Pressure support ventilation allows intubated ICU patients to breathe spontaneously. Among specific settings, the adjustment of the trigger value (or threshold for triggering the ventilator) has not been explored to date. The trigger threshold corresponds to the sensitivity of the ventilator to detect patient's inspiratory effort and then deliver the predefined pressure support to inflate the lungs and deliver a tidal volume. The purpose of this study is to explore the influence of trigger level on pulmonary and ventilatory physio (-patho)logical parameters in spontaneously breathing ICU patients.

The use of invasive mechanical ventilation is one of the most frequent therapies in intensive care units (ICUs). There are several types of indications, depending on the failure: essentially neurological, hemodynamic or respiratory.

In recent years, the notion of lung damage induced by mechanical ventilation (VILI) has led to major changes in ventilator settings in both ICUs and operative rooms (Ors). The reduction of the tidal volume (TV) to 6-8 mL/kg of ideal body weight, the use of an individualized positive end-of-expiratory pressure (PEEP) and the possible use of pulmonary aeration optimization therapies (alveolar recruitment manoeuvres, prone positioning sessions...) have become essential to increase patient's survival.

Withdrawal of invasive mechanical ventilation remains a daily issue and traditionally requires the transition from fully controlled ventilation to pressure support ventilation. Among specific settings of the latter, the adjustment of the trigger value (or threshold for triggering the ventilator) has not been explored to date. The trigger threshold corresponds to the sensitivity of the ventilator to detect patient's inspiratory effort and then deliver the predefined pressure support to inflate the lungs and deliver a tidal volume. The lower (or more sensitive) the trigger threshold, the smallest patient's effort will be rewarded. On the other hand, the higher the threshold, the greater the inspiratory effort required from the patient. Usually, this value is set by default to the minimum level to avoid self-triggering of the ventilator. With the objective to optimize pulmonary aeration, the use of higher trigger levels could increase diaphragmatic work (with a potential re-training and reinforcement effect) and contribute to better alveolar recruitment in the postero-inferior territories that are traditionally the most impacted, following a higher diaphragmatic motion. The authors propose to explore the impact of different trigger levels on pulmonary aeration (evaluated by electrical impedance tomography) and ventilatory parameters, in order to validate our hypotheses and before considering a trial with the objective of defining individualized trigger levels, according to patient's respiratory mechanics and pulmonary parenchyma morphology, with potential benefits on ventilator weaning.

Inclusion Criteria:

• Age ≥18 years
• Patient hospitalized in the Intensive Care Unit of Clermont-Ferrand's Hospital
• Patients with mechanical invasive ventilation in spontaneous ventilation with inspiratory support (intubation or tracheostomy)
• Trigger level set to minimum
• Patient under sedation compatible with spontaneous ventilation (SV) with inspiratory support (AI) and positive end-expiratory pressure (PEP) Patient calm (RASS between -2 and 0) Consent for participation or consent from patient's next of kin or inclusion according to an emergency procedure Patient benefiting from the French social security scheme

Exclusion Criteria:

• Refusal to participate in the proposed study

• Contraindication to the installation of a nasogastric tube:

  • Severe disorder of uncorrected blood clotting
  • Known nasosinus lesion
  • Oesophageal varices recently ligated (<48h)

• Contraindication to the use of the electro-impedancemetry technique by tomography

  • Thoracic lesions
  • Chest dressings
  • Pace-maker / Implantable Defibrillator

• Known lesion of central respiratory centers, including patients with neurological injury

• Patients with Acute Respiratory Distress Syndrome (according to Berlin criteria)

• Patients with restrictive or obstructive pulmonary pathology

• Patients admitted post-operatively for surgery that may affect the diaphragmatic function ( thoracic or abdominal supra-mesocolic)

• Patients with abdominal distention (ileus, intra-abdominal hyperpressure)

• Patient whose BMI is greater than 35 kg.m-2

• Pregnant patient

• Patient under guardianship,

Statistical analysis will be conducted by an independant statistician not involved in data collection