Value of multifrequency bioelectrical impedance in assessing fluid disturbances in patients undergoing major abdominal surgery.
Presentation September 2004 at the ESPEN Congress, Lisbon
INTRODUCTION: The ability of bedside multifrequency bioelectrical impedance analysis to monitor fluid disturbance and predict patients at risk of developing oedema after major surgery was assessed.
- 22 ward-based patients (11 M, 11 F; aged 28-85 yrs) had their whole body bioelectrical impedance (Z) assessed before and on days 1, 3 and 5 after major abdominal surgery, to examine the relationship between Z and fluid balance.
- Measures of Z were obtained at four frequencies (5, 50, 100 and 200kHz) with the Quadscan 4000® (Bodystat Limited, Isle of Man, UK). No patient had oedema prior to surgery.
- Administration of fluids intra- and post-operatively followed routine protocols.
- After surgery, some patients developed oedema (OD group, n=9); the other patients were consistently non-oedematous (NOD group, n=13) throughout the study.
- Parametric tests were used for statistical analysis.
RESULTS: Preoperative BMI was similar in OD and NOD groups (25.8 + 3.1 vs 27.2 + 6.5 kg/m2; p = 0.560).
- A reduction in impedance and an increase in impedance index (height2/Z; ZI) occurred at all frequencies (Figure 1).
- The increase in ZI was greater in OD group compared to NOD group (e.g. at 50kHz, this was 21.3 vs 6.86 cm2/ohm for average increments in ZI on days 1, 3 and 5; p <0.001) (Figure 2).
- The intraindividual changes in ZI were related to fluid balance (r = 0.86 using analysis of covariance with subject as fixed factor).
- Z5/Z200 (the ratio of impedance at 5kHz, mainly influenced by extracellular water, to impedance at 200 kHz, influenced by extra and intracellular water), decreased in all subjects (from 1.266 + 0.047 preoperatively to 1.201 + 0.064 post operatively (mean of days 1, 3 and 5); p <0.001), and tended to change more in the oedematous patients (0.0788 v 0.055; p = 0.078) (Figure 3).
- Preoperatively, Z5/Z200 ratio was significantly lower in those that went on to develop oedema compared to those that did not (1.237 + 0.044 v 1.285 + 0.039; p =0.015).
DISCUSSION: This study suggests:
- Postoperative fluid balance can be detected by changes in whole body bioelectrical impedance, even in patients who do not develop oedema.
- The ratio Z5/Z200 became exaggerated post-operatively in those that developed oedema.
- A predisposition to developing post-operative oedema is suggested by measurements of a bio-electrical impedance ratio (Z5/Z200) obtained pre-operatively.
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