Biomedical subjects

Keefe B Manning

Publications and source records attributed to Keefe B Manning.

14 recordsLinked to original sources

PubMed2006

Wavelet transforms in the analysis of mechanical heart valve cavitation.

Cavitation is known to cause blood element damage and may introduce gaseous emboli into the cerebral circulation, increasing the patient's risk of stroke. Discovering methods to reduce the intensity of cavitation induced by mechanical heart valves (MHVs) has long been an area of interest. A novel approach for analyzing MHV cavitation is presented. A wavelet denoising method is explored because currently used analytical techniques fail to suitably unmask the cavitation signal from other valve closing sounds and noise detected with a hydrophone. Wavelet functions are used to denoise the cavitation signal during MHV closure and rebound. The wavelet technique is applied to the signal produced by closure of a 29-mm Medtronic-Hall MHV in degassed water with a gas content of 5 ppm. Valve closing dynamics are investigated under loading conditions of 500, 2500, and 4500 mm Hg/s. The results display a marked improvement in the quantity and quality of information that can be extracted from acoustic cavitation signals using the wavelet technique compared to conventional analytical techniques. Time and frequency data indicate the likelihood and characteristics of cavitation formation under specified conditions. Using this wavelet technique we observe an improved signal-to-noise ratio, an enhanced time-dependent aspect, and the potential to minimize valve closing sounds, which disguise individual cavitation events. The overall goal of this work is to eventually link specific valves with characteristic waveforms or distinct types of cavitation, thus promoting improved valve designs.

Algorithms↗

PubMed2005

Off-design considerations of the 50cc Penn State Ventricular Assist Device.

Particle image velocimetry is used to study the flow field associated with off-design conditions of the 50cc Penn State Ventricular Assist Device. Phase average data, for 300 consecutive beats, was acquired for 60, 75, and 90 beats per minute (bpm) at 35/65 systolic duration and at 75 bpm for 50/50 systolic duration. Diastolic flow is dominated by the inflow jet and shows more penetration and better wall washing at higher beat rates and shorter diastolic time. A flow separation near the minor orifice of the aortic valve, however, is larger under these conditions. Turbulent energy, which peaks during diastole, is found to be strongest at the edges of the jet and near the valves and scales well with the maximum inlet jet velocity. A region of potential separation on the right lateral wall is identified and will be relieved by geometric modifications of the evolving design.

Blood Flow Velocity↗

PubMed2005

Acoustic and visual characteristics of cavitation induced by mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: A sudden pressure drop and recovery can induce cavitation in liquids. Mechanical heart valves (MHVs) generate such a pressure drop at closure, and cavitation generation around MHVs has been demonstrated many times. Cavitation is suspected as being a cause of blood and valve material damage. METHODS: In this in-vitro experiment, visual images and acoustic signals associated with MHV cavitation were studied to reveal cavitation characteristics. Björk-Shiley Convex-Concave valves, one with a pyrolytic carbon occluder and one with a Delrin occluder, were installed in a single-shot valve chamber. Cavitation intensity was controlled by load (dP/dt) and air content of water. The acoustic signal was measured using a hydrophone and visual images recorded with a high-speed digital camera system. RESULTS: Cavitation images showed that 10 ppm water rarely developed cavitation, unlike the 16 ppm water. A distinct peak pressure was observed at cavitation collapse that was a good indicator of MHV cavitation intensity. The average of the peak pressures revealed that cavitation intensity increased faster with increasing load for the 16 ppm water. CONCLUSION: The use of the peak pressure may be the preferred method for correlating cavitation intensity in structures for which the separation of valve closure noise and cavitation signal is difficult, as for the valves studied here.

