Responses to pacing depend on electrical and structural properties of the reentrant circuit.
Explore the source record for details and available documents.
Action Potentials↗
SEARCH · Search PubMed
Results for “Models, Cardiovascular”
Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
The restitution portrait: a new method for investigating rate-dependent restitution.
INTRODUCTION: Electrical restitution, relating action potential duration (APD) to diastolic interval (DI), was believed to determine the stability of heart rhythm. However, recent studies demonstrate that stability also depends on long-term APD changes caused by memory. This study presents a new method for investigation of rate- and memory-dependent aspects of restitution and for assessment of mapping models of APD. METHODS AND RESULTS: Bullfrog ventricular myocardium was paced with a "perturbed downsweep protocol." Starting from a basic cycle length (BCL) of 1,000 ms, the tissue was paced until steady state was achieved, followed by single beats of longer and shorter cycle lengths. BCL was decreased by 50 to 100 ms and the process repeated. All APDs were plotted as a function of the preceding DI, which allowed simultaneous observation of dynamic, S1-S2, and two constant-BCL restitution curves in a "restitution portrait." Responses were classified as 1:1 (stimulus:response), transient 2:2, or persistent 2:2 (alternans) and were related to the slopes of the restitution curves. None of these slopes approached unity for the persistent 2:2 response, demonstrating that the traditional restitution condition does not predict alternans. The restitution portrait was used to evaluate three mapping models of APD. The models with no memory and with one-beat memory did not produce restitution portraits similar to the experimental one. A model with two-beat memory produced a qualitatively similar portrait. CONCLUSION: The restitution portrait allows a more comprehensive assessment of cardiac dynamics than methods used to date. Further study of models with memory may result in a clinical criterion for electrical instability.
Action Potentials↗
Comparison of conventional and biventricular antitachycardia pacing in a geometrically realistic model of the rabbit ventricle.
INTRODUCTION: ICDs often are programmed with antitachycardia pacing (ATP) as the first response to ventricular tachycardia (VT). Many ICDs have an additional lead available for ventricular pacing. We hypothesized that using the additional lead for ATP would improve therapy by advancing the orthodromic wavefront, thereby reducing the size of the excitable gap and inducing block of all reentrant activity. METHODS AND RESULTS: Monomorphic VT was initiated in a thin-walled model of rabbit ventricular myocardium that included an apical infarct and anatomically realistic dimensions. ATP with up to eight pulses was delivered at 90% of VT cycle length to one (conventional) or two (biventricular) stimulation areas. Stimulation areas were adjusted from 0.017 cm2 to 0.169 cm2 to modulate interactions between the antidromic and VT wavefronts, and between the orthodromic wavefront and the VT's refractory region. During conventional ATP, we found that larger stimulation areas terminated the VT in three pulses. Continued pacing after termination caused VT reinitiation in the reversed direction in some instances. With smaller stimulation areas, conventional ATP simply reset the circuit. During biventricular ATP, larger stimulation areas terminated VT in one pulse. There were no instances of reinitiation with reversal. However, with smaller stimulation areas, prolongation of refractoriness near the additional stimulation area facilitated induction of functional reentry with pathways modified by continued pacing. CONCLUSION: Our modeling suggests that biventricular ATP is superior to conventional ATP under conditions where the additional ventricular lead effectively advances the orthodromic wavefront. Failure to achieve this advancement poses a risk of VT acceleration.
Animals↗
Utility of the proximal jet width in the assessment of regurgitant and stenotic orifices--effect of low velocity filter and comparison to actual vena contracta width: an in vitro and numerical study.
