What are the guidelines for selecting and using a finger tip pulse oximeter (pulse oximetry) for clinical use?

Fingertip Pulse Oximeter Selection and Use in Clinical Practice

Essential Requirement and Availability

Pulse oximetry must be available in all locations where emergency oxygen is being used by healthcare professionals, and all clinical staff who use oximeters must be trained in their use and made aware of the limitations of oximetry. 1

• Emergency ambulances, fast response vehicles, and primary care medical centers should be equipped with appropriate pulse oximeters—hand-held finger oximeter-type devices for fast response settings and portable or finger oximeters for staff initiating oxygen in the home 1
• Pulse oximetry should always be measured in patients with breathlessness or suspected hypoxemia 1
• Within hospital medicine, the oximeter reading is now established as an essential part of the National Early Warning Score (NEWS) 1

Device Selection Considerations

While many cheap, small finger oximeters are available on the internet, there has been no thorough evaluation of the accuracy and reliability of these devices. 1

• Pulse oximetry devices used by Community First Responder schemes should wherever possible reflect that of the ambulance service to which they are affiliated 1
• Standard two-wavelength pulse oximeters (660 nm and 990 nm) cannot distinguish carboxyhemoglobin or methemoglobin from oxyhemoglobin 1, 2, 3
• Pulse oximeters have inherent accuracy limitations of ±4-5% even under optimal conditions 2, 4
• Different pulse oximeter brands show significant bias and variable accuracy—some overestimate SaO2 while others underestimate it, with discrepancies ranging from -3.1% to +0.9% in critically ill patients 4

Critical Limitations That Must Be Understood

Skin Pigmentation Effects

Black patients have almost 3 times the frequency of hypoxemia that is missed by pulse oximetry compared with White patients, and pulse oximetry systematically overestimates oxygen saturation in people with darker skin pigmentation. 1, 2

• This overestimation occurs because carboxyhemoglobin and oxyhemoglobin have similar absorbances at 660 nm, causing pulse oximeters to measure COHb similarly to O2Hb 1
• Accuracy is thought to be less reliable at saturations below 88%, which is further exacerbated in Black patients 2

Perfusion and Anatomical Factors

Poor peripheral perfusion yields falsely low readings because adequate pulsatile arterial flow is required for accurate measurement. 2, 3

• Severe finger clubbing with extensive lipodermatosclerosis can significantly hinder pulse oximetry accuracy by compromising detection of adequate pulsatile arterial flow 2
• Altered tissue architecture in clubbed fingers changes the normal light transmission pathway through tissues 2
• Inadequate surface contact may occur with severe clubbing due to bulbous fingertip morphology, preventing proper probe seating 2
• Finger probes perform better than nose, ear, or forehead probes in patients with poor peripheral perfusion 5

Environmental and Patient Factors

Multiple patient, device, and environmental factors can affect pulse oximeter accuracy. 1

• Patient factors: chronic respiratory disease, nail thickness and nail polish, heart rhythm and cardiac output, skin thickness/perfusion/pigmentation/temperature 1
• Device factors: battery level, device condition (dusty, dirty, damaged), size and orientation of light and sensor, device accuracy and calibration 1
• Environmental factors: extremes of temperature, movement or vibration, moisture and humidity, interference from direct external light sources including sunlight 1

Proper Use Algorithm

Step 1: Verify Signal Quality First

Check 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

Step 2: Optimize Measurement Conditions

• Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements 2
• Ensure the patient's hand is still and not gripping objects tightly, as movement artifact during measurement can cause significant errors and data dropout 2
• Remove nail polish or artificial nails that may interfere with readings 1

Step 3: Use Alternative Sites When Needed

• Use an ear lobe probe as an alternative site, ensuring any jewelry is removed and gently rubbing the lobe to improve local perfusion 2
• Note that nose and forehead probes perform poorly compared to finger probes in poorly perfused patients 5

Step 4: Obtain Arterial Blood Gas When Indicated

If adequate signal cannot be obtained despite these maneuvers, obtain arterial blood gas analysis, as pulse oximetry measures saturation (SaO2) rather than partial pressure (PaO2). 2

When Pulse Oximetry Is Insufficient

The presence of a normal SpO2 does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy. 1

• Pulse oximetry will be normal in a patient with normal oxygen tension (PO2) but abnormal blood pH or carbon dioxide tension (PCO2) or with a low blood oxygen content due to anemia 1
• Blood gases should be checked in all critically ill patients 1
• Blood gases should be checked for unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen 1
• Blood gases should be checked for deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 1
• For critically ill patients or those with shock or hypotension (systolic blood pressure <90 mm Hg), the initial blood gas measurement should be obtained from an arterial sample 1

Integration with Clinical Assessment

A physical examination and history should be the primary assessment methods for first aid providers to evaluate an ill or injured person. 1

• Assessment of respiratory status includes observing for bluish discoloration of the face, lips, or nails; abnormal respiratory rate and effort; cough; sensation of dyspnea; restlessness and discomfort; chest pain or tightness; and increased heart rate 1
• Interpretation of any numerical value provided by a pulse oximeter must include considering these and other potential assessment findings 1
• Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues. 2

Special Clinical Scenarios

Carbon Monoxide Poisoning

Standard pulse oximeters cannot detect carboxyhemoglobin—a patient with COHb 50% may show SpO2 readings approximately 5% higher than actual oxygen saturation. 1

• Carboxyhemoglobin and oxyhemoglobin have similar absorbances at 660 nm, resulting in pulse oximeters measuring COHb similarly to O2Hb 1
• In one series of 30 CO-poisoned patients with COHb at least 25% measured by CO oximeter, simultaneous pulse oximeter oxygen saturation (SpO2) was greater than 90% in all cases 1
• Fingertip pulse CO oximetry is commercially available but requires laboratory-based confirmation by spectrophotometry for patients being considered for hyperbaric oxygen therapy 1

Oxygen Dissociation Curve Limitations

Because of the nature of the HbO2 dissociation curve, saturation measurements will not be sensitive to changes in PaO2 when the PaO2 is greater than 100 torr. 3

• Despite a fall in arterial PaO2 to 70 mmHg, saturation would still remain above 93% because the oxygen dissociation curve at this point is insensitive to changes in PO2 2
• This implies that the pulse oximeter may fail to detect an inadvertent endobronchial intubation in the operating room 3

Hypoxemia Detection

All modern pulse oximeters can detect hypoxemia during motion and low-perfusion conditions, but motion impairs performance at all ranges, with less accuracy at lower SaO2. 6

• Moderate hypoxemia (SaO2 86-90% or PaO2 55-60 mmHg) detection rates vary significantly by device brand, ranging from 11% to 92% in one study 4
• Accuracy is thought to be less reliable at saturations below 88% 2

Common Pitfalls to Avoid

• Do not assume accuracy based on a displayed reading alone—always verify signal quality by comparing displayed heart rate to actual pulse 2
• Do not use pulse oximetry as the sole determinant of oxygen therapy needs in patients on supplemental oxygen—blood gases are required 1
• Do not ignore clinical signs of respiratory distress even when SpO2 appears normal 1, 2
• Do not rely on inexpensive, unevaluated finger oximeters for critical clinical decisions without understanding their limitations 1
• Do not assume all pulse oximeters perform equally—different brands show significant bias and variable accuracy 4