Noninvasive electrical imaging: is it ready for clinical application?
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Cavitation of mechanical heart valves under physiologic conditions.
Cavitation has recently entered the discussion of factors leading to blood and material damage after implantation of mechanical heart valves. Since direct evidence for this phenomenon cannot yet be demonstrated in vivo, an in vitro method had to be developed to permit the investigation whether cavitation actually occurs under (simulated) physiologic conditions and to clarify its clinical relevance. Previous studies have shown that different types of commercially available mechanical valves exhibit different tendencies to generate cavitation in vitro. The present study presents a test protocol for the assessment of cavitation generated by different replacement valves under simulated physiologic conditions. Comparative investigations were performed in order to transfer the in vitro results in vivo conditions. Although mechanical valves with an overlapping closing body/valve ring configuration tend to create cavitation earlier, cavitation could not be demonstrated for any investigated mechanical valve type under simulated resting conditions. The danger of continuous cavitation damage is therefore low. Certain valve types exhibit cavitation only at higher heart rates (more than 120-140 beats/min). Some valve types do not show cavitation even during heavy exercise. Based on mathematical models and experimental investigations, it is very likely that if cavitation does occur, blood and material damage may be expected.
Blood↗
Experimental embolic capture by asymmetric Greenfield filters.
An uncommon complication of percutaneous insertion of the titanium Greenfield filter is angulation at discharge with clustering of the limbs to one side of the vena cava. Potential adverse effects of this asymmetry were tested in a perfusion system with cadaver venae cavae. The stainless-steel Greenfield filter (SGF) was compared with the modified-hook titanium model (TGF-MH) and a percutaneous modified-hook model of the SGF (SGF-MH) with flexible plastic emboli in four sizes and varied sequences. There were 458 runs comprised of 2203 embolic events, with an overall successful capture rate of 81% despite tilting, asymmetry, and accelerated flow rates from 1.0 to 2.5 L/min. Significant differences were observed in vena cava with diameters greater than 22 mm where capture rate was 76% as opposed to 86% in venae cavae less than 22 mm (p = 0.01). Larger emboli (8 x 25 mm and 8 x 100 mm) were trapped uniformly, whereas smaller emboli (5 x 10 mm) had 59% capture (p = 0.001). Similarly, the capture rate was less when smaller emboli were released before the larger ones (71% vs 87%; p = 0.001). Capture in small versus large venae cavae was not significantly different for the SGF (75%) or TGF-MH (84% vs 87%) but differed for the SGF-MH (98% vs 67%; p less than 0.001). Logistic regression was used to develop a mathematic model for interaction of the variables and determined two first-order interactions: caval size and limb position and caval size and type of filter.(ABSTRACT TRUNCATED AT 250 WORDS)
Cadaver↗
Dynamic pumping performance of the Hemopump--a small intraventricular blood pump.
We studied the pumping characteristics of the Hemopump, a commercially available miniature intraventricular blood pump for temporary support of failing hearts. The Hemopump is an axial flow pump of which the characteristics can be described by turbomachine theory. Experiments with water and a mock circulation verified that the pumping characteristics of the Hemopump in terms of both pressure head and flow as a function of rotational speed, very well can be described by a first order differential equation. The influence of blood with its non-Newtonian character is being investigated.
Heart-Assist Devices↗
Aortic wall stress profile after repair of coarctation of the aorta. It is related to subsequent true aneurysm formation?
UNLABELLED: True aneurysm formation at the site of coarctation repair has been increasingly recognized after synthetic patch aortoplasty. A mathematical model was developed to determine the aortic wall stress profile after coarctation repair with this technique. METHODS: A two-dimensional nonlinear mathematical model and a three-dimensional finite element model were developed for different physiologic, geometric, and materials properties variables, which were incorporated into an idealized coarctation repair. RESULTS: The models demonstrated that the major variable affecting stress levels in the aortic wall after coarctation repair was the patch geometry. If the patch was allowed to balloon out, the aortic wall stress increased out of proportion to the increase in aortic diameter because of nonlinear effects. The maximal aortic wall stress concentration occurred opposite the patch. Patch stiffness was also an important variable, with a lower stiffness (subclavian flap repair) leading to a higher aortic wall stress for the same patch geometry as a synthetic patch repair. Inferences: Development of true aneurysms after coarctation repair by synthetic patch aortoplasty is likely to result from excessive aortic wall stress due to patch geometry.
