Cardiopulmonary recirculation during hemodialysis

Intradialytic techniques for automatic and everyday access monitoring

Vascular access dysfunction is associated with reduced delivery of dialysis, unplanned admissions, patient symptoms, and loss of access, making assessment of vascular access a fundamental part of routine care in dialysis. Clinical trials to predict the risk of access thrombosis based on accepted reference methods of access performance have been disappointing. Reference methods are time-consuming, affect the delivery of dialysis, and therefore cannot repeatedly be used with every dialysis session. There is now a new focus on data continuously and regularly collected with every dialysis treatment, directly or indirectly associated with access function, and without interrupting or affecting the delivered dose of dialysis. This narrative review will focus on techniques that can be used continuously or intermittently during dialysis, taking advantage of methods integrated into the dialysis machine and which do not affect the delivery of dialysis. Examples include extracorporeal blood flow, dynamic line pressures, effective clearance, dose of delivered dialysis, and recirculation which are all routinely measured on most modern dialysis machines. Integrated information collected throughout every dialysis session and analyzed by expert systems and machine learning has the potential to improve the identification of accesses at risk of thrombosis.

Reassessing the utility of access recirculation and Kt/V for the prediction of arteriovenous fistula failure using online clearance monitoring: the SHUNT STUDY

BACKGROUND

The arteriovenous fistula (AVF) is prone to thrombosis which can be avoided by use of monitoring and surveillance programmes. Although surveillance imaging techniques have been shown to be more sensitive and specific than clinical monitoring during dialysis, monitoring may have significant advantages in terms of cost and time saving. In this study we evaluate the yield of two monitoring techniques [blood temperature monitoring (BTM) access recirculation (AR) and Kt/V via online-clearance-monitoring (OCM)].

METHODS

In this single-centre prospective observational study, 101 patients were followed-up for one year. The primary outcome measure was a composite of AVF failure. OCM-Kt/V and BTM-AR were recorded at every dialysis session.

RESULTS

Of all baseline characteristics only a prior history of percutaneous transluminal angioplasty (PTA) to the AVF conferred a significant change in AVF survival (failure events/100 pt years with prior PTA vs. without = 64.0 vs. 17.3, log-rank p = 0.0014; unadjusted hazard ratio (HR) 3.74 (95% CI 1.56-8.94) p = 0.003). Participants with baseline AR < 10% vs. > 15% had poorer AVF survival (p = 0.0002) and HR for baseline AR 10-15% group vs. AR > 15% group = 4.5 (95% CI 1.55-13.05). There was no combination of change in (Δ) AR, ΔKt/V or its presence over any number of dialysis sessions that provided an acceptable combination of sensitivity and specificity or discrimination for AVF failure.

CONCLUSIONS

BTM-AR and OCM-Kt/V are specific but insufficiently sensitive tools for the prediction of AVF failure. BTM-AR and OCM-Kt/V use at every dialysis session appears to add little to the traditional, infrequent use of these evaluations.

Should a fistula first policy be revisited in elderly haemodialysis patients?

Life-sustaining haemodialysis requires a durable vascular access (VA) to the circulatory system. The ideal permanent VA must provide longevity for use with minimal complication rate and supply sufficient blood flow to deliver the prescribed dialysis dosage. Arteriovenous fistulas (AVFs) have been endorsed by many professional societies as the VA of choice. However, the high prevalence of comorbidities, particularly diabetes mellitus, peripheral vascular disease and arterial hypertension in elderly people, usually make VA creation more difficult in the elderly. Many of these patients may have an insufficient vasculature for AVF maturation. Furthermore, many AVFs created prior to the initiation of haemodialysis may never be used due to the competing risk of death before dialysis is required. As such, an arteriovenous graft and, in some cases, a central venous catheter, become a valid alternative form of VA. Consequently, there are multiple decision points that require careful reflection before an AVF is placed in the elderly. The traditional metrics of access patency, failure and infection are now being seen in a broader context that includes procedure burden, quality of life, patient preferences, morbidity, mortality and cost. This article of the European Dialysis (EUDIAL) Working Group of ERA-EDTA critically reviews the current evidence on VA in elderly haemodialysis patients and concludes that a pragmatic patient-centred approach is mandatory, thus considering the possibility that the AVF first approach should not be an absolute.

Are high flow arteriovenous accesses associated with worse haemodialysis?

Introduction:

An arteriovenous (AV) access flow (Qa) of 400 mL/min is usually sufficient for an effective hemodialysis (HD), but some accesses continue developing and become high flow accesses (HFA). Some authors postulated that an HFA might shift a significant portion of dialyzed blood from the cardiac output, which could decrease HD efficiency and lead to volume overload.

Objective:

The aim of our study was to evaluate if HFA is associated with reduced HD efficiency and/or volume overload in prevalent HD patients.

Methods:

We performed a 1-year retrospective study and assessed HD efficiency by the percentage of sessions in which the Kt/V > 1.4 and volume overload by bioimpedance spectroscopy.

Results:

The study included 304 prevalent HD patients with a mean age of 67.5 years; 62.5% were males, 36.2% were diabetics, with a median HD vintage of 48 months. Sixteen percent of the patients had a HFA (defined as Qa > 2 L/min). In multivariate analysis, patients with HFA presented higher risk of volume overload (OR = 2.67, 95%CI = 1.06-6.71) and severe volume overload (OR = 4.06, 95%CI = 1.01-16.39) and attained dry weight less frequently (OR = 0.37, 95%CI = 0.14-0.94). However, HFA was not associated with lower Kt/V.

Conclusion:

Our results suggest that patients with HFA have higher risk of volume overload. However, contrarily to what has been postulated, HFA was not associated with less efficient dialysis, measured by Kt/V. Randomized controlled trials are needed to clarify these questions.

Mass Balance Index: An Index for Adequacy of Dialysis and Nutrition

Determining adequacy of dialysis has remained a problem for the nephrologist despite the results of the National Cooperative Dialysis Study published more than 20 years ago. Urea Kinetics Modelling (UKM) which requires computer data entry is time-consuming for the dialysis staff but is the only method that has been rigorously studied. Furthermore, it is unclear today what value of Kt/V represents ideal dialysis; the technique is subject to a number of errors associated with estimation of dialyser clearance (K) and volume of distribution of urea (V) but it is useful for calculating protein catabolic rate (PCR). Methods that use urea reduction ratios (URR) is widely used because it is simpler but not always accurate and suffer from an inability to calculate PCR. Direct dialysis quantification (DDQ) can overcome a number of these problems but it is too cumbersome for routine use. Simpler methods to determine dialysateside kinetics have the advantage of solving a number of these problems and also facilitate the calculation of PCR to determine the patient's nutritional state. In our study, we have demonstrated that by taking two dialysate samples at the beginning and at the end of dialysis (2-DSM), it is possible to determine total urea removal (TUR) which is equivalent to DDQ. By taking blood samples after dialysis and before the next dialysis, it is possible to calculate the total urea generated (TUG). The ratio of TUR/TUG will provide an index of dialysis which places emphasis on removal of solute that has accumulated in the inter-dialytic interval thus re-establishing a state of equilibrium. We refer to this index as the Mass Balance Index (MBI). The MBI is also useful in helping to identify those patients whose PCR is inadequate since the mean MBI for patients with an nPCR <0.8 was 0.93 ± 0.03 vs 1.08 ± 0.02 in those with a PCR >0.8. In these two groups of patients the Kt/V was not significantly different, 1.49 ± 0.07 vs 1.53 ± 0.06, p -0.64. We suggest that the emphasis for adequacy of dialysis should shift away from Kt/V to maintaining a state of equilibrium by removing the solutes that accumulate between dialysis and by identifying those patients with an inadequate PCR.

Potassium-Based Dilutional Method to Measure Hemodialysis access Recirculation

Background

Assessment of access recirculation (AR) is crucial to dialysis efficiency and there is thus a need for a method yielding a highly accurate, fast, easy and economical measurement that can be applied in any busy dialysis clinic. Non-urea based dilutional methods are more accurate than urea based methods and avoid problems with cardiopulmonary recirculation, but they require expensive specialized devices, which limit their applicability.

