Respiratory Gas Analysis

Question 1

What do the AAGBI recommend as essential components during an anaesthetic regarding respiratory gases?

Answer 1

• an O2 analyser with an alarm
• a CO2 analyser
• a vapour analyser if volatile is being delivered
• monitoring of ambient anaesthetic agent concentrations

Question 2

How is O2 normally measured in theatre?

Answer 2

A paramagnetic cell

Question 3

How does a paramagnetic cell work?

Answer 3

Oxygen and nitrous oxide are strongly PARAMAGNETIC. It’s attracted to a magnetic field by having unpaired electrons in the outer electron ring.

A switched electromagnetic field and pressure transducer are used. The electromagnetic field is generated at 110Hz. This creates a pressure differential between the air and the patient sample.

A transducer detects pressure fluctuations of 20-50 micro bar and converts this to DC voltage, which is directly proportional to the concentration of O2.

Question 4

How is blood gas analysis normally performed?

Answer 4

Using a Clarke electrode for O2 and a Severinghaus electrode for CO2 and a glass electrode for pH.

These measure the partial pressure of dissolved O2 or CO2.

Question 5

Can you continuously monitor intravenous O2 and CO2?

Answer 5

The paratrend 7 does this but for routine clinical use is not currently used.

Question 6

How does the Clarke (polarographic) sensor work?

Answer 6

Uses a noble metal (platinum, gold or palladium) cathode and a silver/silver chloride anode, in a potassium chloride or potassium bromide electrolyte buffer in a cellophane compartment around both electrodes.

An external current is required to drive the cathode reaction.

At the cathode (positive) O2 is reduced (O2 + 2H2O + 4e^- = 4OH^-)

At the anode (negative) silver is oxidated (4Ag⁺ + 4CL^- + 4e^- = 4AgCl)

Therefore current flows in the presence of O2 and the strength is directly proportional to the concentration of O2.

Question 7

What kind of membrane does the Clarke sensor use?

Answer 7

Teflon membrane as it allows O2 through but stops other gases

Question 8

What are the limitations of the Clarke electrode?

Answer 8

• over-reads in the presence of N2O as silver contamination will allow reduction of N2O at the cathode
• both the anode and electrolyte solutions degrade, requiring recalibration and replacement
• temperature and pressure sensitive

Question 9

What is a fuel cell (galvanic sensor/hersch)?

Answer 9

Similar to polarographic sensor but electrodes provide their own current.

The cathode is gold/silver and anode is lead.

Electrolyte solution is potassium hydroxide.

The cathode reaction is the same as in the Clarke electrode (O2 + 2H20 + 4e = 4OH-)

The anode reaction is:
2Pb + 6OH^- = 2PbO2H^- + 2H2O + e^-

The flow of electrons is proportional to the concentration of O2 present.

Question 10

What are the drawbacks of a fuel cell?

Answer 10

• system is temperatire and acid sensitive
• can take a while to recover after high concentrations of O2 exposure (O2 shock)
• limited life span
• can be made relatively cheap

Question 11

What is used to measure CO2 in solution?

Answer 11

The Stow-Severinghaus-type sensor.

Question 12

How does the Stow-Severinhaus sensor work?

Answer 12

Has a glass pH electrode to measure partial pressure of CO2 (over range of 1.3-12 kPa, this relationship is linear).

Blood is separated from a buffer by a Teflon membrane - CO2 can diffuse freely into the buffer (usually hydrogencarbonate) with NaCl and AgCl. This is in contact with the H+ sensitive glass.

Inside the glass electrode are KCl and a calomel (Hg/HgCl2) reference electrode. A further Ag/AgCl electrode is in contact with the bufer to complete the circuit.

Question 13

What methods can be used to measure gases? Which method is most used in theatre?

Answer 13

• photoacoustic spectroscopy
• silicone rubber and piezoelectric absorption
• refractometry
• Raman scattering
• mass spectrometry
• infrared absoprtion *in theatre most side sampling benches use this*

Question 14

How does infrared absorption spectroscopy work?

Answer 14

Molecules absorb infrared radiation and convert this energy into molecular vibration. The frequency of the vibration depends on molecular mass and atomic bonding within the molecule.

Most molecules will absorb infrared at specific wavelengths so the molecule can be identified and it’s concentration measured.

