Pressure profile analysis at hemodialysis needle: a new method for early detecion of vascular access stenoses

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The accuracy of volume flow measurements derived from pulsed wave Doppler: a study in the complex setting of forearm vascular access for hemodialysis

Purpose: Maturation of an arterio-venous fistula (AVF) frequently fails, with low postoperative fistula flow as a prognostic marker for this event. As pulsed wave Doppler (PWD) is commonly used to assess volume flow, we studied the accuracy of this measurement in the setting of a radio-cephalic AVF. Methods: As in-vivo validation of fistula flow measurements is cumbersome, we performed simulations, integrating computational fluid dynamics with an ultrasound (US) simulator. Flow in the arm was calculated, based on a patient-specific model of the arm vasculature pre and post AVF creation. Next, raw ultrasound signals were simulated, from which the Doppler spectra were calculated in both a proximal (brachial) and a distal (radial) location. Results: The velocity component in the direction of the US beam, in a centred, small, sample volume, can be captured accurately using PWD spectrum mean-tracking. However, deriving flow rate from these measurements is prone to errors: (i) the angle-correction which is influenced by the radial velocity components in the complex flow field; (ii) the largest error is introduced due to a lack of knowledge on the spatial flow profile

Swirlgraft versus conventional straight graft as vascular access: a full CFD-analysis

Two 3D models of an arterio-venous graft, a connection between an artery and a vein as vascular access for hemodialysis, were studied. One model of a conventional straight loop graft, the other of a graft with helical configuration (e.g. SwirlGraft (Veryan Medical, London, UK)). The statement that the helical design reduces Intimal Hyperplasia (IH) formation was studied by evaluating low wall shear stress and high oscillatory shear stress zones next to the helicity flow index. The IH-inducing zones were reduced but were not eliminated and the helicity of the flow was increased. The statement that the SwirlGraft avoids stenosis should however be considered with care in clinical practice

Detection of hemodialysis vascular access stenosis by intravascular pulse pressure analysis: an in-vitro study

Purpose: Vascular access (VA) thrombosis, due to significant stenoses (>50%), is the main cause of VA failure in hemodialysis patients. Flow measurements enable detection of stenoses >70% and not >50%. Flow measurements regularly fail to prevent thrombosis. The purpose of the study was to test a new technique for detection of significant stenoses (>50%). Methods and Materials: A pulsatile in-vitro model of a radio-cephalic arteriovenous fistula with silicone tubes, a reservoir and a pump was created. A 15G needle was introduced at 5 and 10 cm downstream of the anastomosis. Intravascular pulse pressure amplitude (systolic minus diastolic pressure = PP) was measured in the arterial inflow and at the arterial needle. PP ratios were calculated (PP-needle/PP-inflow*100%). A 50% stenosis was introduced in the arterial inflow, between needles and in the venous outflow, successively. Measurements were repeated at different heart rates (60-90 beats/min) and different flows (500-1.300 ml/min). ANOVA analysis and post-hoc tests were used to evaluate the relation between the PP ratio and the presence of a stenosis in different conditions. Results: PP ratios were 20.26 ± 4.55% (no stenosis), 7.69 ± 2.08% (arterial inflow stenosis), 36.20 ± 2.12% (between needles stenosis) and 32.38 ± 2.17% (venous outflow stenosis). Stenoses can be located upstream and downstream of the needle (P < 0.001). Between needles stenoses and venous outflow stenoses could also be distinguished (P < 0.001). Conclusion: Pulse pressure analysis enables detection of 50% stenosis independent of heart rate and flow volume. It also enables stenoses localization, in contrast to flow measurements. This promising new method needs clinical validation

The multigate Doppler approach for assessing hemodynamics in a forearm vascular access for hemodialysis purposes

We compared the multigate and single gate pulsed wave Doppler (PWD) approach to assess the hemodynamics in an arteriovenous fistula for hemodialysis purposes. In particular, we evaluated volume flow and wall shear rate measurements based on PWD because: (i) volume flow measurements are typically performed to assess the maturation of the arteriovenous fistula (AVF), with low postoperative fistula flow being a prognostic marker for failure of the AVF; (ii) wall shear stress may play an important role in vascular remodeling processes of the fistula. As in-vivo validation of fistula flow measurements is cumbersome, we performed simulations integrating computational fluid dynamics with an ultrasound (US) simulator. Flow in the arm was calculated, based on a patient-specific model of the arm vasculature pre and post AVF creation. Next, raw ultrasound signals were simulated and processed using a multigate and single gate Doppler approach in the radial artery, both for the preop and postop setting. Results showed that the extreme postop flow conditions (high velocity magnitudes and complex flow profiles) hampered accurate assessment of both wall shear rate and volume flow, while reasonable estimates were obtained preop (lower velocity magnitudes and laminar flow)

Experimental Study of a New Method for Early Detection of Vascular Access Stenoses: Pulse Pressure Analysis at Hemodialysis Needle

Hemodialysis vascular access stenosis remains a frequent complication. However; early detection is challenging and costly. The aim of this in-vitro study was to assess a new detection method based on pulse pressure analysis at the hemodialysis needle. A silicon model of a radio-cephalic arteriovenous fistula was built in a mock loop. Pressure profiles were measured at the arterial hemodialysis needle and in the proximal feeding artery. Stenoses (50% and 25% diameter reduction) were created proximal to the anastomosis (proximal artery) and distal to the arterial needle (distal vein and proximal vein). The pulse pressure at the needle was divided by the pulse pressure at the feeding artery to obtain a dimensionless ratio, %PP. Experiments were conducted at different blood flow (500 to 1200 ml/min) and heart rates (60 to 90 beats/minute) to test this new index over a wide range of hemodynamic conditions. In the control model (no stenosis), %PP was 20.26±4.55. A proximal artery 50%-stenosis significantly decreased %PP to 7.69±2.08 (P<0.0001), while presence of 50%-stenosis in the distal (36.20±2.12%) and proximal (32.38±2.17) vein lead to significantly higher values of %PP (P<0.0001). For stenosis of 25% diameter reduction in the proximal artery, the %PP decreased to 15.45±2.13 (P=0.0022) and the %PP increased with a 25%-stenosis in the distal vein to 26.71±3.01 (P=0.0003) and in the proximal vein to 26.53±2.67 (P=0.0004). This in-vitro study shows that the analysis of the pulse pressure at the dialysis needle is useful for early detection and localization of hemodialysis vascular access stenosis, independent of heart rate and flow level