Effect of changes in the intravascular volume during hemodialysis on blood viscoelasticity.

Adoption of high rate of ultrafiltration (UF) during hemodialysis (HD) may affect the hemorhelogical blood profile, by changing Hematocrit (Hct) and the concentration of plasma proteins, which may in turn interfere with tissue perfusion. The aim of this work is to examine the effect of acute volume change during dialysis on the hemorheological variables. The study included 21 hemodialysis patients. Hematocrit (Hct) and percent decrease in blood volume (BV) were recorded by blood volume monitor. Blood samples were taken before and at the end of dialysis, for measuring plasma fibrinogen and haemorheological variables, which included blood viscosity, plasma viscosity, red cells elasticity and aggregation. The UF volume was 3.52±1.54 L. Hct increased from 34.2±6.1 to 42.1±7.3% (P<0.001), and blood volume (BV) decreased to 85.5±6.4% (P<0.001). Blood and plasma viscosity significantly increased from 3.28±0.69 to 5.48±0.85 mPa.s (P<0.001), and from 1.24 ± 0.16 to 1.65±0.24 mPa.s (P<0.001), respectively. Changes in plasma viscosity were correlated to changes in plasma fibrinogen (r=0.63, P<0.05), while the increase in blood viscosity was correlated to the percent reduction in blood volume (r=0.85, P<0.005). Red cells elasticity increased from 0.26±0.12 to 0.48±0.18 mPa.s (P<0.05), and the aggregation index rose from 0.86±0.31 to 1.25±0.26 (P<0.01). This combination of increased plasma viscosity and red cell aggregability may lower the velocity of erythrocyte transfer inside the tissue capillaries after HD, which may affect tissue perfusion. Moreover, increased elasticity may require more energy from the heart to disaggregate the cells, and this may induce problems in the patients with cardiac dysfunction. In conclusion, the hemorheological variables change after dialysis in the direction which may impede the flow inside the microvessels.

The cardiovascular response to lower body negative pressure in humans depends on seal location.

We tested whether seal location at iliac crest (IC) or upper abdomen (UA), before and during lower body negative pressure (LBNP), would affect thoracic electrical impedance, hepatic blood flow, and central cardiovascular responses to LBNP. After 30 min of supine rest, LBNP at -40 mm Hg was applied for 15 min, either at IC or UA, in 14 healthy males. Plasma density and indocyanine green concentrations assessed plasma volume changes and hepatic perfusion. With both sealing types, LBNP-induced effects remained unchanged for mean arterial pressure (-3.0+/-1.1 mm Hg), cardiac output (-1.0 l min(-1)), and plasma volume (-11 %). Heart rate was greater during UA (80.6+/-3.3 bpm) than IC (76.0+/-2.5 bpm) (p<0.01) and thoracic impedance increased more using UA (3.2+/-0.2 Omega) than IC (1.8+/-0.2 Omega) (p<0.0001). Furthermore, during supine rest, UA was accompanied by lower thoracic impedance (26.9+/-1.1 vs 29.0+/-0.8 Omega, p<0.001) and hepatic perfusion (1.6 vs 1.8 l.min(-1), p<0.05) compared to IC. The data suggest that the reduction in central blood volume in response to LBNP depends on location of the applied seal. The sealing in itself altered blood volume distribution and hepatic perfusion in supine resting humans. Finally, application of LBNP with the seal at the upper abdomen induced a markedly larger reduction in central blood volume and greater increases in heart rate than when the seal was located at the iliac crest.

Albumin infusion fails to restore circulatory function following paracentesis of tense ascites as assessed by beat-to-beat haemodynamic measurements.

