Relation between regional electrical activation time and subepicardial fiber strain in the canine left ventricle

Numerical evaluation of cardiac mechanical markers as estimators of the electrical activation time

Recent advances in the development of noninvasive cardiac imaging technologies have made it possible to measure longitudinal and circumferential strains at a high spatial resolution also at intramural level. Local mechanical activation times derived from these strains can be used as noninvasive estimates of electrical activation, in order to determine, eg, the origin of premature ectopic beats during focal arrhythmias or the pathway of reentrant circuits. The aim of this work is to assess the reliability of mechanical activation time markers derived from longitudinal and circumferential strains, denoted by AT_ell and AT_ecc , respectively, by means of three-dimensional cardiac electromechanical simulations. These markers are compared against the electrical activation time (AT_v ), computed from the action potential waveform, and the reference mechanical activation markers derived from the active tension and fiber strain waveforms, denoted by AT_ta and AT_eff , respectively. Our numerical simulations are based on a strongly coupled electromechanical model, including bidomain representation of the cardiac tissue, mechanoelectric (ie, stretch-activated channels) and geometric feedbacks, transversely isotropic strain energy function for the description of passive mechanics and detailed membrane and excitation-contraction coupling models. The results have shown that, during endocardial and epicardial ectopic stimulations, all the mechanical markers considered are highly correlated with AT_v , exhibiting correlation coefficients larger than 0.8. However, during multiple endocardial stimulations, mimicking the ventricular sinus rhythm, the mechanical markers are less correlated with the electrical activation time, because of the more complex resulting excitation sequence. Moreover, the inspection of the endocardial and epicardial isochrones has shown that the AT_ell and AT_ecc mechanical activation sequences reproduce only some qualitative features of the electrical activation sequence, such as the areas of early and late activation, but in some cases, they might yield wrong excitation sources and significantly different isochrones patterns.

Role of coronary flow regulation and cardiac-coronary coupling in mechanical dyssynchrony associated with right ventricular pacing

Mechanical dyssynchrony (MD) affects left ventricular (LV) mechanics and coronary perfusion. To understand the multifactorial effects of MD, we developed a computational model that bidirectionally couples the systemic circulation with the LV and coronary perfusion with flow regulation. In the model, coronary flow in the left anterior descending (LAD) and left circumflex (LCX) arteries affects the corresponding regional contractility based on a prescribed linear LV contractility-coronary flow relationship. The model is calibrated with experimental measurements of LV pressure and volume, as well as LAD and LCX flow rate waveforms acquired under regulated and fully dilated conditions from a swine under right atrial (RA) pacing. The calibrated model is applied to simulate MD. The model can simultaneously reproduce the reduction in mean LV pressure (39.3%), regulated flow (LAD: 7.9%; LCX: 1.9%), LAD passive flow (21.6%), and increase in LCX passive flow (15.9%). These changes are associated with right ventricular pacing compared with RA pacing measured in the same swine only when LV contractility is affected by flow alterations with a slope of 1.4 mmHg/mL² in a contractility-flow relationship. In sensitivity analyses, the model predicts that coronary flow reserve (CFR) decreases and increases in the LAD and LCX with increasing delay in LV free wall contraction. These findings suggest that asynchronous activation associated with MD impacts 1) the loading conditions that further affect the coronary flow, which may explain some of the changes in CFR, and 2) the coronary flow that reduces global contractility, which contributes to the reduction in LV pressure.NEW & NOTEWORTHY A computational model that couples the systemic circulation of the left ventricular (LV) and coronary perfusion with flow regulation is developed to study the effects of mechanical dyssynchrony. The delayed contraction in the LV free wall with respect to the septum has a significant effect on LV function and coronary flow reserve.

The cardiac work-loop technique: An in vitro model for identifying and profiling drug-induced changes in inotropy using rat papillary muscles

The cardiac work-loop technique closely mimics the intrinsic in vivo movement and characteristics of cardiac muscle function. In this study, six known inotropes were profiled using the work-loop technique to evaluate the potential of this method to predict inotropy. Papillary muscles from male Sprague-Dawley rats were mounted onto an organ bath perfused with Krebs-Henseleit buffer. Following optimisation, work-loop contractions were performed that included an initial stabilisation period followed by vehicle control or drug administration. Six known inotropes were tested: digoxin, dobutamine, isoprenaline, flecainide, verapamil and atenolol. Muscle performance was evaluated by calculating power output during work-loop contraction. Digoxin, dobutamine and isoprenaline caused a significant increase in power output of muscles when compared to vehicle control. Flecainide, verapamil and atenolol significantly reduced power output of muscles. These changes in power output were reflected in alterations in work loop shapes. This is the first study in which changes in work-loop shape detailing for example the activation, shortening or passive re-lengthening have been linked to the mechanism of action of a compound. This study has demonstrated that the work-loop technique can provide an important novel method with which to assess detailed mechanisms of drug-induced effects on cardiac muscle contractility.

Low Intensity Epicardial Pacing During the Absolute Refractory Period Augments Left Ventricular Function Mediated by Local Catecholamine Release

BACKGROUND

Biventricular epicardial (Epi) pacing can augment left ventricular (LV) function in heart failure. We postulated that these effects might involve catecholamine release from local autonomic nerve activation. To evaluate this hypothesis we applied low intensity Epi electrical stimuli during the absolute refractory period (ARP), thus avoiding altered activation sequence.

METHODS

Anesthetized pigs (n = 6) were instrumented with an LV pressure (LVP) transducer, left atrial (LA) and LV Epi pacing electrodes, and sonomicrometer segment length (SL) gauges placed proximal and remote to the LV stimulation site. A catheter was placed into the great cardiac vein adjacent to the LV pacing site for norepinephrine (NE) analysis. During LA pacing at constant rate, 3 pulses (0.8 milliseconds, 2-3x threshold) were applied to the LV Epi electrodes during the ARP. An experimental run consisted of baseline, stimulation (10 minutes), and recovery (5 minutes), repeated 3 times before and after β1 - receptor blockade (BB, metoprolol).

