Prolonged nonpulsatile left heart bypass with reduced systemic pulse pressure causes morphological changes in the aortic wall

Reciprocal interferences of the left ventricular assist device and the aortic valve competence

Patients suffering from end-stage heart failure tend to have high mortality rates. With growing numbers of patients progressing into severe heart failure, the shortage of available donors is a growing concern, with less than 10% of patients undergoing cardiac transplantation (CTx). Fortunately, the use of left ventricular assist devices (LVADs), a variant of mechanical circulatory support has been on the rise in recent years. The expansion of LVADs has led them to be incorporated into a variety of clinical settings, based on the goals of therapy for patients ailing from heart failure. However, with an increase in the use of LVADs, there are a host of complications that arise with it. One such complication is the development and progression of aortic regurgitation (AR) which is noted to adversely influence patient outcomes and compromise pump benefits leading to increased morbidity and mortality. The underlying mechanisms are likely multifactorial and involve the aortic root-aortic valve (AV) complex, as well as the LVAD device, patient, and other factors, all of them alter the physiological mechanics of the heart resulting in AV dysfunction. Thus, it is imperative to screen patients before LVAD implantation for AR, as moderate or greater AR requires a concurrent intervention at the time of LVADs implantation. No current strict guidelines were identified in the literature search on how to actively manage and limit the development and/or progression of AR, due to the limited information. However, some recommendations include medical management by targeting fluid overload and arterial blood pressure, along with adjusting the settings of the LVADs device itself. Surgical interventions are to be considered depending on patient factors, goals of care, and the underlying pathology. These interventions include the closure of the AV, replacement of the valve, and percutaneous approach via percutaneous occluding device or transcatheter aortic valve implantation. In the present review, we describe the interaction between AV and LVAD placement, in terms of patient management and prognosis. Also it is provided a comprehensive echocardiographic strategy for the precise assessment of AV regurgitation severity.

A Multi-Domain Simulation Study of a Pulsatile-Flow Pump Device for Heart Failure With Preserved Ejection Fraction

Mechanical circulatory support (MCS) devices are currently under development to improve the physiology and hemodynamics of patients with heart failure with preserved ejection fraction (HFpEF). Most of these devices, however, are designed to provide continuous-flow support. While it has been shown that pulsatile support may overcome some of the complications hindering the clinical translation of these devices for other heart failure phenotypes, the effects that it may have on the HFpEF physiology are still unknown. Here, we present a multi-domain simulation study of a pulsatile pump device with left atrial cannulation for HFpEF that aims to alleviate left atrial pressure, commonly elevated in HFpEF. We leverage lumped-parameter modeling to optimize the design of the pulsatile pump, computational fluid dynamic simulations to characterize hydraulic and hemolytic performance, and finite element modeling on the Living Heart Model to evaluate effects on arterial, left atrial, and left ventricular hemodynamics and biomechanics. The findings reported in this study suggest that pulsatile-flow support can successfully reduce pressures and associated wall stresses in the left heart, while yielding more physiologic arterial hemodynamics compared to continuous-flow support. This work therefore supports further development and evaluation of pulsatile support MCS devices for HFpEF.

Heart, kidney and left ventricular assist device: a complex trio

BACKGROUND

Heart failure (HF) is a complex syndrome affecting the whole body, kidneys included. The left ventricular assist device (LVAD) is a valid option for patients with very severe HF. Focusing on renal function, LVAD implantation could theoretically reverse the detrimental effects of HF syndrome on kidneys. However, implanting an LVAD is a high-risk surgical procedure, and LVAD patients have higher risk of bleeding, device thrombosis, strokes, renal impairment, multi-organ failure and infections. Furthermore, an LVAD has its own particular effects on the renal system.

METHODS

In this review, we provide a comprehensive overview of the complex interaction between LVAD and the kidneys from the pathophysiological and clinical perspectives. An analysis of the different effects of pulsatile-flow and continuous-flow LVAD is provided.

RESULTS

Despite their limitations, creatinine-based estimated glomerular filtration rate (eGFR) formulas help to stratify patients by their post-LVAD placement prognosis. Poor basal renal function, the onset of acute kidney injury or the need for renal replacement therapy after LVAD implantation negatively influences a patient's prognosis. LVAD can also prompt an improvement in renal function, however, with some counterintuitive effects on a patient's prognosis.

CONCLUSION

It is still hard to say whether different trends in eGFR depend on different renal conditions before LVAD placement, on a patient's better overall status or on a particular patient management strategy before and/or after the device's implantation. Steps should be taken to solve this question because finding the best candidates for LVAD implantation is of paramount importance to ensure the best outcomes.

Patient-Specific Computational Fluid Dynamics Reveal Localized Flow Patterns Predictive of Post-Left Ventricular Assist Device Aortic Incompetence

BACKGROUND

Progressive aortic valve disease has remained a persistent cause of concern in patients with left ventricular assist devices. Aortic incompetence (AI) is a known predictor of both mortality and readmissions in this patient population and remains a challenging clinical problem.

METHODS

Ten left ventricular assist device patients with de novo aortic regurgitation and 19 control left ventricular assist device patients were identified. Three-dimensional models of patients' aortas were created from their computed tomography scans, following which large-scale patient-specific computational fluid dynamics simulations were performed with physiologically accurate boundary conditions using the SimVascular flow solver.

