Outflow Conduit Research Articles

Computational fluid dynamics analysis of surgical adjustment of left ventricular assist device implantation to minimise stroke risk

Background. Currently, mechanical support is the most promising alternative to cardiac transplantation. Ventricular assist devices (VADs) were originally used to provide mechanical circulatory support in patients awaiting planned heart transplantation (‘bridge-to-transplantation’ therapy). The success of short-term bridge devices led to clinical trials evaluating the clinical suitability of long-term support (‘destination’ therapy) with left ventricular assist devices (LVADs). The first larger scale, randomised trial that tested long-term support with an LVAD reported a 44% reduction in the risk of stroke or death in patients with an LVAD. In spite of the success of LVADs as bridge-to-transplantation and long-term support, patients managed by these devices are still at risk of several adverse events. The most devastating complication is caused by embolisation of thrombi formed within the LVAD or inside the heart into the brain. Prevention of thrombi formation is attempted through anticoagulation management and by improving LVADs design; however, there is still significant occurrence of thromboembolic events in patients. Investigators have reported that the incidence of thromboembolic cerebral events ranges from 14% to 47% over a period of 6–12 months.Methods and approach. An alternative method to reduce the incidence of cerebral embolisation is proposed by the co-authors, and the hypothesis is that it is possible to minimise the number of thrombi flowing into the carotid and vertebral arteries by an optimal placement of the LVAD outflow conduit, with or without the addition of aortic bypass connecting the ascending aorta and the innominate artery (IA), or left carotid artery. This paper presents the computational fluid dynamics (CFD) analysis of the aortic arch haemodynamics using a representative geometry of the human aortic arch with or without an alternative aortic bypass. In order to study the trajectory of the thrombi within the aortic arch bed, the CFD code, Fluent 6.3, is utilised to resolve the flow field and to solve the Lagrangian particle tracking of thrombi released randomly at the inlet of the LVAD cannula.Results. Results are presented for simulations of thrombi in the range of 2–5 mm. The percentage of individual diameter as well as aggregate diameter thrombi flowing to the carotid and vertebral arteries as a function of LVAD conduit placement and aortic bypass implantation is reported. The influence of the LVAD conduit implantation and bypass reveals a nearly 50% variation in predicted cerebral embolism rates.Conclusions. The adjustment of the location of the anastomosis of the LVAD outflow cannula as well as its angle of incidence plays a significant role in the level of thromboembolisms. By proper adjustment in this CFD study of a synthetic model of an aortic arch bed, we found that nearly a 50% reduction in cerebral embolism could be achieved for a configuration consisting of a shallow angle of implantation over a baseline normal incidence of the LVAD cannula. Within the limitations of our model, we have established that the LVAD implantation geometry is an important factor and should be taken into consideration when implanting an LVAD. It is possible that other parameters such as distance of the LVAD outflow cannula to the root of the IA could affect the thrombi embolisation probabilities. However, the results of this study suggest that the risk of stroke may be significantly reduced by as much as 50% by tailoring the VAD implantation by a simple surgical manoeuvre. The results of this line of research may ultimately lead to techniques that can be used to estimate the optimal LVAD configuration in a patient-specific manner by pre-operative imaging.

Transthoracic Echocardiography of the HeartWare Left Ventricular Assist Device

Transthoracic Echocardiography of the HeartWare Left Ventricular Assist Device

Primary arteriovenous fistula inflow proximalization for patients at high risk for dialysis access-associated ischemic steal syndrome

Establishing a functional vascular access while minimizing the risk of dialysis access-associated ischemic steal syndrome (DASS) may present a challenging problem in patients with severe peripheral vascular disease where even a low-flow arteriovenous fistula (AVF) may lead to severe symptoms and physical findings of DASS. Proximalization of arterial inflow for an existing vascular access is established as an effective treatment for DASS. We hypothesized that a primary proximal arterial inflow procedure for vascular access in patients judged to be at high risk for DASS would result in a successful hemodialysis access and mitigate the risk of steal syndrome. We report four such patients considered to be at significant risk for DASS after construction of a new vascular access. An axillary artery AVF inflow anastomosis was constructed in each patient. The access outflow configuration varied with the available venous outflow conduit identified during the preoperative ultrasound evaluation. In all four patients in this report, a functional autogenous dialysis access was established without DASS.