Methods

We developed a simple dilutional method of AR which does not require any specific device, based on the determination of serum potassium [K+] in two samples. Briefly, a basal sample is drawn at the time of needle insertion (basal [K+]); needles are connected to blood lines and blood flow rate is quickly increased to 300 ml/mm; a second sample (arterial [K+]) is drawn from the arterial line port within 5 to 10 seconds, to avoid errors due to cardiopulmonary recirculation of the normal saline entering the blood stream. At this time, if recirculation is present, part of the normal saline will enter the arterial line and dilute the serum [K+]. The AR formula is: AR (%) = 100 x [1 - arterial K+ / basal K+] We compared our method with the two-needle urea and ultrasound velocity dilution methods. Results: AR values by the ultrasound method > 10% were hypothesized as gold standard for AR, against which values obtained with the potassium method were compared. The potassium based method showed: sensitivity (100%,); specificity (95%); predictive value, positive (91%); predictive value, negative (100%). In addition, the potassium based method appears to be more reliable than the two-needle urea based method.

Conclusion

Our method, similar to other dilutional methods, is not influenced by cardiopulmonary recirculation or veno-venous disequilibrium and is fast and accurate. Moreover it is very simple, economical, and can easily be performed in any dialysis unit.

Model-Based Dialysis Adequacy Prediction by Continuous Dialysate Urea Monitoring

A modeling approach for on-line estimation of urea kinetics from continuous measurement of urea concentration in the effluent dialysate stream (DUN) is presented. On-line identification of urea kinetics response parameters is used to predict and update dialysis adequacy during the treatment. Dialysis adequacy can be quantified in several ways, but its strict dependence on final urea concentration is a major fact. For this reason, a good predictive skill on the time course of DUN may enable better performances in the control of dialysis outcome by treatment parameters adjustment. A post-filter enzymatic sensor performs continuous measurement of DUN on patients undergoing standard haemodialysis. To get an early prediction of the end dialysis urea level, the solution of a variable volume double-pool (VVDP) model is used, whose parameters are identified at each time on the basis of the past DUN history. Unlike the variable volume single-pool (VVSP) model, this enables a prompt and accurate estimation of the final DUN. In fact, after 75 min the estimates always differ by less than 10% from the values measured by the sensor at the end of the treatment. Moreover, values predicted by the model in the last hour always lie within 1% of measured final values. Realtime knowledge of an analytic expression for whole DUN time course also enables the accurate prediction of total removed urea, with no need of cumbersome dialysate collection techniques.

Continuous On-Line Determination of Recirculation by Thermodilution in Hemodialysis Patients

The Consistency Of The Determination Of A-V Fistula Recirculation (R) Using The Thermodilution Method (T) With A New Probe (Blood Temperature Monitor, Btm Fresenius A.G.) Was Studied In 32 Patients (Avf: Proximal 34%, Distal 63%, Graft 3%). We Compared R Calculated By T With Both The Traditional Three-Sample Method (C) And The Low-Flow Three-Sample Method (L); Both Bun And Creatinine (Cr) Were Measured In All Samples At The Beginning And At The End Of The Session. T Was Also Determined At The 2Nd And 3Rd Hour. There Was A Significant Correlation Between T And Either C Or L At The Start Of The Session (Bun And Cr) As Well As At The End (Only Cr). R Was Higher (11.9±10) In Proximal Avf Than In The Distal (5+3.1%; P0.01) When Measured By T At The Same Blood Flow (Qb: 313±45 Vs 343+52 Mls/Min, P=Ns). T Increased But Not Significantly By Increasing Qb From 150 To 300 Mls/Min In Ten Patients. No Correlation Was Found During The Session Between Blood Pressure And T Variations. In Conclusion, T And L Give Very Similar Results While C Overestimates Recirculation. R Is Easy To Perform Repeteadly By T With Results Available Online.

Quantitating Dialysis using two Dialysate Samples: A Simple, Practical and Accurate Approach for Evaluating Urea Kinetics

Urea kinetics is now widely used to determine the adequacy of dialysis. Several simplified formulae are currently in use but only a few have been accepted into clinical practice because of their simplicity and ease of calculation. A recent analysis of these formulae showed that for the same set of blood urea values the calculated Kt/V can range from 1.0 to 1.5. We have developed a new dialysate-based method (2DSM) to estimate the urea kinetic parameters using dialysate and blood samples taken at the beginning and at the end of dialysis. The total urea removed (TUR) was calculated from the geometric mean of the two dialysate samples, dialysate flow rate and the duration of dialysis. The Watson formula was used to determine the volume of distribution of urea. A comparison of the 2DSM and the direct dialysate quantification (DDQ) method showed the following results (mean ± sd, n = 52): for total urea removal (TUR) 697 ± 32 vs 722 ± 37 mmol (p = 0.6, r2 = 0.928, y = 101 + 0.83 ×, mean difference 25 ± 76 mmol, see Bland-Altman plot), dialysate urea concentration (Durea) 5.55 ± 0.25 vs 5.75 ± 0.29 mmol/l (p = 0.6, r2 = 0.928, y = 0.8 + 0.82 x, mean difference 0.2 ± 0.6 mmol, see Bland-Altman plot), dialyser clearance (K) 232 ± 4.4 vs 235 ± 5.6 ml/min (p - 0.54), Kt/V 1.42 ± 0.04 vs 1.51 ± 0.04 (p = 0.21), volume of distribution of urea (Vd) 40.14 ± 1.04 vs 38.74 ± 1.2 L, (p = 0.38), and PCR 64.6 ± 2.6 vs 68.1 ± 3.1 g/day. We have developed a simple method of determining dialysate-based urea kinetics which requires two dialysate samples, one at the beginning and one at the end of dialysis and a blood sample at the midpoint of dialysis. TUR can be calculated using the dialysate flow rate and the dialysis duration and once this is known all the other kinetic parameters can be calculated.

Comparison of approximate and iterative methods to calculate the efficiency of haemodialysis as KT/V

Dialysis dose is given by the index KT/V, where K is the dialyser urea clearance, T the duration of dialysis and V the notional urea distribution volume. KT/V may be calculated using the single pool model and an iterative calculation which requires a computer program. Three estimates of KT/V, each based on simple correction of the log ratio in (C0/C1), where C0, C1 denote the pre- and post-dialysis blood urea concentrations in mg/ml, were developed and compared with the values of KT/V given by iteration. The first estimate was given by adding two terms to the log ratio to account for the effect of ultrafiltration and urea generation while the second and third estimates were given by increasing the log ratio by 12.5% and 15% respectively. All estimates gave good agreement when compared with the iterative technique by Bland-Altman analysis for 27 patients on standard 4 hour dialysis treatment using 4 different dialysers (0.71<KT/V<1.62). The mean difference in all cases was less than 0.025 and the differences were not clinically significant. The use of these approximate formulae to estimate dialysis efficiency avoids the need for complex calculation or additional blood samples.

Concordance of absolute and relative plasma volume changes and stability of Fcells in routine hemodialysis

Central hematocrit (H) measurements are currently used to track the degree of ultrafiltration-induced hemoconcentration with the aim to detect and prevent excessive intravascular fluid depletion during hemodialysis (HD). Failure to maintain hemodynamic stability is commonly attributed to the misinterpretation of H caused by an unaccountable increase in Fcells , the ratio of whole-body hematocrit to H. It was the aim to examine Fcells under everyday conditions in a group of stable HD patients. Absolute plasma volume (Vp ) and H were concomitantly measured during routine HD in the extracorporeal system in hourly intervals by noninvasive and continuous technology (CritLine-Instrument-III) and indocyanine green dye dilution to derive relative plasma volumes from Vp and H (RPVp , RPVH ), respectively, and to calculate Fcells . Thirteen patients were studied during two midweek treatments (n = 26). Both absolute Vp (P < 0.05) and relative plasma volumes RPVH (P < 0.001) decreased during HD. Vp at any time point was positively correlated to RPVH (r = 0.52). Moreover, relative plasma volumes RPVH and RPVp determined by independent techniques were identical and showed negligible bias (-0.2%) but considerable limits of agreement (-15.6% to +15.3%). Fcells was stable and in the range of 0.9 ± 0.05 throughout HD and not different from the value assumed at the beginning of HD. Although Fcells remains constant in patients on routine dialysis and relative plasma volumes (RPVH and RPVp ) determined by independent techniques are therefore comparable, the variability of experimental conditions during dialysis and the limited accuracy of absolute volume measurements using available technology continues to complicate the ultrafiltration control problem.

A simple and feasible method to determine absolute blood volume in hemodialysis patients in clinical practice

BACKGROUND

We developed a simple method to determine the absolute blood volume (V) during hemodialysis in everyday clinical practice and examined its relationship with volume overload, clinical relevance, and accuracy.

METHODS

The increase in relative blood volume (RBVpost - RBVpre) measured before and after infusion of 240 ml of ultra-pure dialysate using the bolus function of a commercial online hemodiafiltration machine incorporating a relative blood volume monitor was applied to determine absolute blood volume. The specific blood volume (Vs, blood volume per kg body mass at dry weight, in ml/kg) was compared to volume status as assessed by bioimpedance analysis and clinical criteria.

RESULTS

The blood volume measured in 30 stable hemodialysis patients was 6.51 ± 1.70 l at the beginning, corresponding to a specific blood volume of 80.1 ± 12.8 ml/kg, and dropped to 5.84 ± 1.61 l or 72.0 ± 12.1 ml/kg at the end of the dialysis session, respectively. Specific blood volume correlated with volume status assessed both clinically and by bioimpedance analysis. Intradialytic morbid events occurred only in treatments where specific blood volume fell below 65 ml/kg. The reproducibility of the technique was better than 4% and the in vitro accuracy corresponds to a resolution in Vs of better than 1 ml/kg.

CONCLUSION

Absolute blood volume can be easily measured at the beginning of the dialysis session using the current dialysis technology. Information about V and Vs could be a promising tool to avoid intradialytic morbid events. This technique could be completely automated without altering the hardware of currently available online dialysis devices. Therefore, it is recommended that this technique be integrated into all hemodiafiltration machines.

Patient clearance time (tp) with different vascular access types

BACKGROUND/AIMS

The dialysis delivered dose is limited by the rate at which urea can be transferred from the different body compartments. The time needed to clear the peripheral compartments of the body has been called the patient clearance time (tp). The aim of the study was to compare delivered dialysis dose using the tp index between patients dialyzed through a permanent central venous catheter (CVC) and patients with an arteriovenous fistula (AVF).

METHODS

The study included 48 stable hemodialyzed patients. Patients were classified into two groups according to their vascular access type. The first group included 24 patients dialyzed through a permanent CVC and the second group consisted of 24 patients with a mature AVF. The following parameters were calculated twice for each patient: tp, Kt/V adjusted for the tp.

RESULTS

tp was lower in the AVF dialysis modality than in CVC (26 ± 7 vs. 42 ± 14 min, p<0.001) while the (eqKt/V)tp was higher in AVF than in CVC dialysis (1.36 ± 0.11 vs. 1.19 ± 0.13, p<0.001).

CONCLUSIONS

The patient clearance time is lower in AVF than in CVC dialysis, and this is accompanied by a higher delivered dialysis dose.

Vascular access for hemodialysis: postoperative evaluation and function monitoring

Vascular access (VA) survival is a crucial issue associated with morbidity and mortality of patients undergoing maintenance hemodialysis. The development of stenosis is the major factor that leads to VA failure. Strategies for early detection of lesions within a VA system before serious complications arise are therefore crucial. The implementation of a VA surveillance program could lead to timely detection of VA dysfunction and referral for correction, reduction in central venous catheter use and decrease in hospitalization and VA-related cost. Suggested methods for arteriovenous fistulae and grafts surveillance include blood flow measurement, static pressure evaluation and duplex ultrasonography. Physical examination is an accepted method in contrast to nonstandardized dynamic pressure measurement for grafts. Access recirculation (not urea based) and dynamic pressure measurements are accepted methods for fistulae. Decreasing URR or Kt/V (otherwise unexplained) and increased (negative) arterial pressure in the dialysis machine are methods of limited sensitivity and specificity for both fistulae and grafts. Measurement of access blood flow has been proposed as the gold standard for the screening of all types of VA. Access flow can be measured by various techniques which are direct or indirect. Several studies about VA surveillance programs have demonstrated conflicting results. Larger, randomized controlled trials need to be carried out in order to clarify whether surveillance programs are necessary and which is the best surveillance strategy for each type of VA.

A model to calculate cardiac output in hemodialysis patients by thermodilution

The Blood Temperature Monitor module (BTM) is used to measure recirculation by thermodilution in dialysis. Numerous studies have confirmed its interest in the measuring of the vascular access flow. In this letter we describe a model to calculate cardiac output in dialysis by the BTM.

[Physical principles of renal replacement therapy applied to end stage renal disease patients]

"Hemodialysis" is the generic term that refers to all forms of renal replacement therapy (RRT) able to restore periodically the "internal milieu" composition in end stage renal disease patients (ESRD). RRT includes several modalities (hemodialysis, hemofiltration, hemodiafiltration) that induce basic physical principles (diffusion, convection, adsorption) via an exchange module (dialyser) and an electrolytic exchange solution (dialysis fluid). The cleansing property of the RRT depends on different factors: the treatment modality itself, the uremic toxin considered, patient's characteristic and the operational conditions (duration of treatment, session frequency, blood and dialysate flow rates). Solute instantaneous clearances reflect the dialyser's performances used in optimal conditions but not necessarily the body clearance. The effective solute body clearance is more difficult to assess in clinical practice since it includes some variables such as the treatment duration, the biological complexity of internal milieu and the variability of the patient/dialysis system interaction. The "dialysis adequacy" concept that governs the treatment efficacy in ESRD patients could not be reduced to the urea Kt/V ratio. It must integrate a selection of pertinent clinical and biological markers covering the complete spectrum of uremic abnormalities. Adequate knowledge of those basic physical principles that control the solute exchange in hemodialysis patient is highly recommended to any nephrologist who looks forward to improve treatment efficacy and reduce mortality in ESRD patients.

How to improve dialysis adequacy in patients with vascular access problems

Blood flow rate is a critical factor in the achievement of an adequate dialysis dose. The aim of this review is to evaluate the possibility of optimizing dialysis dose in terms of Kt/V in patients with reduced vascular access (VA) flow rate, considering effective blood flow (Qb eff), recirculation, access flow and hemodialyzer. In patients where the achievement of adequate blood flow rates are difficult to obtain and no surgical revision is necessary, to avoid under dialysis the increase in the treatment time should be the first choice solution. If such a solution is difficult for various reasons, a forced partial blood flow recirculation, especially in central venous catheters (CVCs) with reversed lines can be useful, on condition that the dialysis session is prolonged. The possibility of increasing the efficiency of dialysis through an increase in filter clearance has to be considered. Monitoring arterial pre-pump pressure (P asp) and optimizing ratio P asp/Qb eff during hemodialysis (HD) is one possible solution to improve blood flow rates, but it is necessary to educate and involve the staff. Recent developments in a new class of highly effective hemodialyzer due to dialysate distribution, has opened up interesting opportunities in terms of dialysis adequacy in patients with reduced VA flow rate.

Vascular access monitoring: methods and procedures--something to standardize?

The article discusses the issue of suitable parameters (pressures, recirculation and access flow) to assess hemodialysis vascular access quality (VAQ), available methods to measure those parameters and the setup of the entire VAQ surveillance system (VAQS) in a dialysis facility. Special attention is paid to factors which need some standardization to enable evaluation of VAQ trends in an individual as well as comparison of data from different patients and different dialysis facilities. The discussed procedures are documented with the authors' own measurement results and the results of the VAQS implemented in their unit. Both dynamic and static pressures exhibit insufficient sensitivity in detecting stenoses in native arteriovenous fistulas. Access recirculation is a late finding because with its non-zero value dialysis quality is already compromised. Timely and reliable detection of a deteriorating access condition is enabled by access flow (QVA) only. No standardization is needed in extracorporeal blood flow used in QVA evaluation by ultrasonic dilution. Multiple measurements may increase the reliability of thermodilutional measurements and are a must in optodilutional ones. Timing of the measurement during dialysis should be standardized. Measurement frequency should take into account access type, QVA value and access history. Shortened intervals are needed in the immediate post-intervention period with regard to risk of re-stenosis incidence and strongly nonlinear QVA decreases in such cases. A significant shift-over from surgical interventions to balloon angioplasties is to be expected with the introduction of a VAQS, and appropriate measures must be taken to ensure their quick availability.

The dose of dialysis in hemodialysis patients: impact on nutrition

Multiple lines of evidence have indicated that the dose of hemodialysis impacts upon patient outcome. Among these outcome measures, nutrition is inextricably linked to the adequacy of the treatment. All of the methods of determining dialysis adequacy are based on assessing the removal of toxic substances retained in renal failure, the majority of which are derivatives of protein metabolism. Urea kinetics, employing urea as a surrogate for quantifying the elimination of small molecular weight nitrogenous substances, is the method that has been most thoroughly validated to date as defining a dose range for thrice-weekly hemodialysis: Both inadequate and optimal levels of hemodialysis dose have been identified by prospective, randomized clinic trials utilizing Kt/V(urea) as the index of adequacy. The impact of urea kinetics on nutritional status during thrice-weekly hemodialysis is discussed. Recently, in an attempt to improve outcome beyond that achievable with thrice-weekly hemodialysis, alternative regimens, consisting of daily treatments, have received increasing interest. In order to compare the dose of hemodialysis associated with these regimens with conventional thrice-weekly regimens in terms of removal of small molecular weight substances, standard Kt/V(urea), a parameter that combines treatment dose with treatment frequency, and thus allows for various intermittent therapies to be compared to continuous therapy, must be used. In addition, membrane flux and middle molecule removal, factors that have not yet been well defined as parameters of adequacy during thrice-weekly regimens, may be shown to be important indices with longer hemodialysis treatments, particularly daily nocturnal hemodialysis. The impact that these alternative regimens have had on nutritional status in hemodialysis patients and how they compare to conventional therapy are important considerations.

Ionic dialysance allows an adequate estimate of urea distribution volume in hemodialysis patients

BACKGROUND

An adequate estimation of urea distribution volume (V) in hemodialysis patients is useful to monitor protein nutrition. Direct dialysis quantification (DDQ) is the gold standard for determining V, but it is impractical for routine use because it requires equilibrated postdialysis plasma water urea concentration. The single pool variable volume urea kinetic model (SPVV-UKM), recommended as a standard by Kidney Disease Outcomes Quality Initiative (K/DOQI), does not need a delayed postdialysis blood sample but it requires a correct estimate of dialyser urea clearance.

METHODS

Ionic dialysance (ID) may accurately estimate dialyzer urea clearance corrected for total recirculation. Using ID as input to SPVV-UKM, correct V values are expected when end-dialysis plasma water urea concentrations are determined in the end-of-session blood sample taken with the blood pump speed reduced to 50 mL/min for two minutes (U(pwt2')). The aim of this study was to determine whether the V values determined by means of SPVV-UKM, ID, and U(pwt2') (V(ID)) are similar to those determined by the "gold standard" DDQ method (V(DDQ)). Eighty-two anuric hemodialysis patients were studied.

RESULTS

V(DDQ) was 26.3 +/- 5.2 L; V(ID) was 26.5 +/- 4.8 L. The (V(ID)-V(DDQ)) difference was 0.2 +/- 1.6 L, which is not statistically significant (P= 0.242). Anthropometric volume (V(A)) calculated using Watson equations was 33.6 +/- 6.0 L. The (V(A)-V(DDQ)) difference was 7.3 +/- 3.3 L, which is statistically significant (P < 0.001).

CONCLUSION

Anthropometric-based V values overestimate urea distribution volume calculated by DDQ and SPVV-UKM. ID allows adequate V values to be determined, and circumvents the problem of delayed postdialysis blood samples.

Mechanisms determining the ratio of conductivity clearance to urea clearance

BACKGROUND

Effective conductivity clearance (K(ecn)) has been reported to be a surrogate for effective urea clearance (K(eu)), where both are usually defined respectively as the dialyzer conductivity and urea clearances (K(cn), K(u)) corrected for access recirculation (R(ac)). However, many investigators have reported K(ecn)/K(eu) to be <1 and postulated anatomic distribution of Na in plasma water, cardiopulmonary recirculation (R(cp)), and high rates of urea clearance (K(u)) as causes. The aims of these studies were to devise analytic models of these mechanisms and to clinically evaluate the modeled relationships.

METHODS

We modeled and measured: (1) Na osmotic distribution volume flow rate (Q(osmNa)) in dialyzer blood flow; (2) the separate and combined effects of R(ac) and R(cp) on K(u) and K(cn); and (3) a novel mechanism reducing the conductivity diffusion gradient during measurement of K(cn) by recirculation through the dialyzer (R(s)) of a change in systemic blood conductivity (Delta Cn(s)) induced by the abrupt changes in dialysate inlet Na (Delta C(diNa)) required for the measurement of K(cn).