Aorta↗
Simulate heart rate variability in different physiological conditions.
Heart rate variability is assumed to result from a multiplicative or cascade mechanism based on its multifractal property. Numerical results on the perturbation of the cascade are given to compare HRV under parasympathetic (PNS) and sympathetic (SNS) autonomic blockades. It was found that qualitative change of the HRV multifractal, which was observed in PNS blockade, is due to the change from a multiplicative to an additive mechanism.
Biophysical Phenomena↗
Statistical interpretation of body surface potential maps; aspects of adequate group representation and parameter selection.
Nonparametric approaches for studying adequate group representation accuracy vs. sample size relationship and two discriminative parameter searching methods of statistical body surface map analysis are discussed. A clinical application of the selection methods supports that body surface potential mapping provides significant additional diagnostic information with respect to the conventional 12 lead ECG.
Action Potentials↗
Automatic decomposition of Wigner distribution and its application to heart rate variability.
OBJECTIVE: We introduce an algorithm for the automatic decomposition of Wigner Distribution (WD) and we applied it for the quantitative extraction of Heart Rate Variability (HRV) spectral parameters during non-stationary events. Early response to tilt was investigated. METHODS: Quantitative analysis of multi-components non-stationary signals is obtained through an automatic decomposition of WD based on least square (LS) fitting of the instantaneous autocorrelation function (ACF). Through this approach the different signal and interference terms which contributes to the ACF may be separated and their parameters (instantaneous frequency and amplitude) quantified. A beat-to-beat monitoring of HRV spectral components is obtained. RESULTS: Analysis of simulated signals demonstrated the capability of the proposed approach to track and separate the signal components. Analysis of HRV data evidenced different dynamics in the early Autonomic Nervous System (ANS) response to tilt. CONCLUSIONS: The novel approach to the quantification of the beat-to-beat HRV spectral parameters obtained from decomposition of Wigner distribution was demonstrated to be effective in the analysis of HRV data. Relevant physiological information about the dynamics of the early sympathetic response to tilt were obtained. The method is a general approach which may be employed for a quantitative time-frequency analysis of non-stationary biological signals.
Algorithms↗
Use of the left ventricular time-activity curve as a noninvasive input function in dynamic oxygen-15-water positron emission tomography.
Noninvasive recording of arterial input functions using regions of interest (ROIs) in the left ventricular (LV) chamber obviates the need for arterial cannulation in PET, but it is compromised by the limited recovery coefficient of the LV chamber and by statistical noise. In the present study, a new mathematical model has been developed, which corrects for the spillover of radioactivity both from the myocardium into the LV ROI and the blood into the myocardial ROI. The method requires the measurement of a time-activity curve in the LV chamber during the dynamic H2(15)O PET study and the measurement of the recovery coefficient of the LV ROI using a 15O-carbon monoxide (C15O) scan and venous blood sampling. This approach was successfully validated against direct measurements of the arterial input function using an on-line beta detector in five greyhounds undergoing dynamic H2(15)O PET imaging. This technique also yielded myocardial blood flow (MBF) values which were not significantly different from those obtained with the beta-probe analyses (maximum difference less than 2%), provided that the LV ROIs were sufficiently large to provide good counting statistics. When this model was not applied for large ROIs (small recovery in LV ROI), systematic overestimations in MBF compared with beta-probe analysis (e.g., a factor by 40% for a recovery coefficient of 0.7) were observed. Thus, this technique enabled the prediction of an accurate input function using the LV time-activity curve, and hence, noninvasive quantification of MBF without arterial cannulation.
