Experimental Analysis of Inferior Vena Cava Diameter Variations under Controlled Respiratory Load

Ultrasonography of the inferior vena cava (IVC) is a non-invasive technique widely employed to assess venous diameter and pulsatility, providing clinically relevant information on intravascular volume status and right atrial pressure (RAP). In routine clinical practice, this evaluation is based on non-standardized respiratory manoeuvres, such as the sniff test, which can limit measurement reproducibility and clinical reliability. This thesis aims to investigate IVC behaviour under controlled inspiratory conditions by combining real-time ultrasonography with simultaneous spirometric acquisition. The experimental setup integrates a custom 3D-printed spirometer with a mechanical airflow resistance designed for this study. The resistance was designed using Autodesk Fusion 360 and printed in PET plastic. It consists of a cylindrical shell with a rotating internal disk featuring a 1mm central hole. This geometry generates controlled airflow resistance while allowing minimal flow to keep the epiglottis open, ensuring effective transmission of pressure changes to the thoracic cavity. The rotation of the disk allows two configurations: open, corresponding to spontaneous breathing, and resisted, where the airflow passes only through the small central hole, increasing respiratory effort. The experimental protocol consists of a 6-second inspiratory trial, divided into two equal phases: the first 3 seconds without resistance and the last 3 with resistance. This design enables the comparison of IVC dynamics before and after the application of an inspiratory load while maintaining a natural, continuous breathing pattern. The acquisition was tested both in supine and PLR (Passive Leg Raising) positions. Ultrasound recordings of the IVC in the longitudinal axis are acquired using the Viper software in real-time mode, allowing continuous segmentation of the venous diameter throughout the acquisition. Simultaneous spirometric data provide complementary information on flow and pressure variations, ensuring precise temporal alignment between respiratory and ultrasound signals. The results show a consistent reduction in the inferior vena cava (IVC) diameter following the application of respiratory resistance across all subjects, with a negative diameter variation (ΔD post–pre) ranging between approximately –1 mm and –5 mm. The Caval Index (CI), which quantifies diameter variability, has higher values under the supine condition (normal), about 30–60%, compared to the PLR condition, where it was about 10–45%. These observations highlight an increased respiratory collapsibility of the IVC during spontaneous breathing, whereas the PLR posture enhances venous return, promoting vessel stabilization and reducing oscillatory behaviour. In conclusion, controlled inspiratory resistance induces a measurable reduction in IVC diameter, reflecting the rise in intrathoracic pressure. The PLR condition mitigates this effect by increasing preload and minimizing collapsibility. These findings suggest that the combination of standardized respiratory loading and postural modulation can improve the physiological interpretation and reproducibility of IVC-based hemodynamic evaluations.