Ahlfors cited:
QuoteLike many other drugs, chloroquine and hydroxychloroquine can cause changes in the electrocardiogram.
At the risk of repeating myself, this is important. In a subject who has a long interval between the Q wave and the T wave of their electrocardiogram (400 msec corrected for heart rate is roughly the upper limit of normal), hydroxychloroquine, chloroquine and possibly antibiotics including azithromycin, may further lengthen this interval. That messes with the heart's conduction system and can result in premature beats, supraventricular tachycardia (SVT) and even ventricular tachycardia, an extremely dangerous rhythm which often leads to ventricular fibrillation which causes death in minutes unless there is intervention with a defibrillator.
You can ask your health care provider to measure your Q-T interval. If you are a bit technically oriented, you can do it yourself. You will need to get a recent EKG. Either from your provider or one you take yourself. To take it yourself is possible but not entirely easy using the Kardia device. This is available in two forms, one lead (Lead I) or standard 6 lead (Leads I, II, III, aVR, aVL, and aVF). Usually lead I is enough but sometimes it's better to have the others. Kardia, in the US, is available without prescription, from Amazon for $90 (one lead) and $150 (six lead). The device is tiny, intrinsically safe (3V battery) and entirely wireless. To use Kardia correctly, make sure your Kardia app has access to blue tooth and to the microphone (on the iPhone anyway, I don't know about the Android app if there is one). Kardia is also available as an accessory to the iWatch. I am not familiar with that. Amazon has other inexpensive EKG DIY devices but I do not know if they work properly. Kardia does.
You can learn how to measure your Q-T interval from this wikipedia entry: https://en.wikipedia.org/wiki/QT_interval and note the correction for heart rate when you determine "normal."
The Kardia app allows you to send a PDF of 30 seconds of EKG along with a calibration grid, to your PC or other device connected to a printer, using email or text messaging. The measurement should be made on a printed copy. Note that the grid made with the printer may not be accurate. On my printer, what should be 25 mm/second turns out to be 22. If that happens, correct appropriately. For example if your Q-T is 9 mm and your one second grid marks are 22 mm apart, your interval is 9/22 or 0.41 seconds (410 msec). Again be sure to correct for heart rate as per the wiki article.
If you elect to take chloroquine or hydroxychlotoquine, with or without azithromycin, I recommend doing this measurement every day. QT prolongation can happen that fast. If it happens, stop the drugs and notify your health care provider. If you are very isolated, you may wish to have a supply of a beta blocker around. It may help protect from arrhythmias associated with prolongation of the QT interval.
Sorry, no time to proofread, please make a post for me if there are typos or questions. If you are planning self medication, you may want to note and print this post.
Remember the old adage, the person who medicates themselves has a fool for a physician! Always consult your medical provider before doing ANYTHING recommended on the internet. Including this!