AIM: The colour Doppler proximal jet width (CDPJW) has been shown to be directly related to the severity of regurgitant and stenotic valve lesions. It is generally assumed that the CDPJW is equivalent to the vena contracta width (VCW). The purpose of this numerical and in vitro study was to evaluate how changing low velocity filter (LVF) settings on colour Doppler imaging devices may affect the CDPJW and its estimate of the VCW. METHODS: Computational fluid dynamic software was used to create models of round orifices (0.785, 1.13, 1.76, 3.14 cm2) at set flow rates (0.37-25 1/min). In vitro experiments were performed with round orifices (0.2, 0.95 and 1.76 cm2) with set flow rates (1.8-3.6 1/min). Laser flow visualization was used to obtain gold standard vena contracta widths for comparison to CDPJW for various LVF settings (4-24 cm/s). RESULTS: With the LVF set 'too low', overestimation errors occur. In contrast, with the LVF set 'too high', underestimation errors occur. Optimal LVF settings are required to avoid over- and underestimation errors of up to 280%. SUMMARY: The VCW is related to regurgitant or stenotic lesion severity, and the CDPJW is an approximation of the VCW. The CDPJW closely resembles the actual VCW only at optimally chosen LVF settings. LVF settings can have a significant impact on the accuracy of the CDPJW. Inter mediate filter settings remove unnecessary background noise while maintaining actual flow regions, thereby providing the best agreement between the CDPJW and the VCW. If treatment decisions are to be based on these measurements, understanding such dependencies becomes quite important.
Blood Flow Velocity↗
Sensitivity of the Doppler rate of pressure rise to changes in the inotropic state: an experimental comparison with invasively obtained dP/dt.
AIMS: To investigate the sensitivity of the rate of pressure rise obtained by Doppler to changes in the inotropic state by comparing it to simultaneous invasive measurements of dP/dt under different conditions of contractility. METHODS AND RESULTS: Mitral regurgitation was provoked in five pigs, and simultaneous measurements of dP/dt and the Doppler-estimated rate of pressure rise were made with a micro-manometer and with continuous-wave Doppler. Changes in the inotropic state were induced by drug infusion and by ischaemia. One hundred and twenty-seven simultaneous measurements were made with a correlation coefficient between the Doppler-estimated rate of pressure rise and dP/dt of 0.85 (P<0.001). Sensitivity to inotropic changes was estimated as the percentage change of each parameter in each condition of contractility, and showed that the Doppler-estimated rate of pressure rise had better sensitivity than dP/dt. CONCLUSION: The sensitivity of Doppler-estimated rate of pressure rise to changes in the inotropic state is greater than that of dP/dt. The correlation between the rate of pressure rise obtained by Doppler and dP/dt is maintained even in extreme conditions of contractility. Therefore, the rate of pressure rise can be considered a good parameter to assess linear changes of contractility.
Animals↗
Effect of aortic compliance on Doppler diastolic flow pattern in coarctation of the aorta.
The spectral Doppler pattern at the site of an aortic coarctation (CoA) generally displays increased maximal velocity (Vmax) during systole with a slow velocity decay, resulting in the characteristic "sawtooth" pattern. If there is rapid velocity decay, the obstruction is often judged to be mild. The purpose of this study was to investigate if velocity decay is affected by proximal aortic compliance (C(p)). The relation between the velocity decay measured from the Doppler pattern and C(p) was studied with the use of an in vitro pulsatile flow model. The time (tau) between Vmax and 33% Vmax was the measure of velocity decay. The C(p) was varied from 0.7 to 2.6 mL/mm Hg for each of 4 levels of CoA severity. The various obstructions produced a Vmax range of 2.7 to 5.5 m/s. There was a positive linear relation between tau and C(p) (r(2) = 0.76). For a low C(p) (compliance = 0.7 mL/mm Hg), velocity decay was rapid (tau = 0.2 to 0.3 seconds) with no diastolic gradient. For equivalent obstructions, a high C(p) (2.6 mL/mm Hg) produced a persistent diastolic gradient and slow velocity decay (tau = 0.5 to 0.6 seconds). The Doppler pattern across a CoA is affected by C(p). Therefore, the absence of a sawtooth pattern should not exclude the diagnosis of significant CoA obstruction.
Aorta↗
Cardiac afferents and neurohormonal activation in congestive heart failure.