Use of a vascular plug to percutaneously occlude the outflow conduit of a dysfunctional left ventricular assist device

Use of a vascular plug to percutaneously occlude the outflow conduit of a dysfunctional left ventricular assist device

Extending Outflow Brachial Vein Tength for Transposition Arteriovenous Fistulas

Transposition arteriovenous fistulas (T-AVF) play an important role in establishing autogenous vascular access for many hemodialysis patients. When the basilic vein is not available, one of the paired brachial veins offers a reliable T-AVF option, generally as a two-staged operation. Uncommonly, the brachial vein outflow conduit dissipates into multiple small branches communicating with the paired brachial vein or occasionally with a residual proximal segment of the basilic vein. We utilized parallel outflow vein component segments to create additional outflow vein length necessary for successful T-AVFs in these patients. We identified four patients where a vein length extension technique utilizing parallel and overlapping vein segments with an end-to-end anastomosis gained adequate length for successful T-AVFs. All transpositions were based on a first stage AVF with access outflow established into a brachial vein. Three of the operations utilized paired brachial vein segments and one gained needed access outflow length with a brachial vein anastomosis to a residual proximal basilic vein segment. All four patients had functional T-AVFs at 4, 5, 7, and 14 month follow-up. None of the patients developed arm swelling. Creating a longer transposition AVF venous outflow segment using parallel and overlapping vein segments with an end-to-end anastomosis gained the adequate length needed for successful T-AVFs in these four patients.

Design Concepts and Principle of Operation of the HeartWare Ventricular Assist System

Implantable left ventricular assist devices provide circulatory support for patients at risk of death from refractory, end-stage heart failure. Rotary blood pumps have been designed for increased reliability and smaller size for use in a broader population of patients than the first-generation pulsatile devices. The design concepts and principle of operation of the HeartWare System are discussed. The HeartWare Ventricular Assist System (HVAD) is a small centrifugal flow pump with a displacement volume of 50 ml and an output capacity of 10 L/min. A unique wide-blade impeller is suspended by hybrid passive magnets and hydrodynamic forces. An integrated inflow cannula is inserted into the left ventricle and is held in position by an adjustable sewing ring; the pump is positioned in the pericardial space. The 10-mm outflow graft is anastomosed to the ascending aorta. External system components include the microprocessor-based controller, a monitor, lithium-ion battery packs, alternating current and direct current power adapters, and a battery charger. Physiologic control algorithms are incorporated for safe operation. Preclinical life cycle tests have shown the HVAD to be highly reliable. This system design offers reliability, portability, and ease of use for ambulatory patients.

Buccal Mucous Membrane Graft-Assisted Lacrimal Drainage Surgery

Dacryocystorhinostomy (DCR) success may depend on mucous membrane alignment across the osteotomy and entire surgical lacrimal outflow conduit. Scar tissue, obliteration of the lacrimal sac, and obstruction at the level of the common canaliculus may result from prior surgery or may occur as an isolated problem. We describe and evaluate the efficacy of a technique of grafting autologous buccal mucous membrane during external DCR in cases complicated by scarring and/or common canalicular obstruction. We retrospectively reviewed 11 nasolacrimal outflow system obstructions (in 9 patients) treated with a buccal mucous membrane graft-assisted DCR. The surgical technique is described in detail. All cases presented with insufficient nasolacrimal outflow. Ten tear ducts failed prior DCR procedures, while one case had isolated common canalicular obstruction, resistant to multiple probing and intubation and was previously recommended a Jones tube procedure. Three tear outflow systems (in 2 patients) exhibited common canalicular obstruction; the rest were obstructed at the level of the lacrimal sac. Two patients had bilateral obstruction. Ten eyes had symptomatic epiphora. One eye had dacryocystitis alone, and 2 eyes had dacryocystitis and epiphora. Six tear ducts had more than one previous DCR procedure. Four tear ducts had prior endonasal DCR procedures, while 6 had a previous external DCR. All eyes with epiphora preoperatively had complete relief of tearing. All cases of dacryocystitis resolved with no recurrences. Nine tear ducts were patent to irrigation testing postoperatively (2 were not tested). Buccal mucous membrane grafting may improve the success of DCR in cases complicated by scarring, mucosal shortage, and/or common canalicular obstruction.

