Effects of intra-abdominal pressure on respiratory system mechanics in mechanically ventilated rats☆

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Abstract

We investigated the effects of intra-abdominal pressure on respiratory system compliance at different PEEP levels. 20 ventilated rats underwent four pressure levels (7, 9, 11, 13 mmHg) of helium pneumoperitoneum and were allocated to one of the four PEEP groups (0, 3, 6 and 9 cmH2O). From the expiratory pressure–volume curve the mathematical inflection point (MIP) was calculated. Volume-dependent compliance was analyzed using the SLICE-method. MIP-pressure correlated to the intra-abdominal pressure (r2 = 0.94, p < 0.001). Peak inspiratory pressure increased with intra-abdominal pressure, and was lower after recruitment-maneuvers (p < 0.001). The compliance gain following recruitment-maneuvers depended on PEEP, intra-abdominal pressure and intratidal volume (all p < 0.001). Mean arterial pressure was independent of PEEP (p = 0.068) and intra-abdominal pressure (p = 0.293). Volume-dependent compliance courses varied according to PEEP and intra-abdominal pressure. The level of intra-abdominal pressure alters respiratory system mechanics in healthy lungs. Intratidal compliance can be used to guide the PEEP adjustment in intra-abdominal hypertension. If counterbalanced by PEEP, elevated intra-abdominal pressure has no negative effects on oxygenation or hemodynamics.

Highlights

► We model intra-abdominal hypertension by helium pneumoperitoneum in rats. ► Intra-abdominal pressure reduces respiratory system compliance. ► PEEP counterbalances increased intra-abdominal pressure during mechanical ventilation. ► The analysis of intratidal compliance could be helpful for guiding the PEEP.

Introduction

Intra-abdominal hypertension (IAH) is strongly related to morbidity and mortality in critically ill patients (Cheatham, 2009, Cheatham and Safcsak, 2010). Intra-abdominal hypertension causing the abdominal compartment syndrome (ACS) is observed in severe abdominal trauma and in aftermath of major abdominal operations. Further reasons for elevated intra-abdominal pressure are morbid obesity (Pelosi et al., 1999, Lambert et al., 2005) which is known to alter respiratory system mechanics by decreasing functional residual capacity and compliance (Safran and Orlando, 1994, Iwasaka et al., 1996, Pelosi et al., 1999, Lambert et al., 2005) and artificially induced pneumoperitoneum during laparoscopy, which is applied to facilitate the minimally invasive (laparoscopic) surgery procedure. Several studies were performed to investigate the effects of a pneumoperitoneum on respiratory system mechanics. Elevated airway pressure and reduced compliance were commonly reported (Bardoczky et al., 1993, Safran and Orlando, 1994, Iwasaka et al., 1996).

There is evidence that PEEP has a positive effect on oxygenation (Pelosi et al., 1999, Hazebroek et al., 2002) in the situation of increased intra-abdominal pressure. However, the use of PEEP in the presence of pneumoperitoneum remains controversial since the application of both may reduce cardiac output (Kraut et al., 1999).

In the present study we investigated the effects of different intra-abdominal pressures at different PEEP levels on intratidal nonlinear respiratory system mechanics, on hemodynamics and on oxygenation in a rat model with uninjured lungs under mechanical ventilation. The application of a pneumoperitoneum served as a model for various conditions accompanied by elevated intra-abdominal pressure.

Section snippets

Materials and methods

This study was approved by the review board for the care of animal subjects of the government executive (Regierungspräsidium, Freiburg, Germany; G-06/3) and was carried out in accordance with the German law for animal protection and in compliance with the animal care guidelines of the European Community (86/609/EC).

Results

Application of intra-abdominal pressure resulted in characteristic changes of the pressure–volume loop of the respiratory system (Fig. 1). Increasing intra-abdominal pressure decreased the slope of the pressure–volume loop and shifted the expiratory MIP towards higher pressures. The pressure at the MIP of the expiratory limb correlated well to the applied intra-abdominal pressure (r2 = 0.94, p < 0.001, Fig. 2). Two factorial ANOVA exhibited a significant dependence of this pressure on

Intratidal compliance

The main findings of our study demonstrate that dynamic intratidal compliance is reduced when the intra-abdominal pressure is increased by helium pneumoperitoneum.

This finding is in accordance with several other studies demonstrating similar impairments of respiratory system mechanics at intra-abdominal hypertension (Bardoczky et al., 1993, Iwasaka et al., 1996, Pelosi et al., 1997, Kraut et al., 1999, Avital et al., 2008). The main reason for such effects appears to be the impeded

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    ☆

    This work was supported by the Deutsche Forschungsgemeinschaft Grant #GU 561/6-2.

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