Individualized Positive End-expiratory Pressure Guided by End-expiratory Lung Volume in the Acute Respiratory Distress Syndrome

During the acute respiratory distress syndrome (ARDS), patients' response to positive end-expiratory pressure (PEEP) is variable according to different degrees of lung recruitability. The search for a tool to individualize PEEP on the basis of patients' individual response is warranted.

Measurement of end-expiratory lung volume (EELV) by the nitrogen washin-washout technique, bedside available from recent ICU ventilators, has been shown to reliably estimate PEEP-induced alveolar recruitment and may therefore help titrate PEEP on patient's individual requirements.

The authors designed an open-label, multicenter, randomized trial to test whether an individualized PEEP setting protocol driven by EELV may improve a composite clinical outcome in patients with moderate-to-severe ARDS.

ARDS patients with a PaO2/FiO2 ratio equal or below 150 mmHg (during mechanical ventilation with PEEP 5 cmH2O) will be enrolled within 24 hours from endo-tracheal intubation.

To standardize lung volumes at study initiation, all patients will undergo mechanical ventilation with tidal volume set at 6 ml/kg of predicted body weight and PEEP set to obtain a plateau pressure within 28 and 30 cmH2O for thirty minutes (Express PEEP).

Afterwards, a 5-step decremental PEEP trial will be conducted (Express PEEP to PEEP 5 cmH2O), and EELV will be measured at each step. PEEP-induced alveolar recruitment will be calculated for each PEEP range as the difference between PEEP-induced change EELV and the predicted increase in lung volume due to PEEP (PEEP-induced overdistension, equal to the product of respiratory system compliance and PEEP change).

Patients will be then randomized to receive mechanical ventilation with PEEP set according to the optimal recruitment observed in the PEEP trial (IPERPEEP arm) trial or according to the Express strategy (Control arm, PEEP set to achieve a plateau pressure of 28-30 cmH2O).

In both groups, tidal volume size, the use of prone positioning and neuromuscular blocking agents will be standardized.

Primary endpoint of the study is a composite clinical outcome incorporating in-ICU mortality, 60-day ventilator free days and the area under the curve of serum Interleukin 6 over the course of the initial 72 hours.

Primary and secondary endpoints will also be analyzed in subgroups, as defined below:

* ∆EELV5-16/FRC ≥ 73% \[18\] during the PEEP trial
* ∆EELV5-16/FRC \< 73%\[23\] during the PEEP trial
* Recruitment-to-inflation ratio (RI) ≥ 1 and \<1 across the range between the lowest and highest PEEP tested during the PEEP trial
* P/F ratio\<100 mmHg at study inclusion
* IL-6\>400 pg/ml at study inclusion

Eligibility Criteria:
Eligibility inclusion criteria, according to the ARDS Berlin definition, will be assessed within the first 24 hours from the initiation of invasive mechanical ventilation:

1. Acute respiratory failure within 1 week of a known clinical insult or new or worsening respiratory symptoms;
2. Bilateral infiltrates at the chest x-ray or CT scan, not fully explained by effusions, lobar/lung collapse, or nodules;
3. Respiratory failure not fully explained by cardiac failure or fluid overload; objective assessment required to exclude hydrostatic edema if no risk factor present.
4. PaO2/FiO2 ratio≤150 mmHg after 30 mins - 1 hour of mechanical ventilation with PEEP=5 cmH2O.
5. Written informed consent.

Exclusion Criteria:

1. Pregnancy;
2. Pneumothorax;
3. Acute brain injury;
4. Clinical signs of history of decompensated heart failure (New York Heart Association class 3-4 before the acute phase of the disease or documented ejection fraction\<35% or pulmonary capillary wedge pressure\>18 mmHg) or acute coronary syndrome;
5. Intubation as a result of an acute exacerbation of chronic pulmonary disease: chronic obstructive pulmonary disease, asthma, cystic fibrosis, etc;
6. Clinically evident intrinsic PEEP (≥2 cmH2O) during screening visit (End-expiratory pause to achieve Flow=0);
7. BMI\>35;
8. BMI\<15 or body weight\<35 Kg;
9. Any chronic disease requiring long-term oxygen therapy or mechanical ventilation at home;
10. Neuromuscular disease of any kind;
11. Severe chronic liver disease (Child-Pugh C or worse);
12. Bone marrow transplantation or chemotherapy-induced neutropenia;
13. History of liver or lung transplant;
14. Decision to withhold life-sustaining treatment;
15. Need for therapy with inhaled nitric oxide due to documented pulmonary arterial hypertension;
16. Life-threatening hypoxemia deemed to require extracorporeal membrane oxygenation (ECMO);
17. Presence of documented barotrauma;
18. High risk of mortality within 3 months from other than ARDS (severe neurological damage, age \>85 years and cancer patients in terminal stages of the disease).
19. Persistent hemodynamic instability, intractable shock (norepinephrine\>1 mcg/kg/h and/or blood lactate\>5 mmol/L and/or considered too hemodynamically unstable for enrolment in the study by the patient's managing physician).
20. More than 24 hours from endotracheal intubation to the time of the screening visit.