The IVC view (3)

Changes in diameter correlate with changes in intrathoracic and intra-abdominal pressure. A collapse index is calculated as the change in diameter between inspiration and expiration divided by the maximal diameter. It may be more useful to measure trends of IVC diameter and collapsibility in response to fluid resuscitation; however, there is evidence for cut-off values which can indicate an underfilled or overfilled status. A maximal IVC diameter of 2 cm with collapse of 40-50% suggests a pressure of >10 mm Hg, although there is conflicting evidence on which exact IVC measurements equate to  right atrial pressures. In patients being invasively ventilated, the distensibility index is used which is the difference between the maxium and minimum IVC diameter divided by the minimum IVC diameter. A distensibility index >18% can suggest a patient will benefit from further fluid resuscitation.

Ref: the dark art of ivc ultrasound