RESULTS

ARP stimulation produced significant increases in cardiac function reflected by elevated LVP, LV, dP/dtmax , and reduced time to LV dP/dtmax . This was accompanied by increased coronary NE levels and increases in LVP versus SL loop area in the remote myocardial segment. In contrast, the proximal segment exhibited early shortening and decreased loop area. BB abolished the changes in SL and LV function despite continued NE release.

CONCLUSION

These results demonstrate that ARP EPI stimulation induces NE release mediating augmented global LV function. This effect may contribute to the beneficial effect of biventricular Epi pacing in heart failure in some patients.

Biomechanics of cardiac electromechanical coupling and mechanoelectric feedback

Cardiac mechanical contraction is triggered by electrical activation via an intracellular calcium-dependent process known as excitation-contraction coupling. Dysregulation of cardiac myocyte intracellular calcium handling is a common feature of heart failure. At the organ scale, electrical dyssynchrony leads to mechanical alterations and exacerbates pump dysfunction in heart failure. A reverse coupling between cardiac mechanics and electrophysiology is also well established. It is commonly referred as cardiac mechanoelectric feedback and thought to be an important contributor to the increased risk of arrhythmia during pathological conditions that alter regional cardiac wall mechanics, including heart failure. At the cellular scale, most investigations of myocyte mechanoelectric feedback have focused on the roles of stretch-activated ion channels, though mechanisms that are independent of ionic currents have also been described. Here we review excitation-contraction coupling and mechanoelectric feedback at the cellular and organ scales, and we identify the need for new multicellular tissue-scale model systems and experiments that can help us to obtain a better understanding of how interactions between electrophysiological and mechanical processes at the cell scale affect ventricular electromechanical interactions at the organ scale in the normal and diseased heart.

Timing and magnitude of systolic stretch affect myofilament activation and mechanical work

Dyssynchronous activation of the heart leads to abnormal regional systolic stretch. In vivo studies have suggested that the timing of systolic stretch can affect regional tension and external work development. In the present study, we measured the direct effects of systolic stretch timing on the magnitude of tension and external work development in isolated murine right ventricular papillary muscles. A servomotor was used to impose precisely timed stretches relative to electrical activation while a force transducer measured force output and strain was monitored using a charge-couple device camera and topical markers. Stretches taking place during peak intracellular Ca(2+) statistically increased peak tension up to 270%, whereas external work due to stretches in this interval reached values of 500 J/m. An experimental analysis showed that time-varying elastance overestimated peak tension by 100% for stretches occurring after peak isometric tension. The addition of the force-velocity relation explained some effects of stretches occurring before the peak of the Ca(2+) transient but had no effect in later stretches. An estimate of transient deactivation was measured by performing quick stretches to dissociate cross-bridges. The timing of transient deactivation explained the remaining differences between the model and experiment. These results suggest that stretch near the start of cardiac tension development substantially increases twitch tension and mechanical work production, whereas late stretches decrease external work. While the increased work can mostly be explained by the time-varying elastance of cardiac muscle, the decreased work in muscles stretched after the peak of the Ca(2+) transient is largely due to myofilament deactivation.

Myofiber prestretch magnitude determines regional systolic function during ectopic activation in the tachycardia-induced failing canine heart

Electrical dyssynchrony leads to prestretch in late-activated regions and alters the sequence of mechanical contraction, although prestretch and its mechanisms are not well defined in the failing heart. We hypothesized that in heart failure, fiber prestretch magnitude increases with the amount of early-activated tissue and results in increased end-systolic strains, possibly due to length-dependent muscle properties. In five failing dog hearts with scars, three-dimensional strains were measured at the anterolateral left ventricle (LV). Prestretch magnitude was varied via ventricular pacing at increasing distances from the measurement site and was found to increase with activation time at various wall depths. At the subepicardium, prestretch magnitude positively correlated with the amount of early-activated tissue. At the subendocardium, local end-systolic strains (fiber shortening, radial wall thickening) increased proportionally to prestretch magnitude, resulting in greater mean strain values in late-activated compared with early-activated tissue. Increased fiber strains at end systole were accompanied by increases in preejection fiber strain, shortening duration, and the onset of fiber relengthening, which were all positively correlated with local activation time. In a dog-specific computational failing heart model, removal of length and velocity dependence on active fiber stress generation, both separately and together, alter the correlations between local electrical activation time and timing of fiber strains but do not primarily account for these relationships.

Simultaneous optical mapping of transmembrane potential and wall motion in isolated, perfused whole hearts

Optical mapping of cardiac propagation has traditionally been hampered by motion artifact, chiefly due to changes in photodetector-to-tissue registration as the heart moves. We have developed an optical mapping technique to simultaneously record electrical waves and mechanical contraction in isolated hearts. This allows removal of motion artifact from transmembrane potential (V(m)) recordings without the use of electromechanical uncoupling agents and allows the interplay of electrical and mechanical events to be studied at the whole organ level. Hearts are stained with the voltage-sensitive dye di-4-ANEPPS and ring-shaped markers are attached to the epicardium. Fluorescence, elicited on alternate frames by 450 and 505 nm light-emitting diodes, is recorded at 700 frames∕ per second by a camera fitted with a 605 ± 25 nm emission filter. Marker positions are tracked in software. A signal, consisting of the temporally interlaced 450 and 505 nm fluorescence, is collected from the pixels enclosed by each moving ring. After deinterlacing, the 505 nm signal consists of V(m) with motion artifact, while the 450 nm signal is minimally voltage-sensitive and contains primarily artifacts. The ratio of the two signals estimates V(m). Deformation of the tissue enclosed by each set of 3 rings is quantified using homogeneous finite strain.

