Physiologic effects and regional ventilation of high-frequency positive-pressure ventilation using a conventional ventilator in a severe ARDS animal model associated with an inspiratory pause or recruitment maneuvers

Introduction

Protective mechanical ventilation (MV) in ARDS is based on reduced stretch of pulmonary tissue, sometimes resulting in severe hypoventilation that can be avoided when using high respiratory rate. High-frequency positive-pressure ventilation (HFPPV) has not been fully explored, especially when associated with other strategies aiming to avoid hypercapnia.

Methods

We induced ARDS in eight pigs by lung lavage with saline plus 3 hours of injurious MV with low PEEP and high driving pressure (DP). We then performed a recruitment maneuver (RM) followed by PEEP titration using the amount of alveolar collapse in electrical impedance tomography (EIT). Then stabilization during 1 hours with tidal volume (VT) at 6 ml/kg, respiratory rate (RR) 35 breaths/minute and PEEP selected with the PEEP-FiO₂ table (ARMA study), which was kept constant during two steps of HFPPV with a RR 60: one without an inspiratory pause (HFPPV-60), and one with a pause of 30% of inspiratory time (HFPPV-60 w/P30%). In another HFPPV step, we used PEEP titrated with EIT after RM (HFPPV-60 w/RM). During each HFPPV step, VT was set to reach a PaCO₂ of 60 ± 3 mmHg. Distribution of regional ventilation was analyzed using EIT. Equilibrium was considered if PaCO₂ was stable (<5% of variation) for >30 minutes.

Results

HFPPV allowed reduction in PaCO₂ levels: 81 (77, 94) versus 60 (58, 61), 59 (58, 60), 60 (58, 61) mmHg, besides using lower VT: 5.2 (5.0, 5.9), 5.1 (4.5, 6.0), 4.7 (4.2, 5.7) and 4.8 (4.5, 5.6) ml/kg during stabilization, HFPPV-60, HFPPV-60 w/P30% and HFPPV-60 w/RM, respectively. It had no significant different results comparing HFPPV-60 with and without an inspiratory pause. HFPPV-60 w/RM allowed a better alveolar homogenization and improvement in oxygenation, shunt, dead space and DP compared with the other steps. See Table 1.

Table 1 Physiological variables

*Tukey's post-hoc analysis, P < 0.05 versus others.

Conclusion

HFPPV with a conventional mechanical ventilator is able to maintain stable PaCO₂ in clinically acceptable values, allowing reductions in VT. HFPPV-60 w/RM and PEEP titration using EIT allowed further physiologic benefits in a severe ARDS model.