What are the risks of untreated bradycardia or tachycardia?

Depends. Depends on how fast or how slow, the cause for the abnormal rate and whether or not it is causing any symptoms. Both abnormally slow and fast heart beats can cause congestive heart failure and syncope (fainting). Persistent fast heart beats can cause weakness of heart muscle referred to as tachycardia induced cardiomyopathy.

Related Questions

What risks are attributed to untreated bradycardia or tachycardia?

Tachy or Brady. Aside from not feeling well with either of these, there is a likelihood that either can cause a cardiomyopathy or a weakness in the muscle. These rhythms can also cause a loss of consciousness or syncope. So, these arrhythmias should be treated and followed regularly. Read more...
Pump Failure. Heart rate & rhythm combine to determine cardiac output, or, in common terms, the pumping ability of the heart. Good cardiac function & pumping ability in a normal heart can decrease with slow or fast heart rates. Usually, low rates protect the heart, but limit cardiac output = exercise tolerance is decreased. Sustained high rates = heart damage & heart failure. Sick heart? All bets are off. Dr. M. Read more...
It depends. In some cases, these can be normal states (an athletic person with bradycardia) or due to another condition (tachycardia with overactive thyroid, too much alcohol, or anemia) that needs to be treated. Otherwise, either can cause dizziness or fainting, fatigue, and inadequate blood flow (which can cause heart damage or even a stroke). The safest approach is treatment. Read more...