Volume 2 Supplement 1
Diagnosis and management of intraabdominal hypertension and abdominal compartment syndrome
Integration of inspiratory and expiratory intraabdominal pressure: a novel concept looking at mean intraabdominal pressure
 Siavash AhmadiNoorbakhsh^{1} and
 Manu LNG Malbrain^{2, 3}Email author
DOI: 10.1186/211058202S1S18
© AhmadiNoorbakhsh and Malbrain; licensee Springer 2012
Published: 20 December 2012
Abstract
Background
The intraabdominal pressure (IAP) is an important clinical parameter that can significantly change during respiration. Currently, IAP is recorded at endexpiration (IAP_{ee}), while continuous IAP changes during respiration (ΔIAP) are ignored. Herein, a novel concept of considering continuous IAP changes during respiration is presented.
Methods
Based on the geometric mean of the IAP waveform (MIAP), a mathematical model was developed for calculating respiratoryintegrated MIAP (i.e. $\mathsf{\text{MIA}}{\mathsf{\text{P}}}_{\mathsf{\text{ri}}}=\mathsf{\text{IA}}{\mathsf{\text{P}}}_{\mathsf{\text{ee}}}+i\cdot \mathrm{\Delta}\mathsf{\text{IAP}}$), where 'i' is the decimal fraction of the inspiratory time, and where ΔIAP can be calculated as the difference between the IAP at endinspiration (IAP_{ei}) minus IAP_{ee}. The effect of various parameters on IAP_{ee} and MIAP_{ri} was evaluated with a mathematical model and validated afterwards in six mechanically ventilated patients. The MIAP of the patients was also calculated using a CiMON monitor (Pulsion Medical Systems, Munich, Germany). Several other parameters were recorded and used for comparison.
Results
The human study confirmed the mathematical modelling, showing that MIAP_{ri} correlates well with MIAP (R^{2} = 0.99); MIAP_{ri} was significantly higher than IAP_{ee} under all conditions that were used to examine the effects of changes in IAP_{ee}, the inspiratory/expiratory (I:E) ratio, and ΔIAP (P < 0.001). Univariate Pearson regression analysis showed significant correlations between MIAP_{ri} and IAP_{ei} (R = 0.99), IAP_{ee} (R = 0.99), and ΔIAP (R = 0.78) (P < 0.001); multivariate regression analysis confirmed that IAP_{ee} (mainly affected by the level of positive endexpiratory pressure, PEEP), ΔIAP, and the I:E ratio are independent variables (P < 0.001) determining MIAP. According to the results of a regression analysis, MIAP can also be calculated as
Conclusions
We believe that the novel concept of MIAP is a better representation of IAP (especially in mechanically ventilated patients) because MIAP takes into account the IAP changes during respiration. The MIAP can be estimated by the MIAP_{ri} equation. Since MIAP_{ri} is almost always greater than the classic IAP, this may have implications on endorgan function during intraabdominal hypertension. Further clinical studies are necessary to evaluate the physiological effects of MIAP.
Introduction
The intraabdominal pressure (IAP) is an important clinical parameter with major prognostic impact [1, 2]. An unrecognised pathological increase in IAP eventually leads to intraabdominal hypertension (IAH) and abdominal compartment syndrome (ACS) [3, 4], which result in significant morbidity and mortality [5]. Thus, recognition and reliable measurement of IAP are the first important steps for prevention and management of IAH and ACS in critically ill patients [6].
According to the current consensus definitions of the World Society of the Abdominal Compartment Syndrome (WSACS), the IAP should be measured at endexpiration (IAP_{ee}) [13], referred to as the 'classic IAP' throughout the text. However, the IAP_{ee} is only a single component of an everchanging trend and thus does not incorporate a considerable portion of this IAP trend (Figure 2). The objectives of this study were to develop and validate a novel IAP measurement concept to consider IAP changes during respiration and to identify independent variables influencing IAP within this novel concept.
Methods
Part A: mathematical model
where 'i' is the decimal fraction of the inspiratory time in a respiratory cycle and can be calculated from the inspiratory/expiratory (I:E) ratio (i = I /(I + E); 0 <i < 1) and ΔIAP = IAP_{ei} − IAP_{ee}. Since IAP_{ee}, i, and ΔIAP can be assumed to be independent, a computerised iteration can be used for a set of values for each parameter to determine their effect on MIAP_{ri} and to compare the MIAP_{ri} with the classic IAP.
Each of the abovementioned data sets was assumed to be a unique case, and the values shown in Figures 3,4,5 should not be considered as a trend in changes that can be obtained in a single patient.
Part B: human pilot study
In six ICU patients that were mechanically ventilated with Evita XL ventilators (Draeger, Lubeck, Germany), the mean IAP was automatically calculated as the geometrical mean (MIAP) via a balloontipped nasogastric tube connected to a CiMON monitor (Pulsion Medical Systems, Munich, Germany). The MIAP_{ri} was also calculated according to Equation 2. Data were collected on respiratory settings, plateau and mean alveolar pressures (P _{plat}, P _{mean}), PEEP, and dynamic compliance (calculated as the tidal volume (TV) divided by (P _{plat}  PEEP)). The C _{ab} was calculated as TV divided by ΔIAP. The thoracoabdominal index of transmission (TAI) was calculated as ΔP _{alv} (= P _{plat} − PEEP) divided by ΔIAP, in which P _{alv} is the alveolar pressure.
The effects of IAP_{ee} on MIAP_{ri} were examined by a gradual increase in PEEP from 0 to 15 cmH_{2}O, with steps of 5 cmH_{2}O during a bestPEEP manoeuvre (20 measurements at each PEEP level in five patients, resulting in 80 measurements). The effects of ΔIAP on MIAP_{ri} were examined by a gradual increase in TV from 250 to 1,000 ml, with steps of 250 ml during a lowflow pressurevolume loop (20 measurements at each TV level in five patients, resulting in 80 measurements). The effects of I:E ratio on MIAP_{ri} were examined by a gradual increase in the I:E ratio from 1:2 to 2:1, with steps of 0.5 during a recruitment manoeuvre (9 measurements at each I:E ratio in one patient, resulting in 45 measurements).
Statistical analysis was performed using SPSS software. Pearson correlation analysis and Bland and Altman analysis were performed. For comparisons between MIAP_{ri} and IAP_{ee} at different levels of IAP_{ee} (PEEP), TV, and I:E ratio, a twotailed paired Student's ttest was performed. Data are expressed as the mean with the standard deviation (SD), unless specified otherwise. A P value below 0.05 was considered statistically significant. The local EC and IRB approved the study, and informed consent was obtained from next of kin.
Results
Part A: mathematical modelling
Part B: human pilot study
Patient characteristics at baseline
Parameter  Mean ± SD 

