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Arterial Blood Gases

From Mediwikis
Blood Gas Analysis machine

Taking an Arterial Sample

  1. Introduce yourself to the patient, explain the purpose of the test, and how it differs from a venous blood sample
  2. Wash your hands
  3. Equipment
    1. ABG Syringe (contains heparin to prevent clotting in the needle)
    2. Needle
    3. Disinfectant swab
    4. Gauze
    5. Gloves/ Apron
  4. Perform Allen's test- Check the collateral circulation of the hand, to ensure any accidental occlusion of the radial artery will not result in ischemia
    1. Apply pressure to both the ulnar and radial arteries for a few seconds. The hand may blanch during this.
    2. Release pressure from the radial artery; the hand should reperfuse.
  5. Assemble equipment- twist the needle onto the ABG syringe, and squirt all of the heparin out of the syringe through the needle.
  6. Locate the radial pulse at its strongest point
  7. Clean the area with an alcohol swab- do not repalpate!
  8. Hold the syringe like a pen
  9. Insert the needle with the bevel facing towards the patient's shoulder. Once you're in the artery, the syringe will flashback, and auto-fill.
  10. Finish off by removing the needle, attaching the bung to the syringe, labelling the sample, and moving it immediately to the lab to prevent a false low O2 value.

Interpretation

By using this systematic approach, you can diagnose most ABGs:

Examine the patient and predict the values of the ABG based on clinical findings

Types of values:

Check PaO2

This should be >10.6 kPa. The ABG should also say what percentage O2 the reading was made at. A good rule of thumb is that the PaO2 should be about 10kPa lower than the atmospheric PO2. Atmospheric pressure is ~100kPa so at ground level %O2 = kPa O2

Check pH

Davenport Fig 12.jpg

pH 7.35-7.45

pH < 7.35 = Acidosis

pH > 7.45 = Alkalosis

Check PaCO2

Davenport equation of Carboxylic acid, that causes acidity in the blood (H+ ions)

Normal- 4.7-6 kPa

  1. If CO2 changes explain pH change, condition is respiratory
    1. High CO2 = Acidosis
    2. Low CO2 = Alkalosis

Check HCO3

Normal- 22-26 mM

  1. If HCO3 explains pH change, condition is metabolic.
    1. High HCO3 = Alkalosis
    2. Low HCO3 = Acidosis

If both HCO3 and CO2 are off, the one that can be seen to be causing the pH change is the primary cause, and the other is compensating for it. Remember, the body never overcompensates; if the pH is 7.36, CO2 is up and HCO3 is up, you know it's a compensated respiratory acidosis and not a compensated metabolic alkalosis. Mixed disorders can also exist; for example, someone in sepsis from pneumonia might have both a respiratory acidosis (from respiratory failure) and a metabolic acidosis (from production of lactate).