RESULTS

The spatial distributions of time-averaged wall shear stress and oscillatory shear index show no significant differences in the aortic root in patients with and without AI (mean difference, 0.67 dyne/cm2 [95% CI, -0.51 to 1.85]; P=0.23). Oscillatory shear index was also not significantly different between both groups of patients (mean difference, 0.03 [95% CI, -0.07 to 0.019]; P=0.22). The localized wall shear stress on the leaflet tips was significantly higher in the AI group than the non-AI group (1.62 versus 1.35 dyne/cm2; mean difference [95% CI, 0.15-0.39]; P<0.001), whereas oscillatory shear index was not significantly different between both groups (95% CI, -0.009 to 0.001; P=0.17).

CONCLUSIONS

Computational fluid dynamics serves a unique role in studying the hemodynamic features in left ventricular assist device patients where 4-dimensional magnetic resonance imaging remains unfeasible. Contrary to the widely accepted notions of highly disturbed flow, in this study, we demonstrate that the aortic root is a region of relatively stagnant flow. We further identified localized hemodynamic features in the aortic root that challenge our understanding of how AI develops in this patient population.

NUMERICAL INVESTIGATION ON PRELOAD AND AFTERLOAD SENSITIVITY FOR USING VENTRICULAR ASSIST DEVICE ON HEART FAILURE PATIENTS

A left ventricular assist device (LVAD) is a surgically implanted mechanical pump being used for patients with end-stage heart failure (HF). One of the significant clinical challenges in using LVADs is its remarkable changes in hemodynamic parameters during a change in body position from supine to standing. In standing position, vasodilatation of veins occurs in the legs, which decreases left ventricular end-diastolic pressure, and, in turn, the preload to the LVAD. In this research, a numerical investigation is carried out to evaluate the effect of LVAD in cardiac hemodynamic parameters such as cardiac output (CO) and stroke work (SW) under preload, normal, and afterload conditions. A Proportional–integral–derivative (PID) controller associated with an LVAD pump model and cardiovascular system (CVS) model is developed to study the cardiac hemodynamic and its performance during HF condition by changing system parameters in one cardiac cycle. The performance of the proposed model is then evaluated using a pump cannulae model, real-time status detection of the aortic valve (av), and left ventricular stroke volume. The model parameters associated with HF, including contractility of the left and right ventricle ([Formula: see text] & [Formula: see text]), systemic peripheral resistance ([Formula: see text]) and total blood volume ([Formula: see text]) were set 0.71[Formula: see text]mmHg.s.mL[Formula: see text], 0.53[Formula: see text]mmHg.s.mL[Formula: see text], 1.11[Formula: see text]mmHg.s.mL[Formula: see text] and 5800[Formula: see text]mL, respectively, to allow simulation of HF conditions. The findings of this study show that the CO is increasing linearly with end-diastolic left ventricular volume (LVEDV) and end-diastolic right ventricular volume (RVEDV). However, other vital parameters behavior has a nonlinear relation to CO. Results of this study prove that the LVAD model is more sensitive to preload than afterload condition under different hemodynamical conditions.

Use of patient-specific computational models for optimization of aortic insufficiency after implantation of left ventricular assist device

OBJECTIVE

Aortic incompetence (AI) is observed to be accelerated in the continuous-flow left ventricular assist device (LVAD) population and is related to increased mortality. Using computational fluid dynamics (CFD), we investigated the hemodynamic conditions related to the orientation of the LVAD outflow in these patients.

METHOD

We identified 10 patients with new aortic regurgitation, and 20 who did not, after LVAD implantation between 2009 and 2018. Three-dimensional models of patients' aortas were created from their computed tomography scans. The geometry of the LVAD outflow graft in relation to the aorta was quantified using azimuth angles (AA), polar angles (PAs), and distance from aortic root. The models were used to run CFD simulations, which calculated the pressures and wall shear stress (rWSS) exerted on the aortic root.

RESULTS

The AA and PA were found to be similar. However, for combinations of high values of AA and low values of PA, there were no patients with AI. The distance from aortic root to the outflow graft was also smaller in patients who developed AI (3.39 ± 0.7 vs 4.07 ± 0.77 cm, P = .04). There was no significant difference in aortic root pressures in the 2 groups. The rWSS was greater in AI patients (4.60 ± 5.70 vs 2.37 ± 1.20 dyne/cm², P < .001). Qualitatively, we observed a trend of greater perturbations, regions of high rWSS, and flow eddies in the AI group.

CONCLUSIONS

Using CFD simulations, we demonstrated that patients who developed de novo AI have greater rWSS at the aortic root, and their outflow grafts were placed closer to the aortic roots than those patients without de novo AI.

