Stability of urea and creatinine in spent hemodialysate

Urea and creatinine levels in spent hemodialysates showed only small declines in spite of incubation at 37 degrees C for 36 hours. In the determination of dialysate-side solute removal, it would seem prudent to keep spent dialysate cold during collection to retard bacterial breakdown of these waste products.

Body size, dose of hemodialysis, and mortality

This study investigates the role of body size on the mortality risk associated with dialysis dose in chronic hemodialysis patients. A national US random sample from the US Renal Data System was used for this observational longitudinal study of 2-year mortality. Prevalent hemodialysis patients treated between 1990 and 1995 were included (n = 9,165). A Cox proportional hazards model, adjusting for patient characteristics, was used to calculate the relative risk (RR) for mortality. Both dialysis dose (equilibrated Kt/V [eKt/V]) and body size (body weight, body volume, and body mass index) were independently and significantly (P < 0.01 for each measure) inversely related to mortality when adjusted for age and diabetes. Mortality was less among larger patients and those receiving greater eKt/V. The overall association of mortality risk with eKt/V was negative and significant in all patient subgroups defined by body size and by race-sex categories in the range 0.6 < eKt/V < 1.6. The association was negative in the restricted range 0.9 < eKt/V < 1.6 (although not generally significant) for all body-size subgroups and for three of four race-by-sex subgroups, excepting black men (RR = 1. 003/0.1 eKt/V; P > 0.95). These findings suggest that dose of dialysis and several measures of body size are important and independent correlates of mortality. These results suggest that patient management protocols should attempt to ensure both good patient nutrition and adequate dose of dialysis, in addition to managing coexisting medical conditions.

Prescription of twice-weekly hemodialysis in the USA

BACKGROUND/AIMS

The purpose of this study was to investigate the frequency and characteristics of two hemodialysis sessions/week, to identify factors which influence or predict this prescription, and to examine the outcomes of patients receiving hemodialysis two times/week as compared to the more common treatment of three times/week.

METHODS

Data from a national sample of 15,067 adult hemodialysis patients were utilized to compare twice-weekly with thrice-weekly therapy by logistic regression.

RESULTS

Patients treated less than one year were more likely to be treated twice-weekly (6.1%) than patients on dialysis for one year or more (2.7%) (AOR = 1.49, p = 0.002). Treatment schedules also varied significantly by geographic region. Factors predictive of twice-weekly hemodialysis (p < 0.05) were older age, Caucasian race, female gender, higher serum albumin, lower serum creatinine levels, and lower body mass index. A higher estimated renal function at the start of ESRD was also predictive of a twice-weekly schedule among incident patients (AOR = 1.05, p = 0.05). In addition, Cox-adjusted survival analysis indicated a lower mortality risk (RR = 0.76, p = 0. 02) for twice-weekly hemodialysis compared to thrice-weekly among prevalent patients. For incident patients, however, the results were not significant when adjusted for GFR at ESRD onset (RR = 0.85, p = 0.31).

CONCLUSION

Geographic differences in prescribed treatment remained unexplained by measured characteristics. The survival advantage associated with twice-weekly hemodialysis is likely to be related to patient selection and greater residual renal function.

Relationship between apparent (single-pool) and true (double-pool) urea distribution volume

BACKGROUND

The volume of urea distribution (V) is usually derived from single-pool variable volume urea kinetics. A theoretical analysis has shown that modeled single-pool V (Vsp) is overestimated when the urea reduction ratio (URR) is greater than 65 to 70% and is underestimated when the URR is less than 65%. The "true" volume derived from double-pool kinetics (Vdp) does not exhibit this effect. An equation has been derived to adjust Vsp to the expected Vdp.

METHODS

To validate these theoretical predictions, we examined data from the Hemodialysis (HEMO) Study to assess the performance of Vdp as estimated from Vsp using the previously published prediction equation. For increased precision, both Vsp and Vdp were factored by anthropometric volume (Va). Patients were first dialyzed with a target equilibrated dialysis dose (eKt/V) of 1.45 during a baseline period and were then randomly assigned to eKt/V targets of either 1. 05 (a URR of approximately 67%) or 1.45 (a URR of approximately 75%). A blood sample was obtained one hour after starting dialysis during one dialysis in each patient.

RESULTS

Vsp/Va was (mean +/- SD) 1.014 +/- 0.127 in 795 patients during the baseline period when the URR was approximately 1.45. During the first modeled dialysis after randomization, the Vsp/Va fell to 0.961 +/- 0.138 in the group with an eKt/V target of 1.05, but did not change significantly under the high eKt/V goal. The correction of Vsp to Vdp using the prediction equation resulted in a Vdp/Va ratio of 0.96 to 0.98 in all three circumstances without significant differences. When a blood sample was drawn one hour after starting dialysis, the apparent Vsp/Va ratio at one hour was much lower at 0.708 +/- 0.139. However, the mean Vdp/Va ratio, computed using the correction equation, was 0.968 +/- 0.322, which was similar to the Vdp/Va ratio calculated from the postdialysis blood urea nitrogen.

