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Lung recruitment is the amount of lung-collapsed tissue that can be reopened by applying high transpulmonary pressure for an adequate period of time. The decrease of the lung gas volume with the development of non-aerated regions mainly in the more dependent part is typical of the ARDS because of the increase of lung edema and of lung weight.

Computed Tomography

Quantitative CT scan analysis is the reference method for computing lung recruitment. It requires dedicated software and manual delineation of the perimeter of the lungs in each CT image.

It can be computed as the amount of non-inflated tissue that regains inflation [29], or as the increase in gas volume in the poorly and non-aerated tissue [30] at two different levels of pressures (Fig. 10.1). However, these two methods are not interchangeable; in fact the first method underestimated lung recruitment compared to the second one probably because it did not take into account lung recruitment in the poorly aerated lung regions. Due to the different amount of lung edema, the lung recruitability, estimated by lung CT scan, was found in whole population of ARDS patients, to range from 0% up to 70% of the total lung weight [19].

In clinical practice, to quantify lung recruitability may be important for assessing the severity of ARDS, planning recruitment maneuvers, and setting adequate PEEP levels during mechanical ventilation. In fact, lung recruitability affects the response to the applied PEEP levels: the higher is the lung recruitability, the higher will be the end-expiratory alveolar collapse that PEEP can prevent.

Recently Chiumello et al. compared CT scan-based and respiratory mechanics- based methods to assess the lung recruitment, and they found that they measure different phenomena related to the pressure increase: the CT scan measures the amount collapsed tissue which regains inflation, while the respiratory mechanics- based methods measure gas entering in already open pulmonary units which improve their mechanical properties at higher PEEP that is the overall improvement of inflation [31].

Obviously, quantitative analysis of the whole lung is labor-intensive and timeconsuming. To increase the feasibility of the use of CT, Chiumello and colleagues demonstrated that visual anatomical analysis of lung CT had a good accuracy in detecting patients with high or low recruitability, although this method is unable to estimate the hyperinflation [32].

Computed tomography and lung recruitment

Fig. 10.1 Computed tomography and lung recruitment. The use of computed tomography scanning to identify potential for lung recruitment. Differences in aerated and non-aerated tissue at different pressure levels are presented. Panel (a) shows CT scan performed at end-expiration at 5 cmH2O of PEEP; Panel (b) shows the CT scan of the same lung slice performed at inspiratory plateau pressure of 45 cmH2O

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