The authors showed that the maximal volume hysteresis obtained fr

The authors showed that the maximal volume hysteresis obtained from pressure-volume curves predicted changes in lung mechanics better than changes in gas exchange in response to recruitment manoeuvres. Similar observations mainly were obtained in a pig model of ARDS induced by oleic acid injection, in which Lambermont and coworkers [11] measured functional residual capacity, statistic pulmonary compliance and PaO2 during a sequential reduction in PEEP from high to low levels. The investigators reported that combined EELV and lung compliance measurements may help to optimize the PEEP level.These interesting observations in animal models were also reported in the clinical setting. Bikker and coworkers [12] measured EELV at PEEP levels that were reduced sequentially from 15 to 5 cmH2O in 45 mechanically ventilated patients with and without lung injury.

In all patients, EELV decreased significantly corresponding to the sequential reduction in PEEP, whereas the PaO2/FiO2 ratio remained unchanged. Interestingly, a correlation between the change in EELV and the change in respiratory system compliance was found in patients with extrapulmonary injury. These clinical results suggest that, in combination with the assessment of pulmonary compliance, measurement of EELV may provide useful information to help optimize the ventilatory settings in ALI/ARDS conditions.Given these findings, an important issue at the bedside is what is the best way clinically to perform an EELV measurement. The helium dilution technique has been used for a decade but the procedure is both time consuming and labour intensive.

Chiumello and coworkers [13] compared EELV measured by the conventional helium dilution technique with that obtained by a modified nitrogen wash-out/wash-in system in 30 patients with ALI/ARDS. EELV was also compared with CT scans, which were considered to be the ‘gold standard’. The authors reported that the EELV measured with either the helium dilution technique or the modified nitrogen wash-out/wash-in system exhibited good correlation with the CT scan data. In particular, the helium dilution technique appeared to be more precise at low lung volumes, and the modified nitrogen wash-out/wash-in system worked well at all lung volumes. This study provides an alternative approach to measurement of EELV at the bedside.

As discussed above, several studies have examined the importance of shunt fraction, dead space and EELV within the context of ALI/ARDS. Varelmann and coworkers [14] conducted an elegant study to address the question of whether spontaneous breathing during pressure-controlled ventilation improves oxygenation, ventilation/perfusion matching, dead space and EELV. ALI was induced Brefeldin_A either by hydrochloric acid aspiration as a direct lung injury model or by increasing intra-abdominal pressure combined with intravenous oleic acid injection as an indirect lung injury model in pigs.

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