1 Nephrology and Dialysis Units, Hospitals of Chioggia (VE), 2 Camposanpiero (PD), and 3 University of Padova, Italy
Keywords: bioimpedance; fluid volume; haemodialysis; hydration; obesity
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Introduction |
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Following the RXc graph method, hydration is established by a comparison of the measured tissue impedance with percentiles (50, 75 and 95% tolerance ellipses) of the reference vector distribution from a healthy population of the same race and gender (Figure 1) [2]. The impedance vectors of both obese and oedematous subjects are shorter than normal, but vectors from obese subjects (body mass index (BMI) 3180 kg/m2) can be discriminated from those of oedematous patients with 91% accuracy, since vectors from obese subjects fall above the straight line of boundary between fat and fluid overload (Figure 1
) [3]. The RXc graph method also allows direct comparison over repeated measurements of intra-subject with inter-subject variability (RXc path graph) [1]. Both in lean and in obese haemodialysis (HD) patients, the pre-post-HD, wet-dry weight cycling is represented on the RXc graph with a cyclical, backwardforward displacement of the impedance vector, with a line of action parallel to the major axis of the reference tolerance ellipses, despite a different point of application [3,4].
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Case |
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During the first dialysis sessions, the patient suffered from hypotensive episodes that were overcome by incrementing the dialysis time and maintaining ultrafiltration rates less than 0.8 l/h. Thereafter, she remained symptom-free and had a normal blood pressure, both during dialysis and at home. Ultrafiltrate volume and interdialytic weight gain were 34 kg. At that time, pre-dialysis impedance vectors were short, below the tolerance ellipses, and were scattered on the verge of the linear threshold discriminating between the obese and the oedematous (Figure 1, path with vectors represented with solid circles, one every 3 weeks). These vectors indicated that, in addition to both fat and fluid overload, there was a modest fluctuation of tissue hydration over time (6 months). BIVA was then used in monitoring long-term fluid balance, using the boundary between fat and fluid overload as the target of pre-dialysis vector position on the RXc graph.
In November 1998 the patient underwent lipectomy (18 kg), bringing her average body weight down from 181 to 163 kg (BMI 56 kg/m2) with a consequent improvement in physical activity and quality of life. Clinically, the patient continued to be symptom-free, both during dialysis and at home. Analysis of the RXc graph after lipectomy (Figure 1, vectors with label b) revealed no definite migration of the impedance vector.
In the subsequent months body weight increased from 162 to 170 kg (Figure 1, vectors with label c), and pre-dialysis blood pressure peaked to 180200/ 90100 mmHg. Both body weight and blood pressure decreased following removal of more fluid over a few sessions, until the pre-dialysis weight reached 159 kg (Figure 1
, vectors with label d). In short, the post-lipectomy path of the impedance vector was characterized by a shortening and down-sloping trajectory during body weight increase with hypertension (indicating body fluid overload) and by a lengthening and steepening trajectory during body weight and blood pressure decrease (indicating a negative fluid balance).
Finally, pre- to post-HD vector migrations (i.e. lengthening and steepening of pre-HD vectors, not shown) after lipectomy were comparable to those before lipectomy, since the amount of fluid removal per session was also comparable.
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Discussion |
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Body weight change in the long-term is not easy to interpret, since it can be altered by changes in fat, muscle mass and tissue hydration. In severe obesity, body weight itself cannot be accurately determined, and interpretation of body weight change is problematic. For instance, it is not easy to discriminate between pitting oedema and pitting fat (Figure 2). In the present patient, estimation of total body water based on body weight (e.g. by Watson formula) [9], would range from 61 (pre-lipectomy) to 56 (post-lipectomy) and to 55 litres (last HD sessions), falsely indicating that more fluid would have been lost after lipectomy (5 of 18 kg) than after vigorous ultrafiltration (1 of 11 kg).
Difficult identification of reference sites for electrode placement on trunk and limbs increases the measurement error of segmental bioimpedance, whose clinical utility, however, has not yet been established in renal patients [10]. Fortunately, the accuracy of whole-body impedance measurements (for both inter-subject and intra-subject measurement errors) is comparable between obese and lean subjects [3]. However, total body water estimates from conventional regression equations of whole-body bioimpedance become inaccurate when impedance vectors shorten out of the 95% tolerance ellipse, due to the hyperbolic relationship between body fluid volume and bioimpedance [6]. For instance, equations from literature [3,6], starting from the same impedance readings and ignoring body weight would predict values ranging from 4250 (pre-lipectomy) to 4245 (post-lipectomy) and 37 litres (last HD sessions), or from 5763 (pre-lipectomy) to 5463 (post-lipectomy) and 50 litres (last HD sessions), when including body weight in the equation.
In our case, long-term monitoring of tissue hydration with BIVA indicated an adequate fluid balance (i.e. asymptomatic HD sessions and normal blood pressure both in dialysis and at home) when vectors fluctuated on the boundary between fat and fluid overload, despite different body weights. An expected result was the small vector migration after lipectomy that replicated findings from a study using energy restriction [3]. Interestingly, a modest downward deviation of vectors from the boundary was associated with hypertension (Figure 1, label c), indicating that body weight increase was due to fluid accumulation instead of soft-tissue increase (which was erroneously attributed to the improved clinical condition following lipectomy). The reverse was also documented by the transient removal of fluid during dialysis, which brought the vector back close to the target boundary (Figure 1
, from c to d label). This included a twofold vector lengthening compared with pre- to post-lipectomy, which indicated a high sensitivity of the method for detecting tissue hydration change and a low sensitivity for detecting anhydrous, fat mass loss. This specific sensitivity can be utilized in monitoring tissue hydration without knowledge of body weight in all clinical conditions, particularly in maintenance HD. It is worth noting that body fluid volume variations in the order of 3 kg in subjects with impedance vectors within the gender-specific 75% tolerance ellipses (e.g. lean HD subjects), are associated with a three- to fourfold vector displacement as compared with subjects with short vectors, which are below the 95% tolerance ellipse (geometric properties of fluid volume-bioimpedance curve) [4,6].
In conclusion, we observed, in a severely obese HD patient, a long-term fluctuation of impedance vector on the boundary between fat vs fluid overload regions of the RXc graph when the clinical course was asymptomatic. A downward deviation of vectors from the boundary was associated with hypertension and body weight increase due to fluid retention. A normal blood pressure was obtained with transient vigorous ultrafiltration that brought vectors back to the boundary. This BIVA pattern may be useful in monitoring hydration of maintenance HD patients with severe obesity, in whom body weight cannot be monitored accurately.
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Notes |
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References |
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