[Abstract] OBJECTIVE: To explore the application of Bioelectrical Impedance Vector Analysis (BIVA) in the evaluation of dry weight (DW) in maintenance hemodialysis (MHD) patients. METHODS: MHD patients with appropriate clinical evaluation of DW in the hemodialysis unit of the Department of Nephrology, Peking University First Hospital were selected for single-frequency (50 KHz), whole-body bioelectrical impedance measurements before and after dialysis, respectively. A healthy check-up population in the Shijingshan area of Beijing was collected as a control. The 95% confidence intervals of the vectors for the healthy population and the MHD patients were plotted on the resistance-electrical resistance vector diagram (RXc diagram) and the tolerance interval for the healthy population, respectively. MHD patients were divided into two groups, A (water overload) and B (normal water load), according to the position of the pre-dialysis patients on the tolerance interval map of the healthy population, and the prevalence of hypertension before dialysis was compared between the two groups. Patients were divided into water overload (OH), normal water load (NH) and dehydration (DH) groups according to the position of their vectors on the tolerance interval chart after dialysis, thus predicting the direction of dry weight adjustment, and comparing the changes in blood pressure before and after dialysis in the three groups. RESULTS: A total of 290 healthy adults, male:female=149:141, and 37 hemodialysis patients, male:female=13:24, were measured. compared to healthy controls, the pre-dialysis vectors of dialysis patients were shorter and had smaller phase angles; after dialysis, all vectors were longer and had larger phase angles than before dialysis, with improved hydration status. the occurrence of hypertension in group A was 39.3%, compared to 22.2% in group B. The mean blood pressure level in the DH group The mean blood pressure level was lower in the DH group than in the OH and NH groups, but there was no statistical difference. the agreement between the BIVA method for evaluating patients with appropriate dry weight and clinical evaluation was 70.3%; BIVA predicted that 11 patients needed to adjust their DW, and after two weeks of observation, five adjusted their dry weight in the same direction as predicted by BIVA. Conclusion: Even patients with appropriate dry weight have heavy pre-dialysis water load. the BIVA method can detect inappropriate dry weight before clinical experience and can be used as a more sensitive aid to estimate dry weight.
Discussion.
In this paper, the BIVA method was applied for the first time in China for the evaluation of DW in MHD patients, using BIVA data obtained from our normal population as a reference. In this study, we found that in a teaching hospital hemodialysis unit, BIVA predicted that nearly 1/3 of the patients who were judged by clinical experience as having appropriate DW needed to redefine DW, and in subsequent observations, the direction of clinical adjustment of DW was found to be largely consistent with the predicted direction of BIVA, indicating that BIVA can be used as an adjunct to clinical judgment of DW.
During hemodialysis, if DW is set too low, post-dialysis hypotension is likely to occur, causing complications such as blockage of internal fistula, accelerated heart rate, and insufficient cardiovascular and cerebrovascular blood supply, increasing the incidence of acute complications during dialysis or even mortality [22]; if it is set too high, water retention increases in hemodialysis patients, and complications such as intractable hypertension, left ventricular hypertrophy, and heart failure can occur [23], affecting patients’ quality of life. And the presence of these complications can increase the hospitalization and mortality rates of patients [24-27] and increase health care expenditures. Therefore, setting an appropriate DW is of great significance for improving patients’ quality of life, preventing complications, and saving national health and economic expenditures.
Evaluation of DW by clinical signs and symptoms is the most common and convenient method, but this method is trial and error (trial and error) and is not accurate. For example, the presence of hypotension or cramping during dialysis is considered to be a low DW setting; however, the presence of hypotension during dialysis is associated with both a vascular refill rate smaller than the ultrafiltration rate [28-29] and may also be associated with reduced ventricular filling and decreased peripheral vascular resistance in patients [30], or decreased cardiac pump function (left ventricular function) and vascular regulation [29,31]. How to objectively evaluate DW has been a difficult problem for clinicians. In this paper, the BIVA method was applied to the assessment of DW in MHD patients for the first time in China.
Before applying the BIVA method to MHD patients, healthy controls must first be established and RXc maps of the healthy control population, including confidence interval maps and tolerance interval maps of the mean vectors, must be drawn. The different vector lengths and phase angles of the healthy population by gender suggest significant differences in the distribution of body fluids by gender in the normal population. The long bioelectrical impedance vector in females relative to males suggests that females contain less water than males, which may be related to less muscle content in females, which is consistent with our previous study [32] and with Professor Piccoli’s studies in Italy [14-15,20], the United States [18] and India [19]. The smaller phase angle in women may be related to the lower body cell mass in women. Our study normal controls were very similar to the Indian population in terms of R/H and Xc/H values [19] and different from the Italian and US populations [14-15,18]. As shown in Table-5, the mean R/H values of the US and Italian populations of the same gender were smaller than those of the Asian population; the mean Xc/H values of the US population were larger than those of the Asian population, while the mean Xc/H values of the Italian population were smaller than those of the Asian population. These suggest that the American population has more muscle mass and more body cell mass than the Asian population; the Italian population has more muscle mass and less cell mass than the Asian population, i.e., the Italian population has less body fat cells.
