Which patient, which pump?

Computations of the shear stresses distribution experienced by passive particles as they circulate in turbulent flow: A case study for vWF protein molecules

The stress distribution along the trajectories of passive particles released in turbulent flow were computed with the use of Lagrangian methods and direct numerical simulations. The flow fields selected were transitional Poiseuille-Couette flow situations found in ventricular assist devices and turbulent flows at conditions found in blood pumps. The passive particle properties were selected to represent molecules of the von Willebrand factor (vWF) protein. Damage to the vWF molecule can cause disease, most often related to hemostasis. The hydrodynamic shear stresses along the trajectories of the particles were calculated and the changes in the distribution of stresses were determined for proteins released in different locations in the flow field and as a function of exposure time. The stress distributions indicated that even when the average applied stress was within a safe operating regime, the proteins spent part of their trajectories in flow areas of damaging stress. Further examination showed that the history of the distribution of stresses applied on the vWF molecules, rather than the average, should be used to evaluate hydrodynamically-induced damage.

Distribution and history of extensional stresses on vWF surrogate molecules in turbulent flow

The configuration of proteins is critical for their biochemical behavior. Mechanical stresses that act on them can affect their behavior leading to the development of decease. The von Willebrand factor (vWF) protein circulating with the blood loses its efficacy when it undergoes non-physiological hemodynamic stresses. While often overlooked, extensional stresses can affect the structure of vWF at much lower stress levels than shear stresses. The statistical distribution of extensional stress as it applies on models of the vWF molecule within turbulent flow was examined here. The stress on the molecules of the protein was calculated with computations that utilized a Lagrangian approach for the determination of the molecule trajectories in the flow filed. The history of the stresses on the proteins was also calculated. Two different flow fields were considered as models of typical flows in cardiovascular mechanical devises, one was a Poiseuille flow and the other was a Poiseuille–Couette flow field. The data showed that the distribution of stresses is important for the design of blood flow devices because the average stress can be below the critical value for protein damage, but tails of the distribution can be outside the critical stress regime.

Pediatric Mechanical Circulatory Support

Over the past two decades clinicians and researchers have sought to bring mechanical circulatory support (MCS) to pediatric patients with heart failure. ECMO, IABPs, and VADs have all been used in infants and children as a bridge to myocardial recovery or as a bridge to transplant. However, until recently, a commitment by industry, government, and researchers towards the development of pediatric MCS has not been present, especially in the United States. Advancements in adult VAD design to smaller, quieter, and fully implantable pumps capable of complete outpatient support have sparked curiosity in the application of this technology to children. Also, the increasing success of palliating congenital heart disease is creating an ever-growing cohort of children and adolescents with heart failure. These changing demographics and technological advances have caused a refocus of attention. This is most clearly demonstrated by the international use of several established MCS pediatric and neonatal systems, by the FDA's increasing proclivity to allow the use of international pediatric VADs in the USA, and by the recent National Heart, Lung, and Blood Institute funding of several institutions to develop pediatric VADs. This review describes the different aspects of pediatric MCS including indications, the features of the various VADs, and their current application in children worldwide.

An insight into short- and long-term mechanical circulatory support systems

Cardiogenic shock due to acute myocardial infarction, postcardiotomy syndrome following cardiac surgery, or manifestation of heart failure remains a clinical challenge with high mortality rates, despite ongoing advances in surgical techniques, widespread use of primary percutaneous interventions, and medical treatment. Clinicians have, therefore, turned to mechanical means of circulatory support. At present, a broad range of devices are available, which may be extracorporeal, implantable, or percutaneous; temporary or long term. Although counter pulsation provided by intra-aortic balloon pump (IABP) and comprehensive mechanical support for both the systemic and the pulmonary circulation through extracorporeal membrane oxygenation (ECMO) remain a major tool of acute care in patients with cardiogenic shock, both before and after surgical or percutaneous intervention, the development of devices such as the Impella or the Tandemheart allows less invasive forms of temporary support. On the other hand, concerning mid-, or long-term support, left ventricular assist devices have evolved from a last resort life-saving therapy to a well-established viable alternative for thousands of heart failure patients caused by the shortage of donor organs available for transplantation. The optimal selection of the assist device is based on the initial consideration according to hemodynamic situation, comorbidities, intended time of use and therapeutic options. The present article offers an update on currently available mechanical circulatory support systems (MCSS) for short and long-term use as well as an insight into future perspectives.

