Individually Titrated Mechanical Ventilation Improves Survival of Severely Obese ARDS Patients

Acute respiratory distress syndrome (ARDS) is a common cause of ICU admission for patients with class III obesity (BMI >40 kg/m2). Unfortunately, obese patients have been excluded from most of the major trials that tested different methods of titrating mechanical ventilation in ARDS, such as positive end-expiratory pressure (PEEP) and lung recruitment maneuvers (LRM).

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Lorenzo Berra, MD, medical director of Respiratory Care at Massachusetts General Hospital, Robert M. Kacmarek, PhD, director of Respiratory Care, and colleagues have verified in multiple studies that pleural pressure is higher in severely obese patients than in those with lean body habitus. Increased pleural pressure significantly reduces lung volume and can lead to atelectasis, and the research team has shown in Critical Care Medicine that PEEP levels commonly used in the ICU are often insufficient for optimal ventilation of severely obese patients.

In 2014, the Mass General Respiratory Care Service and the Critical Care Group jointly introduced the lung rescue team that provides individualized physiologic treatment exclusively to ARDS patients with class III obesity who are being mechanically ventilated. In a retrospective study published in Critical Care, Dr. Berra, Dr. Kacmarek and colleagues determined that the implementation of the specialized team significantly reduced mortality.

Study Methods

The researchers studied adults with ARDS and class III obesity who were admitted to a Mass General surgical or medical ICU between January 1, 2012, and December 31, 2017, and were mechanically ventilated for >48 hours.

From 2012 to 2014, 70 ARDS patients (average BMI 49 kg/m2) were managed according to the standard protocol using ARDSnet PEEP/FiO₂ tables.

From 2015 to 2017, 50 ARDS patients (average BMI 54 kg/m2) were managed by the lung rescue team.

The clinical decisions of the lung rescue team were based on multiple measurements of respiratory mechanics and hemodynamics. Mechanical ventilation was titrated using LRM, and the optimal value of PEEP was selected to match information from the decremental PEEP trial, end-expiratory transpulmonary pressure measurements and the use of electrical impedance tomography. To monitor right heart function of hemodynamically unstable patients, the team performed bedside transthoracic echocardiography before and after titration of ventilation.

Respiratory Mechanics and Oxygenation

• Standard protocol cohort — PEEP levels and respiratory mechanics did not change during the first four days of ventilation. By day four, only 14% of patients had oxygenation >300 mmHg
• Lung rescue team cohort — Throughout the first four days of ventilation, driving pressure decreased an average of 3.4 cmH₂O and compliance of the respiratory system improved an average of 12 mL/cmH₂O. The average PaO₂/FiO₂ ratio improved from 153 mmHg at admission to 282 mmHg on day two. On day four, 56% of patients had oxygenation >300 mmHg

Hemodynamics

• Standard protocol cohort — During the first four days of ICU admission, the average vasoactive-inotropic score, which reflects dosing levels of those medications, did not change substantially. By day four, 56% of patients required vasopressors
• Lung rescue team cohort — Despite an average PEEP increase of 10 cmH₂O, the average vasoactive-inotropic score significantly decreased during the first four days of ICU admission. By day four, only 30% of patients required vasopressors (P = .005 vs. the other cohort)

LRM and PEEP titration had no adverse impact on right heart function in the lung rescue team cohort.

Mortality

Compared with patients in the lung rescue team cohort, those in the standard protocol cohort had about double the rates of death from an ARDS diagnosis in the hospital (18% vs. 41%, respectively), at 28 days after admission (16% vs. 31%) and at three months, six months and one year after admission (22% vs. 41% at all three timepoints).

The mortality difference between cohorts was even greater when corrected for potential confounders such as APACHE score. At 28 days after admission the adjusted hazard ratio was 0.31 (95% CI, 0.13–0.78; P = .001) and at three months, six months and one year after admission, it was 0.35 (95% CI, 0.16–0.74; P = .006).

Research Implications

In the U.S. the prevalence of class III obesity is approaching 10%, so the population of hypoxic ARDS patients with obesity can be expected to increase. The current study justifies the conduct of a randomized control trial to evaluate whether titration of mechanical ventilation based on an individualized strategy, performed by a dedicated team, is superior to a fixed ARDSnet protocol.

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