Acid-Base Disorders

Acid-base balance is a perennial NEET PG favourite, sitting at the crossroads of physiology, biochemistry, medicine and critical care. Mastery of a stepwise arterial blood gas (ABG) approach lets you decode any disorder — simple or mixed — and pick the high-yield clinical association (DKA, salicylate poisoning, vomiting, COPD) the examiner is really testing.

Core definitions and normal values

The body guards arterial pH within a razor-thin range because enzyme function, oxygen delivery and electrolyte balance all depend on it. The master equation is the Henderson–Hasselbalch equation:

$$pH = 6.1 + \log_{10}\frac{[HCO_3^-]}{0.03 \times PaCO_2}$$

This tells you pH is set by the ratio of bicarbonate (metabolic/renal, the "kidney" component) to PaCO₂ (respiratory, the "lung" component). The simplified Henderson equation is more bedside-friendly:

$$[H^+] = 24 \times \frac{PaCO_2}{[HCO_3^-]}$$

High-yield: For pH 7.40, [H⁺] = 40 nmol/L. Between pH 7.20–7.50, every 0.01 fall in pH raises [H⁺] by ~1 nmol/L. So pH 7.30 ≈ 50 nmol/L, pH 7.50 ≈ 32 nmol/L.

Parameter	Normal range	Meaning
pH	7.35–7.45	Net acidity (<7.35 acidaemia, >7.45 alkalaemia)
PaCO₂	35–45 mmHg	Respiratory component
HCO₃⁻	22–26 mEq/L	Metabolic/renal component
Base excess (BE)	−2 to +2 mEq/L	Metabolic deviation
PaO₂	80–100 mmHg	Oxygenation
Anion gap	8–12 mEq/L	Unmeasured anions

A clarifying distinction the examiner loves: -aemia vs -osis. Acidaemia/alkalaemia describe the actual blood pH. Acidosis/alkalosis are the underlying processes that add acid/base or alter PaCO₂ — a patient can have two opposing processes (a mixed disorder) yet a normal pH.

Classification of the four primary disorders

There are four primary disturbances, defined by which parameter moves first and in which direction:

Disorder	Primary change	pH	Compensatory change
Metabolic acidosis	↓ HCO₃⁻	↓	↓ PaCO₂ (hyperventilation)
Metabolic alkalosis	↑ HCO₃⁻	↑	↑ PaCO₂ (hypoventilation)
Respiratory acidosis	↑ PaCO₂	↓	↑ HCO₃⁻ (renal retention)
Respiratory alkalosis	↓ PaCO₂	↑	↓ HCO₃⁻ (renal excretion)

The golden rule: in a simple disorder, the primary parameter and the compensatory parameter move in the SAME direction. If pH is low and both HCO₃⁻ and PaCO₂ are low → metabolic acidosis with respiratory compensation. If HCO₃⁻ and PaCO₂ move in opposite directions, suspect a mixed disorder.

High-yield: Compensation NEVER fully corrects the pH back to normal (7.40). If pH is exactly normal with deranged PaCO₂ and HCO₃⁻, two primary processes are present (a mixed disorder).

Stepwise ABG interpretation — the exam algorithm

This 6-step "flow" is the single most useful tool you can carry into the exam:

1. Look at pH → acidaemia (<7.35) or alkalaemia (>7.45)? → 2. Identify the primary driver → is the abnormality explained by PaCO₂ (respiratory) or HCO₃⁻ (metabolic)? → 3. Check compensation → calculate the expected compensatory value (formulae below) and compare → 4. Calculate the anion gap in every acidosis → 5. If high anion gap, apply the delta-delta ratio to unmask a coexisting disorder → 6. Correlate with the clinical history to pick the cause.

