Elastic Power of Mechanical Ventilation in Morbid Obesity and Severe Hypoxemia

BACKGROUND: To minimize ventilator-induced lung injury, the primary clinical focus is currently expanding from measuring static indices of the individual tidal cycle (eg, plateau pressure and tidal volume) to more inclusive indicators of energy load, such as total power and its elastic components. Morbid obesity may influence these components. We characterized the relative values of elastic subcomponents of total power (ie, driving power and dynamic power) in subjects with severe hypoxemia, morbid obesity, or their combination.

METHODS: We analyzed data from subjects receiving mechanical ventilation divided into 4 groups. / < 150 mm Hg (severe hypoxemia) indicated probable reduction of lung compliance while body mass index > 40 kg/m² (morbid obesity) suggested a possible contribution to reduced respiratory system compliance from the chest wall. Group 1 included subjects with no expected abnormality of lung compliance or chest wall compliance; Group 2 included subjects with expected reduction of lung compliance on the basis of severe hypoxemia but with no morbid obesity; Group 3 included subjects with morbid obesity without severe hypoxemia; and Group 4 included subjects with morbid obesity and severe hypoxemia. All ventilator-induced lung injury predictors were compared among groups using mixed-effects linear models.

RESULTS: Groups 1–4 included 61, 52, 49, and 51 subjects, respectively. Mean body mass index averaged 28.7 kg/m² for nonobese subjects and 52.1 kg/m² for morbidly obese subjects. Mean driving pressure, dynamic power, and driving power of Groups 2 and 3 exceeded the corresponding values of Group 1 but fell into similar ranges when compared with each other. These values were highest in Group 4 subjects. In Group 2, mean dynamic power and driving power values were comparable to those in Group 3.

CONCLUSIONS: In mechanically ventilated subjects, stress and energy-based ventilator-induced lung injury indicators are influenced by the relative contributions of chest wall and lung to overall respiratory mechanics. Numerical guidelines for ventilator-induced lung injury risk must strongly consider adjustment for these elastic characteristics in morbid obesity.