Mechano-energetics of the asynchronous and resynchronized heart

Abnormal electrical activation of the ventricles creates major abnormalities in cardiac mechanics. Local contraction patterns, as reflected by measurements of local strain, are not only out of phase, but often also show opposing length changes in early and late activated regions. As a consequence, the efficiency of cardiac pump function (the amount of stroke work generated by a unit of oxygen consumed) is approximately 30% lower in asynchronous than in synchronous hearts. Moreover, the amount of work performed in myocardial segments becomes considerably larger in late than in early activated regions. Cardiac Resynchronization Therapy (CRT) improves mechano-energetics of the previously asynchronous heart in various ways: it alleviates impediment of the abnormal contraction on blood flow, it increases myocardial efficiency, it recruits contraction in the previously early activated septum and it creates a more uniform distribution of myocardial blood flow. These factors act together to increase the range of cardiac work that can be delivered by the patients’ heart, an effect that can explain the increased exercise tolerance and quality of life reported in several CRT trials.

Ventricular dilation and electrical dyssynchrony synergistically increase regional mechanical nonuniformity but not mechanical dyssynchrony: a computational model

BACKGROUND

Heart failure (HF) in combination with mechanical dyssynchrony is associated with a high mortality rate. To quantify contractile dysfunction in patients with HF, investigators have proposed several indices of mechanical dyssynchrony, including percentile range of time to peak shortening (WTpeak), circumferential uniformity ratio estimate (CURE), and internal stretch fraction (ISF). The goal of this study was to compare the sensitivity of these indices to 4 major abnormalities responsible for cardiac dysfunction in dyssynchronous HF: dilation, negative inotropy, negative lusitropy, and dyssynchronous activation.

METHODS AND RESULTS

All combinations of these 4 major abnormalities were included in 3D computational models of ventricular electromechanics. Compared with a nonfailing heart model, ventricles were dilated, inotropy was reduced, twitch duration was prolonged, and activation sequence was changed from normal to left bundle branch block. In the nonfailing heart, CURE, ISF, and WTpeak were 0.97+/-0.004, 0.010+/-0.002, and 78+/-1 milliseconds, respectively. With dilation alone, CURE decreased 2.0+/-0.07%, ISF increased 58+/-47%, and WTpeak increased 31+/-3%. With dyssynchronous activation alone, CURE decreased 15+/-0.6%, ISF increased 14-fold (+/-3), and WTpeak increased 121+/-4%. With the combination of dilation and dyssynchronous activation, CURE decreased 23+/-0.8%, ISF increased 20-fold (+/-5), and WTpeak increased 147+/-5%.

CONCLUSIONS

Dilation and left bundle branch block combined synergistically decreased regional cardiac function. CURE and ISF were sensitive to this combination, but WTpeak was not. CURE and ISF also reflected the relative nonuniform distribution of regional work better than WTpeak. These findings might explain why CURE and ISF are better predictors of reverse remodeling in cardiac resynchronization therapy.

Cardiac resynchronization: insight from experimental and computational models

Cardiac resynchronization therapy (CRT) is a promising therapy for heart failure patients with a conduction disturbance, such as left bundle branch block. The aim of CRT is to resynchronize contraction between and within ventricles. However, about 30% of patients do not respond to this therapy. Therefore, a better understanding is needed for the relation between electrical and mechanical activation. In this paper, we focus on to what extent animal experiments and mathematical models can help in order to understand the pathophysiology of asynchrony to further improve CRT.

Asynchrony of ventricular activation affects magnitude and timing of fiber stretch in late-activated regions of the canine heart

Abnormal electrical activation of the left ventricle results in mechanical dyssynchrony, which is in part characterized by early stretch of late-activated myofibers. To describe the pattern of deformation during "prestretch" and gain insight into its causes and sequelae, we implanted midwall and transmural arrays of radiopaque markers into the left ventricular anterolateral wall of open-chest, isoflurane-anesthetized, adult mongrel dogs. Biplane cineradiography (125 Hz) was used to determine the time course of two- and three-dimensional strains while pacing from a remote, posterior wall site. Strain maps were generated as a function of time. Electrical activation was assessed with bipolar electrodes. Posterior wall pacing generated prestretch at the measurement site, which peaked 44 ms after local electrical activation. Overall magnitudes and transmural gradients of strain were reduced when compared with passive inflation. Fiber stretch was larger at aortic valve opening compared with end diastole (P < 0.05). Fiber stretch at aortic valve opening was weakly but significantly correlated with local activation time (r(2) = 0.319, P < 0.001). With a short atrioventricular delay, fiber lengths were not significantly different at the time of aortic valve opening during ventricular pacing compared with atrial pacing. However, ejection strain did significantly increase (P < 0.05). We conclude that the majority of fiber stretch occurs after local electrical activation and mitral valve closure and is different from passive inflation. The increased shortening of these regions appears to be because of a reduced afterload rather than an effect of length-dependent activation in this preparation.

Pacing-Induced Dys-Synchrony Preconditions Rabbit Myocardium Against Ischemia/Reperfusion Injury

Background— Because increased mechanical load induces preconditioning (PC) and dys-synchrony increases loading in late-activated regions, we investigated whether dys-synchrony induced by ventricular pacing (VP) at normal heart rate leads to cardioprotection.

Methods and Results— Isolated working rabbit hearts were subjected to 35 minutes of global ischemia and 2 hours of reperfusion. Seven hearts underwent VP PC (3 periods of 5 minutes VP at the posterior left ventricular [LV] wall), 7 hearts underwent ischemic preconditioning (IPC) (3 periods of 5 minutes of global ischemia), and 9 hearts served as control (C). LV pressure and sonomicrometry were used to assess global hemodynamics and segment work (SW) and end-diastolic segment length (EDSL) in anterior and posterior LV myocardium. Myocardial release of lactate and expression of proBNP mRNA were determined to gain insight in molecular processes involved in VP PC (* P <0.05). Infarct size (triphenyl tetrazolium chloride staining) was 18.3±13.0% in group C, and was uniformly reduced in the VP PC and IPC groups (1.8±0.8%*, and 3.5±3.1%*, respectively; and not significant between VP PC and IPC). LV posterior wall pacing (VP PC group) increased EDSL (by 6.3±5.8%*) and SW (to 335±207%*) in the LV anterior wall, whereas posterior wall SW decreased to negative values (−23±63%*). LV pacing did not significantly change lactate release and coronary flow but significantly increased proBNP mRNA expression in both anterior and posterior myocardium as compared with controls.