Acoustics↗

PubMed2005

The effect of dissolved carbon dioxide on cavitation intensity in mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Mechanical heart valves (MHVs) are known to induce cavitation during closure and rebound. Cavitation may lead to blood element damage and stable bubble formation, with the latter introducing emboli into the cranial circulation and increasing the risk of stroke. Previous research has suggested that CO2 is the primary blood gas involved in stable bubble growth, due to its high solubility compared to that of oxygen or nitrogen. The primary objective of this study is to determine the role that CO2 plays in MHV-induced cavitation bubble formation. METHODS: Degassed water (5 ppm) was supplemented with CO2 at partial pressures of 0, 40 and 100 mmHg. Cavitation was visualized using high-speed videography for 29 mm Björk-Shiley Monostrut and Medtronic Hall MHVs in the mitral position. Experimental parameters (heart rate, systolic duration, and left ventricular pressure) were adjusted to provide dp/dt values of 500, 2,500 and 4,500 mmHg/s. High-frequency pressure fluctuations of cavitation bubble collapse were detected using a hydrophone. RESULTS: Root-mean square (RMS) values were calculated to quantify the cavitation intensity for both MHVs at the three loading conditions. The images of cavitation bubble formation and collapse were correlated to their respective RMS values. This study revealed no statistical difference between the cavitation intensities produced by either of the MHVs for the range of CO2-supplemented degassed water tested. For example, at the most physiologic loading condition of 2,500 mmHg/s, the RMS values for the Björk-Shiley Monostrut valve in degassed water containing 0 and 100 mmHg CO2 were 32.7 +/- 3.5 and 34.3 +/- 6.1 mmHg, respectively. CONCLUSION: The results of this in-vitro study show that, despite affecting stable bubble growth, the presence and quantity of dissolved CO2 does not affect the intensity of the cavitation events occurring during impact of the valve occluder with its housing. Therefore, the role of CO2 is limited to stable bubble development.

Carbon Dioxide↗

PubMed2004

Wall shear-rate estimation within the 50cc Penn State artificial heart using particle image velocimetry.

Particle image velocimetry (PIV) has been gaining acceptance as a routine tool to evaluate the flow fields associated with fluid mechanical devices. We have developed algorithms to investigate the wall shear-rates within the 50cc Penn State artificial heart using low magnification, conventional particle image velocimetry (PIV). Wall shear has been implicated in clot formation, a major post-implant problem with artificial hearts. To address the issues of wall scattering and incomplete measurement volumes, associated with near wall measurements, we have introduced a zero masking and a fluid centroid shifting technique. Simulations using different velocity fields were conducted with the techniques to assess their viability. Subsequently, the techniques were applied to the experimental data collected. The results indicate that the size of the interrogation region should be chosen to be as small as possible to maximize resolution while large enough to ensure an adequate number of particles per region. In the current study, a 16 x 16 interrogation window performed well with good spatial resolution and particle density for the estimation of wall shear rate. The techniques developed with PIV allow wall shear-rate estimates to be obtained from a large number of sites at one time. Because a planar image of a flow field can be determined relatively rapidly, PIV may prove useful in any preliminary design procedure.

Algorithms↗

PubMed2004

Fluid dynamic analysis of the 50 cc Penn State artificial heart under physiological operating conditions using particle image velocimetry.

In order to bridge the gap of existing artificial heart technology to the diverse needs of the patient population, we have been investigating the viability of a scaled-down design of the current 70 cc Penn State artificial heart. The issues of clot formation and hemolysis may become magnified within a 50 cc chamber compared to the existing 70 cc one. Particle image velocimetry (PIV) was employed to map the entire 50 cc Penn State artificial heart chamber. Flow fields constructed from PIV data indicate a rotational flow pattern that provides washout during diastole. In addition, shear rate maps were constructed for the inner walls of the heart chamber The lateral walls of the mitral and aortic ports experience high shear rates while the upper and bottom walls undergo low shear rates, with sufficiently long exposure times to potentially induce platelet activation or thrombus formation. In this study, we have demonstrated that PIV may adequately map the flow fields accurately in a reasonable amount of time. Therefore, the potential exists of employing PIV as a design tool.

Blood Flow Velocity↗

PubMed2003

Regurgitant flow field characteristics of the St. Jude bileaflet mechanical heart valve under physiologic pulsatile flow using particle image velocimetry.

