An experimental and numerical study of the flow and mass transfer in a model of the wearable artificial kidney dialyzer.

BACKGROUND: Published studies of the past decades have established that mass transfer across the dialyzer membrane is governed by diffusion, convection and osmosis. While the former is independent of the pressure in the liquids, the latter two are pressure dependent and are enhanced when the pressure difference across the membrane is increased. The goal of the present study is to examine the impact of pulsatile flow on the transport phenomena across the membrane of a high-flux dialyzer in a wearable artificial kidney (WAK) with a novel single small battery-operated pulsatile pump that drives both the blood and dialysate in a counter-phased manner, maximizing the trans-membrane pressure. METHODS: Both in-vitro experimental and numerical tools are employed to compare the performance of the pulsatile WAK dialyzer with a traditional design of a single-channel roller blood pump together with a centrifugal pump that drives the dialysate flow. The numerical methods utilize the axisymmetric Navier-Stokes and mass transfer equations to model the flow in the fibers of the dialyzer. RESULTS: While diffusion is still the dominating transport regime, the WAK pump enhances substantially the trans-membrane pressure and thus increases mass convection that might be as high as 30% of the overall transfer. This increase is obtained due to the design of the pulsatile WAK pump that increases ultrafiltration by increasing the trans-membrane pressure. CONCLUSIONS: The experimental and numerical results revealed that when pumping at similar flow rates, a small battery-operated pulsatile pump provides clearances of urea and creatinine similar as or better than a large heavy AC-powered roller pump.

A wearable haemodialysis device for patients with end-stage renal failure: a pilot study.

BACKGROUND: More frequent haemodialysis can improve both survival and quality of life of patients with chronic kidney disease. However, there is little capacity in the UK to allow patients to have more frequent haemodialysis treatments in hospital and satellite haemodialysis units. New means of delivering haemodialysis are therefore required. Our aim was to assess the safety and efficiency of a wearable haemodialysis device. METHODS: Eight patients with end-stage kidney failure (five men, three women, mean age 51.7 [SD 13.8] years) who were established on regular haemodialysis were fitted with a wearable haemodialysis device for 4-8 h. Patients were given unfractionated heparin for anticoagulation, as they would be for standard haemodialysis. FINDINGS: There were no important cardiovascular changes and no adverse changes in serum electrolytes or acid-base balance. There was no evidence of clinically significant haemolysis in any patient. Mean blood flow was 58.6 (SD 11.7) mL/min, with a dialysate flow of 47.1 (7.8) mL/min. The mean plasma urea clearance rate was 22.7 (5.2) mL/min and the mean plasma creatinine clearance rate was 20.7 (4.8) mL/min. Clotting of the vascular access occurred in two patients when the dose of heparin was decreased and the partial thromboplastin time returned towards the normal reference range in both of these patients. The fistula needle became dislodged in one patient, but safety mechanisms prevented blood loss, the needle was replaced, and treatment continued. INTERPRETATION: This wearable haemodialysis device shows promising safety and efficacy results, although further studies will be necessary to confirm these results.

Role of vortices in growth of microbubbles at mitral mechanical heart valve closure.

This study is aimed at refining our understanding of the role of vortex formation at mitral mechanical heart valve (MHV) closure and its association with the high intensity transient signals (HITS) seen in echocardiographic studies with MHV recipients. Previously reported numerical results described a twofold process leading to formation of gas-filled microbubbles in-vitro: (1) nucleation and (2) growth of micron size bubbles. The growth itself consists of two processes: (a) diffusion and (b) sudden pressure drop due to valve closure. The role of diffusion has already been shown to govern the initial growth of nuclei. Pressure drop at mitral MHV closure may be attributed to other phenomena such as squeezed flow, water hammer and primarily, vortex cavitation. Mathematical analysis of vortex formation at mitral MHV closure revealed that a closing velocity of approximately 12 m/s can induce a strong regurgitant vortex which in return can instigate a local pressure drop of about 0.9 atm. A 2D experimental model of regurgitant flows was used to substantiate the impact of vortices. At simulated flow and pressure conditions, a regurgitant vortex was observed to drastically enlarge micron size hydrogen bubbles at its core.

Optimal vortex formation as an index of cardiac health.

Heart disease remains a leading cause of death worldwide. Previous research has indicated that the dynamics of the cardiac left ventricle (LV) during diastolic filling may play a critical role in dictating overall cardiac health. Hence, numerous studies have aimed to predict and evaluate global cardiac health based on quantitative parameters describing LV function. However, the inherent complexity of LV diastole, in its electrical, muscular, and hemodynamic processes, has prevented the development of tools to accurately predict and diagnose heart failure at early stages, when corrective measures are most effective. In this work, it is demonstrated that major aspects of cardiac function are reflected uniquely and sensitively in the optimization of vortex formation in the blood flow during early diastole, as measured by a dimensionless numerical index. This index of optimal vortex formation correlates well with existing measures of cardiac health such as the LV ejection fraction. However, unlike existing measures, this previously undescribed index does not require patient-specific information to determine numerical index values corresponding to normal function. A study of normal and pathological cardiac health in human subjects demonstrates the ability of this global index to distinguish disease states by a straightforward analysis of noninvasive LV measurements.

Continuous renal replacement therapy for congestive heart failure: the wearable continuous ultrafiltration system.

