- Principle of shining radiation through a sample and determining the quantity of radiation absorbed.
There are two important laws:
Beer's law: absorption of a given thickness of a solution of a given concentration is the same as twice the thickness of half the concentration.
Lambert’s law: each layer of equal thickness absorbs an equal fraction of radiation which passes through it.
The light absorbed by blood depends upon the quantities of deoxy- and oxyhaemoglobin (deoxy- /oxy-Hb) present and the wavelength of light. In infrared light (940 nm), the absorbance of oxy-Hb is greater than that of deoxy-Hb.
The points where the absorbances for the two forms of Hb are identical are known as ‘isobestic points’ and are dependent upon Hb concentration.
The pulse oximeter
Two light emitting diodes, a red (660 nm) and infrared (940 nm) shine through the finger and the photocell detects the transmitted light. The output is processed electronically, to give a pulse waveform and the arterial oxygen saturation.
Diodes are switched on in sequence, with a pause where both diodes are off. This allows for the microprocessor to compensate for ambient light. The diodes are switched off hundreds of times a second – thus, the processor can detect cyclical changes due to arterial blood flow. The non-pulsatile component is disregarded.
Errors and problems
In the following situations, the pulse oximeter readings may not be accurate:
A reduction in peripheral pulsatile blood flow produced by peripheral vasoconstriction (hypovolaemia, severe hypotension, cold, cardiac failure, some cardiac arrhythmias) or peripheral vascular disease. These result in an inadequate signal for analysis.
Venous congestion, particularly when caused by tricuspid regurgitation, may produce venous pulsations which may produce low readings with ear probes. Venous congestion of the limb may affect readings, as can a badly positioned probe. When readings are lower than expected, it is worth repositioning the probe. In general, however, if the waveform on the flow trace is good, then the reading will be accurate.
Bright overhead lights in theatre may cause the oximeter to be inaccurate, and the signal may be interrupted by surgical diathermy. Shivering may cause difficulties in picking up an adequate signal.
Pulse oximetry cannot distinguish between different forms of haemoglobin. Carboxyhaemoglobin (haemoglobin combined with carbon monoxide) is registered as 90% oxygenated haemoglobin and 10% desaturated haemoglobin - therefore, the oximeter will overestimate the saturation. The presence of methaemoglobin will prevent the oximeter from working accurately and the readings will tend towards 85%, regardless of the true saturation.
When methylene blue is used in surgery to the parathyroids or to treat methaemoglobinaemia, a shortlived reduction in saturation estimations is registered.
Nail varnish may cause falsely low readings. However, the units are not affected by jaundice, dark skin, or anaemia.