The central premise of positive pressure ventilation is that gas flows along a pressure gradient between the upper airway and the alveoli. The magnitude, rate and duration of flow are determined by the operator. Flow is either volume targeted and pressure variable, or pressure limited and volume variable. The pattern of flow may be either sinusoidal (which is normal), decelerating or constant. Flow is controlled by an array of sensors and microprocessors. Conventionally, inspiration is active and expiration is passive (although modern ventilators have active exhalation valves).
There are two phases in the respiratory cycle, high lung volume and lower lung volume (inhalation and exhalation). Gas exchange occurs in both phases. Inhalation serves to replenish alveolar gas. Prolonging the duration of the higher volume cycle enhances oxygen uptake, while increasing intrathoracic pressure and reducing time available for CO2 removal.