Biomedical subjects

D Schneditz

Publications and source records attributed to D Schneditz.

48 records · Page 3Linked to original sources

PubMedDate not supplied

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.

Body Fluid Compartments↗

PubMedDate not supplied

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.

Energy Metabolism↗

PubMedDate not supplied

Cardiopulmonary recirculation in dialysis. An underrecognized phenomenon.

Access recirculation can be determined by measuring blood temperature or blood water concentration in the dialyzer inlet after injecting a bolus of cold saline into the venous line. In patients with access recirculation, some of the cooled venous blood re-enters the blood inlet line soon after injection, resulting in a sharp transient drop in its temperature. There is also a prompt increase in blood water concentration at the dialyzer blood inlet caused by the dilution effect of the recirculated saline. In this study, data are reported on four patients studied under conditions where no access recirculation could occur because blood was returned to a second access or into a central vein. In these patients, transient cooling of the blood in the dialyzer inlet and a transient hemodilution after venous line injection of cold saline was still observed. These observations can be explained by passage of the injected saline through the heart and pulmonary blood vessels and return of a portion of the cooled blood to the vascular access, bypassing the systemic capillary microcirculation. This "cardiopulmonary recirculation" can cause dilution of urea in dialyzer inlet blood, with resulting errors in urea kinetic modeling and in computing access recirculation.

Arteriovenous Shunt, Surgical↗

PubMedDate not supplied

Formal analytical solution to a regional blood flow and diffusion based urea kinetic model.

At the beginning of hemodialysis (HD), urea in arterial blood drops rapidly, whereas a pronounced postdialytic urea rebound (PDUR) can be observed when HD is discontinued. In a new approach to this observation, the authors suggest that solute flux from remote body compartments to the dialyzer is governed by 1) diffusion of solutes from the tissue to the blood perfusing the tissue, and 2) by regional blood flow distribution, cardiopulmonary recirculation, and access recirculation, respectively. These concepts were incorporated into a variable volume, two compartment model that could be treated as an eigenvalue problem and solved analytically. The resulting equations were used to model intradialytic and postdialytic urea profiles with the help of a commercial spreadsheet program. The significance of hemodynamic model parameters such as cardiac output (CO) and regional blood flow distribution on PDUR was modeled in simulation runs, where PDUR increased from 5% to 15% when CO fell from 7 to 3 l/min with standard treatment parameters (t = 3h, KD = 0.3 l/min, V = 35l, UFV = 2.8l, fQHFS = 0.8, QAc = 0.8l/min). Thus, this urea kinetic model establishes a previously missing link between hemodynamics and solute removal.

Biological Transport, Active↗

PubMedDate not supplied

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.

Computer Simulation↗

PubMedDate not supplied

Measurement of access flow during hemodialysis using the constant infusion approach.

With reversed placement of blood lines and with a peripheral arteriovenous access, hemodialysis recirculation (R) consists of a local access component, and a central cardiopulmonary component that must be separated for the calculation of access flow (Qac) using indicator dilution principles. With indicator injections that follow constant infusion principles Qac = (1 - Rx)/(Rx(1 - CPR)) x (Qb,x - UFR), where Qb is the extracorporeal blood flow, where UFR is the ultrafiltration rate, and where the index x indicates reversed placement of blood lines. CPR, the amount of cardiopulmonary recirculation (CPR = Qac/CO) is determined from two recirculation measurements with correct (index n) and with reversed (index x) placement of blood lines CPR = Rn(1 - Rx)/Rx(1 - Rn) x (Qb,x - UFR)/Qb,n. Qac was measured in 11 hemodialysis (HD) patients using a thermodilution device tested in an in vitro set-up based on constant infusion principles. Mean Qac was 1.135 L/min and 1.054 L/min for measurements done early and late in dialysis. The coefficient of variation was +/-7.3% and +/-8.6%, respectively. Repeated measurements of access flow in HD patients showed good reproducibility (Qac.1 = 1.01*Qac.0, r2 = 0.98), with the regression line not different from the line of identity; however, in vivo results remain to be validated by an independent technique.

Arteriovenous Shunt, Surgical↗

PubMedDate not supplied

Validation of changes in extracellular volume measured during hemodialysis using a segmental bioimpedance technique.

