Minimization of ventilator induced lung injury in chronic lung disease

Chronic lung disease (CLD) is diffused in very low birthweight (VLBW) infants as a consequence of respiratory distress syndrome (RDS). Ventilator induced lung injury is a significant component of CLD, determined by several factors, such as volutrauma (overdistension of the lung by the delivery of too much gas), barotraumas (excessive pressure, with damage of airway epithelium), atelectotrauma (opening and closing of lung units), biotrauma (infection and inflammation), rheotrauma (injury caused by inappropriate airway flow). Lung damage may be minimised by using lung protective strategies, in order to minimise ventilator induced lung injury by ameliorating the damaging effects of these components.

Continuous positive airway pressure (CPAP) is a form of continuous distending pressure used to maintain alveolar inflation during expiration; its clinical application is still under debate. Permissive hypercapnia is a protective lung strategy capable to decrease volutrauma and increase oxygen unloading. Clinical trials, however, show no reduction in CLD associated with this strategy. Further studies are therefore necessary for both these strategies.

CLD may be determined by patient-ventilator asynchrony even if technological improvements, such as patient triggered ventilation (PTV) may have limited this concern. Unfortunately, trials of PTV have so far been disappointing, although short term benefits have been described. Additional investigation is thus warranted. Volume targeted ventilation allows to choose the volume of gas to be delivered; this strategy reduces volutrauma, regardless of lung compliance. Potential advantages of this methodology have been demonstrated and a recent trial showed a significant benefit in VLBW infants. Another novel technique is proportional assist ventilation, which is based on pulmonary mechanics and on loading of respiratory musculature to regulate the ventilator; this may reduce barotrauma, and further study seems advisable.

High frequency ventilation is based on tiny gas volumes moving in and out of the lung at rapid rates. Two modalities are used: high frequency jet ventilation (HFJV) and high frequency oscillatory ventilation (HFOV). HFJV is used as a rescue therapy; however, one study on HFJV as a primary strategy showed a reduction in the incidence of CLD and a lower need for home oxygen therapy. Results concerning HFOV are much more controversial; therefore, HFOV cannot be recommended as a primary strategy.

Nowadays, it is possible to monitor continuously all respiration and vital parameters. Monitoring has been shown to be a useful help to weaning infants from mechanical ventilation. Although there are no randomised controlled trials, monitoring can be used to customise strategies for each patient.

Avoidance of the ''mechanical'' elements of ventilator induced lung injury can be accomplished by ventilating the lung close to the normal functional residual capacity. Moreover, careful attention must be given to airway and circuit flow. Excessive flow in the ventilator circuit may contribute to an increased elastic load and overdistension; if flow is too low, gas volumes may be too small and air hunger may develop.

In conclusion, clinical investigation has not yet defined the best way to avoid lung injury in preterm infants requiring mechanical ventilation. The reality is that some CLD may be inevitable in VLBW infants. Nevertheless, clinicians must try to avoid inflicting further damage during this time and try to mitigate it with an adequate strategy.

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