Pressures in the superior and inferior vena cava and intra-abdominal pressure

Purpose

To investigate the relationship of pressure in the inferior vena cava (Pivc) with a) pressure in the superior vena cava (Psvc), and b) intra-abdominal pressure as measured in the urinary bladder (Pcyst).

Materials and results

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 ≤ Pcyst < 15 mmHg), group C (Pcyst ≥ 15 mmHg). Statistical analysis was performed with the paired t test, Pearson correlation. Results are expressed as mean ± SEM (Table 1).

Table 1

There was significant correlation of Psvc and Pivc in groups A and B (group A r = 0.93, group B r = 0.87). In Group C, Pivc correlated (P < 0.01) with both Pcyst (r = 0.78) and Psvc (r = 0.7).

When Pcyst > Psvc, Pivc was higher than Psvc (P < 0.01). Furthermore: a) with Pcyst < 15 mmHg, no significant difference was found between Pcyst and Pivc (P = 0.068) 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). b) With Pcyst ≥ 15 mmHg, Pivc was lower than Pcyst (P < 0.01). It highly correlated with Pcyst (r = 0.85, P < 0.01) and less strongly correlated with Psvc (r = 0.701, P < 0.01).

When Pcyst ≤ Psvc, no difference between Pivc and Psvc was observed. Furthermore: a) 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). b) With Pcyst ≥ 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 to a 'waterfall' effect, and Pivc does not accurately reflect Psvc any more.