A single study has shown that antiarrhythmic activity of quinidine can be opposed by amiloride
A study in 10 patients with inducible sustained ventricular tachycardia was carried out to see whether a beneficial interaction occurred between quinidine and amiloride. Patients were given oral quinidine until their trough serum levels reached 10 micromol/L, or maximum well-tolerated dose was reached. After electrophysiological studies,oral amiloride was added at a dosage of 5mg twice daily, increased up to 10mg twice daily (if serum potassium levels remained normal) for 3 days. The electrophysiological studies were then repeated. Unexpectedly, 7 of 10 patients demonstrated adverse responses while taking both drugs. Three developed sustained ventricular tachycardia and 3 others had somatic adverse effects (hypotension,nausea, diarrhoea), which prevented further studies being carried out. One patient had 12 episodes of sustained ventricular tachycardia while taking both drugs. Amiloride had no effect on quinidine levels (See reference number 1).
Not understood. The combination of quinidine and amiloride increased QRS interval, but did not prolong QT interval more than quinidine alone.
So far evidence seems to be limited to this single study but it suggests that amiloride can oppose antiarrhythmic activity of quinidine. The full clinical implications of this interaction are not yet known, but it would now clearly be prudent to consider monitoring to confirm that quinidine continues to be effective if amiloride is present.
1. Wang L,Sheldon RS, Mitchell B, Wyse DG, Gillis AM, Chiamvimonvat N, Duff HJ. Amiloride-quinidine interaction: adverse outcomes. Clin Pharmacol Ther (1994) 56, 659–67.