Acid-base Abnormalities

Definition

Acid–base disorders refer to disturbances in the body's pH balance due to alterations in hydrogen ion concentration. These imbalances are classified based on pH, PaCO₂, and HCO₃⁻ levels into respiratory or metabolic and acidosis or alkalosis.

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Normal Reference Values (ABG)

Parameter	Normal Range
pH	7.35 – 7.45
PaCO₂	4.5 – 6.0 kPa
HCO₃⁻	22 – 26 mmol/L
Base excess	±2 mmol/L
PaO₂	10.5 – 13.5 kPa
O₂ saturation	94 – 98%

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Classification

Disorder	pH	PaCO₂	HCO₃⁻
Metabolic acidosis	↓	↓ (comp)	↓
Metabolic alkalosis	↑	↑ (comp)	↑
Respiratory acidosis	↓	↑	↑ (comp)
Respiratory alkalosis	↑	↓	↓ (comp)

Comp = compensatory response

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Approach to Acid–Base Disorders (Stepwise)

1. Assess pH → Acidaemia (<7.35) or Alkalaemia (>7.45)?
2. Look at PaCO₂ and HCO₃⁻ → Respiratory or Metabolic?
3. Assess compensation (partially, fully, or not at all)
4. Check anion gap (for metabolic acidosis)
   • AG = Na⁺ – (Cl⁻ + HCO₃⁻)
   • Normal: 8–12 mmol/L
5. Use clinical context to identify cause

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Metabolic Acidosis

• pH: Low
• HCO₃⁻: Low
• Compensatory: Hyperventilation → ↓ PaCO₂ (Kussmaul breathing)

High Anion Gap Metabolic Acidosis (AGMA)

Cause	Description
DKA	Diabetes, starvation, alcohol
Lactic acidosis	Sepsis, shock, hypoxia
Renal failure	↓ H⁺ excretion
Toxins	Methanol, ethylene glycol, salicylates

Normal Anion Gap Metabolic Acidosis (NAGMA)

Cause	Description
Diarrhoea	Bicarbonate loss
Renal tubular acidosis	Impaired H⁺ secretion
Ileostomy/fistula	GI bicarbonate loss

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Metabolic Alkalosis

• pH: High
• HCO₃⁻: High
• Compensatory: Hypoventilation → ↑ PaCO₂

Cause	Notes
Vomiting / NG suction	HCl loss
Diuretics (loop/thiazides)	↑ renal bicarbonate reabsorption
Hypokalaemia	Drives H⁺ into cells
Conn's syndrome	Hyperaldosteronism ↑ H⁺ secretion

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Respiratory Acidosis

• pH: Low
• PaCO₂: High
• Compensatory: ↑ HCO₃⁻ (slow renal response)

Acute Causes	Chronic Causes
Opioid overdose	COPD
Airway obstruction	Neuromuscular disease
CNS depression	Obesity hypoventilation

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Respiratory Alkalosis

• pH: High
• PaCO₂: Low
• Compensatory: ↓ HCO₃⁻ (renal excretion)

Cause	Notes
Hyperventilation	Anxiety, pain
PE, pneumonia	Hypoxia-driven hyperventilation
Salicylate overdose	Early phase
Pregnancy, high altitude	Progesterone/hypoxia

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Mixed Acid–Base Disorders

Clues:

• Unexpected pH for level of PaCO₂/HCO₃⁻
• No compensation or overcompensation
• Use of Winter’s formula and delta-delta calculation

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Formulas:

• Anion gap = Na⁺ – (Cl⁻ + HCO₃⁻)
• Winter’s formula (expected PaCO₂ in metabolic acidosis):
  • = (1.5 × HCO₃⁻) + 8 ± 2
• Delta ratio = (measured AG – normal AG) / (24 – measured HCO₃⁻)

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Clues from ABG Compensation

Disorder	Compensation Type
Metabolic	Respiratory (quick)
Respiratory	Renal (slow)
Full compensation	Normal pH with abnormal CO₂/HCO₃⁻
Mixed disorder	Inappropriate compensation or conflicting findings

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Fries Tips 

• Acidosis + high AG? Think DKA, lactic acidosis, toxins
• Alkalosis + vomiting? Suspect H⁺ loss from GI tract
• Kussmaul breathing = metabolic acidosis
• Respiratory causes? Always assess for opioid use, COPD, or anxiety
• Don’t forget to assess anion gap and clinical context for all metabolic disorders

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