Analysis of novel geometry-independent method for dialysis access pressure-flow monitoring

Theoretical Biology and Medical Modelling Analysis of novel geometry-independent method for dialysis access pressure-flow monitoring William F Weitzel 1 Casey L Cotant 1 Zhijie Wen 0 Rohan Biswas 1 Prashant Patel 1 Harsha Panduranga 1 Yogesh B Gianchandani 0 Jonathan M Rubin 1 0 College of Engineering, University of Michigan , Ann Arbor, MI , USA 1 School of Medicine, University of Michigan , Ann Arbor, MI , USA 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 (P) 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 P 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 P as a function of needle position, were used to devise and test a method of estimating Q using P and variable dialysis pump speeds (variable flow) that diminishes dependence on geometric factors and fluid characteristics. Results: In the fluid circuit model, P increased with increasing volume flow rate and with increasing needle-separation distance. A nonlinear model closely predicts this P-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 P waveform data were also observed during the mock dialysis treatment, which may be useful in detecting low or reversed flow within the access. Conclusion: With further experimentation and needle design, this geometry-independent approach may prove to be a useful access flow monitoring method. - Background Dialysis access blood volume flow and pressure may be helpful parameters in end-stage renal disease (ESRD) vascular access monitoring. [1-5] The magnitude of the clinical problem is well recognized, with 330,000 dialysis patients with ESRD in the U.S., and the cost of maintaining dialysis access in the care of these patients is over $1 billion in the U.S. alone, which represents approximately 10% of the total cost of dialysis care.[6,7] The recently updated National Kidney Foundation (NKF) Dialysis Outcomes and Quality Initiative (DOQI) recommendations have reaffirmed the recommendation for monitoring using monthly measurement of flow or static venous pressure as the preferred methods.[8] Monthly flow monitoring may lead to as much as a 50% reduction in access failure,[9] yet this number still represents 25% of patients with grafts experiencing failure (thrombosis or clotting) per year, which requires emergency treatment to re-establish flow. Divergent opinions exist about the utility of flow monitoring, partly fueled by the relatively infrequent (e.g., monthly) flow monitoring interval. [10-12] Since it may be practical to follow access pressure more frequently,[13] some have advocated pressure monitoring over flow monitoring.[14] Additionally, it should be noted that other data support the cost effectiveness of access flow monitoring even when performed less frequently,[15] and that the combined sensitivity and specificity improves,[16] and cost effectiveness improves,[17] when flow monitoring frequency is increased. Our group is investigating the possibility of using differential pressure (P) monitoring to estimate access flow for dialysis access monitoring, with the current study aimed at developing and testing an access geometry-independent algorithm that is convenient to perform throughout dialysis or at least at every dialysis session. The underlying assumption is that flow along with pressure monitoring may be a more complete representation of the hemodynamic status of the access. Furthermore, frequent and convenient flow estimations may improve monitoring by determining each patient's mean access flow and standard deviation in flow. Additionally, this would allow the change in access blood flow with ultrafiltration and blood pressure reduction to be followed, just as blood pressure and various machine parameters are followed during dialysis. However, several engineering problems must be addressed to make this approach clinically practical. While pressure measurements within the access have been used as an indicator of stenosis (which partially obstructs flow and alters access pressure), pressure differences within the dialysis graft or fistula have not typically been used to estimate flow. This is primarily because wellestablished fluid dynamics models require knowledge or estimation of access geometry, needle separation, and fluid properties, such as viscosity, to determine flow.[18] This study derived experimental data on the relationship between access flow and P between two dialysis access needles in a model of the arteriovenous graft (AVG) vascular circuit. This geometry-dependent data was used to devise methods and perform experiments that estimate access flow using P and variable dialysis pump speeds while being mathematically independent of geometric factors and fluid characteristics. We present a potentially useful geometry-independent method, modeling data, and experimental results for flow determination using intra-access P and its dependence on dialysis pump speed. Implementation of this method will require the development of new dialysis needle technology or intraaccess P measurement devices to allow for intra-access pressure measurement during dialysis, work that is currently in progress. These data suggest that this approach or subsequent permutations may result in easy to use, operator-independent alternative methods of access monitoring to improve future access monitoring strategies. Materials and methods Experimental Steady-Flow AVG Circuit A fluid circuit model of the AVG vascular circuit was developed to study the relationship between access flow (Q) and the P between two dialysis access needles placed 2.5, 5, 10, 15, and 20 cm from one another within the circuit. A Masterflex Console Drive non-pulsatile blood roller pump (Cole Parmer, Vernon Hills, IL) was utilized to draw a glycerol-based fluid, with a kinematic velocity of 0.029 cm2/s (corresponding to a hemat (...truncated)