Physiology of Lung Collapse Under One-Lung Ventilation: Underlying Mechanisms

Lung isolation and one-lung ventilation (OLV) have been used for more than 60 years, principally via double lumen endotracheal tubes (DLT). Since the beginning of the 21st century, modernisation of bronchial blockers (BB) has favoured their more frequent use. Meanwhile, video assisted thoracoscopic surgery (VATS) has increased, becoming the standard for the majority of intra-thoracic pulmonary surgeries.

Lung collapse during OLV undergoes two distinct phases. The first phase occurs at the opening of the pleural cavity and corresponds to a quick but partial collapse of the lung due to its intrinsic recoil. This phase probably ends when small airways are closed. Thereafter, the second phase, a slower one, corresponds to the reabsorption, by the capillary bed, of gas contained into the alveoli. The speed of this reabsorption depends on the solubility of the gas contained in the alveoli.

Intriguingly, the physiology of lung collapse under OLV remains poorly understood, especially with the use of BB. Theoretically, many aspects of lung isolation may influence lung collapse, including the ventilation strategy before OLV, the timing and the lung isolation devices being used. While oxygen at 100% is widely used for ventilation before OLV, the timing of initiation of lung isolation varies from centers to centers. Indeed, the most conservative will begin the lung isolation just before the opening of the pleural space, whereas others begin the lung isolation following the appropriate positioning of the patient and confirmation that the lung isolation device is properly positioned by fiberoptic bronchoscopy (FOB) examination. Therefore, the period between initiation of lung isolation and pleural opening may vary from a few minutes to >30 minutes. The mechanic of lung isolation differs between DLT and BB and consequently the physiology of lung deflation may be different. When using DLT, the lumen that corresponds to the collapsed lung is disconnected from the ventilator and is continuously in communication with the ambient air. When using BB a bronchial cuff is inflated within the main bronchus following a 30 seconds apnea period, allowing the initial lung deflation to be mediated by elastic lung recoil. After this initial phase, the only communication with ambient air is through the small (2 mm) and long internal (67 mm) channel, which is completely different from the larger lumen of the DLT.

Rapid and complete lung collapse is essential during lung isolation for VATS otherwise; there is no alternative available for the surgeon to get proper view of the pulmonary hilum. Previous studies suggested that BB allow a less effective lung collapse than the one obtained with DLT. However, the authors recently documented that the use of BB with its internal channel occluded creates a statistically significant shorter time to complete lung collapse during VATS compared to DLT (36.6 ± 29.1 vs 7.5 ± 3.8 min; p<0.001). In contrast to the previous studies, the authors used off-line review videos recorded during the surgery to obtain a more objective evaluation of the complete lung collapse time which probably reflected the second phase of lung deflation. Although, our definition of lung collapse was very strict, meaning complete collapse of all the lung areas, graded using a standardized visual scale and chart. However, authors do not have any data to explain why this internal channel occlusion may have some positive impact. The authors hypothesized that their results could be explained by the optimisation of the reabsorption phases following enhanced atelectasis by gas reabsorption (phase 2) after bronchial blockade. This latter hypothesis is supported by a pilot observation that ambient air (FiO2 at 0.21) was "sucked up" within the collapsing lung when using DLT to a greater extent than with the use of BB (unpublished data). The presence of ambient air (21%) in the alveolar space may likely slowing subsequent gas reabsorption compared to intra-alveolar 100% O2 . However, these hypotheses remain to be confirmed.

The investigators proposed this study to update the knowledge about lung collapse with the actual lung isolation devices: DLT and BB. This protocol will describe the lung collapse physiology and allows getting data for the elaboration of further studies.