Ventricular-assist device therapy in children

Mechanical circulatory support is an important modality in the management of paediatric patients with circulatory failure. The field of ventricular assist device (VAD) is particularly evolving. We review the currently available and developing VADs in the paediatric population, and the anaesthetic management of patients presenting for VAD implantation and with VADs for non-cardiac surgery.

The use of computational fluid dynamics in the development of ventricular assist devices

Progress in the field of prosthetic cardiovascular devices has significantly contributed to the rapid advancements in cardiac therapy during the last four decades. The concept of mechanical circulatory assistance was established with the first successful clinical use of heart-lung machines for cardiopulmonary bypass. Since then a variety of devices have been developed to replace or assist diseased components of the cardiovascular system. Ventricular assist devices (VADs) are basically mechanical pumps designed to augment or replace the function of one or more chambers of the failing heart. Computational Fluid Dynamics (CFD) is an attractive tool in the development process of VADs, allowing numerous different designs to be characterized for their functional performance virtually, for a wide range of operating conditions, without the physical device being fabricated. However, VADs operate in a flow regime which is traditionally difficult to simulate; the transitional region at the boundary of laminar and turbulent flow. Hence different methods have been used and the best approach is debatable. In addition to these fundamental fluid dynamic issues, blood consists of biological cells. Device-induced biological complications are a serious consequence of VAD use. The complications include blood damage (haemolysis, blood cell activation), thrombosis and emboli. Patients are required to take anticoagulation medication constantly which may cause bleeding. Despite many efforts blood damage models have still not been implemented satisfactorily into numerical analysis of VADs, which severely undermines the full potential of CFD. This paper reviews the current state of the art CFD for analysis of blood pumps, including a practical critical review of the studies to date, which should help device designers choose the most appropriate methods; a summary of blood damage models and the difficulties in implementing them into CFD; and current gaps in knowledge and areas for future work.

Mechanical support in childhood heart failure

Despite optimization of standard medical therapy, some patients with chronic heart failure will deteriorate to the point that they require hospitalization for intravenous therapies and inpatient monitoring. Once the condition is recognized, the therapeutic goals are to reverse hemodynamic derangements, correct metabolic abnormalities, and provide symptomatic relief. Achievement of these goals requires individualized care and a familiarity with the risks and benefits of particular therapies.

Using extracorporeal life support to resuscitate adult postcardiotomy cardiogenic shock: treatment strategies and predictors of short-term and midterm survival

BACKGROUND

Postcardiotomy extracorporeal life support (ECLS) is a resource-demanding therapy with varied results among institutions. An organized protocol was necessary to improve the effectiveness of this therapy.

METHODS AND RESULTS

A total of 110 patients received ECLS due to refractory postcardiotomy cardiogenic shock between January 2003 and June 2009, and were eligible for inclusion in this retrospective study. Preoperative, perioperative, and postoperative variables were collected, including the European system for cardiac operative risk evaluation (EuroSCORE) and markers of ECLS-related organ injuries. All variables were analyzed for possible associations with mortality in hospital, and after hospital discharge. The mean age, additive EuroSCORE, and left ventricular ejection fraction (LVEF) for all patients was 60 (+/-14) years, 9 (+/-6), and 43% (+/-20%) respectively. Sixty-seven patients were weaned from ECLS and 46 survived to hospital discharge. The mean duration of ECLS support was 143 h (+/-112 h). Multivariate analysis revealed that an age of >60 years, a necessity for postoperative continuous arteriovenous hemofiltration, a maximal serum total bilirubin >6 mg/dL, and a need for ECLS support for >110 h were independent predictors of in-hospital mortality. In addition, persistent heart failure with LVEF <30% was an independent predictor of mortality after hospital discharge. A risk-predicting score for in-hospital mortality associated with postcardiotomy ECLS was developed for clinical application.

CONCLUSION

Based on the abovementioned findings, a comprehensive protocol for postcardiotomy ECLS was designed. The primary objective was to achieve adequate hemodynamics within the first 24h of initiating ECLS. Other objectives of the protocol included a consistent approach to safe anticoagulation while on ECLS, a process to make decisions within 7 days of initiating ECLS, and patient follow-up after hospital discharge.

Axial magnetic bearing development for the BiVACOR rotary BiVAD/TAH

A suspension system for the BiVACOR biventricular assist device (BiVAD) has been developed and tested. The device features two semi-open centrifugal impellers mounted on a common rotating hub. Flow balancing is achieved through the movement of the rotor in the axial direction. The rotor is suspended in the pump casings by an active magnetic suspension system in the axial direction and a passive hydrodynamic bearing in the radial direction. This paper investigates the axial movement capacity of the magnetic bearing system and the power consumption at various operating points. The force capacity of the passive hydrodynamic bearing is investigated using a viscous glycerol solution. Axial rotor movement in the range of +/-0.15 mm is confirmed and power consumption is under 15.5 W. The journal bearing is shown to stabilize the rotor in the radial direction at the required operating speed. Magnetic levitation is a viable suspension technique for the impeller of an artificial heart to improve device lifetime and reduce blood damage.

Strategies for temporary mechanical support: contemporary experience with pulsatile and non-pulsatile support systems

Despite advances in mechanical circulatory support, cardiogenic shock continues to have a high mortality. We reviewed our experience with pulsatile versus non-pulsatile temporary mechanical support at our institution to determine optimal strategy for survival. From January 2001 to December 2003, mechanical support for cardiogenic shock was instituted in 38 patients. Non-pulsatile devices (NP group) were used in 22 patients and pulsatile devices (P group) in 16 patients. Indications for the NP group were post-cardiotomy shock (PCS) in 17, myocardial infarction in 2, and isolated post-cardiotomy right ventricular failure in 3 patients. In the P group, 9 had the device placed for PCS, 3 for viral myocarditis, 1 after myocardial infarction, and 3 for right ventricular (RV) failure. Overall, bleeding, limb ischemia, and multi-system organ failure were higher in NP group with 5 weaned and 3 surviving to discharge (14%). In the P group, survivors included 7 weaned and 3 transplanted patients (63%). With the exception of isolated RV failure, we obtained a dismal survival result with ECMO/centrifugal circuits for treatment of cardiogenic shock. For refractory pump failure, improved survival was achieved by using intermediate-term pulsatile devices with early transition to a chronic device and/or heart transplantation.

The physiologic basis for the management of ventricular assist devices

Mechanical ventricular assist devices are now approved as destination therapy for terminal heart failure. It is the purpose of this review to discuss the physiology of this technology that is considered in outpatient care. The currently available pulsatile devices are solely dependent of preload volume and, when placed in the automatic mode, can maintain physiologic cardiac outputs with exercise. However, because of their dependence on preload volume, there are unique physiologic consequences; device bradycardia represents volume depletion, device tachycardia reflects volume overload. The differential diagnosis of left ventricular assist device dysfunction includes native right ventricular failure, native left ventricular recovery, or other technical considerations. The management of biventricular mechanical support as well as arrhythmia management and the role of echocardiographic assessment in this unique patient population will be discussed. Expertise in outpatient management of such devices is now a requisite for subspecialists in heart failure, In the future, technical innovations may simplify management for professionals, patients, and their families.

