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
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Abstract |
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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 1.031.50 kg; P<0.0001) and foot bioimpedance increased by 51 ohm after bowel preparation (95% CI 3666; 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 527 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: 1346
Keywords: complications, dehydration; fluid balance, fluid depletion; measurement techniques, bioimpedance analysis; surgery, day; surgery, gastrointestinal
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Introduction |
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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.
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Methods and results |
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Bowel preparation
Forty-three patients (64 (range 3088) yr old; body mass index 21.2 (3.9) kg m2) 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 1975) yr old; body mass index 24.3 (4.8) kg m2) who received standard nil-by-mouth instructions (light meal >6 h, clear fluid >3 h before admission). Repeat measurements (23 weeks after the initial recordings) on the day of surgery were made but always within 24 h of the time of day of the initial measurements.
Control volunteers
Fifteen control volunteers (2536 yr old; body mass index 23.6 (4.2) kg m2) served to assess the variability in bioimpedance/weight measurements over time (23 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 (=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 Students 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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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 (0.050.47 kg); P=0.07), foot bioimpedance increased by 16 ohms (527 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 (0.180.84 litres; P=0.003).
Volunteers
Total body weight increased by 0.33 kg (0.080.58 kg; P=0.01) between readings but foot bioimpedance remained unchanged (2 ohms (12 to 17 ohms); P=0.73).
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Comment |
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Acknowledgement |
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References |
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