Integration of inspiratory and expiratory intra-abdominal pressure: a novel concept looking at mean intra-abdominal pressure

Background: The intra-abdominal pressure (IAP) is an important clinical parameter that can significantly change during respiration. Currently, IAP is recorded at end-expiration (IAP(ee)), while continuous IAP changes during respiration (Delta IAP) 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 respiratory-integrated MIAP (i.e. MIAP(ri) = IAP(ee) + i . Delta IAP), where 'i' is the decimal fraction of the inspiratory time, and where Delta IAP can be calculated as the difference between the IAP at end-inspiration (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 Delta IAP (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.IAP (R = 0.78) (P < 0.001); multivariate regression analysis confirmed that IAP(ee) (mainly affected by the level of positive end-expiratory pressure, PEEP), Delta IAP, 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 MIAP = -0.3 + IAP(ee) + 0.4 . Delta IAP + 0.5 . I/E. 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 MIAPri equation. Since MIAP(ri) is almost always greater than the classic IAP, this may have implications on end-organ function during intra-abdominal hypertension. Further clinical studies are necessary to evaluate the physiological effects of MIAP.