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Clinical utility of functional residual capacity measurement based on a modified nitrogen breath washout technique


Improvement of oxygenation during acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) requires a high level of positive end-expiratory pressure (PEEP) to recruit nonaerated lung zones and decrease pulmonary shunt. However, monitoring of alveolar recruitment at the bedside is difficult, as neither the PaO2/FiO2 ratio, thoracopulmonary compliance or generation of pressure curve are indices of alveolar recruitment and avoidance of lung hyperinflation. Monitoring of the functional residual capacity (FRC) at the bedside may be useful to monitor directly lung recruitment and to optimize the PEEP level [1]. The aims of this study are to evaluate the FRC by a modified nitrogen multiple washout technique (NMBW) in ALI/ARDS patients, and to set PEEP levels on data of FRC values.


Twenty patients with ALI/ARDS were enrolled in the study. All patients were ventilated in pressure-controlled ventilation with an Engstrom carestation ventilator (GE Healthcare, Helsinki, Finland) in accordance with the ARDSnet guidelines. FRC measurement was carried out with the COVX module integrated within the ventilator (GE Healthcare) by a NMBW technique. Every patient had a basal FRC measurement and then three measurements at PEEP 15/10/5 cmH2O during a derecruiting maneuver. After all measurements, PEEP was set as the PEEP at which value FRC started to decrease. At basal time (T0) and after setting the best PEEP (T1) the PaO2/FIO2 ratio and static compliance were measured too. All data are reported as the mean ± SD. A t test was used to compare changes during time.


Table 1 presents the main results of the study.

Table 1 abstract P49


FRC measurement by the NMBW technique integrated in the ventilator is useful to assess functional lung impairment at the bedside. Setting PEEP on FRC measurements may improve lung recruitment and oxygenation, but anatomical studies (CT scan) are also warranted.


  1. Lambermont B, et al.: Comparision of functional residual capacity and static compliance of the respiratory system during a PEEP ramp procedure in an experimental model of acute respiratory distress syndrome. Crit Care 2008, 12: R91. 10.1186/cc6961

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Turani, F., Barchetta, R., Mounajergi, F. et al. Clinical utility of functional residual capacity measurement based on a modified nitrogen breath washout technique. Crit Care 13 (Suppl 1), P48 (2009).

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