Sonomicrometric studies on the effects of long-term pumping of cardiac assist device on the bulk and regional mechanics of the normal left ventricle in goat.

Pneumatically driven, diaphragm type left ventricular assist devices (LVADs) were implanted into 2 goats with normal hearts for approximately 1 month to study the effects of long-term pumping of LVAD on the cardiac mechanics. One sham-operated goat was used to obtain control data. Diameters and myocardial segment lengths of the left ventricle were measured with an ultrasonic displacement meter to calculate the bulk mechanical work (BMW) and regional myocardial mechanical work (RMW), respectively. The LVAD was pumped in the 2:1 drive mode (one counterpulsated pumping in every two cardiac cycles), and was temporarily driven in the 1:1 mode (one pumping in every cardiac cycle) or stopped to obtain the data under these conditions. During the second half of the post-operative period while the animal condition was stable, the BMW in the 2:1 and 1:1 modes were approximately 59% and 72% of that observed under the temporary pump-off condition (0.22 W/(100 g)), respectively. The RMW in the 2:1 and 1:1 modes were 69% and 74% of that obtained during pump-off (6.2 mW/cm3), respectively. The myocyte diameter in the subendocardial layer was reduced by unloading effect of 1-month pumping, whereas those in middle and subepicardial layer showed little change.     

Computational analysis of the effect of mitral and aortic regurgitation on the function of ventricular assist devices using 3D cardiac electromechanical model

Valvular insufficiency affects cardiac responses and the pumping efficacy of left ventricular assist devices (LVADs) when patients undergo LVAD therapy. Knowledge of the effect of valvular regurgitation on the function of LVADs is important when treating heart failure patients. The goal of this study was to examine the effect of valvular regurgitation on the ventricular mechanics of a heart under LVAD treatment and the pumping efficacy of an LVAD using a computational model of the cardiovascular system. For this purpose, a 3D electromechanical model of failing ventricles in a human heart was coupled with a lumped-parameter model of valvular regurgitation and an LVAD-implanted vascular system. We used the computational model to predict cardiac responses with respect to the severity of valvular regurgitation in the presence of LVAD treatment. An LVAD could reduce left ventricle (LV) pressure (up to 34%) and end-diastolic ventricular volume (up to 80%) and maintain cardiac output at the estimated flow rate from the LVAD under the condition of mitral regurgitation (MR); however, the opposite would occur under the condition of aortic regurgitation (AR). Considering these physiological responses, we conclude that AR, and not MR, diminishes the pumping function of LVADs.     

Preload sensitivity of the Jarvik 2000 and HeartMate II left ventricular assist devices

The in vitro sensitivity of continuous flow pumps to preload and afterload pressure has been well characterized. We compared flow in the Jarvik 2000 and HeartMate II continuous flow left ventricular assist devices (LVADs) at different inflow and outflow pressures and different pump speeds. This allowed us to measure the impact of a changing inflow pressure on the pump flow rate at different speeds but against a constant afterload. The resulting preload sensitivity curves showed that, overall, both LVADs have a mean preload sensitivity of 0.07 L/min/mm Hg in the physiologic ranges of pressures and flows encountered during normal operation. The HeartMate II pump had an increased preload sensitivity (up to approximately 0.1 L/min/mm Hg) as the preload was increased. The preload sensitivity of the Jarvik 2000 LVAD was more variable, having several peaks and troughs as the preload was increased. In future LVADs, improved preload sensitivity may allow passive regulation of pump output, optimize ventricular unloading, and decrease the risk of ventricular suction by the pump.     

An implantable Fabry-Pérot pressure sensor fabricated on left ventricular assist device for heart failure

Continuous flow left ventricular assist devices (LVADs) are commonly used as bridge-to-transplantation or destination therapy for heart failure patients. However, non-optimal pumping speeds can reduce the efficacy of circulatory support or cause dangerous ventricular arrhythmias. Optimal flow control for continuous flow LVADs has not been defined and calls for an implantable pressure sensor integrated with the LVAD for real-time feedback control of pump speed based on ventricular pressure. A MEMS pressure sensor prototype is designed, fabricated and seamlessly integrated with LVAD to enable real-time control, optimize its performance and reduce its risks. The pressure sensing mechanism is based on Fabry-Pérot interferometer principle. A biocompatible parylene diaphragm with a silicon mirror at the center is fabricated directly on the inlet shell of the LVAD to sense pressure changes. The sensitivity, range and response time of the pressure sensor are measured and validated to meet the requirements of LVAD pressure sensing.     

