Lung computed tomography during a lung recruitment maneuver on patients with acute respiratory failure: mechanisms and clinical usefulness

Introduction

Lung computed tomography (CT) has been widely used to assess lung morphology, which has led us to a better understanding on the pathophysiology of ARDS, mechanical ventilation and ventilatory induced lung injury. Despite the absence of controlled studies and standardization, LRM are increasingly used in patients with acute respiratory failure. The objective of our study was to assess the effect of different levels of airway pressure on lung morphology by performing a LRM during the lung CT-scan. This way, we could set the best ventilatory strategy for the patient and identify the mechanisms involved during the LRM.

Methods

Ten patients (5 male, 5 female, 58 ± 16 years old) with ARF (PaO₂/FiO₂ 46-214) underwent a thoracic CT scan for diagnostic or therapeutic reasons. Patients were connected to a Siemens 900-C ventilator in the CT-scan facility, under sedatives and muscle relaxants. We used a Picker PQ2000 CT-scan, which has a workstation for the processing of images. At first, a conventional CT scan from the neck down to the lung basis was performed with CT slices 8 mm thick during an inspiratory pause. Then, PEEP was down to ZEEP and a LRM applied with 5 cmH₂O increments in PEEP up to 30-40 cmH₂O. A CT slice from the basal third of the lung during an expiratory pause was performed at each PEEP level. This took 4-6 min and then the patient was back to baseline ventilatory parameters. Arterial blood gases were taken at baseline, ZEEP, 30-40 cmH₂O PEEP and 3 min after LRM, and airway pressures and tidal volumes registered. Lung slice volumes and densities were measured as previously described [1,2].

Results

Decreasing PEEP down to 0 cmH₂O (ZEEP) was associated with an increase in basal densities. The increase in PEEP level from 0 up to 40 cmH₂O was associated with a significant increase in lung volume in both lungs, because of an increase in gas volume and not change in the amount of lung tissue, even with high levels of PEEP. This was associated with increases in PaO₂, PaO₂:FiO₂ ratio and mean airway pressure, and a decrease in tidal volume and total static compliance. Three to five minutes after LRM, there was a significant improvement in oxygenation index, despite similar airway pressures.

Discussion

This study showed that lung CT scan during a LRM in patients with ARF is safe and gives morphologic and functional information that could be useful in setting ventilatory parameters. By the other way, LRM improves lung mechanics and oxygenation in the short-term period, probably by effectively opening previously closed alveoli. The time course of these effects is still unknown.