Cardiac chambers have afferent connections to the brainstem and to the spinal cord. Vagal afferents mediate depressor responses and become activated by volume expansion, increased myocardial contractility and atrial natriuretic factor. Sympathetic afferents, on the contrary, are activated by metabolic mediators, myocardial ischemia and cardiac enlargement. These opposite behaviors may lead to activation or suppression of the sympathetic nervous system and of the renin-angiotensin-aldosterone system. As cardiac diseases progress, the heart dilates, plasma norepinephrine increases, atrial natriuretic factor is released and the renin-angiotensin-aldosterone system is suppressed to maintain water and sodium excretion. This dissociation of the neurohormonal profile of cardiac patients, may be explained by coactivation of sympathetic afferents, by cardiac dilatation, and of vagal afferents by atrial natriuretic factor. In more advanced stages, atrial natriuretic factor suppression of the renin-angiotensin-aldosterone system is overridden by overt sympathetic activation and sodium and water retention ensues. Digitalis, angiotensin-converting enzyme inhibitors and beta-blockers selectively decrease cardiac adrenergic drive. A common mechanism of action, to all three groups of drugs, would be attenuation of sympathetic afferents and partial normalization of vagal afferents. Consequently, heart size and cardiac afferents emerge as the key factors to understand the pathophysiology and treatment of the syndrome of congestive heart failure.
Afferent Pathways↗
Tachycardia: an autosomal, monogenic, biallelic, recessive trait.
A brief commentary of the genetics of blood pressure is presented. The importance of the mechanisms of blood pressure regulation, among which heart rate is relevant, is emphasized. The analysis of a small population to test the quantitative model of the trait heart rate, considered as a metric character, is presented. The analysis of heart rate fitting to a qualitative model of inheritance is carried out. The results displayed might support the hypothesis that tachycardia could be an autosomal, monogenic, biallelic recessive trait.
Alleles↗
Impaired compliance of the intracranial vessels in complicated childhood migraine. Demonstration by transcranial Doppler-sonography--a vascular model approach.
AIM: To increase the diagnostic value of Doppler sonography a uniform lumped parameter model of pulsatile blood flow in human arteries was derived from the serial and/or parallel connection of parallel oscillatory circuits. METHODS AND PATIENTS: In this setting the current (I) in the ohmic resistor (R) represents the volume flow in the vessel, the voltage (V) the blood pressure. The vessel compliance corresponds to the capacitance (C), the inductivity (L) represents the accelerated mass. The flow velocity (v) which is assessable non-invasively is given by v = I/A, where A is the cross-sectional area of the vessel. Mathematically, the model represents a two-dimensional Fourier-series of the input signal by the segments. Thus it seems to be evident that almost all forms of flow and pressure pulses can be generated. The equations of the model were solved numerically. Doppler-sonographically measured flow velocities in the ascending aorta were used as input signal. The parameters for A, R, L and C were optimized to gain the measured flow spectrum in the target vessel as model response. RESULTS: Application in 14 children (aged 3-16 yrs.) with complicated migraine during a symptom-free interval revealed a significantly lowered cross sectional area of the middle cerebral artery on the side of the higher flow velocities. For both middle and posterior cerebral arteries, a decreased vessel compliance (C) (mean -30%) was found, whereas values for R and L were within normal limits. CONCLUSION: These findings suggest an altered vascular compliance even between attacks. This may contribute to the pathogenesis of migraine through increased cerebral impedance resulting in impaired cerebral perfusion.
Adolescent↗
Comparison of transit-time and Doppler ultrasound methods for measurement of flow in aortocoronary bypass grafts during cardiac surgery.
To evaluate the accuracy of flow measurements in aortocoronary bypass grafts with the ultrasound transit-time method, an in vitro and in vivo comparison was carried out. The in vitro comparison with evaluation against both true flow and the ultrasound Doppler method, was carried out with a fresh saphenous vein mounted in a pulsatile flow rig. The two flow probes were placed on the graft 4-5 cm apart to avoid acoustic interference, and blood was pumped through the system at different flow rates. The comparison between the methods showed excellent agreement with a linear correlation coefficient of 0.996, and a mean error of -2.9 ml/min with limits of agreement +/- 13.1 ml/min (+/- 2 SD = 95% of measured differences between these limits). However, against true flow, both methods overestimated flow slightly with mean error 4.4 and 7.3 ml/min for the transit-time and Doppler, respectively. Both methods showed excellent correlation with true flow (correlation coefficient 0.998 for the transit-time and 0.997 for the Doppler method). The in vivo accuracy was evaluated by comparison of the two methods in 9 patients. The two probes were placed on the same saphenous vein grafts 4-5 cm apart, and a total of 34 measurements in 17 grafts were carried out including measurements at baseline and after papaverine injection. The correlation coefficient was 0.990 and linear regression analysis gave the equation: Transit-time flow = 1.00 x Doppler flow + 1.3. In terms of flow, the mean error was 1.5 ml with limits of agreement +/- 17.2 ml.(ABSTRACT TRUNCATED AT 250 WORDS)
Blood Flow Velocity↗
[Experimental studies on magnetic resonance tomographic and magnetic resonance angiographic imaging of poststenotic flow patterns].