Absorption is based on the Beer-Lambert Law. So there’s a logarithmic dependence between the transmission of light through a substance and the concentration of that substance.

Question 15

What is photoacoustic spectroscopy?

Answer 15

Similar to infrared absorption spectroscopy but if the initial radiation is pulsed, the subsequent vibration pulse can be detected using a microphone and then amplified

Question 16

What molecules will interfere with absorption peaks when trying to measure volatile agents?

Answer 16

• CO2
• N2O
• alcohol
• water vapour
• CO

These will all absorb infrared between 3 and 12 micro metres

Question 17

What is collision broadening?

Answer 17

CO2 and N2O will broaden each other’s peaks and those of volatile agents.

This is where the energy absorbed by one molecule is transferred to another, allowing further radiation energy to be taken up by the 1st molecule

Question 18

What is an in-line or mainstream infrared spectrometer?

Answer 18

These are small enough to sit in the patient’s ventilator circuit. They shine infrared light at a specific wavelength through the plastic housing to a photo-detector. They’re only used for CO2 at present.

The do add bulk to the patient end of the circuit but are portable, don’t require any gas to be taken from the circuit and cheap.

Question 19

What is a sidestream infrared spectrometer?

Answer 19

• Commonly used in theatre
• require sampling flow rate of 200ml/min which can be returned to circuit
• water vapour entering the analysis chamber has to be prevented
• lag time for sample to reach the analyser - 2.5s for 3m of tubing
• the position matters, especially in small tidal volumes such as neonates

Question 20

What is refractometry?

Answer 20

• shine beams of a monochromatic light source through a gaseous medium and focus them on a screen
• light and dark bands will appear
• the nature of the bands depends on the light waves arriving in or out of phase with each other - which depends on the gas’s refractive index and concentration
• the Rayleigh refractometer is calibrated for each particular gas
• difficult to use for breath-to-breath analysis
• can be used to calibrate vaporizer output

Question 21

What is piezoelectric absorption?

Answer 21

• piezoelectric compound such as quartz can be made to resonate at a particular frequency
• in the Engstrom Emma analyser, 2 quartz crystals are mounted between electrodes (only one covered in silicone that absorbs anaesthetic vapours - the other is the reference one)
• the oil absorbs the anaesthetic and changes the resonant frequency in proportion to the concentration of vapour present
• N2O is minimally absorbed
• response time is fast but individual vapours can’t be differentiated by the machine

Question 22

What is Raman scattering?

Answer 22

• when light meets an object it’s scattered, usually elastically, with no change in energy state (Rayleigh scattering - responsible for blue sky)
• some of the light’s energy is absorbed and a transformation shift occurs, either by absorption of the energy or release of energy as a photon with a different wavelength
• this is inelastic raman scattering
  
  • laser beam is concentrated in the analysis chamber, the light is passed through to a photo detector and signal processing unit which only allows photons on interest through

Question 23

What are the adv/disadvantages of Raman scattering?

Answer 23

• more expensive than infrared
• fast response times
• no degradation of the molecule under examination
• ability for multi-gas analysis
• greater accuracy than infrared spectroscopy (close to mass spectrometrys accuracy)

Question 24

How does mass spectrometry work?

Answer 24

Sample gas drawn into low pressure chamber then into a another chamber with pressure nearing that of a vacuum - maintained by pumps. A molecular leak pathway is constructed between the 2 chambers.

In the vacuum chamber the molecules are ionized, losing an electron. The resulting ions are accellerated by the cathode to the other side of the chamber where either magnets or electromagnets allow separation by the ions mass and charge. The photo-voltatic receptors detect the ions.

Question 25

What happens in the quadrapole spectrometer?

Answer 25

The magnetic field is a mix of DC field and AC radiofrequency field. If the AC component’s freq and cathode acceleration are altered, only ions of interest will be detected - the others trapped in the magnet.

Question 26

What are the advantages of mass spectrometry?

Answer 26

• very accurate
• require tiny amounts of sample
• able to distinguish between different compounds by looking at both parent compound and predictable subsidiary peaks
  
  • the subsidiary peaks are formed by degradation of the compound - eg N2O becomes NO, O2, N2, N and O

Question 27

What are the disadvantages of mass spectrometry?

Answer 27

• requires powerful vacuum pumps
• sample can’t be returned to patient due to ionic degradation
• response and delay times long
• not cost effective for theatre use