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A 64-year-old man develops hypotension after induction. Cardiac output falls despite a preserved heart rate. Which physiological variable best explains the reduction in stroke volume?
- AIncreased afterload from sympathetic activation
- BReduced venous return and ventricular preload
- CImpaired myocardial contractility from anaesthetic agent
- DBradycardia-induced reduction in coronary perfusion
- ELoss of atrial kick from junctional rhythm
Induction agents reduce sympathetic tone and venodilate, decreasing preload. By the Frank–Starling relationship, reduced end-diastolic volume lowers stroke volume even when contractility and rate are preserved.
- Clear structured approach with early call for help.
- Differential should include PE, atelectasis and opioid-related hypoventilation.
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MRCP Part 1
UK physician membership exam
SBA question bank, syllabus tutorials and per-topic analytics for the MRCP(UK) Part 1 written exam.
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Anaesthesia basic sciences
Pharmacology, physiology and physics questions mapped to the RCoA Primary FRCA syllabus.
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US medical licensing exam
High-yield basic science questions and tutorials built around the USMLE Step 1 content outline.
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Hyponatraemia: classification, osmolality and safe correction
2,480 words · 13 min read · Updated for the 2026 syllabus
- Classify hyponatraemia by serum osmolality and volume status.
- Interpret urine osmolality and urine sodium to reach a diagnosis.
- Apply safe correction limits and recognise osmotic demyelination risk.
1. Start with osmolality
Measure serum osmolality before anything else. Hyperglycaemia and mannitol cause a translocational (hypertonic) hyponatraemia — correct sodium by 2.4 mmol/L for every 5.6 mmol/L (100 mg/dL) rise in glucose. Severe hypertriglyceridaemia or paraproteinaemia produces pseudohyponatraemia with a normal measured osmolality.
| Cause | Urine Na⁺ | Urine osm |
|---|---|---|
| SIADH | >30 mmol/L | >100 mOsm/kg |
| Hypovolaemia | <20 mmol/L | >100 mOsm/kg |
| Primary polydipsia | Variable | <100 mOsm/kg |
| Diuretic-induced | >30 mmol/L | Variable |
2. Rate of correction
In chronic hyponatraemia, limit correction to 8 mmol/L in any 24 hours (4–6 mmol/L if malnourished, alcohol-dependent, hypokalaemic or liver disease). Severe symptoms — seizures or reduced consciousness — justify 100 mL boluses of 3% saline to raise sodium by 4–6 mmol/L quickly, then stop.
A rapid rise in urine output with dilute urine after fluid resuscitation signals resolving ADH stimulus — the classic set-up for overcorrection and osmotic demyelination. Desmopressin plus 5% dextrose can re-lower the sodium.
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