Physics in anesthesia ,Gas measurement, O2,Co2,& ph

1. MEASURMENT OF O2,CO,PH & ANESTHETIC GASES
DR.IBRAHIM HASSAAN,MSC,EDAIC, FRCA (PRIMARY-PART1)

2. Oxygen measurement
specific
Clark polarographic electrode
Galvanic fuel cell
Paramagnetic O2 analyser
Non-specific
Mass spectrometer
Raman spectroscope
Oximetry
Pulse oximeter
Co-oximeter
Cerebral oximeter
Transcutanus O2 electrode

3. Clark polarographic electrode
A platinum cathode and a silver anode in potassium chloride (KCl) solutionsolution .
Voltage of 0.6V is applied across the electrodes .
Anode reaction: electrons (e–) are generated by the reaction of Ag+ with the Cl– ions from the KCl
solution.
Cathode reaction: O2 combines with e– and water to generate hydroxyl (OH–) ions
The greater the amount of O2 available, the greater the rate of electron uptake at the cathode and hence
the greater the flow of current.
Flow of current is therefore proportional to the O2 tension at the cathode.
 O2 + 4e– + 2H2O → 4OH

4. Problems in practice
They give only one reading, which is the average of inspiratory and expiratory concentrations.
Their life expectancy is limited (about 3 years) because of the deterioration of the membrane.
Halothane may cause falsely high O2 readings but this problem is overcome by the use of a
halothane-impermeable membrane.
The silver chloride anode will eventually be consumed
Calibration
By standard gas mixture

5. Galvanic fuel cell
1.It generates a current proportional to the partial pressure of oxygen (so acting as a battery requiring oxygen for
the current to flow). It does not require an external power source.
2. It consists of a gold cathode and a lead anode in a potassium chloride electrolyte solution. An oxygen-
permeable membrane separates the cell from the gases in the breathing system.
3. The oxygen molecules diffuse through the membrane and electrolyte solution to the gold cathode generating
an electrical current proportional to the partial pressure of oxygen:
At the anode: electrons are generated from the reaction between OH– from KOH and the Pb anode.
Pb + 2(OH) → PbO + H2O + 2e–
At the cathode: O2 combines with electrons and water to generate hydroxyl ions .
O2 + 4e → 4(OH)–
4. It reads either the inspiratory or expiratory oxygen concentration. It is usually positioned at the common gas
outlet of the anaesthetic machine.
.

6. Problems in practice
It is depleted by continuous exposure to oxygen because of exhaustion of the cell, so
limiting its lifespan to about 1 year.
The fuel cell has a slow response time of about 20 seconds with an accuracy of ±3%
AND not suitable for breath-to-breath measurements.
The redox reaction at the cathode is temperature sensitive. Hence, temperature
compensation is achieved using a thermistor.
Gas mixtures containing N2O may damage the fuel cell.
Calibration
is achieved using 100% oxygen and room air (21% oxygen).

7. Paramagnetic oxygen analyser
O2 is a paramagnetic gas, which means that it is attracted towards a magnetic field because it has
unpaired electrons in its outer shell. Most other gases (e.g. N2) are diamagnetic and are repelled
from magnetic fields.
Analyser is composed of two nitrogen-filled glass spheres connected in a dumbell arrangement,
suspended from a filament within a gas-tight chamber. A mirror is attached to the dumbell.
Glass spheres are subjected to a non-uniform magnetic field. If O2 is added to the chamber, it is
attracted towards the magnetic field, causing rotation of the glass spheres.
The degree of rotation of the glass spheres can be measured using a simple light beam deflection
principle. A beam of light passing to the mirror gets deflected as the mirror rotates.
This deflected beam is sensed by a photodetector, which is calibrated to match the degree of
rotation of the system to the oxygen concentration within the chamber.

8. Adventges:
Accurate
Sensitive
faster response time (Newer versions use the null deflection )
Instead of the glass spheres rotating, a current is supplied to oppose the movement of the spheres.
The amount of current required to keep the spheres in their resting position is calibrated to O2 concentration.
allow breath to breath O₂ analyzer.
inspired & Expired O₂ Can be measured.
No regular Calibration is required
Disadvantages
- affected by N20 & water vapor So put Silica gel in analysis cell

9. Mass spectrometer
 Mass spectrometer creates positive ions (positively charged particles) from gas molecules by
bombarding them with high speed electrons, knocking out electrons from the outer shells of
atoms within the molecules.
 The ions are then accelerated by passing them through an electric field
 then deflected using a strong, fixed magnetic field.
 Heavier ions are deflected less than lighter ions, so each type of ion is separated according to its
charge to mass ratio.
 Compounds of identical molecular weight (MW) are distinguished by identifying their
breakdown products, e.g. N2O and CO2 both have MW 44, so N2O is identified from its smaller
nitric oxide fragment (MW 30).
Advantages & dis advantages
 A rapid response times of less than 0.1 s means mass spectrometry can be used for continuous
gas analysis.
 It can measure a variety of gases within a mixture.
 Water vapour can interfere with the apparatus.
 It is a very bulky and an expensive piece of equipment.

