Implantation techniques for the Hemopump

The Evolution of Durable, Implantable Axial-Flow Rotary Blood Pumps

Left ventricular assist devices (LVADs) are increasingly used to treat patients with end-stage heart failure. Implantable LVADs were initially developed in the 1960s and 1970s. Because of technological constraints, early LVADs had limited durability (eg, membrane or valve failure) and poor biocompatibility (eg, driveline infections and high rates of hemolysis caused by high shear rates). As the technology has improved over the past 50 years, contemporary rotary LVADs have become smaller, more durable, and less likely to result in infection. A better understanding of hemodynamics and end-organ perfusion also has driven research into the enhanced functionality of rotary LVADs. This paper reviews from a historical perspective some of the most influential axial-flow rotary blood pumps to date, from benchtop conception to clinical implementation. The history of mechanical circulatory support devices includes improvements related to the mechanical, anatomical, and physiologic aspects of these devices. In addition, areas for further improvement are discussed, as are important future directions-such as the development of miniature and partial-support LVADs, which are less invasive because of their compact size. The ongoing development and optimization of these pumps may increase long-term LVAD use and promote early intervention in the treatment of patients with heart failure.

The Enabler Cannula Pump: A Novel Circulatory Support System

Background

The enabler circulatory support system is a catheter pump which expels blood from the left or right ventricular cavity and provides pulsatile flow in the ascending aorta or pulmonary artery. It is driven by a bedside installed pulsatile driving console. The device can easily be implanted by a minimal invasive approach, similar to the Hemopump.

Purpose

To demonstrate the hemodynamic performance of this new intracardiac support system.

Methods

In a series of 9 sheep, hemodynamic evolutions were recorded in various conditions of myocardial contractility (the non-failing, the moderately failing and the severely failing heart). Heart failure was induced by injection of microspheres in the coronary arteries.

Results

Introduction of the cannula through the aortic valve was feasible in all cases. Pump flow by the enabler was gradually increased to a maximum of 3.5 L/min. Diastolic (and mean) aortic blood pressure is significantly increased in the non-failing and moderately failing condition (counterpulsation mode). In heart failure, cardiac output is significantly increased by the pump (p<0.0001). A drop in left atrial pressure (indicating unloading) is achieved in all conditions but reaches significant levels only during heart failure (p=0.0068).

Conclusions

This new circulatory support system contributes to stabilization of the circulation in the presence of cardiac unloading. In heart failure it actually supports the circulation by increasing cardiac output and perfusion pressure.

Hydraulic Refinement of An Intraarterial Microaxial Blood Pump

Intravascularly operating microaxial pumps have been introduced clinically proving to be useful tools for cardiac assist (1–4). However, a number of complications have been reported in literature associated with the extra-corporeal motor and the flexible drive shaft cable (5,6). In this paper, a new pump concept is presented which has been mechanically and hydraulically refined during the developing process. The drive shaft cable has been replaced by a proximally integrated micro electric motor and an extra-corporeal power supply (7). The conduit between pump and power supply consists of only an electrical power cable within the catheter resulting in a device which is indifferent to kinking and small curvature radii. Anticipated insertion difficulties, as a result of a large outer pump diameter, led to a two-step approach with an initial 6,4mm pump version and a secondary 5,4mm version. Both pumps meet the hydraulic requirement of at least 2.5I/min at a differential pressure of 80–100mmHg. The hydraulic refinements necessary to achieve the anticipated goal are based on ongoing hydrodynamic studies of the flow inside the pumps. Flow visualization on a 10:1 scale model as well as on 1:1 scale pumps have yielded significant improvements in the overall hydraulic performance of the pumps. One example of this iterative developing process by means of geometrical changes on the basis of flow visualization is illustrated for the 6.4mm pump.

Predicting the Dimensions of an Intracardiac Partial-Assist Pump for Percutaneous Delivery by Analytical and Numerical Methods

A minimally invasive ventricular assist device is under development for percutaneous insertion into the left atrium via transseptal access from the right atrium (RA). This study aimed to mathematically describe the vascular anatomy along possible insertion pathways to determine the device's maximum outer dimensions. We developed 2-dimensional mathematical models describing the vascular anatomy to the RA from three access points: subclavian vein (SCV), internal jugular vein (IJV), and femoral vein (FV). All pathways terminated by turning from the superior or inferior vena cava (SVC/IVC) into the RA. The model equations were based on restriction points in the pathways and were solved using anatomic size values 1 SD below published mean values so that the device will accommodate most patients. Vessels were considered rigid so that vessel deformation (and therefore risk) is minimized during device insertion. Maximum device length was calculated for a range of device diameters. The length at the most constraining angle in each turn was the maximum allowable device length. The least restrictive pathway was from the right FV, the turn from the IVC through the atrial septum being the most restrictive point. For a 10-mm diameter device, the length restriction for this pathway was 45 mm, whereas those for the right IJV and SCV were 42 and 21 mm, respectively. Medical device developers can apply these models to determine size specifications of new devices, whereas interventional physicians can apply them to determine if an existing device is appropriate for an individual patient.

Design and test of a vascular access device

Transarterial left ventricular assist devices (LVADs), such as the Hemopump, IABP, and PUCA-pump, are meant to be introduced into the body via the femoral or axillary artery without major surgery. For certain applications, introduction is performed directly into the aorta via an open thorax procedure. A prototype of a vascular access device has been realized that allows direct access into the aorta as an alternative for the common surgical graft anastomosis suturing technique. The device consists of a metal tube acting as a circular knife to cut a hole in the aortic wall, a screw to store the removed part of the aortic wall, and a plastic tube that is introduced through the hole and tightly connected to the aortic wall. The device could be placed without aortic clamping. The device has been tested on a slaughterhouse porcine aorta. A low-pressurized aorta appeared to be the worst case; thus, two animal experiments in the low-pressurized pulmonary artery were performed. No leakage occurred for pressures between 40 and 300 mm Hg.

Mechanical unloading with a miniaturized axial flow pump (hemopump): an experimental study

A family of miniaturized axial flow pumps has been developed, including the Hemopump, the 14-F, and the 21-F HP, which were especially designed for cardiological use. We designed an experimental set-up to study the unloading properties of these devices in a model of regional stunning in an anesthetized, open thorax preparation in sheep. Stunning was caused by 15-min occlusion of the diagonal branch of the left anterior descending coronary artery with subsequent 90 min of reperfusion. Regional myocardial function was assessed by sonomicrometry. A control group was compared with 2 groups with either mechanical unloading during part of ischemia (Group 2) or the early phase of reperfusion (Group 3). In either unloading protocol, both Hemopumps were used. It was shown that recovery from asynchrony was significantly faster in Groups 2 and 3 if unloading was performed with the 21-F HP compared with control Group 1 and the groups using the 14-F HP (p > 0.05). Thus, mechanical unloading with the 21-F Hemopump enhances recovery from stunning whereas unloading with the 14-F HP has only minor effects on hemodynamics and no effects on recovery.

Concept, realization, and first in vitro testing of an intraarterial microaxial blood pump

Intravascular operating microaxial pumps have been clinically introduced (Hemopump 21; Hemopump 14) and have proven to be useful tools for cardiac assist. Due to device-related complications that are associated with the drive concept of an extracorporeal motor and a flexible drive shaft cable, a new pump concept is presented and has been refined in the development process. The cable is replace by a proximally attached drive unit and an extracorporeal power supply. In addition to ongoing hydrodynamic studies of the flow inside the pump and improvements of the overall hydraulic performance, a microelectric motor was realized and integrated. In vitro tests revealed the feasibility of such a concept.

Cardiogenic shock complicating acute myocardial infarction: the use of coronary angioplasty and the integration of the new support devices into patient management

