ECG Case 031

Main Abnormalities

• The first half of the ECG shows sinus rhythm with prominent U waves and a long QU interval (520ms).
• An atrial ectopic beat kicks off a run of Torsades de Pointes by landing on the T/U wave during the vulnerable phase of repolarisation and causing “R on T” (or “R on U”) phenomenon.

Diagnosis

The combination of…

• Atrial ectopy
• Prominent U waves
• Long QU interval
• Torsades de Pointes

… is strongly suggestive of severe hypokalaemia. This patient had a K of 1.9 mmol/L.

Hypokalaemia occurs in eating disorders via multiple mechanisms including:

• H+ is lost in vomiting — renal distal tubular reabsorption of H+ occurs in exchange for K+ (primary mechanism)
• Loss of K+ in bodily secretions — vomiting, purging with laxatives or diuretics
• Reduced oral intake
• Metabolic alkalosis from vomiting causing intracellular shifts of K+
• Hypovolaemia causing secondary aldosteronism with renal loss of K+

U wave morphology

The appearance of U waves in hypokalemia may vary. This example demonstrates discrete U waves that are clearly distinguishable from the T wave.

Compare this with Quiz ECG 006, where the whole ST-T-U complex is fused together to produce a “rollercoaster” appearance, with the U wave appearing as a positive deflection that emerges from a negative ST segment and T wave.

Discrete U waves

“Rollercoaster” U waves

Further Reading

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.
• Wiesbauer F. Little Black Book of ECG Secrets. Medmastery PDF