What is the proper amount of fluid to infuse into the urinary bladder to obtain accurate pressure transduction measurements?
Introduction:
The volume of fluid infused into the urinary bladder to obtain intra-abdominal pressure (IAP) measurements varies greatly in the original literature on the topic– ranging from very small amounts (a few ml) to 250 ml or greater. This variability has led to some confusion as to what is the “correct” amount of fluid to infuse. The following discussion with review this research, demonstrating that the right amount is “enough but not too much.”
The literature on the
topic
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.[3]
Bladder pressure versus volume of infusion:
The reason for this finding is fairly simple. If you infuse “large” volumes of fluid into the bladder it will fill up, then begins to stretch. The point at which bladder wall stretching occurs is when a pressure volume threshold is passed and any increase in volume leads to a very rapid increase in pressure within the bladder itself – somewhat like that which occurs when inflating a latex balloon from a flaccid state to the point it begins stretching. Once this bladder wall stretching volume is achieved, the pressure in the bladder no longer reflects passive transmission of intraabdominal pressure.
The solution is to not infuse a large volume of fluid into the urinary bladder when measuring IAP. So how much is enough? To answer this question we need to understand some very basic aspects of modern hemodynamic pressure transducers. When using modern electronic hemodynamic transducers, all the fluid that is required is enough to create a hydrostatic coupling of the transducer with the anatomic structure within which the distal end of the monitoring catheter resides. For example – when transducing arterial pressure or CVP the transducer needs to hydrostatically couple with the lumen of the artery or the superior vena cava. This requires very little fluid infusion – essentially just enough to fill the tubing. This is automatically accomplished by the device with a flow rate of about 3-4 cc per hour. To transduce IAP, the entire system also needs to be fluid filled and the tip of the catheter (the Foley) must reside within a fluid bubble in the urinary bladder.
Research now clearly demonstrates that to accomplish this goal of hydrostatic coupling via the Foley catheter – only a small amount of fluid needs to be infused. In fact, larger volumes may result in erroneous measurements due to overdistention of the bladder and measurement of bladder wall compliance rather than equilibration with the abdominal pressure. 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. Malbrain and Deeren also noted increasing pressures as volumes over 25 ml were infused.[10] They infused volumes of between 0 and 300 ml of saline into 13 different patients. Once they exceeded 50 ml of volume the measured pressure began to rise significantly. They conclude that volumes over 50 ml may result in clinically important overestimation of IAP, and that volumes of approximately 25 ml are sufficient for accurate IAP measurement.(Click here for PDF file of this Open Access article) 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 volume 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). Shuster et al reported their initial data on using volumes of 0 ml and 25 ml in adults (future publications will include data on 50 and 200 ml infusion volumes).[11] They note much tighter limits of agreement for the 25 ml volume than the 0 ml volume, confirming pediatric data (described below) that suggests there has to be at least some volume in the bladder hydrostatically coupled with the fluid within the Foley to reliably measure bladder pressure, and confirming Malbrain and Deeren data suggesting up to 25 ml is adequate for accurate measurements.
Three 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] Suominen conducted a nearly identical study design a came to the same conclusion – 1 ml per kg is the optimal volume of infusion in children.[7] Ejike measured bladder compliance curves in children by infusing increasing volumes and measuring each pressure obtained with that volume.[8] She found that optimal volumes ranged from 3 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 three 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]
Conclusions:
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, and 10-20 ml are sufficient in adults. Balough, et al, used this knowledge to design and conduct a study using a 3-way Foley to continuously measure IAP.[9] 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.
Bottom line:
The amount of fluid needed to accurately transduce bladder pressure through the Foley catheter in children is 6-10 ml maximum (or 1 ml/kg) and in adults is 10-20 ml. Infusing more volume than this risks overestimation of actual intra-abdominal pressure due to compliance issues of the bladder wall.
References:
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. De Waele, J.J., et al., Saline volume in transvesical intra-abdominal pressure measurement: enough is enough. Intensive Care Med, 2006. 32(3): p. 455-9.
5. Kimball, E.J., et al., A comparison of infusion volumes in the measurement of intra-abdominal pressure. J Intensive Care Med, 2009. 24(4): p. 261-8.
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. Suominen, P.K., et al., Comparison of direct and intravesical measurement of intraabdominal pressure in children. J Pediatr Surg, 2006. 41(8): p. 1381-5.
8. Ejike, J.C., K. Bahjri, and M. Mathur, What is the normal intra-abdominal pressure in critically ill children and how should we measure it? Crit Care Med, 2008. 36(7): p. 2157-62.
9. Balogh, Z., et al., Continuous intra-abdominal pressure measurement technique. Am J Surg, 2004. 188(6): p. 679-84.10. Malbrain, M.L.N.G. and D. Deeren, Effect of bladder volume on measuring intravesical pressure: a prospective cohort study. CCForum, 2006. 10(4): p. 1-6. (Click here for open access link to article)
11.
Shuster, M.H., et al.,
Reliability of intrabladder
pressure measurement in intensive care. Am J Crit Care, 2010.
19(4): p. e29-39.