RESULTS

The ratio Q(osmNa)/Q(bi)= 1.00 +.03, N= 19 (Q(bi)= total blood water flow rate). Modeling showed that the effects of R(ac), R(cp), and R(s) on K(cn) can be quantified as K(ecn)= K(cn)(1 -Delta Cn(bi)/Delta Cn(di)), where Delta C(nbi) is any change in conductivity in the dialyzer blood inlet stream during a measurement, and the effect of a combination of these mechanisms is the product of the effects of individual mechanisms. A single-step dialysate profile (with R(ac)= 0) resulted in measured Delta C(biNa)/Delta C(diNa)= 2.5/15, K(ecn)/K(eu)= 0.83, N= 21 because of R(s) and R(cp), but with a two-step, high/low profile (P(h/L)) we found these respective values to be -0.6/20 and 0.97, N= 19. The ratio K(ecn)/K(eu3)= 1.06 +.02, M + SE, N= 35 (K(eu3)= Ku corrected to reflect both access and cardiopulmonary recirculation). The ratio K(ecn)/K(eu1) (K(eu1) is K(u) corrected to reflect access recirculation only) = 1.01 +.07, N= 297, with no bias on Bland Altman analysis.

CONCLUSION

We conclude that (1) the osmotic Na distribution volume in blood is total blood water; (2) K(ecn) measured with a short, high/low, and asymmetric dialysate profile shows R(ac) effect but neither R(cp) nor R(s) effects on K(ecn) and K(ecn)/K(eu)= 1.0; (3) the K(ecn)/K(eu) ratio is strongly dependent on the type of dialysate profile used, which must be optimized to minimize net Na transfer to and from blood during measurement of conductivity clearance to avoid erroneous underestimation of K(ecn) and K(ecn)/K(eu) ratios <1.

Solute removal index correlates more with equilibrated Kt/V than with single pool Kt/V in haemodialysis patients

BACKGROUND AND AIMS

Solute removal index (SRI) is an adequacy index that estimates haemodialysis dose based on urea removal in the spent dialysate. We examined the SRI, single pool Kt/V (spKt/V) and equilibrated Kt/V (eKt/V) in two groups of chronic haemodialysis patients; group A, 25 patients received haemodialysis twice weekly and group B, 11 patients received haemodialysis thrice weekly.

METHOD AND RESULTS

The Ing's partial dialysate collection method was used for spent dialysate collection. The SRI values of the first and second dialysis sessions in a week in group A were 68.7 +/- 4.7 and 69.7 +/- 4.1%, respectively, while that of the first, mid-week and third dialysis sessions in a week in group B were 56.5 +/- 5.3, 55.8 +/- 5.4 and 57.5 +/- 6.2%, respectively. The correlation coefficients (r) between SRI and spKt/V in the first and second dialysis sessions in a week in group A were 0.90 (P < 0.01) and 0.95 (P < 0.01), respectively, and that in the first, mid-week and third sessions for group B were 0.96 (P < 0.01), 0.94 (P < 0.01) and 0.91 (P < 0.01), respectively. The r values between SRI and eKt/V in the first and second sessions for group A were 0.97 (P < 0.01) and 0.99 (P < 0.01), respectively, and that in the first, mid-week and third sessions for group B were 0.98 (P < 0.01), 0.97 (P < 0.01) and 0.98 (P < 0.01), respectively. Therefore, the correlation between SRI and eKt/V (r values approaching one) was better than that between SRI and spKt/V in all dialysis sessions in a week for both group A and B patients.

CONCLUSION

We conclude that the SRI, an index based on dialysate urea removal, correlates more with equilibrated Kt/V (an index that accounted for postdialysis urea rebound) than with single pool Kt/V.

RENAL RESEARCH INSTITUTE SYMPOSIUM: Surveillance of Access Function by the Blood Temperature Monitor

Low access blood flow is considered the most important cause of peripheral vascular access thrombosis, particularly with grafts. The measurement of access flow is time consuming, operator dependent, and may affect the efficiency of dialysis so that it cannot be done with every treatment. The measurement of recirculation, however, is possible with every treatment and can be done automatically when using the thermodilution offered by the blood temperature monitor (BTM). Since BTM recirculation always includes a component related to cardiopulmonary recirculation, the threshold to detect access recirculation in peripheral arteriovenous grafts and native fistulas must be shifted from zero recirculation applicable for bolus techniques to 15% for the slow thermodilution technique. In one study, recirculation in native arteriovenous fistulas was measured with every treatment in 80 patients over a period of 6 months. Nine of 11 interventions performed during the entire observation period were triggered by a BTM recirculation above the threshold. Two fistulas thrombosed in spite of a BTM recirculation below the threshold. BTM recirculation to detect fistulas for revision is sensitive (81.8%) and specific (98.6%) in the presence of cardiopulmonary recirculation and can be done with minimum intervention and without loss of efficient treatment time.