Decreasing tidal volume from 6 to 4 ml/kg: feasibility and effects on repeated opening and closing

Introduction

Low tidal volumes (Vt) are thought to protect the lung by avoiding overdistension. We recently have shown that Vt may also influence repeated opening and closing (O/C) [1]. The goal of this study was to determine whether decreasing Vt from 6 to 4 ml/kg was effective to reduce O/C, and whether it was possible to maintain alveolar ventilation at such low Vt.

Methods

Cross-over study at two Vt levels: 6 versus 4 ml/kg IBW. We included ALI/ARDS patients, ventilated <48 hours, and who would have a chest computed tomography (CT) scan. For the 4 ml/kg arm: we replaced the heat and moisture exchange filter by a heated humidifier, and the respiratory rate was increased to keep minute ventilation constant. The protocol had two parts: one bedside and other in the CT room. Both Vts were applied in a random order. For the bedside protocol each Vt arm was applied for 30 minutes. Data on lung mechanics and gas exchange were taken at baseline and 30 minutes. For the CT scan protocol each Vt arm was applied for 5 minutes and then a dynamic CT (4 images/second for 8 seconds) was taken at each Vt at a fixed transverse region at the lower third of the lungs. Afterwards, CT images were analyzed by software (MALUNA) and repeated O/C was determined as nonaerated tissue variation between inspiration and expiration, expressed as a percentage of lung tissue weight.

Results

We analyzed nine patients (six male), who had a median age of 39 (21 to 72) years, APACHE II score 14 (5 to 23) and SOFA score 9 (6 to 15). All patients had a pulmonary origin of their ARDS and were on their first day of ventilation. At baseline patients had a PaO₂/FiO₂ ratio of 141 (71 to 280), compliance of 32 (17 to 43) ml/cmH₂O, and PEEP of 12 (10 to 16) cmH₂O. In the Vt arms 4 and 6 ml/kg, Vts were 260 (210 to 300) and 350 (310 to 400) ml, respectively (P < 0.01), respiratory rates were 37 (31 to 42) and 25 (21 to 28) breaths per minute (P < 0.01), and PaO₂ levels were 84 (54 to 148) and 83 (61 to 162) mmHg (P = 0.3). PEEP and FiO₂ were kept constant. PaCO₂ did not significantly increase with Vt 4 but repeated O/C (delta nonaerated tissue) consistently decreased (Figure 1).

Figure 1

Conclusions

Decreasing Vt from 6 to 4 ml/kg reduces repeated O/C. Hypercapnia can be effectively prevented by decreasing the instrumental dead space and increasing the respiratory rate.