Reasons for using nitrous oxide in one-lung ventilation,especially for thoracoscopic surgery

Prompt collapse of the non-ventilated (‘operated’) lung during one-lung ventilation (OLV) can facilitate surgery, especially for thoracoscopic procedures. As N₂O has a 34-fold higher blood solubility than nitrogen, ¹ it is absorbed faster, leading to more rapid lung collapse than if nitrogen is present in the fresh gas mixture at the time OLV is initiated. ² I previously found in a sheep model that the ‘operated’ lung collapsed faster when N₂O was used rather than 100% oxygen.^3,4 Subsequently, I found that if a N₂O:O₂ gas mixture was used and nitrogen was meticulously excluded from the non-ventilated lung (by connecting an oxygen source to its open airway to prevent entry of ambient air, ⁵ and by avoiding air in the inspiratory gas), the need for continuous positive airway pressure (CPAP) to the ‘operated’ lung to manage hypoxaemia was rare.^6,7 Because the application of CPAP first interrupts and then impedes thoracoscopic surgery, possibly at a clinically critical time, I recommended that during thoracoscopic surgery, air should be used only after the surgeon has indicated that operative access will not be impaired by re-expansion of the ‘operated’ lung. ⁷

OLV is often initiated before the pleura is opened. In my opinion, the airway should not be left open to air during this time. ⁸ This is because with intermittent positive pressure ventilation (IPPV), the positive pressure in the ventilated lung during the inspiratory phase will be transmitted to the contralateral hemithorax, causing some emptying of the non-ventilated lung (if the airway channel is open to atmosphere). During the subsequent expiratory phase, ambient air (78% of which is slowly diffusing nitrogen) will be drawn into the non-ventilated lung as the external positive pressure falls and the non-ventilated lung passively re-expands. With each cycle of IPPV, further nitrogen will progressively ‘wash’ into the lung.^4,7,8 This will delay its collapse when the chest is opened, and the lung can collapse down. ² Ideally, when OLV is commenced, IPPV should be temporarily interrupted and the oxygen source attached before IPPV is recommenced.^7,8

There may be other advantages in using N₂O rather than nitrogen during OLV. I found, albeit anecdotally, that OLV can be conducted with a lower FiO₂ if N₂O is used rather than nitrogen, even in patients with marked respiratory compromise.^6,9 This observation has not been reported independently and any potential clinical advantage would require investigation in further studies. Second, in the event that hypoxaemia occurs and the FiO₂ has to be increased, the faster washout of N₂O from the blood (versus N₂) could increase the PaO₂ more rapidly than if a proportion of the original gas mixture was nitrogen. However, again, the clinical relevance of this theoretical more rapid increase in PaO₂ has never been formally investigated.

With OLV for open-chest thoracotomy, the use of nitrogen does not create the same degree of problem as it does for thoracoscopic surgery. Any delayed collapse of the ‘operated’ lung, or need for the lung to be temporarily re-expanded (or to remain partially inflated with CPAP), may not be as major an impediment during open-chest surgery.

In clinical practice, when N₂O is used for OLV, the FiO₂ should be increased (possibly even to 1.0) as soon as the pulse oximetry reading (SpO₂) falls to 94%–95% because even with rapidly diffusing N₂O, it takes a minute or so to halt or reverse a downward trend.^4,7 While the contraindications to N₂O are well known, the possibility of known or suspected raised pulmonary vascular resistance during OLV should not be overlooked, since N₂O has been shown to increase high pulmonary vascular resistance further. ¹⁰

It continues to worry me when I hear of thoracic surgical colleagues struggling with a poorly collapsed lung. ‘Can’t you put some suction on the lung?’ they ask. The two questions that I believe they should be asking are: ‘You did meticulously exclude nitrogen from the lung, I trust?’ and ‘You were using N₂O when OLV was initiated, weren’t you?’