Stethopedia | Approach

Approach to

Acid–Base Disorders

Read the pH, name the primary process and check compensation. Act first on shock, poisoning, acute hypercapnia or pH <7.20/>7.60. Always correct the anion gap for albumin, then sort: high-AG, normal-AG, alkalosis, respiratory.

  1. Abnormal pH, HCO3 or PaCO2 on a blood gas or chemistry panel
  2. Gas + lytes: pH <7.35 acidemia, >7.45 alkalemia; primary process; VBG pH ~0.03 below ABG1,3
  3. Winter's PaCO2 = 1.5×HCO3 + 8±2; anion gap = Na − (Cl + HCO3), correct for albumin1,2,12
  4. Shock, toxin, acute ↑CO2 or extreme pH?

Emergency: ABCs, ICU, poison centre1,12

  • pH <7.20 or >7.60: arrhythmia, shock12
  • Drowsy or acute ↑PaCO2: NIV/intubate1
  • Shock or sepsis + lactate >2 mmol/L: fluids7,9
  • Toxic alcohol suspected: fomepizole ± HD6
  • Salicylate: IV NaHCO3; intubated: keep RR high4,10,12
  • Salicylate HD: AMS, >7.2 mmol/L (>6.5 if AKI)10,12

→ Acidemia: NaHCO3 only if able to ventilate; recheck gas, K, iCa

Then: which primary disorder?

CausesCluesWorkupManagement

High-AG acidosis

  • Lactate: shock, ischemia, metformin
  • Ketoacidosis: DKA (incl. SGLT2i), alcohol
  • Toxic alcohols, salicylate; uremia
  • Osm gap >10: toxic alcohol; can be normal6
  • Respiratory alkalosis + tinnitus: salicylate
  • ΔAG vs ΔHCO3: hidden second process1
  • Lactate, β-hydroxybutyrate, creatinine
  • Salicylate, ethanol, measured Osm6
  • Lactate: restore perfusion; DKA protocol7
  • Fomepizole ± HD for toxic alcohol6
  • NaHCO3 if pH ≤7.20 + AKI 2–3 (not DKA)8,9

Normal-AG acidosis

  • Diarrhea, GI fistula, ileal conduit
  • Large-volume 0.9% saline
  • RTA; acetazolamide
  • Diarrhea or saline load in history
  • High K, diabetic CKD: type 4 RTA12
  • UAG neg: GI or proximal RTA; pos: distal5,12
  • Urine pH, K; Ca/Cr for type of RTA12
  • Stop saline; balanced crystalloid9
  • Alkali for RTA 1–2; type 4: lower K12

Metabolic alkalosis

  • Vomiting, NG suction
  • Diuretics; low K or Mg
  • Mineralocorticoid excess
  • Volume-depleted: vomiting, diuretics
  • Hypertension with low K
  • Urine Cl: low = saline-responsive11
  • Renin, aldosterone if hypertensive11
  • 0.9% saline + KCl if Cl-responsive12
  • Overloaded: acetazolamide12
  • Aldosterone excess: amiloride or MRA12

Respiratory

  • High PaCO2: opioids, COPD, weakness
  • Low PaCO2: sepsis, PE, hypoxemia
  • Pain, anxiety, pregnancy, cirrhosis
  • PaCO2 +10: HCO3 +1 acute, +4 chronic1,12
  • PaCO2 −10: HCO3 −2 acute, −4 chronic1,12
  • ABG: venous PCO2 is unreliable3
  • CXR, drug screen; CTPA if PE likely
  • NIV or ventilate; reverse opioids
  • No NaHCO3 for respiratory acidosis12
  • Alkalosis: treat hypoxemia, pain, cause

Can't-miss cause Superscripts link to the references below.

References

  1. Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid–base disturbances. N Engl J Med. 2014;371:1434–1445. PubMed 25295502
  2. Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26:1807–1810. PubMed 9824071
  3. Byrne AL, Bennett M, Chatterji R, et al. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology. 2014;19:168–175. PubMed 24383789
  4. Stolbach AI, Hoffman RS, Nelson LS. Mechanical ventilation was associated with acidemia in a case series of salicylate-poisoned patients. Acad Emerg Med. 2008;15:866–869. PubMed 18821862
  5. Batlle DC, Hizon M, Cohen E, et al. The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis. N Engl J Med. 1988;318:594–599. PubMed 3344005
  6. Kraut JA, Mullins ME. Toxic alcohols. N Engl J Med. 2018;378:270–280. PubMed 29342392
  7. Kraut JA, Madias NE. Lactic acidosis. N Engl J Med. 2014;371:2309–2319. PubMed 25494270
  8. Jung B, Jabaudon M, De Jong A, et al. Sodium bicarbonate for severe metabolic acidemia and acute kidney injury: the BICARICU-2 randomized clinical trial. JAMA. 2025;334:2000–2010. PubMed 41159812
  9. Prescott HC, Antonelli M, Alhazzani W, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2026. Crit Care Med. 2026;54:725–812. PubMed 41869847
  10. Juurlink DN, Gosselin S, Kielstein JT, et al. Extracorporeal treatment for salicylate poisoning: systematic review and recommendations from the EXTRIP Workgroup. Ann Emerg Med. 2015;66:165–181. PubMed 25986310
  11. Emmett M. Metabolic alkalosis: a brief pathophysiologic review. Clin J Am Soc Nephrol. 2020;15:1848–1856. PubMed 32586924
  12. Gay E, Levy N, eds. Acid-Base Disorders. In: Department of Medicine Housestaff Manual, 30th ed. 2024:102–103.

For education only; not a substitute for clinical judgment or local protocols. Where sources differ, the flowchart follows the current guideline.

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