AIMS: To study whether circulatory changes during large volume paracentesis (LVP) in patients with liver cirrhosis and tense ascites as assessed by novel non-invasive haemodynamic measuring technology are reversed by subsequent albumin infusion. MATERIALS AND METHODS: Eleven patients with portal hypertensive ascites secondary to liver cirrhosis of Child's class B or C were studied during LVP (10.7 +/- 4.4 l) and subsequent infusion of albumin. Digital arterial pulse waves were continuously measured by vascular unloading technique providing data for beat-to-beat values of systolic (P(s)), diastolic (P(d)) and mean arterial pressures (P(m)), respectively, as well as for heart rate (F(h)), stroke volume (V(s)), cardiac output (Q(co)) and peripheral resistance (R). Data extrapolated to the end of paracentesis, albumin infusion and follow-up phases were compared with the end of the equilibration phase. RESULTS: At the end of paracentesis, P(s), P(m) and P(d) changed by -14 +/- 15% (p < 0.05), -16 +/- 11% (p < 0.01) and -17 +/- 11% (p < 0.001), respectively, whereas Q(co) and F(h) did not change substantially. There was a highly significant increase in V(s) by +21 +/- 25% (p < 0.01). The largest change was seen in R which significantly decreased by -29 +/- 24% (p < 0.01). This change was not reversed by infusion of albumin and persisted up to the end of follow-up. CONCLUSION: The haemodynamic changes following LVP appear to be first and foremost controlled by changes in peripheral resistance with insufficient cardiac compensation. Further studies combining albumin with vasopressors for prevention of paracentesis-induced circulatory changes are needed.

Access flow measurement by indicator dilution without indicator injection: effect of switch location.

It is well known that the measurement of access flow by one of the various dilution techniques requires the reversal of blood flow drawn from and returned to the peripheral vascular access. But it was only recently recognized that the line switch itself constitutes a dilution experiment for certain blood and dialysate components and properties, so that a subsequent injection of indicator is no longer required. New switches introduced at different locations in the extracorporeal circulation not only simplify manual operation for standard access flow measurement but also provide an essential tool for the new technique, which is based on continuously measuring certain blood and/or dialysate characteristics and their changes caused by switching the bloodlines. In this study, the effects of switching the bloodlines at two different locations were studied when extracorporeal temperatures were used as a marker. The study shows that the temperature changes depend on the location of the switch relative to the extracorporeal temperature sensors, and that different algorithms to calculate access flow have to be used for the two possible switching positions to account for this dependence.

Measurement of hemodialysis vascular access flow using extracorporeal temperature gradients.

A reduction in vascular access flow poses a risk for thrombosis. We present a new technique to measure vascular access flow during dialysis based on extracorporeal temperature gradients, and their changes, on reversing the extracorporeal bloodlines without having to inject an indicator. Fistula temperatures were measured by the blood temperature monitor with normal line position and after manual switching of the bloodlines using the same extracorporeal blood flow. The access flow by our temperature gradient method (TGM) was compared to access flow derived by saline dilution with measurements in the same patients repeated in subsequent weeks. In 70 pairs of TGM and saline dilution measurements in 35 patients, the repeatability of the TGM measurements was not significantly different from that of saline dilution. There was a highly significant correlation between the two techniques with an acceptable confidence level for limits of agreement for the difference between them. It took about 9 min to complete the TGM method and about 5 min for saline dilution. Our studies show that the novel TGM method showed excellent agreement and reproducibility with the saline dilution method without the need for indicator dilution.

The arrow of bioimpedance.

The value of bioimpedance in hemodialysis remains under debate. However, when appropriately used, bioimpedance can provide measures of body hydration characterized by a small error, a high sensitivity to changes in water volume, and, above all, a linear relationship over a wide range of volume changes. These features make it very useful to measure body hydration in hemodialysis patients.

Modeling indicator dispersion in extracorporeal blood lines.

