Lung conditioning

Before setting the appropriate respiratory treatment, a recruitment maneuver is of major importance in every patient presenting with ALI/ARDS. In fact, regardless of the dominant damage underlying ALI/ARDS, it is likely that some degree of lung collapse is always present when the patient is referred to the ICU. The patient has probably had secretions, high respiratory frequency, and low tidal volume, i.e. several risk factors for reabsorption atelectasis, for hours or days before the admission. To achieve the goal of an 'open lung', airway patency must be assured (consider bronchoscopy) and the atelectasis recruited.

There are two types of atelectasis in ALI/ARDS.

1. Compression atelectasis, which develops at the end of each expiration, is due to the squeezing out of gases from the compressed pulmonary units. As some gas is left in the pulmonary units, the atelectasis is 'loose', and the transmural pressure (i.e. the difference between the airway pressure and the pleural pressure) required for opening is of the order of 10 to 20 cmH 2O.

2. In reabsorption atelectasis all gases have been reabsorbed so that the atelectasis is 'sticky', and the transmural pressure required for opening is between 30

and 35 cmH2O.

The transmural pressure is a function of the airway pressure ( Paw) and of the elastances of the lung ( EL) and chest wall EW:

Thus to achieve a transmural pressure of 30 cmH2O when EL= EW, Paw should be 60 cmH2O. However, as EL is high and EW is normal in ALI/ARDS from direct insult, whereas EW is greatly increased in ALI/ARDS from indirect insult, application of the same Paw results in two different transmural pressures. Thus a higher Paw is required in the recruitment maneuver for ALI/ARDS from indirect insult.

Moreover, the recruitment maneuver may be difficult when the compliance of the respiratory system is relatively good, as in moderate ALI, or when one lung has good compliance compared with the other. In these conditions it is difficult to achieve an adequate transmural pressure unless volumes of 2 liters or more are insufflated. Artificial alterations of the total respiratory system compliance (external compressions or positioning) may help to achieve the required transmural pressure without insufflating excessive volumes.

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