Inferior vena cava diameter measurement technique.The inferior vena cava should be measured in two-dimensional mode at the subcostal window using the long axis distal to the hepatic vein (arrow), approximately 1 - 3cm from the entrance of the inferior vena cava in the right atrium (A). Measurements near the right atrium entrance or near the diaphragm should be avoided. Its diameter can also be measured in M mode simultaneously with the two-dimensional mode to ensure the perfect alignment of the probe, perpendicular to the long axis of the inferior vena cava. In patients under invasive mechanical ventilation, the diameter of the inferior vena cava at the end of inspiration (maximal diameter) and at the end of expiration (minimum diameter) is measured to calculate the distensibility index. The probe must be kept in a fixed position during the respiratory cycle. Image obtained with a GE Vivid T8 echocardiograph. IVC - inferior vena cava.