Conceptual basis for inverse ratio and airway pressure release ventilation

A variety of inspiratory maneuvers during mechanical ventilation do not improve arterial oxygenation. Modestly prolonged inspiratory time with a pressure-limited ventilator (using either IRV or APRV) may benefit arterial oxygenation in this population. Available data are not sufficient to be conclusive. It appears that mean airway pressure, rather than end-expiratory or inspiratory pressures, determines arterial oxygenation. Increases in mean airway pressure that are achieved either by increasing end-expiratory or by prolonging inspiratory time with a pressure-limited ventilator affect oxygenation similarly. Therefore, in the apneic patient, improved arterial oxygenation can be obtained by increasing mean airway pressure either with the addition of CPAP or PEEP, by instituting IRV with or without PEEP, or by using APRV. Data gathered from patients who are breathing spontaneously may lead to very different conclusions regarding these various ventilatory interventions.