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Introduction: 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). The present study was designed to clarify whether variations in IVC flow rate and RAP caused by positive pressure ventilation are associated with changes in IVC cross-section (□IVC) during major changes in IAP.Methods: Nine pigs were anesthetized, mechanically ventilated and equipped. IAP was set at 0, 15 and 30 mmHg during two conditions, ie normovolemia and hypovolemia generated by blood removal to obtain a mean arterial pressure value lower than 60 mmHg. At each IAP increment, cardiac output, RAP, IVC flow and surface area were respectively assessed by flowmeters and transesophageal echocardiography.Results: At normal IAP, even in presence of respiratory changes in IVC flows, no □IVC were observed during the two conditions. At high IAP, neither □IVC nor modulations of IVC flow were observed whatever the volemic status. The majority of animals with an IVC area of less than 0.65 cm2 showed evidence of IAP greater than RAP values suggesting that IAP affects RAP- IVC size relationship.Conclusion: The present study shows that RAP should not be estimated from IVC dimensions in case of high IAP.