Can cavitation bubbles generated by mechanical heart valves be detected by transcranial Doppler?

Increasingly, transcranial Doppler (TCD) systems are being considered as potential tools to determine the thrombogenicity of herat valve prostheses and/or to serve as early warnings of impending thromboembolic strokes. TCD is thought to detect the presence of intracranial arterial microemboli based on high intensity signals (HITS). While the exact cause of these HITS is not known, it has been theorized that they are due in part to mechanical valve induced cavitation. To determine if this is correct, in vitro experiments were conducted to find if TCD could detect mechanical valve induced cavitation bubbles. An in vitro flow system was developed in which 29mm Medtronic Hall, St. Jude Medical and Medtronic Intact valves, one each, were simultaneously subjected to validated non-cavitation and cavitation generating conditions. Polystyrene particles as well as air bubbles were infused into the flow system to provide controls for comparison. The flow field proximal to each valve was interrogated using TCD sample volumes located 2.5 cm proximal to, 1.3 cm proximal to, and at the valvular inflow surface. The TCD system accurately detected both infused air bubbles and polystyrene particles at all sample volume locations. Under known cavitation generating conditions without infused air bubbles, the TCD failed to register HITS at any sample volume location. Even though cavitation at the valve surface might have been masked by a "system generated artifact", cavitation formed bubbles must persist and pass through the two proximal sample volumes if they were to be captured intracranially. These data suggest that the TCD is unable to detect mechanical valve induced cavitation bubbles either because the bubbles are too small, too few, or too short lived. It is therefore probable that clinically recorded HITS associated with mechanical valves are from origins other than valve induced cavitation bubbles.

In vitro observations of mechanical heart valve cavitation.

The in vitro cavitation thresholds and locations were studied on ten different heart valve designs. The valves were mounted in a mock circulation flow loop which simulated a cardiovascular system. All the tests were run at 70 beats per minute with a cardiac output varying between 2 l/min and 6 l/min in increments of 1 l/min. In vitro cavitation phenomena generated at the closing instant of mechanical heart valves were captured using a video photographic technique. Cavitation locations and intensity on different valve designs were analyzed from the cavitation images recorded on a video tape. When cavitation occurs on a bileaflet valve, it can occur in the same localized area of the leaflet from cycle to cycle thus producing a cumulative effect. In a single disc valve, the free rotation of the valve disc during operation provides a means of distributing a localized cavitation activity over an ever changing disc surface. Thus any cavitation-induced damage on the disc surface can be reduced or eliminated even though a single disc valve may have a lower cavitation threshold. Cavitation locations and thresholds are primarily a function of valve design. Smaller size valves have higher cavitation thresholds than larger ones. The cavitation thresholds of all the valves tested were above the physiological left ventricular maximum dp/dt at rest. If in vivo cavitation occurs under some extreme conditions, this study suggests possible locations on mechanical heart valves which could be examined for traces of cavitation activity.

A microstructural flow analysis within a bileaflet mechanical heart valve hinge.

BACKGROUND AND AIMS OF THE STUDY: During recent clinical trials, the Medtronic Parallel bileaflet heart valve was found to have an unacceptable thrombosis complication rate. As patient- and material-related factors proved negative causes for this outcome, it was hypothesized that the flow fields within the valve's hinge pocket contributed to the thrombus formation. METHODS: A microstructural flow analysis within the hinge pocket is presented which uses the techniques of flow visualization, computational fluid dynamics (CFD), and laser Doppler velocimetry (LDV). The application of these techniques towards solving this problem has become possible through (i) the ability to manufacture dimensionally correct 1-X transparent heart valve housings, (ii) advances in CFD technology, and (iii) advances in LDV measurement techniques. RESULTS: This analysis showed that a vortex was present at the hinge pocket's inflow channel during forward flow and degenerated to a disturbed three-dimensional structure during reverse flow with zones of turbulent shear stress large enough to cause blood cell damage. In addition, multiple zones of flow stagnation and disturbed flow existed along the leaflet's pivot throughout the entire cardiac cycle. It was felt that these complex fluid structures created conditions which resulted in the formation of thrombus within the hinges of the Medtronic Parallel valve. These findings were supported by limited clinical explant data which illustrated early thrombus formation within the Parallel valve's hinge pocket at sites predicted by the analysis. CONCLUSIONS: This study provides, for the first time, an understanding of the detailed flow structures within the hinges of a mechanical heart valve and demonstrates an analysis technique by which future mechanical heart valve designs may be assessed for the potential of thrombus formation within the valve's hinge regions.

An interlaboratory comparison of the FDA protocol for the evaluation of cavitation potential of mechanical heart valves.

