Use of single-frequency bioimpedance at 50 kHz to estimate total body water in patients with multiple organ failure and fluid overload

Objective: To investigate the relationship between single-frequency bioimpe dance at 50 kHz (both total body impedance and segmental impedance) and tot al body water, measured using tritiated water in the presence of the severe fluid retention seen in multiple organ failure. Design: Prospective, controlled study. Setting: General intensive care unit. Subjects: Twenty patients with multiple organ failure acid 30 normal volunt eers, of whom a subgroup of ten had total body water measured. Interventions: None. Measurements and Main Results: Total body water and total and segmental bio impedance values at 50 kHz were measured using tritiated water and a Holtai n body composition analyzer in 20 patients with multiple organ failure and in ten normal volunteers. An additional 20 normal volunteers also had segme ntal and total body impedance measured. There was no difference in the line ar regression lines constructed for the patients and the volunteers, but th e SEM in the patients (7.6 L) was four times that seen in the normal subjec ts (1.9 L). In a further seven patients, the impedance technique overestima ted the change in total body water, deduced from acute changes in weight, b y between 0% and 46% (median, 12%). In the patients, who were supine, the k nee-ankle segment contributed significantly more to total body impedance th an it did in the normal volunteers (42.5% vs. 34.4%; p < .001), who were up right and mobile immediately before the measurement Conclusions: Although single-frequency bioimpedance does give an indication of total body water and change in total body water, it is neither precise nor accurate enough to he the sole guide to fluid therapy. The proportion o f total impedance contributed by the knee-ankle segment, which contains rel atively little water, was significantly greater in the patients than in the controls, probably reflecting better drainage of fluid from the lower limb in the supine position.