Indications

Pulse oximetry provides important, real-time physiologic data and is the standard noninvasive measure of arterial saturation. It is now widely considered one of the standard measurements in patient vital signs. Continuous monitoring is indicated for any patient at risk or in the midst of acute cardiopulmonary decompensation. Reliable continuous oximetry is mandatory for patients requiring airway management and should be a component of each preintubation checklist. If placed on an extremity, the probe is preferably placed on the opposite side of the blood pressure cuff to avoid interruptions in SaO₂ readings during cuff inflation.

Limitations and Precautions

Even with verified signal detection, measurement bias limits SpO₂ reliability during physiologic extremes. Reliability deteriorates with progressive hypotension below systolic BP of 80 mm Hg, and in these patients, readings generally underestimate true SaO₂. Severe hypoxemia with SaO₂ < 75% is also associated with increased measurement error as comparisons to reference standards are limited below this value. However, patients with this severity of hypoxemia are typically receiving maximized intervention, and closer discrimination in this range rarely imparts new information that alters management.

A number of physical factors affect pulse oximetry accuracy. Signal reliability is influenced by sensor exposure to extraneous light, excessive movement, synthetic fingernails, nail polish, intravenous dyes, severe anemia, and abnormal hemoglobin species. Diligent probe placement and shielding the probe from extraneous light should be routine. Surface extremity warming may improve local perfusion to enable arterial pulse sensing, but SpO₂ accuracy using this technique is not confirmed.

Dyshemoglobinemias such as carboxyhemoglobin (CO-Hgb) and methemoglobin (Met-Hgb) absorb light at different wavelengths and may affect the accuracy of oximetry readings. Co-oximeters (and some new generation pulse oximeters) use four wavelengths of light stimulus to selectively discriminate these species. However, CO-Hgb absorbance is close to oxyhemoglobin such that most conventional pulse oximeters sum up their measurement and give artifactually high SpO₂ readings. Met-Hgb produces variable error, depending on the true oxy- and Met-Hgb levels. SpO₂ classically approximates 85% in severe toxicity.

Response Time

Pulse oximetry readings lag the patient’s physiologic state; signal averaging of 4 to 20 seconds is typical of most monitors. Delay because of sensor anatomical location and abnormal cardiac performance compound the lag relative to central SaO₂. Forehead and ear probes are closer to the heart and respond more quickly than distal extremity probes. Response difference compared to central SaO₂ is also compounded by hypoxemia (i.e., starting on the steep portion of the oxyhemoglobin dissociation curve) and slower peripheral circulation such as low cardiac output states. As such, forehead reflectance probes are often preferred in critically ill patients. All of these response delays become more clinically important during rapid desaturation such as that which may occur during airway management and is the basis for our general recommendation to abort most intubation attempts when the SpO₂ falls below 93%.

Physiologic Insight and Limitations

Hemoglobin saturation is just one part of the assessment of systemic oxygenation. Although monitoring is continuous, SpO₂ provides momentary information on arterial saturation without true insight into systemic oxygenation and respiratory reserve. The physiologic context of oximetry is critical for appropriate interpretation and assists estimation of a patient’s cardiopulmonary reserve for planning and execution of an airway management plan.