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

Abstract

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 (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 respiratory-integrated 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 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 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 end-expiratory 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

$\mathsf{\text{MIAP}}=-0.\mathsf{\text{3}}+\mathsf{\text{IA}}{\mathsf{\text{P}}}_{\mathsf{\text{ee}}}+0.\mathsf{\text{4}}\cdot \mathrm{\Delta }\mathsf{\text{IAP}}+0.\mathsf{\text{5}}\cdot \frac{I}{E}.$

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 MIAP _ri 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.