefact caused by massive increase in platelet numbers due to the myeloproliferative disorder, polycythemia vera (PV). 

The continued consumption of oxygen by platelets after blood was sampled was sufficient to reduce pO₂ from normal to the measured value. Successful treatment of PV resulted in a marked reduction in platelet numbers from 2,425 x 10⁹/L to 721 x 10⁹/L (ref. range 130 - 350 x 10⁹/L) and a concurrent increase in measured pO₂ to a normal value of 11.2 kPa (84 mmHg). 

This case-history report of spurious hypoxemia includes a short historical review of the condition with the reminder that it is most often caused by marked increase in white cell numbers (leucocytosis). Addition of potassium cyanide - which halts cellular aerobic metabolism and thereby in vitro oxygen consumption - to arterial blood immediately after sampling and prior to analysis is helpful in confirming spurious hypoxemia. 

Discrepancy between oxygen saturation calculated during blood gas analysis and oxygen saturation measured by pulse oximetry is also a useful diagnostic feature.