11. Pulse oximetry
a non-invasive measurement of the arterial blood oxygen saturation at the level of the arterioles
Components
A probe is positioned on the finger, toe, ear lobe or nose . Two light-emitting diodes (LEDs)
produce beams at red 660 nm and infrared frequencies 940 nm, on one side and there is a
sensitive photodetector on the other side. The LEDs operate in sequence at a rate of about 30
times/s
The case houses the microprocessor. There is a display of the oxygen saturation, pulse rate and
a plethysmographic waveform of the pulse. Alarm limits can be set for a low saturation value
and for both high and low pulse rates.

12. Mechanism of action
The oxygen saturation is estimated by measuring the transmission of light, through a pulsatile
vascular tissue bed (e.g. finger).
This is based on
Beer’s law
the relation between the light absorbed and the concentration of solute in the solution
Lambert’s law
relation between absorption of light and the thickness of the absorbing layer).

13. The pulsatile vessels, therefore, cause two waveforms to be produced by the sensor.
Oxyhaemoglobin absorbs more infrared light (940 nm) and allows more red light (660 nm) to pass through.
 Deoxyhaemoglobin absorbs more red light (660 nm) and allows more infrared light (940 nm) to pass through.

14. a graph comparing the absorbance of light by oxyhaemoglobin with
deoxyhaemoglobin

15. Problems in practice and safety features
1. It is accurate (±2%) in the 70–100% range. Below the saturation of 70%, readings are extrapolated.
2. The absolute measurement of oxygen saturation may vary from one probe to another but with accurate
trends. This is due to the variability of the centre wavelength of the LEDs.
3. Carbon monoxide poisoning (including smoking), coloured nail varnish, intravenous injections of certain
dyes (e.g. methylene blue, indocyanine green) and drugs responsible for the production of
methaemoglobinaemia are all sources of error

17. Co-oximeter
a device that uses spectrophotometry to measure relative blood concentrations of
oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced haemoglobin
METHOD OF USE (old version)
a laboratory test involving a blood sample heated to 37 C and subjected to light of various
length and assesses absorption spectra.
does not require pulsatile flow
measures MetHb, COHb and other forms of Hb
uses many other wavelengths

18. New version
The pulse co-Oximeter continuously and noninvasively measures the oxygen sauration of
arterial hemoglobin (Sp0,l, carboxyhemoglobin saturation (SpcO, and methemoglobin
saturation (SpMel at a peripheral site, such as the foot, toe, or finger)

19. Cerebral Oximeter (Near-Infrared Spectroscopy)
It monitors regional 02 saturation of Hb in the brain .
A sensor is placed over the forehead and emits two lights of specific wave-lengths of near-infrared
spectrum (730 and 810 nm).
It measures light reflected back to the sensor (near-infrared spectroscopy) (i.e., reflection spectroscopy).
Cerebral oximetry measures venous and capillary blood 02 saturation in addition to arterial blood 02
saturation (unlike pulse oximetry).
Normal cerebral rS02 values range from 55-75%.
Values < 50% for long periods of time and below 40% for short periods of time or change of more than
20% from baseline, are associated with an increased incidence of neurological complications.
Uses:
Dramatic decrease in rS02 occurs in: - cardiac arrest, - cerebral embolization,- deep hypothermia, and -
severe hypoxemia.

20. Transcutaneous oxygen electrode
Enables a continuous non-invasive measurement of P02.
A modified Clark 02 electrode with platinum cathode, silver anode, and electrolyte (retained by a membrane) is heated
by a heater (ideally to z 43°C) and then placed on the surface of the skin as a surface electrode
The skin temperature is measured by a thermistor, and this is used to control the heater power.
Advantages
Transcutaneous 02 electrode is superior to pulse oximetry in patients with carbon monoxide poisoning because it
continues to measure 02 tension accurately.
Disadvantages
1- It needs a warm-up time of 10-15 minutes.
2- It has a slow response time
3- It needs regular calibration before each application to the skin.
4- There is a risk of skin burns especially in neonates (particularly at a temperature of 45 °C for> 4 hours).

21. Co2 measurement
Indirect methods
Infrared analyzer
Gas chromatograph
Mass spectrometry
Colorimetric
Direct methods
the Severinghaus carbon dioxide electrode.

22. Technical terms used in measuring end-tidal CO2
 Capnogram is the graphical plot of CO2 partial pressure (or percentage) versus time.
Capnometer is the device which only shows numerical concentration of CO2 without a waveform.

23. Infrared analyzer
Infrared absorption: this is the main method of measuring CO2 in the operating theatre.
Diatomic gas molecules (i.e. containing two or more different atoms) absorb infrared radiation.
This applies to CO2, as well as to N2O and to all other inhalational agents.
Each diatomic gas absorbs radiation of a particular wavelength. By measuring the proportion of infrared
radiation absorbed by a gas mixture, the partial pressure of a diatomic gas can be inferred.
The system comprises an infrared source, a filter to ensure that only radiation of the desired wavelength
is transmitted, a crystal window (glass absorbs infrared), a sample chamber and a photodetector.

