Premature Ventricular Contractions
These are Premature Ventricular Contractions (PVCs) that seem to be caused by my chemotherapy. They aren't a serious health threat in my case. Just annoying.
Worth consulting your cardiologist if they are occurring frequently.
Here they're shown on the ECG app on my Apple Watch. When they're happening I can usually feel them as a flutter in my chest, as the lower half of my heart does an extra beat.