Age  59.5 ± 14.4 
SAPSII  43.5 ± 11.6 
APACHEII  21.8 ± 8.6 
SOFA  9.5 ± 4 
BMI (kg/m^{2})  28.6 ± 4.7 
IAP_{ei} (mmHg)  15.3 ± 3.7 
IAP_{ee} (mmHg)  11.1 ± 2.8 
ΔIAP (mmHg)  4.3 ± 1.3 
MIAP (mmHg)  12.9 ± 3 
IBP (mmHg)  12 ± 3 
TV (ml)  608 ± 117 
TV (ml/kg)  7.2 ± 1.2 
RR (/min)  17.7 ± 2.1 
P_{plat} (cmH_{2}O)  28 ± 4.1 
PEEP (cmH_{2}O)  9.2 ± 3.3 
Regression analysis and Bland and Altman analysis
In total, 205 paired MIAP and MIAP_{ri} measurements were performed with an identical statistical mean of 12.2 ± 3.8 mmHg. Figure 6A shows an excellent correlation between the MIAP and MIAP_{ri} (R^{2} = 0.99, P < 0.001). Analysis according to Bland and Altman showed a bias and precision of 0 and 0.2 mmHg, respectively, with small limits of agreement ranging from −0.4 to 0.5 mmHg (Figure 6B). The percentage error was 3.5%.
Effect of IAP_{ee}, I:E ratio, and ΔIAP on MIAP_{ri}
The classic IAP of patients was below the IAH grade I threshold; however, the MIAP_{ri} significantly exceeded the threshold in several instances (P < 0.001; Figure 7).
Univariate analysis
Multivariate regression analysis
Multiple regression analysis looking for independent variables influencing MIAP
Unstandardized coefficients  Standardized coefficients  95.0% Confidence interval for B  

