Rationale: During controlled mechanical ventilation (CMV) only the inspiration is controlled by either a set driving pressure (Pressure Controlled Ventilation, PCV) or tidal volume (Volume Controlled Ventilation, VCV). The expiration depends on the passive elastic recoil of the respiratory system and cannot be controlled and lasts until the airway pressure is equal to the positive end-expiratory pressure (PEEP). The exponential decrease in airway pressure during expiration may result in alveolar collapse and hypoxemia. Flow controlled ventilation (FCV) is a mechanical ventilation method that uses a constant flow during both inspiration and expiration. FCV results in a gradual decrease in airway pressure during expiration as flow is controlled. In both animal and prospective crossover studies, controlled expiration resulted in higher mean airway pressures with reduced alveolar collapse. Besides, FCV resulted in a higher ventilation efficiency measured by a decrease in minute volume at stable arterial partial pressures of carbon dioxide (PaCO2). Where a reduction in alveolar collapse may lead to less atelectrauma, a higher ventilation efficiency may lead to a lower mechanical power (MP), which is the amount of energy (Joules) that is transferred to the respiratory system by the mechanical ventilator every minute. Both are important determinants of Ventilator Induced Lung Injury (VILI). This makes FCV a very interesting ventilation mode in patients with the acute respiratory distress syndrome (ARDS) in which VILI is still a major contributor to overall morbidity and mortality. Two prior prospective cross-over studies have been performed in (COVID-19) ARDS patients that did show a lower minute volume with FCV compared to PCV or VCV. However, these studies did not take into account assessments of the MP or end-expiratory lung volume (EELV), which is a measurement of lung aeration.
The investigators hypothesize that FCV in patients with moderate to severe ARDS results in a lower MP and an increased EELV compared to standard CMV modes (PCV or VCV).
Objectives: To study the effect of FCV on the MP and the EELV compared to PCV.
Study design: Randomized crossover physiological pilot study comparing FCV and PCV.
Study population: Patients with moderate to severe ARDS ≥ 18 years old receiving CMV.
Intervention: Patients are mechanically ventilated with PCV mode at baseline. Upon inclusion the EIT-belt and an esophageal balloon are placed to assess the EELV and transpulmonary pressures respectively. Besides, participants are randomized between the sequence of ventilation mode, namely 90 minutes of PCV followed by 90 minutes of FCV or 90 minutes of FCV followed by 90 minutes of PCV. When PCV is switched to FCV the same mechanical ventilator settings are used as in the PCV mode. After half an hour on FCV the PEEP, driving pressure and flow of FCV are optimized based on the highest compliance and lowest flow matching with a stable PaCO2 thereby not exceeding lung protective ventilation limits (transpulmonary driving pressure ≤ 12cmH2O and tidal volumes ≤ 8 ml/kg ideal body weight (IBW)). PCV is always set according to standard of care. Total time of measurements / study time is 180 minutes.
Main study parameters/endpoints: Primary endpoint is the difference in MP after 90 minutes on FCV compared to after 90 minutes of PCV. An important secondary endpoint is the difference in EELV after 30 minutes on FCV compared to after 30 minutes of PCV.
Nature and extent of the burden and risks associated with participation, benefit and group relatedness: All participants are sedated and on CMV, therefore there will be no discomfort for the patient. FCV has been successfully applied during surgery and on the ICU and the patient will be monitored continuously so the clinical team can act directly in case of any adverse event. Lung volume is measured with EIT, a non-invasive, radiation-free monitoring tool. Transpulmonary pressures are measured with an esophageal balloon that is placed in a similar manor as a nasogastric feeding tube. During optimization of FCV no lung protective ventilation limits will be exceeded. Therefore, overall, the risks of this study are limited.