The natural history of vascular access for hemodialysis: A single center study of 2,422 patients

Our objective is to provide provision of primary and secondary patency rates data and incidence of complications. Despite the publication of some review articles and small prospective trials about vascular accesses, controversy still exists regarding the choice of the outflow conduit and especially the choice of the fistula to be formed in secondary and tertiary access procedures. This is a retrospective study of 2,422 consecutive patients who underwent 3,685 vascular access procedures in a tertiary care hospital, including radial-cephalic (RCAVF), brachial-cephalic (BCAVF), brachial-basilic (BBAVF), and prosthetic graft (PTFE) fistulas. Maximum follow-up period was 20 years. Actuarial patency rates were obtained by Kaplan-Meier analysis. The median primary patency (days) of the most common 1st choices for vascular access were 712 (95% CI: 606, 818), 1,009 (95% CI: 823, 1,195), and 384 (95% CI: 273, 945) days for RCAVF, BCAVF, and PTFE, respectively. The median secondary patency was 1809 days (95% CI: 1,692, 1,926) for the RCAVF. The median primary patency of BBAVF (2nd or 3rd choice for vascular access) was 1,582 days (95% CI: 415, 2,749). The cumulative incidence of clinically important complications for the patients who received a RCAVF, BCAVF, BBAVF, and u-PTFE was 0.25, 0.57, 0.33, and 0.61 per patient-year, respectively. We advocate maximal use of autogenous conduits, except probably the case of the older diabetic patient, in whom access at the antecubital fossa should be the first choice. BBAVF is an excellent fistula and should probably be constructed before prosthetic graft placement.

Left Ventricular Assist Device Outflow Cannula Obstruction by the Rare Environmental Fungus Myceliophthora thermophila

Left ventricular assist devices are used to provide mechanical circulatory support during end-stage heart failure either as a destination therapy or as a bridge to heart transplantation. Perioperative transesophageal echocardiography is becoming an invaluable tool to investigate device function during implantation and in case of mechanical malfunction. Most malfunctions are due to inflow graft occlusion, or device malfunction, while outflow graft dysfunction is rare. Here, we present a case of severe outflow conduit obstruction by a rare environmental fungus, Myceliophthora thermophila. After replacement of the infected device and intensive antifungal treatment, heart transplantation was performed 2 yr later.

Conduit from Hypoplastic Right Ventricle to Pulmonary Artery in Tricuspid Atresia

Tricuspid atresia with transposition of the great arteries produces single ventricle physiology. Ultimate goals of neonatal palliative operations are to provide optimum anatomic and physiologic conditions for a Fontan procedure. A modification of the Norwood procedure is reported, with an aorto-pulmonary anastomosis, utilizing the hypoplastic right ventricle as the pulmonary outflow conduit, avoiding a left ventriculotomy and preserving its function with excellent recovery. We believe this technique has not been previously published in the English literature.

Implantation technique for the DuraHeart left ventricular assist system

The DuraHeart is a centrifugal pump with a magnetically levitated impeller. We implant the DuraHeart blood pump, which is 72 mm in external diameter and 45 mm in height, in a preperitoneal pocket in the left upper abdomen. An apical cuff is sutured to the apical hole in the left ventricular apex with 12 mattress sutures of 3-0 Prolene pledgeted with Dacron felt. In the first six mattress sutures, the needle is exited from the apical hole after a full-thickness bite of myocardium, and in six additional mattress sutures, the needle is exited from the epicardial edge of the apical hole. Next, a double purse-string suture with 3-0 Prolene is placed on the Dacron pledgets around the apical hole. After the inflow conduit has been connected to the inlet port of the blood pump, the inflow conduit is secured to the apical cuff. The graft portion of the outflow conduit, which was connected to the blood pump beforehand, is sewn end-to-side to the ascending aorta with a 4-0 Prolene suture. Gentle suture connection of the apical cuff to the left ventricular apex and optimal alignment of the inflow conduit, which is bent by 60 degrees, are crucial in the surgical procedure. A detailed description of the implantation technique is presented to facilitate the use of this system.

