Estimating the optimal bladder volume for intra-abdominal pressure measurement by bladder pressure–volume curves

Introduction

Intra-abdominal pressure (IAP) is an important parameter and prognostic indicator of the patient's underlying physiologic status [1]. Correct IAP measurement therefore is crucial. Because measurement of IAP by an indwelling catheter in the urinary bladder is simple, minimally invasive, and reproducible, it has been forwarded as the gold standard. It was thought that the highly compliant wall of the bladder acts as a passive diaphragm, and intrinsic bladder pressure did not rise when its volume is between 50 and 100 ml. However, considerable variability in the measurement technique has been noted, not only interindividual and intraindividual but also intercentre and intracentre. Variations in IAP from -6 to +30 mmHg have been noted. The measurement technique itself is also not uniform, some authors recommend injecting 50 ml saline, others up to 200 ml. The aim of the present study is to determine the optimal bladder volume for correct IAP transmission but without the risk of 'over inflation' of the bladder and raising intrinsic intrabladder pressure (IBP).

Methods

In six sedated and mechanically ventilated patients sterile saline was injected via a Foley catheter with 25 ml increments up to 300 ml. In total 13 'insufflation' and 'deflation' pressure–volume (PV) curves were constructed in these patients. The male/female ratio was 5/1, age 63.8 ± 14, MODScore 8.6 ± 4, SOFA score 10 ± 4.5, APACHE II score 31 ± 11.4, SAPS II score 59.2 ± 13.2. The number of measurements in each patient was 2.3 ± 0.5. Construction of PV curves was done with the SPSS 10™ software, values are mean ± SD.

Results

The values for IBP with regard to bladder volume are summarized in Table 1. Figure 1 plots the 'insufflation' and 'deflation' PV curve as a curve fit of the means of the 13 measurements. A lower inflection point (LIP) can be seen at a bladder volume of 100 ml and an upper inflection point (UIP) at a bladder volume of 250 ml. The difference in bladder pressure was 2.7 ± 3.3 mmHg between 0 and 50 ml volume, 1.7 ± 1.2 mmHg between 50 and 100 ml, 7.7 ± 5.7 mmHg between 50 and 200 ml, and 16.8 ± 13.4 mmHg between 50 and 300 ml.

Figure 1

Table 1

Conclusions

If IBP is used as an estimate for IAP the volume instilled in the bladder should be between 50 and 100 ml; however, in some patients with a low bladder compliance, intrinsic bladder pressure can be raised at low bladder volumes. Ideally a bladder PV curve should be constructed for each individual patient before using IBP as an estimation for IAP.