What is pulse oximetry (PO) plethysmography?

Pulse Oximetry Plethysmography

Pulse oximetry plethysmography is a noninvasive monitoring technique that combines spectrophotometry to measure arterial oxygen saturation with photoplethysmography to detect pulsatile arterial blood flow, providing continuous measurement of both oxygen saturation (SpO2) and heart rate. 1

Core Operating Mechanism

The device operates by exploiting differential light absorption properties of hemoglobin at two wavelengths:

• Oxygenated hemoglobin absorbs more infrared light (990 nm) while deoxygenated hemoglobin absorbs more red light (660 nm), allowing the device to distinguish between the two hemoglobin states 1, 2
• The pulse oximeter isolates arterial blood by detecting only the pulsatile component of blood flow, which corresponds to arterial pulsations with each heartbeat 1, 2
• This pulsatile signal is the "plethysmographic" component—it represents the rhythmic expansion and contraction of arteries with each cardiac cycle 3, 4

What the Plethysmographic Waveform Reliably Provides

The plethysmographic waveform and heart rate are the most reliable outputs from pulse oximetry:

• Heart rate monitoring via the plethysmographic signal demonstrates accuracy comparable to ECG monitoring across various clinical conditions 1, 5
• The presence of a plethysmographic waveform is valuable in detecting return of spontaneous circulation during resuscitation 5
• The waveform provides real-time information about peripheral perfusion and the interaction of heart and lungs during positive pressure ventilation 3
• Analysis of the photoplethysmographic contour can assess preload dependency by examining breath-by-breath variation of the signals in mechanically ventilated patients 3

Oxygen Saturation Measurement Capabilities

Under optimal conditions, pulse oximetry reliably detects early decreases in oxygen saturation:

• Accuracy is within ±4-5% compared to arterial blood gas analysis when a good pulse signal is obtained 1, 5, 2
• The device excels at monitoring trending phenomena rather than determining absolute values 1, 2
• Accuracy is thought to be less reliable at saturations below 88% 6, 2

Critical Limitations That Cannot Be Overcome

Standard two-wavelength pulse oximeters have fundamental detection limitations:

• Cannot detect carbon monoxide poisoning or methemoglobinemia because COHb and O₂Hb have similar absorbances at 660 nm, yielding falsely reassuring readings 1, 5, 2
• Cannot detect early decreases in adequacy of ventilation or the onset of hypercarbia that may occur before apnea develops 6, 1, 5
• Measures saturation rather than partial pressure (PaO2), which is more relevant for assessing pulmonary gas exchange effects in lung disease 6, 2

The oxyhemoglobin dissociation curve creates inherent insensitivity:

• During normal respiration with oxygen saturation approaching 100%, significant changes in arterial oxygen partial pressure may occur with little alteration in oxygen saturation 6
• Oxygen saturation will maintain a level of 90% or more until the arterial oxygen partial pressure decreases to less than 70 mm Hg 6
• Supplemental oxygen administration further delays the detection of hypoventilation 6

Conditions Causing Unreliable or Inaccurate Readings

Poor peripheral perfusion from any cause yields falsely low readings:

• Hypothermia, hypovolemia, vasoconstriction, shock, or cardiovascular disease decrease pulsatility and interfere with signal detection 6, 1, 5, 2
• Adequate pulsatile flow is required for accurate measurement—without it, the plethysmographic signal cannot be detected 2, 7, 4

Dark skin pigmentation systematically overestimates oxygen saturation:

• Black patients have almost 3 times the frequency of hypoxemia missed by pulse oximetry compared to White patients 6, 1, 5, 2
• This represents a systematic bias that interferes with signal detection 6, 1

Additional factors affecting accuracy include:

• Nail thickness, nail paint or polish 6
• Movement or vibration during measurement 6, 2
• Extremes of temperature, moisture, and humidity 6
• Interference from direct external light sources including sunlight 6
• Battery level, device condition (dusty, dirty, damaged), and size/orientation of light and sensor 6

Practical Troubleshooting Algorithm

Always 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 1, 2
• Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements 1, 2
• Use an ear lobe probe as an alternative site, ensuring any jewelry is removed and gently rubbing the lobe to improve local perfusion 2
• Ensure the patient's hand is still and not gripping objects tightly to minimize movement artifact 2

If adequate signal cannot be obtained despite these maneuvers:

• Obtain arterial blood gas analysis, as this provides PaO2 measurement which is more relevant for assessing pulmonary gas exchange 2

Clinical Integration Principles

Pulse oximetry should supplement, not replace, clinical assessment:

• A physical examination and history should be the primary assessment methods for evaluating an ill or injured person 6
• The device should be utilized as a reliable adjunct, particularly in patients at increased risk of developing hypoxemia 1, 5
• Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise 1, 2

Assessment of respiratory status must include:

• Observing for bluish discoloration of the face, lips, or nails 6
• Abnormal respiratory rate and effort, cough, sensation of dyspnea 6
• Restlessness and discomfort, chest pain or tightness, increased heart rate 6
• Interpretation of any numerical value provided by a pulse oximeter must include considering these and other potential assessment findings 6

Pulse oximetry is most valuable:

• When high doses of drugs or multiple drugs are used 1, 5
• When treating patients with significant comorbidity 1, 5
• During procedural sedation and analgesia 6, 5
• In patients receiving intravenous sedation for endoscopic procedures 6

Common Pitfalls and How to Avoid Them

Do not use pulse oximetry as a proxy for respiratory distress:

• There is very poor correlation between respiratory distress and oxygen saturations among infants with lower respiratory tract infections 1
• Transient desaturation below 90% is a normal phenomenon in 60% of healthy infants between 2 weeks and 6 months of age 1

Be aware that supplemental oxygen masks hypoventilation:

• The administration of supplemental oxygen delays the detection of hypoventilation because patients can maintain adequate saturation despite rising CO2 levels 6
• Consider capnography for patients at risk of hypoventilation, as it detects changes in ventilation before pulse oximetry indicates decreased oxygen saturation 6

Understand device accuracy categories:

• Consumer products, home-use medical devices, and medical devices have different FDA accuracy and calibration standards 6
• Be aware of which category your device falls into when interpreting results 6