Prolonged Nonpulsatile Left Heart Bypass Diminishes Vascular Contractility

We investigated possible functional changes in the vascular system accompanying the morphological change in prolonged nonpulsatile left heart bypass (LHB). Three adult goats underwent pulsatile LHB. Two weeks postoperatively, the pulsatile ventricular assist device was replaced with a centrifugal pump and nonpulsatile LHB was subsequently conducted for 4 weeks. The mean aortic pulse pressure was 39 and 16 mmHg during the pulsatile and nonpulsatile LHB, respectively. Systemic vascular resistance (SVR) and plasma norepinephrine levels were measured at the end of pulsatile LHB (PUL), and at the end of 1st, 2nd, 3rd, and 4th week of nonpulsatile LHB (NP1w, NP2w, NP3w, NP4w, respectively). At each point, 50 μg/kg nitroglycerin and 1μg/kg norepinephrine were injected and the minimal and maximal values of SVR after injection were calculated as parameters reflecting the vascular tonus and contractility. The SVR and plasma nor epinephrine level did not significantly change during the entire course (SVR: 1106, 895, 982, 920, and 938 dyne·sec·cm−5; norepinephrine level: 0.3, 0.2, 0.1, 0.2, and 0.1 ng/ml; at PUL, NP1w, NP2w, NP3w, and NP4w, respectively). The minimal value of SVR after nitroglycerin injection remained unchanged, indicating that vascular tonus was stable during the entire course (618, 687, 623, 560, 653 dyne·sec·cm−5, respectively). In contrast, the maximal value of SVR after norepinephrine injection at NP3w and NP4w (1695 and 1759 yne·sec·cm−5) became significantly reduced compared to that at PUL (2346 dyne·sec·cm−5). These results indicated that prolonged nonpulsatile left heart bypass did not affect the vascular tonus, but significantly diminished the vascular contractility.

Enhancement of Arterial Pressure Pulsatility by Controlling Continuous-Flow Left Ventricular Assist Device Flow Rate in Mock Circulatory System

Continuous-flow left ventricular assist devices (CF-LVADs) generally operate at a constant speed, which reduces pulsatility in the arteries and may lead to complications such as functional changes in the vascular system, gastrointestinal bleeding, or both. The purpose of this study is to increase the arterial pulse pressure and pulsatility by controlling the CF-LVAD flow rate. A MicroMed DeBakey pump was used as the CF-LVAD. A model simulating the flow rate through the aortic valve was used as a reference model to drive the pump. A mock circulation containing two synchronized servomotor-operated piston pumps acting as left and right ventricles was used as a circulatory system. Proportional-integral control was used as the control method. First, the CF-LVAD was operated at a constant speed. With pulsatile-speed CF-LVAD assistance, the pump was driven such that the same mean pump output was generated. Continuous and pulsatile-speed CF-LVAD assistance provided the same mean arterial pressure and flow rate, while the index of pulsatility increased significantly for both arterial pressure and pump flow rate signals under pulsatile speed pump support. This study shows the possibility of improving the pulsatility of CF-LVAD support by regulating pump speed over a cardiac cycle without reducing the overall level of support.

Complications of Continuous-Flow Mechanical Circulatory Support Devices

Left ventricular assist devices (LVADs), more importantly the continuous-flow subclass, have revolutionized the medical field by improving New York Heart Association (NYHA) functional class status, quality of life, and survival rates in patients with advanced systolic heart failure. From the first pulsatile device to modern day continuous-flow devices, LVADs have continued to improve, but they are still associated with several complications. These complications include infection, bleeding, thrombosis, hemolysis, aortic valvular dysfunction, right heart failure, and ventricular arrhythmias. In this article, we aim to review these complications to understand the most appropriate approach for their prevention and to discuss the available therapeutic modalities.

Effect of Rotary Blood Pump Pulsatility on Potential Parameters of Blood Compatibility and Thrombosis in Inflow Cannula Tips

Purpose

Rotary Blood Pump (RBP) pulsatile strategies relative to the native cardiac cycle have been widely studied because of their benefits to hemodynamics. However, the effects that inducing pulses has on the blood compatibility of ventricular assist device (VAD) support have not yet been understood. Inflow cannulae have been found to be associated with thrombosis under conventional constant speed support of RBPs. To prevent further risks to blood compatibility, it is necessary to understand the relationship between cannula tip design and the induced pulsatility. The purpose of this study was to evaluate the flow field of 5 different tip geometries under RBP pulsatile support using stereo-particle image velocimetry (PIV).

Methods

Inflow cannulae with conventional tip geometries (blunt, blunt with 4 side ports, beveled with 3 side ports, and cage) and a custom designed crown tip were studied. All cannulae were interposed between a mixed-flow RBP and a silicone left ventricle. The contractile function and hemodynamics were reproduced in a mock circulation loop (MCL). The RBP was configured to induce synchronous and counter-synchronous pulses relative to cardiac cycles while supporting the failing ventricle.

Results

Between both pulsing strategies, low shear volume ([Formula: see text], potential parameter of thrombus formation) showed no significant difference. However, counter-synchronous pulsatile mode induced less increase of both high shear volume ([Formula: see text], potential parameter of platelet activation) and recirculation volume (Vz>0, potential parameter of thrombus formation).

Conclusions

Although the clinical relationship cannot be inferred from this measurement, when considering the inflow tips only, a necessary trade-off should be made between adverse effects on blood compatibility and benefits for hemodynamics during RBP pulsatile mode.

