Showing papers on "Cardiac cycle published in 2002"

Myocardial elastography—a feasibility study in vivo

Abstract: Early detection of cardiovascular diseases has been a very active research area in the medical imaging field. Assessment of the local and global mechanical functions is one of the major goals of accurate diagnosis. In this study, we investigated the feasibility of elastography for estimation and imaging of the local cardiac muscle displacement and strain in a human heart in vivo. In its noninvasive applications, elastography has been typically used to determine local tissue strain through the use of externally applied compression. For our study, we utilized the cardiac muscle motion during a cardiac cycle as the mechanical stimulus, and acquired successive radiofrequency (RF) data frames of the septal and posterior walls over a few cardiac cycles in parasternal and apical views, respectively. High-quality cine-loop elastograms were obtained due to high frame rates and the resulting low decorrelation noise. Furthermore, the strain contrast was higher in the parasternal case, when only the posterior wall was imaged, and strain estimation was more robust in the apical view. High repeatability of the results was observed through elastographic measurements over several cardiac cycles. Finally, an M-mode version of elastography was used to follow part of the interventricular septum or the posterior wall over the course of two cardiac cycles. Not only do these preliminary results show that elastography is feasible in cardiac applications in vivo, but also that it can provide new information regarding cardiac motion and mechanical function. Future prospects include assessment of the role of elastography in detection of ischemia and infarction.

Multi-electrode apparatus and method for treatment of congestive heart failure

Abstract: An apparatus and method for treatment of congestive heart failure from the right side of the heart. An implantable cardiac stimulation system with a multi-electrode lead having three or more selectable electrodes, together with apparatus for identifying an optimal subset of electrodes, apparatus for shaping a propagating wave front, and apparatus for modifying the intrinsic ventricular cardiac activation sequence, or generating simultaneous or near simultaneous pacing pulses to the septum or right ventricular outflow tract during ventricular systole in order to improve left ventricular cardiac efficiency and reduce mitral regurgitation in patients with dilated cardiomyopathy. A three dimensional map of electrode placement may be calculated. A sub set of the available electrodes in the right side of the heart is selected for stimulation such that septal motion during systole is reduced or the mitral valve area is stiffened to reduce mitral regurgitation.

Anatomy of the mitral valve

Abstract: According to Walmsley,1 it was Andreas Vesalius who suggested the picturesque term “mitral” to describe the left atrioventricular valve owing to its resemblance to a plan view of the bishop’s mitre. Guarding the inlet to the left ventricle, the mitral valve prevents backflow to the left atrium during ventricular systole. In its open state, the valvar leaflets are like a funnel extending from the hinge line at the atrioventricular junction to the free margins. Tendinous cords attach the leaflets to two closely arranged groups of papillary muscles. The interchordal spaces serve as important pathways for blood flow. As emphasised by Perloff and Roberts,2 the mitral valve requires all its components, together with the adjacent atrial and ventricular musculature, in order to work properly. This review examines the component parts to provide the background for a better understanding of the mitral valve as seen in cross sectional anatomy and highlights pertinent features of its anomalies. The valvar complex comprises the annulus, the leaflets, the tendinous cords, and the papillary muscles. Also important for its functioning is the left atrial musculature inserting to the leaflets and the myocardium to which the papillary muscles are inserted. The valve is obliquely located in the heart and has a close relation to the aortic valve (fig 1A). Unlike the tricuspid valve which is separated by muscle from its counterpart, the pulmonary valve, the mitral valve is immediately adjacent to the aortic valve (fig 1B). Figure 1 (A) View of the base of the heart in anatomical orientation shows the spatial relations of the four cardiac valves. The left heart valves are close together whereas the right heart valves are separated by myocardium. Dotted line marks the limit of atrial myocardium around the mitral orifice. (B) This dissection of the heart viewed from the anterior aspect …

A four-dimensional study of the aortic root dynamics.

