What is pulse oximetry?

What is Pulse Oximetry?

Pulse oximetry is a noninvasive monitoring technique that continuously measures arterial oxygen saturation (SpO2) and heart rate by detecting differential light absorption between oxygenated and deoxygenated hemoglobin at two wavelengths—red (660 nm) and infrared (990 nm)—while isolating arterial blood through detection of pulsatile flow with each heartbeat. 1

Core Operating Mechanism

The device exploits a fundamental optical principle: oxygenated hemoglobin absorbs more infrared light while deoxygenated hemoglobin absorbs more red light, allowing the oximeter to calculate the ratio of the two hemoglobin states. 2, 1 The pulse oximeter distinguishes arterial blood from venous blood and tissue by analyzing only the pulsatile component of blood flow that corresponds to arterial pulsations with each cardiac cycle. 3, 1

The technology provides two primary outputs: 1

• Oxygen saturation (SpO2): Percentage of hemoglobin saturated with oxygen
• Heart rate: Detected through arterial pulsations, with accuracy comparable to ECG monitoring 4

What Pulse Oximetry Reliably Provides

The plethysmographic waveform and heart rate are the most reliable parameters from pulse oximetry. 4 The device excels at:

• Detecting early decreases in oxygen saturation under optimal conditions, with accuracy within ±4-5% compared to arterial blood gas analysis 4, 1
• Monitoring trending phenomena in oxygen saturation over time, though it is less reliable for determining absolute magnitude of change 2, 1
• Detecting return of spontaneous circulation during resuscitation through presence of the plethysmographic waveform 4

Critical Limitations That Cannot Be Overcome

Standard two-wavelength pulse oximeters cannot detect carbon monoxide poisoning or methemoglobinemia because carboxyhemoglobin and methemoglobin absorb light similarly to oxyhemoglobin at 660 nm, yielding falsely reassuring readings. 4, 1 This is a fundamental limitation of the two-wavelength technology. 2

Pulse oximetry cannot detect hypoventilation or rising carbon dioxide levels (hypercarbia) until severe hypoxemia develops. 3, 4, 1 The sigmoid-shaped oxyhemoglobin dissociation curve means that during normal respiration with oxygen saturation approaching 100%, significant changes in arterial oxygen partial pressure may occur with little alteration in oxygen saturation. 3 Even with arterial PaO2 falling to 70 mmHg, saturation remains above 90-93%. 3, 2 Supplemental oxygen administration further delays detection of hypoventilation. 3

Conditions Causing Unreliable or Inaccurate Readings

Patient Factors

Dark skin pigmentation systematically overestimates oxygen saturation, with Black patients having almost 3 times the frequency of hypoxemia missed by pulse oximetry compared to White patients. 3, 4, 1 This represents a significant racial disparity in measurement accuracy. 3

Poor peripheral perfusion from hypothermia, hypovolemia, vasoconstriction, or shock yields falsely low readings because adequate pulsatile flow is required for accurate measurement. 3, 4, 1 Conditions affecting perfusion include: 3

• Heart rhythm abnormalities and low cardiac output
• Extremes of temperature
• Decreased skin perfusion and temperature

Severe finger clubbing with altered tissue architecture and lipodermatosclerosis can significantly hinder accuracy by compromising detection of adequate pulsatile arterial flow. 2

Device and Environmental Factors

Multiple factors can interfere with readings: 3

• Movement or vibration during measurement causes significant errors and data dropout 2
• Nail thickness, nail polish, or nail paint interferes with light transmission
• Direct external light sources including sunlight
• Moisture and humidity
• Battery level, device condition (dusty, dirty, damaged)
• Improper size and orientation of sensor

Practical Troubleshooting Algorithm

Always verify signal quality first by checking that the heart rate displayed on the pulse oximeter matches the ECG or palpated pulse rate—if these don't match closely, the reading is unreliable. 2, 1

When readings appear questionable: 2

1. Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements
2. Use an ear lobe probe as an alternative site, ensuring jewelry is removed and gently rubbing the lobe to improve local perfusion
3. Ensure the patient's hand is still and not gripping objects tightly to minimize movement artifact
4. If adequate signal cannot be obtained despite these maneuvers, obtain arterial blood gas analysis, as pulse oximetry measures saturation (SpO2) rather than partial pressure (PaO2)

Clinical Integration Principles

A physical examination and history should be the primary assessment methods for evaluating an ill or injured person, with pulse oximetry used as an adjunct rather than a replacement for clinical assessment. 3 The primary survey should include observing for: 3

• Bluish discoloration of face, lips, or nails
• Abnormal respiratory rate and effort
• Cough and sensation of dyspnea
• Restlessness, discomfort, chest pain or tightness
• Increased heart rate

It is reasonable for providers to use pulse oximetry results in the context of a complete assessment while being aware of the numerous limitations before acting on any results. 3 The device is most valuable in patients at increased risk of developing hypoxemia, such as when high doses of drugs or multiple drugs are used, or when treating patients with significant comorbidity. 4, 1

Pulse oximetry should supplement, rather than replace, direct observation of the patient's respiratory effort. 3 Continuous pulse oximetry monitoring is more likely to detect respiratory depression than periodic monitoring, but it is not more likely to detect respiratory depression than clinical signs alone. 1

Common Pitfalls to Avoid

Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues. 2 The clinical significance of transient desaturation remains uncertain, and the ability of oximetry to reduce the incidence of cardiopulmonary complications remains unproven despite its widespread use. 3

Recognize that accuracy is less reliable at saturations below 88%, which is further exacerbated in Black patients. 2, 1 Pulse oximeters have inherent accuracy limitations of ±4-5% even under optimal conditions. 2

Be aware that whenever higher than normal oxygen fractions are used, SpO2 can mask existing oxygenation defects due to the S-shape of the oxyhemoglobin saturation curve. 5