24. The fraction of radiation absorbed is compared with a reference gas (so regular calibration against
zero and known CO2 concentrations is essential) before the value is displayed.
The infrared wavelength absorbed varies with the gas, thereby allowing its identification.
For CO2, this absorption is maximal at 4.28 μm. There is some overlap between CO2 and N2O 4.5μ
(collision broadening cause falsely elevate the CO2 readings).
A water vapour trap is required (water has high infrared absorbance).

25. Problems in practice and safety features
Dilution of the end-tidal carbon dioxide can occur whenever there are loose connections and
system leaks.
Collision broadening or pressure broadening is a cause of error. The absorption of carbon
dioxide is increased because of the presence of nitrous oxide or nitrogen. Calibration with a gas
mixture that contains the same background gases as the sample solves this problem.

26. SIDE-STREAM ANALYSERS
This consists of a 1.2-mm internal diameter tube that samples the gases (both inspired and exhaled)
at a constant rate (e.g. 150–200 mL/min).
The tube is connected to a lightweight adapter near the patient’s end of the breathing system (with
a pneumotachograph for spirometry) with a small increase in the dead space. It delivers the gases to
the sample chamber.
A variable time delay before the sample is presented to the sample chamber is expected.
Disadvantages:
There is a transit time
The narrow tube is easily obstructed by water vapor.
Calibration is needed using a source of known CO2 concentration

27. MAIN-STREAM ANALYSER
The sample chamber is positioned within the patient’s gas stream, increasing the dead space. In order to
prevent water vapour condensation on its windows, it is heated to about 41°C.
no transport time delay in gas delivery to the sample chamber.
Disadvantages:
It is bulky, so it may cause traction on the endotracheal tube.
It needs sterilization between cases.
It produces radiant heat, so skin burns may occur (new designs avoid this).

29. Diagram of an end-tidal carbon dioxide waveform.
I = inspiration; E = expiration;
A–B represents the emptying of the upper dead space of the airways. As this has not undergone gas
exchange, the CO2 concentration is zero.
B–C represents the gas mixture from the upper airways and the CO2-rich alveolar gas. The
CO2 concentration rises continuously.
C–D represents the alveolar gas and is described as the ‘alveolar plateau’. The curve rises very slowly.
D is the end-tidal CO2 partial pressure where the highest possible concentration of exhaled CO2 is
achieved at the end of expiration. It represents the final portion of gas which was involved in the gas
exchange in the alveoli.
D–A represents inspiration where the fresh gas contains no CO2.

30. ETCO2,Alveolar CO2,& Ar co2
The end-tidal CO2 is less than alveolar CO2 because the end-tidal CO2 is always diluted with
alveolar dead space gas from un perfuse alveoli. These alveoli do not take part in gas exchange
and so contain no CO2.
Alveolar CO2 is less than arterial CO2 as the blood from unventilated alveoli and lung
parenchyma (both have higher CO2 contents) mixes with the blood from ventilated alveoli. In
healthy

32. Transcutaneous Severinghaus electrodes
measure carbon dioxide concentration in vivo.
The Severinghaus electrode must be modified
slightly for it to function as transcutaneous
electrode.
These modifications include the addition of a
heating element and a thermistor to the electrode,
so that the skin can be heated to a temperature of
between 40and 42°C.
This increases capillary blood flow, carbon dioxide
production and carbon dioxide solubility. It is worth
remembering that the transcutaneous carbon
dioxide is usually higher than arterial carbon
dioxide.

33. Transcutaneous Severinghaus electrodes
Advantage
allows continuous measurement of carbon dioxide concentration. there
sponse time is slow
correlation between transcutaneous and arterial concentration is variable.
Disadvantage
carry a risk of skin burns

34. PH measurement
pH stands for ‘power of hydrogen’.
It is a measure of the hydrogen ion activity in an aqueous solution.
pH = negative log to the base 10 of the hydrogen ion concentration
[H+]
For each 1 unit change in pH there is a 10-fold change in [H+].

35. [H+] measured BY pH electrode.
Reference electrode: mercury/mercury chloride electrode within a potassium chloride solution. This
solution is saturated and acts as a salt bridge to complete the circuit between the sample and the
electrode.
The KCl solution is prevented from diffusing into the sample by a porous membrane that is permeable
to only H+ ions.
pH electrode: silver/silver chloride electrode within a buffer solution of hydrochloric acid. The tip of
the electrode is composed of pH-sensitive glass.
The temperature of the system is maintained at 37 °C because dissociation of acids and bases
increases with increasing temperature.

36. [H+] is held constant around the pH electrodes by the buffer solution and so any potential
difference across this electrode is due to [H+] within the blood sample.
Potential difference between the two electrodes is measured and converted to a direct reading
of pH or [H+].
Potential output is linear (60 mV per unit pH).

37. The sources of error in this measuring system
Calibration errors
Drift of the measuring system
Membrane damage resulting in electrode contamination
Temperature – hypothermia increases CO2 solubility resulting in reduced PaCO2 and increased
pH
Sampling errors
Effect of over-heparinisation – acidic heparin lowers pH.