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Biomedical subjects

N H Hwang

Publications and source records attributed to N H Hwang.

At least 37 records · Page 2Linked to original sources
PubMed1988

Dynamic analysis of flutter in disk type mechanical heart valve prostheses.

Parametric study of the low frequency oscillations occasionally observed in certain types of disc type prosthetic heart valves (PHV) are carried out using a finite element technique. The analysis is performed to determine the frequencies of the dynamic fluttering with the help of the 'ANSYS' computer program. The results show that the frequencies of the dynamic fluttering for both the circular occluders and the semi-circular occluders are at least two orders of magnitude higher than that observed in vivo. It is thus concluded that the clinically observed leaflet oscillations should not be a dynamic flutter phenomenon. Rather, the vortex shedding has been assumed to be the cause of these oscillations.

Biomechanical Phenomena↗
PubMed1988

Bursting strength in CO2 laser-assisted microvascular anastomoses.

Iliac artery end-to-end anastomoses were performed in 42 Sprague-Dawley rats, divided into seven groups, to determine the welding effects of CO2 laser radiation in microvascular anastomoses. Conventional suture techniques were performed on right iliac arteries, and left iliac arteries were anastomosed with a laser-assisted technique. Bursting strength and diameters of the anastomotic sites were measured at different intervals (from one day to five weeks) post surgery. The anastomotic patency rate was 100 percent in both groups, and the aneurysm rate was only 2 percent in the laser group. Bursting strength was low at one and three days post surgery in both groups; then, it increased gradually until both groups could withstand higher than physiologic pressures. Results of high patency rates, low aneurysm formation, and the ability to withstand pressures higher than physiologic, suggest that the laser-assisted anastomotic technique can play an important role in microvascular surgery.

Anastomosis, Surgical↗
PubMed1988

Segmental volume distensibility of the canine thoracic aorta in vivo.

Segments of the canine ascending aorta, upper descending thoracic aorta, and middle descending thoracic aorta were instrumented with ultrasonic dimension gauges and a cathetertip manometer simultaneously to measure changes in segment diameter, length, and intravascular pressure. Volume distensibility (EV) was calculated as the sum of circumferential extensibility (EC), longitudinal extensibility (EL), and high order extensibilities (EK) for each segment. The EC and EL were linear expressions that represented percentage volume changes per mmHg pulse pressure due to circumferential and longitudinal dimensional changes. The high order extensibilities (second and third order) accounted for the percentage volume changes per mmHg pulse pressure due to the interactions between circumferential and longitudinal dimensional changes. Mean(SEM) EV values from six dogs were 1.62(0.31), 0.84(0.08), and 0.62(0.08)% delta V/mmHg delta P for the ascending aorta, upper descending thoracic aorta, and middle descending thoracic aorta segments respectively. The EV, EL, and EK of the ascending aorta segment were significantly greater than those of the upper descending thoracic aorta and middle descending thoracic aorta segments, whereas EC was significantly less in the ascending aorta than in both the upper descending thoracic aorta and middle descending thoracic aorta segments. It is concluded that there are regional differences in aortic distensibility and its components in vivo. Longitudinal wall motion is an important determinant of these aortic mechanical properties.

Animals↗
PubMed1988

Arterial stenosis murmurs: an analysis of flow and pressure fields.

The flow field distal to an arterial stenosis is simulated by a confined turbulent jet with moderate Reynolds numbers. The wall pressure fluctuations are related to the momentum fluctuations of the jet by the Poisson equation. A Green's function was derived to satisfy the boundary conditions on a cylindrical surface. This allows the solution of the Poisson's equation to include only a volume integral of the fluctuating momentum, weighed by the relative distance between the source and the sensor. The velocity fluctuations on the jet centerline and at the middle of the shear layer were measured using a laser Doppler anemometer. The wall pressure fluctuations were detected simultaneously by an array of nine wall-mounted pressure transducers along the axial direction. Cross correlation performed between the velocity and pressure fluctuations reveals that the pressure fluctuations were mostly imposed by the passage of turbulent eddies with a convective velocity that is a function of the jet exit velocity. The Strouhal number, defined by the frequency of the passing large-scale structure, is a function of the initial conditions only very close to the jet exit. Further downstream, where the effect of the initial conditions is lost, the Strouhal number approaches a constant irrespect of the jet Reynolds number. The contribution of a source near the jet exit to wall pressure fluctuation near the reattachment is rather weak due to the rapidly decaying weighting function in the axial direction. However, for sources located within one nozzle diameter from the sensor, the cross-spectral density function has a high magnitude with maximum coherence where the pressure spectral changes its slope.

