Ilse Van Tricht

Hemodynamics and complications encountered with arteriovenous fistulas and grafts as vascular access for hemodialysis : A review

This review article describes the current state of affairs concerning in vivo, in vitro and in numero studies on the hemodynamics in vascular access for hemodialysis. The use and complications of autogenous and non-autogenous fistulas and catheters and access port devices are explained in the first part. The major hemodynamic complications are stenosis, initiated by intimal hyperplasia development, and thrombosis. The different in literature proposed conceivable causes of intimal hyperplasia development like surgical interventions, compliance mismatch, wall shear stress (WSS) and shear rate, vessel wall thrill and blood pressure are discussed on the basis of in vivo, in vitro and in numero studies.

Hemodynamics in a compliant hydraulic in vitro model of straight versus tapered PTFE arteriovenous graft

BACKGROUND Hemodialysis patients require a vascular access to deliver sufficient blood flow to the artificial kidney. Of these vascular accesses, 30% are prosthetic (mainly polytetrafluorethylene [PTFE]) graft implants. These grafts are prone to the development of stenosis in the vein due to intimal hyperplasia, subsequently leading to thrombosis and graft failure. AIM We investigated the hemodynamics in a straight and a tapered PTFE-graft and compare the hydrodynamical behavior of both grafts. MATERIALS AND METHODS Two different vascular access geometry models were examined: a 6-mm diameter straight graft and a 4- to 7-mm tapered graft. The grafts were sutured to a compliant silicon model of an artery and vein in a loop configuration. Flow rate varied between 500 and 1500 mL/min. Two conditions were tested: 1). control: mean pressure is 100 mm Hg at the arterial inlet; and 2). low resistance condition: pressure is 20 mm Hg at the venous outlet. Pulse pressure is 60 mm Hg at the arterial inlet for both conditions. Pressure and flow velocity are measured continuously, while flow rate is measured volumetrically. RESULTS The pressure drop at the arterial anastomosis of the tapered graft is three times higher compared to the straight graft model. Intragraft pressure drops are similar in both graft types. Mean pressure and pulse pressure in the graft and vein are decreased in the low resistance condition. Also, the difference between maximum and minimum velocity is smaller in this. CONCLUSIONS No significant differences are noted between the graft geometries: pressure drop over the graft is almost equal. The major difference is the higher pressure drop at the arterial anastomosis of the tapered graft.

Experimental analysis of the hemodynamics in punctured vascular access grafts

The hemodynamics in the vascular access graft are influenced by the flow aspirated and injected through the two needles during hemodialysis. For the first time, the impact of needle flow on vascular access performance, measured in an in vitro set up, is reported. A vascular access model, consisting of a loop polytetrafluoroethylene graft sewn to a compliant artery and vein, simulated the patient. The extracorporeal circuit was connected to the model. Three mean access flow rates (QG; 500, 1,000, and 1,500 ml/min) and five roller pump flow rates (QR; 0, 200, 300, 400, and 500 ml/min) were studied. Mean, systolic, and diastolic pressure and according pressure drops were derived at 14 loci. Systolic, diastolic, and mean pressures drop along the graft decreased with increasing QR and decreasing QG. At QR = 500 ml/min and QG = 500 ml/min, the mean pressure drop over the graft was negative (–10 mm Hg), indicating a reversed pressure profile, originating at the puncture site of the venous needle. Mean pressure in the venous outlet segment was about 100 mm Hg compared with only 75 mm Hg without needle flow. The combination of a low QG (500 ml/min) and high QR (>300 ml/min) must be avoided because venous pressures can rise to 100 mm Hg and load the venous system. The results of this in vitro setup indicate that high QR (>400 ml/min) should be avoided at QG up to 1,000 ml/min; however, in vivo tests have to be performed to prove this thesis. This study demonstrates the need for a well-functioning vascular access (QG > 600 ml/min) to perform adequate dialysis and to avoid venous system loading.

Assessment of the Tilting Properties of the Human Mitral Valve during Three Main Phases of the Heart Cycle: An Echocardiographic Study

Rationale and Objectives: In experimental models of the left heart, the mitral valve (MV) is commonly implanted perpendicular to a central axis of the apex/MV. To adapt this to a more correct anatomical model, as well as for further studies of the left ventricle, we created a database of implantation angles of the MV and annulus during three main phases of the heart cycle, based on standard cardiac ultrasound measurements. Materials and Methods: Twenty‐eight patients were studied with the standard cardiac ultrasound equipment. From the apical echo window, an anteroposterior (AP) plane and a perpendicular commisure‐commisure (CC) plane were generated during three critical moments in the heart cycle: systole (S); diastole early filling (E); and diastole late filling (A). In both planes, the angles between the annular plane and each mitral leaflet, as well as the angle between a theoretical longitudinal axis through the apex and center of the MV orifice and the mitral annulus plane, were measured with a custom‐made application of Matlab R14. Results: We observed an inclination of the angle mitral annulus/central left ventricle axis, with its lowest point in the direction of the aortic valve (AP plane) of 85°± 7° in systole (S), 88°± 8° in early diastole (E), and 88°± 7° in late diastole (A). In the CC plane, we observed an almost horizontal implantation of 91°± 5° in systole (S), 91°± 8° in early diastole (E), and 91°± 7° in late diastole (A).