This single center physiological study was approved by the institutional review board (Comitato etico Milano Area 3, # 179-30032021). Informed consent was obtained according to Italian regulations. Patients admitted to the COVID-19 ICU (Rossini) of the ASST Grande Ospedale Metropolitano Niguarda, Milan for C-ARDS were enrolled. The study was performed on a convenience sample of 11 patients.
Inclusion criteria were the following: (1) > 18 years; (2) diagnosis of C-ARDS (laboratory confirmation of SARS-CoV-2 infection and concomitant ARDS according to Berlin definition ); (3) mechanically ventilated (Evita V800, Dräger, Lübeck, Germany) with sedation and myorelaxation in volume-controlled mode; (4) protective Vt (≤ 6 mL/kg) and (5) Crs ≤ 35 ml/cmH2O on clinical settings.
Exclusion criteria were: (1) Pregnancy; (2) hemodynamic instability; and (3) Contra-indication to electrical impedance tomography (EIT) positioning (e.g., trauma, burns, pace-maker, defibrillator).
The study protocol had four steps (Additional file 2: Figure E1): (1) at baseline (before the positioning of a weight on the chest wall), we ensured that hemodynamics was stable; then, during an expiratory hold maneuver, we performed a brief static chest compression with a 5-L saline bag, and recorded the change in Paw determined by this compression (Additional file 2: Figure E2). (2) ECC, with a 5-L bag placed in the middle of the thorax using the sternum as a landmark, for 60 min; (3) ECC discontinuation, 10 min without compression; and (4) PEEP reduction from baseline by the same amount of static pressure developed by the saline bag (step 1).
At enrollment, clinical ventilator settings were used. No standardized protocol to set PEEP was available in the ICU; therefore, PEEP was set upon clinician’s decision in a tertiary referral hospital.
During all steps, the patients were placed in supine flat position (0° trunk inclination) to standardize every measurement  and the ventilator settings were unchanged (except for PEEP in step 4). No recruitment maneuvers were performed.
Before the protocol was started, a 5 Fr esophageal balloon (Cooper surgical, Trunbull, CT, USA) was positioned in 9 out of 11 patients enrolled in the study to partition respiratory mechanics. The proper position of the esophageal balloon was ensured . Patient hemodynamics was monitored by a central line and invasive arterial pressure.
At the end of each step, and after 5, 30 and 60 min of step 2 (ECC), we performed expiratory and inspiratory holds to obtain static measurements of airway (Paw) and esophageal pressure (Pes). The distribution of tidal volume between ventral (non-dependent, regions of interest 1 + 2) and dorsal (dependent, regions of interest 3 + 4) lung areas was assessed through the analysis of EIT data (PulmoVista 500, Dräger, Lübeck, Germany) to obtain a regional Vt as previously described [18, 19]. In addition, end-expiratory lung impedance (EELI, a surrogate of end-expiratory lung volume ) was analyzed by EIT . The following variables were calculated:
Respiratory system Driving pressure or DP = Plateau Pressure (Pplat) – Total PEEP (set PEEP + intrinsic PEEP).
Respiratory system compliance or Crs = Vt/DP.
Regional Crs = Regional Vt derived from EIT/DP.
Transpulmonary pressure (absolute value) or PL = Paw – Pes.
Lung compliance or Clung = Vt/(PL inspiration – PL expiration).
Chest-wall compliance or Ccw = Vt/(Pes inspiration – Pes expiration).
Paw and Pes waveforms as well as EIT data were prospectively recorded and stored for offline analysis.
Arterial and central venous blood samples were obtained to assess gas exchange and to calculate shunt fraction at baseline, at the end of step 2 (60 min of ECC), at step 3 and step 4. Shunt fraction was calculated as follows: (CcO2 – CaO2)/(CcO2 – CvO2), where CcO2 represents the O2 content of capillary blood, CaO2 the arterial O2 content and CvO2 the O2 content of venous blood. Alveolar dead space was calculated as follows: (PaCO2 – PEtCO2)/PaCO2, where PaCO2 is the arterial partial pressure of carbon dioxide (CO2) and PEtCO2 represents the end-tidal CO2 value.
Our primary endpoint was the DP change after the application of an ECC. We hypothesized that ECC would produce a decrease in DP.
Secondary endpoints were: change in chest-wall compliance (Ccw); change in lung compliance (Clung); change in regional Crs (i.e., of non-dependent and dependent lung); change in gas exchange, shunt fraction and dead space after ECC.
We performed an experimental porcine study using ECC in a two-hit lung injury model. The aim of the study was to measure directly the effect of ECC on pleural pressure. Ventral and dorsal Ppl were, therefore, directly measured to understand the effect of ECC on Ppl gradient, differentiating between non-dependent and dependent lung areas.
The experiments were conducted in the animal facility of The Hospital for Sick Children Hospital (Toronto, ON, Canada). All experimental procedures followed the guidelines of the Canadian Council on Animal Care and were approved by the Animal Care Committee, Research Institute, The Hospital for Sick Children (protocol number 46420).
Four healthy Yorkshire pigs (32.6 ± 2.1 kg) were sedated and paralyzed. Pigs were intubated and mechanically ventilated in volume-controlled mode in supine position. An esophageal catheter (Nutrivent; Sidam, Mirandola, Italy) was inserted to record Pes and positioned as previously described [17, 18]. Pleural pressure (Ppl) was directly recorded in the dorsal and ventral part of the pleural space in the right lung with two balloons (Cooper surgical, Trunbull, CT, USA). To ensure a proper Ppl measurement, the calibration of pleural catheters was done at each PEEP level and the minimal non-stressed volume of the balloon with a stable Ppl measure was selected .
Afterwards, we established lung injury by a two-hit model: surfactant depletion with saline lavage followed by injurious ventilation, as described previously .
Experimental data and measurement
The mechanical ventilator (GE Carestation 620, Boston, MA, USA) was set as follows: Vt 6 mL/kg, respiratory rate 25/min, FiO2 1.0. Ventral and dorsal pleural pressures were measured during respiratory holds both at end-inspiration and at end-expiration for every PEEP step. Every step lasted 20 min and was done without (first 10 min) and with (second 10 min) ECC using a 2.3 kg sandbag on top of the thorax. The pleural pressure gradient was calculated as follows: Ppl gradient = Ppl dorsal–Ppl ventral and averaged between different PEEP steps.
Data were expressed as mean ± SD or median ± interquartile range, as appropriate. Data were compared using one-way repeated measures ANOVA followed by Newman–Keuls or Sidak–Holm post hoc tests. If both the normality and equal variance tests failed, repeated measures ANOVA on ranks was used. Difference in continuous data in the preclinical model between baseline and ECC was assessed using the Wilcoxon signed-rank test. Statistical analyses were performed using STATA/16 MP (TX, USA), GraphPad Prism 8.0.2 (La Jolla, CA, USA) and Systat software Inc. (Sigmaplot 12.0, UK). Statistical significance was set at P < 0.05 (two-tailed).