Veterinary patients present unique challenges, from fur and pigmentation to species-specific anatomy and movement under light planes of anaesthesia. Misinterpreting a reading could lead to unnecessary interventions, or worse, missed warning signs.
Here’s what can throw off pulse oximeters during anaesthesia, and how to get around it.
Ultimately, as with all monitors and measurement devices, the pulse oximeter is designed to “fail safe.” When conditions interfere with accurate measurement too much, the monitor often displays an error message or shows no values; which is sometimes mistaken for a malfunction.
What’s actually happening is that the monitor cannot guarantee the accuracy of the data it’s displaying because it has moved outside its performance envelope due to external factors.
Think of it this way: fail-safe behavior is just as valuable as an alarm. It tells you one thing clearly; it’s time to rely on your manual monitoring skills and direct patient observation. Something about the situation (perfusion, placement, movement, etc.) is outside the limits of what the device can interpret reliably.
Now, let’s explore the most common causes of poor or misleading SpO₂ readings during veterinary anaesthesia — and what to do about them.
Cause:
Hypotension, vasoconstriction (from alpha-2 agonists, for example), hypothermia, or shock can lead to weak peripheral pulses, especially in small or compromised patients.
Effect:
Low signal quality, erratic waveform display, erratic or absent SpO₂ readings.
Fix it:
Warm the patient with a circulating water blanket or forced-air warmer.
Check probe placement and consider using alternative locations (tongue, lip, prepuce, or vulva) or use an internal or reflective sensor like the UYB
Cause:
Darkly pigmented tissues or residual fur can interfere with red/infrared light transmission.
Effect:
No signal or artificially low SpO₂ values.
Fix it:
Use hairless, less pigmented areas like the tongue, pinna, or prepuce.
Ensure the sensor is clean and flush with the tissue surface.
Avoid the lip in heavily pigmented breeds (e.g., black Labs, Chow Chows) unless an alternative is unavailable.
Use a reflectance sensor on the gums or elsewhere (ear canal, rectum)
Cause:
Swallowing, tongue flicking, jaw tone, or light planes of anaesthesia causing minor movement.
This will clearly show as an erratic waveform
Effect:
Signal dropout or fluctuating readings.
Fix it:
Deepen anaesthesia if appropriate.
Reposition the probe securely.
Use a motion-resistant model, especially for small or exotic species.
Cause:
Anaesthetized animals can become cold, especially small patients or those undergoing lengthy or wet procedures.
Effect:
Peripheral vasoconstriction and poor perfusion to sensor sites.
Fix it:
Monitor temperature actively.
Use warming aids early and consistently.
Combine with other monitoring tools if perfusion remains low.
Cause:
Incorrect alignment or excessive pressure can occlude blood flow or skew signal capture.
Effect:
Unreliable or falsely low readings.
Fix it:
Confirm the emitter and detector are aligned directly opposite each other.
Avoid compressing tissue too tightly — especially in cats, small dogs, or exotics.
Consider a reflective or internal probe like the UYB sensor
Use appropriately sized probes for species and sites.
Try and position the probe directly over the artery eg lingual artery
Cause:
Agents like dexmedetomidine, medetomidine, and epinephrine reduce peripheral blood flow reducing the quality of the pulse that the pulse oximeter is looking for.
Effect:
SpO₂ may drop despite adequate oxygen delivery.
Fix it:
Be aware of this expected side effect in otherwise stable patients.
Monitor trends, not just single values.
Cross-reference with heart rate, capillary refill time, mucous membrane color, and end-tidal CO₂.
Try and position the probe directly over the artery e.g. lingual artery
Switch to a central monitoring position and use a reflectance or internal sensor, e.g. a rectal probe
Cause:
Pulse oximeters measure saturation of available hemoglobin — but not total oxygen content. In anemic patients, SpO₂ may appear normal even when oxygen-carrying capacity is dangerously low.
Effect:
False reassurance.
Fix it:
Monitor PCV/TS and overall perfusion status.
Don’t rely solely on SpO₂ in known or suspected anemic patients.
What happens:
Strong external light; like direct sunlight or operating room lights or flickering fluorescent lights, can interfere with sensor readings.
What to do:
Shield the sensor from bright light using a towel, blanket, or opaque cover.
Avoid direct light on the sensor during use.
Position the receiver so it faces away from the light.
Always interpret SpO₂ in context; it’s just one piece of the anaesthetic monitoring puzzle.
Use trends, not isolated values. A drop from 98% to 93% over 2 minutes is more meaningful than a steady 94%.
Check the pulse waveform on the monitor matches the rhythm of the palpated pulse. If the waveform doesn’t look like a pulse waveform, double check probe position and for light interference.
Watch out for fail safe errors.
Back up with Capnography, Blood pressure, temperature, mucous membrane color, capillary refill time (CRT) and a stethoscope whenever possible.
Pulse oximeters are incredibly valuable; but only when we understand their limitations. Knowing what can interfere with readings during anaesthesia helps us respond wisely, avoid false alarms, and keep our patients safe and stable.
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