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BI9.1-3 | Minerals, electrolytes, Water and Acid base balance — Part 3

The Four Primary Acid-Base Disorders

Disorder	pH	Primary Change	Compensation	Common Causes
Metabolic Acidosis	<7.35	HCO3- ↓	Respiratory: pCO2 ↓ (hyperventilation, Kussmaul breathing)	DKA, lactic acidosis, renal failure, diarrhoea, RTA
Metabolic Alkalosis	>7.45	HCO3- ↑	Respiratory: pCO2 ↑ (hypoventilation, limited)	Vomiting (HCl loss), nasogastric suction, diuretics, antacid excess
Respiratory Acidosis	<7.35	pCO2 ↑	Renal: HCO3- ↑ (H+ excretion, HCO3- retention)	COPD, severe asthma, respiratory depression (opioids), neuromuscular disease
Respiratory Alkalosis	>7.45	pCO2 ↓	Renal: HCO3- ↓ (decreased H+ excretion, HCO3- excretion)	Hyperventilation (anxiety, pain, high altitude), salicylate poisoning (early), pregnancy

Use a systematic 5-step approach to interpret blood gas:

The Four Primary Acid-Base Disorders — Multi-panel illustration of acid-base disorders: systematic 5-step diagnostic approach, 2x2 grid of four primary disorders with causes and compensation, anion gap vs normal AG metabolic acidosis with mnemonics, and worked clinical scenario

Step 1: pH — acidosis (<7.35) or alkalosis (>7.45)?
Step 2: pCO₂ and HCO₃⁻ — which is abnormal in the direction of the pH change?
Step 3: Is there compensation? (partial or complete?)
Step 4: Anion Gap = Na⁺ − (Cl⁻ + HCO₃⁻). Normal = 8–12 mEq/L. High AG = added acid metabolites.
Step 5: Clinical context.

The four disorders:

Disorder	pH	pCO₂	HCO₃⁻	Compensation	Common Causes
Metabolic acidosis	↓	↓ (resp. comp.)	↓ Primary	Hyperventilation	DKA, lactic acidosis, diarrhoea, renal failure
Metabolic alkalosis	↑	↑ (resp. comp.)	↑ Primary	Hypoventilation	Vomiting, diuretics, Conn's syndrome
Respiratory acidosis	↓	↑ Primary	↑ (renal comp.)	↑ HCO₃⁻	COPD, pneumonia, respiratory failure
Respiratory alkalosis	↑	↓ Primary	↓ (renal comp.)	↓ HCO₃⁻	Anxiety, altitude, liver failure, pregnancy

High anion gap metabolic acidosis: MUDPILES mnemonic — Methanol, Uraemia, DKA, Paraldehyde, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates

The four disorders: — Multi-panel illustration of acid-base clinical detail: DKA pathophysiology and features, lactic acidosis Type A vs Type B, acute vs chronic respiratory acidosis with compensation kinetics, and mixed acid-base disorders with Winter's formula

Normal anion gap metabolic acidosis: USED CARP — Ureteroenterostomy, Small bowel fistula, Extra Cl⁻, Diarrhoea, Carbonic anhydrase inhibitor, Adrenal insufficiency, Renal tubular acidosis, Pancreatic fistula

Interpreting the scenario patient (Bed 3): pH 7.28 → acidosis. HCO₃⁻ 13 mEq/L ↓ → metabolic component. pCO₂ 28 mmHg ↓ → respiratory compensation (hyperventilating to blow off CO₂). Diagnosis: metabolic acidosis with respiratory compensation. In a patient with diarrhoea: normal AG acidosis from bicarbonate loss in stool (small bowel fluid is rich in HCO₃⁻).

SELF-CHECK — : Acid-Base Balance

A 16-year-old type 1 diabetic is brought unconscious. Blood gas: pH 7.18, pCO₂ 22 mmHg, HCO₃⁻ 8 mEq/L, Na⁺ 135, Cl⁻ 98. Calculate the anion gap and identify the disorder.

A. Normal anion gap metabolic acidosis — diarrhoea

B. High anion gap metabolic acidosis with respiratory compensation — DKA

C. Respiratory acidosis — respiratory failure

D. Mixed metabolic acidosis and respiratory alkalosis

Reveal Answer

Answer: B. High anion gap metabolic acidosis with respiratory compensation — DKA

Which buffer system is most important in EXTRACELLULAR fluid and acts as the primary blood buffer?

A. Phosphate buffer

B. Protein buffer (haemoglobin)

C. Bicarbonate buffer (H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂)

D. Ammonia buffer

Reveal Answer

Answer: C. Bicarbonate buffer (H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂)