Evaluation of a pulsatile pediatric ventricular assist device in an acute right heart failure model

Can the temporary use of right ventricular assist devices bridge patients with acute right ventricular failure after cardiac surgery to recovery?

A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was: Can the temporary use of right ventricular assist devices (RVADs) bridge patients to recovery who suffer acute right ventricular failure after cardiac surgery? More than 183 papers were found using the reported search, of which 13 represented the best evidence to answer the clinical question. The authors, journal, date and country of publication, patient group studied, study type, relevant outcomes and results of these papers are tabulated. Indications for surgical intervention included coronary artery bypass surgery, valve replacement, post-heart transplant and left ventricular assist device insertion. Significant reductions in central venous pressure (P = 0.005) and mean pulmonary artery pressures (P < 0.01) were reported during and after RVAD support. Furthermore, increases in right ventricular cardiac output (P < 0.05), right ventricular ejection fraction (P < 0.05), right ventricular stroke work (P < 0.05) and pulmonary artery oxygen saturations (P < 0.05) were also seen. Assessment by one study showed that on Day 7 after RVAD removal, the right ventricular ejection fraction had increased by up to 40%. Dynamic echocardiography studies performed before, during and after RVAD placement demonstrated that after RVAD implantation, right ventricular end-diastolic dimensions (P < 0.05) and right atrial dimensions decreased (P < 0.05) and right ventricular ejection fraction (P < 0.05) increased. Although several studies successfully weaned patients from an RVAD, there were several complications, including bleeding requiring surgical intervention. However, this may be reduced by using percutaneous implantation (bleeding incidence: 4 of 9 patients) rather than by a surgically implanted RVAD (bleeding incidence: 5 of 5 patients). However, mortality is higher in percutaneous RVAD patients rather than in surgical RVAD (80-44%) patients. Causes of death cited for patients on an RVAD included multiorgan failure, sepsis, thromboembolic events, reoccurring right heart failure and failure to wean due to persistent right ventricular failure. We conclude that RVADs have been successfully used to bridge patients to recovery after cardiac surgery; however, RVADs carry numerous risks and a high mortality rate.

Peristaltic hemodynamics of a new pediatric circulatory assist system for Fontan circulation using shape memory alloy fibers

Fontan procedure is one of the common surgical treatments of congenital heart diseases. Patients with Fontan circulation have single ventricle in the systemic circulation with the total cavopulmonary connection. We have been developing a pulmonary circulatory assist device using shape memory alloy fibers for Fontan circulation with total cavopulmonary connection. It consisted of the shape memory alloy fibers, the diameter of which are 100 µm. The fibers could wrap the ePTFE conduit for Fontan TCPC connection from the outside. We designed the sequential motion control system for sophisticated pulmonary hemodynamics by the pulsatile flow generation. In order to achieve pulsatile flow assistance in pulmonary arterial system, we fabricated a mechanical structure by sequential contraction of shape memory alloy fibers. Then, we developed a sequential contraction controller for the assist system, which could reproduce the wall contractile velocity at 6.0 to 20.0 cm/sec. We examined hemodynamic characteristic of its function using a mock circulatory system, which consisted of two overflow tanks representing venous and pulmonary arterial pressures in Fontan circulation. As a result, the pulmonary circulation assist device with sequential contraction could achieve effective promotion of the pulsatility in pulmonary arterial flow.

Forgotten driving forces in right heart failure (Part II): experimental study

BACKGROUND

Cardiac-assist devices for right ventricular failure remain controversial with poor results. This study evaluated a pulsatile cardiac-assist device in an acute right ventricular failure model vs. current therapies.

