Physiological effects and safety of bed verticalization in patients with acute respiratory distress syndrome

Abstract

Background: Trunk inclination in patients with Acute Respiratory Distress Syndrome (ARDS) in the supine position has gained scientific interest due to its effects on respiratory physiology, including mechanics, oxygenation, ventilation distribution, and efficiency. Changing from flat supine to semi-recumbent increases driving pressure due to decreased respiratory system compliance. Positional adjustments also deteriorate ventilatory efficiency for CO₂ removal, particularly in COVID-19-associated ARDS (C-ARDS), indicating likely lung parenchyma overdistension. Tilting the trunk reduces chest wall compliance and, to a lesser extent, lung compliance and transpulmonary driving pressure, with significant hemodynamic and gas exchange implications.

Methods: A prospective, pilot physiological study was conducted on early ARDS patients in two ICUs at CHU Clermont-Ferrand, France. The protocol involved 30-min step gradual verticalization from a 30° semi-seated position (baseline) to different levels of inclination (0°, 30°, 60°, and 90°), before returning to the baseline position. Measurements included tidal volume, positive end-expiratory pressure (PEEP), esophageal pressures, and pulmonary artery catheter data. The primary endpoint was the variation in transpulmonary driving pressure through the verticalization procedure.

Results: From May 2020 through January 2021, 30 patients were included. Transpulmonary driving pressure increased slightly from baseline (median and interquartile range [IQR], 9 [5-11] cmH₂O) to the 90° position (10 [7-14] cmH₂O; P < 10^-2 for the overall effect of position in mixed model). End-expiratory lung volume increased with verticalization, in parallel to decreases in alveolar strain and increased arterial oxygenation. Verticalization was associated with decreased cardiac output and stroke volume, and increased norepinephrine doses and serum lactate levels, prompting interruption of the procedure in two patients. There were no other adverse events such as falls or equipment accidental removals.

Conclusions: Verticalization to 90° is feasible in ARDS patients, improving EELV and oxygenation up to 30°, likely due to alveolar recruitment and blood flow redistribution. However, there is a risk of overdistension and hemodynamic instability beyond 30°, necessitating individualized bed angles based on clinical situations. Trial registration ClinicalTrials.gov registration number NCT04371016 , April 24, 2020.

Keywords: Acute respiratory distress syndrome; Intensive care unit; Mechanical ventilation; Verticalization.

Fig. 1

Diagram of the progressive verticalization procedure. All patients started the sequence in the 30° semi-seated position (baseline). After 30 min, successive steps at 0°, 30°, 60°, and 90° were reached for 30 min each, allowing progressive bed verticalization without body flexion (standing upright). Then, patients were returned to the 30° semi-seated position for 30 min

Fig. 2

Physiological values evaluated at each position, from baseline (30° semi-seated position) to the standing upright position (90°), and a repositioning to 30° semi-seated. Values are reported as box and whisker plots. A Transpulmonary driving pressure (ΔP_L), computed as the difference between end-inspiratory and end-expiratory transpulmonary pressures, as measured using an esophageal balloon catheter. B Static compliance of the respiratory system (C_RS). C Mechanical power in the different study steps. D End-expiratory lung volume, as expressed in milliliters per kilogram of predicted body weight (EELV_PBW). E Strain, computed as the tidal volume-to-EELV ratio. F Pulmonary shunt measured using the venous-to-arterial difference in oxygen concentrations. Mixed-effects models were used and the overall P value for the effect of position through the experimental procedure is provided