Pitfalls and common errors of anaesthetic monitoring devices. Part 1: Pulse oximetry

Abstract

Pulse oximetry is the most commonly used piece of monitoring equipment in general practice. It gives non-invasive objective information on oxygenation and pulse rate and subjective information on pulse quality. Like all electronic anaesthetic monitoring devices pulses oximeters can be used to improve the safety of anaesthesia; however, over-reliance on their use has a number of pitfalls. Foremost is the potential for erroneous measurement. Poor pulse quality, light interference, movement, pigmented tissue can all effect the accuracy and reliability of pulse oximetry. Understanding how the pulse oximeter works and how to assess signal quality is vital if pulse oximeters are to be used in an appropriate fashion. This allows the anaesthetist to ensure that the readings being given and recorded are realistic, which in turn helps decision making and ensures timely intervention.

This article is part of a series including:

Pitfalls and common errors of anaesthetic monitoring devices part 3: Capnography

Pitfalls and common errors of anaesthetic monitoring devices. Part 2: Non-invasive blood pressure monitoring

Multiple Choice Questions

1.	Pulse Oximetry measures which of the following: (a) Oxygen saturation of haemoglobin (b) Partial pressure of oxygen dissolved in the blood (c) End tidal carbon dioxide (d) Oxygen perfusion
2.	In the McMillan & Darcy, 2016 study how many patients showed significant oxygen desaturation? (a) 1 in 10 (b) 1 in 37 (c) 1 in 54 (d) 1 in 106
3.	Which of the following drugs commonly used in anaesthetic premedication may interfere with perfusion and therefore pulse oximeter readings? (a) Methadone (b) Acepromazine (c) Dexmedetomidine (d) Buprenorphine
4.	Methaemoglobin may be caused by which type of poisoning? (a) Ethylene Glycol (b) Smoke inhalation (c) Theobromine (d) Paracetamol
5.	The two common types of pulse oximetry are: (a) Reflectance and transmittance (b) Transmittance and absorbance (c) Reflectance and absorbance (d) Reflectance and illuminance
6.	What is the ‘penumbra effect’? (a) Where the pulse oximeter double counts (b) Where the light from the illuminated LED clearly bypasses the tissue (c) Interference from fluorescent strip lighting (d) Elastic recoil from the vessel causing the pulse oximeter to misread
7.	Which of the following does NOT typically cause altered absorption when utilising pulse oximetry? (a) Hyperlipidaemia (b) Injection of dyes intravascularly such as new methylene blue (c) Bilirubin in the jaundiced patient (d) Pigmentation of skin
8.	When is measuring SpO2 most useful? (a) Induction and recovery of anaesthesia (b) Monitoring the patient on 100% oxygen (c) Monitoring during the maintenance period of anaesthesia (d) Monitoring the conscious patient
9.	If SpO2 is 95% what is the approximate expected PaO2? (a) 500mmHg (b) 100mmHg (c) 50mmHg (d) 1000mmHg
10.	If an abnormal reading is detected on pulse oximetry, which of the following is the least appropriate action? (a) Repeatedly move the probe (b) Assess the pulse rate reading on the monitor with a manual pulse reading (c) Assess the plethysmographic waveform for clear, visible pulse waveforms (d) Thoroughly assess the patient

Additional information

Notes on contributors

Matthew McMillan

Matthew McMillan, BVM&S, DipECVAA, MRCVS

Matt runs the clinical anaesthesia service at the Queen’s Veterinary School Hospital. He is a European and RCVS recognised specialist in Veterinary Anaesthesia and Analgesia. His main interests include patient safety, especially pertaining to anaesthesia, medical error and veterinary education. He led the team that developed the Association of Veterinary Anaesthetist’s Anaesthetic Safety Checklist.