The effects of hemodialysis on platelet deposition in prosthetic graft fistulas

Vascular access problems in dialysis patients: pathogenesis and strategies for management

Repetitive hemodialysis was made possible through the development of a chronic means of accessing the circulation. This was accomplished through the use of arteriovenous fistulae or grafts, using autologous veins or synthetic materials. Although the arteriovenous fistula remains the access of choice, synthetic arteriovenous grafts are used in most patients because of problems with late referral to a nephrologist and poor vasculature. This article describes the means of accessing the circulation for hemodialysis, the pathogenesis of access failure through progressive stenosis followed by thrombosis, methods of detecting access dysfunction before thrombosis, and therapeutic options. Although angiographic or surgical intervention remain the mainstays of management, medical treatments to decrease stenosis and delay thrombosis are currently under investigation.

Serum thrombopoietin levels in haemodialysis patients: involvement of arteriovenous fistula

BACKGROUND

Thrombopoietin (Tpo) is a recently cloned growth factor which plays a critical role in the regulation of thrombopoiesis. Tpo has also been shown to stimulate in vitro and in vivo erythroid cell growth. Although Tpo transcripts were detected in hepatocytes, proximal tubules and endothelium, mechanisms regulating the level of circulating Tpo have not been fully delineated. Changes in the vessel wall and blood flow in arteriovenous fistula (AVF) might alter Tpo activity.

METHODS

Serum thrombopoietin levels and serum erythropoietin levels in samples concurrently obtained from venous returns of AVF and contralateral peripheral veins in 31 haemodialysis patients were determined and compared with 12 healthy controls. Levels were also compared between 14 haemodialysis patients (group I) treated with recombinant human erythropoietin (rHu-Epo) and 17 haemodialysis patients (group II) not requiring rHu-Epo.

RESULTS

Serum Tpo levels (44.8 +/- 23.9 pg/ml, vs 129.9 +/- 113.6 pg/ml, P<0.05) and platelet counts (194 +/- 55, 10(6)/ml vs 273 +/- 94. 10(6)/ml, P<0.05) of haemodialysis patients were lower than healthy controls. Serum Tpo levels were inversely correlated with platelet counts in the control group (R=-0.61, P<0.05), but not in haemodialysis patients. Tpo concentrations of AVF samples were lower than peripheral venous samples (31.6 +/- 17.7 pg/ml vs 44.8 +/- 23.9 pg/ml, P=0.001). No significant difference was present between the serum Tpo concentrations of haemodialysis patients in group I and group II. Serum Tpo levels were not correlated with haemoglobin levels or serum erythropoietin levels in haemodialysis patients.

CONCLUSION

Decreased serum Tpo levels despite low platelet counts in haemodialysis patients suggest that the proposed feedback mechanism of platelet uptake of Tpo is not fully operative in these patients. Moreover, AVF might affect the local production and/or catabolism of this growth factor.

Effects of antiplatelet drugs on dialysis-associated platelet deposition in polytetrafluoroethylene grafts

Hemodialysis is associated with platelet deposition in polytetrafluoroethylene (PTFE) graft fistulas. We determined whether aspirin or ticlopidine would modify this response. Patients on chronic hemodialysis with forearm loop PTFE fistulas were studied. Platelets labeled with 111indium were injected and a baseline scan of the fistula arm was made with a large field of view gamma camera. After a routine dialysis treatment, a second scan was performed within 1 hour. Four weeks later, a repeat labeled platelet study was conducted after taking either aspirin 325 mg/d or ticlopidine 250 mg/d orally for 7 days. Images were computer analyzed by drawing seven standardized regions along each graft. The counts per second per pixel in postdialysis images were compared with predialysis images for each region and a percent uptake compared with the predialysis image was calculated. Regions with dialysis-induced uptake of more than 1.5-fold compared with the predialysis image before antiplatelet drug therapy were compared with these same regions after therapy. Six patients were studied before and after aspirin therapy. Uptakes larger than 1.5-fold over predialysis images were found in 12 of 40 regions and were 292% +/- 50% (+/-SEM) before and 193% +/- 25% of predialysis values after aspirin (P = 0.02, paired t-test). Uptakes in the remaining regions were 107% +/- 4% before and 115% +/- 6% after aspirin (P = NS). A second group was studied before and after ticlopidine (n = 5). Uptakes increased by more than 1.5-fold compared with predialysis images in 19 of 30 regions and had a median of 286% increased uptake (mean, 785% +/- 374%) before and 160% (153% +/- 10%) after drug therapy (P < 0.001, Wilcoxon). Uptakes in the remaining regions were 116% +/- 5% before and 134% +/- 13% after drug therapy (P = NS). Platelet aggregation studies suggested compliance with both drugs. These studies show that these antiplatelet drugs reduce, but do not completely prevent, dialysis-associated radiolabeled platelet deposition in PTFE grafts.

Increased reticulated platelets in dialysis patients

The purpose of this study was to measure the percent reticulated platelets and platelet counts in patients on chronic hemodialysis, peritoneal dialysis and normal volunteers. The relationship between the percent reticulated platelets and the platelet count can then be used to determine the rate of platelet turnover. Platelet rich plasma was obtained, platelets were fixed and incubated with thiazole orange and analyzed for the percent reticulated platelets by flow cytometry. Normal controls had a mean of 2.77 +/- 0.17% reticulated platelets while peritoneal dialysis patients had a mean percent reticulated platelets of 6.92 +/- 0.68 (P < 0.00001). Chronic hemodialysis patients had a mean percent reticulated platelets of 8.21 +/- 0.36 (P < 0.00001 vs. normal controls and P = 0.05 vs. peritoneal dialysis patients). Platelet counts did not differ significantly among the three groups. The identity of reticulated platelets was confirmed in experiments measuring platelet specific glycoproteins, experiments using RNase, and in mixing experiments with normal and uremic platelets and plasma. We conclude that dialysis patients have a marked increase in circulating reticulated platelets compared to normal controls, indicating accelerated platelet turnover. Increased platelet activation and turnover may contribute to the qualitative platelet dysfunction observed in dialysis patients.