Monitoring Gas Delivery in Anaesthetics Flashcards
What is the purpose of a fuel cell and what are its components?
The fuel cell can only measure O2.
It works on the same principles as the Clark Electrode, but does not require a battery.
- gold cathode
- lead anode
- electrolyte solution is potassium hydroxide.
Again electrons are produced at the anode, and react with O2 at the cathode.
How does a paramagnetic analyser work?
The paramagnetic analyzer works on the principle that O2 is attracted towards a magnetic field (due to the unpaired electrons in its outer shell).
The analyzer measures the pressure differential between:
- a stream of reference gas (e.g. air) on one side
- the sample gas on the other.
-» when exposed to an alternating magnetic field.
A sensitive pressure transducer is used to convert this pressure difference into an electrical signal calibrated to give the partial pressure of O2 on the sample.
What can mass spectrometry measure?
How does it work?
Measures any gas in a sample.
A few molecules of the gas sample enter an ionization chamber where they are bombarded by electrons that are moving from a hot cathode to an anode.
The gas molecules become charged fragments and are accelerated through a magnetic tunnel through to a detector at the end.
The amount of deflection depends on the mass of the gas molecules. Varying the current to the magnet rods, can focus fragments of particular size to measure concentration.
What can gas chromatography measure?
can measure all gases, if correct detector used.
Types of gas chromatography detectors?
- Flame ionization detector measures the current produced by organic vapours ionized in a flame.
- Thermal conductivity detector measures changes in the resistance of a heated wire in the gas flow - suitable for inorganic gases e.g. N2O and O2.
- Electron capture detector - Halogenated compounds reduce the electron flow produced by a radioactive cathode, thereby altering the current measured in proportion to their concentration.
Raman spectometry - what can it measure?
Components?
Can measure all gases/ vapours.
Uses laser bream though the sample, and processes the frequency of resulting scattered radiation to estimate sample concentration.
Infrared absorbtion: principle
Infrared (IR) analyzers rely on the fact that gases with two or more different atoms in their molecules absorb IR radiation at characteristic frequencies.
Gases that absorb IR?
Carbon dioxide, water, nitrous oxide and volatile agents.
What law does IR function upon?
Beer-Lambert Law
What is collision broadening?
in IR, this occurs when energy absorbed by CO2 from the IR radiation is transferred to any N2O also present in the gas sample.
This allows the CO2 to absorb more IR and consequently leads to a falsely high reading. Most IR analyzers account for this.
What can UV analyzers measure?
Principle of working?
Only Halothane
Same principle as IR except use mercury lamp as light source, and detect absorbtion light in the 200nm band.
What do Piezoelectric crystals measure?
can only measure gases/vapours that are soluble in oil, thus limiting them to anaesthetic vapour measurement.
How do Piezoelectric crystals work?
They vibrate at a specific resonant frequency when a current is applied across them.
After coating them with oil, anaesthetic vapour in the gas sample dissolves into it, shifting their resonant frequency in proportion to the concentration of vapour present.
By measuring this frequency shift, the vapour concentration is calculated.
Uses of a refractometer?
The refractometer is not routinely used in anaesthesia gas analysis, but identifies gases according to their differing refractive indices.
Light split through a pair of chambers will produce characteristic interference patterns when recombined.
O2 - commonly used to measure.
Fuel cell and Clark electrode
