Influence of positioning and flow bias on secretion displacement and lung mechanics in pigs under invasive mechanical ventilation

Abstract

Introduction: Invasive ventilatory support is essential in the treatment of patients with acute respiratory failure. However, invasive mechanical ventilation causes adverse effects, such as pulmonary secretion retention that induces alveolar collapse and increases the risk of ventilator-associated pneumonia (VAP). VAP is a pulmonary infection that appears 48h after orotracheal intubation. Among the main recommendations for the prevention of VAP are checking the readiness for extubation daily and raising the bed head at 30-45° to avoid aspiration of gastric contents. Although 30-45° head elevation is recommended by most of the VAP prevention guidelines, it is known that the degree of evidence supporting its use is low. In addition, there is evidence that this position facilitates the aspiration of accumulated secretion in the upper region of the trachea and glottis, above the cuff, which flows into the lung due to gravity, despite the inflated cuff. Furthermore, secretions from the distal lung regions have more difficulty overcoming gravity, impacting in the gravity-dependent portions of the lung. Thus, the head-elevated at 30-45° can paradoxically accelerate the occurrence of VAP. Another important point is that the mechanical ventilator, depending on how its parameters are adjusted, can act as an inducer of pulmonary secretion retention. There is evidence that the presence of an inspiratory flow bias, that is, a peak expiratory flow (PEF) lower than the peak inspiratory flow (PIF), from a certain threshold, can promote the displacement of secretion towards the lungs and favor the occurrence of VAP. Therefore, the head at 30-45° associated with the presence of an inspiratory flow bias can act in synergism and result in greater secretion retention.Objectives: To investigate the interaction between two flow patterns, inspiratory (PIF>PEF) or expiratory (PEF>PIF) flow bias, and different angles of head elevation, in relation to the removal of pulmonary secretions and induction of alveolar collapse in pigs under 52h of mechanical ventilation.Methods: The study will be conducted at the FMUSP animal ICU in partnership with UNIFESP. Four groups will be formed, with 8 animals each: 1- decubitus with head at 30° and inspiratory flow bias (D30-IFB); 2- decubitus with head at 10° and inspiratory flow bias (D10-IFB); 3- decubitus with head at 30° and expiratory flow bias (D30-EFB); and 4- decubitus with head at 10° and expiratory flow bias (D10-EFB). The variables of interest will be respiratory mechanics; distribution of regional ventilation and lung volume at the end of expiration provided by the electrical impedance tomograph; gas exchange; velocity and displacement direction of the tracheal mucus (innovative technique with tantalum discs placed 5cm beyond the distal portion of the orotracheal tube and monitored by computed tomography); hemodynamics; weight and rheology of aspirated secretion; weight of the lungs; and histological and immunohistochemical analysis of lung.Expected results: The D30-IFB animals will present secretion displacement towards the lungs, which will cause alveolar ventilation reduction, alveolar collapse, and shunt, with worsening of gas exchange and lung compliance. Animals will show progressive signs of lung infection such as fever. In the end, they will present more edematous lungs and worse histological lesions. Vice versa, the D10-EFB animals will present displacement of tracheal secretion towards the glottis. Thus, secretion retention will be minimized, which will result in better compliance, gas exchange, hemodynamic stability, and lower lung weight and histological damage. If these results are confirmed, it will be possible to conclude that the 30° head-elevated position, currently considered protective, and the adjustment of the inspiratory flow bias, often ignored in clinical practice, are inducers of pulmonary retention and potentially of VAP, justifying a future translational clinical study (AU)