Episode 2: Pulse Oximetry
Author: Thong Yi Kun
Script Editor: Dr Suneet Sood
Narrator: Thong Yi Kun
The buzzer sounds as you are broken from your reverie. You are the sole registrar working in the red zone of a rural hospital emergency department. Major cases are infrequent but sometimes when it rains it pours.
A patient has arrived via ambulance. You rush out and greet the paramedic with an earnest look.
“What have you got for me today?”
“This lady attempted suicide. She put a pipe connecting the exhaust to her window”
“What are her saturations?”
“98%, but we have been manually ventilating her the whole way”
You quickly assess the patient. Her GCS is 3, appears pink but with minimal spontaneous breaths. You quickly make a provisional diagnosis of carbon monoxide poisoning. Despite her oxygen saturation of 98%, you decide for immediate endotracheal intubation in view of her impaired GCS.
Funny that, in carbon monoxide poisoning, the pulse oximeter shows high saturation levels of haemoglobin!
You decide to read up on how the pulse oximeter works.
The pulse oximeter works on two principles: first, by being able to take readings only the arterial blood, and second by being able to calculate the level of oxygenation.
How does the pulse oximeter take readings only from the arterial blood? Well, arteries are pulsatile, and the light absorbance changes from beat to beat, and the pulse oximeter is programmed to select these changes. Obviously, it will not work if the patient does not have a pulse! Also, in severe tricuspid regurgitation, the venous system acquires a pulse. This confuses the pulse oximeter, and it can read the venous blood, resulting in errors.
The pulse oximeter also calculates the level of oxygenation. Oxygenated haemoglobin absorbs more infrared light. Deoxygenated blood absorbs more red light. By comparing the ratio of absorbed infrared to red light, the percentage of oxyhaemoglobin can be worked out.
But why would a patient with severe CO poisoning have an artificially high SpO2?
The pulse oximeter reports the percentage oxygenation as the ratio of oxygenated haemoglobin to total haemoglobin. The total Hb of course is assumed to be the oxygenated plus deoxygenated Hb. Unfortunately, the pulse oximeter ignores non-functioning hemoglobins like methaemoglobin and carboxyhaemoglobin. Since it presumes the carboxyhemoglobin does not exist, the detected total haemoglobin falls drastically in patients with CO poisoning. This results in a falsely high percentage reading for oxygenated haemoglobin.
Chan ED, Chan MM, Chan MM. Pulse oximetry: understanding its basic principles facilitates appreciation of its limitations. Respiratory medicine. 2013;107(6):789-99.
Stewart K, Rowbottom S. Inaccuracy of pulse oximetry in patients with severe tricuspid regurgitation. Anaesthesia. 1991;46(8):668-70.