title: Analysis of novel geometry-independent method for dialysis access pressure-flow monitoring.
abstract: BACKGROUND
  End-stage renal disease (ESRD) confers a large health-care burden for the United States, and the morbidity associated with vascular access failure has stimulated research into detection of vascular access stenosis and low flow prior to thrombosis. We present data investigating the possibility of using differential pressure (DeltaP) monitoring to estimate access flow (Q) for dialysis access monitoring, with the goal of utilizing micro-electro-mechanical systems (MEMS) pressure sensors integrated within the shaft of dialysis needles.
  
  
  METHODS
  A model of the arteriovenous graft fluid circuit was used to study the relationship between Q and the DeltaP between two dialysis needles placed 2.5-20.0 cm apart. Tubing was varied to simulate grafts with inner diameters of 4.76-7.95 mm. Data were compared with values from two steady-flow models. These results, and those from computational fluid dynamics (CFD) modeling of DeltaP as a function of needle position, were used to devise and test a method of estimating Q using DeltaP and variable dialysis pump speeds (variable flow) that diminishes dependence on geometric factors and fluid characteristics.
  
  
  RESULTS
  In the fluid circuit model, DeltaP increased with increasing volume flow rate and with increasing needle-separation distance. A nonlinear model closely predicts this DeltaP-Q relationship (R2 > 0.98) for all graft diameters and needle-separation distances tested. CFD modeling suggested turbulent needle effects are greatest within 1 cm of the needle tip. Utilizing linear, quadratic and combined variable flow algorithms, dialysis access flow was estimated using geometry-independent models and an experimental dialysis system with the pressure sensors separated from the dialysis needle tip by distances ranging from 1 to 5 cm. Real-time DeltaP waveform data were also observed during the mock dialysis treatment, which may be useful in detecting low or reversed flow within the access.
  
  
  CONCLUSIONS
  With further experimentation and needle design, this geometry-independent approach may prove to be a useful access flow monitoring method.
authors.affiliation: School of Medicine, University of Michigan, Ann Arbor, MI, USA. weitzel@umich.edu
authors.forename: William F
authors.initials: WF
authors.lastname: Weitzel
authors.forename: Casey L
authors.initials: CL
authors.lastname: Cotant
authors.forename: Zhijie
authors.initials: Z
authors.lastname: Wen
authors.forename: Rohan
authors.initials: R
authors.lastname: Biswas
authors.forename: Prashant
authors.initials: P
authors.lastname: Patel
authors.forename: Harsha
authors.initials: H
authors.lastname: Panduranga
authors.forename: Yogesh B
authors.initials: YB
authors.lastname: Gianchandani
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Rubin
mesh_terms: D000465:Algorithms; D001166:Arteriovenous Shunt, Surgical / Q000295:instrumentation* / Q000379:methods*; D001783:Blood Flow Velocity / Q000502:physiology; D019295:Computational Biology / Q000295:instrumentation / Q000379:methods; D003956:Dialysis / Q000295:instrumentation / Q000379:methods; D006801:Humans; D008954:Models, Biological; D011312:Pressure