The measurement of indicators such as saline diluted by blood flow provides important information on transport characteristics during extracorporeal blood treatments. When saline is injected and measured using the extracorporeal system, the effects caused by dispersion within the extracorporeal system have to be taken into consideration in order to adequately identify intracorporeal transport characteristics. It was the aim of this study to quantify the extracorporeal contribution and to obtain a transport function for specified sections of the extracorporeal system. The dispersion of saline following an impulsive input was measured in arterial and venous segments of customary extracorporeal blood lines with different distribution volumes (V = 23-87 mL) using a range of different blood flows (Qb = 200-450 ml/min). The dispersion was analyzed using a modified Gamma distribution function characterized by three parameters n, k, and tau, where n is real, positive, and n > or = 1, where k = Qb/Vt*n2, and where tau refers to the indicator appearance time at the sampling site. The parameters n, k, and tau were identified by fitting the model function to experimental data. The value of n was 2.3+/-0.5 and largely independent of the type of line segment, Qb, or Vt tau showed a strong dependence on Vt and Qb which was described by tau = Vt/Qb*(n-1)/n. Thus, with a given n, and when Vt and Qb are known, the transport function for saline in important sections of the extracorporeal circulation can be determined for specific experimental conditions. With this information indicator dilution curves measured in extracorporeal blood lines can be corrected for extracorporeal effects.

Comparison of numerical methods applied to field stimulated cardiomyocytes.

Different algorithmic approaches to the task of numerically simulating stimplified electro-physiological settings encountered in field stimulation experiments of cardiac preparations are compared. Rapid prototyping environments for dynamic system simulation as well as dedicated liberaries for high performance computing are employed to serve the varying needs.

Increasing blood flow increases kt/V(urea) and potassium removal but fails to improve phosphate removal.

BACKGROUND: Hyperphosphatemia and hyperkalemia are major determinants of morbidity and mortality in hemodialysis patients. Half of the dialysis population suffers from hyperphosphatemia which is now recognized as an important cardiovascular disease risk factor. It is, therefore, necessary to improve the removal of these molecules. In this study, we investigated the effect of enhancing blood flow on Kt/V for urea (Kt/Vu), potassium and phosphate removal. METHODS: Thirteen patients were investigated in a randomized, cross-over, prospective study using 3 blood flows (Qb) of 200,250 and 300 ml/min which gave 39 standardized high-flux hemodialysis treatments. Effective blood flows were measured by ultrasonic flow meter. Quantification of delivered dialysis dose was performed by partial dialysate and ultrafiltrate collection for the determination of potassium and phosphate removal and by blood urea concentrations for determination of Kt/Vu. RESULTS: Kt/Vu rose significantly from 1.10 +/- 0.14 to 1.22 +/- 0.14 and finally to 1.39 +/- 0.16 (p = 0.0001) with increasing Qb similar to the increase in potassium removal from 53.0 +/- 2.4 to 63.4 +/- 2.6 and to 74.2 +/- 3.8 mMol (p = 0.01). Phosphate removal only improved from 28.1 +/- 1.3 to 31.4 +/- 1.5 (p = 0.050) when Qb was increased from 200 to 250 ml/min but remained unchanged at 31.2 +/- 1.5 mMol (NS compared to phosphate removal at Qb = 250 ml/min) when Qb was increased to 300 ml/min. CONCLUSIONS: Increasing delivered Kt/Vu and potassium removal with higher Qb fails to produce the same desired effect with phosphate removal during high-flux hemodialysis.

Temperature and thermal balance in hemodialysis.

