Inspiratory effort increases blood volume in the thoracic cavity and decreases end-expiratory lung impedance: a preliminary prospective study

Purpose: Passive leg raising (PLR) increases intrathoracic blood volume by redistributing blood from the lower to the upper body area. While inspiratory effort is hypothesized to have a similar effect due to pressure differences between the intrathoracic and extrathoracic cavities, direct evidence is scarce. Therefore, this study evaluated whether excessive inspiratory effort increases intrathoracic blood volume using end-expiratory lung impedance (EELI). Methods: Volunteers, fitted with electrical impedance tomography (EIT) belts, underwent a spontaneous breathing procedure in the supine position (control step). They breathed through a specialized face mask with separated inspiration and expiration routes (one-way valves) and their EELI was continuously recorded. First, PLR was performed. Subsequently, resistors (3-mm and 2-mm) were sequentially added to the mask’s inspiration route, requiring volunteers to increase inspiratory effort. A reference EELI was established during spontaneous breathing, and changes in EELI (ΔEELI) were calculated for each step (control, PLR, 3-mm, and 2-mm). ΔEELI values were compared using the Friedman test and Wilcoxon signed-rank test with Holm’s P value adjustment. Results: Across 11 participants, the mean ΔEELI decreased by 13, 18, and 19 units for PLR, 3-mm, and 2-mm resistors, respectively. The Friedman test and Wilcoxon signed-rank test revealed significant differences between the control and each aforementioned intervention. Conclusion: PLR and increased inspiratory effort augment thoracic blood volume, thereby reducing EELI. Registration: UMIN000054238. April/23/2024.