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To the Editor:We read with great interest the review by Dugan et al1 published in this issue of CHEST (August 2017) on the management of persistent air leaks (PALs). The authors note that the highest incidence of PALs occurred in patients undergoing lung volume reduction surgery for emphysema (24%-46%), followed by patients undergoing lobectomies (8.6%). Notably, clinical factors associated with emphysema were also identified as risk factors in patients undergoing lobectomies, including a history of smoking, COPD, lower FEV1, and a lower diffusing capacity for carbon monoxide. The extent of emphysema was also one of the best predictors of PALs in previous studies that used quantification of CT scans.2,3 It would seem the overriding risk factor for PALs is the presence of emphysema.The authors1 describe the morbidity associated with PALs and suggest that the best strategy would be prevention. Identifying patients who are at high risk for PAL following lung resections, and preparing in advance for the potential need of additional therapies (eg, use of sealants) at the time of the procedure, could be an effective strategy. A first step to this end requires clear understanding of the mechanism for PALs in emphysema. We propose that (intralobar and interlobar) collateral ventilation is the primary physiologic mechanism linking emphysema to PAL risk. Collateral ventilation is defined as “the ventilation of alveolar structures through passages or channels that bypass the normal airways.”4,5 Although resistance in collateral channels in normal lungs is greater than resistance in the airways, the opposite is true in emphysema and, consequently, air flows preferentially through collateral pathways. When an air leak occurs via pleuroparenchymal tears in an emphysematous lung, the airflow out through the tear literally represents the pathway of least resistance. Collateral ventilation can extensively connect airspaces of an entire lung when incomplete fissures are present. We have observed complete transpleural exhalation via air leaks in such a situation.Future studies using quantitative analysis of CT scans to determine emphysema severity and distribution, as well as fissure integrity, could provide important information to facilitate anticipation, and thus potentially lead to prevention of PALs.5 Such a quantitative CT scan analysis suggesting the presence of interlobar collateral ventilation would also probably be important for the selection of candidates likely to benefit from endobronchial valve therapy for PALs. With evidence of widespread emphysema and incomplete fissures on quantitative CT scan analysis, interlobar collateral ventilation is likely present and makes endobronchial valve therapy for PALs substantially more challenging.