Pressure support ventilation (PSV) is used to assist the breathing of the intubated patient with some pressure from the ventilator. This support aims at avoiding excessive inspiratory effort, while ensuring a certain degree of training of the patient's inspiratory muscle. Avoiding both minimal and excessive assistance is thus important for the optimal care of the intubated patient ensuring a lung and diaphragm protective ventilation with the goal to liberate the patient from the ventilator as soon as possible.
Recently pressure-muscle-index (PMI), an index of inspiratory effort easy to be measured on the ventilator screen, has been proposed to avoid excessive assistance in PSV. This will be the first prospective study testing the effects of setting pressure support based on PMI to avoid excessive assistance on patients recovering from acute lung injury (acute hypoxemic respiratory failure).
>18 y.o.
Invasively ventilated in Intensive Care Unit.
Diagnosed with acute hypoxemic respiratory failure (P/F<300 mmHg).
Breathing on pressure support ventilation.
<18 y.o.
Contraindication for Electrical Impedence Tomography monitoring (e.g. pacemaker, burns, or - wounds limiting electrode placement).
Severe and previously diagnosed neurological and neuromuscular disease.
Psychomotor agitation (Sedation Agitation Scale > 4).
The Study population will include patients mechanically ventilated in the St. Michael's Intensive Care Unit. Patients who are younger than 18 years of age will be excluded from this study on the basis that the ICUs at St. Michael's Hospital are intended for adults; individuals aged less than 18 years are rarely, if ever, admitted.
Case-crossover
Prospective
Measuring Respiratory Mechanics
Other interventionFirst, for each patient enrolled we will perform inspiratory and expiratory occlusions - routinely performed diagnostic maneuvers to measure patient's breathing effort - at the clinically set level of breathing assistance by the mechanical ventilator. Importantly, we will measure pressure-muscle-index (PMI). Second, we will set the level of assistance (i.e. pressure support, PS) in order to make PMI=0. The PS level at which PMI=0 (PS PMI=0) will be used as reference for the study protocol. After 5 minutes at PS PMI=0 we will perform inspiratory and expiratory occlusions.
Randomized Pressure Support
Other intervention3 steps of PS above (+2, +4, +6 cmH2O) and 3 below (-2, -4, -6 cmH2O) PSPMI=0 done in a randomized order (sequence in envelopes opened blindly by the investigators). Each step will last ≤5 minutes if poorly tolerated (see tolerance rule in description). At the end of each step we will perform 1 inspiratory occlusion and 3 expiratory occlusions. Each of the expiratory occlusion will be separated by 30 seconds in order to restore the patient's breathing pattern. We will resume a 3-minutes clinical PS in between each step to minimize changes in PaCO2. If not yet comprised by the study steps each patient will undergo an adjunctive 5 minutes step at PS 0 (zero) cmH2O.
Inspiratory Occlusion
To investigate the role of the inspiratory occlusion as a diagnostic tool to understand the patient's desired tidal volume and minute ventilation in Pressure Support Ventilation (PSV),
Pressure Muscle Index (PMI)
To investigate the role of a PMI\>0 strategy to avoid over-assistance in Pressure Support Ventilation
Pendelluft
To investigate the relationship between "active" and "passive" pendelluft measured over different levels of assistance in PSV.
Lung and Diaphragm Protective Ventilation
To measure indexes proposed by current literature to target a lung and diaphragm protective ventilation strategy with both inspiratory and expiratory occlusions in PSV, while applying a PMI\>0 strategy.
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