Expected compensation formulae (must memorise)

Primary disorder	Expected compensation
Metabolic acidosis	Winter's formula: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
Metabolic alkalosis	Expected PaCO₂ = (0.7 × HCO₃⁻) + 21 (rises ~0.7 mmHg per 1 mEq/L rise in HCO₃⁻)
Acute respiratory acidosis	HCO₃⁻ ↑ by 1 per 10 mmHg rise in PaCO₂
Chronic respiratory acidosis	HCO₃⁻ ↑ by 3.5–4 per 10 mmHg rise in PaCO₂
Acute respiratory alkalosis	HCO₃⁻ ↓ by 2 per 10 mmHg fall in PaCO₂
Chronic respiratory alkalosis	HCO₃⁻ ↓ by 4–5 per 10 mmHg fall in PaCO₂

High-yield: Winter's formula is the most frequently asked. If measured PaCO₂ > expected → coexisting respiratory acidosis. If measured PaCO₂ < expected → coexisting respiratory alkalosis (classic in salicylate poisoning).

The anion gap

The anion gap (AG) detects unmeasured anions and is the pivot of metabolic acidosis work-up.

$$AG = [Na^+] - ([Cl^-] + [HCO_3^-])$$

Normal AG = 8–12 mEq/L (some labs 3–11 if K⁺ is included). Always correct for albumin, the dominant unmeasured anion: AG falls by ~2.5 mEq/L for every 1 g/dL fall in albumin. A hypoalbuminaemic ICU patient may have a "falsely normal" gap masking a true high-AG acidosis.

High anion gap metabolic acidosis (HAGMA)

Mnemonic GOLD MARK (modern) or the classic MUDPILES:

• Glycols (ethylene glycol, propylene glycol)
• Oxoproline (chronic paracetamol)
• Lactate (sepsis, shock, metformin)
• D-lactate / methanol
• Methanol
• Aspirin (salicylates)
• Renal failure (uraemia)
• Ketoacidosis (DKA, alcoholic, starvation)

MUDPILES = Methanol, Uraemia, DKA, Propylene glycol/Paraldehyde, Iron/INH/Inborn errors, Lactic acidosis, Ethylene glycol, Salicylates.

High-yield: Ethylene glycol → calcium oxalate crystals in urine, raised osmolar gap, renal failure. Methanol → optic disc hyperaemia/blindness, putamen necrosis. Both treated with fomepizole (DOC, an alcohol dehydrogenase inhibitor) or ethanol; haemodialysis for severe cases.

Normal anion gap metabolic acidosis (NAGMA / hyperchloraemic)

Mnemonic HARDASS or USED CARP:

• Hyperalimentation / Acetazolamide / Renal tubular acidosis / Diarrhoea / Addison's / Spironolactone / Saline (normal saline excess)

The key bedside split is the urine anion gap (UAG = Na⁺ + K⁺ − Cl⁻): a negative UAG (high urinary NH₄⁺) points to GI bicarbonate loss (diarrhoea); a positive UAG indicates a renal acidification defect (RTA).

Renal tubular acidosis — a perennial table question

Feature	Type 1 (Distal)	Type 2 (Proximal)	Type 4 (Hyperkalaemic)
Defect	Failure of H⁺ secretion	Failure of HCO₃⁻ reabsorption	Aldosterone deficiency/resistance
Serum K⁺	Low	Low	High
Urine pH	>5.5 (cannot acidify)	Variable (<5.5 when HCO₃⁻ depleted)	<5.5
Min plasma HCO₃⁻	Very low (<10)	12–18	>17
Stones/nephrocalcinosis	Yes	No	No
Classic cause	Sjögren's, amphotericin	Fanconi, multiple myeloma, acetazolamide	Diabetic nephropathy, NSAIDs

High-yield: Type 1 distal RTA → nephrocalcinosis and hypokalaemia; treat with potassium citrate. Type 2 proximal RTA → Fanconi syndrome (glycosuria, phosphaturia, aminoaciduria), needs large alkali doses. Type 4 is the ONLY RTA with hyperkalaemia.