Five laboratories carried out measurements of cavitation threshold for a common set of six mechanical prosthetic heart valves, two each from three different manufacturers. This study was intended to evaluate to what extent FDA's current guidance for cavitation testing would lead to consistent results in a variety of laboratory settings and to seek areas for improvement in the recommended test protocol. The inter-laboratory study protocol specified: (1) characterization of the test fluid by oxygen content and electrical conductivity, (2) location and frequency response of pressure sensors, (3) determination of ventricular and atrial pressures (P) and loading rates (dP/dt) averaged over the time period of valve closure and over the time periods of 1 ms, 5 ms, and 20 ms prior to video visualization. The protocol did not specify: (1) the fluid pumping equipment to be used to generate cavitation, (2) the pump or fluid parameters adjusted to raise or lower the loading rate, (3) the equipment, technique, or sensitivity used to visualize cavitation, and (4) a specific definition of the threshold for cavitation. Results from the five laboratories are reported. Significant differences in results were observed in dP/dt and in the pressure difference across the valves during closure at cavitation threshold. Specific differences in test systems included a wide range of ventricular compliance and single valved versus double valved test systems. Three single valve systems with compliant ventricles produced results in reasonable agreement with one another. Further similarity in test equipment should be specified to assure adequate interlaboratory reliability for cavitation testing. Areas needing better specification include the design of the valve mount, the design of the cavitation generators, and qualitative criteria for detection of threshold cavitation.

Quantification of Doppler color flow images from a stenosed carotid artery model.

Velocity fields at and downstream of graded, axisymmetric stenoses of 0%, 20%, 40%, 60% and 80% diameter reduction were obtained under pulsatile flow conditions simulating that in a carotid artery (Re(mean) = 379, Re(peak) = 1137 and alpha = 5.36) using an ultrasonic Doppler Color Flow Imager. Characteristic features of flow disturbances associated with degree of stenosis, such as jetting, flow separation and reversal and turbulence were quantified using nondimensional indices at peak systole, t0, and early diastole, t1. It was found that the Reverse Area Index (RAI), the Field Profile Index (FPI) and the Velocity Gradient Indices (VGIz and VGIr) were sensitive to these changes, especially at time t1. In particular, the mean values of RAI and VGIr were significantly different for all stenotic cases. These findings may provide a more quantitative and reproducible method of interpreting flow patterns in the region of stenosed carotid arteries.

The closing behavior of Medtronic Hall mechanical heart valves.

The rapid deceleration of mechanical heart valve leaflets or discs at closure, and their rebound after impact between the leaflets or discs and housing may produce conditions that favor cavitation inception. Precision measurements of the disc closing velocity using a laser sweeping technique (LST) were made on two Medtronic Hall (Med Hall) mitral valve models, the 29 mm Standard and D-16 Med Hall valves. The experiment was carried out in a pulsatile mock flow loop (PFL) by installing the tested valve in the mitral position of the PFL. Tests were conducted under physiologic pressures at heart rates of 70, 90, and 120 beats/min with flow rates of 5, 6, and 7.5 l/min, respectively. For each valve tested, the experimental series was carried out to include both a flexible mounting and a rigid mounting mechanism. The time history of the disc closing velocity during the last 3 degrees before final closure was measured. Results show that the disc approached the valve housing at a near constant velocity of approximately 1.3, 1.5, and 2.0 m/sec for the Med Hall Standard valve, and 1.9, 2.5, and 2.6 m/sec for the Med Hall D-16 valve at 70, 90, and 120 beats/min, respectively. After impact, the valve of disc rebounded with velocities that depended on the mounting modalities and heart rates. In the rigid mounting, the disc rebound velocities of the Med Hall Standard and D-16 valves were about 60%-70% and 70%-80% of their respective approaching velocities. For the flexible mounting, the rebound velocities were 15%-25% and 20%-30% of their respective approaching velocities. The results demonstrate that a slight modification in the seat stop geometry of Med Hall valves can significantly affect disc closure behavior.

Phasic flow patterns at a hemodialysis venous anastomosis.

A phase-by-phase analysis of local flow patterns at the venous anastomosis of an arteriovenous hemodialysis angioaccess loop graft (AVLG) was made. The study was carried out in an elastic, transparent Silastic in vitro flow model, which duplicates the detail geometry of the AVLG obtained from an animal model (30+ kg dogs with 12 weeks bilateral femoral AVLG implantation). The flow model was installed in a mock pulsatile flow loop system designed to simulate physiological conditions. Flow visualization was made in laser-illuminated flow fields using a high-speed cine camera. Analysis of the high-speed cine indicates there is a distinct separation region downstream of the anastomotic toe in the median plane and a stagnation region that oscillates along the opposite wall. During inward motion of the vessel wall, accumulation of particles in the separation region and the nearby stagnation region is observed. A large swirl appears in the distal vein during end-systolic period. A double-helical flow pattern occurs further down in the distal vein. Retrograde flow in the distal vein occurs in an "oscillating" manner following each cardiac cycle.

Flow profiles and wall shear stress distribution at a hemodialysis venous anastomosis: preliminary study.

