PULSE OXIMETRY
Pulse oximetry is a non-invasive, in-vivo method of measuring oxygen saturation, pulse rate and provides a plethysmograph.
Physics and principles
BEER'S LAW:
'absorption of radiation increases (transmission decreases) as concentration of substance increases'
LAMBERT'S LAW:
'Absorption of radiation increases exponentially (transmission of light decreases exponentially ) as distance travelled through substance increases.
Components:
Sensor probe:
- Two monochromatic LEDS (660-RED & 940nm- INFRARED) one side
- Photodetector on other → Current proportional to intensity of detected light
Microprocessor: analyses signal input
Display: Shows O2 saturation, pulse rate, plethysmograph
Mechanism:
1) LEDS operate individually then both off
2) Transmitted light detected by photodetector → current generated
3) Light absorbed has two components:
constant (DC) = tissues and venous blood
non-constant (AC) = arterial blood pulsation
4) microprocessor extracts AC component at 660nm and 940nm → display as plethysmograph
5) Differential absorption spectra for oxyHb and deoxyHb - CPU calculates % and gives number for SPO2.
Absorption spectra
Oxyhaemoglobin:
- Absorbs more at infra-red wavelength 940nm
- similar absorption to COHb at red wavelength 660nm - (falsely high SPO2 in CO poisoning)
Deoxyhaemoglobin:
- absorbs more at red wavelength 660nm
- similar absorption to MetHb at infrared wavelength 940nm
- false low SpO2 in methaemoglobinaemia
Isobestic point 810nm : both deoxyHb and oxyHb have same absorption at this point
Problems and Causes of error:
Venous pulsation (e.g. TR)
Interference: diathermy, excess movement.
alternate forms of Hb: COHb (false high), MetHb (false low)
Risk pressure sores, burns if prolonged
Limitations
Altered pulsative fraction (AC): Low CO states, arrhythmia, vasoconstriction.
Acute changes not detected (lag 20s)
Detects saturations but not marker of ventilation!
SPO2 Accurate +/- 2% between 70-100% (inaccurate <70%)