Abstract: Objective Although aortic root expansion has been well studied, its deformation and physiologic relevance remain controversial. Three-dimensional (3-D) sonomicrometry (200Hz) has made time-related 4-D study possible. Methods Fifteen sonomicrometric crystals were implanted into the aortic root of eight sheep at each base (three), commissures (three), sinuses of Valsalva (three), sinotubular junction (three), and ascending aorta (three). In this acute, open-chest model, the aortic root geometric deformations were time related to left ventricular and aortic pressures. Results During the cardiac cycle, aortic root volume increased by mean+/-1 standard error of the mean (SEM) 33.7+/-2.7%, with 36.7+/-3.3% occurring prior to ejection. Expansion started during isovolumic contraction at the base and commissures followed (after a delay) by the sinotubular junction. At the same time, ascending aorta area decreased (-2.6+/-0.4%). During the first third of ejection, the aortic root reached maximal expansion followed by a slow, then late rapid decrease in volume until mid-diastole. During end-diastole, the aortic root volume re-expanded by 11.3+/-2.4%, but with different dynamics at each area level. Although the base and commissural areas re-expanded, the sinotubular junction and ascending aorta areas kept decreasing. At end-diastole, the aortic root had a truncated cone shape (base area>commissures area by 51.6+/-2.0%). During systole, the root became more cylindrical (base area>commissures area by 39.2+/-2.5%) because most of the significant changes occurred at commissural level (63.7+/-3.6%). Conclusion Aortic root expansion follows a precise chronology during systole and becomes more cylindrical - probably to maximize ejection. These findings might stimulate a more physiologic approach to aortic valve and aortic root surgical procedures.

Cardiac diffusion MRI without motion effects.

Abstract: We present a method for diffusion tensor MRI in the beating heart that is insensitive to cardiac motion and strain. Using a stimulated echo pulse sequence with two electrocardiogram (ECG) triggers, diffusion-encoding bipolar gradient pulses are applied at identical phases in consecutive cardiac cycles. In this experiment, diffusion is encoded at a single phase in the cardiac cycle of less than 30 ms in duration. This encoding produces no phase shifts for periodic motion and is independent of intervening strains. Studies in a gel phantom with cyclic deformation confirm that by using this sequence we can map the diffusion tensor free of effects of cyclic motion. In normal human subjects, myocardial diffusion eigenvalues measured with the present method showed no significant change between acquisitions encoded at maximum contractile velocity (peak) vs. at myocardial standstill (end-systole), demonstrating motion independence of in vivo diffusion measurements. Diffusion tensor images acquired with the present method agree with registered data acquired with a previous cardiac diffusion MRI method that was shown to be valid in the normal heart, strongly supporting the validity of MRI diffusion measurement in the beating heart. Myocardial sheet and fiber dynamics measured during systole showed that normal human myocardial sheet orientations tilt toward the radial during systole, and fiber orientations tilt toward the longitudinal, in qualitative agreement with previous invasive studies in canines. These results demonstrate the technique's ability to measure myocardial diffusion accurately at any point in the cardiac cycle free of measurable motion effect, as if the heart were frozen at the point of acquisition.

Aortic Valve Disease

Abstract: It is important to diagnose and treat diseases of the aortic valve. Untreated aortic valve disease can eventually result in heart failure, severe infection, and even sudden death. Some patients present with severe symptoms, whereas others have few, if any symptoms. The diagnosis may be made on a routine physical examination. Regular medical follow-up, treatment to prevent infection of the valve (infective endocarditis), and optimal timing of surgery are necessary to avoid the severe consequences of improper function of the aortic valve. Cardiac valves are structures that are designed to work like one-way doors (Figures 1 and 2⇓). They let blood flow in from one chamber or vessel to another, and then close to prevent the blood from regurgitating backward. The aortic valve consists of 3 half-moon-shaped pocket-like flaps of delicate tissue, referred to as cusps. When the aortic valve is closed, the cusps are perfectly aligned and separate the large pumping chamber of the heart (the left ventricle) from the large artery (aorta) that supplies blood to the body. During the period when the left ventricle contracts and pumps the blood (systole), the aortic valve opens widely and blood flows freely from the left ventricle to the aorta. When the left ventricle then relaxes (diastole), the aortic valve closes completely so that the blood remains in the aorta. During diastole, blood flows into the left ventricle from the lungs through the left atrium across the mitral valve, thus refilling the ventricle for the next contraction. Figure 1. Schematic drawing of the heart during contraction of the heart (systole). A, In the normal individual, the left ventricle contracts and fully opens the thin …