CONCLUSIONS

These data suggest that the previously derived formula for adjusted Vsp is valid experimentally. The Vsp/Vdp correction should be useful for prescribing hemodialysis with either a very low Kt/V (for example, daily and early incremental dialysis) or a very high Kt/V.

Survival advantage in Asian American end-stage renal disease patients

Survival advantage in Asian American end-stage renal disease patients.

BACKGROUND

An earlier study documented a lower mortality risk for end-stage renal disease (ESRD) patients in Japan compared with the United States. We compared the mortality of Caucasian (white) and Asian American dialysis patients in the United States to evaluate whether Asian ancestry was associated with lower mortality in the United States.

METHODS

The study sample from the U.S. Renal Data System census of ESRD patients treated in the United States included 84,192 white or Asian patients starting dialysis during May 1995 to April 1997, of whom 18,435 died by April 30, 1997. Patient characteristics were described by race. Relative mortality risks (RRs) for Asian Americans relative to whites were analyzed by Cox proportional hazards regression models adjusting for characteristics and comorbidities. Population death rates were derived from vital statistics for the United States and Japan by age and sex.

RESULTS

Adjusting for demographics, diabetes, comorbidities, and nutritional factors, the RR for Asian Americans was 0.75 (P = 0.0001). Race-specific background population death rates accounted for over half of the race-related mortality difference. For whites, mortality decreased as the body mass index (BMI) increased. For Asians, the relationship between BMI and survival was u-shaped. The ratio of Asian American/white dialysis death rates and the ratio of Asian American/white general population death rates both varied by age in a similar pattern. The population death rates of Asian American and Japanese were also similar.

CONCLUSION

Among dialysis patients, Asian Americans had a markedly lower adjusted RR than whites. The effect of BMI on survival differed by race. Compared with the respective general population, dialysis patients had the same relative increase in death rates for both races. The difference in death rates between the United States and Japan does not appear to be primarily treatment related, but rather is related to background death rates.

Imprecision of the hemodialysis dose when measured directly from urea removal. Hemodialysis Study Group

BACKGROUND

The postdialysis blood urea nitrogen (BUN; Ct) is a pivotal parameter for assessing hemodialysis adequacy by conventional blood-side methods, but Ct is relatively unstable because of hemodialysis-induced disequilibrium. The uncertainty associated with this method is potentially reduced or eliminated by measuring urea removed on the dialysate side, a more direct approach that can determine adequacy from the fraction of urea removed and by substituting an estimate of the equilibrated postdialysis BUN (Ceq) for Ct. For a patient with a known urea volume (V), Ceq, the equilibrated Kt/V (eKt/V), and the solute removal index (SRI) can be calculated from the predialysis BUN (C0), total urea nitrogen removed (A), and V from simple mass balance calculations (dialysate/volume method). However, a theoretical error analysis showed that relatively small errors in A, C0, or V are magnified when SRI or eKt/V is calculated using this method, especially at higher eKt/V values (for example, if eKt/V = 1.4 per dialysis, a 7% dialysate collection error causes a 20% error in eKt/V).

METHODS

During three to four baseline dialyses in each of 39 patients enrolled in the pilot phase of the HEMO Study, "A" was measured using an instrument that sampled dialysate frequently (Biostat), and V was calculated from A, C0, and Ceq (median CV for V = 5.6%). The mean V was then applied to the dialysate/volume method to estimate eKt/V and SRI during two to five subsequent dialyses per patient (comparison dialyses). The accuracy and precision of these estimates were assessed by comparing them with eKt/V and SRI derived from a direct measurement of Ceq drawn 30 minutes after dialysis (reference method), from mathematical curve-fitting of sequential dialysate urea concentrations (dialysate curve-fit method), and from another blood-side method that estimates eKt/V from single pool Kt/V and the fractional rate of solute removal (rate method): eKt/V = spKt/V - 0.6.K/V + 0.03.

RESULTS

During 128 comparison dialyses, median absolute errors for calculated eKt/V compared with the reference method were 0.169, 0.061, and 0.071 for the dialysate/volume method, the rate method, and the dialysate curve-fitting method, respectively. The corresponding correlation coefficients were 0.47, 0.88, and 0.81. For SRI, median absolute errors were 0.044, 0.018, and 0.027, and the correlation coefficients were 0.54, 0.85, and 0.74 for the three methods.

CONCLUSIONS

The precision of eKt/V and SRI measurements was significantly lower for the dialysate/volume method compared with the blood-side methods. Inclusion of the dialysate curve analysis provided by the Biostat restored precision to the dialysate method to a level comparable to that of the blood-side methods. New techniques employing dialysate urea analysis should include a concentration profile to avoid these inherent methodological errors and assure the accuracy of eKt/V and SRI.

Effects of hemodialyzer reuse on clearances of urea and beta2-microglobulin. The Hemodialysis (HEMO) Study Group

Although dialyzer reuse in chronic hemodialysis patients is commonly practiced in the United States, performance of reused dialyzers has not been extensively and critically evaluated. The present study analyzes data extracted from a multicenter clinical trial (the HEMO Study) and examines the effect of reuse on urea and beta2-microglobulin (beta2M) clearance by low-flux and high-flux dialyzers reprocessed with various germicides. The dialyzers evaluated contained either modified cellulosic or polysulfone membranes, whereas the germicides examined included peroxyacetic acid/acetic acid/hydrogen peroxide combination (Renalin), bleach in conjunction with formaldehyde, glutaraldehyde or Renalin, and heated citric acid. Clearance of beta2M decreased, remained unchanged, or increased substantially with reuse, depending on both the membrane material and the reprocessing technique. In contrast, urea clearance decreased only slightly (approximately 1 to 2% per 10 reuses), albeit statistically significantly with reuse, regardless of the porosity of the membrane and reprocessing method. Inasmuch as patient survival in the chronic hemodialysis population is influenced by clearances of small solutes and middle molecules, precise knowledge of the membrane material and reprocessing technique is important for the prescription of hemodialysis in centers practicing reuse.

Variation in blood sample collection for determination of hemodialysis adequacy. Council on Renal Nutrition National Research Question Collaborative Study Group

Inadequate dialysis has been associated with high morbidity and mortality in end-stage renal disease (ESRD) patients receiving maintenance hemodialysis. The accurate estimation of dialysis adequacy, measured either as a calculated urea kinetics (Kt/V) or a simple urea reduction ratio (URR) is dependent on the proper collection of blood samples for predialysis and postdialysis blood urea nitrogen (BUN) determination. Because no established protocol exists for blood sampling, we surveyed the study cohort of dialysis centers participating in the National Kidney Foundation Council on Renal Nutrition National Research Question Collaborative Study to determine the comparability of BUN data that were collected to calculate URR to determine adequacy of dialysis. Surveys were completed by 100% of the 202 units participating: 195 in the United States (from 43 states) and seven from Canada, treating approximately 15,000 hemodialysis patients in total. The distribution of the sample by the type of facility mirrored that of 1996 United States Renal Data System (USRDS) Annual Report facilities data. Results showed a 5.0% error in predialysis blood draw and an 8.4% to 41.6% error in the postdialysis counterpart. There was a large variability in the observed postdialysis methods in general. Dilution of predialysis sample with either heparin or saline will falsely underestimate Kt/V and URR. The presence of access-derived, recirculated blood in the postdialysis sample will falsely overestimate Kt/V and URR. Excessive delay in drawing postdialysis sample will reduce Kt/V and URR because of urea rebound. Adoption by all dialysis providers of a uniform blood sample draw procedure will result in a consistency necessary to allow reliable and valid comparison of adequacy of dialysis parameters within and between ESRD patients, units, and clinical trials.

Comparison of methods to predict equilibrated Kt/V in the HEMO Pilot Study

The ongoing HEMO Study, a National Institutes of Health (NIH) sponsored multicenter trial to test the effects of dialysis dosage and membrane flux on morbidity and mortality, was preceded by a Pilot Study (called the MMHD Pilot Study) designed to test the reliability of methods for quantifying hemodialysis. Dialysis dose was defined by the fractional urea clearance per dialysis determined by the predialysis BUN and the equilibrated postdialysis BUN after urea rebound is completed (eKt/V). In the Pilot Study the blood side standard for eKt/V was calculated from the predialysis, postdialysis, and 30-minute postdialysis BUN. Four techniques of approximating eKt/V that eliminated the requirement for the 30-minute postdialysis sample were also evaluated. The first adjusted the single compartment Kt/V using a linear equation with slope based on the relative rate of solute removal (K/V) to predict eKt/V (rate method). The second and third techniques used equations or mathematical curve fitting algorithms to fit data that included one or more samples drawn during dialysis (intradialysis methods). The fourth technique (dialysate-side) predicted eKt/V from an analysis of the time-dependent profile of dialysate urea nitrogen concentrations (BioStat method; Baxter Healthcare, Inc., Round Lake, IL, USA). The Pilot Study demonstrated the feasibility of conventional and high dose targets of about 1.0 and 1.4 for eKt/V. Based on the blood side standard method, the mean +/- SD eKt/V for patients randomized to these targets was 1.14 +/- 0.11 and 1.52 +/- 0.15 (N = 19 and 16 patients, respectively). Single-pool Kt/Vs were about 0.2 Kt/V units higher. Results were similar when eKt/V was based on dialysate side measurements: 1.10 +/- 0.11 and 1.50 +/- 0.11. The approximations of eKt/V by the three blood side methods that eliminated the delayed 30-minute post-dialysis sample correlated well with eKt/V from the standard blood side method: r = 0.78 and 0.76 for the single-sample (Smye) and multiple-sample intradialysis methods (N = 295 and 229 sessions, respectively) and 0.85 for the rate method (N = 295). The median absolute difference between eKt/V computed using the standard blood side method and eKt/V from the four other methods ranged from 0.064 to 0.097, with the smallest difference (and hence best accuracy) for the rate method. The results suggest that, in a dialysis patient population selected for ability to achieve an equilibrated Kt/V of about 1.45 in less than a 4.5 hour period, use of the pre and postdialysis samples and a kinetically derived rate equation gives reasonably good prediction of equilibrated Kt/V. Addition of one or more intradialytic samples does not appear to increase accuracy of predicting the equilibrated Kt/V in the majority of patients. A method based on dialysate urea analysis and curve-fitting yields results for equilibrated Kt/V that are similar to those obtained using exclusively blood-based techniques of kinetic modeling.

Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates. The Hemodialysis (HEMO) Study

The dialyzer mass transfer-area coefficient (KoA) for area is an important determinant of urea removal during hemodialysis and is considered to be constant for a given dialyzer. We determined urea clearance for 22 different models of commercial hollow fiber dialyzers (N = approximately 5/model, total N = 107) in vitro at 37 degrees C for three countercurrent blood (Qb) and dialysate (Qd) flow rate combinations. A standard bicarbonate dialysis solution was used in both the blood and dialysate flow pathways, and clearances were calculated from urea concentrations in the input and output flows on both the blood and dialysate sides. Urea KoA values, calculated from the mean of the blood and dialysate side clearances, varied between 520 and 1230 ml/min depending on the dialyzer model, but the effect of blood and dialysate flow rate on urea KoA was similar for each. Urea KoA did not change (690 +/- 160 vs. 680 +/- 140 ml/min, P = NS) when Qh increased from 306 +/- 7 to 459 +/- 10 ml/min at a nominal Qd of 500 ml/min. When Qd increased from 504 +/- 6 to 819 +/- 8 ml/min at a nominal Qh of 450 ml/min, however, urea KoA increased (P < 0.001) by 14 +/- 7% (range 3 to 33%, depending on the dialyzer model) to 780 +/- 150 ml/min. These data demonstrate that increasing nominal Qd from 500 to 800 ml/min alters the mass transfer characteristics of hollow fiber hemodialyzers and results in a larger increase in area clearance than predicted assuming a constant KoA.

Splanchnic erythrocyte content decreases during hemodialysis: a new compensatory mechanism for hypovolemia

Splanchnic and splenic erythrocyte volumes decrease during postural changes and exercise to help maintain central blood volume and cardiac output. The contribution of this compensatory mechanism to hemodynamic stability during dialysis has not been studied, however. In 8 ESRD patients, age 51.0 +/- 4.5 years old, we measured changes in the splanchnic/splenic erythrocyte volume during dialysis by tagging the patients' erythrocytes with technetium and following abdominal radioactivity over time. Splanchnic radioactivity decreased to 90.2 +/- 3.8% (mean +/- SEM) of the baseline value after 2 hr of accelerated fluid removal (3.7 +/- 0.4 liters) during dialysis (DUF), while it remained relatively unchanged after two hours of dialysis without fluid removal (DD) [106.5 +/- 2.3%, P (DUF vs. DD) = 0.03]. Splenic radioactivity decreased to 89.2 +/- 5.0% of the initial value during DUF versus 103 +/- 3.8% during DD, but the decrease was noted only during the last 30 minutes of DUF and did not attain statistical significance. Autonomic nervous system integrity was measured by the spontaneous variation of the R-R interval during deep respiration (E/I ratio) and by the Valsalva ratio. The mean E/I and Valsalva ratios in the eight patients were 1.13 +/- 0.03 (+/-SEM) and 1.42 +/- 0.1 respectively, suggesting reasonably adequate autonomic nervous system functioning. The results suggest that contraction of the splanchnic, and possibly the splenic, vascular beds occurs during fluid removal associated with hemodialysis. The resultant addition of erythrocytes to the circulation may help maintain central blood volume and cardiac output.

Screening for extreme postdialysis urea rebound using the Smye method: patients with access recirculation identified when a slow flow method is not used to draw the postdialysis blood

To look for patients with extreme urea rebound, we drew intradialytic samples one third of the way into dialysis during routine modeling for 3 months. The samples taken postdialysis were obtained after stopping the blood pump, without any slow flow period. Using the Smye equations, the intradialytic urea level was used to predict urea rebound, expressed as Kt/V-equilibrated minus Kt/V-single pool (deltaKt/V). Results were averaged for the 3-month period in 369 patients. Mean estimated deltaKt/V was -0.20 +/- 0.13, which was similar to but slightly higher than the predicted value (-0.6 x K/V + 0.03) of -0.19 +/- 0.04. In 27 patients, extreme rebound (mean deltaKt/V < -0.40) was found. Sixteen of these patients consented to further study, but only after access revision in four patients. In these patients, additional slow flow samples after 15 seconds and 2 minutes of slow flow, respectively, were drawn one third of the way into dialysis and postdialysis, and a sample was drawn 30 minutes after dialysis. On restudy, postdialysis rebound was still high with full flow samples deltaKt/V = -0.40 +/- 25, but was much lower (-0.18 +/- 0.07) and similar to predicted rebound (-0.19 +/- 0.05; P = NS) when based on 15-second slow flow samples. Eight of the 16 had marked (>15%) access recirculation by urea sampling, and deltaKt/V based on full flow post samples correlated with access recirculation (r = -0.91). The results suggest that the Smye method is valuable for identifying patients with aberrantly large postdialysis rebound values. When the postdialysis samples are drawn without an antecedent slow flow period, most patients with extreme rebound values turn out to have marked access recirculation.

Cardiac output and urea kinetics in dialysis patients: evidence supporting the regional blood flow model

The regional blood flow model predicts that urea sequestration occurs in organs rather than cells, and that post-dialysis urea rebound is a function of both cardiac index (CI) and regional blood flow distribution to muscle. We measured cardiac output (CO) in 100 randomly selected dialysis patients using bioelectric impedance three times during a single dialysis. Mean CO was 5.8 +/- 2.1 liter/min and CI averaged 3.1 +/- 1.1 liter/min/M2. CI was negatively correlated with age (r = -0.48, P < 0.01). CI was strongly affected by vasodilator ingestion (yes, N = 36, CI = 3.5 +/- 1.2; no, N = 64, CI = 2.88 +/- 0.92, P < 0.006). CI was not associated with systolic, diastolic, or mean blood pressures, nor with Hct, although very few severely anemic patients were in the cohort. Repeat intra-dialytic CO measurements two to three months later in 15 patients with low CI (2.59 +/- 0.59 liter/min/M2) and in 13 patients with high CI (5.00 +/- 0.9, P < 0.001) during a urea kinetic modeling session including 30 minutes post-dialysis rebound, sampling showed highly reproducible values for CO, with a mean absolute value % difference between CO values measured several months apart of 9.0 +/- 17%, r = 0.92. Urea rebound expressed as the difference (delta Kt/V30) between equilibrated and single-pool Kt/V was lower in the high CI group (-0.099 +/- 0.07) than in the low CI group (-0.16 +/- 0.06, P = 0.026), and delta KT/V30 as well as delta Kt/V30 divided by K/V correlated with CI (r = 0.48 and 0.48, respectively, P < 0.01). The RBF model was used to compute a group mean predicted delta Kt/V30 for the low CI and high CI groups based on measured group mean values for CI and K/V. The predicted delta Kt/V30 values for the high CI group (-0.097) and the low CI group (-0.183) agreed closely with measured values. RBF modeled values of CO (7.46 +/- 2.96 liter/min) were not significantly different from impedance-derived CO (6.93 +/- 2.70 liter/min), and the two CO measures correlated significantly (r = 0.63, P = 0.0003). The results provide support for the regional blood flow model of urea kinetics.

Effect of the dialysis membrane on mortality of chronic hemodialysis patients

Mortality of prevalent chronic hemodialysis patients remains high. The potential effect of the dialysis membrane on this mortality has not been previously investigated in a large population of chronic hemodialysis patients. Using data from the United States Renal Data System (USRDS), we analyzed a random sample of 6,536 patients receiving hemodialysis on December 31, 1990. The study design was a historical prospective study. By limiting the study to patients dialyzed for at least one year with bicarbonate dialysate, in whom the dose of dialysis could be calculated, and in whom dialysis membrane and co-existing morbidities were defined, the sample size was reduced to 2,410 patients. A Cox proportional hazards model was used to estimate relative mortality risk. The types of dialysis membranes used were broadly classified into three categories: unsubstituted cellulose, modified cellulose (generally cellulose membranes that have been modified by substitutions of some or most of their hydroxyl moieties) and synthetic membranes that are not cellulose-based. The results of the study suggest that after adjusting for the dose of dialysis and the presence of co-morbid factors, the relative risk of mortality of patients dialyzed with modified cellulose or synthetic membranes was at least 25% less than that of patients treated with unsubstituted cellulose membranes (P < 0.001). To account for the possibility that these differences were due to regional practice patterns, we further stratified the data for nine different regions. There was still a 20% difference in relative risk of mortality between membrane groups with the mortality statistically significantly less in patients treated with synthetic membranes (P < 0.045) compared to patients dialyzed with unsubstituted cellulose membranes. The results of this study suggest that the dialysis membrane plays an important role in the outcome of chronic hemodialysis patients. However, more definitive studies are needed before a cause and effect relationship can be proven.

The dose of hemodialysis and patient mortality

The relationship between the delivered dose of hemodialysis and patient mortality remains somewhat controversial. Several observational studies have shown improved patient survival with higher levels of delivered dialysis dose. However, several other unmeasured variables, changes in patient mix or medical management may have impacted on this reported difference in mortality. The current study of a U.S. national sample of 2,311 patients from 347 dialysis units estimates the relationship of delivered hemodialysis dose to mortality, with a statistical adjustment for an extensive list of comorbidity/risk factors. Additionally this study investigated the existence of a dose beyond which more dialysis does not appear to lower mortality. We estimated patient survival using proportional hazards regression techniques, adjusting for 21 patient comorbidity/risk factors with stratification for nine Census regions. The patient sample was 2,311 Medicare hemodialysis patients treated with bicarbonate dialysate as of 12/31/90 who had end-stage renal disease for at least one year. Patient follow-up ranged between 1.5 and 2.4 years. The measurement of delivered therapy was based on two alternative measures of intradialytic urea reduction, the urea reduction ratio (URR) and Kt/V (with adjustment for urea generation and ultrafiltration). Hemodialysis patient mortality showed a strong and robust inverse correlation with delivered hemodialysis dose whether measured by Kt/V or by URR. Mortality risk was lower by 7% (P = 0.001) with each 0.1 higher level of delivered Kt/V. (Expressed in terms of URR, mortality was lower by 11% with each 5 percentage point higher URR; P = 0.001). Above a URR of 70% or a Kt/V of 1.3 these data did not provide statistical evidence of further reductions in mortality. In conclusion, the delivered dose of hemodialysis therapy is an important predictor of patient mortality. In a population of dialysis patients with a very high mortality rate, it appears that increasing the level of delivered therapy offers a practical and efficient means of lowering the mortality rate. The level of hemodialysis dose measured by URR or Kt/V beyond which the mortality rate does not continue to decrease, though not well defined with this study, appears to be above current levels of typical treatment of hemodialysis patients in the U.S.

Equations for normalized protein catabolic rate based on two-point modeling of hemodialysis urea kinetics

The normalized protein catabolic rate (PCRn) can be calculated from predialysis and postdialysis BUN measurements in patients receiving intermittent dialysis. This measure of net protein catabolism, adjusted for body size, is a useful clinical measure of nutrition that correlates with patient outcome and, in patients who are in nitrogen balance, is a reasonable estimate of dietary protein intake. Whereas simplified formulae that estimate the per-treatment dose of hemodialysis, expressed as Kt/Vurea (Kt/V), are in common use, simplified methods for determining PCRn have only recently appeared. In the study presented here, equations were derived for calculating PCRn from the predialysis BUN and Kt/V. The equations were of the general form: PCRn = C0/(a + bKt/V + c/(Kt/NLL)) + 0.168, where Co is the predialysis BUN in mg/dL. Three sets of coefficients were developed for patients dialyzed thrice weekly: one for patients dialyzed after the long interval at the beginning of the week, one for patients dialyzed at midweek, and the third for patients dialyzed at the end of the week. Two similar sets of coefficients were developed for patients dialyzed twice weekly. For patients with remaining function in the native kidney remnant, equations were developed and refined for upgrading PCRn by adjusting C0 upward. The equations were validated by comparing the calculated PCRn with PCRn determined by a formal iterative model of urea kinetics in a series of 119 dialyses in 51 patients dialyzed thrice weekly (r = 0.9952; mean absolute error, 1.97 +/- 1.39%) and in a series of 71 dialyses in 25 patients dialyzed twice weekly (r = 0.9956; mean absolute error, 2.17 +/- 1.56%). These simple yet accurate equations should be useful in epidemiologic studies or in clinical laboratories where limited data are available for each patient or when iterative computer techniques cannot be applied.

Effect of access recirculation on the modeled urea distribution volume

The effect of vascular access recirculation (AR) on the modeled urea distribution volume (V) is not straightforward. When blood is sampled properly so that it is not admixed with recirculated blood, AR will cause an unexplained increase in V in cases in which AR is present throughout the dialysis session (when AR is limited to the terminal portion of a dialysis session it will cause little or no change in the modeled V). On the other hand, when blood is sampled from the arterial line after simply stopping the pump, postdialysis blood urea nitrogen (BUN) represents arterial line BUN and does not always reflect the BUN in the patient's blood. Under these conditions, when AR is present throughout the dialysis session the modeled V usually shows an unexplained decrease, but V may be unchanged, depending on the urea reduction ratio (URR). We performed a mathematical analysis to predict when V would be decreased and when it would be unchanged when the postdialysis BUN is contaminated with admixed blood. The analysis revealed that when AR is present uniformly throughout the dialysis session, the modeled V should be underestimated when the URR is < or = O.70. When the URR is greater than 0.70, even severe degrees of AR may not be reflected by a change in V. When AR is limited to the terminal part of the dialysis session or when AR increases during the dialysis session, and when V is based on admixed postdialysis blood, underestimation of V will occur in almost all circumstances. In a cross-sectional comparison of modeled to anthropometric volume ratios in eight patients with severe AR and in 11 controls, and in sequential modeling studies in a single patient in whom severe AR developed gradually over time, the volume ratio was low in most, but not all instances when modeled V was based on an admixed postdialysis BUN sample.

Simplified equations for monitoring Kt/V, PCRn, eKt/V, and ePCRn

Although computer solution of the differential equations used in urea kinetic modeling has its advantages, simplified formulas that are actually approximate algebraic solutions to the same equations in the clinically useful range are also useful. The Kt/V can be resolved from the predialysis to postdialysis urea nitrogen ratio (R), the weight loss (UF), session length in hours (t), and anthropometric or modeled volume (V) using the equation: KtV = In (R - 0.008 x t) + (4 - 3.5 x R) x 0.55 UF/V. The equilibrated Kt/V (eKTV) can be estimated from the single-pool arterial Kt/V (spaKTV) or the single-pool venous Kt/V (spmvKTV) using a rate equation based on the regional blood flow model of urea kinetics: eKTV = spaKTV - (0.60)(spaKTV)/t + 0.03 = spmvKTV - (0.46) (spmvKTV)/t + 0.02. The normalized protein catabolic rate (PCRn) can be determined from either the single-pool or equilibrated Kt/V based on the predialysis urea nitrogen level (C0) and the Kt/V (KTV) using the generalized equation: PCRn = C0/(a + bKTV + c/ + KTV) + 0.168, where the constants a, b, and c vary depending on the dialysis schedule and the time of the week that the predialysis blood specimen has been drawn. Such equations can be used either for retrospective surveys, or for quality assurance purposes, as well as for bedside guidance in an individual patient. The actual modeled urea distribution volume can also be easily computed if some effort is made to estimate the dialyzer urea clearance.

Estimating equilibrated Kt/V from an intradialytic sample: effects of access and cardiopulmonary recirculations

The Smye method has been proposed to estimate the equilibrated post-dialysis BUN based on an additional intradialytic sample obtained approximately one hour into dialysis. However, the effects of access recirculation (AR) and cardiopulmonary recirculation (CPR) on the Smye computation and the corresponding details of how blood is sampled have not been studied. We examined the accuracy of two variations of the Smye technique. In one method, the intradialytic and postdialysis blood samples were obtained at full blood flow. In the other, the samples were obtained after two minutes of slow flow, to obviate the effects of both AR and CPR. Seventeen patients undergoing high efficiency dialysis and three- to four-hour treatment times were studied, in whom substantial AR was excluded based on two-minute slow flow urea rebound measurements during and just after dialysis. In this group equilibrated Kt/V (eKt/V) values computed using the Smye-derived equilibrated postBUN estimates (full flow samples, 1.22 +/- 0.058 SEM, slow flow samples, 1.23 +/- 0.064) were similar to eKt/V calculated from the 30-minute postdialysis BUN specimen (1.23 +/- 0.049, P = NS). In eight other patients with severe AR (mean 35% +/- 4.5), the accuracy of the full flow Smye estimates was poor when the degree of AR was not constant throughout the dialysis session. Accuracy of the slow flow Smye estimates of eKt/V was unaffected by the presence of severe AR. One advantage of using the full flow Smye method, however, was that a large delta Kt/V value ( < -0.40) was highly suggestive of severe AR.(ABSTRACT TRUNCATED AT 250 WORDS)

Overestimation of hemodialysis dose depends on dialysis efficiency by regional blood flow but not by conventional two pool urea kinetic analysis

In 26 patients, a linear relationship between delta Kt/V (equilibrated minus single pool) and dialysis efficiency K/V was noted (r = -0.72). To determine if such a relationship would be supported by formal urea kinetic analysis, t, Kd, and V were randomly varied in 1,400 simulations using both intracellular/extracellular and regional blood flow 2 pool variable volume models. In the intracellular/extracellular model, delta Kt/V was best correlated with Kd/Kc (r = -0.96), where Kc is the intercompartmental clearance. Kc was not correlated with V, which translated into a lack of correlation between delta Kt/V and V, and a better correlation between delta Kt/V and Kd than between delta Kt/V and K/V. In the regional blood flow model delta Kt/V was best correlated with Kd/QL (r = -0.99), where QL is the perfusion of the low flow compartment. QL was correlated with V because QL is a function of cardiac output, which varies with surface area and therefore with V. In the regional blood flow model, delta Kt/V did correlate with V (r = 0.49), and better with K/V (r = -0.76) than with K (r = -0.47), similar to the results in patients. The slope of delta Kt/V on K/V depended upon fQL (the fractional perfusion of the low flow compartment) and on cardiac index. At an fQL of 0.15 and a cardiac index of 2.85, the theoretical slope was similar to that seen in observational data: delta Kt/V = -0.6 x K/V + 0.03. The results show that the regional blood flow model predicts the observed relation between delta Kt/V and K/V, whereas the intracellular/extracellular model fails in this task unless one arbitrarily ties Kc to V.

Effect of dialysate temperature on central hemodynamics and urea kinetics

Use of cool dialysate is associated with increased intradialytic blood pressure, but the hemodynamic mechanism is unknown. Whether changes in dialysate temperature affect muscle blood flow, which may the alter the degree of urea compartmentalization, also is unknown. We measured hemodynamics and blood and dialysate-side urea kinetic indices in nine hemodialysis patients during two cool (35.0 degrees C) versus two warm (37.5 degrees C) dialysate treatments. The % change in mean arterial pressure was different when using the cool (+6.5 +/- 9.7 mm Hg) versus the warm (-13.4 +/- 3.6) dialysate (P < 0.01), despite comparable amounts of fluid removal. Percent changes in cardiac output were similar with the two dialysates, and thus the blood pressure effect was due primarily to changes in total peripheral resistance (% delta TPR, cool +26 +/- 13.6, warm +8.6 +/- 14.5; P < 0.02). During cool dialysate use tympanic membrane temperature changed by -0.51 +/- 0.23 degree C, whereas body temperature increased by 0.52 +/- 0.14 degree C during use of warm dialysate. Measured urea recovery normalized to the predialysis urea nitrogen concentration was similar with the two treatments: cool 31.3 +/- 0.039 liter-1; warm 29.7 +/- 0.021; P = NS. In a second study, post-dialysis urea rebound values from 15 seconds to 30 minutes, expressed as the percent of the post-dialysis SUN, were similar after the two treatments: cool 11.79 +/- 1.4; warm 12.21 +/- 2.27, P = NS.(ABSTRACT TRUNCATED AT 250 WORDS)

Collection of a representative fraction of total spent hemodialysate

We describe a method of obtaining a small representative fraction of spent dialysate by placing a side tube in the dialysate drainage tube. The side tube, capped with a small-gauge needle, is used to collect the specimen. Fractions obtained in this fashion are found to have a composition similar to that of the remaining spent dialysate.

Intracellular acidification associated with changes in free cytosolic calcium. Evidence for Ca2+/H+ exchange via a plasma membrane Ca(2+)-ATPase in vascular smooth muscle cells

The purpose of this study was to define the mechanism whereby agonists that increase free cytosolic calcium (Cai2+) affect intracellular pH (pHi) in smooth muscle. Rat aortic vascular smooth muscle cells grown on coverslips were loaded with BCECF/AM or fura-2/AM for continuous monitoring of pHi or Cai2+, respectively, in a HCO3-/CO2- containing medium. Recovery from rapid increases in Cai2+ produced by 1 microM angiotensin (Ang) II (delta Cai2+ -229 +/- 43 nM) or 1 microM ionomycin (delta Cai2+ -148 +/- 19 nM) was accompanied by a fall in pHi (delta pHi, -0.064 +/- 0.0085 P < 0.01, and -0.05 +/- 0.012 pH units, P < 0.01, respectively). Neither the fall in pHi nor the rise in Cai2+ elicited by Ang II was prevented by pretreatment with agents which block the action of this agonist on pHi via the stimulation of the Cl/HCo3 exchangers (DIDS, 50 microM) or the Na+/H+ antiporter (EIPA, 50 microM). In the presence of DIDS and EIPA, Ang II produced a fall in pHi (delta pHi, -0.050 +/- 0.014, P < 0.01) and a rise in Cai2+ (delta Ca2+ 252 +/- 157 nM, P < 0.01). That the change in pHi was secondary to changes in Cai2+ was inferred from the finding that, when the rise in Cai2+ elicited by Ang II was prevented by preincubation with a Ca2+ buffer, BAPTA (60 microM), the fall in pHi was abolished as well (delta pHi, 0.0014 +/- 0.0046). The pHi fall produced by Ang II and ionomycin was prevented by cadmium at a very low concentration (20 nM) which is known to inhibit plasma membrane Ca(2+)-ATPase activity (delta pHi -0.002 +/- 0.0006 and -0.0016 pH units, respectively). Cadmium also blunted Cai2+ recovery after Ang II and ionomycin. These findings suggest that the fall in pHi produced by these agents is due to H+ entry coupled to Ca2+ extrusion via the plasma membrane Ca(2+)-ATPase. Our results indicate that agonists that increase Cai2+ cause intracellular acidification as a result of Ca2+/H+ exchange across the plasma membrane. This process appears to be mediated by a plasma membrane Ca(2+)-ATPase which, in the process of extruding Ca2+ from the cell, brings in [H+] and thus acidifies the cell.