New modes of ventilator operation are often introduced without rhyme or reason. The dual control modes are, however, a response of the manufacturers to clinician desire to provide constant minute volume/tidal volume while operating in the pressure support or pressure control mode. Dual control modes allow the ventilator to control pressure or volume. The ventilator cannot control pressure and volume at the same time, so it is an either/or phenomenon. Dual control modes can operate "within a breath" or on a "breath to breath" basis. Dual control within a breath refers to a technique whereby the ventilator switches from pressure to volume control in the middle of a breath. In the technique of dual control breath to breath, all breaths are pressure control breaths, but the level of pressure varies from breath to breath in an effort to maintain a constant tidal volume.