Non-invasive method and device to monitor cardiac parameters

Abstract: A method of and a device for non-invasively measuring the hemodynamic state of a subject or a human patient involve steps and units of non-invasively measuring cardiac cycle period, electrical-mechanical interval, mean arterial pressure, and ejection interval and converting the measured electrical-mechanical interval, mean arterial pressure and ejection interval into the cardiac parameters such as Preload, Afterload and Contractility, which are the common cardiac parameters used by an anesthesiologist. The converted hemodynamic state of a patient is displayed on a screen as a three-dimensional vector with each of its three coordinates respectively representing Preload, Afterload and Contractility. Therefore, a medical practitioner looks at the screen and quickly obtains the important and necessary information.

Comparison of permanent left ventricular and biventricular pacing in patients with heart failure and chronic atrial fibrillation: prospective haemodynamic study

Abstract: Background: Left ventricular pacing (LVP) and biventricular pacing (BVP) have been proposed as treatments for patients with advanced heart failure complicated by discoordinate contraction due to intraventricular conduction delay. For patients in sinus rhythm, BVP works in part by modulating the electronic atrial-ventricular time delay and thus optimizing contractile synchrony, the contribution of atrial systole, and reducing mitral regurgitation. However, little is known of the mechanisms of BVP in heart failure patients with drug-resistant chronic atrial fibrillation.

The human nociceptive flexion reflex threshold is higher during systole than diastole.

Abstract: A baroreflex mechanism may explain hypertensive hypoalgesia. At rest, arterial baroreceptors are stimulated during the systolic upstroke of the pressure pulse wave. This study examined the effects of naturally occurring variations in baroreceptor activity during the cardiac cycle on an objective measure of pain, the nociceptive flexion reflex (NFR). Two interleaved up-down staircase procedures determined separate NFR thresholds during systole and diastole in 36 healthy, normotensive young adults. On odd-numbered trials, the sural nerve was stimulated electrocutaneously at R + 300 ms whereas on even-numbered trials, stimulation was delivered at R + 600 ms. The NFR threshold was higher at R + 300 ms than R + 600 ms. In contrast, stimulus intensity ratings did not differ between R + 300 ms and R + 600 ms. Stimulation of baroreceptors by natural increases in blood pressure during the systolic phase of the cardiac cycle was associated with dampened nociception.

Defective cardiac ion channels: from mutations to clinical syndromes.

Abstract: Normal cardiac excitation and relaxation involves a delicate balance of complex dynamic interactions between ionic currents passing through a variety of membrane channels and the cellular environment. Genetic defects, polymorphisms, therapeutic intervention or structural abnormalities can disrupt this balance and underlie severe arrhythmogenic phenotypes that lead to sudden cardiac death. Inheritable gene defects give rise to phenotypic variation and an unpredictable manifestation of syndromes, ranging from silent gene carriers to profoundly symptomatic individuals, even within single families (1–7). As such, realizing the relationship between genetic mutations and clinical syndromes is becoming increasingly complex. In this issue of the JCI, Grant and colleagues (3) investigate the manifestations of phenotypically opposite and overlapping cardiac arrhythmogenic syndromes that surprisingly stem from the same mutation (1–4). Cardiac excitation reflects membrane depolarization of cardiac myocytes, primarily due to the activation of fast voltage-dependent Na+ channels that underlie the action potential upstroke. Activation is followed by a long depolarized plateau phase that permits Ca2+-induced Ca2+ release from the sarcoplasmic reticulum, binding of Ca2+ to contractile proteins on the sarcomeres, and coordinated contraction. Repolarization follows due to the time- and voltage-dependent activation of repolarizing potassium currents. Relaxation of contraction is coupled to the electrical repolarization phase, which allows filling of the ventricles prior to the next excitation. Each of these electrical processes can be detected on the body surface electrocardiogram (ECG) as a signal average of the temporal and spatial gradients generated during each phase (8–11) (Figure ​(Figure1a).1a). Electrical excitation gradients in the atria (atrial depolarization) manifest on the ECG as P waves, while gradients of ventricular depolarization are seen as the QRS complex. Gradients in ventricular repolarization are reflected in the T wave (Figure ​(Figure11). Figure 1 Electrical gradients in the myocardium can be detected on the body surface ECG. (a) An illustrative example of a single cardiac cycle detected as spatial and temporal electrical gradients on the ECG. The P wave is generated by the spread of excitation ... A recently described example of a multi-syndrome genetic defect in the SCN5A gene, encoding the cardiac Na+ channel (Figure ​(Figure2),2), is the insertion of an aspartic acid, 1795insD, in the C-terminus of the cardiac Na+ channel that underlies both Brugada (BrS) and Long-QT (LQTs) cardiac arrhythmic syndromes (1, 2). Figure 2 The predicted transmembrane topology of domains I–IV of the cardiac Na+ channel α subunit encoded by SCN5A showing the location and nature of the mutations inducing LQTs, BrS, and isolated cardiac conduction disease. Grant and colleagues investigate an even more complex mutation (3). The deletion of lysine, ΔK1500, in the III–IV linker of SCN5A (Figure ​(Figure2)2) is associated with BrS, LQTs, and isolated cardiac conduction disease (ICCD). LQTs is typically associated with a gain of Na+ channel function that stems from mutation induced destabilization of channel inactivation, leading to a persistent inward Na+ current (INa) during the action potential (AP) plateau and prolonged repolarization (12, 13). Paradoxically, BrS and ICCD are linked to a loss of Na+ channel function and a resulting reduction in macroscopic current (3, 6, 7, 14, 15). How can multiple and seemingly contradictory arrhythmic syndromes arise from a mutation at a single locus?

