MBBS OSCE · General Medicine
OSCE — Acid-Base Disorders
Eight-minute OSCE station on Acid-Base Disorders: focused history, examination priorities, investigations, emergency and definitive management.
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Brief (to candidate)
You will assess a patient with a presentation consistent with Acid-Base Disorders.[1] You have 8 minutes to take a focused history, outline examination, investigations, and management including red flags.[1]
Clinical context
Acid-base disorders arise from disturbance of the bicarbonate-carbon-dioxide buffer system. There are four primary disorders: metabolic acidosis (low pH, low bicarbonate) — raised anion gap from ketoacidosis, lactic acidosis, renal failure or toxins (MUDPILES) or normal gap from diarrhoea and renal tubular acidosis (HARDUP); metabolic alkalosis (high pH, high bicarbonate — vomiting, diuretics); respiratory acidosis (low pH, high CO2 — COPD, opiates); respiratory alkalosis (high pH, low CO2 — anxiety, pain, sepsis, altitude). The stepwise approach is check the pH, identify the primary disorder, assess compensation (Winter's and the respiratory rules), calculate the anion gap, then treat the c[1]
Candidate tasks
- Clarify onset, severity, associated features, and red-flag symptoms.[1]
- State focused examination priorities.
- List first-line investigations and any named score/criteria.[1]
- Give immediate resuscitation steps.
- Outline definitive management with doses/routes where standard.[1]
- Name complications and disposition (ward / HDU / theatre / discharge safety-net).[1]
- Mention one special-population modifier (pregnancy, child, elderly, CKD).[1]
Examiner checklist
| Domain | Pass behaviours |
|---|---|
| Definition | Correct working diagnosis language |
| Assessment | Focused, prioritised, red flags sought |
| Investigations | Appropriate first-line + interpretation |
| Emergency care | ABC / time-critical actions first |
| Definitive care | Specific drugs/procedures, not generic phrases |
| Safety | Severe metabolic acidosis with pH under 7.1 to 7.15 and haemodynamic instability |
| Safety | High anion-gap acidosis with high osmolar gap — toxic alcohol ingestion (methano |
| Safety | Mixed respiratory alkalosis plus high-gap metabolic acidosis with tinnitus — sal |
| Communication | Clear plan and safety-netting |
Model outline
Lead with the working diagnosis and life threats. Resuscitate before definitive tests when unstable. Use guideline-standard therapy with named agents and doses. Document escalation criteria and follow-up. A safe candidate is specific, structured, and never delays critical care for non-urgent imaging.[1]
References4ShowHide
- [1]Adrogué HJ, Madias NE Management of life-threatening acid-base disorders. First of two parts N Engl J Med, 1998.PMID 9414329
- [2]Wrenn K The delta (delta) gap: an approach to mixed acid-base disorders Ann Emerg Med, 1990.PMID 2240729
- [3]Figge J, Jabor A, Kazda A, et al. Anion gap and hypoalbuminemia Crit Care Med, 1998.PMID 9824071
- [4]Jaber S, Paugam C, Futier E, et al. Sodium bicarbonate therapy for patients with severe metabolic acidaemia in the intensive care unit (BICAR-ICU): a multicentre, open-label, randomised controlled, phase 3 trial Lancet, 2018.PMID 29910040