The regurgitant flow fields of clinically used mechanical heart valves have been traditionally studied in vitro using flow visualization, ultrasound techniques, and laser Doppler velocimetry under steady and pulsatile flow. Detailed investigation of the forward and regurgitant flow fields of these valves can elucidate a valve's propensity for blood element damage, thrombus formation, or cavitation. Advances in particle image velocimetry (PIV) have allowed its use in the study of the flow fields of prosthetic valves. Unlike other flow field diagnostic systems, recent work using PIV has been able to relate particular regurgitant flow field characteristics of the Bjork-Shiley Monostrut valve to a propensity for cavitation. In this study, the regurgitant flow field of the St. Jude Medical bileaflet mechanical heart valve was assessed using PIV under physiologic pulsatile flow conditions. Data collected at selected time points prior to and after valve closure demonstrated the typical regurgitant jet flow patterns associated with the St. Jude valve, and indicated the formation of a strong regurgitant jet, in the B-datum plane, along with twin vortices near the leaflets. Estimated ensemble-average viscous shear rates suggested little potential for hemolysis when the hinge jets collided. However, the vortex motion near the occluder tips potentially provides a low-pressure environment for cavitation.

Blood Flow Velocity↗

PubMed2003

Diaphragm motion affects flow patterns in an artificial heart.

In the sac-driven artificial heart, the flow characteristics are coupled to the dynamics of the sac motion. The opening dynamics of the sac wall can, for example, strongly affect the chamber flow characteristics during diastole by directing or impeding the inflow. Poor sac motion can reduce the volume output of the pump and may increase the potential for thrombus formation within the ventricular chamber. It is particularly important for laboratory studies of the flow fields in artificial hearts that the diaphragm motion properly simulates the sac motion observed in vivo. In the present study, flow visualization was performed to investigate the relationship between the chamber flow characteristics of a Penn State artificial heart and the motion of the diaphragm during the filling phase during in vitro experimentation. The chamber flow pattern and diaphragm motion were recorded as a function of time, using high-speed videography. Experiments were conducted to determine the influence of diaphragm motion on the flow characteristics by altering the filling pressure, diaphragm thickness, and fluid density. Diaphragm motion was quantified by tracking the position of three surface points over the cardiac cycle. The alignment of these three surface trajectories can be used to quantify the uniformity of diaphragm motion. As a result, diaphragm motion was determined to be nonuniform under most operating conditions with the diaphragm opening in a wave-like pattern starting at the bottom of the chamber and propagating toward the inflow/outflow ports. This opening pattern simulates the opening pattern observed in an in vitro study of the clinical blood sac used in the Lionheart LVAD.

Blood Circulation↗

PubMed2003

A new method for evaluation of cavitation near mechanical heart valves.

Evaluation of cavitation in vivo is often based on recordings of high-pass filtered random high-frequency pressure fluctuations. We hypothesized that cavitation signal components are more appropriately assessed by a new method for extraction of random signal components of the pressure signals. We investigated three different valve types and found a high correlation between the two methods (r2: 0.8806-0.9887). The new method showed that the cavitation signal could be extracted without a priori knowledge needed for setting the high-pass filter cut off frequency, nor did it introduce bandwidth limitation of the cavitation signal.

Algorithms↗

PubMed2002

Flow through an outlet cannula of a rotary ventricular assist device.

Rotary ventricular assist devices (VADs) are primarily used during surgery and short-term postoperative assistance. There are issues involving the use of rotary VADs including the behavior of flow emanating from the assist device which might induce flow irregularities that could potentially lead to stroke or cerebrovascular accident. An in vitro flow visualization study using particle image velocimetry was conducted to evaluate flow fields at the bend of outlet cannulae with angles ranging from 30 degrees to 75 degrees. A multiple disk centrifugal blood pump produced steady, laminar flow fields at the outlet. Flow fields at the bend of each cannula demonstrated the development of a boundary layer along the inner wall and shift of fluid momentum to the outer wall. Stagnant zones along the inner wall were prevalent for each cannula and increased in severity with increasing angle. The outlet flow fields of rotary VADs have the potential to produce thrombus, feasibly leading to thromboembolisms should the bend angle of the outlet cannula be acute enough. Therefore, the placement of the outlet cannula or conduit should be taken into consideration during implantation.

Blood Flow Velocity↗

PubMedDate not supplied

Correlation of in vivo clot deposition with the flow characteristics in the 50 cc penn state artificial heart: a preliminary study.