Cardiac rhythm management system and method using time between mitral valve closure and aortic ejection

Abstract: This document discusses cardiac rhythm management systems and methods using the MVC-to-AE time between mitral valve closure (“MVC”) and aortic ejection (“AE”) of the same heart contraction, sometimes referred to as the isovolumic contraction time (“ICVT”). In one example, the MVC-to-AE time is used for predicting which patients will respond to cardiac resynchronization therapy (CRT), or other therapy. In another example, the MVC-to-AE time is used as a wellness indicator. In a further example, the MVC-to-AE time is used to select or control a therapy or therapy parameter. In one example, the MVC and AE are obtained using an accelerometer signal, however, plethysmography, tonometry, or other techniques may alternatively be used.

Dynamic three-dimensional color Doppler ultrasound of human fetal intracardiac flow.

Abstract: Objectives To develop dynamic three-dimensional ultrasound techniques for prenatal imaging of the intracardiovascular flow as well as the cardiovascular structure to address difficulties in assessing the spatially complex hemodynamics and morphology of the fetal heart. Methods Gray-scale and color (velocity) Doppler echocardiography were performed on 12 fetuses to provide serial anatomical and rheological tomograms which were spatially registered in three dimensions. Using a second ultrasound machine simultaneously, spectral Doppler ultrasound was performed to record umbilical arterial waveforms, thus providing the temporal (fourth) dimension in terms of the cardiac cycle and facilitating removal of motion artifacts. Results Acquisitions were successful in eight of 15 attempts. Imaging of the flow of blood in four dimensions was achieved in six of the eight datasets. In one case with complex cardiac malformations, three-dimensional reconstructions at systole and diastole offered dynamic diagnostic views not appreciated on the cross-sectional images. Conclusions Our novel technique has made possible the prenatal visualization of the spatial distribution and true direction of intracardiac flow of blood in four dimensions in the absence of motion artifacts. The technique suggests that diagnosis of cardiac malformations can be made on the basis of morphological and hemodynamic changes throughout the entire cardiac cycle, offering unique and significant information complementary to conventional techniques. Further work to integrate the several non-purpose-built machines into a single system will improve the rate of acquisition of data, and may provide a new means of imaging and modeling structure and hemodynamics, not only for the fetal heart but for many other moving body parts.

