Pulse Oximeter Viva

How does a pulse oximeter work?

It is a device used to determine arterial oxygen saturation. It measures the percentage of haemoglobin saturated with oxygen in the arterial blood.

Components:

· Probe placed on extremity (ear/finger/toe)

· Probe consists of two high intensity monochromatic light emitting diodes (LED) on one side, and a photodetector on the other side

· Photodetector connected to an electronic processor which used the output to produce a pulsatile waveform that is then displayed on a screen

Principles:

1. Application of Beer Lambert law:

a. Beers Law: the absorption of radiation increases as the concentration of a substance increases

b. Lamberts Law: the intensity of transmitted light decreases exponentially as the distance travelled through the substance increases

2. Hence, absorption of light differs between oxy and deoxyhaemoglobin

How does it work…

LED on one side of probe emit light at two wavelengths (660 nm-red and 940nm-infrared) and they turn on and off in sequence, and then are paused with both off. This sequence is done HUNDREDS of times per second. This high frequency enables probe to detect arterial pulsations. Pausing in off position allows for detection of ambient light (corrects for this).

The light passes through the finger, and some gets absorbed by arterial blood, venous blood, capillaries and tissues. The amount that is absorbed depends on the amount of oxy and deoxyHb present. The absorbance at 660nm is higher in deoxyHb, and at 940nm is higher in oxyHb.

The pulsatile component of the signal is analysed and non-pulsatile component ignored. Signal is processed in electronic processor and displayed as a waveform and number.

Can you draw the graph?

Yes!

What is the isobestic point and what is its significance?

It is a point at which two substances absorb a certain wavelength of light to the same extent. For oxy and deoxyHb the isobestic points are 590 and 805nm. The isobestic point can be used as a reference when absorption of light is independent of saturation (old oximeters use this principle). Also point at which one can calculate haemoglobin concentration.

How does it tell it is arterial blood?

Pulsatile component is AC

Non-pulsatile component is DC

The pulsatile expansion of arteriolar blood produces an increased path length thereby causing an increased absorbance.

Oximeter first determines AC component, then divides it by DC component to obtain a ‘pulse-added’ absorbance that is independent of the incident light intensity. It then calculates the ratio of these pulse added absorbances, which is empirically related to SaO2:

R = AC660/DC660

AC940/DC940

What does the R value mean?

See above

In words, it is the absorbance of the pulastile component divided by the non-pulsatile component at 660nm, which is then divided by the pulastile component divided by the nonpulsatile component at 940nm.

When the R ratio = 1 the saturation is approximately 85%

To work out why R <1 = high SaO2 and R >1 = low SaO2, I think of the ratio as:

R = Deoxy absorbance (660)

Oxy absorbance (940)

Are there any conditions that can alter the R value?

Methaemoglobinaemia – trend towards R =1 (see below for explanation)

CarboxyHb – falsely low R values (see below for explanation)

Any substance present in pulsatile blood that absorbs light at either wavelength and is not present in volunteers (those used to establish curve). Intravenous dyes (methylene blue, indigo carmine, indocyanine green) – all falsely lower SaO2

How accurate is a pulse oximeter?

Fairly accurate - within 2% SaO2 between 70 and 100%

Below 70% less accurate, below 50% inaccurate; because measurements were taken against healthy subjects arterial blood saturations and lower readings are merely extrapolations of this data.

What can affect its accuracy?

· Artefact

· Ambient light

· Patient movement eg shivering (large AC/DC signal)

· Low cardiac output/hypoperfusion states (low AC/DC signal)– pulsatile element is 1-5% of total signal so vasoconstriction causes loss of this

· Nail polish/darker skin discolouration

· Tachy/bradyarrhythmias (eg AF)

· Venous pulsation eg severe TR (machine assumes any pulsatile flow is arterial)

· Carbon monoxide – carboxyHb and oxyHb similar absorption at 660nm (hence get higher value for denominator, causing falsely low R value and thus overestimates SaO2)

· IV dyes (eg methylene blue, indocyanine green) give falsely low readings

· Methaemoglobinaemia – sats trend towards 85% (may be higher or lower than real value) – this is because it has high absorbance at both 660 and 940, adding to both numerator and denominator of R ratio, trending R towards 1.

· Inaccurate LED devices

NOT affected by fetal Hb