Oxygen measurement

Regarding spectrophotometric oximetry, which one of the following statements is

CORRECT?

a) 660nm is the wavelength of light emitted by one of the LEDs in the apparatus

because this is one of the isobestic points of the pertinent absorption spectra

b) While placed on the ear, approximately 30% of absorbed energy is due to the

pulsatile component of the tissue

c) Venous blood in the tissues does not contribute to the absorption of the red and

infrared light

d) Of all the potential colours of nail varnish, red-coloured nail varnish will disrupt the

pulse oximeter to the greatest extent

e) Oximetry may determine relative proportions of carboxyhaemoglobin and

methaemoglobin

Answer: e

Explanation

Oximetry is the technique by which the proportions of different varieties of haemoglobin

in a sample are determined by examining the pattern with which red and infrared

light successfully traverse the sample. It can be applied in vitro where a haemolysed

sample may be exposed to a number of wavelengths of light and analysis of the

absorption spectra allows determination of various haemoglobin combinations (deoxyhaemoglobin,

oxyhaemoglobin, carboxyhaemoglobin, methaemoglobin) or in vivo as

pulse oximetry where historically only deoxyhaemoglobin and oxyhaemoglobin are

measured giving a quantification of oxygen saturation in arterial blood. In fact, there

are now commercially available non-invasive monitors that may also determine other

haemoglobins. The extent to which haemoglobin and oxyhaemoglobin absorb light

varies according to the wavelength of the incident light. At around 700 nm, red light is

absorbed well by deoxyhaemoglobin, but not by oxyhaemoglobin. This is intuitive as

on visual inspection, deoxygenated blood appears dark and less red than oxygenated

blood. This is because the red light is being absorbed. Oxygenated blood appears bright

red because it does not absorb red light so well (thus we see it as red); however, at

around 1000nm it absorbs infrared light more than deoxygenated blood. By selecting

light-emitting diodes (LEDs) with wavelengths in this region (and 660nm and 940 nm),

maximum discrepancy allows greatest sensitivity. The isobestic points are those wavelengths

at which absorption by both types of haemoglobin are identical. The tissue

composite through which the light travels is largely non-pulsatile: tissue, venous blood

and that volume of arterial blood that is non-pulsatile. The component of the composite

of interest is just the pulsatile component of the arterial blood. This forms a very small

proportion of total absorption. Red nail varnish disrupts the signal least as it does not

absorb the red light (hence it appears red to the eye).