What information can be reliably obtained from a pulse oximeter waveform?

Pulse Oximeter Waveform: Reliable Clinical Information

The pulse oximeter waveform (photoplethysmographic waveform) reliably provides heart rate, oxygen saturation trends, detection of cardiac arrhythmias, and can indicate return of spontaneous circulation (ROSC) during resuscitation, but it cannot reliably detect early hypoventilation, distinguish carboxyhemoglobin from oxyhemoglobin, or provide accurate readings in states of poor peripheral perfusion. 1, 2

Primary Reliable Parameters

Heart Rate Detection

• The waveform provides continuous, reliable heart rate monitoring by detecting arterial pulsations with each heartbeat. 1, 3
• Verify accuracy by confirming the displayed heart rate matches the ECG or palpated pulse—if these don't correlate closely, the reading is unreliable. 2
• The PPG waveform is excellent for detecting cardiac arrhythmias, particularly when used in conjunction with ECG. 3

Oxygen Saturation Trending

• Pulse oximetry reliably detects early decreases in oxygen saturation under optimal conditions, with accuracy within ±4-5% compared to arterial blood gas analysis. 1, 2
• The device is best used for monitoring trending phenomena rather than determining absolute magnitude of change. 2
• Accuracy becomes unreliable when saturation falls below 80-88%. 1, 2

Detection of ROSC During Cardiac Arrest

• The presence of a plethysmographic waveform on pulse oximetry is valuable in detecting return of spontaneous circulation during resuscitation. 1
• During cardiac arrest, pulse oximetry typically does not provide a reliable signal because pulsatile blood flow is inadequate in peripheral tissue beds. 1

Critical Limitations That Cannot Be Overcome

Cannot Detect Early Hypoventilation

• Pulse oximetry cannot detect early decreases in adequacy of ventilation or the onset of hypercarbia that may occur before apnea develops. 1
• Administration of supplemental oxygen during sedation delays the onset of hypoxemia, further masking hypoventilation. 1
• 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

Cannot Distinguish Carboxyhemoglobin

• Standard two-wavelength pulse oximeters (660 nm and 990 nm) cannot differentiate carboxyhemoglobin from oxyhemoglobin because COHb and O₂Hb have similar absorbances at 660 nm. 1, 2
• In carbon monoxide poisoning, pulse oximeters measure COHb similarly to O₂Hb, resulting in falsely reassuring readings. 1
• In one series of 30 CO-poisoned patients with COHb ≥25%, all had simultaneous SpO₂ >90%. 1

Conditions Causing Unreliable Readings

Poor Peripheral Perfusion States

• Poor perfusion from hypothermia, hypovolemia, vasoconstriction, or shock yields falsely low readings because adequate pulsatile flow is required for accurate measurement. 2, 4, 5
• Cold peripheries compromise detection of arterial pulsations, making it difficult for the device to detect adequate signals. 4
• Severe Raynaud's phenomenon from collagen vascular diseases causes extreme vasoconstriction leading to particularly unreliable readings. 4

Anatomical and Tissue Factors

• Severe finger clubbing with extensive lipodermatosclerosis significantly hinders 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 occurs with severe clubbing due to bulbous fingertip morphology preventing proper probe seating. 2

Skin Pigmentation Bias

• Dark skin pigmentation systematically overestimates oxygen saturation and interferes with signal detection, with Black patients having almost 3 times the frequency of hypoxemia missed by pulse oximetry compared to White patients. 1, 2, 4
• This accuracy limitation is further exacerbated at saturations below 88%. 2

Practical Troubleshooting Algorithm

When Readings Appear Unreliable:

1. First, verify signal quality by checking that the displayed heart rate matches the ECG or palpated pulse. 2
2. Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements. 2
3. Try alternative sites: use an ear lobe probe after removing jewelry and gently rubbing the lobe to improve local perfusion. 2
4. Actively warm the measurement site before and during measurement in cold peripheries. 4
5. Ensure the patient's hand is still and not gripping objects tightly to avoid movement artifact. 2
6. If adequate signal cannot be obtained despite these maneuvers, obtain arterial blood gas analysis. 2

Clinical Integration Principles

When Pulse Oximetry Should NOT Substitute for Clinical Assessment

• Pulse oximetry should not substitute for clinical assessment during procedural sedation and analgesia, but rather be utilized as a reliable adjunct. 1
• Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues. 2, 4
• Assessment of respiratory status must include observing for cyanosis, abnormal respiratory rate and effort, dyspnea, restlessness, chest pain, and increased heart rate. 1

When Pulse Oximetry Is Most Valuable

• Use pulse oximetry 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. 1
• When the patient's level of consciousness is minimally depressed and verbal communication can be continually monitored, pulse oximetry may not be necessary. 1