The recent definitions on intra-abdominal pressure (IAP), intra-abdominal volume (IAV) and abdominal compliance (C<inf>ab</inf>) are a step forward in understanding these important concepts. They help our understanding of the pathophysiology, aetiology, prognosis, and treatment of patients with low C<inf>ab</inf>. However, there is still a relatively poor understanding of the different methods used to measure IAP, IAV and C<inf>ab</inf> and how certain conditions may affect the results. This review will give a concise overview of the different methods to assess and estimate C<inf>ab</inf>; it will list important conditions that may affect baseline values and suggest some therapeutic options. Abdominal compliance (C<inf>ab</inf>), defined as a measure of the ease of abdominal expansion, is measured differently than IAP. The compliance of the abdominal wall is only a part of the total abdominal pressure-volume (PV) relationship. Measurement or estimation of C<inf>ab</inf> is difficult at the bedside and can only be done in a case of change (removal or addition) in IAV. The different measurement techniques will be discussed in relation to decreases (ascites drainage, haematoma evacuation, gastric suctioning) or increases in IAV (gastric insufflation, laparoscopy with CO<inf>2</inf> pneumoperitoneum, peritoneal dialysis). More specific techniques using the interactions between the thoracic and abdominal compartment during positive pressure ventilation will also be discussed (low flow PV loop, respiratory IAP variations, respiratory abdominal variation test, mean IAP and abdominal pressure variation), together with the concept of the polycompartment model. The relation between IAV and IAP is linear at low IAV and becomes curvilinear and exponential at higher volumes. Specific conditions in relation to increased (previous pregnancy or laparoscopy, gynoid fat distribution, ellipse-shaped internal abdominal perimeter) or decreased C<inf>ab</inf> (obesity, fluid overload, android fat distribution, sphere-shaped internal abdominal perimeter) will be discussed as well as their impact on baseline IAV, IAP, reshaping capacity and abdominal workspace volume. Finally, we suggest possible treatment options in situations of unadapted IAV according to existing C<inf>ab</inf>, which results in high IAP. A large overlap exists between the treatment of patients with abdominal hypertension and those with low C<inf>ab</inf>. The C<inf>ab</inf> plays a key role in understanding the deleterious effects of unadapted IAV on IAP and end-organ perfusion and function. If we can identify patients with low C<inf>ab</inf>, we can anticipate and select the most appropriate surgical treatment to avoid complications such as IAH or ACS.

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Anaesthesiology Intensive Therapy
Department of Surgery

Malbrain, M., De Laet, I., De Waele, J. J., Sugrue, M., Schachtrupp, A., Duchesne, J., … Roberts, D. J. (2014). The role of abdominal compliance, the neglected parameter in critically ill patients - A consensus review of 16. Part 2: Measurement techniques and management recommendations. Anaesthesiology Intensive Therapy, 46(5), 406–432. doi:10.5603/AIT.2014.0063