Monitoring slow recruitment manoeuvres with high-frequency oscillatory ventilation in adult acute respiratory distress syndrome patients using electrical impedance tomography

Introduction

Recruitment manoeuvres (RM) during high-frequency oscillatory ventilation (HFOV) are increasingly used in ARDS. However, the changes in lung volume during a RM (lung recruitability) are difficult to quantify at the bedside, and the use of CT is impractical in patients on HFOV. We studied the effects of a standardised protocol of slow RM (SRM) on regional lung volumes assessed noninvasively by electrical impedance tomography (EIT).

Methods

SRM were performed by progressive increases of continuing distending pressure (CDP) starting from the mean airway pressure on CMV + 5 cmH₂O, by increments of 3 cmH₂O every 10 minutes until a CDP of 50 cmH₂O was reached or haemodynamic instability ensued. Subsequently, CDP was reduced by 2 cmH₂O every 5 minutes until optimal CDP was established on gas exchange. EIT measurements were performed using 16 electrodes, acquired via the Goe-MF II EIT system (Viasys Healthcare, USA). Offline analysis of EIT measurements was performed using the AUSPEX software (University of Amsterdam). Changes in impedance (ΔZ) during tidal breathing were calibrated against set tidal volumes during conventional mechanical ventilation. Changes in lung volume after each increase in CDP on HFOV were expressed as the fold change compared with the previous CDP level.

Results

Four patients with ARDS, who underwent rescue HFOV, were enrolled. Following the SRM, there was a mean 2.38-fold increase in PaO₂/FiO₂ and a 19.7% reduction in PaCO₂. EIT showed a mean 4.66-fold increase in global lung volume, with preferential ventilation of the ventral regions (59.4% of global volume change). Despite these differences, both dorsal and ventral regions showed a similar degree of volume change compared with their own baseline (V/D of 4.7/4.5-fold). This may be consistent with a more homogeneous recruitment with HFOV. The inflation limb of the changes in lung volumes during SRM fitted the Venegas–Harris equation (r² = 0.99).

Conclusion

EIT can noninvasively assess lung recruitability and quantify the changes in global and regional lung volume during SRM with HFOV in ARDS patients.