Conclusions— Intermittent dys-synchrony is equally cardioprotective as “classical” IPC. Stretch-mediated signaling is a more likely trigger for VP PC than ischemia. VP PC is potentially applicable in cardiac surgery.

High resolution mechanical function in the intact porcine heart: mechanical effects of pacemaker location

The necessity to quantify the mechanical function with high spatial resolution stemmed from the advancement of myocardial salvaging techniques. Since these therapies are localized interventions, a whole field technique with high spatial resolution was needed to differentiate the normal, diseased, and treated myocardium. We developed a phase correlation algorithm for measuring myocardial displacement at high spatial resolution and to determine the regional mechanical function in the intact heart. Porcine hearts were exposed and high contrast microparticles were placed on the myocardium. A pressure transducer, inserted into the left ventricle, synchronized the pressure (LVP) with image acquisition using a charge-coupled device camera. The deformation of the myocardium was measured with a resolution of 0.58+/-0.04 mm. Within the region of interest (ROI), regional stroke work (RSW), defined as the integral of LVP with respect to regional area, was determined on average at 21 locations with a resolution of 27.1+/-2.7 mm2. To alter regional mechanical function, the heart was paced at three different locations around the ROI. Independent of the pacemaker location, RSW decreased in the ROI. In addition, a gradient of increasing RSW in the outward direction radiating from the pacemaker was observed in all pacing protocols. These data demonstrated the ability to determine regional whole field mechanical function with high spatial resolution, and the significant alterations induced by electrical pacing.

Electromechanics of paced left ventricle simulated by straightforward mathematical model: comparison with experiments

Intraventricular synchrony of cardiac activation is important for efficient pump function. Ventricular pacing restores the beating frequency but induces more asynchronous depolarization and more inhomogeneous contraction than in the normal heart. We investigated whether the increased inhomogeneity in the left ventricle can be described by a relatively simple mathematical model of cardiac electromechanics, containing normal mechanical and impulse conduction properties. Simulations of a normal heartbeat and of pacing at the right ventricular apex (RVA) were performed. All properties in the two simulations were equal, except for the depolarization sequence. Simulation results of RVA pacing on local depolarization time and systolic midwall circumferential strain were compared with those measured in dogs, using an epicardial sock electrode and MRI tagging, respectively. We used the same methods for data processing for simulation and experiment. Model and experiment agreed in the following aspects. 1) Ventricular pacing decreased systolic pressure and ejection fraction relative to natural sinus rhythm. 2) Shortening during ejection and stroke work declined in early depolarized regions and increased in late depolarized regions. 3) The relation between epicardial depolarization time and systolic midwall circumferential strain was linear and similar for the simulation (slope = -3.80 +/- 0.28 s(-1), R2 = 0.87) and the experiments [slopes for 3 animals -2.62 +/- 0.43 s(-1) (R2 = 0.59), -2.97 +/- 0.38 s(-1) (R2 = 0.69), and -4.44 +/- 0.51 s(-1) (R2 = 0.76)]. We conclude that our model of electromechanics is suitable to simulate ventricular pacing and that the apparently complex events observed during pacing are caused by well-known basic physiological processes.

Contraction wave in axial direction in free wall of guinea pig left ventricle

Mechanical activation of the normal left ventricle (LV) is not simultaneous; however, the potential consequences of the ejection function of the ventricle are not entirely known. We studied contraction of the LV free wall to determine whether it reveals a contraction wave in the axial direction during ejection. Seven guinea pig hearts in situ were studied via thoracotomy. In each heart, the ventricular and aortic pressures were measured by two microtipped manometers (2-Fr, Millar). Contraction of the LV free wall was assessed with a video system (Dalsa D6-0256 camera and EPIX PIXCI D32 frame grabber; acquisition rate, 500 frames/s), and 15-18 epicardial markers were used to divide the region into 20-25 triangular areas. The area sizes were studied during contraction to locate the position of the contraction wave. For each triangular area, two variables were determined as follows: the time (t(c)) from the end of diastole until the size of the area reached 80% of maximum size reduction (normalized with the duration of systole) and the normalized latitude (L(ax)) of the area (determined at the end of diastole). A relationship between these two variables was determined by regression analysis. We found that the t(c) at which the contraction wave reached a triangular area was in positive correlation with the L(ax) value for that triangular area with a slope of 0.25 +/- 0.09 and a linear correlation coefficient of 0.41 +/- 0.08. Thus contraction in the guinea pig LV free wall occurs progressively from apex to base with successive areas reaching 80% contraction.

Transmural mechanics at left ventricular epicardial pacing site

Left ventricular (LV) epicardial pacing acutely reduces wall thickening at the pacing site. Because LV epicardial pacing also reduces transverse shear deformation, which is related to myocardial sheet shear, we hypothesized that impaired end-systolic wall thickening at the pacing site is due to reduction in myocardial sheet shear deformation, resulting in a reduced contribution of sheet shear to wall thickening. We also hypothesized that epicardial pacing would reverse the transmural mechanical activation sequence and thereby mitigate normal transmural deformation. To test these hypotheses, we investigated the effects of LV epicardial pacing on transmural fiber-sheet mechanics by determining three-dimensional finite deformation during normal atrioventricular conduction and LV epicardial pacing in the anterior wall of normal dog hearts in vivo. Our measurements indicate that impaired end-systolic wall thickening at the pacing site was not due to selective reduction of sheet shear, but rather resulted from overall depression of fiber-sheet deformation, and relative contributions of sheet strains to wall thickening were maintained. These findings suggest lack of effective end-systolic myocardial deformation at the pacing site, most likely because the pacing site initiates contraction significantly earlier than the rest of the ventricle. Epicardial pacing also induced reversal of the transmural mechanical activation sequence, which depressed sheet extension and wall thickening early in the cardiac cycle, whereas transverse shear and sheet shear deformation were not affected. These findings suggest that normal sheet extension and wall thickening immediately after activation may require normal transmural activation sequence, whereas sheet shear deformation may be determined by local anatomy.

