"heart rhythm" is correct and usable in written English.
You can use it to refer to the beating of the heart over a period of time, for example "The doctor was concerned about her irregular heart rhythm."
Heart rhythm is, of course, a complicated affair.
Antipsychotic drugs can affect heart rhythm in some vulnerable people.
A heart attack, abnormal heart rhythm or a stroke?
Fortunately, they survived the resulting heart rhythm disturbance.
An abnormal heart rhythm raises her risk for stroke.
Anticoagulants showed benefit even if the heart rhythm returned to normal.
The most common cause is a life-threatening abnormal heart rhythm called ventricular fibrillation".
In addition, adrenaline has drawbacks, like depleting the body's oxygen and disrupting heart rhythm.
Researchers recruited 2,305 patients with heart failure and normal heart rhythm.
Defibrillators deliver a jolt of electricity to interrupt a potentially fatal heart rhythm.
In these cases doctors looked for abnormalities, like an unusual heart rhythm.
