Artificial Increase in Chest Wall Elastance as an Alternative to Prone Positioning in Moderate-to-severe ARDS.

Design: Prospective, pilot, physiological study Setting: 20-bed general ICU, 13 bed surgical ICU, 10 bed neurosurgical ICU, "A. Gemelli" University hospital, Rome, Italy. Protocol Screening visit and oxygenation criterion validation Each patient meeting inclusion criteria will be evaluated for the presence of the oxygenation criterion. After endotracheal suctioning, eligible patients will be ventilated for 30 minutes with PEEP=5 cmH2O in the semi recumbent position and an ABG will be performed to compute PaO2/FiO2 ratio. Patients showing PaO2/FiO2≤150 mmHg will be enrolled. Patients showing PaO2/FiO2\<200 and \>150 mmHg will be treated according to the standard clinical practice and reassessed for the presence of oxygenation criterion within 48 hours from the diagnosing of ARDS. To limit the exposure to low PEEP of possibly derecruiting patient with severe oxygenation impairment, the ABG certifying the oxygenation criterion will be permitted at any time during the 30-minute monitoring period. Procedures All patients will be sedated, paralysed with cisatracurium infusion and connected to a ventilator equipped with lung volume measurement module (Carescape R860 - GE Healthcare, USA) through a standard bi-tube low-resistance circuit with a low-dead space, low-resistance, high-efficiency heat and moisture exchanger. For the purpose of the study, the use of heated and humidified bi-tube circuits (Fisher and Paykel healthcare, humidification chamber temperature set at 37 °C, absolute humidity provided 44 mg H2O/L) will be reserved to patients that remain hypercapnic (ph\<7.30 and PaCO2\>50) despite all adequate ventilator settings provided by the study protocol. Each patient will be ventilated in volume-control mode, in the semirecumbent position (or smaller head elevation for patients with spine/pelvis movement limitations), which will not be changed throughout the study. Ventilation settings will be standardized as follows: VT = 6 mL/Kg (predicted body weight, PBW); inspiratory flow set at 60 l/min resulting in an end-inspiratory pause of 0.2-0.5 sec, I:E ratio 1:1 to 1:3, respiratory rate tailored to achieve 45 mmHg\>PaCO2\>35 mmHg, PEEP set according to the clinical judgment, FiO2 set to achieve a SpO2\>88-95%. A Pplat \< 30 cmH2O will be considered as a safety limit. Predicted body weight will be calculate as: Males: PBW (kg) = 50 + 0.91 (height in cm-152) Females PBW (kg) = 45.5 + 0.91 (height in cm-152) In case of hypercapnia with Ph\<7.30 despite a respiratory rate=30-35, an increase in VT up to 8 ml/kg will be allowed. A dedicated orogastric or nasogastric tube provided with an oesophageal balloon (Cooper esophageal catheter) to monitor oesophageal pressure, estimate pleural pressure and compute transpulmonary pressure will be placed in all enrolled patients after inclusion. The adequate positioning of the esophageal catheter will be certified by an occlusion test, as previously demonstrated(15). The GE-dedicated pneumotacograph and differential pressure transducer will be connected to the respiratory circuit to record airway pressure (PAW) and flow. The oesophageal pressure (PES) will be measured using the previously inserted oesophageal catheter, that will be connected to the auxillary pressure port of the ventilator. All the three signals will be continuously acquired by the ventilator with an analog-digital converter at a sample rate of 25 Hz (GE healthcare). A dedicated laptop connected to the ventilator will acquire PAW, PES and Flow signals through a dedicated software over the entire course of the study (Ohmeda research tool, GE healthcare). At study enrolment, an electrical impedance tomography (EIT) belt with 16 electrodes will be placed around the thorax between the 5th or 6th parasternal intercostal space and connected to a dedicated device to record electrical impedance signals of the thorax (Swisstom EIT, Switzerland). After included in the study, each patient will be treated as follow: * 