Pulse oximetry and capnography Flashcards
Which physical principles are used by the pulse oximeter and what is the function of a pulse oximeter?
Pulse oximeters combine principles of pulse oximetry and plethysmography to noninvasively measure the oxygen saturation of haemoglobin in arterial blood.
What are the fundamental components that make up a pulse oximeter
3 light emitting diodes
Photodiode (light detector)
Placed across any tissue that can be transilluminated
What is transmittance oximetry
When the light emitting diodes are directly opposite one another across perfused tissue
What is reflectance oximetry
When the light emitting diodes and photodiode are on the same side of the patient (e.g. the forehead) then reflectance or backscatter of light is detected by the photodiode
What is the Lambert-beer law
The observation that oxyHb absorbs more infrared (660 um) light and deoxyHb absorbs more red light (940 um) and thus appears blue to the naked eye in cyanosis.
The change in light absorption during arterial pulsations is the basis for oximetric determinations
How does the microprocessor determine SaO2
The greater the ratio of red/infrared absorption during arterial pulsations the lower the SaO2. This is calculated via and algorithm using established norms
Why is plethysmography a necessary component of pulse oximetry
Plethysmgraphy identifies arterial pulsations allowing for corrections for light absorption by non-pulsating venous blood
When is cyanosis clinically detectable
When there is > 5g/dL of deoxyHb
Usually corresponds to SaO2 of < 80%
Why is pulse oximetry not sensitive to hypoxia in CO poisoning
O2Hb and COHb absorb light at 660 um identically. So pulse oximeters that compare only two wavelengths will be unable to disticnguish COHb from O2Hb
Why is the SaO2 of MetHb 85% regardless of hypoxia or hyperoxia
Absorbs red and infrared wavelengths equally in a ratio of 1:1 - this equates to SaO2 85%. Therefore in the presence of MEtHb SaO2 is falsely low readings when SaO2 is actually greater than 85% and falsely high SaO2 when actual SaO2 is below 85%
What are the causes of pulse oximetry artifact
- SaO2 < 70% (inaccurate)
- Excessive ambient light
- Motion
- Low perfusion
- Hypothermia
- Low CO
- Profound anaemia
- Increased SVR - Malpositioned sensor
- Leakage of light from LED to photodiode bypassing arterial bed (optical shunting)
What do noninvasive brain oximetry monitors measure?
rSO2 = regional Hb saturation with O2 - Measures arterial, venous and capillary blood SaO2 and the reading represents an average O2 saturation of all regional microvascular Hb (normal is approximately 70%)
What is an isobestic point
The point on the absorption/electromagnetic wavelength graph where the absorption of light for two substances is identical - i.e. the graph intersects.