Animals↗
Effects of valve characteristics on the accuracy of the Bernoulli equation: a survey of data submitted to the U.S. FDA.
BACKGROUND AND AIM OF THE STUDY: In 1988, valve manufacturers petitioned the U. S. Food & Drug Administration (FDA) to replace catheter with Doppler ultrasound measurements of pressure gradient (delta P) in clinical studies. Manufactures agreed to submit bench data validating the Bernoulli equation used to calculate delta P = delta P = K(Vd2 - Vp2), where K = constant, Vd = distal Doppler velocity, and Vp = proximal Doppler velocity. Previous studies suggest that K may vary from the idealized 4.0, which could lead to incorrect valve assessment and clinical errors. METHODS: Variation in K-values in marketing application data submitted to the FDA was assessed. Pulse duplicator data included four bileaflet valves, two stented bioprostheses, and seven stentless bioprostheses, sized from 19 to 33 mm. Effects of valve type, valve size, blood-mimicking fluid used, and distal pressure tap position (DPTP) were evaluated via an analysis of variance. RESULTS: K-values varied from 2.50 to 7.40 (n = 90). K was found to be dependent on valve type (p < 0.0001), blood-mimicking fluid (p < 0.0001) and DPTP (p < 0.0001), but not valve size. At DPTP = 30 mm, K = 3.43 +/- 0.56, 5.15 +/- 0.81, and 4.81 +/- 1.02, for bileaflet, stented and stentless valves, respectively. K averaged 10% less using the 100-mm DPTP, due to pressure recovery. Variations due to blood-mimicking fluid were likely related to the fluid density. CONCLUSION: Variations due to DPTP and fluid used are consistent with physical mechanisms of pressure recovery and fluid density. Results from previous studies have suggested that effects of valve type on K are also real. The magnitude of these effects appeared to be +/- 25%. Extrapolation to patients is difficult, but clinicians should be aware that Doppler measurements may vary by similar amounts. Doppler pressure gradients should be interpreted qualitatively and moderated by other diagnostic measures of valve performance.
Analysis of Variance↗
Comparison of prosthetic valve hydrodynamic function: objective testing using statistical multilevel modeling.
BACKGROUND AND AIM OF THE STUDY: The performance of novel prosthetic heart valves is assessed using in-vitro hydrodynamic function tests. The study aim was to examine the problem of objective discrimination of hydrodynamic performance to determine significant differences between valve designs, and illustrate proposed methodology using data collected from five different polyurethane tri-leaflet valve designs. METHODS: Two engineering designs were manufactured with leaflets of the same polyurethane (GE, LE); design L was manufactured using three further leaflet materials of differing material modulus (LL, L4, L5). Six valves were made in each design, each tested at five flow rates in a standard hydrodynamic test rig, with five test replications for each valve. The data were analyzed using multilevel statistical modeling methods, allowing simultaneous comparison of multiple regression lines describing valve performance. The multilevel model is hierarchical in structure, in this case with two levels of data, describing individual valves at level 2 and test replicates at level 1. In all cases, the multilevel model uses the hydrodynamic function measure of interest, e.g. mean pressure gradient or leakage, with logarithmic transformation as required as the dependent variable, Y. The independent variable, X, is, in all cases, the natural logarithm of the RMS flow measured through the valve. RESULTS: The two-design multilevel model enabled quantitative discrimination of designs GE and LE, showing that design GE had significantly better hydrodynamic function overall than design LE in this case (mean pressure gradient was estimated as 0.93 mmHg lower at low cardiac output, 14.74 mmHg lower at 9.6 l/min). The five-design multilevel model showed clearly the relatively poor hydrodynamic performance of designs L4 and L5 compared with others. The procedure was straightforward, and produced a statistical comparison among valve designs that is not easily achieved by other means. CONCLUSION: This methodology provides a useful means of objective assessment of valve function for valve developers. Variance estimates provided by the analysis also provide a basis for quality control of valve production and testing.
