The need to better understand the physiology of lung collapse during one-lung ventilation

Abstract

In their challenging study conducted on 39 patients undergoing one-lung ventilation (OLV), Moreault et al. have succeeded in ‘‘highlighting the need to better understand the physiology of lung collapse during OLV’’. My letter addresses but one of the many thought-provoking volume and airway pressure findings: following the initiation of OLV and before pleural opening, the volume of ‘‘gaseous influx’’ into the non-ventilated lung from a measuring ambient air reservoir was greater when it occurred via the narrow-bore internal channel of a bronchial blocker (BB) than via the wide lumen of a double lumen endotracheal tube (DL-ETT) (contrary to one of the researchers’ two hypotheses). This totally unexpected finding could conceivably be a consequence of the ‘‘tidal gas movement’’ that will have been occurring via the DLETT lumen, but not to any meaningful degree via the long narrow-bore internal channel of the BB. ‘‘Paradoxical ventilation’’ occurs in association with the ‘‘tidal gas movement’’ and has been shown (as a single picture ‘‘saying a thousand words’’) to be necessarily associated with an influx of ambient nitrogen into the non-ventilated lung. This could well have resulted in an earlier initiation of hypoxic pulmonary vasoconstriction in the nonventilated lung in the studies with the wide-bore DLETT, with a consequent reduction in pulmonary blood flow that resulted in a lesser ongoing ‘‘gaseous influx’’ than might otherwise have occurred. Whether or not this particular explanation is correct, one important message should be clear from the Moreault et al. study: after OLV is initiated and until the pleura is opened, ambient air (with slowly diffusing nitrogen) will certainly enter the non-ventilated lung if its airway is left nonoccluded (open to air), regardless of whether the nonoccluded airway is the lumen of a DL-ETT or the narrowbore internal channel of a BB. Thus, if prompt lung collapse is desired, the open airway should either be connected to an ambient pressure oxygen source or, as has previously been described, occluded as soon as the patient is placed in the lateral position and OLV is initiated. With regard to the latter option, further studies will determine whether the appreciable reduction in airway pressure that occurs with intentional unilateral airway occlusion might prove to be, in some patients at least, an added risk of OLV. Further studies will also determine which of the two above-mentioned options will be the more efficient at facilitating the non-ventilated lung’s prompt collapse.