The volume of fluid infused into the urinary bladder to obtain intra-abdominal pressure (IAP) measurements vary greatly in the literature -- ranging from very small amounts (a few ml) to 250 ml or greater. Several studies report that injecting over 50 ml of fluid into a non-compliant bladder will result in distention and resistance to stretching of the non-complaint bladder wall, resulting in an overestimation of the true intra-abdominal pressure.[1, 2] Similarly, infusing very large volumes (250 ml or more) into a compliant bladder will eventually result in intrinsic bladder wall muscle contraction, again leading to overestimation of the true IAP.[3] Malbrain et al, constructed bladder volume/pressure compliance curves on several patients, elegantly demonstrating this phenomenon and noting that 50 ml was a reasonable volume of infusion to obtain accurate pressure without causing overestimation of IAP (see figure for an example).[3] Based on these studies, most intuitions have reduced the volume of infusion to 50 ml in adult ICU patients.

However, newer researchers suggest that even 50 ml can result in over-estimation of intra-abdominal pressure. DeWaele hypothesized that bladders will become "stiffer" or have less compliance if a Foley catheter continuously drains them. He is concerned that using 50 or 100 ml of saline will result in an overestimation of IAP in many situations. To test his hypothesis he constructed bladder volume/pressure compliance curves in 20 critically ill adult patients who had a Foley catheter in for 5 days or longer.[4] He noted that a minimum volume of 10 ml was required to get accurate pressure transduction, but that measured pressures increased 21% when 50 ml was infused and increased by 40% when 100 ml was infused. He concluded that 10-20 ml infusion volume was adequate, and that higher volumes risked overestimation of the incidence of IAH and ACS. Kimball and colleagues conducted a similar bladder volume/pressure compliance study in all patients in their ICU who were undergoing IAP monitoring.[5] They found volumes of 10-20 ml were adequate to obtain reproducible IAP measurements, and that volumes over 30 ml resulted in higher values being measured (about 2 mm Hg higher at 50 and 60 ml that at 10 or 20 ml).

Two investigators have conducted research in pediatric ICU’s to determine the optimal volume of infusion to accurately transduce intra-abdominal pressure. Davis et al compared intra-abdominal pressure measured via a peritoneal dialysis catheter (gold standard) against IAP measured via the stomach and via the bladder using infusion volumes of 0, 1, 3 and 5 ml/kg. He found 1 ml/kg infused into the bladder to be the most accurate, while smaller or larger volumes were less accurate as was any volume infused when measuring gastric pressure.[6] Ejike measured bladder compliance curves in children by infusing increasing volumes and measuring each pressure obtained with that volume.[7] She found that optimal volumes ranged from 6 ml to 25 ml in children between 3 kg and 50 kg. She concluded that 6 ml was enough in all situations and one never needed more than 20-25 ml in any child to obtain accurate IAP transduction via the bladder. Therefor, these two studies suggest that one can either use 1 ml/kg (not ever needing more than 20-25 ml) or just use 6 ml infusion volume in pediatric patients to obtain accurate pressure transduction. It is also worthy to note that curves plotting intra-abdominal pressure versus volume of infusion are effected by the baseline IAP: A patient with a relatively low IAP will see minimal increases in IAP as increasing fluid is infused into the bladder since the extra volume infused into the bladder/abdominal compartment has little effect on the entire compartmental pressure. However, a patient with very high IAP who has a very tight abdominal compartment, will demonstrate more substantial increases in IAP as increasing volume is infused into the bladder.[2]

Theoretically, all that is necessary to accurately transduce IAP through the urinary bladder is a static fluid column in the Foley and a very small amount of fluid within the bladder that is in direct communication with the static fluid column in the Foley. It is apparent now that infusion volumes of around 6-20 ml are sufficient to measure meaningful IAP in children,[6, 7] and 10-20 ml are sufficient in adults.[4, 5] Balough, et al, used this knowledge to design and conduct a study using a 3-way Foley to continuously measure IAP.[8] Because of the unique properties of the collapsible, complaint human bladder they were able to create a small pocket of fluid at the tip of the Foley with a nearly static fluid column within the flush lumen. They maintained this pocket of fluid through continuous infusion of saline at 4 ml/hour via a standard arterial line set-up attached to a specially modified 3-way Foley catheter. They proved the reliability of this method by showing that the pressures measured continuously were essentially identical to those recorded using the traditional method of IAP measurement using a 50 ml saline infusion.

In summary, the amount of fluid needed to accurately transduce bladder pressure through the Foley catheter in children is 6-10 ml (or 1 ml/kg) and in adults is 10-20 ml. On occasion, infusing more volume than this risks overestimation of actual intra-abdominal pressure due to compliance issues of the bladder wall.

1. Fusco, M.A., R.S. Martin, and M.C. Chang, Estimation of intra-abdominal pressure by bladder pressure measurement: validity and methodology. J Trauma, 2001. 50(2): p. 297-302.
2. Gudmundsson, F.F., et al., Comparison of different methods for measuring intra-abdominal pressure. Intensive Care Med, 2002. 28(4): p. 509-14.
3. Malbrain, M.L.N.G., Different techniques to measure intra-abdominal pressure (IAP): time for a critical re-appraisal. Intensive Care Med, 2004. 30(3): p. 357-71.
4. DeWaele, J.J., et al., Saline volume in transvesical intraabdominal pressure measurement: Enough is enough. Intensive Care Medicine, 2005. 31, Supplement 1(134): p. S59 - Abstract 215.
5. Kimball, E.J., et al., A comparison of infusion volumes in the measurement of intra-abdominal pressure. Critical Care Medicine, 2005. 33(12 supplement): p. A37, Abstract 139-S.
6. Davis, et al., Comparison of indirect methods of measuring intra-abdominal pressure in children. Intensive Care Med, 2005. 31(3): p. 471-475.
7. Ejike, J.C. and M. Mathur, Optimal Bladder Volumes For Intra-abdominal Pressure Measurement In Small Children. Critical Care Medicine, 2005. 33(12 supplement): p. A93, Abstract 150-M.
8. Balogh, Z., et al., Continuous intra-abdominal pressure measurement technique. Am J Surg, 2004. 188(6): p. 679-84.