Assessment of preoperative fluid depletion using bioimpedance analysis

G. L. Ackland*,1, D. Singh-Ranger2, S. Fox3, B. McClaskey3, J. F. Down1, D. Farrar1, M. Sivaloganathan1 and M. G. Mythen1,4

1 Centre for Anaesthesia, 2 Department of Surgery, and 3 Day Surgery Unit, University College London Hospitals, London W1T 3AA, UK. 4 Portex Professor of Anaesthesia and Critical Care, Institute of Child Health, University College London, Guildford Street, London WC1, UK

*Corresponding author. E-mail: g.ackland@rfc.ucl.ac.uk

Accepted for publication: September 1, 2003


    Abstract
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Background. Fluid depletion during the perioperative period is associated with poorer outcome. Non-invasive measurement of total body water by bioimpedance may enable preoperative fluid depletion and its influence on perioperative outcome to be assessed.

Methods. Weight and foot bioimpedance were recorded under standardized conditions in patients undergoing bowel preparation (n=43) or day surgery (n=44). Fifteen volunteers also followed standard nil-by-mouth instructions on two separate occasions to assess the variabilities of weight and bioimpedance over time.

Results. Body weight fell by 1.27 kg (95% CI 1.03–1.50 kg; P<0.0001) and foot bioimpedance increased by 51 ohm after bowel preparation (95% CI 36–66; P<0.0001). Weight change after the nil-by-mouth period in day-surgery patients (mean –0.22 kg, 95% CI –0.05 to –0.47 kg; P=0.07) correlated (r=–0.46; P=0.005) with an increase in bioimpedance (16 ohms, 95% CI 5–27 ohms; P=0.01). No difference between two separate bioimpedance measurements was seen in the volunteer group.

Conclusions. Further work is warranted to determine if bioimpedance changes may serve as a useful indicator of perioperative fluid depletion.

Br J Anaesth 2004; 92: 134–6

Keywords: complications, dehydration; fluid balance, fluid depletion; measurement techniques, bioimpedance analysis; surgery, day; surgery, gastrointestinal


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Perioperative hypovolaemia is associated with poorer outcome.1 2 The administration of large volumes of fluid (>20 ml kg–1) in ambulatory surgical patients, probably in excess of that lost through starvation and insensible losses, is associated with better postoperative recovery.3 Conversely, preoperative fluid depletion induced by bowel preparation may contribute to postoperative morbidity in major lower gastrointestinal surgery.4

Although body weight is regarded as a good measure of total body water, it may vary independently of it in many clinical conditions:5 thus, non-invasive methods of estimating total body water would potentially be useful in clinical practice. Bioimpedance changes may allow the degree of preoperative fluid depletion to be assessed objectively, and hence the influence of fluid depletion on perioperative outcome to be investigated further. Bioimpedance is based on the principle that various tissues have different conductive and resistive properties when a small electrical current is applied at different frequencies. The impedance of the body consists of two components: (i) resistance of the tissues (proportional to fluid volume) and (ii) reactance (the reciprocal of the capacitance of cell membranes, tissue interfaces, and non-ionic tissues). Lower-frequency (5 kHz) impedance reflects extracellular fluid whereas high- frequency impedance (500 kHz) reflects both extracellular and intracellular fluids. Increased bioimpedance reflects total body water depletion, with total body water derived from these values by making certain electrophysical assumptions.6 This technology has been used to measure total body water during rehydration,7 and after major surgery in which large fluid shifts occurred.8 However, although steady state measurements compare favourably with gold-standard measures of total body water,9 using such technology in dynamic scenarios remains questionable.6 This study assessed whether bioimpedance and/or weight changes reflect fluid depletion in two surgical populations (bowel preparation and day surgery) with markedly different preoperative fluid losses. Volunteers also underwent standard nil-by-mouth instructions on two separate occasions to assess the variabilities of weight and bioimpedance measurements over time.


    Methods and results
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
All procedures were performed with local ethical committee approval. Having obtained informed written consent, all subjects were weighed in theatre garments and bioimpedance readings were recorded under standardized conditions. Single frequency 50 kHz leg–leg bioelectrical impedance analysis (TBF-300GS; Tanita, UK) was undertaken primarily for its ease of use, for the comparable results with conventional arm–leg analysis, and most importantly for its close correlation with the gold-standard measurements of body water, that is dual X-ray absorptiometry and radio-labelled water.9 Total body water was derived from the in-built analyser algorithm, using the absolute value of foot bioimpedance, weight, gender, and height2/resistance. Subjects did not see any of the results. Three groups were investigated.

Bowel preparation
Forty-three patients (64 (range 30–88) yr old; body mass index 21.2 (3.9) kg m–2) underwent standardized Citramag bowel preparation. Weight and foot bioimpedance measurements were repeated approximately 60 h after the initial recording, immediately before the procedure. At each weight and bioimpedance recording, venous blood was drawn via an 18 G needle for measurement of haematocrit and plasma electrolytes. Fluid intake was calculated from self-reporting of oral intake (measured by graduated cup) and/or i.v. fluid administered for inpatients (n=8), in whom supplementary potassium chloride was prescribed according to local surgical practice.

