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Biomedical subjects

D B Geselowitz

Publications and source records attributed to D B Geselowitz.

At least 19 recordsLinked to original sources
PubMed1999

Is accurate recording of the ECG surface Laplacian feasible?

Experimental and model studies were performed to measure the electrocardiographic surface Laplacian using a rectangular finite difference approximation. The experimental approach used ten normal subjects with two sites on the torso. Electrode spacing was 2 cm. The surface Laplacian is theoretically independent of rotation of the electrode array. The data showed considerable variation with rotation. Model studies employed a realistic 23-dipole source. A spherical volume conductor showed invariance with rotation, as anticipated theoretically. A realistic torso, however, showed variation with rotation, although not as severe as that measured. A separate experimental study considered the signal-to-noise ratio (SNR) for the surface Laplacian. The average SNR was 3.3 and 2.5 at the two sites. These results raise serious questions about the practical ability to measure the surface Laplacian on the torso.

Adult↗
PubMed1999

In vitro studies of gas bubble formation by mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Recent clinical research using transcranial Doppler ultrasonography has shown the presence of emboli in the cranial circulation of some mechanical heart valve patients. Due to the high-intensity signals produced by these emboli, it has been suggested that they are small gas bubbles. Meanwhile, transesophageal echocardiography of mechanical heart valve patients has shown images of bright, mobile particles (also considered to be gas bubbles) near the valve. Motivated by these reports, a series of in vitro studies was performed to investigate the relationship between dissolved gas concentration and the incidence of bubble formation after valve closure. METHODS: A mock circulatory loop was used to study a Medtronic Hall tilting disc valve in the mitral position of the Penn State Electrical Ventricular Assist Device (EVAD). The valve was videotaped as it operated in saline with various levels of dissolved CO2. A Doppler ultrasound probe served as a bubble detector on the outflow side of the EVAD. Measurements of vaporous cavitation intensity with a high-fidelity pressure transducer were also made. Similar experiments were then performed in porcine blood, using an imaging ultrasound transducer to detect bubbles. RESULTS: Bubbles were seen moving off the valve in the retrograde direction just after closure. The ultrasound probe detected these bubbles downstream, indicating a bubble lifetime on the order of seconds. It was observed with high-speed video that bubble formation and cavitation are separate events and occur at different times during valve closure. Bubbles were more likely to be observed when CO2 levels were higher. Experiments in blood provided images of bubbles near the valve, predominantly at higher CO2 levels and high valve loading conditions. CONCLUSIONS: These results show that stable gas bubbles can form during mechanical heart valve operation. The bubbles likely form from the combined effects of gaseous nuclei formed by cavitation, low-pressure regions associated with regurgitant flow, and the presence of CO2, a highly soluble gas.

Animals↗
PubMed1998

A comparison of the cavitation potential of prosthetic heart valves based on valve closing dynamics.

BACKGROUND AND AIMS OF THE STUDY: This study compares the cavitation potential of prosthetic heart valves based on valve closing dynamics. METHODS: A laser sweeping technique measured valve closing dynamics (average closing velocity and deceleration) immediately before valve closure. A high-fidelity, piezoelectric pressure transducer was mounted proximal to the mitral valve and measured the high-frequency pressure fluctuations caused by cavitation bubble formation and collapse after valve closure. The band-pass filtered root mean squared (RMS) value of the mitral pressure signal was used as a measure of cavitation intensity. The combination of these two techniques allowed the direct correlation of valve dynamics and cavitation intensity for each valve closure. The effects of three parameters on prosthetic heart valve dynamics and cavitation were examined: valve geometry (Medtronic Hall and Björk-Shiley Monostrut), occluder material (pyrolytic carbon and Delrin), and gap width between the occluder and housing. A dimensional analysis was also performed to investigate the general form of the relationship between valve dynamics and cavitation intensity. RESULTS: For all of the valves investigated in this study, the RMS pressure increased (signifying an increase in cavitation) as the average closing velocity and deceleration increased. In order to compare the cavitation potential of the valves, the RMS pressure was estimated at specific closing velocities using the linear regression of RMS pressure versus average closing velocity for each valve. The effects of valve geometry, occluder material and gap width were then examined at high valve loading conditions (closing velocity of 4.0 m/s). For both pyrolytic carbon and Delrin, the Medtronic Hall valves had significantly higher RMS pressures than did the Björk-Shiley Monostrut valves. For a given valve geometry, the pyrolytic carbon occluder had a significantly higher RMS pressure than the Delrin occluder. The valve gap width did not have a significant effect on RMS pressure. The dimensional analysis revealed the general relationship among average closing velocity, occluder material properties and cavitation intensity. CONCLUSIONS: The results presented here contribute to our fundamental understanding of cavitation on mechanical heart valves.

