Ultralow volumes in transvesical intra-abdominal pressure measurement

Background

Intra-abdominal pressure (IAP) is measured routinely in patients at risk for intra-abdominal hypertension (IAHT) and abdominal compartment syndrome. Transvesical measurement of the IAP has become most widely used technique, but concerns remain about its accuracy. We recently showed that the use of 50 or 100 ml saline for measurement may lead to overestimation of the IAP. The goal of this study was to determine the minimal instillation volume at which an IAP curve can be obtained, and to study bladder compliance at volumes below 20 ml.

Methods

Intra-abdominal pressure was measured transvesically using a custom-designed IAP monitoring set (BBraun, Zaventem, Belgium) in 10 critically ill patients at risk for IAHT. Mean age of the patients was 56 years. Four patients were admitted to the ICU after an emergency abdominal surgical procedure or abdominal trauma, the other patients were referred to the ICU because of acute liver failure with ascites (n = 3), severe sepsis (n = 1), hemorrhagic shock (n = 1) or retroperitoneal bleeding and hematoma (n = 1). Measurements were performed after a median 1 day of bladder drainage.

After priming the system with normal saline the IAP was measured, starting without any extra instillation of saline, and continued with 1 ml increments up to 10 ml, after a 1-min equilibration period after each instillation. After each instillation, an 'oscillation test' was performed, by gently tapping the abdomen until an oscillating curve could be observed on the monitor. The minimal volume at which the oscillation test was positive was recorded. These values were compared to the IAP obtained using 20 ml saline. Data are presented as the mean (± SD).

Results

At 2 ml installed saline volume an oscillation curve could be obtained in all patients. Mean IAP2 ml was 9.3 mmHg (± 3.9), mean IAP10 ml was 9.6 mmHg (± 4.2) and mean IAP20 ml was 10.1 mmHg (± 4.2).

In three patients there was no difference between IAP2 ml and IAP20 ml, but in six patients the IAP reading at 20 ml was 1 mmHg higher than that at 2 ml (an average increase of 11.4%), and in one patient the difference was 2 mmHg (14.2%).

Conclusion

In this sample of ICU patients at risk for IAHT, 2 ml saline was sufficient for IAP signal transduction. Higher volumes for transvesical IAP measurement resulted in higher pressure readings in some ICU patients, even at instillation volumes below 20 ml. Using ultralow volumes for IAP measurement may therefore be preferable.