Recruitability during General Anesthesia: Comment

To the Editor:

We read with great interest the physiologic study by Rosà et al.,¹ which provides a valuable validation of the recruitment-to-inflation ratio for assessing lung recruitability in the operating room. However, we wish to explore two critical aspects of the methodology that may limit the direct clinical translation of these findings.

The first point concerns the phenomenon of “delayed lung deflation” and its impact on the simplified, single-breath recruitment-to-inflation ratio measurement. The authors astutely observe that the simplified recruitment-to-inflation ratio systematically underestimates the value derived from direct end-expiratory lung volume measurements, reporting a bias of −0.25. They attribute this to a slow collapse in some patients, a process not fully captured by the initial postmaneuver breath. This observation implies that some lung units possess longer time constants for collapse.² While the authors suggest that this discrepancy does not alter the recruitment-to-inflation ratio’s overall clinical reliability, this conclusion warrants scrutiny. For a tool intended to guide precise positive end-expiratory pressure (PEEP) titration, a known and systematic underestimation is a particular concern, especially at higher recruitment-to-inflation ratio values. The strong correlation (r = 0.82) demonstrates a physiological relationship, but the inherent inaccuracy raises questions about its reliability for making fine clinical adjustments.

The second, and perhaps more critical, issue is the generalizability of these findings beyond the highly controlled, homogenous patient cohort. The study enrolled nonobese patients (body mass index less than 30 kg/m²) undergoing open abdominal surgery in the supine position, excluding the very populations where PEEP titration is most challenging and vital: patients with obesity, those undergoing laparoscopic surgery with pneumoperitoneum, or positioned in a steep Trendelenburg position.^3,4 In these scenarios, a significant decrease in chest wall compliance becomes a dominant factor in respiratory mechanics. The recruitment-to-inflation ratio calculation relies on total respiratory system compliance, which is mathematically related to both lung compliance and chest wall compliance (respiratory system compliance = 1/lung compliance + 1/chest wall compliance). In the study’s cohort, using respiratory system compliance as a proxy for lung size is reasonable. However, in a patient with a stiff chest wall from obesity or pneumoperitoneum, the markedly reduced chest wall compliance artificially lowers the measured respiratory system compliance. Since respiratory system compliance is the denominator in the recruitment-to-inflation ratio formula, this could paradoxically inflate the calculated recruitment-to-inflation ratio value, potentially misclassifying a patient with a noncompliant chest wall as having high lung recruitability. While a low chest wall compliance might also reduce the numerator, this complex interaction remains a crucial confounding factor, which was not discussed, and could fundamentally undermine the utility of the recruitment-to-inflation ratio ratio precisely where it is needed most.

To advance this promising line of inquiry, future investigations must validate the recruitment-to-inflation ratio ratio in these higher-risk, complex patient populations. Crucially, such studies should incorporate techniques like esophageal manometry to partition lung and chest wall mechanics, isolating the true potential for alveolar recruitment from the confounding effects of chest wall stiffness. Recent work underscores that individualized PEEP levels in obese patients often exceed 15 cm H₂O, highlighting the need for precise assessment tools that account for their unique physiology.⁵

Competing Interests

The authors declare no competing interests.