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Novel Pulsed Laser Imaging of Ventricular Assist Device Flow Fields
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Hypertension During Support with Ventricular Assist Devices: Incidence, Risk Factors and Management
- Use of the TLC-II Portable Ventricular Assist Device (VAD) Driver System in the Home Discharge Setting
FDA approved left ventricular assist devices can operate in various triggering modes. These include internally triggered automatic synchronous counterpulsation, electrocardiogram triggered synchronous operation, and full to empty or fixed rate asynchronous operation. The aim of this pilot study was to determine the extent to which washing of the blood contacting surfaces of the pump may be optimized by suitable choice of operating mode. Visualization of flow fields adjacent to surfaces in confined areas requires small, neutrally buoyant tracer particles for feature extraction. The Artificial Heart Program worked with researchers at the USDOE National Energy Technology Laboratory to develop a novel pulsed laser imaging technology for this purpose. The technology uses fluorescent tracer particles (100 microns), a pulsed argon laser, and a low pass optical filter. Particle motion was tracked from video images and calculations were made of velocity. Flow visualization was performed under conditions that simulated clinically observed hemodynamic conditions, typical of the immediate post-implant period. At a given LVAS output, fluid speed in the vicinity of the inflow valve tended to increase at higher LVAS beat rates (and consequently lower LVAS stroke volumes). This and future work may well be useful in selecting the optimum modes of LVAS operation as a function of the hemodynamic status of the patient.
This visualization technique was also utilized to map flow fields in three proposed outflow valve housing designs: 1) a triple sinus; 2) an axisymmetric concentric sinus (CS); and 3) a modified triple sinus (MS). The 21 mm Carpentier-Edwards trileaflet pericardial heart valve prosthesis was used for all experiments done on the three housing designs (see video clip). The LVAD was connected to a mock flow loop with an adjustable afterload system to provide physiologic pressures and flows (Pao, 120/80 mmHg; pump output [PO], 2-6 L/min). Visualization derived velocity was verified by laser Doppler velocimetry at several selected points in the field. Formation of vortices behind the leaflets during the LVAS ejection phase was observed in each of the housing designs. They were well organized, and they circulated with the greatest strength in MS. These vortices tended to lie in a plane parallel to the main flow axis, with the rotational velocity increasing with the stroke volume of the LVAS. In the CS and the MS housings, a circumferential flow that provided good washing of this region was observed behind the stents.
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In order to study the clinical variation in ventricular assist device generated arterial pressure waveforms arterial pulse waveforms were broken down into quantifiable components. Specifically, we identified the time and pressure at particular waveform points including the foot of the pulse (Tf), first (T1) and second (T2) shoulders, incisura (Ti) and end diastole (Td). dP/dt, and pulse pressure integrals (PI) were calculated between the points. FIGURE #1. This study revealed that the various VAD systems (Novacor, Thoratec, Heartmate) impart differing pressure waveforms on the systemic circulation.
Recently, we tested the hypothesis that data obtained from non-invasive quantitative echocardiography used during VAD weaning trials would correlate with MVO2 data obtained from exercise testing. 10 patients on chronic mechanical circulatory support were studied including 4 Novacor and 6 Thoratec. % ? in LV fractional area change (FAC) and % ? in stroke area (SA) were calculated between full VAD support and decreases in VAD flow using automated border detection echocardiography. Left ventricular pre-load adjusted maximal power (PA-PWRmax) was calculated from the unassisted beats during VAD weaning. MVO2 values were obtained from exercise physiology testing. Correlation coefficients were calculated between data sets to determine significance. Quantitative echocardiography used during VAD weaning trials yields data consistent with results obtained via exercise physiology testing.
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Ventricular assist devices (VAD) are routinely used to “bridge” end-stage heart failure patients to cardiac transplantation. Adverse events observed with VAD support include infection, and thromboembolic events. Recently, systemic hypertension has been reported in several patients on VAD support. The etiology, incidence, predisposing risk factors, implications and management of systemic hypertension in patients on VAD support has not been properly defined. The objective of this study is to evaluate the incidence, risk factors, implications, and management of systemic hypertension in heart failure patients on temporary VAD support.
The findings were as follows:
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The incidence of hypertension in VAD recipients is significant, yet is not predicted by pre-implant risk factors.
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The incidence varies according to etiology of heart failure and type of VAD used.
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Once present, hypertension tends to remain a chronic problem, but can be managed by a single agent most of the time.
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Although, neurologic events are more frequent in hypertensive subjects, the overall clinical outcome after VAD placement is not affected by hypertension.
- Hypertension has significant implications for quality of life with regard to numbers and cost of medications that chronic VAD patients may need to cope with.
These preliminary observations have important implications as Ventricular assist devices are being considered as chronic therapy for end-stage heart failure. The pathogenesis of hypertension in VAD recipients needs further study.
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The Thoratec VAD System including
the Dual Drive Console (DDC) was approved by the FDA in December
1995 for the indication of bridge to cardiac transplant and
in May 1998 for the indication of post-cardiotomy recovery.
The portable driver, TLC-II, received FDA approval in June
2001 (LINK to FIGURE #4). The TLC-II Driver is interchangeable
with the DDC for any approved Thoratec VAD System. This protocol
focuses on home discharge of subjects while the subjects await
either cardiac transplantation or myocardial recovery. We
were the first center nationally to discharge Thoratec patients
while supported on the DDC at Family House. Increased mobility
and independence has been shown to improve VAD patients’
sense of well-being and quality of life. Therefore, if the
Thoratec VAD System is shown to be safe and effective for
home use, clinical utility will also have been demonstrated.
The objective of this study is to evaluate the safety and effectiveness of the Thoratec Ventricular Assist Device (VAD) System and the TLC-II Driver in the home discharge setting without an untoward effect on the rate of adverse events or patient survival to transplantation or myocardial recovery. The effectiveness of the driver in maintaining circulatory support will be assessed by observing VAD flow index. The hypothesis for effectiveness is that the TLC-II Driver in home use will maintain adequate flow index of 2.0 L/min/m2. The study objective for safety is to demonstrate that the risks presented by the types and frequencies of adverse events experienced by subjects using the Thoratec VAD System in the home discharge setting are offset by the benefits of the subject being able to return to familiar surroundings, people, and activities.
This study is a multi-center, prospective, controlled clinical trial. Up to 15 centers may participate in the study. The study will enroll up to 35 subjects with the objective of acquiring a minimum of 365 days of experience in the home discharge setting. Up to 10 subjects may be enrolled at this site. Subjects who meet criteria are discharged to home. A member of the Artificial Heart Program is available by pager 24 hours a day. Local emergency medical services are instructed on the emergency care of the VAD. Each subject is provided with VAD equipment (primary and back-up) as part of his discharge. After home discharge, subjects will be required to fill out data sheets on a daily basis. Subjects will return to the implant center for follow-up at weeks 1,2,3,4, and monthly thereafter. In the event that a subject is readmitted to the hospital the investigator may use either the TLC-II Driver or the DDC. Patients will continue to be followed as part of the study during any period of rehospitalization unless the subject is permanently withdrawn from the study. If the situation or event requiring rehospitalization resolves, the patient may be discharged from the hospital again. Once discharged home the subject will continue outpatient evaluation with resumption of the follow-up schedule. One subject has been enrolled at this center to date.
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Future Directions of the Artificial Heart Program
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