Design features, developmental status, and experimental results with the Heartmate III centrifugal left ventricular assist system with a magnetically levitated rotor

A long-term left ventricular assist system for permanent use in advanced heart failure is being developed on the basis of a compact centrifugal pump with a magnetically levitated rotor and single-fault-tolerant electronics. Key features include its "bearingless" (magnetic levitation) design, textured surfaces similar to the HeartMate XVE left ventricular assist device (LVAD) to reduce anticoagulation requirements and thromboembolism, a sensorless flow estimator, and an induced pulse mode for achieving an increased level of pulsatility with continuous flow assistance. In vitro design verification testing is underway. Preclinical testing has been performed in calves demonstrating good in vivo performance at an average flow rate of 6 L/min (maximum: >11 L/min) and normal end-organ function and host response. Induced pulse mode demonstrated the ability to produce a physiological pulse pressure in vivo. Thirteen LVADs have achieved between 16 to 40 months of long-term in vitro reliability testing and will be continued until failure. Both percutaneous and fully implanted systems are in development, with a modular connection for upgrading without replacing the LVAD.     

A novel counterpulsation mode of rotary left ventricular assist devices can enhance myocardial perfusion

The effect of rotary left ventricular assist devices (LVADs) on myocardial perfusion has yet to be clearly elucidated, and several studies have shown decreased coronary flow under rotary LVAD support. We have developed a novel pump controller that can change its rotational speed (RS) in synchronization with the native cardiac cycle. The aim of our study was to evaluate the effect of counterpulse mode, which increases the RS in diastole, during coronary perfusion. Experiments were performed on ten adult goats. The EVAHEART LVAD was installed by the left ventricular uptake and the descending aortic return. Ascending aortic flow, pump flow, and coronary flow of the left main trunk were monitored. Coronary flow was compared under four conditions: circuit-clamp, continuous mode (constant pump speed), counterpulse mode (increased pump speed in diastole), and copulse mode (increased pump speed in systole). There were no significant baseline changes between these groups. In counterpulse mode, coronary flow increased significantly compared with that in continuous mode. The waveform analysis clearly revealed that counterpulse mode mainly resulted in increased diastolic coronary flow. In conclusion, counterpulse mode of rotary LVADs can enhance myocardial perfusion. This novel drive mode can provide great benefits to the patients with end-stage heart failure, especially those with ischemic etiology.     

Comparison of the effects of continuous and pulsatile left ventricular-assist devices on ventricular unloading using a cardiac electromechanics model

Left ventricular-assist devices (LVADs) are used to supply blood to the body of patients with heart failure. Pressure unloading is greater for counter-pulsating LVADs than for continuous LVADs. However, several clinical trials have demonstrated that myocardial recovery is similar for both types of LVAD. This study examined the contractile energy consumption of the myocardium with continuous and counter-pulsating LVAD support to ascertain the effect of the different LVADs on myocardial recovery. We used a three-dimensional electromechanical model of canine ventricles, with models of the circulatory system and an LVAD. We compared the left ventricular peak pressure (LVPP) and contractile ATP consumption between pulsatile and continuous LVADs. With the continuous and counter-pulsating LVAD, the LVPP decreased to 46 and 10%, respectively, and contractile ATP consumption decreased to 60 and 50%. The small difference between the contractile ATP consumption of these two types of LVAD may explain the comparable effects of the two types on myocardial recovery.     

Advancements in left ventricular assist devices to prevent pump thrombosis and blood coagulopathy

Mechanical circulatory support (MCS) devices, such as left ventricular assist devices (LVADs) are very useful in improving outcomes in patients with advanced-stage heart failure. Despite recent advances in LVAD development, pump thrombosis is one of the most severe adverse events caused by LVADs. The contact of blood with artificial materials of LVAD pumps and cannulas triggers the coagulation cascade. Heat spots, for example, produced by mechanical bearings are often subjected to thrombus build-up when low-flow situations impair washout and thus the necessary cooling does not happen. The formation of thrombus in an LVAD may compromise its function, causing a drop in flow and pumping power leading to failure of the LVAD, if left unattended. If a clot becomes dislodged and circulates in the bloodstream, it may disturb the flow or occlude the blood vessels in vital organs and cause internal damage that could be fatal, for example, ischemic stroke. That is why patients with LVADs are on anti-coagulant medication. However, the anti-coagulants can cause a set of issues for the patient—an example of gastrointestinal (GI) bleeding is given in illustration. On account of this, these devices are only used as a last resort in clinical practice. It is, therefore, necessary to develop devices with better mechanics of blood flow, performance and hemocompatibility. This paper discusses the development of LVADs through landmark clinical trials in detail and describes the evolution of device design to reduce the risk of pump thrombosis and achieve better hemocompatibility. Whilst driveline infection, right heart failure and arrhythmias have been recognised as LVAD-related complications, this paper focuses on complications related to pump thrombosis, especially blood coagulopathy in detail and potential strategies to mitigate this complication. Furthermore, it also discusses the LVAD implantation techniques and their anatomical challenges.     

