BACKGROUND/AIMS: To investigate the relationship of pressure in the inferior vena cava (Pivc) with a) pressure in the superior vena cava (Psvc), b) intra-abdominal pressure as measured in the urinary bladder (Pcyst). METHODOLOGY: A prospective study of 38 mechanically ventilated patients. Simultaneous measurements of Psvc, Pivc and Pcyst (151 sets of measurements) were performed. Measurements were divided in: Group A (Pcyst < 10 mmHg), group B (10 mmHg < or = Pcyst < 15 mmHg), group C (Pcyst > or = 15 mmHg). Statistical analysis was performed with paired t-test, Pearson correlation. Results are expressed in mean +/- SEM. RESULTS: In Groups A and B, Psvc and Pivc were not significantly different and they were highly correlated (10.8+/-0.5 mmHg vs. 10.9+/-0.5 mmHg, r=0.93 for Group A and 14.4+/-0.7 mmHg vs. 14.7+/-0.6 mmHg, r=0.87 for Group B). Pcyst was significantly lower than Pivc in both groups. In Group C, Pivc was significantly higher than Psvc (18.9+/-0.7 mmHg vs. 16.4+/-0.7 mmHg). There was no significant difference between Pivc and Pcyst (19.2+/-0.6 mmHg). Pivc significantly correlated with Pcyst (r=0.78) and Psvc (r=0.7). A. When Pcyst > Psvc, Pivc was higher than Psvc (p<0.01). With Pcyst < 15 mmHg, no significant difference was found between Pcyst and Pivc and they were correlated (r=0.766, p<0.05). Pressures in the superior and inferior vena cava were also correlated (r=0.764, p<0.05). With Pcyst > or = 15 mmHg, Pivc was lower than Pcyst (p<0.01). It correlated highly with Pcyst (r=0.85, p<0.01) and less strongly with Psvc (r=0.701, p<0.01). B. When Pcyst < or = Psvc, no difference between Pivc and Psvc was observed. With Pcyst < 15 mmHg, Pivc was higher than Pcyst (p<0.01) and highly correlated with Psvc (r=0.932, p<0.01). Pivc also correlated with Pcyst (r=0.69, p<0.01). With Pcyst > or = 15 mmHg, Pivc was higher than Pcyst (p<0.01) and correlated with Psvc (r=0.74, p<0.01) and Pcyst (r=0.789, p<0.01). CONCLUSIONS: Although Psvc and Pivc are interchangeable in the absence of an increased Pcyst, when Pcyst is high, measurements of Pivc are misleading. A Pcyst > Psvc results in a "waterfall" effect, and Pivc does not accurately reflect Psvc any more.