MATERIALS AND METHODS

Pulmonary regurgitation was created in 12 piglets by valve avulsion and external transfixation of 2 pulmonary artery cusps suspended to the pulmonary arterial wall. The piglets were divided into 2 treatment groups: a pulsatile group P and a non-pulsatile group NP. Management started when severe right ventricular failure was observed (48.1 ± 24.5 min). In group P, pulsatile trousers driven by a pneumatic generator were pulsated intermittently at 40 beats min(-1). Group NP was treated with oral tadalafil 1 mg kg(-1), intravenous fluids, and adrenaline 0.3 μg kg(-1). After 1 h of therapy, cardiac output was significantly better in group P than group NP (1 ± 0.2 vs. 0.7 ± 0.2 L min(-1)). Mean right ventricular pressure (16 ± 6 vs. 24 ± 2 mm Hg) and pulmonary arterial pressure (22 ± 1 vs. 31 ± 2 mm Hg) were lower in group P. Vascular resistances indices were lower in group P than group NP: pulmonary resistance index was 174 ± 60 vs. 352 ± 118 dyne sec cm(-5)kg(-1); systemic resistance index was 611 ± 70 vs. 1215 ± 315 dyne sec cm(-5)kg(-1). Western-blot analysis showed higher endogenous NO synthase expression in group P pulmonary arteries.

CONCLUSIONS

The pulsatile suit can be used safely as a noninvasive cardiac-assist device in acute right ventricular failure. This represents a cost-effective nearly physiological method, suitable for adults and children.

Ovine models for chronic heart failure

PURPOSE

Testing and optimizing of surgical therapies for chronic heart failure (CHF) requires large animal models. CHF has been induced in several large animal species. Sheep have modest body mass increase and demonstrate docile behavior and are therefore a preferred species in research on surgical therapies for CHF METHODS: A literature search for existing ovine CHF models was performed, using search terms "sheep" and "heart failure". Relevant secondary references were traced.

RESULTS

Rapid ventricular pacing produces rapid-onset CHFE Its severity ranges from moderate left ventricular failure to severe biventricular failure, depending on length and frequency of pacing. Its counterpart in human CHF is tachycardia-induced HF since it is reversible upon cessation of pacing. Myocardial damage models include CHF induced by cardiototoxic drugs and ischemia. Ischemia-based models include coronary microembolization, occlusion and ischemia/reperfusion models. The microembolization model is relevant to diabetic cardiomyopathy. Coronary occlusion models exhibit variable functional impairment, some with aneurysm formation, and some with mitral valve regurgitation, depending on occlusion localization. They are relevant to CHF following non-reperfused myocardial infarction. Coronary occlusion/reperfusion models are relevant to the occurrence of human ãã despite coronary artery recanalization. Pressure overload of left and right ventricle is induced by aortic and pulmonary artery banding, respectively. Hypertrophy precedes CHF as in patients with valve stenosis and hypertension. Volume overload is induced by valve damage or shunt creation. Atrioventricular valve regurgitation is the most important clinical counterpart.

CONCLUSION

Several ovine CHF models exist. Since they exhibit important cardiac pathology differences, the choice of model should be based on the specific experimental question.

Structural design of a newly developed pediatric circulatory assist device for Fontan circulation by using shape memory alloy fiber

Total cavopulmonary connection (TCPC) is commonly applied for the surgical treatment of congenital heart disease such as single ventricle in pediatric patients. Patients with no ventricle in pulmonary circulation are treated along with Fontan algorithm, in which the systemic venous return is diverted directly to the pulmonary artery without passing through subpulmonary ventricle. In order to promote the pulmonary circulation after Fontan procedure, we developed a newly designed pulmonary circulatory assist device by using shape memory alloy fibers. We developed a pulmonary circulatory assist device as a non-blood contacting mechanical support system in pediatric patients with TCPC. The device has been designed to be installed like a cuff around the ePTFE TCPC conduit, which can contract from outside. We employed a covalent type functional anisotropic shape memory alloy fiber (Biometal, Toki Corporation, Tokyo Japan) as a servo actuator of the pulmonary circulatory assist device. The diameter of this fiber was 100 microns, and its contractile frequency was 2-3 Hz. Heat generation with electric current contracts these fibers and the conduit. The maximum contraction ratio of this fiber is about 7% in length. In order to extend its contractile ratio, we fabricated and installed mechanical structural units to control the length of fibers. In this study, we examined basic contractile functions of the device in the mock system. As a result, the internal pressure of the conduit increased to 63 mmHg by the mechanical contraction under the condition of 400 msec-current supply in the mock examination with the overflow tank of 10 mmHg loading.