Cleveland Clinic PediPump Lamb Cadaver Fitting Studies

We are developing the PediPump, a magnetically suspended, mixed-flow pump, as an implantable pediatric ventricular assist device (VAD). Lamb cadaver fitting studies were performed to determine the optimal pump location and optimal design of the inflow and outflow conduits for chronic in vivo studies. A prototype of the PediPump right and left ventricular assist devices (RVAD and LVAD, respectively) were implanted via a sternotomy or left thoracotomy in four lamb cadavers (3.7-34.2 kg). Via a sternotomy, the RVAD and LVAD required long inflow cannulas when placed into the right or left thoracic cavities, respectively. Via a left thoracotomy, with both pumps implanted in the left thoracic cavity, the RVAD required a 70 degrees inflow cannula and a bent outflow graft while the LVAD required a 130 degrees inflow cannula and a straight outflow graft. In conclusion, left thoracotomy provided optimal fitting for both LVAD and RVAD for the small lamb model.

Urgent Pump Exchange for Stroke Resulting From a Distorted HeartMate XVE Inflow Conduit

Long-term left ventricular assist device (LVAD) implantation is associated with an inherent risk of transient ischemic attacks (TIAs). When TIAs occur in patients who have undergone placement of a HeartMate XVE LVAD, the pump must be assumed to be the likely source of the TIA. The HeartMate XVE is unique in that it has a pumping surface of flocked Biomer and a pump housing and inlet cannula coated with sintered titanium. This design facilitates coating of the surfaces with the patient's own cells, usually within 48 hours of implantation, effectively eliminating the pumping surface as a source of thromboemboli. Similarly, neither the outflow conduit nor the porcine outflow valve have been shown to be sources of thromboemboli, as they are both subjected to the wash-out of pulsatile systolic pressure. In our experience, the only components vulnerable to pannus formation are the inflow conduit and its associated valve, and then only when the inflow conduit is distorted at implantation. We describe an illustrative case of a patient supported with a HeartMate XVE LVAD who suffered a device-related stroke that was attributed to the inflow conduit, which was distorted during implantation to configure the pump for the patient's narrow body habitus. The resultant disruption in laminar flow was the most likely cause of the TIAs and stroke. Therefore, documented TIAs in patients with an implanted HeartMate XVE are an indication for urgent pump replacement, even if pump performance is otherwise normal. In this case, the pump was exchanged for a smaller, continuous-flow HeartMate II LVAD.

Effect of Left Ventricular Assist Device Outflow Conduit Anastomosis Location on Flow Patterns in the Native Aorta

Computational fluid dynamics (CFD) models were developed to investigate the altered fluid dynamics of the native aorta in patients with a left ventricular assist device (LVAD). The objective of this study was to simulate the effect of LVAD aortic outflow conduit location on the 3-D flow in the native aorta over a range of boundary conditions. The fluid mechanics of three different surgical geometries [(P), proximal, (D), distal and (IP), in-plane] were studied and the implications for short- and long-term medical consequences explored by evaluating the flow fields, wall shear, and hemolysis. The greatest disruptions in the normal aortic flow pattern occurred with series flow conditions, when flow through the aortic valve was minimal. Under series conditions, circulation in the proximal aorta is retrograde, originating from the LVAD outflow conduit. The (P) geometry provided the most blood washout of the proximal aorta, with a larger region of slow-moving flow observed in the (D) and (IP) models. Wall shear stress was reduced for the (IP) geometry, which lacks the direct flow impingement present in the (P) and (D) models. Clinically, the (D) and (IP) geometries require less traumatic surgeries and probably are better tolerated by the patient. In this situation, the (IP) geometry suggests improvement in both increased flow to the proximal aorta and decreased shear stress compared with (D). However, the (D) and (IP) configurations are not recommended for patients with low or no flow from the heart because of the lack of blood washout near the aortic valve and therefore possible thrombus formation in that area.

