Ultrasound evaluation of the respiratory changes of the inferior vena cava and axillary vein diameter at rest and during positive pressure ventilation in healthy volunteers

Abstract

Introduction: Ultrasound assessment of the inferior vena cava (IVC), a large compliant capacitance vein, has gained favour in aiding fluid management of critically ill patients. The respiro-phasic changes in IVC diameter have given clinicians a tool for assessing patient volume status. There are however, limitations as to when and under what circumstances this tool can be reliably used. The aim of this study was to assess IVC and axillary vein (Axv) diameter size and respiratory variation in spontaneously breathing participants at rest and with the application of increasing positive end-expiratory pressure (PEEP) via non-invasive ventilation (NIV). The Axv was studied as an alternative vein to the IVC because it is accessible for ultrasound visualisation and independent of intra-abdominal pressure. Methods: The IVC and Axv diameters of 28 healthy adult volunteers were measured using ultrasound, at rest and during positive pressure ventilation via NIV. The collapsibility index (CI) and distensibility index (DI) of these vessels were calculated and compared. The correlation between increasing PEEP levels and DI was evaluated. Results: Positive pressure ventilation delivered via NIV produced equivalent respirophasic diameter changes in the IVC and Axv. Both vessel diameter variations at baseline (collapsibility index) and with increasing levels of PEEP (distensibility index) were non-uniform and unpredictable in this study population. The respiro-phasic diameter changes of the IVC and Axv showed no correlation with the level of PEEP. Conclusion: These findings confirm the current clinical limitations of determining volume responsiveness using the IVC in spontaneously breathing patients with and without PEEP. The Axv cannot be used as an alternative vessel to the IVC as it also displays similar unpredictability.