tachycardia
Tachycardia means a fast heart rate — by convention, more than 100 beats a minute in a resting adult. Like its slow counterpart, it is a finding rather than a single disease, and a racing heart can be entirely appropriate or a warning sign depending on the circumstance.
Often the heart is simply doing its job: exercise, fever, fear, pain, dehydration or anaemia all make a normal heart beat faster to deliver more oxygen. This “sinus tachycardia,” driven by the natural pacemaker, is a response to a demand rather than a fault in the wiring. The cure there is to treat the underlying cause, not the rate itself.
Tachycardia becomes a primary problem when the heart's electrical system misfires — as in atrial fibrillation, supraventricular tachycardia or ventricular tachycardia. Clinicians distinguish these by whether the rhythm is regular or irregular and whether the QRS complexes are narrow (supraventricular) or wide (more often ventricular), because that distinction drives both the urgency and the treatment.
A patient with a fever of 39 °C has a heart rate of 120; rather than a heart drug, the right treatment is to lower the fever and replace fluids, after which the rate settles on its own.
Sinus tachycardia is usually a symptom of something else, treated at its source.
The first question with any tachycardia is whether it is sinus tachycardia (a reaction the body needs) or a true arrhythmia. Blindly slowing an appropriate compensatory rate can do harm.