In Vivo Experimental Testing of a Microaxial Blood Pump for Right Ventricular Support

The incidence of isolated right ventricular (RV) failure is rare in postcardiotomy patients, but high in patients undergoing implantation of a left ventricular assist device or cardiac transplantation. Therefore, we have developed a new microaxial flow device and report on our first in vivo animal trials. Six healthy adult female sheep weighing 80-90 kg underwent implantation of the microaxial blood pump for partial unloading of the right ventricle. This pump is a miniaturized rotary blood pump with a diameter of only 6.4 mm and a weight of 11 g. The inner volume of the pump is limited to 12 mL, and the inner artificial blood contacting surface is 65 cm(2). The pump consists of a rotor driven by an incorporated brushless direct current motor, the housing of the rotor, the inflow cage, the outflow cannula, and the driveline. At the maximum speed of 32,500 rotations/min, a flow of 6 L/min can be delivered. The inflow and outflow conduit were anastomosed to the right atrium and the main pulmonary artery, respectively. Hemodynamic and echocardiographic data as well as blood samples were measured over the whole test period of 7 days. The hearts and lungs as well as the pump were explanted for a thorough examination at the end of the trial. Systemic arterial blood pressures remained unchanged during the entire test period. RV cardiac output was diminished significantly as demonstrated by the echocardiographic studies. The number of platelets decreased perioperatively, but recovered within the test period. The free hemoglobin was not enhanced postoperatively indicating no significant hemolysis. Liver function was only slightly impaired due to operative reasons (increase in bilirubin on the first postoperative day but normalization within the test period). The pathologic examination revealed some clots at the inflow cage and fibrin depositions on the impeller as well as on the inner surface of the outflow graft without an impairment of pump function. Our results demonstrate that this newly developed microaxial blood pump is a promising device for RV support, but it cannot be driven without any anticoagulation.

Feasibility of a Miniature Centrifugal Rotary Blood Pump for Low-Flow Circulation in Children and Infants

In this study, a seal-less, tiny centrifugal rotary blood pump was designed for low-flow circulatory support in children and infants. The design was targeted to yield a compact and priming volume of 5 ml with a flow rate of 0.5-4 l/min against a head pressure of 40-100 mm Hg. To meet the design requirements, the first prototype had an impeller diameter of 30 mm with six straight vanes. The impeller was supported with a needle-type hydrodynamic bearing and was driven with a six-pole radial magnetic driver. The external pump dimensions included a pump head height of 20 mm, diameter of 49 mm, and priming volume of 5 ml. The weight was 150 g, including the motor driver. In the mock circulatory loop, using fresh porcine blood, the pump yielded a flow of 0.5-4.0 l/min against a head pressure of 40-100 mm Hg at a rotational speed of 1800-4000 rpm using 1/4" inflow and outflow conduits. The maximum flow and head pressure of 5.25 l/min and 244 mm Hg, respectively, were obtained at a rotational speed of 4400 rpm. The maximum electrical-to-hydraulic efficiency occurred at a flow rate of 1.5-3.5 l/min and at a rotational speed of 2000-4400 rpm. The normalized index of hemolysis, which was evaluated using fresh porcine blood, was 0.0076 g/100 l with the impeller in the down-mode and a bearing clearance of 0.1 mm. Further refinement in the bearing and magnetic coupler are required to improve the hemolytic performance of the pump. The durability of the needle-type hydrodynamic bearing and antithrombotic performance of the pump will be performed before clinical applications. The tiny centrifugal blood pump meets the flow requirements necessary to support the circulation of pediatric patients.

PRELIMINARY IN-VITRO EVALUATION OF THE NOVELCOR II VAD

A mock circulatory loop was developed to assess the flow characteristics of NovelCor II, a newly designed Ventricular Assist Device (VAD). NovelCor II, developed by Dr. Domingo Liotta at the University of Moron, Argentina, is an implantable, pneumatically driven VAD designed primarily for myocardial recovery with atrial and abdominal aorta cannulations for the inflow and outflow conduits, respectively. The mock circulatory loop was designed to control changes on the preload and afterload pressures and monitor systemic pressure and pump output with the aid of pressure sensors and flow meters located at the inflow and outflow positions of the VAD. Data was acquired and analyzed using MAX and LabVIEW data acquisition system (National Instruments, TX). The NovelCor II was compared to a Thoratec paracorporeal VAD while both were operated with their respective pneumatic driver units. Pump outputs for the NovelCor II and Thoratec were determined by varying the following parameters: pump rate, preload and afterload pressures, driver eject pressure, and driver vacuum level. The pump rate was ranged from 30 to 120 bpm in intervals of 10 bpm; the preload was varied from 4 to 19 mmHg and the afterload from 80 to 120 mmHg; the eject pressure was set at 180, 200 or 220 mmHg and the vacuum level was set at -20 or 0 mmHg. Preliminary results showed that at variable preload pressures, the highest pump output was achieved at pump rates of 55–60 bpm for both VADs, but in the case of NovelCor II the pump output decreased faster at higher pump rates than with the Thoratec.

