BIOELECTRIC IMPEDANCE DETECTS FLUID RETENTION IN PATIENTS UNDERGOING CARDIOPULMONARY BYPASS

Excessive fluid accumulation is associated with increased morbidity an d prolonged convalescence after cardiopulmonary bypass. However, water fluctuations are difficult to assess solely on the basis of changes i n body weight and fluid balance. Bioelectric impedance analysis is a s imple, rapid, noninvasive bedside technique that measures the resistan ce of the body to a weak alternating current (50 kHz). The change in r esistance is inversely proportional to the change in total body water, To investigate the relationship between body weight, fluid balance, a nd resistance, 18 patients who had cardiopulmonary bypass (9 men, 9 wo men, aged 61 +/- 3 years, weighing 80 +/- 4 kg, with ejection fraction 54% +/- 3% and bypass time 113 +/- 8 minutes [mean plus or minus stan dard error of the mean]) were followed up for 7 postoperative days. Bo dy weight, fluid balance, and whole body and regional resistance were determined at 24-hour intervals. In the immediate postoperative period , fluid retention was accurately detected by simultaneous measurements of weight gain and decreased resistance (p < 0.001). Both measurement s detected the initiation of diuresis by postoperative day 2 (p < 0.01 ). Whole body resistance returned to baseline values by day 7 (p > 0.0 5), and body weight returned to baseline on day 4 (p > 0.05), Change i n weight and change in whole body resistance were highly correlated wi th cumulative fluid balance (r = 0.84, p < 0.001, and r = -0.81,p < 0. 001, respectively), and these two measures were also related to each o ther throughout the study (r = -0.89, p < 0.001). The initial change i n resistance was the best measurement associated with postoperative ou tcome (p less than or equal to 0.01). The data suggest that the measur ement of electric resistance across the body can accurately detect acu te changes in total body water and in fluid redistribution through the body. However, determining the relative day-to-day change in whole bo dy resistance seems more appropriate than calculating absolute fluid c hanges over time. Bioelectric impedance offers a simple, rapid, noninv asive method to monitor serial changes in total body water, This techn ique can be useful in situations in which rapid alterations in water c ompartments occur, and it may be useful in predicting outcome after ca rdiopulmonary bypass.