Flow stasis in an artificial heart may provide a situation where thrombus develops. Should part, or all, of the clot dislodge, a thromboembolism may lead to stroke(s), neurologic deficits, or even death. In an effort to determine if the regime of low shear or stasis exists, a two-dimensional particle image velocimetry (PIV) system was implemented to measure the velocity field within the 50 cc Penn State Artificial Heart. The velocity measurements were decomposed nearest the wall to obtain wall shear rates along the bottom of the chamber. The PIV measurements were made in three image planes across the depth of the chamber to reconstruct a surface distribution of the wall shear rates at the bottom over the entire heart cycle. The wall shear rate is shown to be spatially nonuniform, with persistently low wall shear rates. An area near the front edge of the chamber at the bottom showed wall shear rates not exceeding 250 s(-1). This was an area of clot formation seen in vivo, suggesting a link may exist between the low wall shear rate zone and thrombus formation.

Animals↗

PubMedDate not supplied

Near field flow characteristics of the Bjork-Shiley Monostrut valve in a modified single shot valve chamber.

In certain mechanical heart valves, cavitation has been shown to develop during closure and rebound, leading to valve damage, blood damage, and strokes. Whereas it is uncertain what causes mechanical heart valve related strokes, some evidence suggests that stable bubbles may be the culprits. Previous work has indicated that vortex cavitation may contribute to stable bubble growth. Therefore, in an effort to understand the vortex cavitation, laser Doppler velocimetry data are collected in a plane parallel to and 3 mm away from the major orifice during closure and rebound of a Bjork-Shiley Monostrut mechanical heart valve. A modified single shot chamber is used that incorporates a more realistic near valve geometry than those used in previous studies. The results show the formation of a vortex during closure, which intensifies during rebound and dissipates during the final closing cycle. A regurgitant jet with mean velocities up to 3 m/s through the clearance gap of the valve provides energy to the vortex. During the final closing cycle, the vortex breaks up into asymmetrical, small scale flow patterns. This study provides further evidence that stable bubble formation may stem from the intense vortex cavitation occurring during valve closure and rebound.

Heart Valve Prosthesis↗

PubMedDate not supplied

Viscoelasticity of pediatric blood and its implications for the testing of a pulsatile pediatric blood pump.

Red blood cell hematocrit, aggregation and deformability, and plasma protein concentration influence the viscosity and elasticity of whole blood. These parameters affect the flow properties, especially at low shear rates (< 50 s(-1)). In particular, we have previously shown that the viscoelasticity of fluid affects the inlet filling characteristics and regions of flow separation in small pulsatile blood pumps. Although the viscosity of pediatric blood has been thoroughly studied, its elasticity has not been previously measured. Here we present the viscosity and elasticity of pediatric blood against shear rate for hematocrits from 19-56, measured using an oscillatory rheometer. There is little effect of patient age on blood viscoelasticity. A statistical analysis showed that when compared at constant hematocrit, blood from adult and pediatric patients had similar viscoelastic properties. We present blood analog solutions, as a function of hematocrit, constructed on the basis of the pediatric measurements. Flow field results for viscoelastic analogs of 20, 40 and 60% hematocrit and a Newtonian analog will be compared in the initial, in vitro testing of the Penn State pediatric blood pump, to determine the importance of incorporating a viscoelastic analog into the desigh interaction.

Blood Flow Velocity↗

PubMedDate not supplied

The 50cc Penn State left ventricular assist device: a parametric study of valve orientation flow dynamics.

We investigated the flow fields associated with the Bjork-Shiley Monostrut mechanical heart valve in the mitral position of the 50 cc Penn State left ventricular assist device. The valve orientation was adjusted whereby flow field data was collected using planar particle image velocimetry. The mitral valve was rotated from 0 to 45 degrees, in 15-degree increments. For each valve orientation, measurements were made in three planes (3, 5, and 8 mm from the front wall) parallel to the device pusher plate. Penetration of the inlet jet was affected by the valve orientation with more intense and longer duration wall washing motion occurring at 45 degrees. As a result, the 45-degree mitral valve orientation is recommended to help prevent areas of thrombus deposition. Valve orientation is an important aspect of assist device design.

Blood Flow Velocity↗