Novel technique for cardiac electromechanical mapping with magnetic resonance imaging tagging and an epicardial electrode sock

Near-simultaneous measurements of electrical and mechanical activation over the entire ventricular surface are now possible using magnetic resonance imaging tagging and a multielectrode epicardial sock. This new electromechanical mapping technique is demonstrated in the ventricularly paced canine heart. A 128-electrode epicardial sock and pacing electrodes were placed on the hearts of four anesthetized dogs. In the magnetic resonance scanner, tagged cine images (8-15 ms/frame) and sock electrode recordings (1000 Hz) were acquired under right-ventricular pacing and temporally referenced to the pacing stimulus. Electrical recordings were obtained during intermittent breaks in image acquisition, so that both data sets represented the same physiologic state. Since the electrodes were not visible in the images, electrode recordings and cine images were spatially registered with Gd-DTPA markers attached to the sock. Circumferential strain was calculated at locations corresponding to electrodes. For each electrode location, electrical and mechanical activation times were calculated and relationships between the two activation patterns were demonstrated. This method holds promise for improving understanding of the relationships between the patterns of electrical activation and contraction in the heart.

The mechanical and metabolic basis of myocardial blood flow heterogeneity

Precise measurements of regional myocardial blood flow heterogeneity had to be developed before one could seek causation for the heterogeneity. Deposition techniques (particles or molecular microspheres) are the most precise, but imaging techniques have begun to provide high enough resolution to allow in vivo studies. Assigning causation has been difficult. There is no apparent association with the regional concentrations of energy-related enzymes or substrates, but these are measures of status, not of metabolism. There is statistical correlation between flow and regional substrate uptake and utilization. Attribution of regional flow variation to vascular anatomy or to vasomotor control appears not to be causative on a long-term basis. The closest relationships appear to be with mechanical function, but one cannot say for sure whether this is related to ATP hydrolysis at the crossbridge or associated metabolic reactions such as calcium uptake by the sarcoplasmic reticulum.

Effects of 1- or -adrenoceptor stimulation on work-loop and isometric contractions of isolated rat cardiac trabeculae

1. We studied the effects of alpha1- or beta-adrenoceptor stimulation on the contractility of isolated rat ventricular trabeculae at 24 degrees C using the work-loop technique, which simulates the cyclical changes in length and force that occur during the cardiac cycle. Some muscles were injected with fura-2 to monitor the intracellular Ca2+ transient. 2. Comparison of twitch records revealed that peak force was greater and was reached earlier in work-loop contractions than in corresponding isometric contractions. This was attributed to the changes in muscle length and velocity during work-loop contractions, since the Ca2+ transients were largely unaffected by the length changes. 3. Stimulation of alpha1-adrenoceptors (with 100 microM phenylephrine) increased net work, power production, the frequency for maximum work, and the frequency for maximum power production (fopt). The increase in net work was due to the positive inotropic effect of phenylephrine, which was similar at all frequencies investigated (0. 33-4.5 Hz). The increase in fopt was attributed to an abbreviation of twitch duration induced by alpha1-stimulation at higher frequencies (> 1 Hz), even though the twitch became longer at 0.33 Hz. 4. beta-Adrenoceptor stimulation (with 5 microM isoprenaline) produced marked increases in net work, power output, the frequency for net work, and fopt. These effects were attributed both to the positive inotropic effect of beta-stimulation, which was greater at higher frequencies, and to the reduction in twitch duration. beta-stimulation also abolished the frequency-dependent acceleration of twitch duration. 5. The increase in power output and fopt with alpha1- as well as beta-adrenoceptor stimulation suggested that both receptor types may contribute to the effects of catecholamines, released during stress or exercise, although the greater effects of beta-stimulation are likely to predominate.

Three dimensional registration of mechanical bladder activity using polystyrene fluorescent spheres: A technical note

Optical marker tracing methods have been applied successfully in recent years to quantify local material deformation of heart tissue, skin and striated muscles. In this study, polystyrene fluorescent spheres (d = 0.6 mm) are glued to the ventral serosal bladder wall in the rabbit. Three dimensional video registration of the polystyrene spheres is used to calculate two directions of principal strain (epsilon (1), epsilon (2) ) on the bladder surface in vivo. The aim is to investigate the feasibility of the technique for this new application in two experimental circumstances: during spontaneous bladder wall activity and after electrical stimulation of bladder innervating nerve fibers. During spontaneous activity, random contraction and relaxation occurred simultaneously and separately across the bladder wall for the two principal strains epsilon (1) and epsilon (2). After extradural electrical stimulation of sacral nerve root S2, the principal strains epsilon ( 1) and epsilon (2) synchronized in time in such a way that epsilon ( 1) and epsilon (2) both represented contraction or both represented relaxation. One and the same bladder wall area passed through phases of contraction followed by relaxation and vice versa. After multiple stimulation periods, the coordination between the two principal strains during stimulation was reduced. This technique allows to identify local areas of contraction and relaxation in the intact bladder wall in vivo. Three dimensional video registration of polystyrene fluorescent spheres to study bladder wall contraction and its relaxation proved to be a feasible technique, with which electrical stimulation effects and spontaneous activity could be measured.

Heterogeneities in myocardial flow and metabolism: exacerbation with abnormal excitation

Because regional myocardial blood flows are remarkably heterogeneous-with a 6- to 10-fold range of flows in normal hearts-and because the spatial profiles of the flows are stable over long periods and over a range of conditions, the relation between flows and other physiologic functions has been explored. Local fatty acid uptake and oxygen consumption are almost linearly related to the flows. Coronary network structure and hydrodynamic resistances give suitable flow heterogeneity but are thought to be a response to local needs rather than being causative. Presumably the cause is the need for adenosine triphosphate (ATP) synthesis locally, and therefore flows, substrate delivery, and oxygen utilization are driven primarily by local rates of ATP hydrolysis, mainly by contractile proteins. This hypothesis is by no means fully tested. Data on pacing dog hearts from different sites, on patients with left bundle branch block, and on unloading transplanted rat hearts, all point in the same direction: unloading ventricular muscle leads to diminished flow and exaggeratedly diminished glucose uptake. The mechanism is likely to be that discovered by Taegtmeyer and colleagues, namely, the expression of fetal genes in regions where the muscle is unloaded and particular metabolic enzymes and transporters are downregulated.

