We read with great interest a recent article on the association between body mass index (BMI) and outcomes in patients supported by venoarterial extracorporeal membrane oxygenation (VA-ECMO).¹ In a review of 22,825 patients from the Extracorporeal Life Support Organization (ELSO) registry from 2015 to 2021, the authors found that patients with BMIs of 35, 45, and 55 would be expected to have respective odds ratios of 1.15, 1.46, and 2.12 for mortality compared with patients with a BMI of 25.¹ Mechanical and renal complications were more frequent with obesity, whereas pulmonary complications were less common. Absolute mortality for patients with overweight, class I, class II, and class III obesity were 53%, 56%, 58%, and 65%, respectively (compared with 50% in normal BMI patients, p < 0.001).¹ The findings are enlightening but require thoughtful interpretation. Specifically, the etiology of reduced pulmonary complications is difficult to interpret due to the lack of pre-extracorporeal life support (pre-ECLS) ventilator data, and the authors did not control for mechanical unloading strategies.

Recent literature reviews examining the association between obesity and outcomes in the VA-ECMO population have shown that most studies do not demonstrate an association between obesity and mortality in the VA-ECMO population.^2,3 Until the current large review, most studies contained subjects numbering at most in the several hundred range and likely lacked sufficient statistical power because of their sample size. Despite the seemingly equivalent outcomes in the VA-ECMO population of obese patients with nonobese patients, concerns are regularly raised about challenges, including technical difficulties with cannulation, vascular access complications, and the need to place larger cannulas (or consider central VA-ECMO cannulation) to achieve adequate blood flow rates.⁴ Additionally, clinicians may opt to offer VA-ECMO to obese patients with fewer or less severe comorbidities than those without obesity. Clinician selection is an important confounder in database analyses and retrospective reviews that is difficult to control for because detailed documentation of comorbidities and their severity is highly variable.

 

Ventilator management for the obese population is challenging and often requires higher positive end-expiratory pressure (PEEP) due to mechanical issues such as reduced chest wall compliance.⁵ While studies comparing the outcomes of obese and nonobese patients with acute respiratory distress syndrome supported by venovenous-ECMO (VV-ECMO) have suggested an association with improved survival, it is evident from examining pre-ECLS mechanical ventilation parameters that PEEP is not used effectively, and poor oxygenation is, in part, due to under-recruitment.^6,7 In the current study, reviewing the supplemental index finds that, of the patients with available ventilator data, 15,569 of 17,649 (88.2%) were mechanically ventilated before VA-ECMO initiation.¹ Although the authors state that they included “ventilator type and parameters,” pre-ECLS PEEP, peak ventilator pressure, fraction of inspired oxygen, and arterial blood gas analysis were not documented as analyzed variables.¹ While the VA-ECMO population is different and more critically ill than the VV-ECMO population, ventilator optimization is important to achieve adequate oxygenation and acid-base status. Ventilator optimization may be especially poor in the obese population with cardiogenic shock because of the concern that PEEP has potentially negative effects on pulmonary vascular resistance and right ventricular function, despite the fact that the relationship between PEEP and these two parameters is more likely “U-shaped.”⁸ Finally, erroneous ventilator management during VA-ECMO could result in mistakes such as adding unnecessary cannulas for venoarterial-venous ECMO (VAV-ECMO) with potentially deleterious consequences.