Is Pulse Oximetry Accurate in Sickle Cell Disease?
Is Pulse Oximetry Accurate in Sickle Cell Disease?
Abstract & Commentary
Synopsis: As long as strong and regular photoplethysmographic waves are present, pulse oximetry is clinically accurate in patients with sickle cell disease.
Source: Ortiz FO, et al. Am J Respir Crit Care Med 1999;159:447-451.
In 17 adult patients with sickle cell disease (SCD) who were hospitalized with acute vasoocclusive crises, Ortiz and colleagues compared 22 pulse oximetry readings (SpO2) with oxygen tension and arterial oxyhemoglobin saturation (SaO2, oxyhemoglobin divided by oxyhemoglobin plus reduced hemoglobin), determined from simultaneously drawn arterial blood specimens. SpO2 readings were accepted only when they were stable and characterized by strong and regular photoplethysmographic waves on the oximeter screen. Ortiz et al also plotted arterial and venous oxygen saturation against oxygen tension to produce oxyhemoglobin dissociation curves and p50, the saturation at which half the hemoglobin was saturated.
All of the patients in this study were anemic (mean hemoglobin level, 7 g/dL) and had mildly to moderately elevated carboxyhemoglobin levels. Most of them were hypoxemic. Their oxyhemoglobin curves were right-shifted, with p50 values of 28-38 mmHg (normal, 27 mmHg). Pulse oximetry generally overestimated FaO2Hb (the ratio of oxyhemoglobin to total hemoglobin) by an average of 3.4 ± 3.4 percentage points. On the other hand, it generally underestimated SaO2, on average by 1.1 ± 0.8 percentage points. The error in SpO2 was never of sufficient magnitude to classify a hypoxemic patient erroneously as normoxemic, or a normoxemic patient erroneously as hypoxemic. Ortiz et al conclude that, as long as strong and regular photoplethysmographic waves are present, pulse oximeters can be relied on not to misdiagnose either hypoxemia or normoxemia in patients with SCD.
Comment BY DAVID J. PIERSON, MD, FACP, FCCP
Patients with SCD may develop pulmonary infiltrates and/or hypoxemia, both acutely during crises and on a chronic basis. Because hypoxia facilitates sickling and vasoocclusion, its prevention and detection when present are important. Measurements of oxyhemoglobin saturation may suggest hypoxemia even when the latter is not present, since sickled red blood cells have a strongly right-shifted oxyhemoglobin saturation curve. Previous studies of SpO2 in SCD have yielded conflicting results, reporting that the pulse oximeter gives readings that are either correct, too high, or too low in this condition. This carefully done study demonstrates that, as long as a good signal can be obtained, pulse oximetry is clinically accurate in patients with SCD, detecting significant hypoxemia when present and avoiding falsely diagnosing hypoxemia when it is absent.
Pulse oximetry in patients with sickle cell disease:
a. does not correlate with arterial blood gases.
b. accurately detects hypoxemia when present but falsely classifies normoxemic patients as hypoxemic 20% of the time.
c. misses hypoxemia when present 20% of the time.
d. may be inaccurate if strong and regular photoplethysmographic waves are not present.
e. None of the above
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