I recently read a textbook and a guidebook for veterinary nurses which looked at the use of pulse oximetry, and it appears that averaging time is still largely misunderstood. The general understanding of Pulse Oximiters is that they shine different wavelengths of red light through tissue to measure and display a numerical vale for the ‘redness’ of the blood (because the more oxygen it is carrying, the brighter red it gets). Without a clearly definable pulse, the oximeter should fail-safe and stop displaying numbers and deliver a warning to the user.
Sadly it is a little more complicated than that – and this is where some textbook authors go astray such as in the example of “averaging time”.
Averaging time is not the average as many people understand it. It doesn’t mean that a pulse oximeter set with an averaging time of 20 seconds is 20 seconds behind any physiological changes and only updates every 20 seconds.
It is important that we understand a little of how the critical equipment we use actually works to avoid misunderstandings. So what exactly is averaging time and why do we have it?
Pulse Oximiters: How do they actually work?
Firstly, lets take a look at how a pulse oximeter gets it’s data and calculates pulse rate (Pulse) and O2 Saturation (Oximiter).
A pulse ox derives its’ data by measuring the light absorption through tissue at two different wavelengths. The precise nature of these wavelengths is largely unimportant, provided that they are either side of the Isobestic point. One will be measuring maximal absorption from the brighter red range of the light spectrum (haemoglobin bound with O2), while the other where the darker red (unbound haemoglobin) is.
In this case the wavelengths are 660nm and 910 but the exact wavelength doesn’t matter (for example, they could be 660 and 920 or 700 and 990).
The LED Cycle
The LEDs flash and go through a very precise cycle: red on, both off, Infra red on, both off, Red and Infra red on, both off. This cycle operates at 60Hz which means the monitor is picking up 6 data points (the cycle) 60 times a second. This is why the light appears constant to the human eye.
The period of ‘off’ allows the oximeter to asses for ambient light, the other points are measuring the absorption of light at that wavelength.
The light absorbed by non-pulsatile tissue (like the tissue thickness/volume it is measuring) is constant, but the absorption from pulsatile tissue varies with the change of volume (from the arterial pulse).
The pulsatile component makes up between only 1% and 5% of the total signal, depending on the blood/pulse pressure. This can vary with movement, which is where averaging times come in.
Averaging Time Explained
Set in standard mode on a perfectly still subject (eg. An anaesthetised patient) the oximeter is able to get fast and accurate data using a 5-second averaging time.
It uses 5 x 60 = 300 data points to calculate and display its readings. If one or two readings are beyond the expected levels or averages that the oximeter is seeing it makes very little difference to the displayed results.
The critical point here is that the averaging time of 5 seconds is being continuously updated – 60 times a second. It is a moving average – and it is moving fast. The oximeter can change the displayed data every second or faster.
Looking at the other end of the spectrum, with the oximeter set at 20-second “averaging” time, 20 x 60 = 1200 data points, and the difference in displayed time and data/results doesn’t change significantly.
Yes, it is slower – but it isn’t 20 or 30 seconds (the averaging time set) slower. Let’s consider for a moment a massive de-saturation. How long will it take for the pulse oximeter to react?
With a 5-second averaging time, it will likely react in 2-3 seconds.
With a 20-second averaging time, the pulse oximeter will start displaying the changes in saturation level after about 4-5 seconds, and from here it will continue to track the de-saturation.
There is not a 20-second delay in readings as stated by the textbook and guidebook.
The author Mike Brampton, started working on oximetry in 1979 and then spent 8 years working for one of the two companies who released the 1st commercially available pulse oximeter. He holds 2 patents in pulse oximeter design including the 1st multi-wavelength oximeter (now marketed by Masimo under the name Rainbow) and transflectance oximetry. Mike continues to work in this area and is known for making specialised pulse oximeter probes for research.