Mapping of regional myocardial strain and work during ventricular pacing: experimental study using magnetic resonance imaging tagging

OBJECTIVES

The purpose of this study was to determine the spatial distribution of myocardial function (myofiber shortening and work) within the left ventricular (LV) wall during ventricular pacing.

BACKGROUND

Asynchronous electrical activation, as induced by ventricular pacing, causes various abnormalities in LV function, perfusion and structure. These derangements may be caused by abnormalities in regional contraction patterns. However, insight into these patterns during pacing is as yet limited.

METHODS

In seven anesthetized dogs, high spatial and temporal resolution magnetic resonance-tagged images were acquired in three orthogonal planes. Three-dimensional deformation data and LV cavity pressure and volume were used to determine midwall circumferential strain and external and total mechanical work at 192 sites around the left ventricle.

RESULTS

During ventricular pacing, systolic fiber strain and external work were approximately zero in regions near the pacing site, and gradually increased to more than twice the normal value in the most remote regions. Total mechanical work, normalized to the value during right atrial pacing, was 38 +/- 13% (right ventricular apex [RVapex] pacing) and 61 +/- 23% (left ventricular base [LVbase] pacing) close to the pacing site, and 125 +/- 48% and 171 +/- 60% in remote regions, respectively (p < 0.05 between RVapex and LVbase pacing). The number of regions with reduced work was significantly larger during RVapex than during LVbase pacing. This was associated with a reduction of global LV pump function during RVapex pacing.

CONCLUSIONS

Ventricular pacing causes a threefold difference in myofiber work within the LV wall. This difference appears large enough to regard local myocardial function as an important determinant for abnormalities in perfusion, metabolism, structure and pump function during asynchronous electrical activation. Pacing at sites that cause more synchronous activation may limit the occurrence of such derangements.

Mapping propagation of mechanical activation in the paced heart with MRI tagging

The temporal evolution of three-dimensional (3-D) strain maps derived from magnetic resonance imaging (MRI) tagging were used to noninvasively evaluate mechanical activation in the left ventricle (LV) while seven canine hearts were paced in situ from three different sites: the base of the LV free wall (LVb), the right ventricular apex (RVa), and the right atrium (RA). Strain maps plotted against time showed the evolution of shortening over the entire LV midwall and were used to generate mechanical activation maps showing the onset of circumferential shortening. RA pacing showed rapid synchronous shortening; LVb pacing showed a wave front of mechanical activation propagating slowly and steadily from the pacing site, whereas RVa pacing showed regions of rapid and slower propagation. The mechanical (M) activation times correlated linearly with the electrical (E) activation (M = 1.06E + 8.4 ms, R = 0.95). The time for 90% activation of the LV was 63.1 +/- 24.3 ms for RA pacing, 130.2 +/- 9.8 ms for LVb pacing, and 121.3 +/- 17.9 ms for RVa pacing. The velocity of mechanical activation was calculated for LVb and RVa pacing and was similar to values reported for electrical conduction in myocardium. The propagation of mechanical activation for RVa pacing showed regional variations, whereas LVb pacing did not.

Estimates of regional work in the canine left ventricle

Assessment of the magnitude of regional myocardial work requires knowledge of regional fiber stress and fiber shortening. The theoretical development and experimental validation of a method is presented which used values of estimated active and passive fiber stress according to a fluid-fiber model, and measured fiber strain values. This enables the construction of regional stress-strain diagrams, a regional analog of the pressure-volume area model by Suga and co-investigators, which can be linked to regional oxygen consumption. In the left ventricle, either normally or asynchronously activated, the method yields reliable data on strain and active and passive fiber stress. The relation between estimated regional work and myocardial oxygen demand is in quantitative agreement with previously reported relations between global oxygen demand and measured pressure-volume area. During coronary artery occlusion, however, these values were less reliable, which might be due to inaqdequate knowledge of the (passive) material properties of the myocardium.

Time-dependent changes in matrix metalloproteinase activity and expression during the progression of congestive heart failure: relation to ventricular and myocyte function

The development of congestive heart failure (CHF) is associated with left ventricular (LV) dilation and myocardial remodeling. However, fundamental mechanisms that contribute to this remodeling process with the progression of CHF remain unclear. The matrix metalloproteinases (MMPs) have been demonstrated to play a significant role in tissue remodeling in a number of pathological processes. The present project tested the hypothesis that the LV dilation and remodeling during the progression of CHF is associated with early changes in MMP expression and zymographic activity. LV and myocyte function, collagen content, and MMP expression and zymographic activity were serially measured during the progression of CHF caused by pacing-induced supraventricular tachycardia (SVT) in pigs. After 7 days of SVT, LV end-diastolic dimension and myocyte length both increased by 15% from control values, and LV fractional shortening fell by 20%. At the level of the myocyte, percent shortening fell by 16% after 7 days of SVT, with no change in the steady-state velocity of shortening. Longer durations of SVT caused progressive LV dilation, LV pump failure, and myocyte contractile dysfunction. Specifically, 21 days of SVT resulted in a >50% increase in LV dimension, a 56% fall in LV fractional shortening, and a 33% decline in myocyte velocity of shortening. The decline in LV and myocyte function with 21 days of SVT was accompanied by signs and symptoms of CHF. Thus, SVT causes time-dependent changes in LV geometry and function and the subsequent development of CHF. LV myocardial collagen content and confluence fell by >25% after 7 days of SVT and were accompanied by an 80% increase in LV myocardial MMP zymographic activity against the substrate gelatin. After 14 days of SVT, total LV myocardial collagen content was reduced by 24%, and LV myocardial MMP zymographic activity increased by >100% from control values. Interstitial collagenase (MMP-1), stromelysin (MMP-3), and 72-kD gelatinase (MMP-2) were increased by approximately 2-fold after 7 days of SVT. LV MMP zymographic activity and abundance remained elevated with longer durations of SVT. The results of the present study demonstrated that in this model of CHF, early changes in LV myocardial MMP zymographic activity and protein levels occurred with the initiation and progression of LV dilation and dysfunction. These findings suggest that an early contributory mechanism for the initiation of LV remodeling that occurred in this model of developing CHF is enhanced expression and potentially increased activity of LV myocardial MMPs.

