EM · ABG interpretation (approach)
Arterial blood gas interpretation — the systematic emergency department approach
Also known as ABG interpretation · Blood gas analysis · Acid-base interpretation · Arterial blood gas
The systematic approach to the arterial blood gas for the emergency medicine trainee — the six values (pH, PaO2, PaCO2, HCO3, base excess, lactate), the respiratory-versus-metabolic rule (pH and PaCO2 move opposite in respiratory disorders, together in metabolic), the compensation formulas (Winter's, the respiratory rules), the anion gap Na minus Cl plus HCO3, the delta-delta for mixed disorders, and the venous-versus-arterial comparison. Includes the gas-driven drug doses — bicarbonate for severe acidaemia on BICAR-ICU terms, insulin 0.1 units per kg per hour for DKA, fomepizole for toxic alcohol. ACEM-primary, globally tagged.
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Target exams
Red flags
- A pH under 7.1 with haemodynamic instability is immediately life-threatening — treat the cause; reserve bicarbonate for severe acidaemia
- A lactate at or above 4 mmol per litre in the septic patient signals severe disease and a high mortality — start the sepsis bundle and resuscitate to clear it
- A high-anion-gap metabolic acidosis with an unexplained osmolar gap is a toxic alcohol until proven otherwise — start fomepizole before the level returns
- A respiratory acidosis with a rising PaCO2 and a falling pH is ventilatory failure — prepare for non-invasive ventilation or intubation
- A mixed disorder is easy to miss: always calculate the anion gap, the delta ratio, and the expected compensation before declaring a single primary disorder
Meet the patient
A 72-year-old with severe COPD arrives drowsy, with purulent sputum and a respiratory rate of 30. The blood gas comes back: pH 7.10, PaCO2 10.6 kPa, HCO3 18, lactate 5.2. The registrar calls it "a respiratory acidosis" and reaches for non-invasive ventilation.[3]
That label is half-right and it is half system exists to catch. Run the six steps and you find two disorders — a respiratory acidosis AND a high-anion-gap metabolic acidosis from the lactate — and the management of each is different. Pattern recognition misses the second one; a system does not. That is the whole point of this topic.[3]
References19ShowHide
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- [2]Adrogué HJ, Madias NE. Management of life-threatening acid-base disorders. First of two parts N Engl J Med, 1998.PMID 9414329
- [3]Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances N Engl J Med, 2014.PMID 25295502
- [4]Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine Clin J Am Soc Nephrol, 2007.PMID 17699401
- [5]Kraut JA, Madias NE. Lactic acidosis N Engl J Med, 2014.PMID 25494270
- [6]Kitabchi AE, Umpierrez GE, Miles JM, Fisher JN. Hyperglycemic crises in adult patients with diabetes Diabetes Care, 2009.PMID 19564476
- [7]Jaber S, Paugam C, Futier E, et al. Sodium bicarbonate therapy for patients with severe metabolic acidaemia in the intensive care unit (BICAR-ICU): a multicentre, open-label, randomised controlled, phase 3 trial Lancet, 2018.PMID 29910040
- [8]Middleton P, Kelly AM, Brown J, Robertson M. Agreement between arterial and central venous values for pH, bicarbonate, base excess, and lactate Emerg Med J, 2006.PMID 16858095
- [9]Jansen TC, van Bommel J, Schoonderbeek FJ, et al. Early lactate-guided therapy in intensive care unit patients: a multicenter, open-label, randomized controlled trial Am J Respir Crit Care Med, 2010.PMID 20463176
- [10]Narins RG, Emmett M Simple and mixed acid-base disorders: a practical approach Medicine (Baltimore), 1980.PMID 6774200
- [11]Wrenn K. The delta gap: an approach to mixed acid-base disorders Ann Emerg Med, 1990.PMID 2240729
- [12]Bass ME, Paavola N, Kiser TH, et al. Evaluation of insulin infusion rates for the treatment of diabetic ketoacidosis in the emergency department J Biosci Med, 2022.PMID 35386489
- [13]Barceloux DG, Krenzelok EP, Olson K, Watson W. American Academy of Clinical Toxicology practice guidelines on the treatment of ethylene glycol poisoning J Toxicol Clin Toxicol, 1999.PMID 10497633
- [15]Brent J, McMartin K, Phillips S, Aaron C, Kulig K. Fomepizole for the treatment of methanol poisoning N Engl J Med, 2001.PMID 11172179
- [16]Brent J, McMartin K, Phillips S, et al. Fomepizole for the treatment of ethylene glycol poisoning N Engl J Med, 1999.PMID 10080845
- [17]Taylor ZC, Carroll VG, Parker HG. Proximal tubule dysfunction secondary to salicylate intoxication Glob Pediatr Health, 2022.PMID 35224141
- [18]Austin MA, Wills KE, Blizzard L, Walters EH, Wood-Baker R. Effect of high flow oxygen on mortality in chronic obstructive pulmonary disease patients in prehospital setting: randomised controlled trial BMJ, 2010.PMID 20959284
- [19]Plant PK, Owen JL, Elliott MW. Early use of non-invasive ventilation for acute exacerbations of chronic obstructive pulmonary disease on general respiratory wards: a multicentre randomised controlled trial Lancet, 2000.PMID 10859037
- [20]Wilson RF, Spencer AR, Tyburski JG, Dolman H, Zimmerman LH. Bicarbonate therapy in severely acidotic trauma patients increases mortality J Trauma Acute Care Surg, 2013.PMID 23271076