Ambulatory surgical patients
Weight and foot bioimpedance measurements were made in 44 ambulatory surgical patients (40 (range 19–75) yr old; body mass index 24.3 (4.8) kg m–2) who received standard nil-by-mouth instructions (light meal >6 h, clear fluid >3 h before admission). Repeat measurements (2–3 weeks after the initial recordings) on the day of surgery were made but always within 2–4 h of the time of day of the initial measurements.

Control volunteers
Fifteen control volunteers (25–36 yr old; body mass index 23.6 (4.2) kg m–2) served to assess the variability in bioimpedance/weight measurements over time (2–3 weeks apart). Weight/foot bioimpedance recordings were made at 09:00 hours on two separate occasions having followed standard nil-by-mouth instructions overnight.

The study was powered ({alpha}=0.05; ß=0.8; n=36 each group) assuming an expected weight difference of 1 kg after bowel preparation/nil-by-mouth between the two patient groups, based on previous,10 and preliminary data where impedance and mean 0.5 kg weight change (SD 0.17 kg) correlated in 19 day-surgery patients. Data are presented as mean (SD) or 95% confidence intervals of the mean difference. Two-tailed Student’s t-test, one-way ANOVA and partial correlation were used for statistical analysis (SPSS version 9.0), with P<0.05 considered significant.

Figure 1 summarizes the body weight and bioimpedance changes for bowel preparation and ambulatory surgical patients. Baseline total body weight (bowel prep 68.5 (15.0) kg, day-surgery 71.2 (10.8) kg, volunteers 71.4 (10.8) kg; P=0.57) and foot bioimpedance measurements (bowel prep 529 (96) ohms, day-surgery 539 (65) ohms, volunteers 525 (60) ohms; P=0.78) did not differ between groups.



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Fig 1 Changes in mean (95% CI) (A) foot bioimpedance and (B) total body weight after bowel preparation and the nil-by-mouth period for day-surgery patients. P-values (Student’s t-test) refer to between group comparisons: (A) P=0.007; (B) P=0.01.

 
Bowel preparation
Total body weight fell by 1.27 kg (1.03–1.50 kg; P<0.0001). Mean body weight loss expressed as a percentage of initial body weight was 1.8 (1.0)%. Mean total fluid intake was 2.1 (0.85) litre. Foot bioimpedance post-bowel preparation increased by 51 ohms (36–66 ohms; P<0.0001). There was no significant correlation between impedance change, weight change or fluid intake. As derived from the inbuilt algorithm, total body water decreased by 0.98 litres (0.54–1.42 litres; P<0.0001). There were no changes in haematocrit (0.40 (0.06) vs 0.40 (0.06); P=0.49), plasma sodium (140 (3) vs 139 (4) mmol litre–l; P=0.11), potassium (4.0 (0.3) vs 3.9 (0.4) mmol litre–l; P=0.62) or chloride (101 (3) vs 101 (4) mmol litre–l; P=0.52).

Day-surgery patients
Although body weight did not change between the measurements taken at the pre-admission clinic and on the operative days (–0.22 kg (0.05–0.47 kg); P=0.07), foot bioimpedance increased by 16 ohms (5–27 ohms; P=0.005) and correlated with body weight change (r=–0.46, P=0.005), controlling for age. Fluid depletion was calculated as a decrease in total body water of 0.51 litres (0.18–0.84 litres; P=0.003).

Volunteers
Total body weight increased by 0.33 kg (0.08–0.58 kg; P=0.01) between readings but foot bioimpedance remained unchanged (2 ohms (–12 to 17 ohms); P=0.73).


    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Fluid depletion in patients undergoing bowel preparation is difficult to assess. Previous work suggests that even with up to 4 litres of fluid intake, a substantial fluid deficit may still occur.10 Deficits of a similar magnitude adversely influence perioperative outcome.1 2 The interpretation of bioimpedance changes after bowel preparation and many forms of acute hypohydration6 is complicated because of concomitant electrolyte changes, which theoretically change impedance. However, no such change was seen in this study. The lack of correlation between bioimpedance and weight change is not surprising as part of the body weight loss constitutes faecal loss (up to 75% faecal weight is water) and there was also a wide variation in fluid intake. In the day-surgery patients, weight decrease correlated with increased bioimpedance, suggesting fluid depletion. Because of the 2–3 week interval between measurements, the lack of total body weight change implies that this is a poor measure of total body water over time. Furthermore, volunteers showed similar bioimpedance readings having undergone similar nil-by-mouth periods on two separate occasions over a similar interval. Given the ease of use and clinical experience with this technology (particularly in haemodialysis units), bioimpedance measurements could be used on a regular clinical basis. However, although foot bioimpedance can discriminate between clinical scenarios where there are markedly different fluid losses, further work is needed to establish how foot bioimpedance compares with gold-standard measures of total body water for assessing perioperative fluid shifts.


    Acknowledgement
 
Tanita UK provided the bioimpedance analyser on an unconditional basis to support this study.


    References
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
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