Conventional therapy for cardiogenic shock complicating acute myocardial infarction continues to be associated with a high in-hospital mortality rate. Hemodynamic support with new mechanical devices and emergency coronary revascularization may alter the long-term prognosis for patients with this complication. Between July 1985 and March 1990, 68 patients presented to the University of Michigan with acute myocardial infarction and cardiogenic shock. Interventions performed included thrombolytic therapy (46%), intraaortic balloon pump counterpulsation (70%), cardiac catheterization (86%), coronary angioplasty (73%), emergency coronary artery bypass grafting/ventricular septal defect repair (15%), Hemopump insertion (11%), percutaneous cardiopulmonary support (4%) and ventricular assist device (3%). The 30-day survival rate was significantly better in patients who had successful angioplasty of the infarct-related artery than in patients with failed angioplasty (61% vs. 7%, p = 0.002) or no attempt at angioplasty (61% vs. 14%, p = 0.003). This difference was maintained over the 1-year follow-up period. The only clinical variable that predicted survival was age less than 65 years. The early use of the new support devices in 10 patients was associated with death in 8 (80%), but this poor outcome may reflect a selection bias for an especially high risk population. Collectively, these recent data continue to suggest that emergency revascularization with angioplasty may reduce the mortality rate, but further study is required to define optimal utilization and integration of new support devices.

Preoperative and postoperative Hemopump support for patients undergoing orthotopic heart transplantation

The Hemopump (Johnson & Johnson Interventional Systems, Rancho Cordova, CA) can be used successfully as a bridge to cardiac transplantation in patients with advanced cardiogenic shock that proves to be irreversible. Some patients, however, may also benefit from maintaining Hemopump support for a period of time after transplantation. A technique has been developed for continuing Hemopump support after transplantation that does not require repositioning of the device.

Left ventricular assistance without thoracotomy: mediastinal and transseptal approaches to the left heart

Two methods to cannulate the left atrium for initiating mechanical left ventricular circulatory assistance using a centrifugal pump were investigated in 25 sheep. A modified Dennis transatrial septal approach produced flow rates of 88.6 +/- 14 mL.kg-1.min-1 through 21F catheters inserted during fluoroscopy through the jugular vein. In 8 animals the septal perforation was plugged after decannulation with a modified Rashkind umbrella plug. Fibroendothelial tissue covered the plug by 4 week. In 7 other animals, the septal defect was not plugged. The septal defect reached pinpoint size by 2 weeks and was completely closed by 4 weeks. In 10 sheep, the left atrium was cannulated from the neck through the mediastinum. Left ventricular assistance flow averaged 71.6 +/- 14 mL.kg-1.min-1. Mean blood loss during 1 hour of left ventricular assistance was 47 mL. In 8 animals, the atrial perforation was plugged with a mean blood loss of 253 +/- 194 mL. In 2 animals, the perforation was intentionally not plugged; mean blood loss was 700 mL. All animals survived. The modified Dennis transatrial method is recommended as a safe, expeditious, cost-effective method to implement left ventricular assistance without thoracotomy. The mediastinal approach, which is technically possible in humans, is more difficult but feasible. Left ventricular assistance has been proven to be the most effective way to rest the failing, ejecting left ventricle. Implementation without thoracotomy potentially expands applications of left ventricular assistance for temporary support of patients with severe manifestations of ischemic heart disease.

Directions in cardiac assistance

The use of mechanical circulatory support devices came to prominence with the use of the Jarvik 7 total artificial heart, both as a permanent implant and as a bridge to transplantation. Over the past decade, however, interest in the use of left ventricular assist devices has overshadowed that of the total artificial heart and great strides have been made, both in the use of such devices as temporary support, and towards the ultimate goal of permanent implantation. A variety of devices are available to support either or both ventricles with a great range of complexity and expense. This test discusses the use of ventricular assist devices and briefly describes the options available. The era is rapidly approaching when the use of implantable circulatory support devices will become commonplace and may outpace, and possibly outperform, the results currently obtained with cardiac transplantation.

Bio-medicus ventricular assist device for salvage of cardiac surgical patients

Over a 5-year period, 41 (1%) of 4,193 patients undergoing cardiac operations underwent intraoperative or early postoperative insertion of a Bio-Medicus ventricular assist device when it became apparent that the patient could not otherwise survive. Fourteen patients were in cardiogenic shock and 7 were in cardiac arrest at the time of initiation of their primary cardiac surgical procedure, and in no instance was the device planned as a bridge to cardiac transplantation. Bleeding, sepsis, and thromboembolism were frequent postoperative complications. Central nervous system deficits were observed in 16 patients during their postoperative course. Eight patients (19.5%) were long-term survivors. Of the preoperative risk factors evaluated only age was significantly associated with survival, with 7 (33%) of the 21 younger (39 to 63 years) patients surviving. Blood product usage and hospital cost were analyzed in an attempt to assess cost/effectiveness of use of this device for attempted salvage of such desperately ill patients.