The delta-delta (Δ/Δ) ratio — unmasking mixed disorders

In HAGMA, every unit rise in AG should drop HCO₃⁻ by roughly one unit. The ratio:

$$\frac{\Delta AG}{\Delta HCO_3^-} = \frac{(AG - 12)}{(24 - HCO_3^-)}$$

• < 0.4–1 → concomitant NAGMA present (HCO₃⁻ fell more than the gap rose)
• 1–2 → pure HAGMA
• > 2 → coexisting metabolic alkalosis or pre-existing chronic respiratory acidosis (HCO₃⁻ higher than expected)

Clinical features of the disorders

Acidaemia depresses myocardial contractility, causes arteriolar vasodilation, hyperkalaemia (H⁺/K⁺ exchange), and Kussmaul (deep sighing) respiration in severe metabolic acidosis. Severe acidosis (pH <7.1) blunts catecholamine response and risks arrhythmia.

Alkalaemia causes tetany (carpopedal spasm, Trousseau/Chvostek signs from reduced ionised calcium), paraesthesiae, seizures, and a left-shifted oxyhaemoglobin curve impairing tissue O₂ delivery. Alkalosis drives hypokalaemia and hypocalcaemia.

High-yield: Hyperventilation (anxiety, high altitude) → respiratory alkalosis → perioral and acral tingling, carpopedal spasm. Acute correction of chronic respiratory acidosis can precipitate post-hypercapnic metabolic alkalosis.

Diagnosis and investigation of choice

The arterial blood gas is the investigation of choice, ideally drawn from the radial artery (after a modified Allen's test) into a heparinised syringe, analysed promptly or kept on ice to prevent ongoing glycolysis falsely lowering pH and raising lactate. Always pair it with serum electrolytes (for AG), albumin, lactate, glucose and renal function. For toxic alcohols, calculate the osmolar gap (measured − calculated osmolality; >10 is significant). Venous gases approximate pH (≈0.03 lower) and HCO₃⁻ well but are unreliable for PaCO₂/PaO₂.

Management — the principle and the drug of choice

The cardinal rule of acid-base management is treat the underlying cause, not the number.

• Metabolic acidosis: fluid resuscitation, treat sepsis/shock, insulin + fluids for DKA. Sodium bicarbonate is reserved for severe acidaemia (pH <7.1–7.2), and is genuinely indicated in NAGMA (RTA, diarrhoea) and certain intoxications (salicylates, TCA) where urinary alkalinisation helps. It is generally avoided in lactic acidosis and DKA (risk of paradoxical CNS acidosis, hypokalaemia, volume/sodium load).
• Metabolic alkalosis: classify by urine chloride. Chloride-responsive (UCl <20, e.g. vomiting, diuretics, NG suction)** → treat with normal saline + KCl. **Chloride-resistant (UCl >20, e.g. hyperaldosteronism, Cushing's, Bartter/Gitelman) → address mineralocorticoid excess; acetazolamide can aid. Severe cases may need dilute HCl or dialysis.
• Respiratory acidosis: reverse the cause — bronchodilators, naloxone for opioids, non-invasive or invasive ventilation to lower PaCO₂; in chronic CO₂ retainers, lower PaCO₂ slowly to avoid post-hypercapnic alkalosis and seizures.
• Respiratory alkalosis: treat anxiety, hypoxia, sepsis, salicylate toxicity; correcting hypoxaemia is paramount.

High-yield: Salicylate poisoning produces a classic mixed primary respiratory alkalosis + high-anion-gap metabolic acidosis — adults often present with a near-normal pH. Treatment: urinary alkalinisation with sodium bicarbonate (targets urine pH 7.5–8 to trap ionised salicylate), correct hypokalaemia, and haemodialysis for severe levels, altered sensorium, renal failure or pulmonary oedema.

