Pulmonary and Ventilatory Effects of Bed Verticalization in Patients With Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is defined using the clinical criteria of bilateral pulmonary opacities on a chest radiograph, arterial hypoxemia (partial pressure of arterial oxygen [PaO2] to fraction of inspired oxygen [FiO2] ratio ≤ 300 mmHg with positive end-expiratory pressure [PEEP] ≥ 5 cmH2O) within one week of a clinical insult or new or worsening respiratory symptoms, and the exclusion of cardiac failure as the primary cause. ARDS is a fatal condition for intensive care unit (ICU) patients with a mortality between 30 and 40%, and a frequently under-recognized challenge for clinicians. Patients with severe symptoms may retain sequelae that have recently been reported in the literature. These sequelae may include chronic respiratory failure, disabling neuro-muscular disorders, and post-traumatic stress disorder identical to that observed in soldiers returning from war.

The management of a patient with ARDS requires first of all an optimization of oxygenation, which relies primarily on mechanical ventilation, whether invasive or non-invasive (for less severe patients). Since the ARDS network study published in 2000 in the New England Journal of Medicine, it has been internationally accepted that tidal volumes must be reduced in order to limit the risk of alveolar over-distension and ventilator-induced lung injury (VILI). A tidal volume of approximately 6 mL.kg-1 ideal body weight (IBW) should be applied. Routine neuromuscular blockade of the most severe patients (PaO2/FiO2 < 120 mmHg) is usually the rule, although it is increasingly being questioned. Comprehensive ventilatory management is based on the concepts of baby lung and open lung, introduced respectively by Gattinoni and Lachmann. According to these concepts, it must be considered that the lung volume available for mechanical ventilation is very small compared to the healthy lung for a given patient (baby lung) and that the reduction in tidal volume must be associated with the use of sufficient PEEP and alveolar recruitment maneuvers to keep the lung "open" and limit the formation of atelectasis.

In addition to this optimization of mechanical ventilation, it is possible to reduce the impact of mechanical stress on the lung. The prone position, for example, makes it possible to free from certain visceral and mediastinal constraints, to optimize the distribution of ventilation as well as the ventilation to perfusion ratios.

Thanks to the technological progress of intensive care beds, it is now possible to verticalize ventilated and sedated patients in complete safety. Verticalization could reduce the constraints imposed to the lungs, by reproducing the more physiological vertical station, and thus modifying the distribution of ventilation.

Indeed, in two physiological studies published in 2006 and 2013 in Intensive Care Medicine, 30 to 40% of patients with ARDS appeared to respond to partial body verticalization at 45° and 60° (in a semi-seated or seated position). In addition to improving arterial oxygenation, verticalization appeared to decrease ventilatory stress, related to supine position, and increase alveolar recruitment, with improved lung compliance and end-expiratory lung volume (EELV) over time. Nevertheless, 90° verticalization has never been studied, nor have positions without body flexion (seated or semi-seated). In these studies, only patients with the highest lung compliance appeared to respond. These data support the current hypothesis of subgroups of patients with ARDS with different pathophysiological characteristics (morphological and phenotypic) and therapeutic responses.

The investigators hypothesize that verticalization of patients with ARDS improves ventilatory mechanics by reducing the constraints imposed on the lung (transpulmonary pressure), pulmonary aeration, arterial oxygenation and ventilatory parameters.

The first objective is to study the influence of the bed position of the patient with early ARDS on the variations in respiratory mechanics represented by the transpulmonary driving pressure (ΔPtp). The second objective is to evaluate changes in ventilatory physiology, tolerance and feasibility of verticalization in patients with early ARDS.