Development of counterpulsation algorithm for a moving-actuator type pulsatile LVAD

A pulsatile left ventricular assist device (LVAD) was used to support the aortic blood pumping function of an injured left ventricle, and as a result helped its recovery. It is important to observe a left ventricle's pumping status and to adjust the operating status of a LVAD to reduce the left ventricle's pumping load and thus to enhance its recovery. To observe the left ventricle's pumping status, an electrocardiogram (ECG) signal is generally used because it is a result of the natural heart's blood pumping function. In this paper, we describe the development of an ECG based counterpulsation control algorithm that prevents simultaneous aortic blood co-pumping by a left ventricle and a moving-actuator type pulsatile LVAD and as a result, reduces the natural heart's pumping load. In addition, to verify the algorithm's applicability for LVAD control we designed three ECG based automatic pump control algorithms that use a developed counterpulsation control algorithm. These algorithms control the operating status of a LVAD automatically and, at the same time, maintain a counterpulsing status. The results of in vitro experiments show that the counterpulsing effect between a left ventricle and a LVAD was successfully produced and that the newly designed automatic pump control algorithms met their own control purposes with a counterpulsing effect.     

HeartMate II implantation technique that spares the sternum and ascending aorta

Left ventricular assist devices (LVADs) have become the standard therapy for patients with end-stage heart failure, and the use of LVADs for long-term support has grown exponentially over the past decade. As the number of LVAD implantations has increased, surgeons have faced more challenging cases, such as those in which the patient has previously undergone a sternotomy. The HeartMate II is one of the most widely implanted LVADs. The standard procedure for HeartMate II implantation is median sternotomy and sewing the outflow graft to the ascending aorta. However, in patients with sternal comorbidities, it can be advantageous to use a less invasive approach that avoids this procedure. We describe the case of a 64-year-old man with a history of end-stage ischemic cardiomyopathy who had previously undergone a median sternotomy and a coronary artery bypass grafting operation and had patent grafts. He required a HeartMate II LVAD (destination therapy), which was implanted via a left subcostal incision; the pump was placed subdiaphragmatically, and the outflow graft was sewed to the descending aorta to avoid a complicated redo cardiac operation via median sternotomy and to minimize the risk of injuring the patent bypass grafts. The patient survived for more than 500 days postoperatively. This approach is feasible and could be a safer method for implanting a HeartMate II device in patients with serious comorbidities that preclude the use of the traditional implantation techniques.     

A novel counterpulse drive mode of continuous-flow left ventricular assist devices can minimize intracircuit backward flow during pump weaning

Bridge to recovery has become a major goal after left-ventricular-assist-device (LVAD) implantation thanks to recent development in adjunctive therapies. Precise assessment of native heart function under minimum LVAD support is the key for successful LVAD explantation. However, weaning of centrifugal LVADs normally generates diastolic intracircuit backward flow. This retrograde flow may become excessive load for the native heart during off-pump test. The flow itself is an inevitable characteristic of centrifugal pumps. Therefore, evaluating this retrograde flow in vitro is of considerable significance, even if its amount is different from that in clinical settings. The purpose of this study was to assess diastolic backflow of continuous-flow centrifugal LVADs in a mock circulation model. A centrifugal LVAD (EVAHEART, Sun Medical Technology) was installed in a mock circulation model by the left ventricle uptake and the ascending aortic return. Pump flow was measured at the pump rotational speed of 1000, 1500, 2000, and 2500 rpm, and pulse rate of the virtual native heart was varied to 60, 90, and 120 beats/min. After data collection, pump flow was integrated, and forward and backward intracircuit flow were calculated. As a result, nonphysiological reverse flow of approximately 2.0 L/min exists at the rotational speed, providing 0 L/min mean pump flow. An ideal off-test trial condition should be realizing both ±0 L/min pump flow and no intracircuit backward flow at the same time. We are developing a novel EVAHEART drive mode that can change its rotational speed in synchronization with cardiac cycle with the aim of controlling this retrograde flow with the new drive mode and creating an ideal off-test condition.     

