Procedures

Reliable blood gas analysis is essential for assessing a patient’s respiratory and metabolic health

Analytical Principles

Blood gas measurements assess pH, oxygen (Pa\(O_2\)), and carbon dioxide (Pa\(CO_2\)) levels
- pH: Potentiometric measurement using a pH-sensitive electrode
- Pa\(O_2\): Amperometric measurement using a Clark electrode
- Pa\(CO_2\): Potentiometric measurement using a Severinghaus electrode
Bicarbonate (\(HCO_3^-\)) is calculated, not measured directly, using the Henderson-Hasselbalch equation
Some analyzers measure electrolytes (\(Na^+\), \(K^+\), \(Cl^-\), \(Ca^{2+}\)) using ion-selective electrodes

Patient Preparation
- Steady state: Ensure the patient’s condition and therapy have stabilized
- Supplemental oxygen: Document the patient’s oxygen level
Specimen Collection
- Arterial sample preferred: For accurate assessment of oxygen and acid-base status
- Site selection: Radial artery is common due to collateral circulation
- Technique:
  - Perform Modified Allen Test
  - Use correct needle angle
  - Prevent air bubbles
  - Proper mixing: Promptly mix specimen
- Syringe Type: Pre-heparinized syringe

Timing: Analyze promptly (within 15-30 minutes) or store on ice to slow metabolism
Storage: If delay is unavoidable, store on ice (analyze within 1-2 hours)
Air Exposure: Prevent air exposure
Clotting: Reject clotted samples

Unexpected Results
- Check patient preparation
- Review collection/processing
- Evaluate reagent quality
- Consider interferences
- Repeat the test
Out-of-Control QC
- Check QC materials
- Recalibrate the instrument
- Review reagents
Inconsistent Results
- Check electrode performance
- Perform instrument maintenance