Magnetic resonance imaging analysis of cardiac cycle events in diabetic rats: the effect of angiotensin‐converting enzyme inhibition

Abstract: Non-invasive magnetic resonance imaging (MRI) was used to characterize changes in left and right ventricular cardiac cycles following induction of experimental, streptozotocin (STZ)-induced, diabetes in male Wistar rats at different ages. The effects of the angiotensin-converting enzyme (ACE) inhibitor captopril upon such chronic physiological changes were then evaluated, also for the first time. Diabetes was induced at the age of 7 weeks in two experimental groups, of which one group was subsequently maintained on captopril (2 g l−1)-containing drinking water, and at 10 and 13 weeks in two further groups. The fifth group provided age-matched controls. All groups (each n = 4 animals) were scanned consistently at 16 weeks, in parallel with timings used in earlier studies that employed this experimental model. Cine magnetic resonance (MR) image acquisition provided transverse sections through both ventricles at twelve time points covering systole and most of diastole. These yielded reconstructions of cardiac anatomy used to derive critical functional indices and their dependence upon time following the triggering electrocardiographic R waves. The left and right ventricular end-diastolic (EDV), end-systolic (ESV) and stroke volumes (SV), and ejection fractions (EF) calculated from each, control and experimental, group showed matching values. This confirmed a necessary condition requiring balanced right and left ventricular outputs and further suggested that STZ-induced diabetes produced physiological changes in both ventricles. Absolute left and right ventricular SVs were significantly altered in all diabetic animals; EDVs and EFs significantly altered in animals diabetic from 7 and 10 but not 13 weeks. When normalized to body weight, left and right ventricular SVs had significantly altered in animals diabetic from 7 and 10 weeks but not 13 weeks. Normalized left ventricular EDVs were also significantly altered in animals diabetic from 7 and 10 weeks. However, normalized right ventricular EDVs were significantly altered only in animals made diabetic from 7 weeks. Diabetic hearts showed major kinetic changes in left and right ventricular contraction (ejection) and relaxation (filling). Both the initial rates of volume change (dV/dt) in both ventricles and the plots of dV/dt values through the cardiac cycle demonstrated more gradual developments of tension during systole and relaxation during diastole. Estimates of the derived left ventricular performance parameters of cardiac output, cardiac power output and stroke work in control animals were comparable with human values when normalized to both body (or cardiac) weight and heart rate. All deteriorated with diabetes. Comparisons of experimental groups diabetic from 7 weeks demonstrated that captopril treatment relieved the alterations in critical volumes, dependence of SV upon EDV, kinetics of systolic contraction and diastolic relaxation and in the derived indicators of ventricular performance. This study represents the first demonstration using non-invasive MRI of early, chronic changes in diastolic filling and systolic ejection in both the left and the right ventricles and of their amelioration by ACE inhibition following STZ-induction of diabetes in intact experimental animals.

Cardiac treatment apparatus

Abstract: A cardiac harness for treating congestive heart failure is disclosed The harness applies elastic, compressive reinforcement on the left ventricle to reduce deleterious wall tension and to resist shape change of the ventricle during the mechanical cardiac cycle Rather than imposing a dimension beyond which the heart cannot expand, the harness provides no hard limit over the range of diastolic expansion of the ventricle Instead, the harness follows the contour of the heart throughout diastole and continuously exerts gentle resistance to stretch Also disclosed is a method of delivering the cardiac harness to the heart minimally invasively

Non-invasive magnetic resonance imaging assessment of myocardial changes and the effects of angiotensin-converting enzyme inhibition in diabetic rats

Abstract: A non-invasive cine magnetic resonance imaging (MRI) technique was developed to allow, for the first time, detection and characterization of chronic changes in myocardial tissue volume and the effects upon these of treatment by the angiotensin-converting enzyme (ACE) inhibitor captopril in streptozotocin (STZ)-diabetic male Wistar rats. Animals that had been made diabetic at the ages of 7, 10 and 13 weeks and a captopril-treated group of animals made diabetic at the age of 7 weeks were scanned. The findings were compared with the results from age-matched controls. All animal groups (n = 4 animals in each) were consistently scanned at 16 weeks. Left and right ventricular myocardial volumes were reconstructed from complete data sets of left and right ventricular transverse sections which covered systole and most of diastole using twelve equally incremented time points through the cardiac cycle. The calculated volumes remained consistent through all twelve time points of the cardiac cycle in all five experimental groups and agreed with the corresponding post-mortem determinations. These gave consistent myocardial densities whose values could additionally be corroborated by previous reports, confirming the validity of the quantitative MRI results and analysis. The myocardial volumes were conserved in animals whose diabetes was induced at 13 weeks but were significantly increased relative to body weight in animals made diabetic at 7 and 10 weeks. Captopril treatment, which was started immediately after induction of diabetes, prevented the development of this relative hypertrophy in both the left and right ventricles. We have thus introduced and validated quantitative MRI methods in a demonstration, for the first time, of chronic myocardial changes in both the right and left ventricles of STZ-diabetic rats and their prevention by the ACE inhibitor captopril.