Heart Valve Prosthesis↗
PubMed1997

Effects of tilting disk heart valve gap width on regurgitant flow through an artificial heart mitral valve.

While many investigators have measured the turbulent stresses associated with forward flow through tilting disk heart valves, only recently has attention been given to the regurgitant jets formed as fluid is squeezed through the gap between the occluder and housing of a closed valve. The objective of this investigation was to determine the effect of gap width on the turbulent stresses of the regurgitant jets through a Björk-Shiley monostrut tilting disk heart valve seated in the mitral position of a Penn State artificial heart. A 2 component laser-Doppler velocimetry system with a temporal resolution of 1 ms was used to measure the instantaneous velocities in the regurgitant jets in the major and minor orifices around the mitral valve. The gap width was controlled through temperature variation by taking advantage of the large difference between the thermal expansion coefficients of the Delrin occluder and the Stellite housing of Björk-Shiley monostrut valves. The turbulent shear stress and mean (ensemble averaged) velocity were incorporated into a model of red blood cell damage to assess the potential for hemolytic damage at each gap width investigated. The results revealed that the minor orifice tends to form stronger jets during regurgitant flow than the major orifice, indicating that the gap width is not uniform around the circumference of the valve. Based on the results of a red blood cell damage model, the hemolytic potential of the mitral valve decreases as the gap width increases. This investigation also established that the hemolytic potential of the regurgitant phase of valve operation is comparable to, if not greater than, the hemolytic potential of forward flow, consistent with experimental data on hemolysis.

Blood Flow Velocity↗
PubMed1996

Mitral heart valve cavitation in an artificial heart environment.

BACKGROUND AND AIMS OF THE STUDY: The formation and subsequent collapse of vaporous cavities in the fluid around mechanical heart valves at valve closure can create stresses large enough to damage both the valve itself and blood cells. Improved understanding of cavitation mechanisms should lead to a reduction in the cavitation potential of future valve designs. MATERIALS AND METHODS: This study compares eight mechanical mitral valves of two different geometries (Monostrut and Medtronic Hall), occluder housing gaps (tight, medium, and leaky), and occluder materials (Delrin and pyrolytic carbon). The valves were evaluated in a model ventricle of the Penn State Electric Ventricular Assist Device (EVAD) operating within a mock circulatory loop. The EVAD represents one half of a total artificial heart. The mock loop consisted of silicone tubing connected to elements designed to mimic the compliant and resistant properties of the natural circulation. Cavitation was controlled by varying the degree of filling of the ventricle: low filling caused higher valve closing velocities resulting in greater cavitation intensities than complete filling of the ventricle. The intensity of cavitation was quantified using a parameter derived from the high frequency fluctuations in the mitral pressure that occur around the valve during cavitation events. The shape of the cavitation pressure signature and that of the power spectrum of the cavitation pressure signature were used in addition to the cavitation intensity parameter to make comparisons between valves. RESULTS: Of the three valve characteristics studied, occluder material showed the most significant influence on cavitation intensity: valves with pyrolytic carbon occluders demonstrated greater cavitation than did those with Delrin discs. CONCLUSION: It is hypothesized that the dominant form of cavitation on the valves studied is related to vortex formation and that occluder material influences the intensity of cavitation through the strength of the tension wave generated at valve closure, while geometry and gap have only secondary effects. Future studies are planned to incorporate this technique in an in vivo environment.

Biophysical Phenomena↗
PubMed1995

Computation of heart surface potentials using the surface source model.

