ABG Interpretation

Author: Michelle Lin, MD
Updated: 4/2/2010

ABG Interpretation

Rules of thumb

Look at pH: Whichever side of 7.4 the pH is on, the process that caused it to shift to that side is the primary abnormality

Calculate the anion gap. If AG ≥ 20, there is a primary metabolic acidosis regardless of the pH or serum bicarbonate concentration

Calculate the excess anion gap = total anion gap - 12 + measured bicarb

  • If sum is > 30, then metabolic alkalosis
  • If sum is < 23, then nongap metabolic acidosis

Respiratory compensation occurs almost immediately in response to metabolic disorders.

Metabolic compensation occurs over 3-5 days in response to respiratory disorders.

Primary respiratory alkalosis

eg. 7.50/ 29 / 80 / 22 (pH / PaCo2 / PaO2 / [HCO3-])

Differential

  • Anxiety
  • Lung disease
  • CNS disease
  • Mechanical ventilation
  • Drug use (aspirin, catecholamines, progesterone)
  • Pregnancy
  • Hepatic encephalopathy
  • Sepsis
  • Hypoxia

Primary respiratory acidosis

eg. 7.25 / 60 / 80 / 26 = ACUTE because no bicarbonate compensation

eg. 7.34 / 60 / 80 / 31 = CHRONIC because bicarbonate conensation

Differential

  • Acute airway obstruction
  • Severe pneumonia or pulm edema |
  • CNS depression
  • Thoracic cage injury-flail chest
  • Impaired lung motion (PTX)
  • Ventilator dysfunction
  • Neuromuscular disorder

Primary metabolic alkalosis

eg. 7.50 / 48 / 80 / 36

Differential

  • Urine Cl = low | Urine Cl = normal-high |
  • Diuretic use in past
  • Current diuretic use
  • NG suction
  • Excess alkali administration
  • Post-hypercapnia
  • Excess mineralocorticoid activity
  • Vomiting
  • Refeeding alkalosis

Primary metabolic acidosis

eg. 7.20 / 21 / 80 / 8

Nonanion gap: CAGE

  • Chloride (HCL) administration
  • Acetazolamide/Addisons
  • GI Loss
  • Extras – RTA, ingestion of oral acidifying salts, recovery phase of DKA

Anion gap: MUDPILES

  • Methanol
  • Uremia
  • DKA
  • Paraldehydes, paint sniffing
  • INH, iron, ibuprofen, inborn error of metabolism
  • Lactic acidosis
  • Ethanol, ethylene glycol
  • Salicylates

Pearl: If anion gap > 20 exists, there is a primary metabolic acidosis

Pearl: If anion gap exists, calculate the excess anion gap to determine if there is an underlying metabolic alkalosis or nongap metabolic acidosis exists.

Examples

7.4 / 40 / 80 / 24, Na 145, Cl 100 --> Metab acidosis + metab alkalosis

7.5 / 20 / 80 / 15, Na 145, Cl 100 --> Resp alkalosis + metab acidosis + metab alkalosis

7.1 / 50 / 80 / 15, Na 145, Cl 100 --> Resp acidosis + metab acidosis + metab alkalosis

7.15 / 15 / 80 / 5, Na 140, Cl 110 --> Metab acidosis, gap AND nongap

References

  • Haber RJ. A practical approach to acid-base disorders. West J Med 1991; 155:146-51. [Source]