Model  B  Standard error  Beta  t  Significance  Lower bound  Upper bound 
(Constant)  −0.27  0.20  −1.4  0.164  −0.66  0.11  
PEEP  0.01  0.00  0.01  2.1  0.040  0.00  0.01 
ΔP _{alv}  0.01  0.01  0.02  1.5  0.133  0.00  0.02 
I:E ratio  0.48  0.04  0.04  12.3  0.000  0.40  0.56 
IAP_{ee}  0.99  0.01  0.86  144.6  0.000  0.98  10.01 
ΔIAP  0.35  0.03  0.16  11.3  0.000  0.29  0.41 
TAI  0.00  0.00  0.01  1.1  0.259  0.00  0.01 
C _{ab}  0.00  0.00  −0.01  −1.6  0.105  0.00  0.00 
Discussion
A novel concept of IAP measurement based on the geometric mean of the IAP waveform was presented. The independent parameters determining the IAP in this concept were defined. The human pilot study validated the mathematical modelling with an excellent correlation. A significant difference was observed between the classic IAP and the MIAP_{ri} in our clinical study.
The human study confirmed that MIAP_{ri} is as accurate as an automated geometric MIAP calculation by a CiMON monitor. More importantly, the higher the MIAP or IAP_{ee}, the higher the ΔIAP since ΔIAP acts as an indirect marker of C _{ab}. The ΔIAP is correlated with ΔP _{alv} or is thus inversely correlated with dynamic compliance. As well, the higher the C _{ab}, the lower the TAI. The human study confirmed the predictions of the mathematical modelling in which IAP_{ee} (affected by different PEEP settings), ΔIAP, and I:E ratio were recognised as the major independent determinants of MIAP_{ri}. We also showed that in patients with IAH and under mechanical ventilation, the IAP may be influenced by ventilator settings.
The critical difference between the MIAP_{ri} and the classic IAP near the ACS threshold in our mathematical modelling, as well as the significantly higher MIAP_{ri} than the IAP_{ee} around the IAH threshold in our human study, calls for future studies. The dissimilar intensity in MIAP_{ri} changes under changes in the I:E ratio in Figure 4 may implicate the existence of critical points in the I:E ratio, wherein changing this ratio may cause a more intense change in the MIAP_{ri}. Furthermore, since MIAP_{ri} seems to be almost always larger than the classic IAP, relying only on the classic IAP may place some patients at risk of silent IAH or ACS. Although the aim of the current study was not to address these implications clinically, these findings indicate that further investigations should be performed on respiratory manoeuvres to manage IAH in mechanically ventilated patients (e.g. decreasing the I:E ratio and/or the ΔIAP, or maintaining the I:E ratio in a predefined range).
A limitation of our study was the lack of data to evaluate the physiological difference between the MIAP_{ri} and the classic IAP. However, this study only aimed to prove the concept and to set the stage for further studies. Therefore, we believe that the lack of physiological data does not limit our findings. Nonetheless, further studies on the clinical effects of this concept are necessary before it can be introduced in clinical practice.
Conclusions
A novel concept MIAP_{ri} was presented to consider the IAP changes during respiration and was based on the geometric mean (MIAP) of the IAP waveform. An excellent correlation was observed between the results of the mathematical modelling and those obtained in real patients. Substantial differences were observed between the two IAP methods (the classic IAP measured at end expiration and the novel MIAP). Based on our findings, we believe that the novel concept of MIAP_{ri} may be a better representation for the pressure concealed within the abdominal cavity. Further clinical studies are necessary to reveal the physiological effects of this novel concept.
Authors' information
SA is aveterinary surgeon (DVM, DVSc) and a medical research consultant in laboratory animal researches in the field of trauma, haemorrhage, critical care, and anaesthesia. MLNGM is a former president and treasurer of the World Society of the Abdominal Compartment Syndrome and is the ICU and High Care Burn Unit Director of the Department of Intensive Care in Ziekenhuis Netwerk Antwerpen Stuivenberg.
Addendum
See additional file 1.
Abbreviations
 ACS:

