ABG interpretation

By Dr Jessica

It is important to interpret arterial blood gases in a systematic manner, taken within the context of the patient’s clinical context.

Steps:

1. PaO₂ (partial pressure of oxygen in arterial blood)

2. pH

3. PaCO₂ (partial pressure of carbon dioxide in arterial blood)

4. Bicarbonate and base excess

1. Oxygenation: PaO₂

Is the patient hypoxaemic?

• PaO₂ = Partial pressure of oxygen in arterial blood

• Principally determined by inspired oxygen concentration and alveolar gas exchange

• >70mmHg is considered ‘safe’

• Always check A-a (alveolar-arterial gradient) for evidence of VQ mismatch, shunt or diffusion abnormality

2. pH

Does the pH show 1. Acidaemia, 2. Alkalaemia 3. Normal pH?

• Usually tightly maintained between 7.35-7.45

• <7.35 = acidaemia

• >7.45 = alkalaemia

3. Respiratory component PaCO2

Is the PaCO2 consistent with, or attempting compensate for the acid-base disturbance?

• Normal values: 35-45mmHg

• Raised PaCO₂ contributes toward an acidosis

• Lowered PaCO₂ contributes toward an alkalosis

4. Bicarbonate HCO_3-

Is the HCO3 or BE consistent with, or attempting compensate for the acid-base disturbance?

• • Normal values: HCO_3- 22-26mmol/L

  • Raised HCO₃ contributes toward an alkalaemia (consistent with positive BE)

  • Lowered HCO₃ contributes toward an acidaemia (consistent with negative BE)

4a) Base excess (BE)

amount of acid required to return to normal pH

• Normal values: +2 to -2mmol/L

• More negative BE indicates a metabolic acidosis

• More positive BE indicated metabolic alkalosis

*BE measures all bases, as opposed to solely bicarbonate. It will tell you if there is any metabolic component regardless of what the pH, compensation and other factors are doing.

Table of interpretation

Acidaemia (pH<7.35)
PaCO2	High			Normal	Low
HCO3-	Low	Normal	High	Low	Low
Diagnosis	Mixed respiratory and metabolic acidosis	Respiratory acidosis	Respiratory acidosis with metabolic compensation	Metabolic acidosis	Metabolic acidosis with respiratory compensation

Alkalaemia (pH>7.45)
PaCO2	Low			Normal	High
HCO3-	Low	Normal	High	High	High
Diagnosis	Respiratory alkalosis with metabolic compensation	Respiratory alkalosis	Mixed respiratory and metabolic alkalosis	Metabolic alkalosis	Metabolic alkalosis with respiratory compensation

Anion Gap

• Anion gap is used to differentiate between causes of metabolic acidosis: High anion gap versus normal anion gap metabolic acidosis.

• It is the difference between the charges of plasma anions and cations

• Normal is approx. 12 +/- 4

• Anion gap = (Na⁺) – [Cl^– + HCO₃^–]

• Eg: (140 ) – (100-24) = 16

Wide anion gap causes (DULE)

• Diabetic ketocacidosis

• Uraemia

• Lactic acidosis

• Ethanols (methanol, ethylene glycol)

Normal anion gap causes (CAGE)

• Chloride excess

• Addisons, azetazolamide

• GI causes – diarrhea/vomiting, fistulae (pancreatic, ureters, billary, small bowel, ileostomy)

• Extra – RTA

References

Cowley, N. J., et al. “Interpreting Arterial Blood Gas Results.” BMJ, vol. 346, no. jan16 1, 16 Jan. 2013, pp. f16–f16, www.bmj.com/content/346/bmj.f16, 10.1136/bmj.f16.

Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med. 2010 Apr;14(2):57-64. doi: 10.4103/0972-5229.68215. PMID: 20859488; PMCID: PMC2936733.