Bioelectrical impedance analysis in ICU patients

Introduction

Bioelectrical impedance analysis (BIA) is a widely used method for calculating body compartments in healthy subjects and chronically ill patients. The resistance (R) – which is correlated with total body water – and the reactance (Xc) – depending on the capacitance of cell membranes – are measured by BIA. The phase angle α, a mathematical relation between R and Xc, has qualified as a prognostic marker in several severe diseases [1]. We aimed at monitoring body composition during the clinical course of ICU patients and to investigate the impact of illness severity on BIA.

Methods

In this observational study we performed BIA testing at the frequencies of 50 kHz unilaterally between the wrist and ankle with a body impedance analyzer. BIA of ICU patients took place on three different measuring points: M1, M2 and M3 (days 2 to 3, days 10 to 11 and days 14 to 15 after admission). Severity of illness was monitored by Simplified Acute Physiology Score (SAPS) II. BIA results of each patient were matched with a standard collective according to sex, age, and body mass index. Spearman's rho (rs) has been calculated to analyze correlation between α and SAPS II. Nonparametric analysis of longitudinal data was carried out to analyze two groups differentiated by SAPS II (Group 1: ≤ 45 points, Group 2: >45 points).

Results

Forty-eight patients received one measurement at M1, 28 patients were measured at all three measuring points. BIA of our patients differed considerably from the normal collective. There were significant differences between the patients grouped by primary disease (abdominal cancer patients, nontumor patients, neurosurgical patients, multiple trauma and acute respiratory distress syndrome). α is correlated with the ICU score SAPS II on admission at all three measuring points (M1: n = 48, r s = -0.296, P < 0.05; M4: n = 28, r s = -0.368, P < 0.005; M5: n = 28, r s = -0.525, P < 0.005). While we did not find a significant change of R in our subsequent measurement, there was a decrease of Xc (P < 0.05). Especially in the more serious ill patients (Group 2), this has been observed (P < 0.05).

Conclusion

BIA helps to estimate body composition at ICU admission. It is associated with primary disease and severity of illness. During the clinical course we observed a decrease of Xc, indicating loss of body cell mass.