Mechanical ventilation is the medical term for artificial ventilation where mechanical means is used to assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by an anesthesiologist, certified registered nurse anesthetist, physician, physician assistant, respiratory therapist, paramedic, EMT, or other suitable person compressing a bag or set of bellows. Mechanical ventilation is termed "invasive" if it involves any instrument penetrating the trachea through the mouth, such as an endotracheal tube or the skin, such as a tracheostomy tube.