Phasic three-dimensional impedance imaging of cardiac activity.

Abstract: Electrical impedance images were made using the ACT 3 instrument, which applies currents simultaneously to 32 electrodes and measures the resulting voltages on those same electrodes. A reconstruction algorithm was written for a three-dimensional cylinder having electrodes in two or four layers, using current patterns that pass current among different planes of electrodes, as well as within each plane. We have previously reported useful vertical resolution by the use of added layers of electrodes. The aim of the present study was to demonstrate that physiologically useful information can be obtained by examining cephalo-caudal differences in three-dimensional images. Phasic changes throughout the cardiac cycle are seen to be markedly different at the heart compared to lung region, both above and beside it. We formed hydrogel electrodes each 3 cm tall and 7 cm wide and applied them to the thorax of an upright human subject in four horizontal rows; each row contained eight electrodes. During breath-holding, cardiac activity was seen in all layers. With systole, conductivity in the anterior of the lowest layers decreased, but not in the upper layer. In the upper layers, conductivity increased with systole in many regions. These observations are consistent with the opposite changes in blood volume of the heart and lungs and the locations of these organs. This paper demonstrates the feasibility of producing and displaying physiologically interpretable three-dimensional images of the chest in real time.

Assessment of synchrony relationships between the native left ventricle and the HeartMate left ventricular assist device.

Abstract: Background: It has been suggested that the cardiac cycle becomes synchronized with the LVAD. Synchronization between the left ventricle and the LVAD may be important for ventricular unloading and coronary flow. In this study, we assessed the synchrony between the cardiac and LVAD cycles. Methods: We studied 24 patients with HeartMate LVAD support. Native heart rate from an electrocardiogram and LVAD rate were measured at rest and peak exercise. Three patients underwent simultaneous invasive pressure measurement from the left ventricle and the aorta, and 3 patients underwent simultaneous recording of electrocardiogram and LVAD electrical signal. Results: Resting heart rate was significantly higher than LVAD rate (96 ± 17 vs 66 ± 15 beats [b]/min, p r = 0.25). Peak heart rate was significantly higher than LVAD rate (142 ± 16 vs 102 ± 14 b/min, p r = 0.31). Electrical signal recording confirmed the absence of cardiac–LVAD synchrony. Pressure measurements revealed a cyclical intraventricular pressure variation, determined by the relationship between the cardiac and LVAD cycles. Intraventricular pressure was lowest when left ventricular systole occurred during pump filling and highest when left ventricular systole occurred during pump ejection. Conclusions: The cardiac and LVAD cycles are not in synchrony at rest or at peak exercise. However, a cyclical variation in left ventricular pressure exists, dependent upon the phasic relationship of the cardiac–LVAD cycles, which significantly effects ventricular loading. Better understanding of this relationship may be important in developing assist devices for optimal left ventricular unloading and improvement of myocardial recovery.

Delayed depolarization of the cog-wheel valve and pulmonary-to-systemic shunting in alligators.

