SN7 SHORT NOTE 5M ★ High-yield

Write a short note on Pulse Oximetry: Sources of Error.

General Surgery · Final Professional

Principle Recap

The pulse oximeter derives SpO₂ from the differential absorption of red and infrared light by oxygenated and deoxygenated haemoglobin in pulsatile arterial blood. Anything that disturbs the signal, the light path, or the assumptions about haemoglobin can cause error.

Sources of Error

Poor pulsatile signal: hypotension, hypothermia, vasoconstriction, cardiac arrest (no reading). Movement/shivering and bright ambient light cause artefact. Nail varnish and dyes (e.g. Methylene blue) reduce readings. Abnormal haemoglobins: carboxyhaemoglobin reads falsely high (it absorbs like oxyhaemoglobin — dangerous in CO poisoning), and methaemoglobin drives the reading toward ~85%. It does not detect hypoventilation (a patient on oxygen can have a normal SpO₂ while retaining CO₂) and responds late.

DANGER / REMEMBER

The dangerous error is a falsely normal/high SpO₂ in carbon monoxide poisoning (carboxyhaemoglobin) — the oximeter cannot distinguish it from oxyhaemoglobin. And SpO₂ does not reflect ventilation, so it must not be relied on to detect hypoventilation.

CLINICAL PEARL

Remember pulse-oximetry errors as three groups: poor signal (hypotension, cold, motion), optical interference (nail varnish, dyes, light), and abnormal haemoglobins (carboxyhaemoglobin falsely high, methaemoglobin ~85%).

Clinical Vigilance

Because of these many potential errors, the pulse-oximetry reading is always interpreted in the context of the whole patient rather than trusted blindly: a low reading in a warm, pink, well-perfused patient may be artefact from movement or a poor probe position, whereas a ‘normal’ reading must not be reassuring in a patient who could have carbon monoxide poisoning or who is being pre-oxygenated and hypoventilating. The safe habit is to check the plethysmograph trace for a good pulsatile signal, correlate the number with the clinical picture, and remember that oximetry says nothing about the adequacy of ventilation.

CLINICAL PEARL

Group oximetry errors as poor signal, optical interference and abnormal haemoglobins, and never be reassured by a normal SpO₂ in possible carbon monoxide poisoning or in a pre-oxygenated, hypoventilating patient.

Reads falsely normal in carbon monoxide poisoning.

Cause	Effect on SpO₂
Carboxyhaemoglobin	Falsely high (reads as oxyhaemoglobin)
Methaemoglobin	Tends towards 85% regardless of true value
Methylene blue, indocyanine	Falsely low
Nail polish, dyes	Falsely low
Motion, shivering, poor perfusion	Unreliable or absent trace
Severe anaemia	May read normal despite low oxygen content

KEY POINT

Key points TO remember

Pulse oximetry needs a good pulsatile signal & normal haemoglobin.
Poor signal: hypotension, hypothermia, vasoconstriction, motion.
Optical: nail varnish, dyes, bright light.
Abnormal Hb: carboxyhaemoglobin (falsely high — CO poisoning), methaemoglobin (~85%); misses hypoventilation, responds late.

EXAM TIP

Sources: Morgan & Mikhail’s Clinical Anesthesiology; Miller’s Anesthesia; Ajay Yadav’s Short Textbook of Anaesthesia.

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