Recently, in my practice, I have come across an increasingly popular drug interaction – amiodarone and ondansetron (Zofran). Amiodarone is a heart medication used to treat irregular heart rhythms, such as ventricular tachycardia and atrial fibrillation. Ondansetron is used to treat or prevent nausea.
The scenario usually goes like this:
You have been taking amiodarone for years to keep your heart rhythm in check…and it’s been working fine. You come to the pharmacy to fill your new prescription for ondansetron because you have awful nausea that is completely unrelated to your heart troubles. The pharmacist tells you that she can’t fill your prescription because it interacts with your heart medicine and causes heart arrhythmia.
You think, “But I already take medication for my heart rhythm, and what does my nausea medicine have to do with my heart?” You feel like you’re going to lose your lunch.
How Does Amiodarone Work?
A single heartbeat occurs through a delicate balance of calcium, sodium and potassium ions flowing in and out of heart cells. When potassium leaves the cell, calcium and sodium enter. This is called depolarization. When potassium flows back into the cell and calcium and sodium leave, it is called repolarization. The time that it takes for the heart cells to depolarize and repolarize is the QT interval.
The purpose of amiodarone is to bring rapid heart rhythms back to normal. It does this by blocking the channels that allow potassium to move back into the heart cells, thereby increasing the time of the QT interval. This is called QT prolongation.
What Does Ondansetron Have to Do with Your Heart?
The problem with ondansetron is that it can also block potassium channels and lead to QT prolongation. If amiodarone is left alone, prolonging the QT interval is simply a treatment for rapid heart rhythm. When you add ondansetron, things can get dangerous.
As I mentioned before, ondansetron can prolong QT intervals. It does so in a dose-dependent manner. This means that the higher your dose of ondansetron, the higher the chance that it will cause QT prolongation.
Enter the Liver
Amiodarone and ondansetron are both metabolized in the liver by a series of enzymes. When the same enzyme metabolizes two drugs, one may slow down the metabolism of the other. This leads to higher levels of the drug that is metabolized slower and increases its effects.
Such is the case with ondansetron. Amiodarone slows down the metabolism of ondansetron, increasing its levels in your body. Because ondansetron causes QT prolongation in a dose-dependent manner, increased levels mean you have a higher chance of having QT prolongation.
So what’s wrong with QT prolongation?
The Twisting of Points
QT prolongation can lead to a special type of arrhythmia known as Torsades de Pointes. In French, it means “the twisting of points”. It got its name because, on an electrocardiogram (ECG), it appears as though the points are twisted. Any time that you have two medications that prolong the QT interval, you are at a higher risk for Torsades de Pointes.
During Torsades de Pointes, your heart rate can go up to 250 beats per minute (normal heart rate is 60 to 100 bpm). Your heart is beating so fast that the ventricles don’t get a chance to fill up with blood before contracting. Your heart is pumping very rapidly but not sending out blood to the body. This may lead to the following:
- Palpitations
- Dizziness
- Fainting
- Sudden cardiac death
None of the above sound pleasant, but personally, I would try to avoid the last symptom. Listen to your pharmacist about drug interactions, and let your doctor change your nausea medicine as needed. There are other options.
