Acute respiratory distress syndrome (ARDS) is characterized by impaired
ventilation-perfusion matching, which not only indicates the severity of the condition
but also contributes to ventilation-induced lung injury. Higher positive end-expiratory
pressure (PEEP) and prone position could improve ventilation-perfusion mismatch by
recruiting collapsed lungs and facilitating more homogeneous ventilation, but these
benefits might depend on lung recruitability. The present study aims to elucidate the
regional effect of PEEP(low and high) and body position(supine and prone) on the
ventilation-perfusion matching. Also endeavors to establish correlations between
alterations in ventilation-perfusion matching patterns and the inherent lung
recruitability.
Participants will be deeply sedated and paralyzed, ventilated in volume-controlled with
protective ventilation (tidal volume=6-8 mL/Kg of predicted body weight and respiratory
rate set to obtain normal pH). Then the patients will be sequentially assigned to each of
four conditions as follows:
Low PEEP, supine position; High PEEP, supine position; Low PEEP, prone position; High
PEEP, prone position. High PEEP and low PEEP is defined as 15 cmH2O and 5 cmH2O (or
airway opening pressure, either of which was higher) respectively. Each measurement
(e.g., arterial blood gas analysis, respiratory parameters, hemodynamics, EIT
measurements) will be performed at least 15 minutes after changing ventilator settings
and at least 1 hour after changing body positions. The timing of turning patients from
supine to prone position is determined by the clinical team.
To assess lung recruitability, a single-breath derecruitment maneuver will be performed
by changing PEEP 15 to 5 cmH2O (or airway opening pressure, either of which was higher)
in supine position. Patients with recruitment-inflation ratio over the median value are
defined as high recruiters.
EIT data will be collected by standard device (Infinity C500, Drager, Germany) with a
sample rate of 50 Hz. The EIT belt will be placed directly below the armpits, between the
third and fifth intercostal spaces. This positioning of the EIT belt will be maintained
consistently during both supine and prone positions. A bolus of 10 ml 5% NaCl will be
injected during a respiratory pause (≥8 s) through the central venous catheter to assess
lung ventilation and perfusion distributions. The primary endpoint is EIT-based
ventilation-perfusion matching (V/Q match%).
