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Biomedical subjects

N H Hwang

Publications and source records attributed to N H Hwang.

At least 55 records · Page 3Linked to original sources
PubMed1977

Dynamic evaluation of the viscoelastic properties of a biomedical polymer (biomer).

This paper deals with the experimental determination of the dynamic constants, storage and loss moduli of a viscoelastic material currently used as membranes in the artificial heart or other types of cardiac bypass pumps. A brief discussion of the viscoelastic theory is given, starting with the relaxation and creep function leading to the determination of the stress-strain relationship in integral equation form. It is shown that when dealing with periodical strain histories, the integral equation form may be reduced to a complex form. The complex moduli are used to describe the stress-strain relationship. The "Hysteresis Loop" method was used in this investigation to experimentally determine the material constants. This method was found to be convenient and direct for the experimental evaluation of the constants. These dynamic constants may then be used in a computer analysis, i.e., finite elements analysis, to determine the stress distribution in the membrane when subjected to loads or deformations.

Acetamides↗
PubMed1975

Hemodynamics and metabolic determinants of distribution of myocardial flow.

A computer simulation of the left coronary outflow response to branch occlusion was carried out, using physiological data derived from canine left coronary artery generated on an IBM 360/44 in order to provide multidimensional analysis of the problem of the distribution of flow within the myocardium. During coronary occlusion in this computer model, distal impedance varied little during the occlusion of one or the other arterial brance. This observation was taken as a boundary condition in the simulation exercise. Some of the important factors in the intramyocardial distribution of flow are discussed.

Animals↗
PubMed1975

Hydraulic studies of aortic cannulation return nozzles.

An in vitro investigation of 8 different types of clinical aortic return nozzles was conducted. The importance of cannula geometry, tip location and direction was demonstrated in preventing abnormal flow conditions in the major aortic arch arteries. Alternative methods to correct these abnormalities were studied. This report has presented a preliminary study of the flow conditions produced by aortic cannulae during cardiopulmonary bypass. More detailed analysis of the pressure field as well as in vivo studies of the effects of a non-rigid geometry of the flow behavior are needed to confirm the findings of this investigation.

Aorta↗
PubMed1974

Critical arterial stenosis: a theoretical and experimental solution.

The mechanics of critical stenosis of a blood vessel are studied by means of a comprehensive theoretical model in terms of energy changes and dissipation. These theoretical assumptions correlate well with experimental data obtained in vivo. Previous work in this field is analyzed. This new treatment of the phenomenon of blood vessel stenosis allows explanation of apparent contradictions in previous studies. When the velocity of flow in the unstenosed portion and the geometry of the stenosis are known, the drop in pressure at flow can be predicted.

Animals↗
PubMedDate not supplied

Fluid dynamics of venous valve closure.

In vitro experiment was performed on a stented bovine jugular vein valve (VV, 14 mm I.D. x 2 cm long) and a stentless bovine jugular vein valve conduit (10 mm I.D. x 6 cm long) in a hydraulic flow loop with a downstream oscillatory pressure source to mimic respiratory changes. Simultaneous measurements were made on the valve opening area, conduit and sinus diameter changes using a specially designed laser optic system. Visualization of flow fields both proximal and distal to the venous valve, and the valve opening area were simultaneously recorded by using two video cameras. Laser Doppler anemometer surveys were made at three cross sections: the valve inlet, the valve exist, and 2 cm downstream of the venous valve to quantity flow reflux at valve closure. The experiment confirmed that the VV is a pressure-operated rather than a flow-driven device and that little or no reflux is needed to close the valve completely. The experiment further demonstrated that the VV sinus expands rapidly against back pressure, a critical character to consider in venous prosthesis design.

Animals↗
PubMedDate not supplied

Graft compliance and anastomotic flow patterns.

The oscillatory flow patterns at the venous anastomosis of a hemodialysis angioaccess loop graft system were studied using two new compliant vascular prostheses: a longitudinally compliant polytetrafluoroethylene-composite (Baxter Ultraflex PTFE-Plus) graft (BA) and a radially compliant ultrafine polyester fiber (TORAY-UFPF) graft (TR). A non-compliant Gore-Tex polytetrafluoroethylene graft was used as the control. The experimental grafts were 8 mm inside diameter x 25 cm long. Flow experiments were done in a transparent, elastic bench-top flow model; fabrication was based on silicone rubber casts obtained from femoral-to-femoral arteriovenous loop grafts surgically implanted in dogs. The loop graft constructed in the dog model was made to mimic the branchial-to-cephalic angioaccess loop graft commonly used in hemodialysis patients. The flow model was connected to a pulse generator, an adjustable arterial afterload, and a venous afterload. Under identical input conditions, the pressure and flow waveforms were monitored simultaneously at the proximal and distal ends of both the arterial and venous anastomoses. For each graft studied, the anastomotic flow field was visualized using laser illuminated hydrogen bubbles as tracers. At pulse rates of 60 and 90 beats/min, graft flow rates were 2.2 and 2.5 L/min, respectively. Among the grafts studied, measurable differences in pressure and flow wave attenuation and their respective phase lags resulted in characteristically dissimilar flow patterns at the venous anastomosis. Growth of the separation zone at the toe of the anastomosis, and the pattern of retrograde flow in the distal vein are visibly different in all three grafts.