Sum of segmental bioimpedance analysis (BIAs) has recently been introduced as a more accurate estimation of extracellular volume (ECV). The current study was designed to compare the changes in ECV estimated by whole body bioimpedance (BIA(W)) and BIAs and to determine whether BIAs could be used to accurately estimate changes in ECV compared with the ultrafiltration volume (UFV) as measured by the dialysis machine. Ten men (age, 49 +/- 10 years; dry weight, 78 +/- 13 kg) were studied during hemodialysis (HD). A multifrequency bioimpedance analyzer (BIS4000B; Xitron Technologies, San Diego, CA) and a digital switch developed by the authors were used to automatically collect data from three body segments (arm, trunk, and leg) and the whole body using the stated technique. There was a significant difference in delta ECV(W) and UFV at the end of the treatment (2.75 +/- 0.74 L vs 3.95 +/- 0.73 L, p < 0.05) because fluid removal from the trunk has almost no effect on whole body resistance. However, delta ECVs was not significantly different from UFV (3.76 +/- 0.65 L vs 3.95 +/- 0.73 L, p = NS). BIAs is a more accurate approach to monitor changes in ECV during HD than is BIA(W) because changes in local resistance can be allocated to segments with uniform geometry and resistivity. This is a prerequisite for future studies on dynamics of regional fluid distribution during hemodialysis.

Biophysical Phenomena↗

PubMedDate not supplied

Predictive value of access blood flow in detecting access thrombosis.

The aim of this study was to evaluate whether repeated measurement of access blood flow (Qac) using the ultrasound dilution technique could predict access failure in patients on hemodialysis. One hundred thirty-one patients were evaluated at intervals of 8 weeks for a period of 6 months. The incidence of thrombosis was determined within each study period. During the 6 month follow-up, 36 thrombotic events occurred in 27 of 68 polytetrafluoroethylene (PTFE) grafts, and only six thrombotic events in 5 of 63 arteriovenous (AV) fistulas. The relative risk for access thrombosis for patients with PTFE grafts was 5.6 times greater than for patients with AV fistulas. Qac was significantly lower in thrombotic compared with patent PTFE grafts (958 +/- 506 ml/min vs 1141 +/- 482 ml/min, p < 0.05). A significant relationship was found between the incidence of subsequent PTFE graft thrombotic events and Qac (p < 0.001). Compared with accesses with high blood flow (1100-1400 ml/min), the risk for subsequent thrombosis tripled in grafts with a Qac of less than 500 ml/min. This relationship was not seen with AV fistulas. In patent PTFE grafts, Qac remained unchanged within each 2 month interval, whereas it decreased in thrombotic PTFE grafts. Thus, repeated measurements of Qac have the potential to predict future access failure in PTFE grafts; however, an increased measuring frequency might improve the predictive value of graft failure with high Qac.

Female↗

PubMedDate not supplied

Exercise and extracorporeal blood cooling during hemodialysis.

Intradialytic exercise may improve hemodialysis efficiency. Because exercise interferes with thermal energy and fluid balance, relative blood volume changes (deltaBV%), arterial blood temperatures (T(art)), mean arterial blood pressures, and heart rates (HR) were measured using different dialysate temperatures (Tdia). Four stable patients (age, 49.9 +/- 7.7 years) were studied during 22 treatments that either maintained Tdia at 35.9 degrees C +/- 0.1 degrees C (standard) or provided maximum extracorporeal cooling (cool, Tdia = 34.8 degrees C +/- 0.8 degrees C) in attempts to maintain a constant T(art). Patients exercised for 1 hr at a resistance of 21 +/- 5 W on a stationary bicycle ergometer. Energy expenditure monitored by indirect calorimetry increased from 117 +/- 38 W (baseline) to 338 +/- 116 W (exercise). Mean arterial blood pressures increased by 7 +/- 7 mmHg with cool Tdia, but remained unchanged (-1 +/- 4 mmHg) with standard Tdia (p < 0.05). However, the increase in T(art) was smaller with cool (0.1 degrees C +/- 0.3 degrees C) than with standard (0.3 degrees C +/- 0.2 degrees C) Tdia (p < 0.05). The larger increase in O2 uptake per change in HR (68 +/- 56 vs 38 +/- 17 ml/beat) indicated an increase in stroke volume when cool dialysate was used (p = NS). Exercise produced a small (0.95% +/- 0.95%), but significant, decrease in deltaBV% that reversed at the end of exercise. Intradialytic exercise was well tolerated, especially when Tdia was lowered such that hemodynamic stress to dissipate excess heat through the cutaneous circulation was reduced and blood pressure stability was improved.

