What are the principles and applications of pulse oximetry (PO) and continuous non-invasive blood pressure (CNBP) monitoring in clinical settings?

Pulse Oximetry: Principles and Clinical Applications

Core Operating Mechanism

Pulse oximetry measures arterial oxygen saturation by exploiting the differential light absorption properties of oxygenated versus deoxygenated hemoglobin at two specific wavelengths. 1, 2

• 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, 3
• The device isolates arterial blood by detecting only the pulsatile component of blood flow, which corresponds to arterial pulsations with each heartbeat 1, 2, 3
• This photoplethysmographic waveform provides two critical pieces of information: oxygen saturation (SpO2) and heart rate 1, 4

What Pulse Oximetry Reliably Provides

The plethysmographic waveform and heart rate are the most reliable outputs from pulse oximetry, with accuracy comparable to ECG monitoring. 1

• Heart rate monitoring via pulse oximetry detects arterial pulsations with each heartbeat and demonstrates high accuracy across various clinical conditions 1
• The device reliably detects early decreases in oxygen saturation under optimal conditions, with accuracy within ±4-5% compared to arterial blood gas analysis 1, 2, 5
• Pulse oximetry excels at monitoring trending phenomena rather than determining absolute values 2, 5
• The presence of a plethysmographic waveform is valuable in detecting return of spontaneous circulation during resuscitation 1

Critical Limitations That Cannot Be Overcome

Standard two-wavelength pulse oximeters cannot detect carbon monoxide poisoning or methemoglobin because these dyshemoglobinemias absorb light similarly to oxyhemoglobin, yielding falsely reassuring readings. 6, 1, 2

• COHb and O₂Hb have similar absorbances at 660 nm, making differentiation impossible with standard devices 6, 1
• Pulse oximetry cannot detect early decreases in adequacy of ventilation or the onset of hypercarbia that may occur before apnea develops 6, 1
• The device measures saturation (SaO₂) rather than partial pressure (PaO₂), and PaO₂ is more relevant for assessing pulmonary gas exchange effects in lung disease 6, 2
• 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

Conditions Causing Unreliable or Inaccurate Readings

Poor peripheral perfusion from any cause yields falsely low readings because adequate pulsatile flow is required for accurate measurement. 6, 1, 2

• Hypothermia, hypovolemia, vasoconstriction, shock, or cardiovascular disease decrease pulsatility and interfere with signal detection 6, 1, 2
• 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
• Accuracy is thought to be less reliable at saturations below 88%, which is further exacerbated in Black patients 2
• Movement artifact and stray light can yield significant errors and data dropout 6, 2
• Nail varnish, severe Raynaud's phenomenon, and altered tissue architecture (such as severe finger clubbing) can interfere with accurate readings 6, 2

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

• Ensure adequate surface contact and perfusion by repositioning the probe and repeating measurements 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 2

Clinical Integration Principles

Pulse oximetry should not substitute for clinical assessment but rather be utilized as a reliable adjunct, particularly in patients at increased risk of developing hypoxemia. 1

• The device is most valuable when high doses of drugs or multiple drugs are used, or when treating patients with significant comorbidity 1
• Continuous pulse oximetry monitoring is more likely to detect respiratory depression than periodic monitoring 6
• However, pulse oximetry monitoring is not more likely to detect respiratory depression than clinical signs alone 6
• Never rely solely on pulse oximetry when clinical assessment suggests respiratory compromise, especially in patients with known perfusion issues 2

Specific Clinical Applications

In bronchiolitis, supplemental oxygen is indicated if SpO2 falls persistently below 90% in previously healthy infants, with oxygen discontinued if SpO2 is at or above 90% and the infant is feeding well with minimal respiratory distress. 6

• Transient desaturation below 90% (to values as low as 83%) is a normal phenomenon in 60% of healthy infants between 2 weeks and 6 months of age 6
• Continuous pulse oximetry measurement in bronchiolitis is potentially problematic, as 1 in 4 patients incur unnecessarily prolonged hospitalization due to perceived need for oxygen outside of other symptoms 6
• Pulse oximetry has been erroneously used as a proxy for respiratory distress, but there is very poor correlation between respiratory distress and oxygen saturations among infants with lower respiratory tract infections 6

In emergency oxygen therapy, pulse oximetry should be available to all healthcare staff managing patients receiving oxygen, with measurements recorded on observation charts along with the oxygen delivery system being used. 6

• Arterial or arterialised capillary blood gases should be measured on arrival at hospital for most patients requiring emergency oxygen therapy 6
• If oxygen saturation fails to rise following 5-10 minutes of increased oxygen therapy or if there is clinical concern, blood gas measurements should be repeated 6

Advanced Monitoring Capabilities

The photoplethysmographic waveform provides real-time, noninvasive information about heart-lung interaction during positive pressure ventilation, including assessment of preload dependency and arterial impedance. 4

• Analysis of breath-by-breath variation of photoplethysmographic signals can assess preload dependency in mechanically ventilated patients 4
• Changes in plethysmographic amplitude, contour, and derived indexes provide information about arterial impedance 4
• New multiwaveform sensors and advanced signal processing techniques can differentiate between different types of hemoglobin and may be useful for continuous measurement of total hemoglobin, as well as for detecting blood loss and cardiac output 7

---

Continuous Non-Invasive Blood Pressure (CNBP) Monitoring

Note: The provided evidence does not contain specific guidelines or research regarding continuous non-invasive blood pressure (CNBP) monitoring principles or applications. The evidence focuses exclusively on pulse oximetry. For CNBP information, arterial blood pressure monitoring during exercise testing was briefly mentioned, noting that automated blood pressure measurement systems using the oscillometric method have been developed specifically for use during exercise, as auscultation becomes difficult due to increased ambient noise 6.