Mechanical support of the functionally single ventricle

Children with a functionally single ventricle constitute just over 1% of congenital cardiac defects.1A majority of children with the functionally univentricular circulation undergo a three-staged reconstruction to achieve completion of the Fontan circulation. The first stage is usually performed in the neonatal period, and is either banding of the pulmonary trunk, an aorto-pulmonary shunt alone, or the shunt included as part of the first stage of reconstruction. In recent years, a conduit placed from the right ventricle to the pulmonary arteries is being used as an alternate source of flow of blood to the lungs. The second stage is the bidirectional cavopulmonary anastomosis, the two surgical variations being the so-called “hemifontan”, and “bidirectional Glenn” procedures, while the third stage is the completion of the Fontan circulation, the two surgical variations being either construction of a lateral tunnel, or placement of an extra-cardiac conduit, each being possible with or without a fenestration. In many centres, patients with the functionally univentricular circulation make up one-fifth of the total surgical volume. The syndrome of low cardiac output is quite common in this population through all three stages of reconstruction, and some of these patients will eventually require cardiac transplantation. While conventional therapy, with inotropic support and afterload reduction, remains the mainstay of therapy for the failing heart in children, mechanical support is being increasingly used.3Most of this experience is limited to extracorporeal membrane oxygenation.2–5In this review, we discuss the current experiences with extracorporeal membrane oxygenation in patients with a functionally univentricular circulation, and describes some of their unique features. We also focus on the pulsatile ventricular assist devices capable of providing support over the longer term, and other new devices that may have a role in the future in these patients.6

Perioperative management of ventricular assist devices in children and adolescents

Ventricular assist devices are an integral part of therapy for patients with end-stage heart failure. Devices can either bridge to recovery or to transplantation. Options for ventricular assist devices include those that are centrifugal, pulsatile, and new rotary/axial devices. Care of the patient on a ventricular assist device is multifaceted, involving pharmacologic or mechanical support of the right ventricle, management of systemic vascular resistance, and manipulation of the hematologic system to avoid bleeding or thrombosis. In addition, care of these patients involves support of all end organs and avoidance of infectious complications. Care of these patients is complex, requiring a highly integrated team for optimal outcome.

The PediPump: a new ventricular assist device for children

Options for mechanical circulatory support for the treatment of end-stage heart failure in children are limited. Ventricular assist devices (VADs), which have revolutionized cardiac care in adults, remain largely unavailable for pediatric applications. The PediPump is a new rotary dynamic VAD designed to provide support for the entire range of patient sizes encountered in pediatrics. Despite being much smaller than currently available VADs, which makes it suitable for even newborn circulatory support, the PediPump demonstrates excellent hemodynamic performance.

Computational Design and Experimental Testing of a Novel Axial Flow LVAD

Thousands of cardiac failure patients per year in the United States could benefit from long-term mechanical circulatory support as destination therapy. To provide an improvement over currently available devices, we have designed a fully implantable axial-flow ventricular assist device with a magnetically levitated impeller (LEV-VAD). In contrast to currently available devices, the LEV-VAD has an unobstructed blood flow path and no secondary flow regions, generating substantially less retrograde and stagnant flow. The pump design included the extensive use of conventional pump design equations and computational fluid dynamics (CFD) modeling for predicting pressure-flow curves, hydraulic efficiencies, scalar fluid stress levels, exposure times to such stress, and axial fluid forces exerted on the impeller for the suspension design. Flow performance testing was completed on a plastic prototype of the LEV-VAD for comparison with the CFD predictions. Animal fit trials were completed to determine optimum pump location and cannulae configuration for future acute and long-term animal implantations, providing additional insight into the LEV-VAD configuration and implantability. Per the CFD results, the LEV-VAD produces 6 l/min and 100 mm Hg at a rotational speed of approximately 6300 rpm for steady flow conditions. The pressure-flow performance predictions demonstrated the VAD's ability to deliver adequate flow over physiologic pressures for reasonable rotational speeds with best efficiency points ranging from 25% to 30%. The CFD numerical estimations generally agree within 10% of the experimental measurements over the entire range of rotational speeds tested. Animal fit trials revealed that the LEV-VAD's size and configuration were adequate, requiring no alterations to cannulae configurations for future animal testing. These acceptable performance results for LEV-VAD design support proceeding with manufacturing of a prototype for extensive mock loop and initial acute animal testing.