Pulmonary hemodynamics and endothelin levels in pacing-induced heart failure: during rest and exercise

Elevated plasma levels of endothelin (ET) have been reported to accompany the development of heart failure (HF), and therefore, this potent vasoconstrictive peptide has been postulated to contribute to the altered pulmonary hemodynamics that occur in this disease process. The overall goal of this study was to examine more carefully the relationship between ET levels in the pulmonary system and pulmonary hemodynamics in the normal and HF states, during both rest and exercise. This study used a porcine model of chronic rapid pacing that has been shown in previous studies to produce left ventricular dysfunction and neurohormonal system activation consistent with the syndrome of HF. Pigs (n = 10) were chronically instrumented to measure pulmonary and systemic hemodynamics, parenchymal flow, and ET content and to obtain blood samples from the pulmonary circuit in the conscious state. Measurements were performed in the normal control state and again following the development of pacing-induced HF (240 beats/min per 21 days), both at rest and during treadmill exercise (3 mph, 15 degrees incline, 12 minutes). With HF, under ambient resting conditions, a threefold increase in pulmonary plasma ET occurred and was accompanied by a fivefold increase in pulmonary vascular resistance. During treadmill exercise, pulmonary plasma ET and pulmonary vascular resistance remained elevated in the HF group when compared with the normal state and were associated with a sixfold decrease in pulmonary parenchymal flow. Pulmonary parenchymal ET content was increased with HF when compared with values for normal control subjects (8.5 +/- 0.6 vs 5.6 +/- 0.8 fmol ET/mg protein, P < .05). Thus, the findings of this study suggest that in this model of HF, increased ET within the pulmonary circuit contributed to abnormalities in resistive properties and parenchymal flow.

Comparison of right ventricular outflow tract and apical lead permanent pacing on cardiac output

Cardiac output was measured in 89 patients using transthoracic continuous-wave echo Doppler comparing right ventricular outflow tract pacing with the right ventricular apex at the time of permanent pacemaker implantation. Overall, cardiac output improved 18.8% (p <0.0001) and cardiac index 21.0% (p <0.0001) with outflow tract placement; patients with a lower baseline cardiac index had a greater percent improvement with outflow tract placement.

Effects of permanent dual chamber pacing on myocardial perfusion in symptomatic hypertrophic cardiomyopathy

OBJECTIVE

Angina and the presence of myocardial ischaemia are common in hypertrophic cardiomyopathy. Dual chamber pacing results in clinical improvement in these patients. This study evaluates the effects of permanent dual chamber pacing on absolute regional myocardial perfusion and perfusion reserve.

SETTING

University hospital.

PATIENTS AND DESIGN

Six patients with hypertrophic cardiomyopathy and severe symptoms of angina received a dual chamber pacemaker. Absolute myocardial regional perfusion and perfusion reserve (dipyridamole 0.56 mg/kg) were measured by dynamic positron emission tomography with 13N-ammonia both during sinus rhythm and 3 months after pacemaker insertion. Results were compared with those from 28 healthy volunteers.

RESULTS

Pacing resulted in a reduction of anginal complaints and a reduction in intraventricular pressure gradient from 65 (SD 30) mm Hg to 19 (10) mm Hg. During sinus rhythm, baseline perfusion was higher in patients with hypertrophic cardiomyopathy than controls (184 (31) v 106 (26) ml/min/100 g, P < 0.01), and perfusion reserve was lower (1.6 (0.4) v 2.8 (1.0), P < 0.05). During pacing myocardial perfusion decreased to 130 (27) ml/min/100 g (P < 0.05), with variable responses in terms of perfusion reserve. Pacing caused a redistribution of myocardial stress perfusion and perfusion reserve. The coefficient of regional variation of myocardial stress perfusion decreased from 19.7 (7.0)% to 14.6 (3.9)% during pacing (12.9 (3.8)% in controls, P < 0.01). The coefficient of regional variation of perfusion reserve decreased from 16.7 (6.6)% to 11.4 (2.6)% during pacing (9.8 (4.1)% in controls, P < 0.01).

CONCLUSIONS

Pacing caused a decrease of resting left ventricular myocardial blood flow and blood flow during pharmacologically induced coronary vasodilatation. Although global perfusion reserve remained unchanged, myocardial perfusion reserve became more homogeneously distributed.

High resolution electrical mapping in the gastrointestinal system: initial results

High resolution electrical mapping in the gastrointestinal system entails recording from a large number of extracellular electrodes simultaneously. It allows the collection of signals from 240 individual sites which are then amplified, filtered, digitized, multiplexed and stored on tape. After recording, periods of interest can be analysed and the original sequence of activity reconstructed. This technology, originally developed to study normal rhythms and abnormal dysrhythmias in the heart, has been modified to allow recordings from the gastrointestinal tract. In this report, initial results are presented describing the origin and propagation of the slow wave in the isolated stomach and the isolated duodenum in the cat. These results show that in both organs it not uncommon to have more than one focus active during a single cycle. The conduction of slow waves from such a multiple pacemaker environment can become quite complex, and this may play a role in determining the contractile pattern in these organs.

