Renal, Fluids, Electrolytes & Acid-Base · Topic 18 of 22

ABG Interpretation & Compensation

Renal · acid–base

ABG Interpretation & Compensation

Read the pH, name the primary change, then test the compensation: Winters formula for metabolic acidosis, 1-4-2-5 for respiratory disorders. A miss means a second disorder.

Key numbers

Normal PaCO₂ 35–45, HCO₃⁻ 22–26pH 7.35–7.45

Winters formula: expected PaCO₂ in metabolic acidosis1.5 × HCO₃⁻ + 8 ± 2

Respiratory compensation: the 1-4-2-5 ruleHCO₃⁻ 1-4-2-5 per 10 mmHg

Metabolic alkalosis: PaCO₂ up 0.7 per HCO₃⁻PaCO₂ ↑ 0.7 per 1 HCO₃⁻

01 · The numberNormal PaCO₂ 35–45, HCO₃⁻ 22–26

Normal arterial values are pH 7.35–7.45, PaCO₂ 35–45 mmHg and HCO₃⁻ 22–26 mEq/L; acidemia is pH below 7.35, alkalemia above 7.45. Compensation formulas use pH 7.40, PaCO₂ 40 and HCO₃⁻ 24.

02 · CalculateWinters formula: expected PaCO₂ in metabolic acidosis

In metabolic acidosis, expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2) mmHg (Winters formula); a higher measured PaCO₂ means a coexisting respiratory acidosis, a lower one a coexisting respiratory alkalosis.

03 · CalculateRespiratory compensation: the 1-4-2-5 rule

Per 10-mmHg change in PaCO₂, HCO₃⁻ rises 1 mEq/L in acute and 4 in chronic respiratory acidosis, and falls 2 in acute and 5 in chronic respiratory alkalosis.

What it is

Core idea

Lungs compensate in minutes; kidneys take daysRespiratory compensation for metabolic disorders occurs over minutes to hours and is about 50–75% effective; renal compensation for respiratory disorders begins over hours and takes days to reach the chronic values.

Presentation

What you'll see

Normal pH can hide ketoacidosis plus vomitingA vomiting patient with ketoacidosis can have a normal pH and HCO₃⁻ yet a high anion gap — metabolic acidosis plus metabolic alkalosis; mixed disorders are present in about one-third of DKA presentations.

Diagnosis & monitoring

Work-up

Metabolic alkalosis: PaCO₂ up 0.7 per HCO₃⁻ must knowIn metabolic alkalosis, expected PaCO₂ = 0.7 × HCO₃⁻ + 21 (± 2) — about 0.7 mmHg per 1 mEq/L HCO₃⁻ rise, rarely above 55 mmHg; higher means added respiratory acidosis, lower added respiratory alkalosis.

Delta gap uncovers a hidden metabolic alkalosisIn a high-gap acidosis, add the delta gap (measured minus normal anion gap) to the measured HCO₃⁻; a result above the normal HCO₃⁻ range means a coexisting metabolic alkalosis.

Management

Orders

Treat each primary disorder separatelyOnce a mixed disorder is identified, explain each primary process from the clinical picture (eg, chronic lung disease, kidney failure, drug overdose, vomiting) and treat each one.

The rules

Know these

Read pH, primary change, compensation, then gap must knowRead every ABG in order: pH; the value that explains it (HCO₃⁻ = metabolic, PaCO₂ = respiratory); whether compensation matches the expected formula; the anion gap; and, if the gap is high, the delta gap.

Compensation never overshoots or fully normalizes pHCompensation moves pH toward normal but never returns it fully to normal or overshoots; in mixed disorders the pH and values can look deceptively normal, so always test compensation against the formulas.

Calculate the anion gap on every gasCalculate the anion gap, Na⁺ − (Cl⁻ + HCO₃⁻), with every ABG: an elevated gap almost always means metabolic acidosis, even when pH and HCO₃⁻ look normal.

Respiratory disorders: off-target HCO₃⁻ means two disordersIn respiratory acidosis, a bigger HCO₃⁻ rise than expected means added metabolic alkalosis, a smaller rise an acute process or added metabolic acidosis; in respiratory alkalosis, a bigger fall means added metabolic acidosis.

Metabolic or respiratory acidosis?

Commonly confused

Primary change	HCO₃⁻ falls
Expected compensation	PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2
Who compensates, how fast	Lungs, minutes to hours
Clue to a second disorder	PaCO₂ outside the Winters range

Commonly confused

Tell apart

Opposite PaCO₂ and HCO₃⁻ moves mean two disordersCompensation always moves PaCO₂ and HCO₃⁻ in the same direction; low HCO₃⁻ with high PaCO₂ is combined metabolic and respiratory acidosis, and high HCO₃⁻ with low PaCO₂ is combined alkalosis.

More detail

Reference

Show 5 reference facts

ABG HCO₃⁻ is calculated from pH and PaCO₂ (Henderson–Hasselbalch), whereas serum chemistry HCO₃⁻ is measured; when they disagree, trust the measured value.

In circulatory failure or during CPR, venous blood may reflect tissue acid–base status better than arterial blood and better guide bicarbonate and ventilation.

Internal consistency check: [H⁺] (nEq/L) = 24 × PaCO₂ / HCO₃⁻ (Kassirer–Bleich); [H⁺] of 40 nEq/L corresponds to pH 7.40.

Imprecise bedside shortcuts for metabolic acidosis: expected PaCO₂ ≈ HCO₃⁻ + 15, or ≈ the last two digits of the pH; Winters formula is preferred.

Severe acidemia impairs contractility and catecholamine response, lowers BP and causes hyperkalemia and arrhythmias; severe alkalemia lowers ionized calcium, potassium, magnesium and phosphate, constricts arterioles and causes tetany, seizures and arrhythmias.

Pearl

Compensation never overshoots and never fully normalizes the pH — if it looks like it did, look for a second disorder.

Sources · Merck Manual Professional — Acid-Base Disorders (compensation table, diagnosis, worked example) · Merck Manual Professional — Acid-Base Regulation · Deranged Physiology — Causes of respiratory acidosis and alkalosis (Boston 1-4-2-5 compensation rules) · Merck Manual Professional — Metabolic Acidosis · AMBOSS — Delta gap / delta ratio · Umpierrez GE et al. Hyperglycaemic crises in adults with diabetes: a consensus report (ADA/EASD/JBDS/AACE/DTS). Diabetologia 2024, doi 10.1007/s00125-024-06183-8 · Michigan Medicine Pathology Laboratory Handbook — Blood gas reference ranges (arterial pH 7.35–7.45, PCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mmol/L) · Sood P, Paul G, Puri S. Interpretation of arterial blood gas. Indian J Crit Care Med 2010;14:57–64 (Winters, 0.7 × HCO₃⁻ + 21, respiratory compensation rules) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).