Caval Index

Longitudinal ultrasound images of the IVC in an asymptomatic patient demonstrate a normal inferior vena cava (IVC) during inspiration and expiration, in which the diameters (yellow double-headed arrows) do not change significantly. In this case, the diameters of the IVC were measured 2-3 cm below the right atrial border (yellow lines).

Facts: IVC Diameter

• IVC diameter changes following total body volume (increases with increasing total body volume, and decreases with volume depletion)
• IVC normally collapses with inspiration (decreased intra-thoracic pressure) and expands with expiration (but this collapsibility should not exceed 50%)

Caval Index

• Caval Index = 100 x (diam expiration - diam inspiration)/diam expiration
• Where to measure the IVC? Several ways exist, and none is perfect yet. Easy way is to measure with a longitudinal view of the IVC - find the junction of the atrium and IVC and measure the IVC at 2-3 cm below the junction
• Interpretation: studies vary greatly as to significance of values in different patient populations. In general, if caval index is greater than 50% it suggests low central venous pressure (CVP less than 8 mmHg) and high probability of fluid responsiveness

Reference:

Nagdev AD, Merchant RC, Tirado-Gonzalez A, et al. Emergency department bedside ultrasonographic measurement of the caval index for noninvasive determination of low central venous pressure. Ann Emerg Med 2010; 55:290-295.