Arterial Occlusive Diseases↗
PubMed1987

Developing oscillatory flow in a circular pipe: a new solution.

The problem of oscillatory flow in a circular pipe was analyzed by Atabek and associates more than two decades ago. Their formulas for velocity and pressure distributions in developing pipe flows under oscillatory conditions have been often cited. However, the application of these formulas for flow field computations requires a rather complex procedure involving plotting of a set of curves and predetermination of the phase angles. This paper presents a method using the imaginary argument of the Bessel function to solve the Navier-Stokes equations. A different set of solution formulas are obtained. A comparison of the formulas obtained in this paper with those of Atabek shows that the former is considerably simpler and more convenient to use in flow computations. Numerical results computed using this paper's formulas are consistent with Atabek's and with the experimental measurements.

Biomechanical Phenomena↗
PubMed1987

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.

Animals↗
PubMed1987

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.

Arteriovenous Anastomosis↗
PubMed1985

Wall shear stress distribution in a model human aortic arch: assessment by an electrochemical technique.

Wall shear stress (WSS) distribution in a human aortic arch model is studied using 130 cathode electrodes flush-mounted on the model walls. Flow visualizations are made in a transparent geometry model to identify the regions of fluid mechanical interests, e.g. regions of flow separation, eddy formation and flow stagnancy. The 130 electrodes are strategically positioned in the arch based on information obtained from the flow visualizations. The measured data indicate that the aortic arch may be categorized into eight regions: three along the inner wall of the arch (A,B,C); and five near the outer wall (D,E,F,G,H). (1) The regions of low WSS are distributed along the inner wall of the ascending aorta A; the inner wall of the descending aorta C; and the upstream inner wall of the innominate and the common carotid branchings F. (2) The high WSS regions are distributed along the outer wall of the arch E; and the inner wall in the arch opposite to the left subclavian branching B. (3) In certain regions, high and low WSS may be found next to each other (e.g. G and H) without a definable boundary in between; and (4) as the Reynolds number increases, the areas of low WSS decrease, while the high WSS areas increase with no obvious change in magnitude of the stress along the inner wall of the arch. At the branchings, the WSS distribution is not affected by the Reynolds number within the range of observations. The measured WSS distribution is compared with Rodkiewicz's map of early atherosclerotic lesions in the aortic arch of cholesterol fed rabbits.

Animals↗
PubMed1984

Human red blood cell hemolysis in a turbulent shear flow: contribution of Reynolds shear stresses.

Various previous models used in studying red blood cell (RBC) hemolysis in turbulent shear flows are reviewed from a fluid dynamic point of view. The effect of turbulent shear stress (Reynolds shear stress, tau R) on RBC hemolysis is investigated utilizing a submerged axisymmetric jet flow field. A detailed survey of the flow field is made with a laser Doppler anemometer system to obtain contour maps of the mean velocity distributions, relative turbulence intensities, and tau R distributions in the field prior to conducting the experiment of sampling and analyzing the cells free-hemoglobin in the field. A new two-point sampling technique, developed in this laboratory, allows collections of RBC samples from selected locations in the flow field so that a relationship between the local shear stress level and the cell damage may be established. The threshold level of tau R responsible for incipient hemolysis is found to be approximately 400 Newtons per square meter (N/m2), below which a sublethal region of zero hemolysis is observed.

Adult↗
PubMed1983

A model investigation of the velocity and pressure spectra in vascular murmurs.

