Oxygen measurement Flashcards
When does cyanosis occur
When the concentration of deoxyhaemoglobin is > ± 5 g/dL
Why is cyanosis a poor indicator of arterial oxygenation
- Subjective - Interobserver variation
- Ambient light
- Anaemia (reduces conc. of all types Hb)
- Low CO orcapillary stasis - isolated cyanosis unrelated to PaO2
What are spectrophotometric techniques
Passing radiation, of specific wavelength relevant to measured variable, through a sample and determining the quantity of radiation absorbed.
What is Beer’s law?
Beers law states that the absorption of radiation by a given thickness of solution of given concentration is the same as that of twice the thickness of the solution at half the concentration.
Think Beers –> glass of beer versus no beer in the glass
What is Bouguer’s or Lamberts law
This law states that each layer of equal thickness absorbs an equal fraction of radiation passing through it
Think Lamberts LENGTH
How does absorption of radiation change with concentration of the sample of interest
The absorption of radiation by the compound of interest increases in proportion to the concentration of the compound of interest.
The higher the concentration, the higher the absorption
At sufficiently low concentrations, the absorption is proportional to the concentration.
How does an oximeter work?
Light of various wavelengths is transmitted through a haemolysed blood sample and a photocell detects the light absorbances so that the O2 saturation can be calculated.
The light absorbed by blood depends on the quantities of oxyhaemoglobin and deoxyHb present as well as the wavelength.
Red light 660nm
- Absorbance less by oxyHb vs. deoxyHb
Infrared light 940mm
- Absorbance less by deoxyHb vs oxyHb
Draw graph with 2 isobestic points
Comparison of the absorbances at these wavelengths enables an oximeter to measure SaO2
What is an isobestic points
Points on the Wavelength (nm) - Absorbance graph where the absorbances for the two forms of Hb are identical. Absorbance at any of these points depends only on the Hb concentration.
How does an oximeter correct for the presence of carboxyHb in smokers and city dwellers
Careful selection of wavelengths used in analysis and by electronic computation.
CarboxyHb and MetHb can also be measured by using a series of light filters or diodes for differing frequencies of light
What is the principle used in a pulse oximeter
The spectrophometric technique as used in the oximeter just without the haemolysed blood sample.
Radiation from a red (660 nm) and infrared (940nm) LED passes through finger and the radiation transmitted is detected by a photocell.
The output of the sensor is is processed electronically to give a pulse waveform and the SaO2.
How do pulse oximeter’s compensate for penetration of ambient light onto the photocell
Red (660nm) on then off. Infrared (940nm) on then off. Then both off. The pause when both lights are off allows the photocell and microprocessor to detect and compensate for any ambient light which may have penetrated around the sensor cover.
How does the pulse oximeter detect arterial rather than venous blood or tissue oxygenation?
The diodes are switched on and off at high frequency, several hundred times a second. Hence the device can detect cyclical changes in the signal caused by pulsations during the cardiac cycle. A microprocessor is programmed to analyse these changes of pulsation during arterial pulsatile flow and to ignore nonpulsatile component. Therefore, Arterial O2 can be measured.
Do pulse oximeters compensate for the presence of abnormal Hbs (CarboxyHb and MetHb)? Explain why
No, They use only two diodes which do not compensate full for COHb and other abnormal Hb
What can cause an abnormal pulse oximeter reading
- Excess ambient light
- Poor contact
- Abnormal Hbs (COHb and MetHb)
- Vasoconstriction
- Low peripheral skin temperature
- fingernail / skin staining
- Excessive movement –> movement artefact
Does pulse oximetry measure venous O2?
Yes. It measures both arterial and venous but then just excludes the lower value from display
What is NIRS monitoring. How does it work?
Cerebral Oximeters use NIRS (Near Infrared Spectroscopy) to obtain continuous non-invasive measurements of cerebral oxygenation values.
The skull is transparent to light in the near-infrared range. LEDs emit near infra-red light through the skull into the brain tissue. This light is either absorbed, reflected, scattered or redirected. When infrared light contacts Hb, a change in the wavelength occurs dependent on the oxygenation status of the Hb molecule. Reflected light is detected by photo detectors on the NIRS apparatus.
What are the isobestic points used for by the pulse oximeter
Used to calculate total Hb concentration
Describe the components and function of an oximeter
Light source –> monochromatic filter –> Beam splitter –> reference detector (reads fraction of deflected light by beam splitter) –> through cuvette (with sample and connected to pump and haemolyser) –> radiation absorbed –> remaining radiation received by detector –> electrical signal transduced by photocell and sent to microprocessor –> comparison of output of two detectors (reference and photocell) to establish how much radiation was absorbed at a specific wavelength.
How is error introduced by traces of COHb in smokers and city dwellers reduced?
- Careful selection of specific light wavelengyhs
- Electronic computation
Can get filters specific for COHb and measure it in more modern oximeters
How does a pulse oximeter work?
Radiation from red (660nm) and infrared (940nm) LED passes through the finger and the photocell detects the transmitted radiation. The output of the sensor is processed electronically to give a pulse waveform and the arterial O2 saturation.
How do pulse oximeters detect arterial rather than venous O2 saturation
The microprocessor is programmed to only analyse the light absorption changes during arterial pulsatile flow and to ignore the non-pulsatile component of the signal in tissues and venous blood
What is the oxygen content equation?
CaO2 = 1.34 x Hb x SaO2%/100 + [PaO2 x 0.023]
How can O2 content of blood be measured more accurately than can be calculated by the CaO2 equation
O2 measured in blood sample directly:
O2 flushed out of a haemolysed blood sample by means of an O2 free carrier gas and the total O2 content in the gas measured with a fuel cell
What is a Clarke electrode, what is an alternative name for this and how does it work
The Clark electrode or polarographic electrode is the apparatus incorporated into a blood gas machine to measure the oxygen tension in blood.
A platinum cathode and a Ag/AgCl anode submerged in a KCl solution
- A voltage of 0.6 V is applied between the electrodes and the current is measured.
- Anode: Ag reacts with Cl (from KCl) –> AgCl + e-
- Cathode: 4e- from the anode combine with O2 and 2H2O to produce 4OH-
THE MORE O2 around, the more e- can be taken up at the cathode and hence the greater the current that flows. so the current flow through the cathode is dependent on the O2 tension at the platinum cathode.
