Hi everybody. I would like to understand from a pathophysiological point of view how long does it take to a nasal flow reduction-associated desaturation to be visible on the pulse oximeter SpO2% signal. In Literature I found that such time depends mainly on functional residual capacity (FRC) and oxygen consumption (VO2), and that the delay for the average person is 60 seconds. Considering that OSA patients show often a higher BMI, would it be correct to expect a reduced delay time, since weight affects, among other factors, FRC? If patients had CV comorbidities, would it be correct to expect a reduced delay time as well?
Thank you in advance for your time and advice.
Source: https://www.sciencedirect.com/science/article/pii/S1743181617303025?via%3Dihub
I think, though I haven't got a reference at my fingertips to back this up, that it is also influenced by the cardiac output, which would in turn be influenced by the heart rate, which is also affected by apneic events.
Belinda Suthers thank you for your answer. Supposing to have a patient with chronic heart failure, I think that such delay would be reduced (e.g. the O2 partial intra-alveolar pressure would drop faster than subjects without heart conditions). Therefore, the maximal mean delay could be considered 60 seconds. In my opinion, I don't see any common condition that would prolong such delay.
Very Good and interesting question e.g. for work physiology!
1. From physiological reasons the delay takes 2-3 sec up to 10 sec or even more depending on the manifold reasons you mentioned (e.g. FRC, oxygen storage capacity, O2 consumption etc).
2. What people don't realize is the filtering technique the SpO2 device uses to smooth the digital values. Depends on # of pulsations (3 -10) to assess actual SpO2 value, and in addition on the time constant for plotting these values.
Some years ago I tried to find out these algorithms the producer use. For most of the companies it turned out to be a secret kept carefully… But try for your own device...
Friedhart Raschke thank you for your kind answer. What I am interested in is knowing the highest physiological delay possible, in order to include every nasal flow reduction-associated desaturation event, excluding at the same time the highest number possible of artifacts such as sensor displacement (desaturation not preceded by a "physiologically near" nasal flow reduction). Therefore, if the delay is reduced by conditions that accelerate O2 alveolar reserve consumption, that would not be a problem. The sampling frequency of my pulse oximetry device is 1 Hz by the way. I think that 60 s might be a physiologically sound value in order to include every nasal flow reduction-related desaturation without including too many artifacts.
Dear Matteo, the sampling rate is not the problem. They nearly always use 1 Hz. Several (how many?) pulsations are averaged, before the 1Hz value is calculated (sliding averaging, leading to not known time delays).
Thank you Friedhart, I get your point. Unfortunately, I am afraid we are not able to take this variable into account and have to speculate on the physiological changes of the delay according to the possible occurring conditions.
I don't know if this will help, but when arterial blood gases are drawn during a cardiopulmonary exercise test, the results of the blood in the syringe are 15 - 20 seconds behind what the heart and lungs are pumping out.
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