Equipment Failure Analysis↗
Measurement of the material properties of viscous liquids using ultrasonic guided waves.
The theoretical basis for a testing tool in the form of a circular waveguide for measuring both the dynamic viscosity and the longitudinal bulk velocity of viscous liquids is presented. It is based on the measurements of the attenuation of the fundamental torsional mode, T(0,1), and the fundamental longitudinal mode, L(0,1), of the waveguide when immersed in the liquid. The modeling techniques to extract the shear viscosity and the longitudinal bulk velocity are explained and experimentally verified. Results for two viscous liquids are presented: good agreement between theory and experiment was obtained.
Blood↗
Statistical characteristics of mechanical heart valve cavitation in accelerated testing.
BACKGROUND AND AIM OF THE STUDY: Cavitation damage has been observed on mechanical heart valves (MHVs) undergoing accelerated testing. Cavitation itself can be modeled as a stochastic process, as it varies from beat to beat of the testing machine. This in-vitro study was undertaken to investigate the statistical characteristics of MHV cavitation. METHODS: A 25-mm St. Jude Medical bileaflet MHV (SJM 25) was tested in an accelerated tester at various pulse rates, ranging from 300 to 1,000 bpm, with stepwise increments of 100 bpm. A miniature pressure transducer was placed near a leaflet tip on the inflow side of the valve, to monitor regional transient pressure fluctuations at instants of valve closure. The pressure trace associated with each beat was passed through a 70 kHz high-pass digital filter to extract the high-frequency oscillation (HFO) components resulting from the collapse of cavitation bubbles. Three intensity-related measures were calculated for each HFO burst: its time span; its local root-mean-square (LRMS) value; and the area enveloped by the absolute value of the HFO pressure trace and the time axis, referred to as cavitation impulse. These were treated as stochastic processes, of which the first-order probability density functions (PDFs) were estimated for each test rate. RESULTS: Both the LRMS value and cavitation impulse were log-normal distributed, and the time span was normal distributed. These distribution laws were consistent at different test rates. CONCLUSION: The present investigation was directed at understanding MHV cavitation as a stochastic process. The results provide a basis for establishing further the statistical relationship between cavitation intensity and time-evolving cavitation damage on MHV surfaces. These data are required to assess and compare the performance of MHVs of different designs.
Acceleration↗
Compact mock loops of the systemic and pulmonary circulation for blood pump testing.
Mock loops are an important tool for in vitro investigations of artificial blood pumps. The simple windkessel, throttle, and atrium principle was used for the mock loop design presented. The components of the systemic and the pulmonary mock loop were designed according to calculated numerical simulation parameters. The loops offer a compact design and simple handling. For simulating biventricular assist or total artificial heart (TAH), both loops can be coupled correspondingly. The numerical simulation and the first results with the loops show very good similarity to physiological data of systemic and pulmonary circulation. The measurements of pump characteristics are significant for quantitative comparison of different pump sizes and types, or driving systems.
Blood Circulation↗
A mathematical analysis of vasomotion in the peripheral vascular bed.
Arterioles and microvascular venules often show rhythmic spontaneous changes in diameter, called vasomotion. In this study, we analyze the possibility that vasomotion originates from the activity of the local myogenic mechanism. This analysis uses an original mathematical model of the peripheral circulation. The peripheral vascular bed has been represented as a series of three consecutive segments, each characterized by its value of vascular resistance per unit weight of tissue. The internal radius of the vessels in the last two segments, and hence their hydraulic resistance, has been assumed to be affected by the local myogenic response of the vascular smooth muscle. This dependence has been reproduced using the Laplace law. Both the static and dynamic (i.e. rate-dependent) components of the myogenic response have been included in the model, in accordance with recent experimental results. Simulations demonstrate that rhythmic, self-sustained oscillations can develop when the dynamic component of the myogenic response of terminal arterioles is much greater than that of more proximal microvessels. A moderate increase in arterial pressure favors the occurrence of oscillations, whereas vasodilatory stimuli tend to suppress vasomotion and contribute to the stabilization of vascular diameters.
