This study will test the validity of a new approach to assess alveolar recruitability at the bedside in comparison to direct measurements of lung volume and derecruitment.

Background: Acute respiratory distress syndrome (ARDS) is characterized by a major loss of lung volume due to alveolar flooding, atelectasis and consolidation[1]. Positive end-expiratory pressure (PEEP) is applied to recruit non-aerated or poorly aerated lung tissue, and thus may improve gas exchange in patients with ARDS[2]. However, PEEP may also be harmful by overdistending the previously open lung tissue. The potential effectiveness and benefit of high PEEP levels depends on the patient's recruitability. Previous techniques for assessing recruitability are usually reserved for clinical research (computed tomography, multiple pressure-volume curves)[3-4] or need special equipment. A simple, feasible bedside method to assess the individual needs for PEEP in terms of recruitability seem essential to improve the prognosis of this syndrome. The investigators therefore propose a simplified approach derived from our previous study[5] to estimate alveolar recruitability at the bedside.

Objectives: The major aim is to test the hypothesis that this simplified approach is a valid method for assessing alveolar derecruitment at the bedside in comparison to direct measurements of lung volume and derecruitment.

Methods: This physiological study will enroll 30 patients from the three Intensive Care Units at St. Michael's Hospital in two years. All patients will be passively ventilated at two PEEP levels (15 cmH2O and 5 cmH2O) and the plateau pressure will be limited to less than 35 cmH2O. Respiratory mechanics, lung volumes (using the nitrogen washout/washin technique), quasi-static Pressure-Volume curves, lung ultrasound, and arterial blood gases will be assessed at each PEEP level. In patients who already have an esophageal catheter in place, Pes will also be measured.

Data Analysis: The primary endpoint is the correlation and bias between the Vder estimated by our approach and the one measured by the multiple P-V curves technique through paired tests, correlations and Bland and Altman analysis

Inclusion Criteria:

• Patients > 16 years of age
• Moderate-to-severe ARDS as per the Berlin definition (Partial pressure of oxygen (PaO2)/Fraction of inspired oxygen (FiO2) < 200 mmHg) and within 10 days of onset
• Receiving assist/control mechanical ventilation (volume or pressure) with continuous sedation and an established arterial line

Exclusion Criteria:

• Pneumothorax with fistula or likely to require a chest tube
• Severe hemodynamic instability (> 30% increase in vasopressors in the last 6 hours or norepinephrine > 0.5 µg/kg/min)
• PaO2/FiO2 < 80 mmHg
• Severe or very severe Chronic Obstructive Pulmonary Disease according to the GOLD criteria (stage III: Forced expiratory volume at one second (FEV1) 30-50% predicted; stage IV: FEV1 < 30% predicted)
• Known or highly suspected elevated intracranial pressure (> 18 mmHg)

Moderate-to-severe ARDS patients as per the Berlin definition