Key clinical associations to memorise

Scenario	Disorder
Prolonged vomiting / NG suction	Metabolic alkalosis (hypokalaemic, hypochloraemic)
Severe diarrhoea	NAGMA (negative UAG)
Diabetic ketoacidosis	HAGMA (ketones)
Sepsis / shock / metformin	Lactic acidosis (HAGMA)
COPD exacerbation	Chronic respiratory acidosis
High altitude / pregnancy / anxiety	Respiratory alkalosis
Pulmonary embolism	Respiratory alkalosis (early)
Aspirin overdose	Mixed resp. alkalosis + HAGMA
Acetazolamide / proximal RTA	NAGMA
Conn's syndrome	Chloride-resistant metabolic alkalosis

Complications

Untreated severe acidosis leads to refractory shock, arrhythmias, coma and death; chronic acidosis (CKD) causes bone demineralisation and muscle wasting. Severe alkalosis precipitates seizures, tetany, arrhythmias and reduced cerebral/coronary perfusion. Iatrogenic complications include bicarbonate-induced hypernatraemia, hypokalaemia, volume overload and paradoxical CSF acidosis; over-rapid correction of chronic respiratory acidosis causes post-hypercapnic alkalosis.

Key differentials and pitfalls

• A normal pH never excludes a serious acid-base disturbance — always check AG and compensation.
• A high lactate is not always tissue hypoperfusion; consider type B (metformin, beta-agonists, liver failure, malignancy).
• An osmolar gap with HAGMA strongly suggests toxic alcohol ingestion before crystals or visual loss appear.
• Pseudohyperchloraemia from bromide or salicylate can distort the calculated AG.
• Distinguish DKA from alcoholic ketoacidosis (normal/low glucose, recent binge then starvation) and from starvation ketosis (mild, HCO₃⁻ rarely <18).

Recently asked / exam angle

NEET PG and INI-CET have repeatedly tested: (1) identify the disorder from a given ABG + electrolyte panel — practice plugging numbers into Winter's formula and the AG; (2) the salicylate poisoning mixed picture and its bicarbonate/dialysis management; (3) RTA type identification from serum K⁺ and urine pH (Type 1 vs Type 2 vs Type 4); (4) the urine anion gap to separate diarrhoea from RTA; (5) toxic alcohol clues — calcium oxalate crystals (ethylene glycol), visual loss (methanol), and fomepizole as DOC; (6) recognising chloride-responsive vs chloride-resistant metabolic alkalosis from urinary chloride; (7) interpreting acute vs chronic respiratory acidosis in a COPD patient using the HCO₃⁻ rise per 10 mmHg PaCO₂. Image/clinical-vignette stems pairing Kussmaul breathing with fruity breath (DKA) or carpopedal spasm with anxiety (respiratory alkalosis) are common.

Rapid revision

1. pH set by HCO₃⁻ : PaCO₂ ratio; [H⁺] = 24 × PaCO₂ / HCO₃⁻.
2. Simple disorder = HCO₃⁻ and PaCO₂ move in the SAME direction; opposite = mixed.
3. Compensation never normalises pH — a normal pH with deranged values means a mixed disorder.
4. Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (metabolic acidosis).
5. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); normal 8–12; correct +2.5 per 1 g/dL fall in albumin.
6. HAGMA = GOLD MARK / MUDPILES; NAGMA = HARDASS, split by urine anion gap.
7. Negative UAG → diarrhoea (GI loss); positive UAG → RTA.
8. RTA: Type 1 distal (high urine pH, stones, low K⁺); Type 2 proximal (Fanconi); Type 4 (HIGH K⁺).
9. Salicylate poisoning = respiratory alkalosis + HAGMA; treat with urinary alkalinisation ± dialysis.
10. Toxic alcohols → raised osmolar gap; DOC fomepizole; ethylene glycol → oxalate crystals, methanol → blindness.
11. Metabolic alkalosis: chloride-responsive (saline-correctable) vs chloride-resistant (mineralocorticoid excess).
12. Treat the cause first; reserve sodium bicarbonate for pH <7.1, NAGMA, and specific toxins.