Improving Arterial Pulsatility by Feedback Control of a Continuous Flow Left Ventricular Assist Device via in Silico Modeling

Purpose

Continuous flow left ventricular assist devices (CF-LVADs) generally operate at a constant speed, which causes a decrease in pulse pressure and pulsatility in the arteries and allegedly may lead to late complications such as aortic insufficiency and gastrointestinal bleeding. The purpose of this study is to increase the arterial pulse pressure and pulsatility while obtaining more physiological hemodynamic signals, by controlling the CF-LVAD flow rate.

Methods

A lumped parameter model was used to simulate the cardiovascular system including the heart chambers, heart valves, systemic and pulmonary arteries and veins. A baroreflex model was used to regulate the heart rate and a model of the Micromed DeBakey CF-LVAD (Micromed Technology, Houston, TX, USA) to simulate the pump dynamics at different operating speeds. A model simulating the flow rate through the aortic valve served as reference model. CF-LVAD operating speed was regulated by applying proportional-integral (PI) control to the pump flow rate. For comparison, the CF-LVAD was also operated at a constant speed, equaling the mean CF-LVAD speed as applied in pulsatile mode.

Results

In different operating modes, at the same mean operating speeds, mean pump output, mean arterial pressure, end-systolic and end-diastolic volume and heart rate were the same over the cardiac cycle. However, the arterial pulse pressure and index of pulsatility increased by 50% in the pulsatile CF-LVAD support mode with respect to constant speed pump support.

Conclusions

This study shows the possibility of obtaining more physiological pulsatile hemodynamics in the arteries by applying output-driven varying speed control to a CF-LVAD.

Morphologic changes in the aortic wall media after support with a continuous-flow left ventricular assist device

BACKGROUND

Continuous-flow left ventricular assist devices (LVADs) provide durable, reliable, energy-efficient long-term support. However, the biologic effects of continuous flow are not completely known. Therefore, we examined aortic wall morphology in patients with heart failure before and after prolonged circulatory support with a continuous-flow LVAD.

METHODS

After applying a partial aortic occlusion vascular clamp in the lower half of the ascending aorta, we removed samples of aortic wall tissue and then attached the outflow graft of the pump. Samples were obtained from 11 patients (9 men and 2 women, mean age 65 ± 7 years) with severe heart failure at the time of LVAD implantation. We obtained matched specimens at explantation after heart transplantation (n = 5) or autopsy (n = 6). These specimens were removed from the distal ascending aorta, remote from the aortic anastomotic site. Tissue sections were stained with hematoxylin and eosin, Movat's pentachrome and Masson's trichrome. Smooth muscle actin immunohistochemistry was performed on all sections. To evaluate the morphology of the aortic wall media, we quantitatively graded tissue sections for medial thickness, medial degenerative changes, smooth muscle cell (SMC) disorientation and depletion, elastic fiber fragmentation and depletion, medial fibrosis and atherosclerotic changes.

RESULTS

The mean duration of support was 140 ± 136 days (range 87 to 580 days). The histologic evaluation and comparison of specimens obtained before and after LVAD support showed significantly increased foci of medial degeneration, SMC depletion, elastic fiber fragmentation, medial fibrosis and atherosclerotic changes after LVAD support. Mean medial thickness was not significantly different after LVAD support. We observed similar changes between samples obtained at transplantation and those obtained at autopsy.

CONCLUSIONS

After continuous-flow LVAD support, the morphology of the aortic wall media was altered in all of our patients. The clinical relevance of these findings is unknown.

Advances and future directions for mechanical circulatory support

Although cardiac transplant remains the gold standard for the treatment of end-stage heart failure, limited donor organ availability and growing numbers of eligible recipients have increased the demand for alternative therapies. Limitations of first-generation left ventricular assist devices for long-term support of patients with end-stage disease have led to the development of newer second-generation and third-generation pumps, which are smaller, have fewer moving parts, and have shown improved durability, allowing for extended support. The HeartMate II (second generation) and HeartWare (third generation) are 2 devices that have shown great promise as potential alternatives to transplantation in select patients.

Less frequent opening of the aortic valve and a continuous flow pump are risk factors for postoperative onset of aortic insufficiency in patients with a left ventricular assist device

BACKGROUND

Postoperative development of aortic insufficiency (AI) after implantation of left ventricular assist devices (LVADs) has recently been recognized, but the devices in the previous reports have been limited to the HeartMate I or II. The purposes of this study were to determine whether AI develops with other types of LVADs and to elucidate the factors associated with the development of AI.

METHODS AND RESULTS

Thirty-seven patients receiving LVADs without evident abnormalities in native aortic valves were enrolled (pulsatile flow LVAD [TOYOBO]: 76%, continuous flow LVAD [EVAHEART, DuraHeart, Jarvik2000, HeartMate II]: 24%). Frequency of aortic valve opening and grade of AI were evaluated by the most recent echocardiography during LVAD support. None of the patients had more than trace AI preoperatively. During LVAD support AI >- grade 2 developed in 9 patients (24%) across all 5 types of devices. More severe grade of AI correlated with higher plasma B-type natriuretic peptide concentration (r = 0.53, P < 0.01) and with less frequent of the aortic valve (r = 0.45, P < 0.01). Multivariate analysis revealed that lower preoperative left ventricular ejection fraction and a continuous flow device type were independent risk factors for higher incidence of AI.

CONCLUSIONS

AI, which is hemodynamically significant, develops after implantation of various types of LVADs. Physicians need to be more alert to the development of AI particularly with continuous flow devices.

Aortic valve pathophysiology during left ventricular assist device support

The increased applicability and excellent results with left ventricular assist devices (LVADs) have revolutionized the available treatment options for patients with advanced heart failure. Pre-existing valve abnormalities are common in this population, and subsequent development of valve abnormalities after LVAD placement is also often noted. Although native mitral and tricuspid valve disease is more common in heart failure patients before LVAD placement, aortic valves are much more likely to generate abnormal pathophysiology in the LVAD patient during as well as after LVAD placement. The aim of this comprehensive review is to review aortic valve function in LVAD patients and highlight the consideration of pre-existing valve disease on patient treatment at the time of LVAD implant. The basis for structural changes leading to valve pathophysiology during and after LVAD placement will be described, providing a basis for improved clinical understanding and new strategies to prevent these conditions.

Reduced pulsatility induces periarteritis in kidney: role of the local renin-angiotensin system

OBJECTIVE

The need for pulsatility in the circulation during long-term mechanical support has been a subject of debate. We compared histologic changes in calf renal arteries subjected to various degrees of pulsatile circulation in vivo. We addressed the hypothesis that the local renin-angiotensin system may be implicated in these histologic changes.

METHODS AND RESULTS

Sixteen calves were implanted with devices giving differing degrees of pulsatile circulation: 6 had a continuous flow left ventricular assist device (LVAD); 6 had a continuous flow right ventricular assist device (RVAD); and 4 had a pulsatile total artificial heart (TAH). Six other calves were histologic and immunohistochemical controls. In the LVAD group, the pulsatility index was significantly lower (0.28 +/- 0.07 LVAD vs 0.56 +/- 0.08 RVAD, vs 0.53 +/- 0.10 TAH; P < 0.01), and we observed severe periarteritis in all cases in the LVAD group. The number of angiotensin II type 1 receptor-positive cells and angiotensin converting enzyme-positive cells in periarterial areas was significantly higher in the LVAD group (angiotensin II type 1 receptor: 350 +/- 139 LVAD vs 8 +/- 6 RVAD, vs 3 +/- 2 TAH, vs 3 +/- 2 control; P < .001; angiotensin-converting enzyme: 325 +/- 59 LVAD vs 6 +/- 4 RVAD, vs 6 +/- 5 TAH, vs 3 +/- 1 control; P < .001).

CONCLUSIONS

The reduced pulsatility produced by a continuous flow LVAD implantation induced severe periarteritis in the kidneys. The local renin-angiotensin system was up-regulated in the inflammatory cells only in the continuous flow LVAD group.

Quantification of Pulsatility as a Function of Vascular Input Impedance: An In Vitro Study

The physiological benefits of pulsatility generated by ventricular assist device (VAD) support continue to be heavily debated as application of VAD support has been expanded to include destination and recovery therapies. In this study, the relationship between input impedance (Zart) and vascular pulsatility during continuous flow (CF) or pulsatile flow (PF) VAD support was investigated. Hemodynamic waveforms were recorded at baseline failure and with 50%, 75%, and 100% CF or PF VAD support for nine different Zart test conditions (combination of three different resistance and compliance settings) in a mock circulatory system simulating left ventricular failure. High-fidelity hemodynamic pressure and flow waveforms were recorded to calculate mean arterial pressure (MAP), Zart, energy equivalent pressure (EEP), and surplus hemodynamic energy (SHE) as metrics for quantifying vascular pulsatility. MAP and EEP were elevated with increasing resistance whereas SHE was reduced with increasing compliance. Vascular pulsatility was restored with increasing PF VAD support, but diminished by up to 90% with increasing CF VAD support. The nonpulsatile energy component (MAP) of the pressure waveform is dependent on resistance whereas the pulsatile energy component (SHE) is dependent on compliance. The impact of Zart and vascular pulsatility on patient recovery with VAD support warrants further investigation.

Vascular Thrombosis During Support With Continuous Flow Ventricular Assist Devices: Correlation With Computerized Flow Simulations

Continuous flow pumps are increasingly used to treat severe heart failure. These pumps alter flow physiology by lowering pulsatility in the arterial circulation. In patients with peripheral stenosis, continuous flow pumps may lead to thrombosis of peripheral vessels, possibly predisposing to vascular thrombosis in areas of non-flow-limiting stenosis. The authors performed a computerized flow modeling simulation to analyze the effects of altered hemodynamics in a stenotic area. Drawing on previous clinical experience, we modeled a stenotic area in the common carotid artery. Computerized flow modeling revealed blood stagnation zones with low shear stress and velocity adjacent to the stenotic area during nonpulsatile flow. Such stagnation was not present during pulsatile flow. These results indicate a mechanism by which altered physiologic flow may accelerate occlusion of arterial conduits in patients with preexisting stenosis. This finding may be important for patients with continuous flow devices who have peripheral vascular disease; therefore, further study is warranted.

Arteriolar blood flow pulsatility in a patient before and after implantation of an axial flow pump

In a patient with end stage ischemic heart failure scheduled for implantation of an axial flow pump small arteriolar flow pattern was recorded using a novel intravital microscope. Preoperative arteriolar blood flow velocity was highly pulsatile, ranging from about 7 to 16 mm per second in a 12.8 microm diameter arteriole. After implantation of the pump, this pulsatility was abrogated and arteriolar blood flow velocity changed instantaneously with changes in pump speed (eg, 2 mm/s at 5,000 rpm vs 3.5 mm/s at 8,000 rpm in an 8.9 microm diameter arteriole). This lack of flow velocity oscillations may have profound long-term effects on shear stress regulated arteriolar remodeling.

In Vivo Assessment of a Rotary Left Ventricular Assist Device-induced Artificial Pulse in the Proximal and Distal Aorta

The increasing clinical use of rotary left ventricular assist devices (LVADs) suggests that chronic attenuation of arterial pulse pressure has no clinically significant detrimental effects. However, it remains possible that modulating LVAD rotor speed to produce an artificial pulse may be of temporary or occasional benefit. We sought to evaluate a pulse produced by a continuous-flow, centrifugal pump in an ovine thoracic and abdominal aorta. Both ventricles of an adult sheep were resected to eliminate all native cardiac contributions to pulsatility, each replaced by a continuous-flow Thoratec HeartMate III blood pump (Burlington, MA, USA). An LVAD-induced pulsatile flow was achieved by sharply alternating the speed of the magnetically levitated rotor of the left pump between 1,500 rpm (artificial diastole) and 5,500 rpm (artificial systole) at a rate of 60 bpm at a "systolic" interval of 30%. A catheter was advanced from the ascending aorta to the iliac bifurcation via the ventricular assist device outflow graft for pressure measurement and data acquisition. The mean LVAD-induced pulse pressures were 34, 29, 27, and 26 mm Hg in the ascending, thoracic, and abdominal aorta, and the iliac bifurcation, respectively. The maximum rate of pressure rise (deltap/deltat) was between 189 and 238 mm Hg/s, approaching that of the native pulse, although the energy equivalent pressure did not exceed the mean arterial pressure. The HeartMate III's relatively stiff speed control, low rotor mass, and robust magnetic rotor suspension result in a responsive system, enabling very rapid speed changes that can be used to simulate physiologic pulse pressure and deltap/deltat.

Peripheral Vascular Reactivity in Patients With Pulsatile vs Axial Flow Left Ventricular Assist Device Support

BACKGROUND

Left ventricular assist devices (LVADs) are either pulsatile or axial flow devices. The latter can be operated at a low-speed setting to allow pulsatility or at a high-speed setting to create continuous flow. The purpose of this study was to compare the effect of continuous flow and pulsatile flow on peripheral vascular reactivity.

METHODS

Twenty consecutive patients were divided into two groups based on the type of LVAD they received. Ten patients had a pulsatile flow LVAD, and 10 had an axial flow LVAD. For the purpose of the study protocol, the axial flow devices were operated at a high speed to ensure continuous flow. The patients' peripheral artery vasoreactivity was assessed with an ultrasound vascular transducer that measured flow-mediated dilation (FMD).

RESULTS

The FMD of the patients supported with pulsatile flow (15.6 +/- 5%) was higher than the FMD of the patients supported with temporary continuous flow (1.8 +/- 3%). The difference was statistically significant (p < 0.0001).

CONCLUSIONS

Pulsatile flow is associated with a better peripheral vascular reactivity than continuous flow. Patients supported by axial flow devices should be kept on the lowest speed setting to allow maximum pulsatility.

Chronic nonpulsatile blood flow is compatible with normal end-organ function: implications for LVAD development

Evolving blood pump technology has produced user-friendly continuous-flow left ventricular assist devices, but uncertainty exists about the safety of chronic nonpulsatile circulation. Recent experimental and clinical evidence suggest that pulse pressure is not required from a blood pump. End-organ function is well maintained with nonpulsatile systems, although pulse pressure may accelerate recovery from cardiogenic shock. Form follows function, so the effects of reduced pulse pressure on the arterial wall are not surprising. The ability to alter aortic wall morphology by reducing pulse pressure may have important implications for the future treatment of arterial pathology. Both centrifugal and axial-flow pumps can be miniaturized and are reliable and silent. Doubts about the feasibility of long-term circulation with reduced pulse pressure are disappearing.

Optimum control of the Hemopump as a left-ventricular assist device

A general framework for designing an optimum control strategy for the Hemopump is described. An objective function was defined that includes four membership functions, each constructed based on the desired values of one of the four members: stroke volume, mean left atrial pressure, aortic diastolic pressure and mean pump rotation speed. The Hemopump was allowed to operate either at a constant speed or at two different speeds during a cardiac cycle. The goal was to maximise the objective function by varying the magnitude and timing of the pump speed. Using a canine circulatory model, it was demonstrated that, in general, different cardiac conditions or different clinical objectives require different operation parameters. For example, when a left ventricle with minor ischaemia was simulated, and the main objective was to increase stoke volume, the objective function was maximised, from a value of 0.877 when the pump was off, to 0.946 when the pump was operated at speed 2 (18 500 revolutions min(-1)). On the other hand, for a severely ischaemic heart, the optimum pump speed became speed 3 (20 000 revolutions min(-1)), which maximized the objective function to 0.943 (from 0.707 when the pump was off). The results also suggest that it is more beneficial to operate the Hemopump at two different speeds during a cardiac cycle (a higher speed during systole and early diastole, and a lower speed during late diastole) than to maintain a constant speed throughout the cardiac cycle.

