Noninvasive positive pressure ventilation, also called noninvasive ventilation (NIV), is delivered through a noninvasive device, such as a full-face mask. Unlike the systems for invasive ventilation, which is delivered through an endotracheal or tracheostomy tube, the NIV delivery system is not a closed system, and so it leaks. The size of the leak affects the tidal volume delivered and the ability of the patient to trigger the machine into inspiration or cycle the machine into expiration. The acceptable leak for most systems is less than 35% of the peak inspiratory flow (leakage may be expressed as a percentage or as liters per minute).