Operation of oximeters using red/infrared densitometry

 

1.                  Transmission [spectrophotometric] oximetry

a)                  Involves shining radiation through a sample & determining the quantity of radiation absorbed

b)                 The wavelength of radiation is chosen so that it is absorbed by the compound of interest in the sample

c)                  The absorption of radiation by a sample of interest increases as the concentration increases

d)                 At sufficiently low concentrations, the absorption is proportional to the concentration

e)                  The light absorbed by blood depends on the quantities of oxyHb & deoxyHb present and the wavelength of light

f)                   The absorbance of oxyHb in red light (at 660 nm) is less than that of deoxyHb — accounts for its red colour

g)                  The absorbance of deoxyHb in infrared light (at 940 nm) is less than oxyHb

h)                  Comparison of the absorbances at these wavelengths enables the oximeter to determine oxygen saturation

i)                    The difference between the absorption at 800 nm (isobestic point & reference point) and 650 nm is proportional to the degree of saturation

j)                   Method based on the fact that if a mixture of oxyHb & deoxyHb is read at two wavelengths, one where there is a significant difference between the oxy & deoxyHb, the other at the isobestic point, the % saturation can be derived

2.                  In Vitro

a)                  Light of various wavelengths are transmitted through a diluted haemolysed blood sample and a photocell detects the light absorbances to determine the oxygen saturation

b)                 Light passes through a filter to provide a beam of one wavelength

c)                  This monochromatic beam passes via the blood sample to be detected by a photocell

d)                 A second photocell acts as a reference detector picking up a fixed fraction of the beam deflected by a beam splitter

e)                  A comparison of the output of the two detectors at different wavelengths allows computation of the oxygen saturation & Hct if required

 

3.                  In vivo — pulse oximeter

a)                  Pulse oximeters estimate arterial Hb saturation by measuring the transmission of light at two wavelengths through a pulsatile vascular bed

b)                 A red light emitting diode (LED) and an infrared LED shine light through a finger and a photocell detects the transmitted light

c)                  The LEDs are switched on in a sequence with a pause with both diodes off

i)                    The pause allows the photocell with microprocessor to detect & compensate for any ambient light penetrating around the sensor cover

ii)                  The LEDs are switched on & off at several hundred times per second; therefore cyclical changes due to pulsatile blood flow are still detected

d)                 A microprocessor analyses the changes of light absorption during the arterial pulsatile wave and ignore the non-pulsatile component of the signal due to tissues and venous blood

e)                 

i)                    Light absorption can be divided into a constant or ‘direct current’ (DC), and a pulsating of ‘alternating current’ (AC),

ii)                  AC component is due to arterial blood pulsation

a)                  Systolic volume expansion of arteriolar bed increases the optical pathway thereby increasing absorbance

iii)                 Pulse oximetry is based on the assumption that arterial blood is the only pulsatile absorber

iv)                The oximeter measures the AC component at each of the two wavelengths & then divides it by the corresponding DC component Žpulse added absorbances

v)                  By determining a ratio R  of the two pulse added absorbances the SaO2 can be determined from a nomogram

4.                   Maintenance of accuracy independent of Hct

a)                  Isobestic points:  points at which the absorbaces for oxyHb & deoxyHb are identical

b)                 Earlier models used isobestic points to compensate for Hct

i)                    A measurement of the light absorbed at the isobestic point is independent on the degree of oxygenation & standardises the system in terms of the quantity of Hb

c)                  Current oximeters use the various wavelengths to derive the results and give a direct reading of saturation (& Hct if required)