abdominal compartment syndrome
 C _{ab} :

abdominal compliance
 IAH:

intraabdominal hypertension
 IAP:

intraabdominal pressure
 IAP_{ee} :

endexpiratory IAP
 IAP_{ei} :

endinspiratory IAP
 MIAP:

mean intraabdominal pressure (geometrical mean)
 MIAP_{ri} :

respiratoryintegrated mean intraabdominal pressure
 P _{alv} :

alveolar pressure
 P _{mean} :

mean airway pressure
 P _{plat} :

plateau airway pressure
 PEEP:

positive endexpiratory pressure
 TAI:

thoracoabdominal index of transmission
 TV:

tidal volume
 WSACS:

World Society of the Abdominal Compartment Syndrome.
Declarations
Acknowledgements
This article has been published as part of Annals of Intensive Care Volume 2 Supplement 1, 2012: Diagnosis and management of intraabdominal hypertension and abdominal compartment syndrome. The full contents of the supplement are available online at http://www.annalsofintensivecare.com/supplements/2/S1.
Authors’ Affiliations
References
 Cheatham ML, White MW, Sagraves SG, Johnson JL, Block EF: Abdominal perfusion pressure: a superior parameter in the assessment of intraabdominal hypertension. J Trauma 2000,49(4):621–626. discussion 626–627 10.1097/0000537320001000000008PubMedView ArticleGoogle Scholar
 Malbrain ML, De laet IE: Intraabdominal hypertension: evolving concepts. Clin Chest Med 2009,30(1):45–70. 10.1016/j.ccm.2008.09.003PubMedView ArticleGoogle Scholar
 Ivatury RR, Cheatham ML, Malbrain ML, Sugrue M: Abdominal Compartment Syndrome. Georgetown: Landes Bioscience; 2006.Google Scholar
 Malbrain ML, Cheatham ML: Definitions and pathophysiological implications of intraabdominal hypertension and abdominal compartment syndrome. Am Surg 2011,77(Suppl 1):S6–11.PubMedGoogle Scholar
 Malbrain ML, Chiumello D, Pelosi P, Bihari D, Innes R, Ranieri VM, Del Turco M, Wilmer A, Brienza N, Malcangi V, Cohen J, Japiassu A, De Keulenaer BL, Daelemans R, Jacquet L, Laterre PF, Frank G, de Souza P, Cesana B, Gattinoni L: Incidence and prognosis of intraabdominal hypertension in a mixed population of critically ill patients: a multiplecenter epidemiological study. Crit Care Med 2005,33(2):315–322. 10.1097/01.CCM.0000153408.09806.1BPubMedView ArticleGoogle Scholar
 Malbrain ML: Different techniques to measure intraabdominal pressure (IAP): time for a critical reappraisal. Intensive Care Med 2004,30(3):357–371. 10.1007/s0013400321072PubMedView ArticleGoogle Scholar
 De Keulenaer BL, De Waele JJ, Powell B, Malbrain ML: What is normal intraabdominal pressure and how is it affected by positioning, body mass and positive endexpiratory pressure? Intensive Care Med 2009,35(6):969–976. 10.1007/s0013400914450PubMedView ArticleGoogle Scholar
 Hunter JD: Abdominal compartment syndrome: an underdiagnosed contributory factor to morbidity and mortality in the critically ill. Postgrad Med J 2008,84(992):293–298. 10.1136/pgmj.2007.064212PubMedView ArticleGoogle Scholar