A phantom has been developed, which permits the perfusion of defined stenosis with variable flow rates. The post-stenotic flow patterns were imaged by means of conventional MRI sequences and special angiographic sequences using a 1.5 total body scanner (Gyroscan S15, Philips). For conventional MRI images, spin-echo-sequences (TR 500 ms, TE 27 ms with reconstruction of the magnitude and phase images) and gradient-echo-sequences (TR 60 ms, TE 27 ms, flip angle 15 degrees with reconstruction of the magnitude images) were used. For MR angiography 2D inflow (multiple-single-slice-technique, TR 40 ms, TE 14 ms, flip angle 60 degrees) and flow-adjusted-gradient-sequences (TR 24 ms, TE 10 ms, flip angle 60 degrees) were performed. In addition to an analysis of the appearances, the length of the changed signal intensity beyond the stenosis was evaluated. For constant flow rates it increases with the severity of the stenosis. For a constant stenosis the length of the post-stenotic signal change alters with flow rate. The changes also depend on the direction of flow and the gradients of the chosen sequences. The comparison of magnitude and phase images allows the discrimination between turbulent and non-turbulent flow. It therefore appears possible to estimate the severity of arteriosclerotic occlusive disease by means of MRI in the future.
Blood Flow Velocity↗
[The rapid magnetic resonance tomography measurement of the contrast medium dilution kinetics (gadolinium-DTPA) in a circulatory phantom].
We studied first-pass MRI contrast dilution to compute flow and volume of distribution in a realistic flow phantom. Pulsatile flow was provided by a one-chamber artificial heart. Physiological stroke volume, rate, pressure, and flow were adjustable. An elastic tube with dimensions similar to that of the human aorta was imaged at a rate of 2.4 Hz. After contrast injection, an initial increase in signal intensity was followed by a decrease. Signal-intensity time plots demonstrated slightly skewed curves as expected from dispersion theory. After calibration at different gadolinium-DTPA concentrations, signal intensities were converted into true gadolinium concentrations, and flow was calculated from the concentration-time curves. Flow was varied between 2.5 and 10.0 l/min and a significant correlation was found between the MRI estimate and true flow. Volume of distribution between injection and detection site was reliably estimated. This study demonstrates rapid 2-D imaging of a paramagnetic contrast bolus in a realistic flow phantom. Reliable estimates of flow and volume are obtained.
Contrast Media↗
Interpretation of heart rate variability via detrended fluctuation analysis and alphabeta filter.
Detrended fluctuation analysis (DFA), suitable for the analysis of nonstationary time series, has confirmed the existence of persistent long-range correlations in healthy heart rate variability data. In this paper, we present the incorporation of the alphabeta filter to DFA to determine patterns in the power-law behavior that can be found in these correlations. Well-known simulated scenarios and real data involving normal and pathological circumstances were used to evaluate this process. The results presented here suggest the existence of evolving patterns, not always following a uniform power-law behavior, that cannot be described by scaling exponents estimated using a linear procedure over two predefined ranges. Instead, the power law is observed to have a continuous variation with segment length. We also show that the study of these patterns, avoiding initial assumptions about the nature of the data, may confer advantages to DFA by revealing more clearly abnormal physiological conditions detected in congestive heart failure patients related to the existence of dominant characteristic scales.
Adult↗
Efficient simulation of three-dimensional anisotropic cardiac tissue using an adaptive mesh refinement method.
A recently developed space-time adaptive mesh refinement algorithm (AMRA) for simulating isotropic one- and two-dimensional excitable media is generalized to simulate three-dimensional anisotropic media. The accuracy and efficiency of the algorithm is investigated for anisotropic and inhomogeneous 2D and 3D domains using the Luo-Rudy 1 (LR1) and FitzHugh-Nagumo models. For a propagating wave in a 3D slab of tissue with LR1 membrane kinetics and rotational anisotropy comparable to that found in the human heart, factors of 50 and 30 are found, respectively, for the speedup and for the savings in memory compared to an algorithm using a uniform space-time mesh at the finest resolution of the AMRA method. For anisotropic 2D and 3D media, we find no reduction in accuracy compared to a uniform space-time mesh. These results suggest that the AMRA will be able to simulate the 3D electrical dynamics of canine ventricles quantitatively for 1 s using 32 1-GHz Alpha processors in approximately 9 h.
