Relationship of body mass index (BMI), lactate and intra-abdominal pressure (IAP) to subsequent mortality in ICU patients

Introduction

Excess body weight increases the risk of death from any cause and from cardiovascular disease in adults [1]. In the majority of population studies, the relationship of BMI to mortality is a U-shaped curve, with increased risk in the lowest and highest percentiles of the distribution. In acutely ill patients however BMI below the 15th percentile remains an independent predictor of mortality whereas a high BMI (>85th percentile) was not significantly related to risk of mortality [2]. We wanted to study in a prospective clinical trial the relationship between IAP and lactate and BMI and their relationship to subsequent mortality in ICU patients. The results of an interim analysis are presented in this abstract.

Methods

Over a 12 month period 405 patients, hospitalised in a seven bed mixed ICU, were screened for increased IAP >12 mmHg (normal 0-5 mmHg) with the standardised intravesical pressure recording method. Data collected within the first 24 h of ICU admission were: age, gender, MODScore. APACHE-II and SAPS-II score and BMI. Maximal IAP and lactate levels were recorded within the first 72 h. Study endpoints were: duration of ICU and hospital stay, ICU and hospital mortality and cost of ICU and hospital stay. Statistical analysis was done with Fisher exact and two-tailed unpaired Student's t-test, values are mean ± SD.

Results

The percentage of female patients was 55.3, age 66 ± 17.4, MODScore 3.4 ± 3.3, APACHE-II score 16.4 ± 6.2, SAPS-II score 35.1 ± 17.5, BMI 25.1 ± 4.8, IAP 8.3 ± 4.7 mmHg, lactate 3.3 ± 4.2 mEq/l, ICU-stay 6.3 ± 9.5 and hospital stay 22.4 ± 22.9 days. Raised IAP was present in 71 patients (17.5%). The incidence of IAP = 12 and the mean IAP values were higher in patients who underwent emergency surgery: 39.4% (mean 11.5 ± 5.3) versus 19.8% (8.6 ± 49) in medical versus 6.1% (6.9 ± 3.5) in scheduled surgical patients. The ICU and hospital mortality were respectively 18% and 27.2%. The IAP was significantly higher in patients who died in the ICU: 13.2 ± 5.2 versus 7 ± 3.6 (P < 0.0001) as well as in patients who died in the hospital: 11.5 ± 5.3 versus 6.9 ± 3.6 (P < 0.0001). The Table lists the parameters studied in patients with high and normal IAP. The ICU and hospital mortality was significantly higher in patients with high IAP; respectively 64.8% versus 8.1% (P < 0.0001, OR 20.9, 95% CI 11.2-39) and 70.4% versus 18% (P < 0.0001, OR 10.9, 95% CI 6.1-19.5). With a cut-off at 12 IAP had 64.8% sensitivity, 78.6% specificity, 75.8% accuracy, 38.7% positive predictive value and 91.3% negative predictive value for ICU mortality. There was a poor but significant correlation between BMI and IAP: BMI-0.2106 × IAP + 23.268 (R ² =0.0413, P < 0.0001) and between lactate and IAP; lactate = 0.4851 × IAP-0.3885 (R ²= 0.2847, P < 0.0001). There was a trend towards lower ICU mortality with higher BMI but none of this reached statistical significance: 25.8% in the first, 15% in the second, 16.3% in the third, and 16.2% in the fourth BMI quartile. In patients within the first BMI quartile (<22) ICU mortality was significantly higher when compared to the total group of other BMI quartiles: 25.8% versus 15.8% (P = 0.04, OR 1.9, 95% CI 1.1-3.3).

Conclusion

The interim results of an ongoing prospective clinical trial show that increased IAP can be expected in about 17.5% of cases. It seems to be a predictor of mortality and causes a considerable extra-cost and prolonged ICU-stay. High IAP docs not correlate well with high BMI or lactate. There is no U-shaped (concave) mortality curve associated with BMI, on the contrary, patients with higher BMI had lower mortality compared to patients within the first BMI quartile, and this is in accordance with the results from others [2]. We suggest that IAP should be used as part of routine monitoring in the ICU and that future studies examining variables predictive of ICU-mortality should include IAP and BMI.

Table 1