Ultrafiltration is effective in the treatment of fluid and sodium overload in congestive heart failure. There is no available device to provide this therapy to ambulatory patients. We built and tested in vivo a wearable belt that can provide continuous ultrafiltration, 168 hours a week. Nine pigs underwent ureteral ligation and subsequently were allowed fluids ad lib, producing fluid overload. Next day, ultrafiltration was performed for 8 hours. The device consists of a hollow-fiber filter, a 9 V battery-operated pulsatile blood pump, a micro pump for heparin infusion, and another micro pump to control ultrafiltration rate. Blood flow was 65 ml/min and the weight of the device is less than 2.5 lb. Fluid removal rate ranged from 0 to 700 ml/h and averaged 106 ml/h. Salt removed was 7.6 g. No complications were observed. The potential impact on the quality of life of these patients by reducing the shortness of breath, leg swelling, and returning their ability to enjoy salt in their food might be significant, and a reduction in morbidity could be expected. The economic impact in reducing hospital admissions and length of stay, intensive care unit utilization, and drug consumption could be significant. Further studies are needed to compare this innovative approach with traditional drug-based therapy.

Mitral mechanical heart valves: in vitro studies of their closure, vortex and microbubble formation with possible medical implications.

OBJECTIVE: The goal of the present work was to create the closest possible in vitro fluid dynamic environment in which prosthetic mitral valves in the patients' hearts function, in order to demonstrate whether microbubbles are generated, and if yes, under what conditions and at which stage of the cardiac cycle. Microbubbles were observed in the blood of patients with mitral mechanical heart valves (MHV) by means of echocardiography. The phenomenon, often referred to as high-intensity transient signals (HITS), appears as bright, intense, high-velocity and persistent echoes detected by Doppler echocardiography at the instant of valve closure. The question is no longer whether microbubbles are being formed in patients with MHV. as an inherent aspect of their design, but rather how they evolve and when. The answer to this question was the objective of the present paper. METHODS: Hemodynamic conditions in which microbubbles were observed in patients with mitral MHV were simulated in our laboratory. We were able to describe the bubble formation process, as one consisting of nucleation and microbubble growth. While mild growth of nuclei is governed by diffusion, extensive growth of microbubbles is controlled by pressure drop during deceleration of the leaflets on the housing on the atrial side of the mitral MHV. RESULTS: The present study has shown that bubbles form in a fluid at the instant of closure of mechanical valves. The formation of vortices after valve closure, although clinically not yet observed, was also demonstrated in the present in vitro studies. We believe that impact of such vortices on the endothelial layer of the left atrial wall may have clinical significance. These two phenomena were not observed in bioprosthetic valves. CONCLUSIONS: As demonstrated, there exist two distinct phenomena characteristic of mechanical heart valves, which take place during valve closure, namely, that of vortex formation and that of microbubble growth. Both phenomena may have far reaching clinical implications.

Detection of calcium deposits on heart valve leaflets by vibro-acoustography: an in vitro study.

The presence of calcium deposits on heart valve leaflets constitutes a clinically significant diagnostic indication. A novel method for imaging and detecting calcium deposits on tissue heart valves is presented. The method, called vibro-acoustography, uses the radiation force of ultrasound to vibrate the tissue at low (kHz) frequency and records the resulting acoustic response to produce images that are related to the hardness of the tissue. The method is tested on excised human heart valve tissues. Resulting images clearly show calcium deposits with high contrast and are in agreement with the corresponding radiographs of the specimens.

Three-dimensional imaging of fractures in outlet struts of Björk-Shiley convexo-concave heart valves by microcomputed tomography in vitro.

BACKGROUND AND AIMS OF THE STUDY: For implanted Björk-Shiley convexo-concave (BSCC) heart valves, structural failure of the valve's U-shaped outlet strut results in embolization of its blood flow-regulating disc (occluder), with consequent patient morbidity and mortality. After a variable and unpredictable number of cardiac cycles, one strut leg may fatigue ('single-leg separation'; SLS); subsequently the other strut leg may also fatigue, resulting in full structural failure ('outlet strut failure'; OSF). Some BSCC valves are believed to be at more risk of SLS and OSF than others. As valves may function in the SLS condition for some time before OSF occurs, several investigators have sought non-invasive methods to differentiate valves with SLS struts from valves with intact struts in order to provide a rationale for prophylaxis. Herein, we report the use of X-ray microcomputed tomography (micro-CT) to image and characterize SLS strut fractures, including fracture faces otherwise visible only by means of physical sectioning. METHODS: An X-ray micro-CT system was adapted to provide high-resolution, three-dimensional (3D) images of intact and fractured BSCC valve outlet struts in vitro. System modifications included use of a tungsten anode X-ray source to achieve sufficiently high X-ray energies to overcome attenuation within the metal structures, and a hafnium filter to minimize the imaging artifact caused by X-ray beam hardening. For rotating the valve for tomographic scanning, special alignment procedures were developed to maintain the region of interest within the field of view. Typical 3D images of the outlet struts were composed of cubic voxels, 10 microm on a side. Image analysis and display software was used to view the outlet struts and the fractures from several perspectives, including en-face images of fracture surfaces. RESULTS: 3D volume data representations of the SLS and intact outlet struts were obtained, facilitating identification of fracture location and geometry. Enface images of the fracture surfaces were also generated. Several different fracture geometries were observed, such as fractures with and without longitudinal gaps between the fracture faces, and fractures with and without lateral displacement between the faces. En-face views showed varying degrees of roughness on fracture faces. CONCLUSION: This application of micro-CT to image outlet strut fractures in BSCC valve explants demonstrates the value of this method for fracture characterization in vitro, including visualization of fracture faces of SLS struts without physical sectioning. Although the method is not suitable for clinical use because it requires high-intensity X-rays, micro-CT can serve as a tool to understand further any failure mechanisms, and to aid the development of clinical differentiation methods.