Describe the principles of measuring oxygen concentration
As oxygen is a molecule containing two similar atoms, its partial pressure cannot be determined using infrared techniques (unlike CO2). Oxygen content of a gas is instead determined using:
- Paramagnetic analyses
- Fuel Cells
Principles of paramagnetic analysis:
- Oxygen is paramagnetic
This means it is attracted by magnetic fields, but does not propagate the field.
- This is because its two unpaired valent electrons have the same spin.
Many other gases weakly repelled by magnetic fields (diamagnetic)
The attaction of a gas mixture to a magnetic field is therefore proportional to its oxygen content
- Many different methods exist which use this property to determine oxygen content
- Gas tested flows into a tube
- A reference gas flows into a parallel tube
- Both gases then pass through:
- Flow restrictors
- Magnetic field
This is being turned on and off at ~100Hz.
- The gases combine in the magnetic field
- The greater the oxygen content of the gas, the more it will move into the magnetic field
This movement creates a negative pressure behind the gas.
- The pressure difference between the tested gas and the reference gas is proportional to the oxygen content of the test gas.
Used in many modern devices.
- Gas flows through a magnetic field, causing the particles to align
This changes the thermal conductivity of the oxygen molecules.
- The change in thermal conductivity of the gas mixture is proportional to the oxygen content
- This is detected by measuring current passing through a heated wire
- Rapid response time
Modern analysers can identify breath-to-breath variation in FiO2.
- Don't require regular calibration
- Water vapour reduces accuracy
- Interference from other paramagnetic gases
- Nitric oxide
Effect is minimal as nitric is delivered in far smaller volumes than oxygen, and is only weakly paramagnetic.
- Nitric oxide
Fuel cells rely on reduction of oxygen to measure oxygen partial pressure. They consist of:
- Oxygen permeable membrane
- KOH solution
- Lead anode
Lead is consumed as the fuel cell operates.
- Gold cathode
- Lead anode
- Oxygen diffuses across the membrane into the potassium hydroxide solution
- At the cathode:
$$O_2 + 4e^- + 2H_2O \rightarrow 4OH^-$$
- At the anode:
$$Pb + 2OH^- \rightarrow PbO + H_2O + 2e^-$$
- The oxygen consumption is proportional to the current generated, which is measured with an ammeter
- No power required
- Will accumulate nitrogen in the presence of N2O
Results in an under-reading of PO2.
- Must be replaced after 6-12 months
- Requires regular two-point calibration
21% and 100% oxygen are used.
- Relatively slow response time compared to paramagnetic analysers
- Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.