Login
Search

Search
Search
  1. Category
  2. Acute and Emergency
  3. Acid-Base Abnormalities

Acid-Base Abnormalities

Arterial Blood Gas (ABG)

1. Check pH
  • <7.35 → acidosis
  • >7.45 → alkalosis

2. Check PaCO2 and HCO3-
  • If acidosis
    • PaCO2 is high → respiratory acidosis
    • HCO3- is low → metabolic acidosis
  • If alkalosis​​​
    • PaCO2 is low → respiratory alkalosis
    • HCO3- is high → metabolic alkalosis
 

ROME mnemonic: Respiratory Opposite, Metabolic Equal 
In respiratory disorders pH and PaCO₂ move opposite (↑pH + ↓PaCO₂ = resp. alkalosis; ↓pH + ↑PaCO₂ = resp. acidosis)
In metabolic disorders pH and HCO₃⁻ move equal/same direction (↑pH + ↑HCO₃⁻ = metab. alkalosis; ↓pH + ↓HCO₃⁻ = metab. acidosis)

Abnormality pH PaCO₂ HCO₃⁻ Base Excess (mmol/L)
Respiratory acidosis ↓ (<7.35) ↑ ↑ (if compensation) -
Respiratory alkalosis ↑ (>7.45) ↓ ↓ (if compensation) -
Metabolic acidosis ↓ (<7.35) ↓ (if compensation) ↓ Negative (< -2)
Metabolic alkalosis ↑ (>7.45) ↑ (if compensation) ↑ Positive (> +2)


 

Base Excess (BE) is a blood value you may come across. A simplified definition is that it shows if there's too much acid or base in the blood (Normal range: -2 to +2 mmol/L)

BE interpretation is similar to looking at HCO₃⁻

  • Low HCO₃⁻ or negative BE (< –2): either primary metabolic acidosis (if pH <7.35) or compensation for respiratory alkalosis (if pH >7.45 & low PaCO2)

  • High HCO₃⁻ or positive BE (> +2): either metabolic alkalosis (if pH >7.45) or compensation for respiratory acidosis (if pH <7.35 & high PaCO2)

Types of Acid-Base Abnormalities

Type ABG findings
Acute respiratory acidosis (no compensation)
  • pH <7.35
  • PaCO2 ↑
  • HCO3- normal
Chronic respiratory acidosis (with compensation)
  • pH <7.35
  • PaCO2 ↑
  • HCO3- ↑
 

Mechanism: alveolar hypoventilation → CO2 retention
Type Causes
Acute respiratory acidosis (no compensation)
  • Respiratory depression
    • Drugs (e.g. opiates)
    • CNS trauma
 
  • Airway obstruction
    • Foreign body aspiration
    • Laryngospasm
    • Acute severe asthma / COPD
Chronic respiratory acidosis (with compensation)
  • COPD - most common
  • Obesity hypoventilation syndrome
  • Neuromuscular disorders (e.g. motor neurone disease, multiple sclerosis, myasthenia gravis
  • Severe restrictive lung disease (e.g. kyphoscoliosis, interstitial lung disease)
 

Type ABG findings
Acute respiratory alkalosis (no compensation)
  • pH >7.45
  • PaCO2 ↓
  • HCO3- normal
Chronic respiratory alkalosis (with compensation)
  • pH >7.45
  • PaCO2 ↓
  • HCO3- ↓
 

Mechanism: hyperventilation → excessive CO2 exhalation

Essentially any cause of tachypnoea:
  • Hypoxia (e.g. pulmonary embolism, pneumonia, anaemia, carbon monoxide poisoning)
  • Panic attack
  • Pain
  • Salicylate overdose (early phase - direct stimulation of the respiratory centre)
  • Excessive mechanical ventilation

Type ABG findings
Acute metabolic acidosis (no compensation)
  • pH <7.35
  • PaCO2 normal
  • HCO3- ↓
  • BE: < -2 mmol/L
Chronic metabolic acidosis (with compensation)
  • pH <7.35
  • PaCO2 ↓
  • HCO3- ↓
  • BE: still < -2 mmol/L
 

Causes depend on the anion gap 
  • Formula: [Na+] – ([Cl−] + [HCO3−​])
  • Also: ([Na+] + [K+]) – ([Cl−] + [HCO3−​])
  • Reference range: 8-12 mmol/L (without K+) / 12-16 mmol/L (with K+)
 

Reference ranges for the anion gap differ between laboratories. For exam purposes, the “normal” range is usually given in the question, or the abnormality will be obvious (very high or clearly normal).

Essentially excess endogenous / exogenous acid (CAT MUDPILES):
  • Carbon monoxide or cyanide poisoning
  • Amioglycosides
  • Toluene (glue-sniffing), Theophylline
  • Methanol, Metformin 
  • Urea 
  • DKADiabetic ketoacidosis / AKAAlcoholic ketoacidosis / starvation ketoacidosis
  • Paracetamol
  • Iron, Isoniazid
  • Lactic acidosis 
  • Ethylene glycol, Ethanol
  • Salicylates

ABCD:

  • Adrenal insufficiency
  • Bicarbonate loss
    • GI loss - diarrhoea, pancreatic fistula, small bowel fistula
    • Renal loss - renal tubular acidosis
  • Chloride excess (e.g. excessive NaCl, ammonium chloride)
  • Drugs - carbonic anhydrase inhibitor (e.g. acetazolamide)

Type ABG findings
Acute metabolic alkalosis (no compensation)
  • pH >7.45
  • PaCO2 normal
  • HCO3- ↑
  • BE: > +2 mmol/L
Chronic metabolic alkalosis (with compensation)
  • pH >7.45
  • PaCO2 ↑
  • HCO3- ↑
  • BE: still > +2 mmol/L
 

4 groups of causes:
 
Group Examples
GI loss of H+
  • Vomiting
  • Excess NG tube output
Renal loss of H+
  • Aldosterone excess (e.g. hyperaldosteronism, Cushing's syndrome, exogenous steroids)
  • Diuretics (loop and thiazide)
  • Tubular disorder (e.g. Bartter syndrome, Gitelman syndrome)
Hypokalaemia
  • Range of causes...
Alkali load
  • Excess antacid / bicarbonate ingestion
  • Milk-alkali syndrome
Author: Adams Lau
Reviewer:
Last edited: 02/09/25