This study aims to compare lung perfusion distribution between inspiratory breath-hold and expiratory breath-hold phases using the saline indicator method combined with electrical impedance tomography (EIT). A self-controlled crossover design will be used in which each participant undergoes both breath-hold conditions with standardized rapid intravenous saline injections. The primary objective is to evaluate changes in global and regional pulmonary perfusion under different lung volume states. The findings may improve understanding of perfusion redistribution with lung inflation and support optimized ventilatory strategies in clinical practice.

Electrical impedance tomography (EIT) with saline bolus injection is a noninvasive technique capable of generating real-time regional lung perfusion images based on transient impedance changes. Lung perfusion is highly dependent on lung volume, vascular resistance, and gravity, yet quantitative bedside evaluation remains limited.

This prospective self-controlled study investigates the differences in lung perfusion between inspiratory breath-hold (near total lung capacity) and expiratory breath-hold (near functional residual capacity). Each participant will undergo two standardized procedures: (1) expiratory breath-hold followed by a rapid intravenous injection of saline, and (2) inspiratory breath-hold followed by the same injection protocol. The order will be randomized.

Perfusion images will be reconstructed using time-impedance curves derived from saline indicator dilution. Regional perfusion parameters, including dependent vs nondependent zones and global perfusion distribution, will be compared between the two phases.

This study may provide foundational physiological evidence for understanding perfusion redistribution across lung volumes and support the clinical application of EIT in respiratory monitoring, ventilatory management, and rehabilitation in critically ill patients.

Inclusion Criteria:

• Age 18 to 65 years
• Able to provide written informed consent
• Able to perform standardized inspiratory and expiratory breath-hold maneuvers for at least 8 seconds
• No known acute cardiopulmonary disease
• Suitable for noninvasive electrical impedance tomography monitoring and intravenous saline administration

Exclusion Criteria:

• Known severe cardiopulmonary disease (e.g., advanced chronic obstructive pulmonary disease, severe pulmonary hypertension, or heart failure)
• Hemodynamic instability or clinically significant cardiac arrhythmia
• Contraindication to intravenous saline administration or fluid bolus
• Pregnancy or breastfeeding
• Skin conditions or chest wall abnormalities preventing proper placement of EIT electrodes