Background: Bruton s tyrosine kinase inhibitors (BTKi) are used to treat a form of leukemia. But taking BTKi can also increase a person s risk of developing an abnormal heart rhythm. This can cause sudden death. In this natural history study, researchers want to learn how BTKi affects the heart.

Objective: To identify and monitor the effects of BTKi on the heart.

Eligibility: People aged 18 and older currently receiving or planning to receive BTKi or venetoclax.

Design: Participants who have not yet started BTKi will have 2 required clinic visits: 1 before they start taking BTKi, and 1 about 6 months later. Participants who are already taking BTKi will have 1 required visit. Participants will undergo multiple tests: A physical exam, including collection of blood and saliva. A test that measures heart activity via stickers placed on the chest. A test that uses sound waves to capture images of the heart. An exercise stress test that monitors heart activity and blood pressure while the participant works on a treadmill or stationary bike. Sound wave images of the heart may also be taken while the participant exercises. Stress magnetic resonance imaging (MRI) may be done in place of an exercise test. Participants will lie on a table that slides into a tube. They will be given drugs to stress the heart while images are taken. Participants may wear a device to monitor their heart at home. Participants may have repeat visits if they develop heart symptoms or if they need to stop taking BTKi. They will have follow-up phone calls each year for up to 3 years.