In vitro characterization of aortic retrograde and antegrade flow from pulsatile and non-pulsatile ventricular assist devices

BACKGROUND

Many advances have been made in left ventricular assist device (LVAD) development including the introduction of smaller, non-pulsatile pumps. However, controversy exists over the potential significance of non-pulsatile blood flow. In addition, some newer LVADs incorporate descending aortic anastomosis (and therefore retrograde ascending aortic flow) for outflow rather than the traditional ascending aortic anastomosis. This, combined with non-pulsatile flow, may significantly increase the risks of ascending aortic thrombus formation, especially if native cardiac function is negligible and the aortic valve remains closed. The purpose of this study was to compare pulsatile and non-pulsatile flow generated by LVADs with outflow to the ascending aorta and descending aorta.

METHODS

An in vitro mock circulatory loop, driven by either a pulsatile or a non-pulsatile LVAD, was anastomosed to transparent aortic models at either the ascending or descending aortic position. The aortic valve was kept closed, modeling no native cardiac output. Normal saline was used as a blood analog. Methylene blue dye was injected into the ascending aorta and aortic arch to demonstrate flow patterns. Dye washout time (in seconds) was used as a marker of flow stagnation and potential thrombogenicity. LVAD flow, rate, after-load and coronary flow were measured.

RESULTS

Dye washout times at a flow rate of 5 liters/min were 1.7 +/- 0.75, 2.1 +/- 0.71, 4.7 +/- 0.82 and 9.9 +/- 4.4 seconds for pulsatile ascending (PA), non-pulsatile ascending (NPA), pulsatile descending (PD) and non-pulsatile descending flow (NPD), respectively. Coronary flow averaged 294 ml/min over all set-ups. Dye washout times at a flow rate of 4-liters/min were 3.0 +/- 1.0, 3.0 +/- 0.8, 14.0 +/- 3.8 and 25.0 +/- 9.1 seconds for PA, NPA, PD and NPD, respectively. Coronary flow averaged 227 ml/min over all set-ups. Ascending aortic anastomoses were associated with shorter dye washout times compared with descending aortic anastomoses, regardless of flow type (p < 0.001). There was no difference in washout time between pulsatile and non-pulsatile flow in the ascending aortic position (p = 0.23 and 0.12 for 5 and 4 liters/min, respectively). Pulsatile flow in the descending aorta had shorter washout times than non-pulsatile flow in the descending aorta (p < 0.001 and p = 0.004 for 5 and 4 liters/min, respectively).

CONCLUSIONS

LVAD descending aortic anastomosis and retrograde aortic flow is associated with increased flow stagnation in the ascending aorta. This may increase the risk for thrombus formation in patients relying solely on retrograde aortic flow, especially if cardiac function and antegrade blood flow returns.

Hemodynamics of chronic nonpulsatile flow: implications for LVAD development

Both experimental and clinical evidence suggest that pulse pressure is not required from a blood pump. End-organ function is well maintained with nonpulsatile systems, though pulse pressure may accelerate recovery from cardiogenic shock. Form follows function, so the effects of reduced pulse pressure on the arterial wall are not surprising. The ability to alter aortic wall morphology by reducing pulse pressure may have important implications for the future treatment of arterial pathology. Both centrifugal and axial-flow pumps can be miniaturized and are silent. Their reliability and user-friendly status may soon allow implantation at an earlier stage of cardiac deterioration. Doubts about the feasibility of long-term pulseless circulation are disappearing.

The roles of vascular smooth muscle cells in the aortic wall thinness under prolonged continuous flow left heart bypass

Aortic wall thinness was one of the most characteristic changes observed in experimental animals under prolonged continuous flow left heart bypass. The goal of this study was to determine the roles of smooth muscle cells in the vascular remodeling process in cases demonstrating aortic wall thinness under prolonged continuous flow left heart bypass. The aortic samples from three goats in which continuous flow left heart bypass was performed were subjected to histological and immunohistochemical analyses. After 4 weeks of observation, the pulse pressure in the goats under the continuous flow left heart bypass was clearly lower than that in the normal healthy goats. The aortic walls of these goats became thinner, an effect caused by the dilation of their internal diameter. These aortic smooth muscle cells maintained contractile formation due to the fact that they contained abundant alpha-smooth muscle actin (SMA) and smooth muscle myosin heavy chain (SMMS). These cells also synthesized redundant matrix metalloproteinase-2 and -9, and the ratio of the SMMS-positive to the SMA-positive area was significantly lower (0.76) than that observed in the control goat (1.00; P < 0.05). The smooth muscle cells demonstrated synthetic-dedifferentiated formation, which is one of the phenotypes of smooth muscle cell function. In conclusion, aortic wall thinness under prolonged continuous flow left heart bypass is caused by over-synthesis of matrix metalloproteinase in smooth muscle cells, and this refers the vascular remodeling process of the extracellular matrix.

