Optical methods for the nondestructive evaluation of collagen morphology in bioprosthetic heart valves.

The aim of the present study was to assess the suitability of nondestructive optical methods as a means of evaluating collagen morphology in bioprosthetic heart valve leaflets. The results of this study demonstrate that transmitted polarized light and incident polarized light optics facilitate the imaging of the inherent birefringence of valvular collagen fibers. Polarized light optics readily document the different patterns of collagen orientation and configuration in porcine aortic valvular (PAV) and bovine pericardial valvular (BPV) bioprostheses. Incident polarized light optics also provide information on leaflet surface morphology. Verification that the birefringence observed by polarized ligh optics represents leaflet collagen was provided by conventional histologic and transmission electron microscopic methods. Quantitative determinations of the spacing of collagen bundle waves gave similar values in intact and in sectioned BPV leaflets. Potential applications of polarized light optics in the assessment of bioprosthetic valve collagen are as follows: the selection of the desired orientation of collagen bundles within pericardium intended to be configured into bioprosthetic leaflets; evaluation of the effects of mechanical stresses and leaflet motion on collagen morphology in bioprosthetic valve leaflets; and initial screening of leaflet specimens and selection of the desired collagen orientation for embedding and sectioning of samples for conventional morphologic studies.

Relative performance of prosthetic heart valves based on power measurements.

The in vitro steady and pulsatile flow results of tests of a series of valves are reported. Fifteen mechanical and two tissue valves from 23 mm to 27 mm nominal size from six manufacturers, plus two prototype experimental valves, were tested in the aortic position in a circulation system simulator. Steady-flow pressure drop/flow rate measurements were direct-recorded on an X-Y plotter. Pulsatile flow rate, LVP, AoP, pressure drop, and pressure-drop power were recorded. Input (left ventricle pumping) power was also recorded directly as the multiplier output from the product of flow rate and LVP. Results are presented in tables and graphs. Power loss at 70/min varied from 3% to 6%; backflow from 2.5% to 11%. Valve evaluation is based on the relative valve power loss that the heart must overcome. Backflow is shown to be as important as pressure drop in determining valve performance. A performance index (PI) equal to the percentage pressure-drop power loss plus the percentage backflow is presented as a means for performance rating. PI values from 6 to 17 were obtained on the 19 valves tested.

Comparison of in vitro valve pressure drop results from different investigators.

Steady-state pressure-drop vs flow-rate measurements on prosthetic valves have been shown to correlate well with mean cyclic pressure drops vs RMS flow rate during systolic ejection from pulsatile flow data. The use of steady-state data is therefore validated as a means of assessing performance characteristics. Pressure-drop results reported in the literature were compared for valves of the same size and manufacture. Significant differences were noted, which are attributed to: (a) different geometries in the region of the valves, (b) different approach flow conditions, (c) different placement of pressure measurement taps, and (d) different methods of pressure measurement. Differences in pressure drop were observed to vary by as much as a factor of five among different investigators. The exponent n in the relation delta p infinity Q" was also observed to vary, depending on the above factors. The reasons for and some quantitative calculations on the observed differences are presented here.

In vitro durability of Hancock Model 242 porcine heart valve.

Two Hancock Model 242 prostheses, tissue anulus diameter 21 mm., were tested in a closed, low-volume, accelerated fatigue tester. The fluid media was sterils fresh-frozen plasma. The normal human aortic root was simulated. The cyclic rate was 20 Hz at 37 degrees C. The prostheses developed severe fatigue at 77 million cycles. Fraying of the free edges was found after 2 million cycles. Small tears near the commissures and then holes between collagen bundles at the base of the leaflets appeared at 7 million cycles. At 71 million cycles the leaflets began to tear and complete prolapse, with gross valvular insufficiency occurring at 77 million cycles. The accelerated wear of Hancock procine prosthesis is frequency dependent and independent of media and the flow geometry of the testing device.

Durability of prosthetic heart valves.

Accelerated fatigue testing of clinical heart valves has been performed at cyclic rates of 33 to 35 cycles per second at 37 degrees C using water for non-biological valves and glutaraldehyde solutions for tissue valves. Flows were in the physiological range, and the pressure difference across each valve during closure was 100 +/- 25 mm Hg. The results showed that major fatigue occurred for the Starr-Edwards 2320 at 150 million cycles, the Hufnagel trileaflet at 124 million cycles, the Björk-Shiley Delrin disc at 140, the Björk-Shiley Pyrolite disc at 973, the Beall 103 at 60, the Hancock porcine at 62, the Carpentier-Edwards porcine at 34, and the Ionescu-Shiley porcine pericardial prosthesis at 65 million cycles. The Lillehei-Kaster was removed after 762 million cycles without discernible wear. Three facts emerged from the testing data: (1) the component worn in vitro wears in vivo; (2) the sites of in vitro fatigue on the component are identical to clinical specimens; and (3) those valves that have high durability in vitro have given similar performance in patients. The in vitro and clinical data for tissue valves do not correlate. The possible reasons for the discrepancy are discussed, and a note of caution is made regarding realistic expectations of clinical durability of tissue valves.

Cardiovascular system simulation requirements.

Circulatory system characteristics are considered with respect to specifying model design parameters for simulators and pulse duplicators. The requirements are investigated to determine what characteristics and parameters are important in design and construction. The specific design depends on the functions of the device to be investigated. The single most important quantity is the modeled proximal capacitance or compliance of the large vessels. More detailed study specifications require more detailed models.

A simple cardiovascular system simulator: design and performance.

A simple simulator has been constructed, evaluated and used for performance studies of prosthetic aortic valves, balloon assist devices and Koroktof sounds and ausculatory cuff blood pressure measurements. A direct drive piston pump is used. Elastic soft rubber tubes with distributed resistances allow modeling of normal and diseased pressure pulse waves. Pressure pulse amplitude amplification is modeled with tube segments of decreasing diameters. Satisfactory proximal pressure pulse shapes are obtained with a soft rubber tube of uniform diameter.