The analysis of thermal balance and temperature in hemodialysis patients reveals both striking similarities and important differences to urea kinetics. Both urea and thermal energy need to be removed during hemodialysis, however, for different reasons. Urea accumulates between hemodialysis treatments, whereas thermal energy accumulates within hemodialysis treatments. Urea concentration is ideally reduced by approximately 70% during a treatment, whereas temperature is ideally kept constant by removing up to 50% of resting energy expenditure because heat dissipation from the body surface is obstructed as a result of ultrafiltration-induced hypovolemia. Extracorporeal heat removal is controlled by several factors. Dialysate and patient temperatures play the main role. Low body temperatures are not uncommon with hemodialysis patients, so that dialysate temperatures less than 36 degrees C are often required to maintain reasonable temperature gradients. Another important role is played by extracorporeal blood flow. At the same temperature gradient, heat transfer by extracorporeal blood flow used with high-efficiency dialysis is approximately six times more efficient than the dissipation of heat across the body surface. And, last but not least, the venous section of the extracorporeal circulation provides constant cooling of approximately 10 W. Almost all dialysis treatments provide extracorporeal cooling, even those using dialysate at 37 degrees C. Therefore it is probably better to define the thermal aspects of hemodialysis with regard to the physiologic effects on the patient. Since thermoregulation responds to changes in body temperature, treatments should be characterized as isothermic, hypothermic, and hyperthermic.

Compartment effects in hemodialysis.

Compartment effects in hemodialysis are important because they reduce the efficiency of removal of the compartmentalized solute during dialysis. The dialyzer can only remove those waste products that are presented to it, and then only in proportion to the concentration of the solute in the blood. Classically a two-compartment system has been modeled, with the compartments arranged in series. Because modeling suggests that the sequestered compartment is larger than the accessible compartment, an assumption has been made that the sequestered compartment is the intracellular space. For urea and other solutes that move easily across many cell membranes, compartmentalization may be flow related, that is, related to sequestration in organs (muscle, skin, bone). Although mathematically urea rebound and mass balance can be described with either model, the flow-related model best explains data showing that urea rebound after dialysis is increased during ultrafiltration, diminished during high cardiac output states, and also reduced during exercise. Whether compartmentalization is increased in vasoconstricted intensive care unit patients receiving acute dialysis remains an open question.

Effect of ultrafiltration on peripheral urea sequestration in haemodialysis patients.

BACKGROUND: Ultrafiltration (UF) is assumed to enhance urea removal during haemodialysis (HD) because of convective transport and because of contraction of urea distribution volume. However, UF-induced blood volume reduction has been hypothesized to enhance peripheral urea sequestration and post-dialysis urea rebound (PDUR), possibly reducing HD effectiveness. The effect of UF on PDUR was investigated in this study. METHODS: Nine HD patients were studied on two subsequent treatment days. The first HD was performed with UF (UF-rate=0.78+/-0.27 l/h), and the second treatment without UF. Serial measurements of serum water urea nitrogen concentration, arterial blood pressures (BP), and relative blood volume changes (BV%) were obtained over the duration of HD. RESULTS: BP and BV% decreased with UF (BP(sys)= -9%, BP(dia)=-8%, BP(mean)=-9%, BV%=-15%) but increased or remained unchanged without UF (BP(sys)= 9%, BP(dia)=12%, BP(mean)=11%, BV%=1%). PDUR was 28.6+/-9.6% without UF, and increased in every single patient with UF (40.7+/-13.2%, P<0.01). Modelled perfusion of the peripheral low-flow compartment decreased from 1.45+/-0.54 l/min without UF to 0.91+/-42 l/min with UF (P<0.05), thereby explaining an enhanced two-compartment effect and increasing PDUR. CONCLUSION: The significant increase in the two-compartment effect of urea kinetics observed in current HD accompanied by UF can be explained by compensatory, intradialytic blood flow redistribution induced by blood volume reduction. Because of the link between UF and blood flow, limited solute clearance treatment modes that optimize fluid removal such as variable UF will also have favourable effects on delivered dose of dialysis.

Low-potassium and glucose-free dialysis maintains urea but enhances potassium removal.