Blood Volume↗

PubMedDate not supplied

Relative underestimation of fluid removal during hemodialysis hypotension measured by whole body bioimpedance.

Whole body bioimpedance is considered helpful in monitoring the removal of excess body water by ultrafiltration in hemodialysis patients. In this study, the cumulative, estimated decrease in extracellular volume (V(est)) modeled from whole body bioimpedance data was compared with measured volume (Vmeas) removed by ultrafiltration (UFR = 1.01 +/- 0.31 L/hr) in 12 patients during 36 high efficiency hemodialysis treatments. In the mean, estimated (V(est) = 3.0 +/- 1.4 L) and measured volumes (Vmeas = 3.4 +/- 1.1 L) correlated linearly: V(est) = 1.05 x Vmeas - 0.60, r2 = 0.68. Patients developed hypotension in half the treatments. Except for a larger decrease in systolic blood pressures in hypotensive (34 +/- 24 mmHg) vs. stable (14 +/- 15 mmHg) treatments, patient and treatment characteristics were not different between groups. However, at the end of hemodialysis, the difference V(est) - Vmeas was -0.8 +/- 0.9 L in hypotensive, and only 0.1 +/- 0.4 L in stable patients (p < 0.05). The difference between V(est) and Vmeas can be explained by a predominant removal of excess body water from central body compartments such as the trunk and the central blood volume during hypotension. These compartments are not adequately measured by whole body bioimpedance techniques. However, this information could be helpful in identifying patients with delayed peripheral fluid removal that may occur when either target weight is too low or UFR rates are too high.

Adult↗

PubMedDate not supplied

Theoretical and practical issues in recirculation; assessment of vascular access.

Haemodialysis recirculation is defined as the fraction of cleared extracorporeal blood flow which returns to the inlet of the extracorporeal blood line without systemic equilibration. There are two components of haemodialysis recirculation: the local component is related to access function and placement of access needles; the cardiopulmonary component is a characteristic of the peripheral arterio-venous access where access blood flow bypasses systemic tissue compartments. Identification of access problems requires separation of the two components using newly developed indicator dilution techniques such as ultrasound dilution. If such techniques are not available and recirculation is determined by techniques which measure combined effects of recirculation such as the urea technique, a second recirculation measurement with reversed placement of blood lines will permit us to distinguish between correct and reversed placement of blood lines. The larger of the two recirculation values can be used to identify accesses with insufficient access flow and access recirculation which require immediate intervention.

Arteriovenous Shunt, Surgical↗

PubMedDate not supplied

A regional blood circulation alternative to in-series two compartment urea kinetic modeling.

Assuming that the clearance of urea from total body water (TBW) is flow limited, the authors developed a parallel flow model using physiologic data. Organ systems with a blood flow to water volume ratio of greater than 0.2 min-1 were allocated to the high flow system. Remaining organs were represented in the low flow system. In end-stage renal disease patients with minimal renal blood flow, the high flow system contained 20% TBW and received 70% of the systemic blood flow. The authors used this flow heterogeneity to predict the post-dialysis urea rebound (R) in 12 patients after 1 hr of hemodialysis. Dialyzer clearance was 248 +/- 14.5 ml/min (mean +/- SEM) Access recirculation was obviated by returning cleared blood into a central vein. In these patients, R at 1, 3, 5, 7, 10, and 15 minutes. after slowing dialyzer blood flow (Qb) from 383 +/- 18 to 50 ml/min was 3.8 +/- 2.9, 6.2 +/- 3.4, 7.6 +/- 3.1, 8.8 +/- 3.9, 9.0 +/- 4.1, and 9.9 +/- 4.4%, respectively. CO and QAc were modeled with values of 5.5 and 0.5 L/min, respectively. The modeled TBW was 35 L. Total body water derived by nomogram was 38.1 +/- 2.0 L. Our results suggest that the parallel-flow model for urea transport can be used to explain the amount and time course of post dialysis R on a physiologic basis.

Blood Flow Velocity↗