Is Tidal Volume Another Piece of the “Rest Setting on ECMO” Puzzle?

To the Editor: We wish to commend the authors of this interesting article comparing outcomes in children who received conventional tidal volume (CTV) and low tidal volume (LTV) ventilation during the first 24 hours of extracorporeal membrane oxygenation (ECMO).1 Although this research aimed to address a crucial knowledge gap related to the tidal volume (Vt) related aspect of mechanical ventilation during ECMO, we wish to highlight a few points for the readers to consider. A key goal of supportive therapy in severe pediatric acute respiratory distress syndrome (pARDS), including the use of ECMO, is to prevent or limit ventilator-induced lung injury (VILI). Mechanical ventilation strategies before, during, and after ECMO initiation may contribute to VILI. The focus of ventilation-related variables was restricted to the first 24 hours of ECMO only. Although that is an important period of therapy, it may be no more or less important than the other days on, before or after ECMO. The literature is filled with a myriad of different ways of analyzing mechanical ventilation strategies on ECMO. For example, Extracorporeal Life Support Organization (ELSO)-based studies typically report mechanical ventilation settings 24 hours after ECMO (although without Vt data, which is typically not reported to ELSO)2; a multicentre North American study reported ventilation on days 1, 3, and 53; a study based on bleeding and thrombosis on ECMO dataset reported day 1 and the day before separation from ECMO4; a multicentre European study reported ventilation data on days 1, 3, 7, and 14 of ECMO.5 We wonder if such heterogeneity in analyses may be counterproductive. The report states that nearly all children were managed on a mode of ventilation that requires a clinician to actively “choose” rather than passively observe a Vt (i.e., pressure regulated volume control [PRVC] or volume controlled ventilation [VCV]). Readers are likely to conclude that important differences in the children with CTV and LTV groups may exist, regardless of similarities, that may have contributed to the rationale for choosing a particular Vt. Positive end-expiratory pressure (PEEP), plateau, peak, and driving pressures were similar in both groups. It is possible that the LTV group of children had significantly impaired compliance in comparison to the CTV group. Did the clinicians therefore choose an LTV strategy to limit “pressure” related aspects of VILI? This may well be an important unmeasured confounder. Given the heterogeneity of disease syndromes such as pARDS and cardiogenic shock, further granularity about the etiology underlying these syndromes may provide further context to the readers. For instance, children with bronchiolitis, asthma may be at much lower odds of mortality compared with children with malignancy, or pertussis. The authors have highlighted the importance of considering Vt as a piece of the “optimal rest setting on ECMO” puzzle. However, we humbly suggest that this study is merely the beginning of the exploration of this puzzle. Until further evidence emerges, we suggest that clinicians adhere to published international guidelines, despite the relative paucity of high-quality evidence base behind such recommendations.6