BACKGROUND: The influence of potassium (K) removal on dialysis efficiency as measured by urea elimination is not clear. In this prospective, randomized, cross-over study we investigated the magnitude of K removal and its effect on urea (u) elimination during high-flux haemodialysis (HD). METHODS: Twelve stable, non-diabetic HD patients were investigated during three one-week standardized HD periods (1.8 m(2) high-flux polysulphone dialyser, treatment time 240 min, Qb = 300 ml/min, Qd = 500 ml/min, dialysate without glucose, bicarbonate 40 mmol/l), using dialysates containing 0 (0K), 1 (1K), and 2 (2K) mmol/l of K. Mass removal of K (M(K)) and u (M(U)) were measured during the mid-week treatment by partial dialysate collection. Urea reduction rate (URR) and Kt/V were determined. RESULTS: 0K, 1K and 2K treatments were perfectly comparable. Plasma K (PK) continuously declined reaching stable concentrations after 180 min. While 0K dialysate removed 117.1 mmol, 80.2 and 63.3 mmol (P < 0.001) were removed by 1K and 2K baths respectively. M(U) was not influenced by M(K) (r = 0.22) and amounted to 491.1 (0K), 508.6 (1K), and 506.2 (2K) mmol (NS) respectively. Accordingly, urea clearance, URR and Kt/V were constant during 0K, 1K and 2K treatments. CONCLUSIONS: Potassium-free dialysate significantly enhances potassium elimination. Potassium removal has no influence on urea elimination. High potassium removal, when needed, does not impair dialysis efficiency as measured by urea kinetics in high-flux, glucose-free, 40 mmol/l bicarbonate HD.

Predictive value of access blood flow and stenosis in detection of graft failure.

AIMS: Low access flow and the diagnosis of high degrees of venous stenosis have been recommended as indications for prophylactic angioplasty. However, recent studies have shown that prophylactic angioplasty for > 50% stenosis did not prolong graft patency, and that a single flow measurement may not accurately predict graft failure. In this study we compared the value of monthly measurement of access flow and of the maximal degree of stenosis in the detection of graft failure over a three-month period. METHODS: Thirty-nine hemodialysis patients with polytetrafluoroethylene (PTFE) grafts were evaluated by Doppler ultrasound at monthly intervals for three months. Graft failures were defined as thrombosis, or surgical and angioplastic revisions required because of the presence of access recirculation, and patients with graft failure were followed within the subsequent one-month periods of observation. RESULTS: Twelve graft failures occurred during the three-month period of observation. The risk for subsequent graft failure significantly increased at flows < 300 ml/min. Nine (20%) graft failures occurred with stenoses of 30 to 50%, and three (13%) with stenoses of> 50%. The grafts that failed in the second and the third study months had a 25.8% (380 +/- 62 vs. 287 +/- 190 ml/min, p < 0.05) and a 36.5% (393 +/- 142 vs. 226 +/- 41 ml/min, p < 0.05) decrease in access flow, respectively. There was no significant change in access flow for the grafts patent throughout the study (911 +/- 333, 794 +/- 302, and 919 +/383 ml/min, p = ns). No significant increases in maximal stenosis were found for the grafts that failed in the second month (44 +/- 6.1 vs. 48 +/- 15%, p = ns) and the third month (48 +/- 9 vs. 51 +/- 16%, p = ns). There were no significant changes in the maximum stenosis for the grafts patent throughout the three-month study periods (37 +/- 15,43 +/- 11, and 44 +/- 15%, p = ns). CONCLUSIONS: Access flow is a more sensitive predictor of graft failure than stenosis. Examination of trend in decline of access flow is a more powerful indicator to detect graft dysfunction than an individual single flow value.

Isothermic hemodialysis and ultrafiltration.

