Ultrasonography of the inferior vena cava (IVC) is a technique widely used in clinical practice to assess its size and pulsatility, providing important information on the volemic status of patients. In addition, it is used to predict fluid responsiveness. Finally, it provides a non-invasive estimate of right atrial pressure (RAP), which is usually measured by an invasive procedure called right heart catheterisation (RHC). Non-invasive estimation of RAP is obtained as a function of IVC maximum diameter and Caval Index (CI), defined as the ratio of the difference between the maximum and minimum diameter and the maximum diameter of the IVC. This is achieved by using a respiratory manoeuvre called sniff, which involves a quick, strong inhalation through the nose by the patient, but is non-standardised.