Initial clinical experience with the Haemopump left ventricular assist device

The Haemopump is an intra-arterial, axial flow, temporary left ventricular assist device. The intra-cardiac pump assembly is connected by a flexible drive shaft to a high speed motor and a drive console. The pump is placed in the left ventricle via the femoral artery, iliac artery or abdominal aorta. Blood is withdrawn from the left ventricle and pumped in a continuous, non-pulsatile fashion into the descending thoracic aorta. We report the use of the Haemopump to provide circulatory assistance in 2 patients with severe graft dysfunction following cardiac transplantation. Both patients were successfully weaned from the Haemopump after 6 and 3 days of support. The first patient subsequently died of overwhelming fungal sepsis and the other remains well 3 months after transplantation with normal left ventricular function. The Haemopump is an effective temporary cardiac assist device for application in severe left ventricular failure.

Rapid placement of the Hemopump and hemofiltration cannula

Hypervolemia, a potential complication in patients on ventricular assist device support, can be managed by use of continuous arteriovenous hemofiltration. The Hemopump, a new catheter-mounted, transaortic axial-flow ventricular assist device, and the vascular access catheter for the Diafilter-30 Hemofilter system, used in continuous arteriovenous hemofiltration, are both usually inserted by way of the femoral artery. Because placing two large catheters in the femoral artery of a patient with peripheral vascular disease can compromise circulation, a technique for placing them in the abdominal aorta was developed. Two patients have undergone combined Hemopump and hemofiltration treatment, and neither experienced complications. Such techniques may benefit more patients in the future, as the usefulness of the Hemopump is proved.

The role of mechanical devices

Devices to support the circulation and directly assist the ventricle during surgery are now both reliable and essential. The intraaortic balloon counterpulsation device is used in patients who fail to wean easily from cardiopulmonary bypass, who develop ischemia in the immediate postoperative period, or who have low output in the maximal edema phase, 4 to 8 hours postoperatively. Complications relate to the insertion of the device, which may cause major arterial disruption, lower leg ischemia, or distal arterial thromboemboli. Ventricular assist device (VAD) support is indicated when, despite the use of the intraaortic balloon, the patient's hemodynamics remain severely depressed and the patient is unable to wean from bypass. There are several contraindications to the use of VADs, including pulmonary hemorrhage. Nevertheless, as experience with the many different types of VADs increases, their use may be extended to periods of days or even months. Various mechanical support devices have been used to bridge to cardiac transplantation, with ranging degrees of success. Decisions concerning the use of mechanical devices for this purpose must take into account both the standard contraindications to transplantation, and contraindications that develop or are acquired during intervention with the bridging device. Current and future experience will decide which devices will be accepted for common clinical use.

First human use of the Hemopump, a catheter-mounted ventricular assist device

The Hemopump, a catheter-mounted, temporary ventricular assist device, consists of an external electromechanical drive console and a disposable, intraarterial axial-flow pump (21F). Power is transmitted percutaneously to the pump by a flexible drive shaft within the catheter. The device is positioned in the left ventricle by way of the femoral artery approach or through the ascending aorta. Blood is drawn from the left ventricle through the transvalvular inlet cannula and pumped into the aorta. As of December 1988, the Hemopump had successfully supported the circulation of 7 patients (5 men, 2 women) ranging in age from 44 to 72 years (mean age, 59 years) and suffering from cardiogenic shock (cardiac index less than 2.0 L/min/m2). Indications for use included failure to be weaned from cardiopulmonary bypass in 4 patients, acute myocardial infarction in 1, severe cardiac allograft rejection in 1, and donor heart failure in 1. Duration of support ranged from 26 to 113 hours (mean, 66 hours). Although 5 patients demonstrated transient hemolysis, none experienced infection, thrombosis, or vascular injury. Hemodynamic variables improved in all patients during support by the device. As of December 1988, 5 of the 7 patients were alive more than 30 days after support had been discontinued, and 3 of these patients were discharged from the hospital. On the basis of our initial clinical results, the Hemopump, which does not require a major surgical procedure for insertion, provides effective, temporary circulatory support in patients with potentially reversible cardiac failure.