Flow of gas is calculated in liters per minute. Flow commences at the beginning of a breath and stops at the end of the breath. Gas flows into the lungs in inspiration and out of the lungs in expiration. The pattern of expiratory flow is more or less the same for different modes of ventilation, as long as the expiratory phase is long enough to prevent gas trapping. The normal flow pattern of gas moving in and out of the lungs is sinusoidal. In volume control ventilation a variety of different wave patterns can be used. In clinical practice, constant and decelerating flow patterns are used; the latter is preferred. In constant, decelerating and sinusoidal flow patterns, the inspiratory flow rate is equal to the peak flow rate, but the mean flow rate is higher in constant flow patterns rather than the other two. This suggests that this pattern will cause more shearing injury to the lung parenchyma. Therefore a decelerating flow pattern is probably the most effective flow pattern - it ensures peak flow early in inspiration, while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it.