Internal Medicine
Core concepts
Internal medicine questions reward a disciplined approach: identify the syndrome, assess severity, then choose the intervention that changes outcome. Know ACS (ECG within 10 minutes, aspirin, reperfusion), acute heart failure (congestion versus perfusion), COPD and asthma exacerbations (controlled oxygen, bronchodilators, steroids, avoid sedatives), diabetes emergencies (DKA versus HHS), thyroid disease, AKI and electrolyte disorders, anaemia, sepsis, and anticoagulation.
Traps
- Correcting sodium too fast in hyperglycaemia: use corrected Na = measured Na + 1.6 x (glucose - 5.5)/5.5 in mmol/L or 0.3 x (glucose - 100)/100 in mg/dL; aim for less than 8-10 mmol/L per 24 hours.
- Giving insulin before fluids in DKA: start isotonic saline first; insulin drives potassium into cells.
- Withholding potassium in DKA because serum K looks normal; total-body potassium is low, so replace once K is below 5.5 and urine output is adequate.
- Missing a normal anion gap acidosis (diarrhoea, renal tubular acidosis) versus a high anion gap acidosis (ketoacidosis, lactate, toxins).
- Beta-blockade in acute decompensated heart failure or cocaine-associated chest pain.
- Treating anaemia of CKD with transfusion instead of iron and an erythropoiesis-stimulating agent.
Worked example
A 24-year-old with type 1 diabetes has glucose 28 mmol/L, pH 7.12, bicarbonate 10 and potassium 4.1. The anion gap is raised and ketones are positive: this is DKA. Give isotonic saline, then a fixed-rate insulin infusion, add potassium to the fluid, and look for the precipitant. Do not give bicarbonate unless pH is below 6.9. Recheck potassium and glucose hourly; add dextrose when glucose falls below about 14 mmol/L.
Rule of thumb
Treat the patient, not the number: in DKA, fluids and potassium come before insulin, and the precipitant is part of the diagnosis.