Combination of variability with pressure support ventilation enhances lung protection and function in experimental acute lung injury

Introduction

Protective ventilation with low tidal volumes became the standard of care in acute lung injury (ALI). Although spontaneous breathing activity may be beneficial even in early ALI, protective ventilation is usually performed as controlled ventilation. Theoretically, pressure support ventilation (PSV) may be advantageous over pressure-controlled ventilation (PCV), particularly if combined with variability in the form of noisy PSV [1, 2].

Methods

Two protocols (A and B) were performed in pigs (20 to 30 kg). Animals were anesthetized, intubated, mechanically ventilated and ALI was induced by surfactant depletion. In protocol A, animals (n = 12) were randomly assigned to a sequence of PCV, PSV and noisy PSV, being ventilated for 1 hour in each mode (crossover design). The distribution of lung aeration and perfusion were determined. In protocol B, animals (n = 24) were randomly assigned to 6 hours of mechanical ventilation with PCV or PSV or noisy PSV, and lungs were extracted for quantification of inflammation and lung damage. In both protocols, gas exchange and respiratory parameters were determined. Statistical analysis was performed with univariate and multivariate tests, as appropriate. Significance was accepted at P < 0.05.

Results

Compared with PCV, arterial oxygenation, intrapulmonary shunt, mean airway pressure and elastance of the respiratory system were improved by PSV and further improved by noisy PSV. Also, noisy PSV reduced the work of breathing and respiratory drive compared with PSV. Lung damage and inflammation were reduced by assisted mechanical ventilation, but comparable between noisy PSV and PSV. The distribution of lung aeration did not differ among the three modes, but PSV and noisy PSV were associated with more redistribution of pulmonary blood flow towards better aerated ventral areas compared with PCV.

Conclusion

The combination of variability with PSV enhances lung protection and function in experimental ALI. The main mechanism of improvement of lung function by assisted mechanical ventilation using pressure support is not recruitment of dorsal areas, but rather redistribution of pulmonary blood flow to ventral zones.