NOVEL LONG-TERM IN-VIVO HEMOCOMPATIBILITY TEST FOR CIRCULATORY ASSIST DEVICES - A NEW APPROACH TO THE DEVELOPMENT OF ANTITHROMBOGENIC BLOOD CONTACTING SURFACES

Background: Thrombus formation and resulting embolic complications remain a major problem of ventricular assist devices. Therefore, the development of efficient antithrombogenic surfaces has a high priority. However, long-term tests of ventricular assist devices in large animal models are very expensive. Method: We have developed a novel in-vivo test to investigate several coatings of the MEDOS-H1A assist device. This blood pump is completely made of polyurethane. The outflow conduits, a very common location of thrombi, were excised and implanted into the thoracic aorta of pigs. Results: Due to the small diameter of the descending aorta the aortotomy had to be closed with a patch to avoid extreme tension of the vascular tissue. The surgical approach via a left lateral thoracotomy was well tolerated by the animals. The movement of the polyurethane valve provided sufficient blood flow through the implanted conduit without significant pressure gradient. Conclusion: This simple and inexpensive large animal model provides excellent conditions for long-term testing of new antithrombogenic surfaces. Further investigations have to assess it's clinical relevance.Figure

Effect of LVAD outflow conduit insertion angle on flow through the native aorta

Background and aim: The goal of this study was to evaluate the effect of surgical anastomosis configuration of the aortic outflow conduit (AOC) from a continuous flow left ventricular assist device (LVAD) on the flow fields in the aorta using CFD simulations. The geometry of the surgical integration of the LVAD is an important factor in the flow pattern that develops both in series (aortic valve closed, all flow through LVAD) and in parallel (heart pumping in addition to LVAD).Methods: CFD models of the AOC junctions simulate geometry as cylindrical tubes that intersect at angles ranging from 30° to 90°. Velocity fields are computed over a range of cardiac output for both series and parallel flow.Results: Our results demonstrate that the flow patterns are significantly affected by the angle of insertion of the AOC into the native aorta, both during series and parallel flow conditions. Zones of flow recirculation and high shear stress on the aortic wall can be observed at the highest angle, gradually decreasing in size until disappearing at the lowest angle of 30°. The highest velocity and shear stress values were associated with series flow.Conclusions: The results suggest that connecting the LVAD outflow conduit to the proximal aorta at a shallower angle produces fewer secondary flow patterns in the native cardiovascular system.

IN VITRO SIMULATOR FOR AN IMPLANTED REPLACEMENT HEART

A fully automated and highly configurable mock-loop has been developed to provide a simulated physiological environment in which an implanted replacement heart, such as the AbioCor® Implantable Replacement Heart (AbioCor), could be fully tested. This loop introduces minimal inductance to all inflow and outflow conduits of the replacement heart, a challenging part in mock-loop design and engineering. Atrial pressure variations as a result of respiration can affect the stability of control algorithms. All compliances (aortic, pulmonary arterial, left atrial and right atrial) can be adjusted to generate a wide range of pulsatilities with ΔAOP: 10–100 mmHg, ΔPAP: 5–60 mmHg, ΔLAP and ΔRAP: 2–20 mmHg. Atrial pressure variations at respiratory rates can be adjusted in the range of 0–25 mmHg for both RAP and LAP. A PC-based system controls other parameters: mean AOP (range for AbioCor: 40–200 mmHg), mean PAP (10–100 mmHg), mean RAP (3–25 mmHg), bronchial shunt flow (0–0.8 LPM), respiratory rate (0–60 breaths/min). All the parameters can be programmed to follow separate courses defined in a script file to simulate real life situations such as circadian cycles. The control system also samples and logs multi-channel data: AOP, PAP, RAP, LAP, flow, shunt flow, barometric pressure, etc. This system has been proven to be very effective in challenging control algorithms by automatically going through targeted operation boundarics.