The recognition and treatment of IAH and the abdominal compartment syndrome (ACS) are clearly reliant on an accurate and reliable system for the measurement of IAP. The technique for IBP measurement has undergone much refinement over the last decade  and has now been presented, by an international panel of experts, as the gold standard for intra-abdominal pressure measurement . In addition to the effects of gravity and sheer stress , the value of bladder pressure relies on two key assumptions which have been widely accepted without direct evidence of their validity.
The first assumption is that the bladder wall will act as a passive diaphragm to the transmission of pressure, and therefore, the pressure measured within the urinary bladder will accurately reflect the pressure immediately outside within the peritoneal cavity. Several studies, in both animal and human models, have shown good agreement between directly and indirectly measured intra-abdominal pressure [22, 23, 32, 33]. All of these studies, however, have measured direct IAP at a site distant to the urinary bladder and followed artificial elevation of IAP by means of either saline or gas insufflation, or by insertion of a mechanical prosthesis. Our data is the first to directly compare the pressure measured at the intra-vesical and intra-peritoneal sides of the bladder wall and confirms that the pressure measured within the urinary bladder demonstrates excellent agreement with the pressure to be measured within the pelvic peritoneal cavity.
The second assumption relates to the mechanical properties of the peritoneal contents. It has been suggested that the abdominal contents are primarily fluid in composition and, therefore, that pressure transmission can be expected to follow Pascal's law such that measurement of the IAP at any point will reflect the pressure contained within the entire abdominal cavity . In reality, however, the abdominal contents remain a heterogeneous mix of solid, liquid and gaseous components with the exact composition influenced by several disease processes such as paralytic ileus, visceral oedema, or the presence of ascites. Pressure transmission characteristics are, therefore, likely to be rather more complex.
The implications of a regional ACS are profound with the gold-standard technique for pressure measurement occurring at the lowest point in the abdominal cavity, whilst the organs that have been shown to be most susceptible to raised IAP all lie in the upper abdomen. Separate studies have all clearly shown the deleterious effects of raised IAP on the splanchnic circulation [11–13, 34, 35], cardiac [8, 36], respiratory [9, 37, 38], renal [5, 32, 39] and neurological [40, 41] functions in both human and animal models.
The possibility of a regional variation between the upper and lower IAP was identified, but not explored in detail in 1994 . In this study, IGP was measured in nine patients undergoing laparoscopic cholecystectomy at a variety of different insufflation pressures. The study was designed to validate the measurement of IGP against the pneumoperitoneum but also showed that IGP could also be up to 4 mmHg higher or 3 mmHg lower than the measured IBP. A further small study has identified differences in gastric and bladder pressure in two patients within a general ICU population  and suggested that such a variation could provide clues as to any underlying pathophysiological process.
Our study is the largest to compare the two compartmental pressures within a clinical setting, without artificial manipulation of IAP. In keeping with the above study, we showed a significant difference between compartmental pressures but with a much broader and more clinically significant range of variation of up to 16 mmHg and a mean difference between the compartments of around 20% which equates to a maximal inter-compartmental mean difference of 5.3 mmHg.
Clearly, such a magnitude of variation, coupled with the observation that compartmental pressures were seen to vary by 4 mmHg or more for an average of 23% of the time, means that relying on the measurement of one compartmental pressure only may lead to a significantly elevated pressure in the other compartment being missed. The positive relationship that we have demonstrated between compartmental pressures should mandate separate measurement of UIAP in patients in whom the IBP is adopting an upward trend.
It was interesting to observe that the range of variation in inter-compartmental pressure was greater in those patients concealing a higher UIAP, and this may be related to the previous data which suggest that upper abdominal incisions result in measurable changes to abdominal wall contractile properties which may contribute to the generation of a locally raised IAP .
Body position and regional IAP
Previous clinical studies have considered the influence of patient positioning on IAP. In the largest , a multi-centre study of 132 ventilated patients, the mean difference between supine and 30° IBP was 3.7 mmHg with a range of 3.4 to 4.0 mmHg. The largest reported difference in positional pressures was seen in a study of 37 patients at a range of bed positions between 0 and 45° . It was found that IBP increased with head-up tilt with a mean increase of 5 mmHg at 30°, and 7.4 mmHg at 45°.
Our data have also demonstrated a statistically significant increase in the IBP with head-up positioning to 30°. This was, however, a far smaller increase of just 0.43 mmHg rather than the 5 mmHg seen in the above study. This would lend support to the theory that LIAP will increase as the result of a more upright posture . The most likely explanation for this is that an erect posture leads to an increase in the hydrostatic weight exerted by the abdominal organs and body habitus pressing downwards on the bladder much in the same manner as increasing the height of a standing column of fluid would increase the measurable pressure at the bottom of the column.
A more interesting observation in our own data, however, is the fact that despite accurate re-zeroing of a patient mounted transducer UIAP was significantly increased in the supine position compared to a 30° head-up tilt. The reason for this observation remains unclear but may be related to the re-positioning of the more mobile hollow abdominal viscera along with both their fluid contents and any free intra-peritoneal fluid with a more upright posture. This observation would suggest that a simple change in posture could provide a clinically significant improvement in the UIAP, which in turn, may improve hepatic, renal and splanchnic blood flow. Such positive effects on organ perfusion would need to be demonstrated by further specific studies; however, it does raise the possibility that a head-up position may be advantageous for reasons other than simple ventilatory mechanics. It is also particularly encouraging to note that a larger reduction in UIAP can be expected in those patients with a higher upper, rather than lower, baseline intra-abdominal pressure. The lack of collection of other body anthropomorphic data to further examine these two groups is accepted to be an unfortunate limitation of the study.
The fact that it was impossible to predict which of the two compartments would conceal the higher pressure suggests that, for this subgroup of patients, dual compartmental pressure monitoring may be required based upon the clinical condition of the patient. It remains unclear, however, whether the observed variation in inter-compartmental pressure is specific to the procedure of liver transplantation, or whether the findings could be generalised to all upper abdominal surgery, local inflammatory conditions such as severe acute pancreatitis, or indeed the measurement of IAP in general. It is also a shortcoming that various anthropomorphic data and details of illness severity scores were not collected, as these have been shown to impact on baseline IAP.
Further study with a larger sample size will be required to elucidate the relationship between the location of the higher compartmental pressure, the magnitude of variation in compartmental pressure and the duration for which there is a significant difference between compartments with clinical outcome. Such a study, with higher numbers, may be facilitated by the recent introduction of a commercially available non-invasive device for the measurement of IGP (CiMON, Pulsion Medical Systems, Munich, Germany). It would also be extremely interesting to measure the retroperitoneal compartmental pressure within the upper abdomen which very much contains the 'anatomical terminus' for the arrival and departure of the abdominal blood supply, as well as the kidneys themselves.