Radiological pattern in ARDS patients: partitioned respiratory mechanics, gas exchange and lung recruitability

The ARDS is characterized by different degrees of impairment in oxygenation and distribution of the lung disease. Two radiological patterns have been described: a focal and a diffuse one. These two patterns could present significant differences both in gas exchange and in the response to a recruitment maneuver. At the present time, it is not known if the focal and the diffuse pattern could be characterized by a difference in the lung and chest wall mechanical characteristics. Our aims were to investigate, at two levels of PEEP, if focal vs. diffuse ARDS patterns could be characterized by different lung CT characteristics, partitioned respiratory mechanics and lung recruitability.

CT patterns were analyzed by two radiologists and were classified as focal or diffuse. The changes from 5 to 15 cmH₂O in blood gas analysis and partitioned respiratory mechanics were analyzed. Lung CT scan was performed at 5 and 45 cmH₂O of PEEP to evaluate lung recruitability.

One-hundred and ten patients showed a diffuse pattern, while 58 showed a focal pattern. At 5 cmH₂O of PEEP, the driving pressure and the elastance, both the respiratory system and of the lung, were significantly higher in the diffuse pattern compared to the focal (14 [11-16] vs 11 [9-15 cmH₂O; 28 [23-34] vs 21 [17-27] cmH₂O/L; 22 [17-28] vs 14 [12-19] cmH₂O/L). By increasing PEEP, the driving pressure and the respiratory system elastance significantly decreased in diffuse pattern, while they increased or did not change in the focal pattern (Δ_15-5: - 1 [- 2 to 1] vs 0 [- 1 to 2]; - 1 [- 4 to 2] vs 1 [- 2 to 5]). At 5 cmH₂O of PEEP, the diffuse pattern had a lower lung gas (743 [537-984] vs 1222 [918-1974] mL) and higher lung weight (1618 [1388-2001] vs 1222 [1059-1394] g) compared to focal pattern. The lung recruitability was significantly higher in diffuse compared to focal pattern 21% [13-29] vs 11% [6-16]. Considering the median of lung recruitability of the whole population (16.1%), the recruiters were 65% and 22% in the diffuse and focal pattern, respectively.

An early identification of lung morphology can be useful to choose the ventilatory setting. A diffuse pattern has a better response to the increase of PEEP and to the recruitment maneuver.