A physical model consisting of an axisymmetrical jet in a rigid plexiglass pipe was used to study the flow and pressure fluctuations downstream from an aortic stenosis. The fluctuating velocity components, u and v, at several locations in the steady liquid jet were measured using a laser Doppler anemometer system. Simultaneous wall pressure fluctuations were monitored by an array of nine miniature pressure transducers wall mounted in the axial direction. This paper presents the detailed measurements of mean velocity profiles, turbulent intensity distributions and RMS pressure fluctuations. The energy spectra obtained for the pressure fluctuations and the u and v velocity components are compared. Contrary to earlier works, we found that the differences between peak frequencies of the pressure spectra and the characteristic frequencies of the velocity spectra vary with positions downstream from the nozzle. These differences are discussed in light of pseudosound generation by the eddy structures in the stenotic flow field.

Aortic Valve Stenosis↗
PubMed1983

Influence of red blood cell concentrations on the measurement of turbulence using hot-film anemometer.

Measurement of local velocity fluctuations was made with an L-shaped conical hot-film probe in a submerged circular jet. The experiment was carried out in solutions of washed human red blood cells (RBC) in a phosphate buffer solution (PBS), at hematocrit concentrations (Ht percent) of 10, 19, 29, and 38 percent. The viscosity of the testing solutions was kept at 3.2 c.p. by adding proper amount of dextran. The experiment was conducted at Reynolds numbers (NR) 674, 963, 1255 and 1410, based on the jet exit velocity and exit diameter. Statistical analyses were performed on the recorded instantaneous velocity signals to obtain the root-mean-square (rms) values, the probability density functions (PDF) and the power spectral density functions (PSDF) of the signals. Within the range tested, we noticed an incidental rise in rms values at 19 to 29 Ht percent for NR = 963 similar to those reported earlier in the literature. Further analyses using PDF and PSDF, however, showed neither a trend nor any physical significance of this rise. Based on the analyses of both the PDF and the PSDF, we believe that the incidental rise in rms value can be partially attributed to the high spikes registered by the probe in a high RBC concentrations fluid flow. The bombardment of RBC on the probe thermal boundary layer may cause a characteristic change in the probe response to certain flow phenomenon, at least within the Reynolds number range used in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Flow Velocity↗
PubMed1980

Physical characteristics of pulmonary artery stenosis murmurs in calves.

To better understnad the characteristics of cardiovascular murmurs and turbulent energy transmission, we made simultaneous measurements of pressure (sound) and velocity fluctuations in the vicinity of surgically produced stenoses in the pulmonary artery of calves. We did a spectral analysis of these data and found an identifiable break frequency at which the slopes of the pressure fluctuations changed. By use of these break frequencies and the corresponding blood jet velocity and vessel diameter measurements, Strouhal numbers were calculated. The jet velocity, Reynolds number, Strouhal number, and unsteadiness parameter values changed when the vessel was stenosed. When the power spectrum curves for the velocity fluctuations and the sound were compared, they tended to show a correlation between break frequencies. We speculate that this apparent correlation indicated a transfer from turbulent to acoustic energy.

Aging↗
PubMed1979

Comparative study of rigid vs flexible pneumatic artificial blood pumps.

For HR in the range of 70--110 beats/min, and same driving conditions, the pump with rigid housing produced a maximum of 15% greater CO than that with the soft housing, against an arterial pressure of 125/75 mm Hg. All 3 pumps exhibited greater sensitivity in function curves for HR in the range of 90--110 beats/min. The greatest sensitivity point in the function curve, for the rigid housing pump was found to be independent of HR change in the range of 90--110 beats/min. However, with an increase in HR the function curves shifted to the right in the case of soft housing pumps. In other words, greater atrial pressure was required to obtain the same CO with an increase in HR. The linear relationship found between CO and HR in flexible and soft housing pumps was very advantageous for controlling CO, whenever such change was required. In all 3 hearts there was a decrease in pulse pressure with an increase in HR. The decrease in pulse pressure was primarily due to the increase in diastolic pressure, while systolic pressure remained almost constant at all HRs. Since these experiments were conducted in a test circulatory system using an aqueous-glycerin solution, information on the degree of hemolysis was not addressed in this report.

Blood Circulation↗