Table-5 R/H and Xc/H values (m±s) of normal control populations in Beijing, China, and India, Italy, and the United States (including non-Caucasian whites, non-Caucasian whites, and Mexican-Americans)
Group
Age
Height
Body weight
BMI
Resistance/height (R/H)
Reactance/height (Xc/H)
Male
Beijing, China
62.4±8.9
169.36±5.87
69.88±9.93
24.32±2.86
322.8±40.5
35.1±4.7
India
39±12
170.4±6.8
70.9±13.0
24.4±4.1
320±42
37±8.0
Italy
49±17
170±8
72.6±11.5
24.9±2.9
298.6±43.2
30.8±7.2
American non-Caucasian whites
20-69
177±7
–
19-30
277.2±33.6
38.1±6.2
American non-Caucasian blacks
20-69
176±7
–
19-30
282.9±37.3
41.4±7.0
Mexican-Americans
20-69
169±6
–
19-30
293.1±36.3
42.2±6.7
Female
Beijing, China
56.3±8.49
158.62±5.37
61.65±10.36
24.48±3.74
406.7±44.2
40.2±5.3
India
37±12
157.5±6.2
62.4±12.4
25.1±5.0
400±58
42±0.01
Italy
48±18
158±7
61.5±9.5
24.5±3.3
371.9±49.0
34.4±7.7
American non-Caucasian whites
20-69
163±6
–
19-30
372.5±44
46.9±7.1
American non-Caucasian blacks
20-69
164±6
–
19-30
372.5±45.8
50.6±8.2
Mexican-Americans
20-69
157±6
–
19-30
390.6±45.8
51.1±8.0
The tracing points of MHD patients at RXc were compared with healthy controls of the corresponding sex to draw conclusions about the hydration status of the patients. Compared to healthy controls, the pre-dialysis vectors of dialysis patients were shorter (suggesting a heavy water load for the patients) and had a smaller phase angle (relatively less body cell mass as a percentage of body weight); post-dialysis vectors were both longer (suggesting a reduced water load) and had a larger phase angle (relatively more body cell mass as a percentage of body weight) than pre-dialysis, in line with Professor Piccoli’s 1998 study of the Italian hemodialysis population [15] The trend is consistent. In the MHD population we studied, the elliptical region of the 95% confidence interval was larger in MHD patients than in healthy controls, both before and after dialysis, suggesting a greater heterogeneity in MHD patients, due to the large differences in water load status between patients and the smaller individual differences in body water load in healthy controls.
Applying the BIVA tolerance interval map to further evaluate the hydration status of individuals, it was found that 1/3 of patients with clinically evaluated appropriate DW were still judged by BIVA to have inappropriate DW. the agreement between the DW estimated by the BIVA method and the DW evaluated by clinical experience was 70.3%. nearly half of the 1/3 patients whose BIVA method conclusion did not match the dry weight estimated by clinical experience had their DW adjusted, and the direction of adjustment was consistent with the direction predicted by BIVA, suggesting that the BIVA method has some reliability and can be used as an adjunct to DW evaluation in the hemodialysis unit. the other half of the patients predicted by BIVA to require dry weight adjustment did not undergo dry weight adjustment during our observation period, and some were patients who were in a dehydrated state after dialysis, and these patients may have been able to tolerate some degree of These patients may have been able to tolerate a certain degree of dehydration and will also reduce the discomfort of dehydration by ingesting water after dialysis. Patients who drink water after dialysis will have a marked sense of pleasure, and this sense of pleasure also makes patients want to shed more water during dialysis rather than adjust their dry weight upward; the other part of the patients are overloaded with water, but the patients’ blood pressure is easily controlled and can still tolerate the current inappropriate dry weight.
Although patients who still have hypertension after dialysis tend to have excessive water load, but according to this paper, there are still non-hypertensive people in the OH group and also normotensive people in the DH group, indicating that the water load of normal blood pressure is not necessarily appropriate, and clinical symptoms will appear when it exceeds the limit that the patient can tolerate, while some patients with OH who do not have high blood pressure before dialysis take antihypertensive drugs, resulting in a low blood pressure level before dialysis, while their fluid load is obviously heavier than normal. The BIVA method is one of such tools.
The BIVA method also has its limitations. It can only report whether a patient has more or less water in their body compared to a healthy control, and it cannot tell exactly how much more or less, which compromises its clinical usefulness. However, if we can determine the relationship between the amount of water removed and the displacement of the vector as a function of the data obtained from a single dialysis session, we can accurately predict the amount of water that needs to be removed to reach the center of the ellipse on the tolerance interval graph.
The BIVA method allows the evaluation of DW in MHD patients independently of body weight and does not require any regression equation thus reducing the resulting error. In this paper, the direction of clinical adjustment of DW was found to be consistent with the direction predicted by BIVA, indicating that the BIVA method has good clinical utility. However, this paper is an observational study, and further data accumulation is needed to verify the reliability and practicality of the BIVA method.