 Malbrain ML, Cheatham ML, Kirkpatrick A, Sugrue M, Parr M, De Waele J, Balogh Z, Leppaniemi A, Olvera C, Ivatury R, D'Amours S, Wendon J, Hillman K, Johansson K, Kolkman K, Wilmer A: Results from the International Conference of Experts on Intraabdominal Hypertension and Abdominal Compartment Syndrome. I. Definitions. Intensive Care Med 2006,32(11):1722–1732. 10.1007/s0013400603495PubMedView ArticleGoogle Scholar
 Sturini E, Saporito A, Sugrue M, Parr MJ, Bishop G, Braschi A: Respiratory variation of intraabdominal pressure: indirect indicator of abdominal compliance? Intensive Care Med 2008,34(9):1632–1637. 10.1007/s001340081155zPubMedView ArticleGoogle Scholar
 Papavramidis TS, Marinis AD, Pliakos I, Kesisoglou I, Papavramidou N: Abdominal compartment syndrome  intraabdominal hypertension: defining, diagnosing, and managing. J Emerg Trauma Shock 2011,4(2):279–291. 10.4103/09742700.82224PubMed CentralPubMedView ArticleGoogle Scholar
 Verzilli D, Constantin JM, Sebbane M, Chanques G, Jung B, Perrigault PF, Malbrain M, Jaber S: Positive endexpiratory pressure affects the value of intraabdominal pressure in acute lung injury/acute respiratory distress syndrome patients: a pilot study. Crit Care 2010,14(4):R137. 10.1186/cc9193PubMed CentralPubMedView ArticleGoogle Scholar
 Malbrain ML, De laet I, Cheatham M: Consensus conference definitions and recommendations on intraabdominal hypertension (IAH) and the abdominal compartment syndrome (ACS)the long road to the final publications, how did we get there? Acta Clin Belg 2007,62(Suppl 1(1)):44–59.PubMedView ArticleGoogle Scholar
 Fischbach M, Terzic J, Laugel V, Escande B, Dangelser C, Helmstetter A: Measurement of hydrostatic intraperitoneal pressure: a useful tool for the improvement of dialysis dose prescription. Pediatr Nephrol 2003,18(10):976–980. 10.1007/s0046700311999PubMedView ArticleGoogle Scholar
 Stewart J: Calculus: Concepts and Contexts. 3rd edition. Belmont: Thomson Brooks/Cole; 2005.Google Scholar
 Kanani M, Elliott M: Applied Surgical Physiology Vivas. London: Greenwich Medical Media Ltd; 2004.View ArticleGoogle Scholar
 Raff H, Levitzky MG: Medical Physiology: A Systems Approach. New York: McGrawHill; 2011.Google Scholar
 Klabunde RE: Cardiovascular Physiology Concepts. Philadelphia: Lippincott Williams & Wilkins; 2005.Google Scholar
 Chemla D, Antony I, Zamani K, Nitenberg A: Mean aortic pressure is the geometric mean of systolic and diastolic aortic pressure in resting humans. J Appl Physiol 2005,99(6):2278–2284. 10.1152/japplphysiol.00713.2005PubMedView ArticleGoogle Scholar
 Moran D, Epstein Y, Keren G, Laor A, Sherez J, Shapiro Y: Calculation of mean arterial pressure during exercise as a function of heart rate. Appl Human Sci 1995,14(6):293–295.PubMedGoogle Scholar
 AhmadiNoorbakhsh S, Malbrain MLNG: Addendum: Mathematical model for calculation of mean intraabdominal pressure, taking into account integration of inspiratory and expiratory intraabdominal pressure. Ann Intensive Care 2012. Supplement 2 (in press)Google Scholar
Copyright
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.