INTRODUCTION: The bidomain model of the heart leads to the result that the volume density of cardiac current source moment is proportional to the gradient of the macroscopic transmembrane action potential distribution. If the anisotropy ratios of the inner and outer domains (syncytia) of the myocardium are equal, then the volume distribution of cardiac sources can be replaced by an appropriate double layer on the heart surface. The double layer source distribution (heart surface source model) provides a basis for calculating heart surface potentials from cardiac sources. METHODS AND RESULTS: The heart surface model was used to calculate epicardial potentials for the normal heart as well as for a case of ischemia and of infarction. The model was also used to determine the effect of insulating the heart surface. Insulating the heart surface caused an almost fourfold increase in peak-to-peak amplitude of simulated electrograms, with little change in waveshape. Simulated electrograms showed good agreement with recorded electrograms reported in the literature. CONCLUSION: The heart surface source model appears to provide a basis for relating heart surface potentials to the distribution of cellular action potential.

Action Potentials↗
PubMed1994

A new mock circulatory loop and its application to the study of chemical additive and aortic pressure effects on hemolysis in the Penn State electric ventricular assist device.

A new mock circulatory loop was developed for hemolysis studies associated with the Penn State electric ventricular assist device (EVAD). This flow loop has several advantages over previously designed loops. It is small enough to accommodate experiments in which only single units of blood are available, it is made out of biocompatible materials, it incorporates good geometry, and it provides normal physiological pressures and flows to both the aortic outlet and the venous inlet of the pumping device. Experiments with reduced aortic pressure but normal cardiac output showed that hemolysis in a loop with normal aortic blood pressure was significantly higher than that in a loop with lowered aortic pressure, thereby illustrating the importance of maintaining loop pressures as close as possible to those found in vivo. This data also imply that blood traveling through the left ventricle in an artificial heart may be subject to higher hemolysis rates than that traversing the right ventricle. Another set of experiments to determine the effects of 4 hemolysis or drag-reducing agents (Pluronic F-68, Dextran-40, Polyox WSR-301, and Praestol 2273TR) on blood trauma due to the EVAD and associated valves was performed. Results indicated that none of the additives significantly reduced hemolysis under the conditions found in the mock loop. Finally, a compilation of data gathered in these experiments showed that the index of hemolysis (IH) is dependent on hematocrit (HCT), which suggests that another parameter, IH/HCT, may be more suited to the quantification of hemolysis.

Acrylic Resins↗
PubMed1994

Identifying the end of ventricular activation: body surface late potentials versus electrogram measurements in a canine infarction model.

INTRODUCTION: Identification of the end of the QRS is perhaps the single most important feature obtained from the high resolution signal-averaged electrocardiogram (SAECG). This point relies on computer algorithms to select a point above the noise levels. Prior studies to substantiate this approach using electrograms for comparison have demonstrated many examples of the body surface recordings failing to detect the full extent of the late potentials. METHODS AND RESULTS: An animal model that generates late potentials was used in conjunction with epicardial cardiac mapping system to systematically examine the reasons for these failures. In 11 of 13 dogs we found a concordance between the signal-averaged recordings and the epicardial recordings within 5 msec. The two discordant studies were attributed to a failure of epicardial mapping to record all late potential sources. Also, a means of accurately comparing measurements from the two recording technologies was required in this study as well as a new definition for identifying the end of activation currents in epicardial electrograms. CONCLUSION: To achieve these results required approaches different from those used in the clinical setting to record the SAECG. These include: (1) the analysis of individual XYZ leads as opposed to the vector magnitude derived from these leads; (2) visual identification of very low level signals, as automatic algorithms often fail to detect low level signals; and (3) the use of finite impulse response digital filters instead of the bidirectional Butterworth filter.

Action Potentials↗
PubMed1994

LDA measurements of mean velocity and Reynolds stress fields within an artificial heart ventricle.

Laser Doppler Anemometry measurements of mean (ensemble average) velocities and turbulent (Reynolds) stresses at 140 locations within the left ventricle of the Penn State 70 cc electric artificial heart/ventricular assist device are reported at 8 times during the cardiac cycle. Mean velocity patterns indicate that the surfaces of the blood sac and valve tracts are exposed to significant levels of wall shear stress (good wall washing) during some portion of the flow cycle, and there is no location where the flow is stagnant over the entire flow cycle. This implies that thrombus deposition within the artificial heart should be suppressed. Turbulent stresses in the main pumping chamber and the outflow tracts of the tilting disk valves do not exceed 2000 dynes/cm2. The highest turbulent stresses (20,000 dynes/cm2) and smallest turbulent microscales (6 microns) are found in the regurgitant jets on the minor orifice side of the aortic valve during diastole and the mitral valve during systole. Taken together, the data suggest that improvements in artificial heart fluid mechanics will come through valve design and pump operating conditions, not pumping chamber design.

