Improved clearance of iohexol with longer haemodialysis despite similar Kt/V for urea

Dialysis and iodinated contrast media

Contrast media are excreted mainly by glomerular filtration. There is thus, a significant correlation between both body and renal clearances of contrast media and glomerular filtration rate, and their renal excretion will be delayed in patients with renal insufficiency. Contrast media can be efficiently removed from blood by hemodialysis (HD). Since most contrast media are middle-sized molecules, the main factors potentially influencing their removal by HD are blood flow, membrane surface area, molecular size, transmembrane pressure, and dialysis time. Peritoneal dialysis is also effective in removing contrast agents from the body but takes longer than HD. Dialysis immediately after radiographic contrast studies has been suggested for two groups of patients. Those on chronic HD and those at very high risk for contrast nephropathy. Three studies have examined the necessity of immediate dialysis after intravascular injection of contrast media in chronic HD patients; the authors found no evidence that it is effective at preventing contrast nephropathy. The reasons why HD treatment was not beneficial in those three studies are not known. Perhaps, the rapid onset of renal injury after administration of contrast media is one answer. It is also possible that HD per se was nephrotoxic and might have offset the beneficial effect of the removal of contrast media. Marenzi et al. randomized 114 consecutive patients with chronic renal failure undergoing coronary interventions to either hemofiltration in an intensive care unit or isotonic saline hydration. The authors concluded that periprocedural hemofiltration given in an intensive care unit setting appears to be effective in preventing the deterioration of renal function due to contrast agent induced nephropathy and is associated with improved in-hospital and long term outcomes. The concentration of contrast media can effectively be reduced by HD and peritoneal dialysis. HD does not offer any protection against contrast media induced nephrotoxicity. Hemofiltration may decrease the risk of contrast induced nephropathy and have some long-term benefits, but additional studies are needed to better define the appropriate population for this treatment.

Associations of hemodialysis dose and session length with mortality risk in Australian and New Zealand patients

The optimal combination of hemodialysis (HD) dose and session length remains uncertain, and previous studies have not conclusively shown session length to be an important independent determinant of patient mortality. The objective of this study was to examine associations between HD dose and session length with mortality risk using data from the Australian and New Zealand Dialysis and Transplant Registry. Analyses were performed using a prospective inception cohort comprising all incident adult patients treated by thrice-weekly maintenance HD, who commenced renal replacement therapy with HD between 1 April 1997 and 31 March 2004. In all, 6593 patients were identified, of whom 4193 had sufficient data for multivariate analyses. HD dose (single pool fractional clearance of urea, Kt/V) and session length were included in analyses as those recorded 12 months after HD inception to reduce confounding by residual renal function. The outcome examined was patient mortality. Survival analyses included Kaplan-Meier calculations of survival and Cox regression for multivariate analyses. Covariates in Cox models included patient demographics, co-morbid medical conditions at HD inception, and HD operating parameters. After adjustment for covariates and each other, Kt/V of 1.30-1.39 and session length of 4.5-4.9 h were associated with the lowest mortality risk. There was no interaction between HD dose and session length. Thus, the optimal combination for mortality appears to be Kt/V of > or = 1.3 and session length of > or = 4.5 h. These data suggest a randomized controlled trial to test these hypotheses, and support the inclusion of criteria relating to session length in definitions of adequate HD practice.

Radionuclide method for evaluating the performance of hemodialysis in vivo

BACKGROUND

Specifications of dialyzer performance are generally based on in vitro measurements. There is, however, a shortage of data on dialyzer performance in vivo. The aim of this study was to use continuous measurement of technetium-99m-diethyltriaminepentaacetic acid (Tc-99m-DTPA) blood concentration as a means of continuously monitoring dialyzer function in vivo in patients undergoing routine hemodialysis.

METHODS

The study population comprised 15 patients (45 to 80 years old; 13 males). Tc-99m-DTPA was administered intravenously 90 minutes before obtaining a blood sample and starting dialysis. Blood Tc-99m-DTPA activity was continuously monitored by passing the line carrying blood from the patient to the dialyzer close to a scintillation probe mounted in a shielded housing. At the end of hemodialysis, lasting 180 to 300 minutes, chromium-51-ethylenediaminetetraacetic acid (Cr-51-EDTA) was given intravenously and a blood sample taken 90 minutes later. Baseline dialyzer blood flow (Q(b)) and dialysate flow (Q(d)) were 250 to 350 mL/min and 500 mL/min, respectively. The rate constant, alpha, of the decrease in blood Tc-99m-DTPA activity was used as the measure of moment-to-moment dialyzer function. Pre- and postdialysis extracellular fluid volumes were calculated from the blood Tc-99m-DTPA and Cr-51-EDTA concentrations (V(DTPA) and V(EDTA)) before and after dialysis. Tc-99m-DTPA clearance was measured as the product of alpha and V(DTPA). Dialyzer urea clearance was calculated from pre- and postdialysis urea nitrogen concentrations and the time of dialysis. The effects of brief changes in Q(b) and Q(d) on dialyzer function were assessed from the associated changes in alpha.

RESULTS

The Tc-99m-DTPA clearance profile was biexponential, becoming monoexponential about 1 hour after starting hemodialysis, with alpha remaining constant for as long as dialysis continued in five patients in whom Q(b) and Q(d) were left unaltered. Mean (SEM) plasma Tc-99m-DTPA clearance averaged over the entire period of dialysis in all 15 patients was 110 (3.1) mL/min. It correlated with urea clearance (r= 0.71) (P < 0.01) which was 225 (9.5) mL/min based on a total body water of 2.5 that of V(DTPA) and 212 (13) mL/min scaled to 40 L/1.73 m(2). Extracellular fluid volume decreased by 1.73 (0.74) l over dialysis, which was comparable to the change in weight [1.48 (0.57) kg]. The extraction fraction of Tc-99m-DTPA across the artificial kidney, directly measured from afferent and efferent blood samples under baseline Q(b) and Q(d), was 0.5 (0.013). Average extraction fraction indirectly estimated from Tc-99m-DTPA blood clearance and Q(b) was 0.54 (0.019). These two measurements of extraction fraction correlated with each other under conditions of varying Q(b) and Q(d) (r= 0.74) (N= 27) (P < 0.001). Changes in alpha resulting from changes in Q(b) and Q(d) were similar to changes predicted from computerized modeling. The ratio of mass transfer coefficients of urea and Tc-99m-DTPA with respect to the dialyzer, calculated as if they were permeability-surface area products, was 3.3, similar to the ratio, obtained from the literature, in continuous capillary endothelium.

CONCLUSION

Tc-99m-DTPA is a useful agent for continuously monitoring dialyzer function in vivo and provides a platform for the use of other radio-pharmaceuticals of different molecular sizes that could be used in an analogous fashion.

Contrast agents in patients on dialysis

Contrast media are used to enhance the structural and functional information that is provided by imaging methods. They are used particularly in conventional radiographic (X-ray) investigations, but are also used in magnetic resonance imaging (MRI) and increasingly in ultrasound (US). The water-soluble, iodinated, intravascular radiographic agents are commonly used, and in large doses. They are excreted almost exclusively by the kidney and may have nephrotoxic effects. The use of these agents in patients with renal impairment may potentially be problematic. In this article the chemical composition and pharmacokinetics of these contrast agents are reviewed, as are their potentially toxic effects on the heart, neural tissues, kidney, and endothelium. Their safety for use in the patient with end-stage renal disease (ESRD) and those on dialysis is discussed. This aspect is also addressed briefly for the contrast agents used to augment MRI and US.

Unrestricted pore area (A0/Deltax) is a better indicator of peritoneal membrane function than PET

BACKGROUND

How to measure the peritoneal exchange in uremic patients treated with peritoneal dialysis (PD) is still a matter of controversy. Most clinics use the peritoneal equilibration test (PET), but from a theoretical point of view, it would be more appropriate to determine the "area" parameter, A0/Deltax. The latter reflects the total unrestricted pore area per centimeter diffusion distance and can be obtained by three-pore analysis using, for example, the PD capacity test (PDC). To evaluate the different estimates of peritoneal function, PET data and the A0/Deltax parameters were compared with the independently determined uptake of a small diffusible tracer, iohexol (molecular weight of 821 D), from the abdominal cavity to blood.

METHODS

Fourteen patients on routine PD underwent determinations of PET and A0/Deltax using PDC. Within a month, the two-hour uptake of iohexol (6 mg/mL) was also determined from the plasma iohexol concentration following abdominal filling.

RESULTS

A strong correlation was found between the rate of iohexol plasma concentration increase (k30-120) and A0/Deltax (A0/Deltax = 76,300. k30-120 - 1.56; r2 = 0.799; N = 14) for the 2 L dwell, while the PET data were far less related to iohexol uptake (D/DPurea, r2 = 0.409; D/Pcreatinine, r2 = 0.436; and D/D0glucose, r2 = 0.015, respectively).

CONCLUSION

The "area" parameter, A0/Deltax, is superior to the more widely used routine PET as an indicator of peritoneal membrane function. In addition, the concept of A0/Deltax has the virtue of supplying quantitative information about the peritoneal pathophysiology and physiology.