Extrasystole and Systole

Extrasystole noun - A premature systole resulting in a momentary cardiac arrhythmia.

Systole and extrasystole are semantically related. In some cases you can use "Systole" instead a noun "Extrasystole".

Systole noun - The contraction of the chambers of the heart (especially the ventricles) to drive blood into the aorta and pulmonary artery.

Extrasystole and systole are semantically related. Sometimes you can replace term "Extrasystole" with "Systole", this nouns are similar.