Animals↗
Refractoriness of cardiac muscle as affected by intercalated disks: a model study implications for fibrillation and defibrillation.
It is generally agreed that inhomogeneities of the recovery process in cardiac tissue play an important role in the genesis of reentrant arrhythmias. Regarding cardiac muscle as an assembly of discrete cells connected by gap junctions, differences in recovery may result from a nonuniformity of membrane or cable properties. In this study, a computer model of a one-dimensional cardiac muscle fiber including a periodic intercalated disk structure is used to study the influence of disk resistance (Rj) and stimulus strength (J) on refractoriness. Stimulating currents are applied externally in a bipolar arrangement. The basic effect of a current pulse is local de- and hyperpolarizations at the ends of an individual cell. Polarization develops very rapidly and increases with increasing values of Rj or J so that an interaction with membrane current kinetics becomes possible. When a premature stimulus is applied during repolarization of a conditioning action potential, multiple Na currents can occur, either caused by depolarization of the cathodal end of a cell or in the form of anode break excitation at the hyperpolarized end. Those currents affect the response of a fiber such that, at a given value of J, the refractory period is shortened by an increase in Rj. In a ring fiber model with different Rj values in the two halves of ring an extrastimulus timed between the refractory periods of the two branches results in a sustained circus movement. Varying stimulus strength yields an upper limit of vulnerability characterized by a "synchronized extrasystole". The ring model also implies the suppression of circus movement by an external shock. The minimal shock strength required for suppression is close to the upper limit of vulnerability. The simulations suggest that discrete effects of junctional resistance may be involved in fibrillation and defibrillation.
Action Potentials↗
[Critique of Starling's hypothesis of the microcirculation].
By means of average estimation of the parameters on the capillary wall in experiments the capillary is supposed to content one pore only. This situation is not relevant to the practice of blood supply and leads to misinterpretation of the Starling's equation which can not represent the theoretical conception of the microcirculation in tissues. In order to avoid mistakes the usage of system theoretical analytic methods is indicated. This is the alternative way which is able to cover all kinds of preconditions of microcirculation in different tissues.
Blood Pressure↗
Effect of red cell clustering and anisotropy on ultrasound blood backscatter: a Monte Carlo study.
When flowing at a low shear rate, blood appears hyperechogenic on ultrasound B-scans. The formation of red blood cell (RBC) aggregates that also alters blood viscosity is the microscopic mechanism explaining this acoustical phenomenon. In this study, Monte Carlo simulations were performed to predict how RBC clustering increases ultrasound scattering by blood. A bidimensional Gibbs-Markov random point process parameterized by the adhesion energy epsilon and an anisotropy index nu was used to describe RBC positions for a hematocrit H = 40%. The frequency dependence of the backscattering coefficient chi(f) was computed using Born approximation. The backscattering coefficient chi0 at 5 MHz and the spectral slopes n(x) and n(y) (chi alpha f(nx) or f(ny)) measured, respectively, when the insonification is parallel and perpendicular with the RBC cluster axis were then extracted. Under isotropic conditions, chi0 increased up to 7 dB with epsilon and n(x) = n(y) decreased from 4.2 to 3.4. Under anisotropic conditions, the backscattering was stronger perpendicularly to aggregate axis, resulting in n(x) < n(y). The anisotropy in scattering appeared more pronounced when epsilon or nu increased. These two dimensional results generally predict that low-frequency blood backscatter is related to cluster dimension, and higher-frequency properties are affected by finer morphological features as anisotropy. This numerically establishes that ultrasound backscatter spectroscopy on a large frequency range is pertinent to characterize in situ hemorheology.
Algorithms↗