Acid-base and ABGs for FCPS Part 1: a method that works every time
Every acid-base question can be solved with the same four steps. Here is the method, the anion gap, the compensation rules and worked gases from the ward situations that examiners copy.
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Why the ABG question is a gift
On the ward, a blood gas comes with noise: a sick patient, a delayed sample, a senior waiting. In the exam it comes clean, with a story that tells you the answer before you read the numbers. A drowsy asthmatic, a diabetic with deep sighing breaths, a man who has been vomiting for a week. Learn the four steps and the story, and the numbers just confirm what you already suspect.
Normal values
| Measure | Normal |
|---|---|
| pH | 7.35 to 7.45 |
| PaCO₂ | 4.7 to 6.0 kPa (35 to 45 mmHg) |
| HCO₃⁻ | 22 to 26 mmol/L |
| Anion gap | About 8 to 12 mmol/L (without potassium) |
To convert PaCO₂ from kPa to mmHg, multiply by 7.5.
The four steps
- pH: acidaemia (below 7.35) or alkalaemia (above 7.45)?
- Primary cause: does the PaCO₂ or the HCO₃⁻ explain the pH? A high CO₂ explains an acidosis (respiratory). A low HCO₃⁻ explains an acidosis (metabolic).
- Compensation: has the other system moved in the expected direction, and by the expected amount?
- If it is a metabolic acidosis, calculate the anion gap.
Compensation rules
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | PaCO₂ (mmHg) ≈ 1.5 × HCO₃⁻ + 8 (± 2), Winter's formula |
| Acute respiratory acidosis | HCO₃⁻ rises about 1 mmol/L for every 10 mmHg rise in PaCO₂ |
| Chronic respiratory acidosis | HCO₃⁻ rises about 3.5 to 4 mmol/L for every 10 mmHg rise |
| Respiratory alkalosis | HCO₃⁻ falls (more if it is chronic) |
If the measured value is outside the expected range, there is a second disorder.
Compensation never overshoots. If a patient with a metabolic acidosis has a normal or high pH, there is a second, alkalotic process.
Worked gas 1: the asthmatic who goes quiet
A man with acute severe asthma becomes drowsy. pH 7.23, PaCO₂ 62 mmHg (8.3 kPa), HCO₃⁻ 25 mmol/L.
- Acidaemia.
- CO₂ is high, so the cause is respiratory.
- HCO₃⁻ is normal, so there is no meaningful compensation yet. This is acute respiratory acidosis.
The clinical message matters as much as the label. In acute asthma the CO₂ should be low, because the patient is breathing fast. A normal or rising CO₂ in a drowsy asthmatic means exhaustion, and that patient needs critical care now. An opioid overdose gives the same gas for a different reason: the drive to breathe is suppressed.
Worked gas 2: the diabetic with deep breathing
An 18-year-old with type 1 diabetes is drowsy, with deep sighing breaths and fruity breath. The pH is low and the HCO₃⁻ is very low.
- Metabolic acidosis with a high anion gap from ketones.
- The deep, sighing breathing is Kussmaul respiration: the lungs blowing off CO₂ to compensate.
Worked gas 3: the child with diarrhoea
Na⁺ 136, Cl⁻ 114, HCO₃⁻ 12 mmol/L.
- Anion gap = 136 − (114 + 12) = 10. Normal.
- This is a normal anion gap (hyperchloraemic) metabolic acidosis. The child is losing bicarbonate in the stool, and chloride rises to take its place.
The anion gap in one table
| High anion gap (an acid has been added) | Normal anion gap (bicarbonate has been lost) |
|---|---|
| Lactic acidosis (shock, sepsis, hypoxia) | Diarrhoea |
| Ketoacidosis (diabetes, starvation, alcohol) | Renal tubular acidosis |
| Renal failure | Acetazolamide |
| Toxins: methanol, ethylene glycol, salicylate | Ureteric diversion into the bowel |
| Large volumes of normal saline |
Worked gas 4: a week of vomiting
A man with gastric outlet obstruction from a pyloric ulcer has vomited for a week.
He loses hydrochloric acid, so he develops a hypochloraemic, hypokalaemic metabolic alkalosis. The kidney, defending volume, holds on to sodium and swaps out potassium and hydrogen ions instead. That is why the urine can be acidic despite a blood alkalosis (paradoxical aciduria), and why the potassium keeps falling.
The treatment is intravenous normal saline with potassium chloride. The same picture in an infant with projectile non-bilious vomiting is pyloric stenosis, and the fluids must be corrected before any surgery.
Do not treat this alkalosis with acid. Replacing chloride, volume and potassium lets the kidney excrete the excess bicarbonate on its own.
Mixed and special patterns
- Salicylate poisoning: respiratory alkalosis early (direct stimulation of breathing), then a high anion gap metabolic acidosis.
- Pregnancy: a mild, compensated respiratory alkalosis is normal. Progesterone drives breathing, so a PaCO₂ around 30 mmHg is expected.
- Early pulmonary embolism, anxiety and high altitude: respiratory alkalosis.
Buffers
- The most important buffer in the extracellular fluid is bicarbonate.
- Inside cells, proteins and phosphate do more of the work. Haemoglobin is an important buffer in blood.
- In urine, phosphate and ammonia carry hydrogen ions out of the body.
How to practise
Do two gases a day for two weeks, always using the four steps in order and writing each step down. By the end, the steps take thirty seconds.
The FCPS-1 Prep question bank includes ABG questions built on these ward situations, each with the steps worked out in the explanation.
Read next: Physiology high-yield · Surviving night duty · Pathology high-yield
Frequently asked questions
Do I need to learn the Henderson-Hasselbalch equation?
Understand that pH depends on the ratio of bicarbonate to CO₂. You will rarely need to calculate it.
Is the anion gap calculated with potassium?
Most exam questions use Na⁺ − (Cl⁻ + HCO₃⁻), with a normal range of about 8 to 12. If the question includes potassium, the normal range is a few points higher.
Are ABG values given in kPa or mmHg?
Both appear. Multiply kPa by 7.5 to get mmHg, and check which unit the options use.