The elliptic shape of inferior vena cava (IVC) sections, while hindering the ultrasound measurement of IVC diameter, may provide a useful tool to estimate central venous pressure (CVP). Hypothetically, the higher is CVP and more the ratio R between minimum and maximum diameters approximates 1. The purpose of this study is to determine R values in four different sections before and after a fluid load test, in order to evaluate if R is affected by the test and at which level the variation is larger.

The secondary endpoint is to search for the parameter best predictive of the positivity of the fluid load test among IVC minimum and maximum diameters, eccentricity, section area, and blood velocity at the level that presented the largest variations after the fluid load.

The study will be carried out during the first 6 hours after the end of cardiac surgery when patients are mechanically ventilated and sedated with propofol and remifentanil to achieve a Ramsey sedation score of 3-4. The following four steps will be carried out:

A) A baseline hemodynamic and ultrasound assessment with the head of the bed positioned at 30 °.

B) The measurement of abdominal pressure with the head of the bed placed in a horizontal position for 3 minutes.

C) A rapid fluid load test with the head of the bed raised again to be 30°. The test will be performed by infusing 250-300 mL/m2 Body Surface Area of 6% hydroxyethyl starch 6% 130/0.4 (VoluvenR) in 30 minutes and will be considered positive if the cardiac index increases by at least 15% above the baseline value.

D) A second hemodynamic and ultrasound assessment with the head of the bed positioned at 30 °.

Hemodynamic evaluation The hemodynamic assessment will be run out by evaluating heart rate, systemic arterial and pulmonary pressures, central venous pressure, and pulmonary wedge pressure. Cardiac output will be measured by the thermodilution technique; the mean of three measures will be taken; then cardiac index will be calculated by dividing the cardiac output by the body surface area.

Ultrasound measurements

IVC maximum and minimum diameters (which will be defined as anteroposterior and lateral) will be measured at the end of inspiration and of expiration at four levels:

1. 1 cm after the confluence of the iliac veins
2. At the confluence of the renal veins
3. Immediately below the confluence of the hepatic veins
4. Immediately above the confluence of the hepatic veins (also in long axis)

At level 3 maximum and average blood velocity will be measured by Doppler provided that the angle of insonation is 60° or less.

At each level, we will compute the ratio R between the anteroposterior and lateral diameters (eccentricity) and the sectional area, calculated with the formula for the ellipse.

Statistical analysis The data obtained will be presented as mean (standard deviation). Statistical analysis will be performed with ANOVA for repeated measures and post-hoc comparisons with Student Newman Keuls test. ROC (Receiver Operating Characteristic) curves will be calculated to test the predictivity of a positive fluid load test by the aforementioned parameters; the analysis will be performed at the level at which each parameter shows the largest variations after the load test.

The a priori power analysis was performed on the primary endpoint, i.e. on changes of R in relation to IVC levels and fluid volume test with the program Gpower (17). The analysis was carried out on ANOVA with a 4 x 2 design by assuming an α error of 0.05, a power of 1-β of 0.95, and an f of 0.40, which corresponded to a large-size effect. The index f is a standardized measure of the effect size and is equal to the minimum difference considered clinically relevant divided by the standard deviation in the population. Since, the standard deviation of R was 0.1 in a previous study on healthy volunteers, an f value of 0.4 corresponded to a minimal difference of 0.04 between R values. On this basis, we obtained 112 determinations over all groups of the design, which corresponded to a sample of 14 patients (112/8=14); such value was increased by 10%, and the final sample of 16 patients was obtained.

Inclusion Criteria:

• the need determined by the attending physician to perform a rapid fluid load test to guide infusions
• a condition of adequate sedation and good adaptation to mechanical ventilation
• the presence of an arterial catheter and a catheter in the pulmonary artery

Exclusion Criteria:

• obesity (BMI> 30 kg/m2)
• age <18 years
• women of childbearing age
• tricuspid insufficiency
• hemodynamically significant right ventricular failure
• bleeding from the surgical drains greater than 150 mL in the hour preceding the enrollment
• history of allergy to colloids
• administration of more than 1000 mL of hydroxyethyl starch in the last 24 hours

Patients undergoing cardiac surgery