Detection of Left Ventricle Function From a Magnetically Levitated Impeller Behavior

The magnetically levitated (Mag-Lev) centrifugal rotary blood pump (CRBP) with two-degrees-of-freedom active control is promising for safe and long-term support of circulation. In this study, Mag-Lev CRBP controllability and impeller behavior were studied in the simulated heart failure circulatory model. A pneumatically driven pulsatile blood pump (Medos VAD [ventricular assist device]-54 mL) was used to simulate the left ventricle (LV). The Mag-Lev CRBP was placed between the LV apex and aortic compliance tank simulating LV assistance. The impeller behavior in five axes (x, y, z, theta, and phi) was continuously monitored using five eddy current sensors. The signals of the x- and y-axes were used for feedback active control, while the behaviors of the other three axes were passively controlled by the permanent magnets. In the static mock circuit, the impeller movement was controlled to within +/-10-+/-20 microm in the x- and y-axes, while in the pulsatile circuit, LV pulsation was modulated in the impeller movement with the amplitude being 2-22 microm. The amplitude of impeller movement measured at 1800 rpm with the simulated failing heart (peak LV pressure [LVP] = 70 mm Hg, mean aortic pressure [AoP(mean)] = 55 +/- 20 mm Hg, aortic flow = 2.7 L/min) was 12.6 microm, while it increased to 19.2 microm with the recovered heart (peak LVP = 122 mm Hg, AoP(mean) = 100 +/- 20 mm Hg, aortic flow = 3.9 L/min). The impeller repeated the reciprocating movement from the center of the pump toward the outlet port with LV pulsation. Angular rotation (theta, phi) was around +/-0.002 rad without z-axis displacement. Power requirements ranged from 0.6 to 0.9 W. Five-axis impeller behavior and Mag-Lev controller stability were demonstrated in the pulsatile mock circuit. Noncontact drive and low power requirements were shown despite the effects of LV pulsation. The impeller position signals in the x- and y-axes reflected LV function. The Mag-Lev CRBP is effective not only for noncontact low power control of the impeller, but also for diagnosis of cardiac function noninvasively.

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.

Lessons learned from the first application of the DeBakey VAD Child: An intracorporeal ventricular assist device for children

We report and describe the design and the first clinical implantation of the DeBakey ventricular assist device (VAD) Child, a pediatric intracorporeal left ventricular assist device, in a 6-year-old girl. The risk-benefit of novel technologies in advanced heart failure and the lessons learned by our experience are important to consider in hopes that other centers (where this device is now available) may benefit.

Left ventricular assist devices: evolving devices and indications for use in ischemic heart disease

PURPOSE OF REVIEW

The mortality with end-stage heart failure is extremely high, especially when patients become refractory to conventional medical therapy and require frequent hospitalization. Ischemic heart disease remains the primary cause of advanced heart failure. Mechanical pumps or devices have been developed called ventricular assist devices and are being used to support an increasing number of patients with refractory heart failure.

RECENT FINDINGS

The use of ventricular assist devices has evolved from initially only support of patients unable to be weaned from a heart-lung machine after cardiac surgery to use now as a bridge to a heart transplant, including patients with acute myocardial infarction and shock and severe pulmonary hypertension. More recently, they have been proven as a definitive alternative for patients not eligible for heart transplantation. There are new devices being examined in clinical trials, including a change from pusher-plate to devices with axial flow technology that are much smaller and easier to implant. Outcomes with their use are improving rapidly as the devices become more reliable and more is learned about the importance of candidate selection.

SUMMARY

This review describes current indications for the use of these devices, the types of pumps now available, criteria for initiating ventricular assist device support, complications of their use, and new applications such as a platform for stem cell therapy for treatment of end-stage heart failure.

Management of chronic congestive heart failure in children

The medical management of congestive heart failure involves manipulating myocardial contractility and loading conditions to achieve optimal performance. Medication may be used to counteract potentially deleterious neurohumoral changes that are associated with congestive heart failure. When appropriate, the correction of the underlying cardiac defect by surgery or catheter intervention is usually the most effective treatment for congestive heart failure in children.