This 58-year-old hypertensive man with known dilated cardiomyopathy and severe left ventricular dysfunction presents to the emergency department with a two day history of weakness. His medications include digoxin, furosemide, spironolactone, and KCl 20 mEq bid:

• No P waves are seen
• Regular wide QRS rhythm, 49/min – QRS duration 226 msec
• Left axis deviation
• Left bundle branch block

The differential diagnosis for this ECG includes hyperkalemia and an idioventricular rhythm. The ECG findings that favor hyperkalemia include the absence of P waves, very wide QRS complexes, and T waves that are not as wide as would be expected in an idioventricular rhythm (where typically the T waves would be much wider than the QRS).

• Hyperkalemia delays myocardial conduction (wide QRS) while it shortens the action potential
• It was noted that the patient had been on spironolactone and potassium chloride because of a previous episode of hypokalemia
• The serum potassium was found to be 7.4 mmol/L (normal = 3.5 – 5.0)

The patient received IV calcium chloride, glucose and insulin. The following ECG is recorded one hour later:

• No P waves are seen
• Regular wide QRS rhythm, 63/min – QRS duration 146 ms. (markedly reduced compared to the previous ECG)
• Left bundle branch block

These changes are compatible with resolving hyperkalemia.