Size and shape of the inferior vena cava before and after a fluid challenge: a pilot study.

BACKGROUND: Recent meta-analyses failed to support the reliability of ultrasound assessment of the inferior vena cava (IVC) to predict fluid responsiveness. However, the techniques utilized were heterogeneous. We hypothesized that the variability of the elliptic section and caliber of the IVC along its course may influence ultrasound evaluation. Therefore, we investigated IVC size and shape at four levels, before and after a fluid challenge. METHODS: Twenty mechanically-ventilated adult patients who received a fluid challenge after cardiac surgery were enrolled. They were regarded as responders if the cardiac index increased more than 15%. Before and after the fluid challenge, IVC anteroposterior (AP) and lateral (LA) diameters, the flat ratio, and the distensibility index were assessed by ultrasound just above the iliac veins, at the confluence of the renal veins, before the confluence of the hepatic veins (where blood flow velocity was also measured), and after it. RESULTS: At all levels, IVC section was elliptical, so that IVC diameters varied between a minimum and a maximum according to the measurement angle. Such interval increased in correspondence of the renal veins, where IVC section was more eccentric. The distensibility index was higher when assessed on AP diameters. After the fluid challenge, non-responders showed a diffuse increase of AP diameters, whereas responders showed an increase of blood velocity before the confluence of the hepatic veins. CONCLUSIONS: The elliptic section should be considered when assessing IVC size. AP diameters are shorter and more affected by the respiratory cycle. After a fluid challenge, an increase of blood velocity associated with unchanged AP diameters may suggest fluid responsiveness.

Is the pCO2 gap obtained from the superior vena cava in agreement with that from the pulmonary artery?

BACKGROUND: The partial pressure of carbon dioxide (pCO2) gap is the difference between pCO2 values in the arterial and mixed venous blood; values higher than six mmHg may be predictive of tissue hypoperfusion. It is still doubtful if central venous blood can be used to assess the gap. This study was aimed to compare the values obtained with blood collected from the superior vena cava and the pulmonary artery. METHODS: Data were obtained from a previous study. In 30 patients who underwent on-pump myocardial revascularization, blood samples from a radial artery, the pulmonary artery, and the superior vena cava were collected eight times in the perioperative period. Two-hundred determinations were utilized to calculate the pCO2 gap from central and mixed venous pCO2. RESULTS: The pCO2 gap was 8.7±2.6 mmHg in the superior vena cava, and 6.0±2.7 in the pulmonary artery. The difference between the two values fell within a wide interval between -4.1 mmHg and +9.5 mmHg in 95% of cases. pCO2 values were significantly higher in the superior vena cava than in the pulmonary artery (mean difference 2.7±2.4 mmHg; P<0.0001), causing a systematic bias between the two estimates; such bias increased during spontaneous breathing (P=0.0249). CONCLUSIONS: pCO2 values are higher in the superior vena cava than in the pulmonary artery. As a consequence, the pCO2 gap calculated with the blood taken from the superior vena cava is overestimated, probably because it reflects the pCO2 arteriovenous difference of the upper part of the body, in particular of the brain.