Animals↗
PubMedDate not supplied

In vitro performance of venous valve prostheses. An experimental model study.

The performance of fresh and glutaraldehyde fixed bovine jugular vein valves (10 mm diameter) was investigated in an experimental flow loop that provides adjustable flow rates and a downstream oscillatory pressure. Three different venous valve (VV) conduit geometries (curved [C], straight [S], and tapered [T]), were tested. The flow loop consisted of two independently adjustable components, with the mean flow generated by adjusting the elevation difference between the head tank and outflow chamber. An adjustable sinusoidal pressure pulse was superimposed on the downstream of a VV to mimic the respiratory effects. Flow visualizations were made using 100 microns mica chip tracers in the laser illuminated flow fields. To assess VV performance under various flow conditions, the closure opening (CO), partial opening (PO), and leaflet fluttering (LF) were evaluated. At a given pulse pressure, the three conduits required different flow rates to reach CO mode. At 12 cm H2O pulse pressure, the fresh valve in C-conduit exhibited stable CO operation at a flow rate of 1.01 ml/sec. That in S and T conduits required 1.67 ml/sec and 2.25 ml/sec, respectively. At higher flow rates, PO and LF performances were observed in all three conduits. Different threshold values of pulse pressure were needed to reestablish the CO operation mode for the C, S, and T conduits, individually. These observations provide some insight into the role of conduit geometry and sinus configuration in the function of VV.

Animals↗
PubMedDate not supplied

Fluid mechanics of left ventricular assist system outflow housings.

Using the Novacor (Baxter Novacor, Oakland, CA) Left Ventricular Assist System (LVAS) as a test bed, phasic flow patterns were analyzed for three 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. The LVAS was actuated by a laboratory model of the Novacor LVAS control console, and it 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). Laser illuminated flow visualization techniques were used to investigate the phasic flow patterns of the housings, and the 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.

Biomechanical Phenomena↗
PubMedDate not supplied

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.

Biomechanical Phenomena↗
PubMedDate not supplied

Ventricular pressure slope and bileaflet mechanical heart valve closure.

The maximum left ventricular pressure slope (dP/dt) value has been used by several investigators as the criterion for studying mitral valve closure. In this article, the relationship between the ventricular pressure slope (dP/dt) and the leaflet closing behavior of bileaflet mechanical heart valves (BMV) is investigated. Two current BMVs, the St. Jude Medical 29 mm and CarboMedics 29 mm, installed in the mitral position of a mock circulatory pulsatile flow loop were used as the study model. Under simulated physiologic pressures and flow conditions, the experiment was conducted at 70, 90, and 120 beats/min with corresponding flow rates of 5.0, 6.0, and 7.5 liters/min, respectively. A laser sweeping technique was used to monitor the leaflet closing motion within the last 3 degrees excursion at valve closure. A modified dual beam laser sweeping technique system was used to register the difference of leaflet/housing impact time between the two BMV closing leaflets in asynchronous closure. Common BMV asynchronous closures were found in both BMVs at all three heart rates tested. The second closing leaflet was found to always close at higher velocity than the first. Simultaneous measurements of the ventricular pressure (Pv) and the leaflet closing time showed that Pv exhibited three stage characteristics. In the first stage, Pv gradually increased as the ventricle was filled. A sudden rise of Pv occurred immediately after closing of the first leaflet. The maximum dp/dt occurred in the third stage after closure of both BMV leaflets. The BMV closing behavior and the corresponding Pv pattern were found to depend strongly upon valve type and heart rate. The time averaged ventricular pressure slope (dp/dt) values at 70, 90, and 120 beats/min were about 40, 70, and 150 mmHg/sec for the St. Jude Medical valve and 40, 105, and 205 for the CarboMedics valve during the first closing stage. The maximum dp/dt values were 2670, 4350, and 5000 mmHg/sec for the St. Jude Medical valve and 1210, 2530, and 3210 mmHg/sec for the CarboMedics valve at the three heart rates tested, respectively. The study showed that the left ventricular pressure patterns (dP/dt) at valve closure were the result of valve operation under given driving conditions. The dp/dtmax cannot be used as the criterion for studying BMV closure.

Biomechanical Phenomena↗