Hydrogen Ion and Carbon Dioxide Measurement Flashcards
Define pH
pH is a measure of the H ion activity in a liquid. H ion activity is not exactly the same as H ion concentration but, for practical purposes in the clinical situation, they may be regarded as equivalent.
pH = - log [H+]
Hence a pH of 9 = [H+] of 10^-9 mol.L^-1
and
pH of 7.4 is 10^-7.4 = 40 nmol/L (normal [H])) at 37 deg C
What is the relationship between pH and [H] over the middle of the clinical range
Practically linear, the pH decreasing by 0.1 for each 10 nmol/L. Thus [H] of 50 nmol/L corresponds to pH 7.3
How can {H} ion be measured in blood?
Using the H+ ion electrode. Most of these devices include a PCO2 and PO2 electrode too.
The H electrode is an example of an ion selective electrode and depends for its operation on the H ion sensitive glass at its tip. A potential develops across the glass which depends on the [H+] difference across it.
- [H] within electorde is maintained at constant value by buffer solution. So potential across glass tip is dependent on H+ gradient across glass tip
- To measure potential difference it is necessary to make electrical contact with the blood and buffer solution. A reference electrode is used which has a membrane at its tip to avoid contamination
How is the electrical circuit completed in a H ion electrode in order to get a current reading and hence an H ion concentration measurement
In both H ion and reference electrode, stability of the metal to solution contact is accomplished through a special electrode of silver in contact with its chloride which is in turn in contact with a solution containing Cl- ions.
The H+ electrode is connected to the reference electrode and the potential difference between these electrodes is measured representing the H ion concentration
How is the stability of the metal to solution contact in both the H+ electrode and Reference electrode achieved
Using a special electrode of silver in contact with its chloride, which is in turn in contact with a solution containing Cl- ions
Describe the mechanical layout of these electrode systems
Sample port containing blood which is in contact with: 1. Reference electrode
- H+ Electrode glass tip
- PO2 electrode
- PCO2 electrode
Which is then attached to a suction control mechanism downstream which moves the sample past the reference electrode and then these electrodes
There is a temperature control unit as with increasing temperature influences the dissociation of H+ ions in solution an will also effect measurements in the Clark Electrode and PCO2 electrode
How is error prevented with the H ion electrode
- Calibration with a fixed concentration of 2 phosphate buffers, for which the H ion concentration has been decided by international agreement
- Maintenance (no holes/protein deposits)
What is the Severinghaus CO2 electrode
- H ion sensitive glass barrier separating two electrodes
- Outer chamber filled with NaHCO3 and in contact with plastic membrane which is in contact with blood.
- CO2 diffuses into outer chamber and combines with water to favour the forward reaction for the CO2 equation. This produces H+ ions. The barrier between the outer and inner chamber only permits H ions.
- The change in H+ concentration is measured by the electrodes and displayed as the PCO@ tension
Why is calibration of PCO2 electrodes NB for dry gas mixtures vs fully humidified mixtures
Must correct for SVP of H2O which is 6.3 kPA. Subtract this from barometric pressure before multiplying by the partial pressure.
Why is the Severinghause electrode slower than the HHydrogen Ion electrode?
The Severing hause electrode response time is 2 - 3 minutes.
The H ion electrode response time is about 1 minute
The Severinghaus electrode takes longer as time is taken for CO2 to diffuse through the plastic membrane.
calculate the partial pressure of 5% CO2 at atmospheric pressure at temperature of 37 deg C in completely humidified gas mixture
5/100 x (100 - 6.3)kPa = 4.7 kPa
6.3 is the saturated vapour pressure of water
What is the principle by which infrared analysers can determine the partial pressure of a gas in a mixture
Gases that have two or more different atoms in the molecule absorb infrared radiation.
By measuring the fraction of radiation absorbed by the by the gas mixture, the partial pressure of a particular gas can be determined.
Each gas absorbs radiation at a characteristic wavelength.
The proportion of gas absorbed also depends on the wavelength
How is intereference from other gases reduced in an infrared analyser reduced
Using a specific wavelength which is characteristically absorbed by the gas of interest. E.g. CO2 at 4.28 nm (minimum intereference from N2O which absorbs 4.35 to 4.5 nm)
How is the specific wavelength of radiation emitted through the sample chamber of an infrared analyzer
Infrared radiation is emitted by a hot wire which passes through a filter which filters all but the wavelength of interest.
How are devices which measure the partial pressures of multiple gases configured
These incorporate variable filters for the various wavelengths of interest. The filters are mounted on a rotating disc (as in spectrophotometry) to permit simultaneous analysis of several sample compounds
What additional advantages does the rotating disc on which the multiple different filters are mounted, apart from that of the ability to measure multiple gases in a gas mixture
Alternating signal
- Alternating signal falls on the detector which is:
1. Less prone to drift
2. Easier to amplify
Versus a steady signal
Are the sample chamber windows made from glass?
No. Glass absorbs infrared radiation
Sapphire is transparent to infrared radiation (none absorbed)
What is a double beam instrument and why is it necessary?
Variations infrared output
Variations in sensitivity
Variations in transmission of the optical components
These variations will all affect the measurement of the specific gases in the sample
They are overcome with a double beam instrument.
Same radiation is passed through air (without any of the gases of interest). Any absorbance or variation that occurs here is subtracted from the detection in the measuring beam arm of the system hence correcting for all these possible variations
What is the collision broadening effect. Why is it relevant
The intermolecular forces vary as the distance between the individual molecules varies. As the molecules approach and recede from each other their energies change, and this affects the wavelength of radiation absorbed.
The interaction between different types of molecules (N2O and CO2) is different from the interaction between the same types of molecules (CO2 alone)
So automatic adjustments for the presence of N2O are present in modern devices
Describe the difference between side-stream and mainstream capnographs
Side-stream
- Sample drawn (150 ml/min)
- goes via fluid trap in sample line to
- infrared analszer
- Electronic processor –> display
- Pump with sample exhaust (back to circuit or to scavenging)
Mainstream
- Circuit adaptor containing infrared analyzer
- Circuit adaptor contains sapphire windows
- Cable connects to electronic processor
- Display
Compare pros and cons of side stream vs mainstream capnographs
Side stream
- less bulky but delay time of 1 second
Mainstream
- Bulky but no delay time
What is response time with regards to a capngraph
Response time needs to be rapid for accuracy
Response time is composed of:
1. Transit time (time for sample to be transferred to analyser)
- Rise time (time taken for rise between 10% and 90% of final value)
What does the rise time of the capnograph depend on
Size of the sample chamber and the gas flow:
Small chamber –> more rapid rise time (<400 ms)
Flow too low –> long rise time
Flow too high –> pressure drop across sampling line –> incorrect results
What is the problem with having an infrared analyzer measuring multiple gases, not just CO2 (i.e. volatile and N2O too)
REquires a longer infrared beam as more infrared will be absorbed
Option 1
Increase sample chamber size –> slow rise time –> slow response time –> reduced accuracy
Option 2
Angled mirrors used to flod the beam so that it passes a number of times through the sample –> same chamber size –> no change in rise time –> no change in response time. (Still larger chamber but better)
