There are two fundamental methods to control the delivery of a breath. The clinician can choose to keep either volume or pressure constant from breath to breath. The control variables most commonly used to describe modes of ventilation are volume-controlled (VC) ventilation and pressure-controlled (PC) ventilation. Within these two categories are multiple ways to tailor specific modes. In volume-controlled ventilator, clinician sets flow magnitude and pattern; the pressures used to deliver the set tidal volume will vary as a function of the patient’s pulmonary resistance and compliance and the his or her efforts in order to deliver that flow.