Morphological changes of the arterial systems in the kidney under prolonged continuous flow left heart bypass

We investigated morphological changes of the arterial systems in the kidneys under prolonged continuous flow left heart bypass. Twelve goats were subjected to 2 weeks of pulsatile left heart bypass followed by 4 weeks of continuous flow left heart bypass (group CF). After autopsy, the kidneys underwent pathological evaluation. Six normal healthy goats were used as controls. The media of the afferent arterioles of group CF were frequently thickened by an increase in the number of the mature smooth muscle cells (SMCs). The juxtaglomerular areas (JGA) were expanded because of an increase in the number and size of SMCs and/or SMC-like cells. Furthermore, the percentage of anti-proliferating cell nuclear antigen antibody-positive cells in the JGA of group CF (9.9 +/- 1.9%) was significantly higher (p = 0.025) than that of the control group (4.6 +/- 3.4%), indicating active proliferation in group CF. We concluded that prolonged continuous flow left heart bypass causes proliferation of SMCs and/or SMC-like cells in the afferent arterioles and their perivascular tissue.

End-organ function during chronic nonpulsatile circulation

BACKGROUND

Evolving blood pump technology has produced user-friendly continuous flow left ventricular assist devices, but uncertainty exists about the safety of chronic nonpulsatile circulation. We established consistently nonpulsatile blood flow in a sheep model using the Terumo magnetically suspended centrifugal pump. We then compared end-organ function between pulseless and control animals.

METHODS

Fifteen healthy sheep (65 to 85 kg) were allocated to either left ventricular assist device (n = 9) or control (n = 6) groups. We implanted the device through a left thoracotomy and determined the flow rate at which pulse pressure was absent. The flow rate was then adjusted to exceed that rate (4.2 +/- 1.5 L/min), and all variables of pump function were continuously monitored by computer. Blood tests were taken serially for hepatic and renal function and plasma renin levels. The sheep were sacrificed electively at 30 (n = 3), 90 (n = 4), 180 (n = 1), and 340 (n = 1) days. Detailed histologic examination was made of the brain, liver, kidney, myocardium, and major arteries.

RESULTS

All animals remained in good condition until sacrifice. All measures of end-organ function remained within normal limits for both groups. There were no histologic differences between the organs of pulsatile and nonpulsatile animals. Although there was no significant difference in mean blood pressure, plasma renin levels were substantially elevated in pulseless animals (1.4 +/- 0.3 pg/mL versus 2.9 +/- 0.3 pg/mL; p < 0.05). We also identified thinning of the medial layer of the ascending aorta in nonpulsatile sheep (1.8 +/- 0.4 mm in left ventricular assist device animals versus 2.6 +/- 0.6 mm in control sheep; p < 0.05).

CONCLUSIONS

Chronic nonpulsatile circulation was well tolerated, and we found neither functional nor histologic changes in major end organs. The renin-angiotensin system was upregulated, but this did not provide a significant rise in blood pressure. The changes in the aortic wall merit further investigation. As a result of these findings, we consider that nonpulsatile devices can be used safely for long-term circulatory support.

Simulation study of the Hemopump as a cardiac assist device

A dynamic model was developed for a Hemopump that withdraws blood from the left ventricle and discharges it to the aorta through a miniature axial-flow pump. Incorporation of the Hemopump model in a previously established model for the canine circulatory system enabled the effects of the Hemopump on various haemodynamic variables of the circulatory system to be studied, and the benefit of the Hemopump to the failing heart was investigated. In addition, the influence of the physiological status of the right ventricle on the Hemopump performances was examined, and the synchronous and non-synchronous operations of the Hemopump were compared. Results verified that the Hemopump assists the failing heart by increasing the oxygen supply, while reducing the oxygen consumption of the heart through a reduction in the workload of the left ventricle. These beneficial effects were enhanced when the pump's rotation speed was increased. When pump speed was increased from 17,000 to 23,000 revolutions min-1, the oxygen supply increased 101%, and the oxygen consumption decreased 60%. However, when the pump rotation speed was too high, the inflow to the pump could be impaired and the pump performance could be negatively affected. Predications from the model were in good agreement with the results previously obtained in animal experiments and in vitro measurements.

Change in vasoconstrictive function during prolonged nonpulsatile left heart bypass

We investigated changes in vasoconstrictive function accompanying prolonged nonpulsatile left heart bypass (NLHB). After 2-week pulsatile left heart bypass (PLHB) in 11 goats, NLHB was conducted for another 4 weeks (Group N) in 6 goats. In the other 5 goats, PLHB was continued for another 4 weeks (Group P). Systemic vascular resistance at rest (rSVR) was measured on the last days of the second and sixth postoperative week (W2 and W6, respectively). Subsequently, phenylephrine was injected, and the maximum values (SVRmax) and the maximum increasing change in SVR (DeltaSVR) were measured. No significant difference was observed in rSVR between groups at W2 or W6. The SVRmax and the DeltaSVR at W2 were consistent in both groups. However, at W6, the SVRmax and the DeltaSVR of Group N were significantly lower than those of Group P. In conclusion, prolonged NLHB caused a significant decrease in the SVR response to phenylephrine, indicating a dimunition of vasoconstrictive function.