Conflicting results have been found regarding correlations between right atrial pressure (RAP) and inferior vena cava (IVC) diameter in mechanically ventilated patients. This finding could be related to an increase in intra-abdominal pressure (IAP). This study was designed to clarify whether variations in IVC flow rate caused by positive pressure ventilation are associated with changes in the retrohepatic IVC cross-section (ΔIVC) during major changes in volume status and IAP. Nine pigs were anesthetized, mechanically ventilated and equipped. IAP was set at 0, 15 and 30 mmHg during two conditions, i.e. normovolemia and hypovolemia, generated by blood removal to obtain a mean arterial pressure value lower than 60 mmHg. At each IAP increment, cardiac output, IVC flow and surface area were respectively assessed by flowmeters and transesophageal echocardiography. At normal IAP, even in presence of respiratory changes in IVC flows, no ΔIVC were observed during the two conditions. At high IAP, neither ΔIVC nor modulations of IVC flow were observed whatever the volemic status. The majority of animals with an IVC area of less than 0.65 cm(2) showed evidence of IAP greater than RAP values. Negative RAP-IAP pressure gradients were found to occur with an IVC area of less than 0.65 cm(2), suggesting that IVC dimensions determined using standard ultrasound techniques may indicate the direction of the RAP-IAP gradient. The clinical relevance of the present findings is that volume status should not be estimated from retrohepatic IVC dimensions in cases of high IAP.