Arterial blood gas instrumentation

What parameters are measured during blood gas analysis?

• pH

• pCO2

• pO2

• O2 sat

• HCO3

• BE

What parameters are measures via electrode?

• pH

• pCO2

• pO2

What parameters are calculated on ABG?

• O2 sat

• HCO3

• BE 

What is potentiometry?

The measure in the change in potential between a measuring an reference electrode

What is the application of potentiometry?

Ion selective electrodes “select” the ion measured by an ion selective surface designed to capture the “unknown” ion.

Are ISEs specific?

No they are selective

How does an ISE work?

Unknown sample comes in contact with the measuring electrode and changes the ionic concentration at the electrode surface thus also changing the potential at the surface of the electrode. This change in potential can be detected by a voltmeter and is proportional to the concentration of the ion in the unknown sample 

What are the components of the pH ISE?

• reference electrode

• measuring or glass electrode

What is the pH reference electrode made of?

A potentiometric electrochemical cell with a constant voltage

• ABG instruments: Ag/AgCl2 

• Calomel electrode: Hg/HgCl2

What is the glass electrode in the pH electrode?

In a potentiometric electrochemical cell, the cell with the variable voltage

When is a salt bridge required?

When there are 2 separate half cells

What does a salt bridge contain?

Concentrated potassium or sodium chloride solution

What is the purpose of a salt bridge?

Connects the 2 half cells, current does not move through the bridge but the salt keeps the charge balanced between the two half cells  

In the blood gas instrument the pH electrode is?

Both the measuring and reference electrodes are combined

In a potassium ISE what is used?

Valinomycin

In a sodium ISE what is used?

Lithium aluminum silicate

In a calcium ISE what is used?

Solid state poly vinyl chloride (PVC) polymer matrix membrane

What are the components of a modified pCO2 electrode?

• modified pH electrode

• Interior pH electrode pair

• Outer jacket: weak sodium bicarbonate buffer solution

• CO2 enters the weak buffer and is converted to H+

What is the most common configuration of pO2 electrodes?

Clark electrode

What does the Clark electrode use?

The principles of polarography or current flow between cathode and anode during an oxidation-reduction reaction when O2 is introduced into the system to measure O2 concentration in the blood

Cathodes?

Reduction half reaction (gain of electrons)

Anode?

Oxidation half reaction (loss of electrons)

What voltage is maintained across the anode and cathode in the Clark electrode?

0.6 V

What is the driving force that pushed electrons from the Ag/AgCl2 anode towards the cathode platinum wire when O2 is introduced in the Clark electrode?

Biasing voltage

What occurs at the cathode in the Clark electrode?

Oxygen diffuses from the sample into the electrode and reduction occurs at the cathode 

What happens as electrons migrate to the cathode?

Current flows

What is the amount of current flow proportional to?

O2 concentration

What is a coulometry/amperometry electrode used for?

Chloride

What is the principle of coulometry?

The measurement of the amount of electricity (in coulombs) passing between two electrodes per unit of time where titration is used to determine the amount of unknown

How is the titrant generated?

electrochemically 

What is the principle of amperometry?

Measures current flow produced by an oxidation-reduction reaction

What is the analyte added to in amperometry?

A dilute electrolyte solution of H2SO4

What is added to the titration vessel?

Cl

What does the Cl combine with in the generator pair of electrodes?

Combines with free Ag from the anode of the generator pair in the titration vessel 

What reaction occurs at the anode in the generator pair?

Ag+ + Cl - → AgCl2

What reaction occurs at the cathode in the generator pair?

2e- + 2H+ → H2 gas 

What happens with the indicator pair of electrodes?

When all Cl- is titrated, excess Ag is detected by the indicator pair shutting of a timer

• Ag+ +e- → Ag0

What is the concentration of chloride proportional too?

The amount of time the titration takes

What is the equation for Cl concentration?

Cl = [time for titration _unk']/[time for titration _std] X standard concentration 

What are the precision buffers used in ABG measurments?

• 6.838

• 7.384

What is used to calibrate an arterial blood gas instrument?

Gas and precision buffers

What are the two levels of gases?

• O2: slope gas = 0% Cal gas=12%

• CO2: slope gas = 5%  Cal gas =10%

How is the pH calibrated on the ABG?

Via an electronic curve set up with the two levels of precision buffers

How is the gas calibration set up on the ABG?

Electronic curve set up with two level of gases for pO2 and pCO2

What is the formula for the pp of gas?

pp of gas = (BP mmHg - pWV at 37 degrees)/100% x%gas

Was is the gas calibration?

% gas converted by partial pressure of gas

What is the partial pressure of water vapor at 37 degrees?

47 mm Hg

What is the pO2 equal to?

O2 dissolved in plasma

What is the oxygen saturation equal to?

% saturation of the hemoglobin with oxygen 

What happens to O2 sat as pO2 increases?

It increases

What is the reference range for pO2?

80-110 mm Hg

What is the reference range for SO2?

>95%

What is the relationship between pO2 and O2 sat?

Non-linear

What is the saturation of normal hgb at a pO2 of approximately 27 mm Hg?

50%

What is the pO2 and O2 sat in normal venous blood?

• pO2: 40 mm Hg

• O2 sat: 75%

What is the pO2 and O2 sat in normal arterial blood?

• pO2: 97 mm Hg

• O2 sat: 97% (sea level) 

What is the oxygen dissociation curve?

A measure of the affinity of hemoglobin for oxygen

What does a high p50 mean?

Means a low affinity of the hgb for oxygen (right shift)

What does a low p50 mean?

Means a high affinity of the hgb for oxygen (left shift)

A shift to the right means?

Decreased hgb affinity for oxygen and a lower oxygen content at any given pO2

What are some causes of a right shift?

• temperature increases (hyperthermia =fever)

• pH decrease (acidosis)

• increase in 2,3 DPG

• increase in pCO2

A shift to the left means?

Increased hgb affinity for oxygen at a higher oxygen content at any given pO2

What are some cause of left shifts?

• temperature decreases (hypothermia)

• pH increase (alkalosis)

• decrease in 2,3 DPG

• decrease in pCO2

What is the oxygen status of the patient affected by?

• respiration

• cardiac

• anaerobic conditions

Why must temperature corrections be performed to give correct ABG measurements on some patients?

Some patients have decreased body temperatures like those on heart lung machines so pH must be corrected

What happens to the pH as temperature goes down?

It increases

What are the corrections to pH for temperature?

• >37 C: subtract 0.015/each degree

• <37 C: add 0.015/ each degree 

What is the base excess related to?

Total buffer capacity of the body

What does the base excess correlate to?

HCO3 concentration

What is the base excess impacted by?

Protein concentration

What is the reference range for BE?

-2.5 to +2.5

What do ABG reports sometimes include?

TCO2 = pCO2 + HCO3 + CO2 ion complexes + H2CO3 

What is most CO2 content in the body in the form of?

HCO3

What is TCO2 approximately equal to?

HCO3

What is true of point of care ABG instruments?

• use electrode in a miniaturized hand-held instrument

• test methods come in self-contained cartridges

• expensive but convenient