Patient determinants of VILI

The condition of the ventilated lung is of considerable importance in determining susceptibility to VILI. At one extreme,

Figure 8.2 (A) CT scan of a 25 year old man with ARDS showing the typically heterogeneous distribution of opacification within the lungs, mostly in the posterior dependent regions. (B) CT scan of the same patient 8 months later showing remarkably little abnormality in the posterior regions but with reticular changes anteriorly (large arrows). Reproduced with permission from Desai et al.44

Figure 8.2 (A) CT scan of a 25 year old man with ARDS showing the typically heterogeneous distribution of opacification within the lungs, mostly in the posterior dependent regions. (B) CT scan of the same patient 8 months later showing remarkably little abnormality in the posterior regions but with reticular changes anteriorly (large arrows). Reproduced with permission from Desai et al.44

VILI does not appear to be a clinical problem in patients with normal lungs who can undergo prolonged periods of mechanical ventilation without detrimental effect.37 In these instances the pressures and flows within the lung closely resemble the physiological situation. At the other extreme, the grossly abnormal lungs of patients with ARDS are highly susceptible to VILI, and it may be that in some patients no mechanical ventilation strategy is entirely devoid of detrimental effects.

Ventilating the injured lung

Animal studies using isolated lungs and intact animals have indicated that injured lungs are more susceptible to VILI.38-40 An important factor underlying this predisposition to VILI is the uneven distribution of disease and inflation seen in injured lungs. Based on the diffuse relatively homogeneous distribution of shadowing on a plain chest radiograph, it was thought that the lung was uniformly affected in ARDS. However, CT scanning showed that the posterior dependent portions of the lung are more severely affected (fig 8.2A), a distribution determined largely by gravity. The greater compliance of less affected areas means that they receive a much greater proportion of the delivered tidal volume.41 This may result in substantial regional overdistension and hence injury.

In a recent study, piglets with multifocal pneumonia were ventilated using a tidal volume of 15 ml/kg for 43 hours.42 Approximately 75% of the lung volume was consolidated, so the residual 25% of normally ventilated lung may have received a tidal volume equivalent to 40-50 ml/kg. Histological examination showed emphysema-like lesions in these areas, whereas in consolidated areas the alveoli were "protected" against overdistension, but the bronchioles that remained patent were injured through overdistension and by the forces generated through interdependence and recruitment-derecruitment (fig 8.3). Lesions similar to those described above have been reported in a necropsy series of patients with ARDS43; furthermore, CT scans of ARDS survivors have shown greatest residual abnormality in the anterior parts of the lung, even though the posterior areas had been most abnormal in the acute phase (fig 8.2B).44 These changes may be due to injury caused by overdistension.

Other factors that may promote VILI in already injured lungs are surfactant abnormalities and the presence of an activated inflammatory infiltrate which may be further stimulated by mechanical ventilation.

Lung immaturity

The immature lung may be particularly susceptible to VILI.45 The volume of the lung relative to body weight and the number of alveoli are lower in premature infants, making a tidal volume based on weight potentially more injurious. The resilience of the lung tissue is lower, due to less well developed collagen and elastin elements, and surfactant deficiency leads to alveolar instability and favours airway closure. At delivery the fluid filled, surfactant deficient airways of the preterm neonate require high inflation pressures to establish patency, with potential generation of high shear stress. Preterm lambs show evidence of lung injury after only six high volume insufflations.46

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