Effects of different body positions on intra-abdominal pressure and dynamic respiratory compliance

Introduction

Intra-abdominal pressure (IAP) is an important parameter and prognostic indicator of the patient's underlying physiologic status [1]. Measurement of IAP via the bladder has been forwarded as the gold standard. The abdomen is considered primarily fluid in character and therefore it follows the law of Pascal. If this is true IAP should be the same regardless of the body position since fluid is not compressible. Putting the patient in the upright position is a common practice in patients with acute respiratory failure (e.g. lung oedema). This study will look at the effects of different body positions on IAP and dynamic compliance (Cdyn) in mechanically ventilated patients.

Methods

In total, 79 paired IAP and Cdyn measurements were performed at each body position in 10 mechanically ventilated ICU patients. The four positions studied were: supine, anti-trendelenburg, trendelenburg and the upright position. The IAP was calculated using the gold standard via an indwelling bladder catheter with a pressure transducer [1]. The Cdyn was calculated by dividing tidal volume (ml) by plateau pressure minus PEEP (cmH₂O). The male/female ratio was 3/2, BMI 26.3 ± 5.4, age 64.5 ± 15.7 years, APACHE II score 30.3 ± 10.5, SAPS II score 59.5 ± 11.1, MODS score 8.3 ± 3.8, and SOFA score 10.7 ± 3.6. The number of measurements in each patient was 7.9 ± 4. Statistical analysis was performed with SPSS 10™ software, and values are expressed as mean ± SD.

Results

Table 1 presents the mean values for IAP and Cdyn in the different positions. The IAP was significantly higher in the anti-trendelenburg and upright positions versus the supine, and significantly lower in the trendelenburg position versus the supine (P < 0.0001, one-way ANOVA). The Cdyn was lowest in the upright position (P = NS). There was only in the upright position a poor but slightly significant correlation between IAP and Cdyn (P = 0.047). In five patients with a BMI > 25 (mean BMI 30.8 ± 3.2) the effect on IAP of the upright position versus the supine was significantly greater compared with patients with a BMI < 25 (mean BMI 21.8 ± 1.6): 10.1 ± 4.4 versus 6.8 ± 3.3 (P = 0.001), but the effect on Cdyn was the opposite: 6.8 ± 12.3 versus 21.1 ± 35.6 (P = 0.02).

Table 1

Conclusions

Putting a patient in different body positions has significant effects on IAP. This is in contradiction with the hypothesis that the abdominal compartment is primarily fluid in character, since IAP would then remain constant regardless of body position. Assessment of IAP should therefore always be done in the complete supine position. The upright position significantly increases IAP, and lowers Cdyn although not significantly. The effects on IAP are more pronounced in obese patients, and the effects on Cdyn more in the nonobese. Putting a patient with acute respiratory failure in the upright position may further deteriorate respiratory function, caused by the acute increase in IAP.