Although the World Society for Abdominal Compartment Syndrome in its guidelines recommends midaxillary line (MAL) as zero reference level in intra-abdominal pressure (IAP) measurements in aiming at standardizing the technique, evidence supporting this suggestion is scarce. The aim of this study is to study if the zero reference position influences bladder pressure measurements as estimate for IAP.

The IAP of 100 surgical patients was measured during the first 24 h of admission to the surgical intensive care unit of General Calixto Garcia Hospital in Havana (Cuba) following laparotomy. The period was January 2009 to January 2010. The IAP was measured twice with a six-hour interval using the transurethral technique with a priming volume of 25 ml. IAP was first measured with the zero reference level placed at MAL (IAP _MAL) , followed by a second measurement at the level of the symphysis pubis (SP) after 3 minutes (IAP _SP ). Correlations were made between IAP and body mass index (BMI), type of surgery, gender, and age.

Mean IAP _MAL was 8.5 ± 2.8 mmHg vs. IAP _SP 6.5 ± 2.8 mmHg ( p < 0.0001). The bias between measurements was 2.0 ± 1.5, 95% confidence interval of 1.4 to 3.0, upper limit of 4.9, lower limit of -0.9, and a percentage error of 35.1%. IAP _MAL was consistently higher than IAP _SP regardless of the type of surgery. The BMI correlated with IAP values regardless of the zero reference level ( R ² = 0.4 and 0.3 with IAP _MAL and IAP _SP respectively, p < 0.0001).

The zero reference level has an important impact on IAP measurement in surgical patients after laparotomy and can potentially lead to over or underestimation. Further anthropometric studies are needed with regard to the relative MAL and SP zero reference position in relation to the theoretical ideal reference level at midpoint of the abdomen. Until better evidence is available, MAL remains the recommended zero reference position due to its best anatomical localization at iliac crest.