Arterial Blood Gas Instrumentation Flashcards
Parameters measured
pH
pCO2
pO2
O2 sat
HCO3-
BE
pH measured by
electrodes
pCO2 measured by
electrodes
pO2 measured by
electrodes
O2 Saturation Measured
Calculation
HCO3- Measured by
Calculated - ABG
Measured - electrodes - lytes
BE Measured by
Calculated
Electrodes Called
Potentiometry & Ion Selective Electrodes (ISE)
Potentiometry Measures
the change in potential between measuring and reference electrode
Application of Electrodes
ISEs select the ion measured by an ion selective surface designed to capture the “unknown” ion
ISEs are not specific but selective
Change Potential Detected
Voltmeter and is proportional to concentration of ion in the unknown sample
ISE Electrode: pH
Uses two probes
1) A calomel electrode: Hg/HgCl2 reference
2) ABG instruments: Ag/AgCl2 reference
Salt Bridge
1) Concentrated K+ or NaCl solution
ISE Electrode: pH on ABG
ISE measuring and reference electrodes are combined
- Ag/AgCl2 reference electrode
- AgCl covered silver wire
- Glass electrode internal solution
ISE Electrode: Measuring Other Ions
Common applications
- Potassium ISE
- Sodium ISE
- Calcium
Potassium ISE
Valinomycin (an antibiotic)
Sodium ISE
Lithium aluminum silicate
Calcium ISE
Solid state poly vinyl chloride (PVC) polymer matrix membrane
Modified ISE Electrode: pCO2
Modified pH electrode
Interior pH electrode pair
Outer jacket: weak Na+ and HCO3- solution
CO2 enters weak buffer and converted to H+
Polarographic Electrode: pO2
Clark Electrode - Most common
Principle of pO2 Electrode
Polarography flow between a cathode and anode during oxidation-reduction reaction O2
Cathode: Reduction half reaction (gain of electrons)
Anode: Oxidation half reaction (loss of electrons)
pO2 Electrode Equation
Use of 0.6 V to cause oxidation
Anode
2 Ag + 4 Cl- = 2 AgCl2 + 4 e-
Cathode
1/2 O2 + H20 + 2 e- = 2 OH-
Coulometry/Amperometry Electrode: Chloride
Measurement of Cl- ions for the sweat chloride test
Principle of Chloride Electrode Coulometry
Measurement of amount of electricity passing between two electrodes per unit of time and titration amount of unknown
- Titrant (Ag+) is electrochemically generated
Principle of Chloride Electrode Amperometry
Measures current flow produced by an oxidation-reduction reaction
- Analyte added to a dilute electrolyte of solution
- H2So4 = H+ + SO4-
Chloride generator pair
Cl- is added to titration vessel
Combines with free Ag which comes from anode
- Ag+ + Cl- = AgCl2
Cathode reaction occurs
- 2e- + 2H+ = H2 gas
Chloride indicator pair
Cl- is titrated, excess Ag is detected by indicator pair shutting off timer
- Ag+ + e- = Ag without charge
Calculation of Chloride Concentration
Cl = ((time of titration) unknown/(time for titration) std) x STD
std and STD = standard
ABG Measurement
Calibrated with gas and precision buffers
Buffers for pH in ABG
6.838
7.384
Levels of Gas for ABG
Two levels of gas
O2: Slope Gas = 0%, Cal Gas = 12%
CO2: Slope Gas = 5%, Cal Gas = 10%
Gas Calibration
% gas converted by partial pressure of gas
pp of Gas = (BP mmHg - pWV at 37 C)/100 x % gas
Partial Pressure of H2O Vapor
At 37 C = 47 mmHg
ABG Parameters for Oxygen
Oxygen dissolved in Plasma
Oxygen saturation (O2 Sat) = % saturation of Hgb with O2
O2 Saturation increases as pO2 increases
Oxygen Dissociation Curve
Non-linear relationship of pO2 to O2 Saturation
Explains O2 reserves in blood
Normal Hgb O2 Levels
50% saturated at plasma pO2 of 27 mmHg
Normal Venous Blood O2
Partial pressure O2 is 40 mmg Hg
O2 Saturation is 75%
Normal Arterial Blood O2
Partial Pressure O2 is 97 mm Hg
O2 Saturation is 97% (sea level)
Oxygen Dissociation Curve: p50
Measure of affinity of Hgb for O2
High p50 means low affinity of Hgb for O2 (right shift)
Low p50 means high affinity of Hgb for O2 (left shift)
O2 Dissociation Right Shift
Decrease Hgb affinity for O2 and lower O2 content
Curve is shifted to right and O2 affinity of Hgb is decreased
Causes of Right Shifts
Temperature increase (Hyperthermia = fever)
pH decrease (acidosis)
Increase in 2,3 DPG
Increase in pCO2
O2 Dissociation Left Shift
Increased Hgb affinity for O2 at higher O2 content
Curve is shifted to left and O2 affinity of Hgb is increased
Causes of Left Shifts
Temperature decrease (Hypothermia)
pH increase (alkalosis)
Decrease in 2,3 DPG
Decrease in pCO2
Oxygen Status of Patient Affected By
Respiration
Cardiac
Anaerobic condition
Problems with exposing Blood Gas to Air
Oxygen
pCO2
pH
Temperature Corrections
> 37 C - Substrate 0.015/each degree
<37 C - Add 0.015/each degree
Base Excess
Related to Total Buffer Capacity in Body
- Correlates strongly with HCO3 concentration
- Impacted by Proteins
Positive # - Excess Base Present
Negative # - Deficient in Base
