Background

Complete airway closure during expiration may underestimate alveolar pressure. It has been reported in cases of acute respiratory distress syndrome (ARDS), as well as in morbidly obese patients with healthy lungs. The authors hypothesized that complete airway closure was highly prevalent in obese ARDS and influenced the calculation of respiratory mechanics.

Methods

In a post hoc pooled analysis of two cohorts, ARDS patients were classified according to body mass index (BMI) terciles. Low-flow inflation pressure–volume curve and partitioned respiratory mechanics using esophageal manometry were recorded. The authors’ primary aim was to compare the prevalence of complete airway closure according to BMI terciles. Secondary aims were to compare (1) respiratory system mechanics considering or not considering complete airway closure in their calculation, and (2) and partitioned respiratory mechanics according to BMI.

Results

Among the 51 patients analyzed, BMI was less than 30 kg/m2 in 18, from 30 to less than 40 in 16, and greater than or equal to 40 in 17. Prevalence of complete airway closure was 41% overall (95% CI, 28 to 55; 21 of 51 patients), and was lower in the lowest (22% [3 to 41]; 4 of 18 patients) than in the highest BMI tercile (65% [42 to 87]; 11 of 17 patients). Driving pressure and elastances of the respiratory system and of the lung were higher when complete airway closure was not taken into account in their calculation. End-expiratory esophageal pressure (ρ = 0.69 [95% CI, 0.48 to 0.82]; P < 0.001), but not chest wall elastance, was associated with BMI, whereas elastance of the lung was negatively correlated with BMI (ρ = −0.27 [95% CI, −0.56 to −0.10]; P = 0.014).

Conclusions

Prevalence of complete airway closure was high in ARDS and should be taken into account when calculating respiratory mechanics, especially in the most morbidly obese patients.

Editor’s Perspective
What We Already Know about This Topic
  • Plateau and driving pressures have been shown to correlate with mortality in adult respiratory distress syndrome (ARDS). However, these static airway pressures may not always accurately reflect alveolar pressure.

  • It has recently been recognized that in ARDS, airway closure may occur while some alveoli are still inflated. This may result in a biased estimate of mean alveolar pressure.

  • Complete airway closure can only be measured by the inflection point on the initial portion of a low-flow inflation pressure–volume or pressure–time curve with the absence of cardiac oscillations and very low compliance, most likely in the terminal bronchioles.

  • In 25 to 33% of patients with ARDS, airway opening pressure (the inflection point value) is greater than the total positive end-expiratory pressure measured by an end-expiratory maneuver.

What This Article Tells Us That Is New
  • In a post hoc analysis of two cohort studies of respiratory mechanics in ARDS, the authors compared the prevalence of complete airway closure stratified by body mass index and its effects on respiratory mechanics.

  • Complete airway closure was present in 41% of patients, increasing with body mass index tercile (65% in the highest).

  • Driving pressure and respiratory system elastances (lung, chest wall) were higher when complete airway closure was not adjusted for.

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