Ventilators are used commonly in the operating theatre and in the ICU to deliver mechanical ventilation to the lungs.
In the operating theatre, ventilation is in anaesthetized and often pharmacologically paralysed patients with predominantly normal lungs.

These ventilators are relatively simple and are designed to deliver varying concentrations of oxygen, air, nitrous oxide and volatile agents to patients through an anaesthetic circuit.

In the ICU, ventilators provide respiratory support to patients with respiratory failure. Respiratory failure is a state in which the pulmonary oxygen uptake is so severely disturbed that the supply of oxygen to the tissues and/or the removal of carbon dioxide from them is inadequate. Respiratory failure can be caused by relative hypoventilation, characterized by an increase in arterial carbon dioxide tension, or failure of diffusion at the alveolar–capillary membrane, characterized by decreased arterial oxygen tension.