Pulse Oximetry 

Pulse oximetry is a useful, non-invasive way to estimate oxygen saturation, but it has several important limitations: 

Technical & Measurement Limitations 

Measures only oxygen saturation, not ventilation 
It does not detect hypoventilation or rising carbon dioxide (CO₂) levels. A patient can have normal SpO₂ and still be in respiratory failure. 

Accuracy decreases at low saturations 
Readings are less reliable when SpO₂ falls below ~80–85%. 

Motion artifact 
Patient movement, tremors, or shivering can cause false readings. 

Poor peripheral perfusion (cool peripheries) 
Low blood flow (shock, hypothermia, vasoconstriction, hypotension) can lead to inaccurate or absent readings. 

Physiological & Patient-Related Factors 

Skin pigmentation 
Darker skin tones may lead to overestimation of oxygen saturation, especially at lower SpO₂ levels. 

Nail conditions 
Nail polish (especially dark colors), artificial nails, or dirty nails can interfere with light transmission. 

Anaemia 
Severe anemia can give misleadingly normal SpO₂ despite reduced oxygen-carrying capacity. 

Dyshaemoglobinemias 

Carbon monoxide poisoning: Carboxyhaemoglobin falsely elevates SpO₂. 
Methaemoglobinemia: Can cause readings to hover around ~85% regardless of true saturation. 
Arrhythmias – arrhythmias can cause discrepancy in the conduction of beats to the pulse oximeter giving false. 

Environmental & External Factors 

Ambient light interference 
Bright light (e.g., surgical lamps) can affect sensor accuracy. 

Improper probe placement or size 
Poor fit or incorrect placement reduces reliability. 

Clinical Interpretation Limitations 

No information about oxygen delivery to tissues 
SpO₂ does not reflect cardiac output, haemoglobin level, or tissue oxygen utilization. 

Delayed detection of hypoxia 
In patients receiving supplemental oxygen, oxygen saturation may remain normal despite worsening respiratory status. 

Key points: 
Pulse oximetry is best used as a screening and monitoring tool, not a standalone diagnostic test. It should always be interpreted alongside clinical assessment and, when needed, arterial blood gas (ABG) analysis. 

Pulse Oximetry: Setup, Use, and Assessment 

Purpose:  
Pulse oximetry measures arterial oxygen saturation (SpO₂) and pulse rate as a noninvasive indicator of oxygenation and perfusion. 

Equipment:  
Pulse oximeter (probe + monitor or integrated unit) – Alcohol wipe (if needed) – Warm blanket or towel (if extremities are cold) 

Site Selection:  
Choose a site with adequate perfusion and minimal motion.  

Common sites:  
– Fingertip (most common)  
– Toe  
– Earlobe (special probe clip) 
– Forehead (special probe) 

Avoid sites with:  
Poor perfusion (can be difficult in shock, hypothermia)  
– Nail polish, acrylic nails  
– Oedema  
– Tremors or excessive movement – 
 Constricting BP cuffs on the same limb 

Setup and Application:  
1. Explain the procedure to the patient.  
2. Assess the extremity before placing the probe (color, temperature, skin integrity).  
3. Check capillary refill time.  
4. Assess proximal pulses.  
5. Clean the site if necessary.  
6. Place the probe so the light emitter and sensor directly oppose each other.  
7. Ensure the probe fits snugly but does not constrict circulation.  
8. Turn on the device and allow the reading to stabilize. 

Capillary Refill Time (CRT): 
– Press on the nail bed or skin until it blanches and hold for 5 seconds.  
– Release and count time for color to return.  
– Normal: ≤ 2 seconds. 

Proximal Pulses: –  
For finger probe: Radial pulse  
– For toe probe: Posterior tibial or dorsalis pedis  
– For ear/forehead: Carotid pulse  
– Assess presence, rate, regularity, and strength (weak, strong, bounding);  

Interpretation of Readings:  
– Normal SpO₂: 95–100% (88-92% in a small percentage of patients with underlying respiratory diseases eg; severe COPD – note, this allowance must be guided by the physician in charge of the patients care; do not assume). 

Note: Do not rely on the pulse oximetry to measure heart rate (particularly in irregular rhythms/heart block). 
Compare pulse oximeter pulse rate with palpated pulse – Ensure a strong, regular waveform – Inaccurate readings may occur with poor perfusion, motion, hypothermia, carbon monoxide, or severe anaemia. 

Documentation Example:  
Pulse oximeter applied to right index finger. SpO₂ 97% on room air. Radial pulse strong, regular. Capillary refill <2 seconds. Good trace (waveform) noted.