Direct measurement of IAP has been considered to be invasive  but is routinely applied for validation of indirect techniques [12–15]. Brooks recently evaluated a device for a direct and continuous assessment of IAP and reported no complications. Moreover, intraperitoneal measurement – though difficult- has been considered to be needed to fully address the accuracy of IAP measurement in clinical practice . In patients undergoing abdominal surgery for other reasons placement of an intraabdominal measurement probe would be no more hazardous than the placement of abdominal drains .
After abdominal surgery, liver transplantation and decompressive laparotomy due to ACS, it has been observed, that IAH occurs often and is an independent risk factor of organ impairment [1, 3] while recurrent ACS has an even increased mortality . During the definitive closure of the abdominal cavity after laparostomy, an increase of IAP has been observed . In these patients, direct IAP measurement could help to identify the right time for reconstruction and could also help to monitor postoperative IAP. In addition, direct IAP measurement would be a good alternative in the case that contraindications for intermittent intravesicular pressure measurement are present e.g. local infection, cystic or urethral trauma and cistostomy .
Agreement of the air-capsule probe with the intravesicular pressure was 0.4 mmHg with limits of agreement ranging from -4.1 to 5.1 mmHg. This probe has not been validated clinically in the intraabdominal position yet. In a porcine model mean difference to applied pressure was 0.5 with limits of agreement ranging from -4.5 to 5.4 mmHg . In a clinical study, Malbrain et al. placed the Spiegelberg probe intra-gastrically and validated it against laparoscopic pressure measurement. In their study, mean difference to laparoscopic pressure was 0.9 with limits ranging from -0.7 to 2.5 mmHg . It was concluded that ACM is a method of high accuracy and reproducibility and is comparable to intravesical pressure measurement. In a recently published review article, it was stated that a new IAP measurement technique should have a mean difference from -1 to 1 mmHg and limits of agreement within 4 mmHg . Accordingly, the agreement of ACM with IVP in the underlying study was acceptable but less when compared to the aforementioned transgastric use. Furthermore, we have to mention that the good agreement is associated with relatively low pressure ranges (<17 mmHg).
In a previous experimental study, ACM displayed a high measurement to measurement drift which was probably due to a falsified capsule volume caused by the CO2 used to increase IAP in that model . In the underlying study, drift was little and unlikely to be the reason for a reduced agreement since values after recalibration were used for comparison of ACM with IVP. A possible cause could be the fact that measurements were performed in two different compartments as already pointed out in other clinical investigations [23, 24]. Lastly, absolute values of IAP appear to be less relevant than a reproducible and reliable registration of IAP trend especially in serial measurement .
Serial measurement of IAP have been recommended for patients at risk to develop IAH . Intermittent bladder pressure measurement however has been characterized as time- and personnel- consuming  which is likely to be the cause for not routinely using it . Although continuous intravesical pressure measurement is available and could be done without extra instruments , some issues remain to be investigated: These are the effect of pelvic trauma, detrusor activity and variable bladder compliance  but probably more important the influence of reference point and patient positioning. Consequently the latter two issues were subject of a recently completed trial of the world society of the abdominal compartment syndrome (WSACS).
The amount of measurement volume has also been questioned . Malbrain and De Waele investigated the effect of measurement volume on bladder pressure and observed a significant increase with a volume of 25 ml. In the investigation of Malbrain, the increase of IVP only became clinically relevant at a volume of 75 ml for most of the patients and it was concluded that larger instillation volumes than the usually recommended 50 ml to estimate IAP by bladder pressure may cause clinically relevant overestimation of IAP. Kimball recently published a study in which bladder pressure measurement in critically ill patients using 50 ml displayed high reproducibility and reliability . Consequently, the 50 ml used as measurement volume for IVP in the patients of the underlying study appear to be appropriate.
Patients in the underlying study were followed according to the standard postoperative protocols. An increased morbidity due to the use of the intraabdominal measurement probe could not be observed in the patients for a period of up to 5 days. All probes could be easily withdrawn without any complications. A control group without intraabdominal probes was not part of the study as we aimed to basically assess feasibility and agreement with intravesicular pressure measurement. The short observation period and the small number of patients restrict the expressiveness according to direct intraabdominal measurement related complications. Comparable studies are missing but to exlude an additional risk of ACM-catheters further studies are needed.
Another limitation of the underlying study might be that IVP did not exceed 17 mmHg. This was probably caused by the fact that patients were investigated after elective abdominal surgery. Consequently, agreement of ACM with IVP at higher IAP levels cannot be derived. Also, the prognostic value of ACM values – as already known from IVP [1, 2, 28] – values remains to be confirmed.