Electrical Impedance Tomography-Guided Identification of the Optimal Lateral Position in Postoperative ARDS

Postoperative abdominal surgery patients commonly develop atelectasis, diaphragmatic dysfunction, and heterogeneous ventilation. When complicated by ARDS, ventilation-perfusion mismatch may worsen oxygenation and ventilatory efficiency. Prone positioning may be limited in this population because of abdominal incisions, drains, and intra-abdominal pressure concerns. Lateral positioning is frequently used in critical care and may improve V/Q coupling by changing gravitational gradients, chest wall mechanics, and pulmonary blood flow distribution; however, quantitative bedside evidence comparing left and right lateral positions is limited.

EIT is a radiation-free bedside functional imaging technique that can continuously monitor regional ventilation. In combination with a brief breath-hold and an intravenous tracer bolus, EIT can derive perfusion-related signals, enabling bedside quantification of V/Q matching.

In this randomized crossover study, each participant undergoes three body positions-supine position, left lateral position, and right lateral position-in a randomized order. The sequence (including whether left lateral or right lateral is performed first) is determined by random-number-based randomization. Each position stage lasts 60 minutes. After entering each position, participants are stabilized for 45 minutes before EIT measurements. EIT ventilation recording is obtained after stabilization, followed by an EIT perfusion assessment performed during a breath-hold of at least 8 seconds (actual breath-hold time recorded). During the breath-hold, a rapid intravenous tracer bolus is administered: 10 mL of 10% sodium chloride solution or 10 mL of 5% sodium bicarbonate solution, injected over 2-3 seconds via a central or peripheral venous line. Between position stages, participants return to the supine position for a 45-minute washout period. The primary endpoint is the difference in global normal V/Q (%) among the three positions. Secondary endpoints include other EIT-derived V/Q indices, oxygenation indices, respiratory mechanics, hemodynamic changes, and feasibility/safety outcomes. The primary statistical analysis required a minimum of 22 evaluable participants. Recruitment was planned to achieve at least this number to account for potential dropout. Recruitment would be terminated once sufficient evaluable subjects were obtained.

• EIT-derived V/Q mismatch components (shunt-like lung pixel percentage, dead-space-like lung pixel percentage). [Time frame: Single assessment time point after respiratory and hemodynamic stabilization within each 60-minute positional stage (left lateral, right lateral); all V/Q mismatch data will be reported separately per positional time point for inter-position comparison.]
• EIT-derived regional ventilation distribution indices [Time frame: Single assessment time point after hemodynamic and respiratory stabilization within each 60-minute positional stage (left lateral, right lateral); comparative data of all indices will be reported separately for each positional assessment time point.]
• EIT-derived regional perfusion distribution indices [Time frame: Single perfusion assessment time point after respiratory and hemodynamic stabilization within each 60-minute positional stage (left lateral, right lateral); all perfusion index data will be reported separately for each positional assessment time point.]
• Oxygenation index (PaO₂/FiO₂ ratio if arterial blood gas sampling is feasible; SpO₂/FiO₂ ratio as an alternative surrogate index). [Time frame: Single assessment time point after respiratory and hemodynamic stabilization within each 60-minute positional stage (supine, left lateral, right lateral); oxygenation index values will be reported separately for each positional assessment time point.]
• Ventilator-related pressure parameters (plateau pressure, driving pressure). [Time frame: Single assessment time point after respiratory and hemodynamic stabilization within each 60-minute positional stage (supine, left lateral, right lateral); all pressure values will be reported separately for each positional assessment time point.]
• Respiratory system compliance (ventilator-derived parameter). [Time frame: Single assessment time point after respiratory and hemodynamic stabilization within each 60-minute positional stage (supine, left lateral, right lateral; all compliance values will be reported separately for each positional assessment time point.]
• Circulatory monitoring parameters (heart rate, mean arterial pressure, vasopressor use if available). [Time frame: Two predefined assessment time points within each 60-minute positional stage (supine, left lateral, right lateral): 1) after full respiratory and hemodynamic stabilization; 2) around breath-hold and contrast bolus injection for perfusion acquisition.]
• Proportion of participants completing all randomized 60-minute positional stages and inter-position washout periods. [Time frame: Through study positional phase completion (from initiation of the first positional stage to the end of the final washout period following the last positional assessment); completion status data will be summarized across all participants.]
• Number of participants with successful acquisition of analyzable EIT imaging datasets. [Time frame: Through study positional phase completion (from initiation of the first positional stage to the end of the final washout period following the last positional assessment); successful EIT acquisition counts will be aggregated for all enrolled subjects.]
• Proportion of participants capable of maintaining breath-hold for at least 8 seconds during EIT perfusion imaging. [Time frame: Through positional phase completion (from first positional stage start to final post-position washout end); breath-hold compliance proportions summarized across all perfusion assessment time points.]
• Number of participants with any procedure-related adverse event (desaturation, hypotension, arrhythmia, accidental tube/line/drain displacement). [Time frame: Through positional phase completion (from first positional stage start to final post-position washout end); all adverse events captured and tallied during positional testing.]
• Number of participants with early study termination secondary to positional intolerance. [Time frame: Through positional phase completion (from first positional stage start to final post-position washout end); all early termination events documented and counted for pooled safety analysis.]