30-minute period in the supine position (T0); * 120-minute period in the supine position with a 100 g/kg soft and smooth weight (warm saline bags) placed and secured on each emithorax (T1); * 120-minute period in the supine position after the weight is removed (T2). Mechanical ventilation settings will be kept unchanged over the course of the entire study. In case of drops in the SpO2, an increase in the FiO2 will be allowed to achieve the previously described oxygenation target. Measurements Patient's demographics will be collected at study entry: initials, age, sex, height, weight, BMI, cause of hospital and ICU admission, SAPSII, Apache, SOFA score, date and time of ICU admission, date and time of enrolment, comorbidities, NYHA category before respiratory failure, body temperature, chest x-ray (jpeg images), chest CT scan (whether available). During the study, each patient will undergo a standard ICU monitoring: ECG; Invasive blood pressure, SpO2, respiratory rate, diuresis. All the relevant data follow described will be collected at prespecified timepoints. The prespecified timepoints are: 1. baseline, T0: after the 30-minute period in the supine position (study enrolment, no weight); 2. T1a: 60 minutes after weights are positioned on the thorax; 3. T1b 120 minutes after weights are positioned on the thorax, end of T1; 4. T2a 60 minutes after weights are removed; 5. T2b 120 min after weights are removed, end of T2, end of the study; At each timepoint the following data will be collected. * Adverse events, if any. * Respiratory rate, SpO2, pH, PCO2, PaO2, SaO2, PaO2/FiO2; * Heart Rate, arterial blood pressure, central venous pressure; dosage of vasoactive or inotrope agents. * End expiratory lung impedance (EELI) and tidal volume distribution. A ten-minute period EIT signals will be recorded and offline reviewed using a dedicated software (Swisstome EIT). Image acquisition rate will be 30 Hz. Lungs will be divided into four regions (ventral, mid-ventral, mid-dorsal and dorsal): the % of impedance variation related to tidal volume and the % EELI in the four regions as compared to the absolute values will be calculated (Appendix 3, Figures 4-5)(16). * Respiratory mechanics End-expiratory airway pressure (PEEPAW) and the end-expiratory esophageal pressure (PEEPES) will be recorded during an 8-second expiratory hold. End-inspiratory airway pressure (PplatAW) and end-inspiratory esophageal pressure (PplatES) will be measured during a 2-second end inspiratory hold. Tidal volume (VT) will be measured as the integration of the flow-time curve during expiration. The following parameters will be calculated offline while reviewing signals: Airway driving pressure (∆P)=PplatAW-PEEPAW Transpulmonary end-inspiratory pressure (PplatL)=PplatAW-PplatESO Transpulmonary end-expiratory pressure (PEEPL)=PEEPAW-PEEPES Lung driving pressure (∆PL)= PplatL-PEEPL Lung plateau pressure, elastance derived (PplatL,EL)=PplatAW X (∆PL/∆P) Static respiratory system compliance (CstRS)=VT/∆P Static Lung compliance (CstL)=VT/∆PL Static Chest-wall compliance=VT/(PplatES-PEEPES) Oxygenation stretch index=PaO2/(FiO2x∆P) • Stress and strain (measured only at T0, T1b and T2b) End-Expiratory lung volume (EELV) at the set PEEP (PEEPSET) will be measured by nitrogen washin-washout technique with a 0.2 change in the FiO2 (0.1 change in the FiO2 only allowed in case of baseline FiO2=0.9-1). A one-breath derecruitment maneuver from PEEPSET to PEEP 0 (ZEEP) will be conducted to assess baseline functional residual capacity (FRC), that will be measured as the difference between EELV at set PEEP and the lung volume increase above FRC, measured as the difference in expired tidal volume as PEEP is decreased from 0 cmH2O in one breath with respiratory rate set at 8-10 breaths per minute. In particular, the lung volume due to the presence of set PEEP (PEEPvolume) will be measured by subtracting the insufflated VT from the expired VT (i.e. the integration of the flow signal after 5s exhalation) during a 5-second exhalation just after PEEP is reduced from set PEEP to 0.