Abstract: SUMMARY Alligators and other crocodilians have a cog-wheel valve located within the subpulmonary conus, and active closure of this valve during each heart beat can markedly and phasically increase resistance in the pulmonary outflow tract. If this increased resistance causes right ventricular pressure to rise above that in the systemic circuit, right ventricular blood can flow into the left aorta and systemic circulation, an event known as pulmonary-to-systemic shunting. To understand better how this valve is controlled, anaesthetized American alligators ( Alligator mississippiensis ) were used to examine the relationships between depolarization of the right ventricle, depolarization/contraction of the cog-wheel valve muscle and the resultant right ventricular, pulmonary artery and systemic pressures. Depolarization swept across the right ventricle from the apex towards the base (near where the cog-wheel valve muscle is located) at a velocity of 91±23 cm s -1 (mean ± S.E.M., N =3). The cog-wheel valve electrocardiogram (ECG) (and thus contraction of the valve) trailed the right ventricular ECG by 248±28 ms ( N =3), which was equivalent to 6-35 % of a cardiac cycle. This long interval between right ventricular and valve depolarization suggests a nodal delay at the junction between the base of the right ventricle and the cog-wheel valve. The delay before valve closure determined when the abrupt secondary rise in right ventricular pressure occurred during systole and is likely to strongly influence the amount of blood entering the pulmonary artery and thus to directly control the degree of shunting. Left vagal stimulation (10-50 Hz) reduced the conduction delay between the right ventricle and cog-wheel valve by approximately 20 % and reduced the integrated cog-wheel ECG by 10-20 %. Direct application of acetylcholine (1-2 mg) also reduced the integrated cog-wheel ECG by 10-100 %; however, its effect on the conduction delay was highly variable (-40 to +60 %). When the cog-wheel valve muscle was killed by the application of ethanol, the cog-wheel ECG was absent, right ventricular and pulmonary pressures remained low and tracked one another, the secondary rise in right ventricular pressure was abolished and shunting did not occur. This study provides additional, direct evidence that phasic contraction of the cog-wheel valve muscle controls shunting, that nervous and cholinergic stimulation can alter the delay and strength of valve depolarization and that this can affect the propensity to shunt.

Physiological mechanisms influencing cardiac repolarization and QT interval.

Abstract: Cardiac repolarization primarily results from activation of outward currents carried by potassium ionswhich are heterogenously distributed throughout the myocardial layers which can lead to changes in various phasesof cardiac cycle. QT interval is a reflector of cardiac repolarization and can be influenced by heart rate,autonomic nervous system, gender and time of the day. T wave alternans (TWA), specifically thediscordant pattern is a feature of abnormal cardiac repolarization that can be used as a predictor of ventriculararrhythmias.

Study of the myocardial contraction and relaxation velocities through Doppler tissue imaging echocardiography: A new alternative in the assessment of the segmental ventricular function.

Abstract: Objective - Doppler tissue imaging (DTI) enables the study of the velocity of contraction and relaxation of myocardial segments. We established standards for the peak velocity of the different myocardial segments of the left ventricle in systole and diastole, and correlated them with the electrocardiogram. Methods - We studied 35 healthy individuals (27 were male) with ages ranging from 12 to 59 years (32.9 ± 10.6). Systolic and diastolic peak velocities were assessed by Doppler tissue imaging in 12 segments of the left ventricle, establishing their mean values and the temporal correlation with the cardiac cycle. Results - The means (and standard deviation) of the peak velocities in the basal, medial, and apical regions (of the septal, anterior, lateral, and posterior left ventricle walls) were respectively, in cm/s, 7.35(1.64), 5.26(1.88), and 3.33(1.58) in systole and 10.56(2.34), 7.92(2.37), and 3.98(1.64) in diastole. The mean time in which systolic peak velocity was recorded was 131.59ms (±19.12ms), and diastolic was 459.18ms (±18.13ms) based on the peak of the R wave of the electrocardiogram. Conclusion - In healthy individuals, maximum left ventricle segment velocities decreased from the bases to the ventricular apex, with certain proportionality between contraction and relaxation (P<0.05). The use of Doppler tissue imaging may be very helpful in detecting early alterations in ventricular contraction and relaxation.

Atrial arrhythmia detection for an active implantable medical device

Abstract: Apparatus for detecting atrial arrhythmias in the treatment of disorders of the heartbeat rate in active implantable medical devices of the pacemaker, cardioverter, defribillator and/or multisite type. This device includes conventional systems, circuits and control algorithm for detecting spontaneous and stimulated ventricular events (R, V) and atrial events (P, A), indicating the delivery of a ventricular and/or atrial event, and inhibiting the detection of atrial events after detection of a ventricular event throughout a post-ventricular atrial absolute refractory period (PVAARP). Detecting atrial events includes protecting against the detection of atrial signals that do not correspond to an atrial event that has actually occurred, in particular protecting against atrial detection of far-field signals caused by a ventricular event. The protection operates by a dynamic adjustment of the sensitivity of detection by temporarily raising (t1, t2) a detection threshold (ΔS1, ΔS2) after detection of a ventricular event without detection of a preceding (spontaneous or stimulated) atrial event during the same cardiac cycle.