Strain distribution on rat medial gastrocnemius (MG) during passive stretch

Deformation of the surface of passive medial gastrocnemius muscle (MG) was measured in vivo while performing a hysteresis test. The gastrocnemius muscle of male rats were dissected free and the distal tendon was cut. The lateral head was separated from the medial head. The muscle origins were left intact. 60-70 fluorescent, polystyrene spheres (diameter 0.7 mm) were attached to the surface of the MG. During the experiment, two-dimensional video recordings of the movements of the MG were made. The coordinates of the marker centroids were obtained by computer processing of digitized images and marker displacements as a function of time were calculated. Green-Lagrange strains in two principal directions were calculated (epsilon 1, epsilon 2) for three specimens. epsilon 1 had approximately the same direction as the muscle fibers. The longitudinal strain of the fibers (20-30%) was larger than the strain of the aponeurosis (1-5%); p < 0.001. No significant difference was found between the values of the transverse strains of muscle fibers and aponeurosis; the value of epsilon 2 was -6 to -9% for both tissue structures.

Redistribution of myocardial perfusion during permanent dual chamber pacing in symptomatic non-obstructive hypertrophic cardiomyopathy: a quantitative positron emission tomography study

Dual chamber pacing causes significant symptomatic improvement in many patients with hypertrophic cardiomyopathy. The mechanism behind this beneficial response is not fully understood. Positron emission tomography showed a redistribution of myocardial flow during pacing in a patient with non-obstructive hypertrophic cardiomyopathy. Early septal activation reduced septal fibre strain and blood flow and increased septal perfusion reserve.

Asymmetric thickness of the left ventricular wall resulting from asynchronous electric activation: a study in dogs with ventricular pacing and in patients with left bundle branch block

Various kinds of abnormal, asynchronous electric activation of the left ventricle (LV) decrease mechanical load in early versus late activated regions of the ventricular wall. Because myocardium usually adapts its mass to changes in workload, we investigated by echocardiography whether regional differences in wall thickness are present in two kinds of asynchronous electric activation of different origin and conduction pathway: epicardial ventricular pacing in dogs and left bundle branch block (LBBB) in patients. In six dogs, 3 months of epicardial LV pacing at physiologic heart rates decreased the thickness of the early activated anterior wall by 20.5 +/- 8.1% without significantly changing LV cavity area and septal thickness. In a retrospective study of 228 LBBB patients, the early activated septum was significantly thinner than the late activated posterior wall. The asymmetry most pronounced was as large as 10% in 28 patients with LBBB and paradoxic septal motion. No difference in regional wall thickness was present in 154 control patients. In conclusion, chronic asynchronous electric activation in the heart induces redistribution of cardiac mass. This redistribution occurs in hearts, which differ in impulse conduction pathway, disease, and species and is characterized by thinning of early versus late activated myocardium.

A model approach to the adaptation of cardiac structure by mechanical feedback in the environment of the cell

The uniformity of the mechanical load of the cardiac fibers in the wall is maintained by continuous remodeling. In this proposed model the myocyte changes direction in optimizing systolic sarcomere shortening. Early systolic stretch and contractility increases the mass of contractile proteins. Cyclic strain of the myocardial tissue diminishes passive stiffness, resulting in the control of ventricular end-diastolic volume. Utilizing these rules of remodeling in our mathematical model yields that the natural helical pathways of the myocardial fibers in the wall are formed automatically.

Regional fibre stress-fibre strain area as an estimate of regional blood flow and oxygen demand in the canine heart

1. In the present study the relation between regional left ventricular contractile work, regional myocardial blood flow and oxygen uptake was assessed during asynchronous electrical activation. 2. In analogy to the use of the pressure-volume area for the estimation of global oxygen demand, the fibre stress-fibre strain area, as assessed regionally, was used to estimate regional oxygen demand. The more often used relation between the pressure-sarcomere length area and regional oxygen demand was also assessed. 3. Experiments were performed in six anaesthetized dogs with open chests. Regional differences in mechanical work were generated by asynchronous electrical activation of the myocardial wall. The ventricles were paced from the right atrium, the left ventricular free wall, the left ventricular apex or the right ventricular outflow tract. Regional fibre strain was measured at the epicardial anterior left ventricular free wall with a two-dimensional video technique. 4. Regional fibre stress was estimated from left ventricular pressure, the ratio of left ventricular cavity volume to wall volume, and regional deformation. Total mechanical power (TMP) was calculated from the fibre stress-fibre strain area (SSA) and the duration of the cardiac cycle (tcycle) using the equation: TMP = SSA/tcycle. Regional myocardial blood flow was measured with radioactive microspheres. Regional oxygen uptake was estimated from regional myocardial blood flow values and arteriovenous differences in oxygen content. 5. During asynchronous electrical activation, total mechanical power, pressure-sarcomere length area, myocardial blood flow and oxygen uptake were significantly lower in early than in late activated regions (P < 0.05). 6. Within the experiments, the correlation between the pressure-sarcomere length area and regional oxygen uptake was not significantly lower than the one between total mechanical power (TMP) and regional oxygen uptake (VO2,reg). However, variability of this relation between the experiments was less for total mechanical power. Pooling all experimental data revealed: VO2,reg = k1 TMP+k2, with k1 = 4.94 +/- 0.31 mol J-1 k2 = 24.2 +/- 1.9 mmol m-3 s-1 (means +/- standard error of the estimate). 7. This relation is in quantitative agreement with previously reported relations between the pressure-volume area and global oxygen demand. The results indicate that asynchronous electrical activation causes a redistribution of mechanical work and oxygen demand and that regional total mechanical power is a better and more general estimate of regional oxygen demand than the regional pressure-sarcomere length area.

Asymmetrical changes in ventricular wall mass by asynchronous electrical activation of the heart

Ventricular pacing causes asynchronous electrical activation of the ventricular wall, because impulse conduction occurs via muscle fibers rather than via the Purkinje system. Chronic (up to 3 months) ventricular pacing caused about 30% decrease of wall mass in early activated regions but did not change wall mass in late activated regions. These are the first data indicating that chronic asynchronous activation induces asymmetrical structural adaptations. This asymmetry is likely to be evoked by regional differences in contractile work, as demonstrated in previous experiments from our laboratory. The nature of the structural adaptation as well as its clinical implications deserve more detailed investigation.