The increase in patient temperature during hemodialysis is explained by hemodynamic compensation during ultrafiltration and hypovolemia that leads to peripheral vasoconstriction and reduced heat losses. We analyzed 51 stable high-efficiency hemodialysis treatments in 27 patients during isothermic dialysis in which body temperature was maintained at a constant level (+/-0.1 degrees C) using the temperature-control option of the Blood Temperature Monitor (BTM; Fresenius Medical Care, Bad Homburg, Germany). Hemodialysis was delivered using ultrapure water (limulus amebocyte lysate test < 0. 06 endotoxin units/mL) at mean blood flows of 410 +/- 40 mL/min. During treatments lasting 178 +/- 23 minutes, 4.8% +/- 1.4% of postdialysis body weight (W%) and 9.5% +/- 2.5% of postdialysis body water were removed using mean ultrafiltration rates of 1.1 +/- 0.3 L/h. Dialysate temperatures significantly decreased from 35.9 degrees C +/- 0.3 degrees C to 35.6 degrees C +/- 0.6 degrees C during hemodialysis. During these treatments, 187 +/- 69 kJ of thermal energy were removed from the patients through the extracorporeal circulation using cool dialysate. Extracorporeal heat flow was 17 +/- 6 W. Energy expenditure (H) estimated from anthropometric data was 65 +/- 12 W. Thus, 28% +/- 10% of estimated energy expenditure (H%) was removed during isothermic dialysis. A highly significant correlation was observed between H% and W% (H% = -5.6 * W%; r(2) = 0.91; P < 0.0001). This result is in support of the volume hypothesis of intradialytic heat accumulation and provides a rule of thumb to estimate extracorporeal cooling requirements for isothermic dialysis. Approximately 6% of H must be removed through the extracorporeal circulation for each percent of ultrafiltration-induced body-weight change. The importance of body temperature control during hemodialysis increases with increased ultrafiltration requirements.

Sensitivity and specificity of the thermodilution technique in detection of access recirculation.

BACKGROUND/AIM: Recirculation measured by thermodilution includes effects caused by access and cardiopulmonary recirculation. The aims of this study were to illustrate the accuracy of thermodilution in measurement of hemodialysis recirculation and also to identify a sensitive and specific threshold to detect access recirculation. METHODS: 110 studies were performed in 19 patients. Recirculation obtained directly by the blood temperature monitor (BTM) was compared to that calculated from access blood flow, pump blood flow, and cardiac output determined by ultrasound dilution using the hemodialysis monitor (HDM). RESULTS: A highly significant linear correlation was obtained between repeated BTM recirculation measurements (R(BTM, 2) = 0.99.R(BTM, 1) - 0.22%, r(2) = 0.99). There were no significant differences between repeated BTM recirculation measurements with correct placement (11.4+/-7.1 vs. 10.9+/-7.4%, p = NS) or reversed placement (30.0+/-15.6 vs. 30.2 +/-15.9%, p = NS) of blood lines. A strong linear relationship was obtained between the recirculation determined by thermodilution and the recirculation calculated from HDM measurements (R(calc) = 0.98. R(BTM) - 1.49%, r(2) = 0.95). The mean recirculation obtained by BTM was not significantly different from the recirculation calculated by HDM with correct placement (9.5+/- 2.2 vs. 8.6+/-2.5%, p = NS) or with reversed placement (25.4+/-7.8 vs. 23.8+/-7.7%, p = NS) of blood lines. When a recirculation greater than 15% measured by the BTM was considered as the threshold at which true access recirculation occurred, sensitivity and specificity of the thermodilution method to detect access recirculation were 93 and 98%, respectively. CONCLUSIONS: Recirculation measurements made by the BTM are accurate and precise. Even though BTM thermodilution includes effects of cardiopulmonary recirculation, so that low levels of access recirculation might not be detected, a BTM recirculation >15% represents a highly significant access recirculation.

Access recirculation in a native fistula in spite of a seemingly adequate access flow.

