Acid-Base Abnormalities
Arterial Blood Gas (ABG)
Approaching an 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)
ABG Interpretation Summary Table
| 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
Respiratory Acidosis
ABG Findings
| Type | ABG findings |
|---|---|
| Acute respiratory acidosis (no compensation) |
|
| Chronic respiratory acidosis (with compensation) |
|
Causes
Mechanism: alveolar hypoventilation → CO2 retention
| Type | Causes |
|---|---|
| Acute respiratory acidosis (no compensation) |
|
| Chronic respiratory acidosis (with compensation) |
|
Respiratory Alkalosis
ABG Findings
| Type | ABG findings |
|---|---|
| Acute respiratory alkalosis (no compensation) |
|
| Chronic respiratory alkalosis (with compensation) |
|
Causes
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
Metabolic Acidosis
ABG Findings
| Type | ABG findings |
|---|---|
| Acute metabolic acidosis (no compensation) |
|
| Chronic metabolic acidosis (with compensation) |
|
Causes
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).
High-Anion Gap Metabolic Acidosis Causes
Essentially excess endogenous / exogenous acid (CAT MUDPILES):
- Carbon monoxide or cyanide poisoning
- Amioglycosides
- Toluene (glue-sniffing), Theophylline
- Methanol, Metformin
- Urea
- DKA / AKA / starvation ketoacidosis
- Paracetamol
- Iron, Isoniazid
- Lactic acidosis
- Ethylene glycol, Ethanol
- Salicylates
Normal-Anion Gap Metabolic Acidosis Causes
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)
Metabolic Alkalosis
ABG Findings
| Type | ABG findings |
|---|---|
| Acute metabolic alkalosis (no compensation) |
|
| Chronic metabolic alkalosis (with compensation) |
|
Causes
4 groups of causes:
| Group | Examples |
|---|---|
| GI loss of H+ |
|
| Renal loss of H+ |
|
| Hypokalaemia |
|
| Alkali load |
|