Pediatric Mechanical Circulatory Support

Extracorporeal membrane oxygenation (ECMO) is based on technology that has been used clinically for decades. Largely because of its widespread use in pediatric patients with respiratory failure, familiarity with this technology has established ECMO as the most commonly used form of circulatory support for children. However, clinical applications and technical aspects of ECMO support continue to evolve, ensuring the ongoing importance of this modality for pediatric circulatory support. Although traditionally lagging behind development for adult applications, the use of ventricular assist devices (VADs) is expanding in pediatrics. Of particular interest, a number of new VADs designed specifically for children are becoming available, while early development of some innovative devices for pediatric circulatory support is currently underway.

The PediPump: Development Status of a New Pediatric Ventricular Assist Device

The PediPump is a new rotary dynamic ventricular assist device designed specifically for pediatric applications. Although it is capable of providing support for adults, the small size of the PediPump makes it suitable for newborn circulatory support while retaining excellent hemodynamics. Current and future development plans include: (1) determination of the basic engineering requirements for hardware and control logic, including design analysis for system sizing, evaluation of control concepts and bench testing of prototypes; (2) performance of preclinical anatomical fitting studies using computed tomography-based three-dimensional modeling; and, (3) evaluation with animal studies to provide characterization and reliability testing of the device.

Bridge-to-recovery from Acute Myocarditis in a 12-year-old Child

Fulminant myocarditis causes substantial morbidity and mortality, especially in children and young adults. Mechanical circulatory support has become the standard therapy to bridge patients with intractable heart failure to either transplantation or myocardial recovery. Yet, successful weaning from biventricular support with full recovery is extremely rare in the pediatric population. This report describes the successful use of the MEDOS HIA ventricular assist device to bridge a 12-year-old girl to myocardial recovery in a biventricular bypass configuration. The left and right ventricle were completely off-loaded by the pumps and the device provided sufficient cardiac output to normalize end-organ function. Anticoagulation was maintained with i.v. heparin infusion. No neurological complications were detectable and the pump system was free of any macroscopic thrombi. After 19 days of support, cardiac function had recovered and the patient was successfully weaned from the device. Following physical rehabilitation, the patient was discharged home.

Mechanical Cardiopulmonary Support of Infants and Children With Congenital Heart Disease

Cardiac indications for the use of mechanical cardiopulmo nary support techniques in infants and children include short-term circulatory support during reversible myocardial failure, cardiopulmonary support before and after cardiac surgery, and a bridge to cardiac transplantation. For practi cal purposes, 3 modalities are currently available for these patients: extracorporeal membrane oxygenation, use of ven tricular assist devices, and intra-aortic balloon pump coun terpulsation. Although a variety of devices are available for larger patients, the need for smaller sizes and a wider range of sizes has delayed their use in children. This article sum marizes the current systems available for children as well as indications and outcome data related to their use in infants and children with congenital or acquired heart disease. Copyright © 2001 by W.B. Saunders Company.

Pulsatile pediatric ventricular assist devices: Current results for bridge to transplantation

While pulsatile ventricular assist devices have gained widespread use in adult patients awaiting heart transplantation, only very limited experience with these devices exists in the pediatric population. In the past mostly non-pulsatile systems such as ECMO have been used to support pediatric patients with heart failure for very limited periods of time. Only recently have miniaturized pulsatile devices became available. This article describes the technical characteristics of these devices which have been implanted at the Deutsches Herzzentrum Berlin since 1992. The Berlin Heart assist device has since been used in 34 children between the ages of 6 days and 16 years. We report our strategy in patient selection, perioperative care and subsequent heart transplantation. The current literature on mechanical circulatory support in children is reviewed. Copyright 1999 by W.B. Saunders Company