Low tidal volume in association with low positive end expiratory pressure in acute respiratory distress syndrome: a suboptimal strategy? A computed tomography-based analysis

Introduction

In a recent trial, ARDSNet investigators found no benefit with the association of an intermediary PEEP to a low-tidal-volume (6 ml/kg) ventilatory strategy. The goal of maximal recruitment strategy (MRS) guided by thoracic CT scan is to minimize alveolar collapse and tidal recruitment (TR).

Objectives

To analyze the amount of lung collapse in ARDS patients ventilated in three situations: A – minimum PEEP (10–20 cmH₂O), B – maximum PEEP (35–50 cmH₂O) and C – titrated PEEP (15–25 cmH₂O) after maximal recruitment. To analyze the occurrence of regional tidal recruitment during the MRS and in two situations: minimum PEEP and titrated PEEP.

Methods

Thirty-one ARDS patients under mechanical ventilation were transported to the CT room and sequences of CT scans at expiratory and inspiratory pause were performed during the recruitment maneuver (RM). The RM consisted of 2-minute steps of tidal ventilation with fixed ΔPCV = 15 cmH₂O and progressive PEEP levels (10-50-30-10 cmH₂O), RR = 15, I:E = 1:1, and FiO₂ = 1.0. CT images between the carina and the diaphragm were performed in all steps of the RM. The amount of lung parenchyma collapse as well as potentially recruitable lung were calculated and compared.

The lungs were divided into four regions according to the sternum-vertebral axis (one anterior and four posterior). The amount of collapsed tissue as well as the difference of collapsed tissue between expiration and inspiration (TR) were calculated and compared during all phases of the protocol.

Results

See Figures 1, 2 and 3.

Figure 1

Data expressed as the median and 25–75 percentiles.

Figure 2

Tidal recruitment region 3.

Figure 3

Tidal recruitment region 4.

Conclusion

MRS and PEEP at 25 cmH₂O were able to minimize collapse and TR, while ventilation with PEEP 10 cmH₂O did not prevent TR. This finding suggests that low PEEP, even associated with low volumes and pressures, is a suboptimal protective strategy.