True access recirculation (AR) measured by ultrasound dilution technique is usually absent in well-working shunts. It occurs with low access flows (Qa). High access flow rates are assumed to prevent AR. Two major exceptions to these rules are known: presence of intra-access strictures and inadvertently reversed blood lines. We present an additional exception in which true access recirculation occurred in a native arteriovenous (AV) fistula with correct placement of bloodlines. Surprisingly, access blood flow exceeded pump blood flow (Qb) almost threefold. The situation was clarified by a magnetic resonance angiogram showing a collateral forming a functional loop. This loop led to true access recirculation in one branch, although overall blood flow through both branches appeared to be adequate. The different findings in this shunt over time give insight into the often complex pathophysiology of native fistulae. This case proves that seemingly adequate access flow does not necessarily prevent access recirculation in native AV fistulae. We suggest monitoring both access flow and recirculation in hemodialysis accesses on a regular basis.

Estimation of body fluid changes during peritoneal dialysis by segmental bioimpedance analysis.

UNLABELLED: Estimation of body fluid changes during peritoneal dialysis by segmental bioimpedance analysis. BACKGROUND: Commonly used bioimpedance analysis (BIA) is insensitive to changes in peritoneal fluid volume. The purpose of this study was to show, to our knowledge for the first time, that a new segmental approach accurately measures extracellular fluid changes during peritoneal dialysis (PD). METHODS: Fourteen stable PD patients were studied during a standard exchange with fluids of known conductivity. Bioimpedance was continuously measured in the arm, trunk, and leg and from wrist to ankle. Volume changes were calculated using both a newly developed sum of segmental BIA (SBIA) and current wrist-to-ankle BIA (WBIA) and were compared with actual volume changes measured gravimetrically. RESULTS: When 2.19 +/- 0.48 L were removed from the peritoneal cavity during draining, 95.2 +/- 13.8% of this volume was detected by SBIA compared with only 12.5 +/- 24. 3% detected by WBIA. When 2.11 +/- 0.20 L of fresh dialysate was infused into the peritoneal cavity during filling, 91.1 +/- 19.6% of this volume was detected by SBIA compared with only 8.8 +/- 21.1% detected by WBIA. CONCLUSION: The good agreement between measured and calculated data using SBIA was due to: (a) improved placement of electrodes, (b) estimation of trunk extracellular volume based on a new algorithm, and (c) consideration of changes in dialysate conductivity. Correct estimation of fluid volume in the trunk is a prerequisite for applications in which direct analysis of fluid changes cannot be performed such as with peritoneal equilibration tests and continuous flow PD.

Sum of segmental bioimpedance analysis during ultrafiltration and hemodialysis reduces sensitivity to changes in body position.

BACKGROUND: Bioimpedance, a noninvasive technique to analyze body composition, has attracted interest in determining body hydration in hemodialysis patients. However, the so-called whole-body (wrist-to-ankle) bioimpedance analysis (WBIA) is sensitive to changes in regional fluid distribution and tends to underestimate fluid changes during ultrafiltration in hemodialysis patients. The aim of this study was to show that volume changes calculated from a new approach, that is, segmental bioimpedance analysis (SBIA), are not affected by changes in body position. METHODS: Ten male patients (age 44 +/- 8 years, target weight 70.8 +/- 10 kg) were studied during their regular hemodialysis treatment while maintaining either a sitting or a supine body position throughout the study. Extracellular volume was calculated from extracellular resistance obtained from bioimpedance data measured for a range of frequencies (5 to 500 kHz) using the Xitron BIS4000B analyzer. Wrist-to-ankle measurements were compared with segmental arm, trunk, and leg measurements. RESULTS: Changes in extracellular volume estimated from wrist-to-ankle measurements only reached 80 +/- 13% and 65 +/- 17% of the actual change in body mass during sitting and supine dialysis treatments, respectively. However, when segmental measurements were analyzed, the calculated change in extracellular volume was 101 +/- 6% and 100 +/- 3% of the actual change in body mass during the sitting and supine treatments, respectively. CONCLUSIONS: SBIA properly identifies regional fluid changes and provides an appropriate measure of fluid changes caused by ultrafiltration and hemodialysis. The volume estimation based on the sum of segmental bioimpedance measurements is independent of body position, which is a prerequisite for applications in everyday practice.