Algorithms↗
What does measure the scaling exponent of the correlation sum in the case of human heart rate?
It is shown that in the case of human heart rate, the scaling behavior of the correlation sum (calculated by the Grassberger-Procaccia algorithm) is a result of the interplay of various factors: finite resolution of the apparatus (finite-size effects), a wide dynamic range of mean heart rate, the amplitude of short-time variability being a decreasing function of the mean heart rate. This is done via constructing a simple model of heart rhythm: a signal with functionally modulated Gaussian noise. This model reproduces the scaling behavior of the correlation sum of real medical data. The value of the scaling exponent depends on all the above-mentioned factors, and is a certain measure of short-time variability of the signal.
Algorithms↗
The role of M cells and the long QT syndrome in cardiac arrhythmias: simulation studies of reentrant excitations using a detailed electrophysiological model.
In this numerical study, we investigate the role of intrinsic heterogeneities of cardiac tissue due to M cells in the generation and maintenance of reentrant excitations using the detailed Luo-Rudy dynamic model. This model has been extended to include a description of the long QT 3 syndrome, and is studied in both one dimension, corresponding to a cable traversing the ventricular wall, and two dimensions, representing a transmural slice. We focus on two possible mechanisms for the generation of reentrant events. We first investigate if early-after-depolarizations occurring in M cells can initiate reentry. We find that, even for large values of the long QT strength, the electrotonic coupling between neighboring cells prevents early-after-depolarizations from creating a reentry. We then study whether M cell domains, with their slow repolarization, can function as wave blocks for premature stimuli. We find that the inclusion of an M cell domain can result in some cases in reentrant excitations and we determine the lifetime of the reentry as a function of the size and geometry of the domain and of the strength of the long QT syndrome.
Action Potentials↗
Modeling wave propagation in realistic heart geometries using the phase-field method.
We present a novel algorithm for modeling electrical wave propagation in anatomical models of the heart. The algorithm uses a phase-field approach that represents the boundaries between the heart muscle and the surrounding medium as a spatially diffuse interface of finite thickness. The chief advantage of this method is to automatically handle the boundary conditions of the voltage in complex geometries without the need to track the location of these boundaries explicitly. The algorithm is shown to converge accurately in nontrivial test geometries with no-flux (zero normal current) boundary conditions as the width of the diffuse interface becomes small compared to the width of the cardiac action potential wavefront. Moreover, the method is illustrated for anatomically realistic models of isolated rabbit and canine ventricles as well as human atria.
Action Potentials↗
Restitution in mapping models with an arbitrary amount of memory.
Restitution, the characteristic shortening of action potential duration (APD) with increased heart rate, has been studied extensively because of its purported link to the onset of fibrillation. Restitution is often represented in the form of mapping models where APD is a function of previous diastolic intervals (DIs) and/or APDs, A(n+1)=F(D(n),A(n),D(n-1),A(n-1),...), where A(n+1) is the APD following a DI given by D(n). The number of variables previous to D(n) determines the degree of memory in the mapping model. Recent experiments have shown that mapping models should contain at least three variables (D(n),A(n),D(n-1)) to reproduce a restitution portrait (RP) that is qualitatively similar to that seen experimentally, where the RP shows three different types of restitution curves (RCs) [dynamic, S1-S2, and constant-basic cycle length (BCL)] simultaneously. However, an interpretation of the different RCs has only been presented in detail for mapping models of one and two variables. Here we present an analysis of the different RCs in the RP for mapping models with an arbitrary amount of memory. We determine the number of variables necessary to represent the different RCs in the RP. We also present a graphical visualization of these RCs. Our analysis reveals that the dynamic and S1-S2 RCs reside on two-dimensional surfaces, and therefore provide limited information for mapping models with more than two variables. However, constant-BCL restitution is a feature of the RP that depends on higher dimensions and can possibly be used to determine a lower bound on the dimensionality of cardiac dynamics.
Action Potentials↗