Ventilatorinduced lung injury

If the lung is unable to return to its resting state during expiration, the alveolar pressure remains higher than the airway pressure measured by the ventilator. This auto-PEEP due to gas trapping increases the risk of alveolar overdistension and ventilator-induced lung injury ( Marcy„.and„.M,arini1991). The gas volume (Vt) above functional residual capacity left in the lung at time t after the start of expiration is calculated using the formula

where V0 is the volume at time zero and RC is the respiratory time constant (the product of resistance and compliance). Therefore as t is reduced, i.e. inspiratory time is lengthened, the possibility of gas trapping increases. Thus inverse ratio ventilation is associated with a high risk of auto-PEEP development. The above formula also shows that as the respiratory time constant increases, the risk of gas trapping also increases. Conditions with increased compliance (e.g. emphysema) and/or increased resistance (e.g. chronic obstructive pulmonary disease or acute asthma) lead to a prolonged respiratory time constant. A reduction in inspiratory time to allow adequate time for alveolar emptying during expiration may reduce auto-PEEP ( SJutsky 1994). However, this reduction in inspiratory time may lead to increased peak inspiratory pressure in volume-targeted ventilation or reduced tidal volume in pressure-targeted ventilation.

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