Non-variable pressure support ventilation (PSV) is one of the most commonly used modes of assisted ventilation worldwide. PSV is a ventilatory strategy which supports spontaneous breathing efforts with a fixed amount of positive airway pressure by the mechanical ventilator. In other words, in non-variable PSV, pressure support is constant breath-by-breath, and will be adjusted according to individual needs. Variable pressure support ventilation is a further development of the conventional PSV mode. The mean pressure support is equivalent to that used in conventional PSV, but single values vary randomly from breath to breath.