Blood gas machine & gas monitoring Flashcards

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1
Q

How is pH measured on a gas machine?

A
  • Using pH probe
  • Constructed with 2 electrodes (mercury reference electrode & silver sensing electrode)
  • Mercury in contact with blood
  • Silver at constant pH with KCl buffer
  • Potential difference measured by sensitive glass bulb at silver
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2
Q

How is paO2 measured on a gas machine?

A
  • Clark electrode made of platinum cathode & silver anode
  • Form circuit via electrolyte solution
  • Powered by battery
  • e- formed at anode
  • React with O2 + H2O at cathode
  • Produces hydroxyl ions & generates current
  • Current measurement = paO2
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3
Q

How is paCO2 measured on a gas machine?

A
  • Severinghaus electrode (similar to pH electrode)
  • Blood separated from electrode by NaHCO3 semi-permeable membrane
  • CO2 diffuses across membrane
  • Reacts with H2O on other side creating H+ ions
  • Change in pH measured by electrode
  • PaCO2 measured due to linear relationship between PCO2 & pH
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4
Q

What is the Henderson Hasselbach equation?

A

CO2 + H2O = H2CO3 = H+ +HCO3-

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5
Q

How is actual bicarb measured on a gas machine?

A
  • Using measured pH & PaCO2 & Hasselbach equation
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6
Q

Define standard bicarb

A

The plasma bicarb conc after the sample has been corrected to PaCO2 of 5 at 37 degrees

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7
Q

Why use a standarb bicarb instead of actual?

A

Removes any resp component of acid/alkalosis
Shows only metabolic component in any derangement

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8
Q

How is Hb measured on a gas machine?

A
  • Using co-oximeter (spectrophotometer)
  • Uses 4 wavelengths of electromagnetic radiation to measure
  • Works by knowing absorption characteristics of substances
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9
Q

What do the 4 wavelengths in Hb measuring monitor?

A
  • Total Hb
  • OxyHb
  • CarboxyHb
  • Methaemoglobin
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10
Q

How is BE calculated?

A
  • Siggaard- Andersen nomogram
  • Number of milimoles of acid required to titrate 1L blood to a pH 7.4 at 37 degrees with PCO2 5
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11
Q

What is standard BE?

A

BE value calculated for blood with a Hb = 5

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12
Q

How does excess Heparin effect an ABG?

A
  • Heparin is acidic
  • Causes low PaCO2, pH, HCO3
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13
Q

How does a delay in analysis effect an ABG?

A
  • Continued metabolic activity of erythrocytes
  • Low pH, paO2
  • Increased CO2
  • HCO3 changes secondary to CO2 changes
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14
Q

How do air bubbles effect an ABG?

A
  • Increase O2
  • Decrease CO2 with resulting pH change
  • If PaO2 of sample >21 (air) then will cause decrease in measured paO2
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15
Q

How does temperature effect an ABG?

A
  • Due to increasing dissociation of H+ ions
  • pH decreases as temp rises
  • Solubility of all gases decreases with increasing temp
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16
Q

How does a fuel cell work?

A
  • Same principle as Clark electrode
  • Does not require battery
  • Anode = lead, cathode = gold
  • e- produced at anode & react with O2 at cathode
17
Q

How does a paramagnetic analyzer work?

A
  • Principle that O2 is attracted towards magnetic field
  • Measures pressure differential between stream of reference gas & sample gas when exposed to alternating magnetic field
  • Pressure transducer used to convert pressure difference into electrical signal to give paO2
18
Q

How does mass spectrometry work?

A
  • Molecules of sample gas enter ionization chamber
  • Bombarded with e- moving from cathode to anode
  • Forms charged fragments
  • These accelerated & focused on to detector using rods/magnetic field
  • By varying current different fragments can be analysed
19
Q

How does gas chromatography work?

A
  • Stationary phase: Column of tiny particles coated in silicone oil
  • Mobile phase: Carrier gas (He) sample is injected into
  • Separates gas sample into constituent components
  • Separation dependent on their differential solubility in the 2 phases
  • Detector records the Conc of the components
20
Q

What are the 3 main detector types in gas chromatography?

A
  • Flame ionization: Current produced by organic vapours ionized in a flame
  • Thermal conductivity: Changes in resistance of a heated wire in gas flow (useful for N2O/O2)
  • Electron capture: Halogenated compounds reduce e- flow produced by radioactive cathode altering current in proportion to conc
21
Q

How does Raman spectrometry work?

A
  • Pass laser beam through sample
  • Process frequency of resultant scattered radiation
22
Q

How does IR absorption work?

A
  • Gases with 2+ atoms with absorb IR at characteristic frequencies
  • ## CO2, H2O, NO, Volaties all absorb IR
23
Q

Which gases do not absorb IR?

A

O2
He
N2

24
Q

What is collision broadening?

A

Energy absorbed by CO2 from IR radiation transferred to N2O.
Allows CO2 to absorb more IR leading to falsely high reading

25
Q

What can UV analyzers measure?

A

Halothane only
Work similar to UV analyzers

26
Q

How do piezoelectric crystals work?

A
  • Only measure gas/vapour that is soluble in oil
  • Vibrate at specific resonant frequency when current applied
  • Once vapour dissolved frequency shifts in proportion to conc present
27
Q
A