Blood Flow Velocity↗
PubMed1994

A method for real-time in vitro observation of cavitation on prosthetic heart valves.

A method for real-time in vitro observation of cavitation on a prosthetic heart valve has been developed. Cavitation of four blood analog fluids (distilled water, aqueous glycerin, aqueous polyacrylamide, and aqueous xanthan gum) has been documented for a Medtronic/Hall prosthetic heart valve. This method employed a Penn State Electrical Ventricular Assist Device in a mock circulatory loop that was operated in a partial filling mode associated with reduced atrial filling pressure. The observations were made on a valve that was located in the mitral position, with the cavitation occurring on the inlet side after valve closure on every cycle. Stroboscopic videography was used to document the cavity life cycle. Bubble cavitation was observed on the valve occluder face. Vortex cavitation was observed at two locations in the vicinity of the valve occluder and housing. For each fluid, cavity growth and collapse occurred in less than one millisecond, which provides strong evidence that the cavitation is vaporous rather than gaseous. The cavity duration time was found to decrease with increasing atrial pressure at constant aortic pressure and beat rate. The area of cavitation was found to decrease with increasing delay time at a constant aortic pressure, atrial pressure, and beat rate. Cavitation was found to occur in each of the fluids, with the most cavitation seen in the Newtonian fluids (distilled water and aqueous glycerin).

Blood Viscosity↗
PubMed1994

An in-vitro investigation of prosthetic heart valve cavitation in blood.

Vapor cavities produced by low pressure fluid flow conditions have been observed in the vicinity of mechanical heart valves for many years. As cavities collapse during pressure recovery, they can produce stresses large enough to cause pitting of the valve occluders and lysing or activation of blood cells. To date, no method has been presented for the quantification of mechanical heart valve cavitation in blood because it has only been detected optically in transparent blood analog fluids. This paper describes a novel method for quantifying cavitation intensities in opaque fluids such as blood. It is based on the detection of high frequency pressure oscillations (35-350 kHz) at a location 4.5 cm proximal to a Björk-Shiley monostrut mitral valve in a mock circulatory loop driven by a Penn State Electric Ventricular Assist Device. The pressure oscillations which result from cavity collapse are used to quantify cavitation intensities in blood. One time domain and three frequency domain parameters have been developed to quantify cavitation intensity during a single valve closure event and over an ensemble of closure events. The time domain parameter is the Root Mean Squared (RMS) value of the pressure signal after it has been high-pass filtered at 35 kHz. The other three parameters are derived from the power spectrum of the pressure signal. One is the maximum value of the power spectrum between 100 and 200 kHz, another is the area under the power spectrum between 35 and 400 kHz, and the last is the volume under a 3-dimensional time vs. frequency vs. power spectrum plot. The parameters are averaged over a random sample of pressure traces to determine an average cavitation intensity for each operating condition studied. In addition, cavitation pressure fluctuations and hemolysis rates were determined simultaneously at several different mock flow loop operating conditions using porcine blood, and the relationships between various measures of cavitation intensity and the associated index of hemolysis have been established. Hemolysis was shown to increase with cavitation intensity.

Animals↗
PubMed1993

Aortic pressure estimation with electro-mechanical circulatory assist devices.

An adaptive technique for the estimation of the time history of aortic pressure (from applied voltage and position feedback) has been designed, implemented, and bench tested using the Penn State Electric Ventricular Assist Device (EVAD). This method, known in the field of automatic control as a dynamic observer, utilizes gains which were determined using experimental data collected while the EVAD was running on a mock circulatory system. An adaptive scheme provides the observer with a method of changing its initial conditions on a stroke-by-stroke basis which improves observer performance. In both determining the feedback gains and developing the adaptation scheme, a range of beat rates and pressure loads was taken into account to yield satisfactory observer performance over a range of operating conditions. The observer was implemented, its performance was verified in vitro and results are reported. In the six experimental operating conditions, the beat rate ranged from 56-104 beats per minute (bpm) and the span of the mean systolic aortic pressure was 10.7-18.7 kPa (80-140 mmHg). For these cases, the mean deviation between the actual and estimated aortic pressure during the latter two-thirds of systole was 0.41 kPa (3.1 mmHg).

Algorithms↗