General Medicine
Fluids, Electrolytes and Acid-Base
Fluids, electrolytes and acid-base for MBBS: hyponatraemia, hyperkalaemia, hypercalcaemia and arterial blood gas interpretation, mapped to NMC codes IM22.1 to IM22.13.
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Fluids, Electrolytes and Acid-Base
Fluids, electrolytes and acid-base for MBBS: hyponatraemia, hyperkalaemia, hypercalcaemia and arterial blood gas interpretation, mapped to NMC codes IM22.1 to IM22.13.
This chapter covers disorders of sodium, potassium, calcium and acid-base balance, integrating their causes, clinical features and management. It emphasises the safe correction of hyponatraemia, the emergency treatment of hyperkalaemia and a stepwise approach to arterial blood gas interpretation.
High-yield: Fluids, Electrolytes and Acid-Base
- Hyponatraemia is classified by volume status into hypovolaemic, euvolaemic and hypervolaemic types.
- The syndrome of inappropriate antidiuretic hormone secretion causes euvolaemic hyponatraemia with concentrated urine.
- Overrapid correction of chronic hyponatraemia risks osmotic demyelination, so the sodium is raised slowly.
- Hyperkalaemia is a medical emergency because of its risk of fatal cardiac arrhythmia.
- The ECG in hyperkalaemia progresses from peaked T waves to a widened QRS and finally a sine-wave pattern.
- Emergency hyperkalaemia treatment gives calcium to protect the heart, then insulin and dextrose to shift potassium.
- Hypokalaemia causes muscle weakness and characteristic ECG changes including U waves.
- Magnesium must be corrected to allow effective correction of refractory hypokalaemia.
- Metabolic acidosis is subdivided by the anion gap into high and normal anion gap types.
- A high anion gap acidosis is caused by lactate, ketones, renal failure and certain toxins.
- Hypercalcaemia is most often caused by primary hyperparathyroidism or malignancy.
- Symptomatic hypercalcaemia is treated with intravenous fluids and, when needed, bisphosphonates.
- Arterial blood gas interpretation follows a stepwise approach to the pH, carbon dioxide, bicarbonate and compensation.
- Compensation is a predictable physiological response, and a value outside the expected range signals a mixed disorder.
- Multiple endocrine neoplasia syndromes link parathyroid, pancreatic and other endocrine tumours.
Fluids and acid-base essentials
- **Hyponatraemia:** Classify by volume status; correct chronic cases slowly to avoid osmotic demyelination.
- **Hyperkalaemia:** Calcium to protect the heart, then insulin-dextrose to shift potassium; watch the ECG.
- **Metabolic acidosis:** Divide by anion gap; high gap from lactate, ketones, renal failure or toxins.
- **Blood gas:** Stepwise pH, carbon dioxide, bicarbonate and compensation; a deviation signals a mixed disorder.
NMC competencies in this chapter
- **IM22.1:** Hypercalcaemia: parathyroid-mediated and non-parathyroid causes
- **IM22.5:** Hyponatraemia: causes, classification and clinical approach
- **IM22.7:** Hypokalaemia: causes, features and management
- **IM22.8:** Hyperkalaemia: recognition and emergency treatment
- **IM22.9:** Metabolic acidosis and the anion gap
- **IM22.10:** Metabolic alkalosis: generation, maintenance and management
- **IM22.11:** Respiratory acidosis: hypoventilation and compensation
- **IM22.13:** Integrated arterial blood gas and acid-base diagnosis
Frequently Asked Questions
Why must chronic hyponatraemia be corrected slowly?
Rapid correction of chronic hyponatraemia can cause osmotic demyelination syndrome, so the serum sodium is raised gradually within recommended daily limits.
How is severe hyperkalaemia treated?
Calcium is given first to stabilise the myocardium, followed by insulin with dextrose and other measures to shift and remove potassium, with dialysis if refractory.
How is metabolic acidosis classified?
It is divided by the anion gap into high anion gap acidosis, caused by lactate, ketones, renal failure and toxins, and normal anion gap acidosis from bicarbonate loss.
How does this chapter help for NEET-PG?
Hyponatraemia correction, hyperkalaemia ECG changes and treatment, and stepwise blood gas interpretation are common exam themes that map directly to questions.
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