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Respiratory monitoring 

Respiratory monitoring
Chapter:
Respiratory monitoring
Author(s):

Sheila K. Adam

and Sue Osborne

DOI:
10.1093/med/9780199215904.003.06
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date: 29 July 2021

Pulse oximetry 118

Capnography 120

Pulmonary function tests 122

Blood gas analysis 124

This refers to continuous, non-invasive measurement of oxygen saturation in arterial blood (SpO2). A probe is placed over a digit, earlobe, cheek, or the bridge of the nose. It emits light at two specific wavelengths – red and infrared. Light passes through the tissue and is sensed by a photodetector at the base of the probe. Most of the emitted light is absorbed by skin (including pigment), bone, connective tissue, and venous vessels (baseline measurement). This amount is constant so the only relevant fluctuations are caused by increases in blood flow during systole. The peaks and troughs of the pulsatile and baseline absorption for each wavelength are detected and the ratios of each compared. This provides the ratio of oxyhaemoglobin to total haemoglobin, i.e. the saturation. Pulse oximetry does not measure the oxygen content of the blood as this depends on how much haemoglobin is present. An anaemic patient may still have an oxygen saturation of 100%....

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