Inspiratory Ratio: Predictor of Inspiratory Effort Response to High PEEP in Patients Recovering From ARDS

Spontaneous Breathing (SB) can be potentially harmful in patient with Acute Respiratory Distress Syndrome (ARDS) during the transition phase of passive ventilation to partial ventilatory support. The application of high Positive End Expiratory Pressure (PEEP) during SB has shown to ameliorate the progression of lung injury by decreasing the TP and esophageal pressure (EP) swings and the stress / strain applied to the lung. The mechanisms proposed to be responsible for these effects are the activation of Hering Breuer reflex, the recruitment of previously collapsed tissue, the homogenization of lung and the improvement of respiratory system compliance and the impairment in the length - tension relationship of the diaphragm. If all the previously explained mechanisms have an effect on the control of inspiratory effort, a decrease in the intensity of effort is expected during an end-inspiratory occlusion in patients who will respond to high PEEP application. Based on this rationale, the investigators developed an index called "Inspiratory Ratio" (IR) to predict the response of patient's inspiratory effort to the application of high PEEP without need of esophageal manometry.

Spontaneous Breathing (SB) can be potentially harmful in patient with Acute Respiratory Distress Syndrome (ARDS) during the transition phase of passive ventilation to partial ventilatory support. A high respiratory drive and consequently, a strong inspiratory effort, may produce large transpulmonary pressure (TP) swings mainly in dependent lung regions closer to the diaphragm and cause alveolar rupture and inflammatory mediators release.

The application of high Positive End Expiratory Pressure (PEEP) during SB has shown to ameliorate the progression of lung injury by decreasing the TP and esophageal pressure (EP) swings and the stress / strain applied to the lung. The mechanisms proposed to be responsible for these effects are the activation of Hering Breuer reflex caused by a greater stretch of the lung parenchyma at the end of inspiration; the recruitment of previously collapsed tissue, the homogenization of lung ("fluid like behavior") and the improvement of respiratory system compliance (Crs); and the impairment in the length - tension relationship of the diaphragm which produces mechanical disadvantage to generate force due to a higher lung volume. However, it is uncertain which patient will respond adequately to the application of high PEEP and consequently will reduce the inspiratory effort.

If all the previously explained mechanisms have an effect on the control of inspiratory effort, in patients who will respond to high PEEP application, a decrease in inspiratory effort is expected during an end-inspiratory occlusion. At end-inspiration lung parenchyma is more homogeneous, the lung volume is higher and the diaphragmic dome is flatter compared to the physiological condition end of expiration, where the lung volume is lower, the parenchyma is more heterogeneous and the diaphragmatic neuromechanical coupling is better. Based on this rationale, the investigators developed an index called "Inspiratory Ratio" (IR) to predict the response of patient's inspiratory effort to the application of high PEEP without having to measure esophageal pressure.

The IR will be calculated using the following formula: (IPSexp - IPSinsp ) / (IPSexp) x 100

IPSexp = negative deflection in airway pressure expiratory pause; IPSinsp = negative deflection in airway pressure end inspiratory pause

Eligibility Criteria:
Inclusion Criteria:

* Need of invasive mechanical ventilation
* Patients who had fulfill ARDS criteria based on Berlin definition during any time of invasive mechanical ventilation.
* Patient ventilated in pressure support ventilation.
* Time of invasive ventilation expected to be longer than 24 hs after the day of enrollment.

Exclusion Criteria:

* Neuromuscular diseases (e.g., amyotrophic lateral sclerosis, Duchenne Erb)
* previous diagnosis of chronic obstructed pulmonary disease
* not resolved pneumothorax
* bronchopleural fistula
* suspicion of central respiratory drive alteration (e.g., benzodiazepines intoxication).