Arterial Blood Gases Flashcards
How many electrodes does blood gas analyser have? And what are they called?
3 electrodes:
1. PaCO2 - severinghaus carbon dioxide electrode.
- PaO2 - Clark electrode
- pH - pH electrode
Several other electrode to measure metabolites and electrolytes.
How is temperature standardised?
On aspiration all blood samples heated to 37 degrees before analysis.
What is polarography?
It is based upon that oxygen will react with a cathode (negatively charged metal), and this allows measurement of PaO2 from an ABG sample.
What are the components of a polarographic oxygen electrode?
- Platinum cathode
- Silver/ silver chloride anode
- Sodium chloride solution
- Oxygen permeable Teflon surround solution, separating it from the ABG sample.
- Power source or 0.7V (polarisation source)
How does the oxygen electrode work?
At the anode an electron is donated by: Ag + Cl- = AgCl + e-
At the cathode electrons are accepted:
O2 + 2H2O + 4e- = 4OH- (hydroxyl ions)
To complete the e- flow, in the electrolyte solution: NaCl + OH- = NaOH + Cl -
A small electric current flows when the polarisation potential is applied across the electrode which can only happen in the presence of oxygen. And the amount of current is linearly proportional to the partial pressure of oxygen in the sample at 37 degrees.
What is a co-oximeter?
A spectrometer that uses four different wavelengths to measure total Hb, oxyhaemoglobin, carboxyhaemoglobin, and methaemoglobin.
How does pH change with temperature?
With every degree Celsius in temperature rise the pH falls by 0.015
How is pH measured in ABG?
Two electrodes:
1. pH electrode - a glass electrode (Ag/ AgCl) incorporating a bulb made of pH sensitive glass holding a buffer solution at constant pH.
- Reference electrode - calomel in contact with potassium chloride solution via a cotton plug.
Arterial blood in contacts with KCL solution.
Potential difference across both electrodes measured and displayed
What electrical output is produced by pH electrode?
60 Mac of linear electrical output..
How is PaCO2 measured?
Carbon dioxide is allowed to diffuse through a permeable membrane into an electrolyte solution and reacts with water to produce H+ ions. Then this change in pH is measured by the glass electrode.
The number of H+ ions produced is proportional to partial pressure of carbon dioxide. (0.01 pH units for every 0.1kPa)