Initial report of bridge to recovery in a patient with DuraHeart LVAD

Continuous-flow left ventricular assist devices (LVADs) provide acceptable clinical results, but the long waiting period for heart transplantation leads to diverse complications. LVAD support can cause reverse left ventricular (LV) remodeling that results in the improvement of LV function and allows LVAD removal. We present a case of successful removal of a DuraHeart LVAD because of sufficient recovery of LV function. Before LVAD removal, we conducted an “LVAD weaning test” by decreasing pump speed and performing an additional normal saline infusion test. We consider that the LVAD weaning test can be used in place of the “pulsatile LVAD off test.”     

Synchrony relationships between the left ventricle and a left ventricular assist device: an experimental study in pigs

Background: Synchronization between the left ventricle and a left ventricular assist device (LVAD) may be important for ventricular unloading and coronary perfusion. We assessed the synchrony between cardiac and LVAD cycles by increasing delays in steps of 100 msec throughout the cycle, under conditions of total and partial left ventricular support. Methods: We studied 7 healthy minipigs weighing 30-40 kg. A 60-cc Berlin Heart Excor LVAD was implanted and connected to a BCM 1200 console, making it possible to synchronize the LVAD systole and the EKG signal with a prefixed delay. We recorded hemodynamic parameters (including aortic, pulmonary, and left ventricular pressure) and LVAD flow for each delay. Results: Intraventricular pressure during LVAD systole was minimized with delays of around 40-80% of one cycle. In addition, total flow was higher under these conditions. Conclusions: This study shows that the synchronous mode of LVAD operation is feasible. Moreover, a delay in device contraction until the second half of the cardiac cycle optimizes ventricular unloading and may eventually improve myocardial recovery.     

Circadian variation of motor current observed in fixed rotation speed continuous-flow left ventricular assist device support

The algorithm for the physiological control provided by left ventricular assist devices (LVADs) has been controversial. In particular, little is known about the physiological control algorithm (such as for achieving physiological circadian rhythms) in continuous-flow LVADs. To investigate the existence of circadian variation, we retrospectively evaluated the LVAD flow-correlated motor current of patients supported by continuous-flow LVADs. The motor current and the pump speed were collected from the external controller every 10 min after device implantation, and the data were divided for every 30-day period, which began on midnight on the first post-operative day. The subjects were 18 patients (mean age 37.7, mean body surface area 1.71 m² at the time of operation) with dilated cardiomyopathy or dilated phase of hypertrophic cardiomyopathy. As of August 1, 2013, the patients’ median support duration was 889 days. The mean calculated dominant period of motor current variation was 24.0 h and the mean amplitude was 11.7 mA for the entire duration. The amplitude of the motor current circadian variation tended to be increased until around the fifth month. The motor current had a tendency to be relatively low during the night time and high during the day time. A significant difference was found between the night-time and day-time mean motor current for the entire duration (p < 0.05). In conclusion, the circadian variation of the motor current could be observed over long term in patients with fixed rotation speed continuous-flow LVAD support.     

Multicenter experience: prevention and management of left ventricular assist device infections

Implantable left ventricular assist devices (LVADs) have demonstrated clinical success in both the bridge-to-transplantation and destination-therapy patient populations; however, infection remains one of the most common causes of mortality during mechanical circulatory support. Thus, serious LVAD infections may negate the benefits of LVAD implantation, resulting in decreased quality of life, increased morbidity and mortality, and increased costs associated with implantation. Prevention of device-related infection is crucial to the cost-effective use of mechanical circulatory support devices. Therefore, adherence to evidence-based infection control and prevention guidelines, meticulous surgical technique and optimal postoperative surgical site care form the foundation for LVAD associated infection prevention.     

Development of a new inflow valve for a 20cc semisoft ventricle: preliminary results

We remodeled and tested our semisoft 20cc ventricle and made a new bileaflet flap inflow valve. Housings, bases, outflow valve, and a newly designed diaphragm were all made by vacuum forming and put together by radiofrequency welding or glue. In vitro, the ventricle produced a cardiac output of 2.5 to 3.0 L/min and showed reliable durability results. Hematological testing showed no important thrombogenicity of the new valve. Cardiac output was higher than expected for the volume of the ventricle, perhaps because of stretching or flow through. Animal experiments with the left ventricular assist device (LVAD) version was done at Ohio State University. Earlier in Utah, we did 20 cc total artificial heart (TAH) implantations and LVAD experiments in lambs and recently in calves with the 60cc TAH version. A soft ventricle is easy to implant and low in production costs.     

Mechanical support of the left ventricle in ischemia induced left ventricular failure: an experimental study.