The phasic velocity field in the vicinity of the venous anastomosis in a hemodialysis angioaccess arteriovenous fistula loop graft (AVLG) is investigated employing a laser Doppler anemometer (LDA) system. Detailed LDA velocity profiles are obtained by sectional survey performed in a transparent, elastic flow model which was fabricated to represent the geometry of the AVLG system under physiological pressure and flow waveforms. The geometry of the flow model was based on a silicone rubber cast obtained from an experimental dog model. In the present study, detailed distribution of velocity profiles is obtained. The distribution of wall shear stress in the model is computed from the slope of the local velocity profiles near the wall. The relationship between the results obtained by flow visualization and the LDA measurement is discussed.

Haemodynamics of angioaccess venous anastomoses.

A large percentage of arteriovenous haemodialysis angioaccess loop grafts (AVLG) fail within the first year after surgery, the occlusive lesions being found predominantly at the venous anastomosis site. This paper presents a detailed flow dynamic study of the AVLG system using three elastic, transparent bench-top flow models, which were based on the geometry of silicone rubber casts obtained at different times from a chronic animal model. Each model thus represented a different stage of the lesion development. Flow visualization and laser Doppler anemometer surveys of the flow field confirmed that the hydrodynamic factors favour lesion development near the stagnation point opposite the anastomotic toe, where the momentum of the impinging jet stream, combined with the oscillating wall shear stress generated in the vicinity of the stagnation point, acts in both directions. The accumulation of tracer particles in the region of flow separation is believed to be a combined contribution from the hydraulic forces and the inward motion of the vessel wall. As these hydrodynamic factors are enhanced upon further development of the occlusive lesion, a vicious cycle may be formed.

Flow phenomena in compliant and noncompliant arteriovenous grafts.

Oscillatory flows at venous anastomoses of two end-to-side angioaccess grafts were studied using: 1) a noncompliant Gore-Tex polytetrafluoroethylene (PTFE) graft and 2) a longitudinal compliant Ultraflex graft. The study was done in a transparent flow model fabricated from a femoral-to-femoral arteriovenous loop graft (AVLG) surgically implanted in dogs. The 6 mm x 25 cm loop graft reproduces that of the brachial artery-cephalic vein hemodialysis angioaccess loop graft commonly used in patients. At a pulse rate of 70 beats/min and blood flow of 0.7 L/min, flow visualization was done using a 15 mW He-Ne laser and a cylindrical lens system which converts the incident laser beam into a thin parallel light plane. Hydrogen bubble technique was used to illustrate the flow field at selected planes. Phasic pressure and flow oscillations were observed in the hypertensive host vein. The pressure wave arrived at the venous anastomosis with higher amplitude and shorter time of travel in the Gore-Tex graft than in the Ultraflex graft. The differences in wave attenuations and wave transmission time between grafts resulted in apparent changes of flow pattern in both the proximal (heart side) and distal (foot side) veins. The negative lags between the flow peaks in the distal vein and proximal artery are explained by the phasic pressure gradients measured at the anastomoses. Spectral analysis was done on the venous thrills, which are characteristic of almost all clinical AVLG implants. The power spectra taken at 60, 70, 90, and 120 beats/min clearly demonstrated a concentration of fluctuation energy at about 100 Hz in the Gore-Tex AVLG.

Doppler color-flow images from a stenosed arterial model: interpretation of flow patterns.

The capability of the recently introduced Doppler color-flow mapping devices to accurately detect flow patterns in the region of an arterial stenosis was evaluated by use of an in vitro flow model. Pulsatile flow simulating that in a low-resistance vessel was induced through a straight acrylic tube, which alternatively contained axisymmetric stenoses of 0%, 20%, 40%, 60%, and 80% diameter reduction. Doppler color-flow mapper images were taken in realtime along the tube midplane from 0 to 8 diameters downstream of each stenosis. Comparison of the Doppler color-flow mapping results with similarly recorded flow visualization (hydrogen bubble) images showed a close correspondence of key features of the flow, including detection of a high-velocity, centerline jet and near-wall separated flow zones. Distinctive flow patterns exist with each stenotic case, and these should be of considerable value in diagnosing clinical disease conditions.

The effect of angle and flow rate upon hemodynamics in distal vascular graft anastomoses: an in vitro model study.

A steady flow, in vitro model of distal arterial bypass graft junctions was used to examine the effects of junction angle and flow rate on the local velocity field. Three test sections were fabricated from Plexiglas tubing having anastomotic junction angles of either 30, 45, or 60 deg. Flow visualization revealed velocity profiles skewed toward the outer wall with a flow split around a clear stagnation point along the outer wall. Laser Doppler anemometry [LDA] measurements confirmed a distinct stagnation point at the outer wall and both reverse and forward shear were detected immediately upstream and downstream, respectively, of this site. Axial velocities and shear rates along the outer wall were higher than along the inner wall and occurred in the junction angle order: 45, 60, and 30 deg. This study clearly identified changes in wall shear which varied with the anastomotic angle and flow rate.