Complex sentence: It is also distinct from a cardiopulmonary bypass machine, which is an external device used to provide the functions of both the heart and lungs, which are only used for a few hours at a time, most commonly during heart surgery.
Split into simpler sentences: It is also distinct from a cardiopulmonary bypass machine, which is an external device used to provide the functions of both the heart and lungs. These machines are only used for a few hours at a time, most commonly during heart surgery.