OBJECTIVE: this study compares the hemodynamic effects of intra-aortic balloon pumping (IABP), left ventricular assist device (LVAD), and extracorporeal membrane oxygenation (ECMO) in left ventricular failure in pigs. METHODS: In 29 pigs weighing 12 +/- 0.7 kg left ventricular failure was induced by ligating the left anterior descending coronary artery. Eight animals served as controls. Eight pigs were treated by IABP, seven by LVAD, and six by ECMO. The study period lasted four hours. Hemodynamic and oxygen transport/uptake parameters were measured continuously or intermittently. RESULTS: Six animals of the ECMO and LVAD groups survived the 4 hour period, but only 3 and 4 animals of the IABP and control groups survived (p less than 0.05). Cardiac index decreased about 48% and 22% in the control and IABP groups (p less than 0.05), whereas there was only a slight decrease in the ECMO (9%) and LVAD (14%) groups. Oxygen delivery fell significantly in the control and IABP groups (p less than 0.05), compared with only a slight change in the LVAD and ECMO groups. CONCLUSION: ECMO is the most effective system for temporary circulatory support in severe ventricular failure. LVAD maintains cardiac output when pulmonary blood flow is provided. IABP is less efficient in supporting the failing heart, especially in the presence of severe ventricular arrhythmias.     

Left ventricular outflow tract obstruction associated with chronic ventricular assist device support

Favorable long-term patient outcome after insertion of a left ventricular assist device (LVAD) as a bridge to recovery or destination therapy for the treatment of end-stage cardiomyopathy is adversely affected by pathophysiologic changes affecting the heart. Alterations in the native aortic valve apparatus, specifically aortic valve cusp fusion, is an example of such a phenomenon and may especially affect patients in cases of bridge to recovery, a rare but reported event. A retrospective review of the last 33 LVAD placements at our institution was conducted, including reviews of operative reports and pathologic examinations of the native hearts. Seven hearts were found to have varying degrees of aortic valve cusp fusion after chronic LVAD support (63-1, 339 days). Five of these patients had native aortic valves, and two had bioprosthetic valves. The left ventricular outflow tracts in two patients were surgically occluded at the time of LVAD insertion. Aortic valve cusp fusion occurs in roughly 25% of patients on chronic LVAD support. This phenomenon may prove to be clinically significant by creating a potential source of emboli and infection. In addition, in the case of myocardial recovery, left ventricular outflow tract obstruction could limit parallel flow and produce suprasystemic ventricular pressures that in turn would elevate left ventricular end diastolic pressures. The latter may contribute to further myocardial injury, ultimately limiting the ability of an otherwise recovered heart to be weaned from LVAD support.     

Use of gated cardiac computed tomography angiography in the assessment of left ventricular assist device dysfunction

The purpose of this study is to describe the utility and limitations of gated contrast-enhanced cardiac computed tomography angiography in assessing left ventricular assist device function. Computed tomography angiography (CTA) was used in 14 patients with left ventricular assist devices (LVADs) who had persistent heart failure symptoms, hemodynamic instability, or potential problems with LVAD flows. Retrospectively gated contrast-enhanced CTA was performed on 64-detector scanner, and the CTA images were postprocessed in multiple curved projections on TeraRecon workstation. This study describes the use of CTA to identify LVAD-related issues that altered clinical management and explores the role of CTA and other techniques in evaluating LVAD function. Six of 14 LVAD patients who demonstrated no abnormality on CTA remained stable with medical management. In the remaining eight patients, CTA was abnormal, including abnormalities specifically related to the LVAD cannula. As a result of findings detected by CTA, six patients underwent surgical intervention, including device exchange and heart transplant. Computed tomography angiography is a noninvasive method that enhances diagnostic evaluation of patients with suspected LVAD dysfunction and can lead to changes in patient management.     

In vitro evaluation of an implantable left ventricular assist device

The development of implantable left ventricular assist devices (LVADs) has almost reached the stage of providing permanent circulatory support in patients who are unsuitable for, or denied, the transplant option. As part of our ongoing haemodynamic evaluation of the Thermo Cardiosystems Inc. (Boston, USA) Mark 14 pneumatic LVAD, pressure-volume loops have been produced from in vitro studies using a modified National Heart Lung and Blood Institute (NHLBI, USA) mock circulatory loop. These studies have demonstrated that during certain phases of the pump cycle non-physiologically high and low pressures are generated within the LVAD. Such abnormal pressures may damage either the bioprosthetic valves in the LVAD or the native heart, and may have adverse effects on cardiovascular control mechanisms.