The time relationships of the constituent components of the human electrocardiogram

Abstract: This paper reports the results of an examination of the timing relationships of the principal constituent components of the human electrocardiogram (ECG). ECG recordings were obtained from 21 healthy subjects, 10 male and 11 female aged between 13 and 65 years, over a wide range of heart rates extending from 46 to 184 beats per minute (bpm). A wavelet transform method based on the Mexican Hat wavelet was then used to precisely locate the positions of the onset, peak, termination and the duration of individual components in the ECG. Component times were then classified according to the heart rate associated with the cardiac cycle to which the component belonged. Second-order equations in the square root of the cardiac cycle time, TR-R of theform AT(1/2)R-R+BT R-R+C were fitted to the data obtained for each component to characterize its timing variation. These equations may be used to synthesize an ECG signal having a profile that varies with heart rate in a manner which reflects the in vivo variation.

The role of short- and long-axis function in determining late diastolic left ventricular filling in patients with hypertension: Assessment by pulsed Doppler tissue imaging

Abstract: Left ventricular (LV) wall motion velocity during atrial systole is mediated by both transmitral flow and LV myocardial compliance at end-diastole. LV wall distensibility along the long- and short-axis during atrial systole and late diastolic LV filling may vary according to the remodeling of LV morphology. We measured LV wall motion velocities along the long and short axes using pulsed Doppler tissue imaging in 127 patients with hypertension to evaluate the relationship between the hemodynamic changes and LV morphology and to determine the role of both long- and short-axis function in late diastolic LV filling. Participants were classified into 3 groups according to LV dimension and end-diastolic wall thickness determined by M-mode echocardiography: group A (n = 62) without LV dilation or hypertrophy, group B (n = 55) with LV hypertrophy, and group C (n = 10) with LV dilation and systolic dysfunction. The time constant of the LV pressure decay during isovolumic diastole and the LV end-diastolic pressure were longest and greatest, respectively, in group C, compared with groups B and A. There were no significant differences in active left atrial emptying volume during atrial contraction determined by computerized echocardiographic 3-dimensional reconstruction among patient and control groups. The peak atrial systolic motion velocity of the LV posterior wall along the long axis was significantly lower in groups B and C, particularly in the latter group, than in group A. The peak atrial systolic motion velocity of the LV posterior wall along the short axis was greatest in group B and was lowest in group C compared with the other groups, respectively. The peak atrial systolic motion velocity of the LV posterior wall was greater along the long axis than the short axis in group A, but was less than the short axis in group B. In conclusion, the long- and short-axis function of the LV wall during atrial systole varies in patients with hypertension according to the severity of hemodynamic and morphologic abnormalities. The degree of LV wall expansion along the short axis is an important factor resulting from the atrial kick, and a determinant of its effectiveness. (J Am Soc Echocardiogr 2002;15:1211-7.)

Four-dimensional reconstruction of the left ventricle using a fast rotating classical phased array scan head: preliminary results.

Abstract: The evaluation of left ventricular function by noninvasive methods is still a major problem in cardiology. Two-dimensional echocardiography requires mental reconstruction of the heart by the physician and is always based on approximation of heart shapes and volumes. Three-dimensional echocardiography is promising but has rhythmic and function constraints because of the acquisition during many cardiac cycles. This article reports a study carried out to validate a new 4-dimensional echocardiography method. With the use of a classical phased-array sensor with a fast rotating motorized motion and a standard ultrasound system, many slices at different angulations are obtained in a single cardiac cycle. After manual endocardial delineation and computation, a representation of the left ventricle (beating heart) and a volume quantification are obtained at each instant of the cardiac cycle. This method has been tested on 11 healthy volunteers and the results are in agreement with those obtained with standard 2-dimensional echocardiography. Because of its simplicity of operation and short time acquisition, this new imaging modality is highly valuable in left ventricle evaluation, even if further studies on pathologic hearts need to be performed. (J Am Soc Echocardiogr 2002;15:593-600.)