Functional residual capacity measurement during mechanical ventilation in order to find the optimal positive end-expiratory pressure

Introduction

In patients with ALI/ARDS, a protective ventilation strategy has been introduced in order to diminish ventilator-induced lung injury. It has become clear that these patients require sufficient levels of PEEP to prevent alveolar derecruitment, but also not too much PEEP that alveolar overdistension occur. To achieve the optimal level of PEEP in patients with ALI/ARDS, different concepts have been introduced. GE Healthcare, along with Dr Ola Stenqvist, has developed a technology to measure functional residual capacity (FRC) in ventilated patients without interruption of the ventilation. The aim of this study was to test the feasibility of this device and to test whether decreasing the PEEP affects FRC in mechanically ventilated patients with and without lung disease.

Methods

For this survey we examined 10 patients under mechanical ventilation. The FRC examinations were performed with the Engström Carestation equipped with the FRC Inview™ monitoring feature. FRC is determined using the change of lung nitrogen volume after a step change in the inspired oxygen fraction. With this system, there is no need to use supplementary gases or specialized gas monitoring devices. Furthermore, a series of FRC measurements can automatically be obtained at different PEEP levels that can be chosen prior to the measurement. During this procedure, all ventilator settings will remain constant other than the FiO₂ and the PEEP settings. In patients with ALI, the PEEP was decreased from 25 to 5 cmH₂O in five steps and the FRC was measured. In the patients without lung disease, PEEP was decreased from 15 to 0 cmH₂O in four steps and the FRC was measured.

Results

The best FRC measurements were obtained in well-sedated patients during controlled mechanical ventilation. During pressure support ventilation, a constant breathing pattern is necessary for accurate FRC measurements. In patients that received pressure support ventilation, FRC values were lower at the highest studied PEEP level. In two patients that received controlled ventilation, lower levels of FRC were found at the highest PEEP level but this was due to a pneumothorax that was diagnosed a day later. In patients with ALI, the FRC decreased after each PEEP reduction step. However, the FRC decreased more when PEEP was lowered from 15 to 10 cmH₂O in these patients. In patients without lung disease, the FRC did not decrease after PEEP was reduced from 15 to 5 cmH₂O but decreased after PEEP was reduced from 5 to 0 cmH₂O.

Conclusion

Accurate measurements of FRC are obtained during a constant breathing pattern that is easier to obtain during controlled ventilation in comparison with pressure-support ventilation. In patients with ALI/ARDS, the FRC decreased during each PEEP reduction, but whether the largest change in FRC indicates the optimal PEEP needs further research.