A man on diuretics presents with weakness. An ECG was done which showed flat T waves and prominent U waves. What is the most likely diagnosis?

Explanation

## Correct Answer: C. Hypokalaemia Hypokalaemia (serum K⁺ <3.5 mEq/L) is the classic cause of flat T waves and prominent U waves on ECG. The patient is on diuretics—loop and thiazide diuretics are the most common culprits in Indian clinical practice for causing hypokalaemia by increasing urinary potassium excretion. The ECG changes reflect altered repolarization: flat or inverted T waves occur due to prolonged repolarization, and U waves (deflection after T wave, best seen in precordial leads) are pathognomonic for hypokalaemia. These changes typically appear when K⁺ falls below 3.0 mEq/L. The weakness described is due to hypokalaemia-induced muscle dysfunction and impaired neuromuscular transmission. Diuretic-induced hypokalaemia is a common iatrogenic problem in India, especially in patients on long-term loop diuretics for heart failure or hypertension without potassium supplementation or concurrent potassium-sparing agents. The combination of diuretic use + weakness + characteristic ECG findings (flat T + prominent U) makes hypokalaemia the definitive diagnosis. ## Why the other options are wrong **A. Hypomagnesemia** — While hypomagnesemia can coexist with diuretic use and may cause weakness and arrhythmias, it does NOT produce the pathognomonic flat T waves and prominent U waves. Hypomagnesemia causes prolonged PR and QT intervals, peaked T waves, and widened QRS—a different ECG pattern. NBE may trap students who know diuretics cause both hypomagnesemia and hypokalaemia but forget the specific ECG signatures. **B. Hypernatremia** — Hypernatremia causes neurological symptoms (confusion, seizures) and muscle weakness, but the ECG findings are non-specific and do NOT include flat T waves or U waves. Hypernatremia is rare with diuretic use alone (requires concurrent water loss). The flat T + U wave pattern is incompatible with hypernatremia pathophysiology. **D. Hyperkalaemia** — Hyperkalaemia produces peaked (tented) T waves, prolonged PR interval, and widened QRS—the opposite ECG pattern from what is described. Diuretics typically LOWER potassium, not raise it (except potassium-sparing agents). This is a classic NBE distractor for students who confuse hypokalaemia and hyperkalaemia ECG changes. ## High-Yield Facts - **Flat T waves + prominent U waves** = pathognomonic ECG pattern for hypokalaemia (K⁺ <3.0 mEq/L). - **Loop and thiazide diuretics** are the most common cause of hypokalaemia in Indian outpatient practice; potassium-sparing agents (spironolactone, amiloride) prevent this. - **Hypokalaemia weakness** results from impaired muscle membrane potential and neuromuscular transmission; severe cases risk rhabdomyolysis. - **U wave** is a small deflection after T wave, best seen in precordial leads (V2–V4), and is virtually diagnostic of hypokalaemia. - **Diuretic-induced hypokalaemia** is preventable with KCl supplementation (20–40 mEq/day) or concurrent potassium-sparing diuretic in Indian guidelines for chronic heart failure. ## Mnemonics **ECG Changes in Hypokalaemia: FLAT-U** **F**lat T waves, **L**owered ST segment, **A**ugmented U waves, **T**ented appearance lost. **U** = U wave prominent. Use this when you see flat T + U waves on ECG in a diuretic patient. **Diuretic-Induced Electrolyte Loss: HALT** **H**ypomagnesemia, **A**lkalosis, **L**ow K⁺, **T**hirst (hypernatremia risk). Loop/thiazide diuretics cause all four; remember K⁺ loss is the most clinically urgent. ## NBE Trap NBE pairs diuretic use with multiple electrolyte abnormalities (hypomagnesemia, hyponatremia, hypokalaemia) to test whether students can distinguish ECG patterns. The flat T + U wave combination is pathognomonic for hypokalaemia alone; students who know "diuretics cause electrolyte loss" but confuse the specific ECG signatures will pick hypomagnesemia or hypernatremia.</trap> <parameter name="textbookRef">Harrison Ch. 280 (Electrolyte Disorders); KD Tripathi Ch. 12 (Diuretics); Robbins Ch. 8 (Cellular Injury)</textbookRef> <parameter name="clinicalPearl">In Indian primary care, diuretic-induced hypokalaemia is a leading preventable cause of sudden cardiac arrhythmias and syncope in elderly hypertensive patients. Always check serum K⁺ at baseline and 2 weeks after starting loop/thiazide diuretics; concurrent spironolactone or KCl supplementation is standard practice in Indian heart failure guidelines (CSCI/IAC).</clinicalPearl> </invoke>