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

Differentiation between left bundle branch block (LBBB) preceded dilated cardiomyopathy and dilated cardiomyopathy preceded LBBB by cardiac magnetic resonance imaging

BACKGROUND

Dilated cardiomyopathy (DCM) may be a result of or the cause of left bundle branch block (LBBB) in patients with DCM and LBBB. It is almost impossible from the history alone to know which came first in clinical work.

METHODS

Patients with LBBB and DCM who had cardiac magnetic resonance (CMR) examination were analyzed. Occurrence sequence of DCM and LBBB was determined by chart reviewing. Diastolic lateral/septal wall thickness ratio (DLSWTR) and lateral wall thickening (LWT) were compared between patients with different time sequences. Response to CRT was analyzed according to medical history and CMR manifestation.

RESULTS

Sixty-three patients were divided into two groups by cluster analysis. DLSWTR and LWT were significantly higher in group 1 (preserved lateral wall thickness and function), compared to those in group 2 (reduced lateral wall thickness and function) (1.06 ± 0.13 vs. 0.8 ± 0.12, 34.57 ± 11.48% vs. 11.18 ± 5.56%, respectively, both P < .001). Occurrence sequence was clear in 14 patients and further analyzed. In group 1, seven patients were clearly having no evidence of DCM when LBBB was first diagnosed (defined as LBBB-precede-DCM) and in group 2, seven patients did not have LBBB when DCM was diagnosed (defined as DCM-precede-LBBB). Among 10 patients who received CRT therapy, all seven patients in group 1 responded well whereas none of three patients in group 2 responded well.

CONCLUSIONS

Occurrence sequence of DCM and LBBB can be discriminated by CMR. Preserved lateral wall morphology and function in CMR suggested LBBB preceded to DCM. Such features may be predictors of good response to CRT.

Left bundle branch block increases left ventricular diastolic pressure during tachycardia due to incomplete relaxation

We investigated whether tachycardia in left bundle branch block (LBBB) decreases left ventricular (LV) diastolic distensibility and increases diastolic pressures due to incomplete relaxation, and if cardiac resynchronization therapy (CRT) modifies this response. Thirteen canines were studied at baseline heart rate (120 beats/min) and atrial paced tachycardia (180 beats/min) before and after induction of LBBB and during CRT. LV and left atrial pressures (LAP) were measured by micromanometers and dimensions by sonomicrometry. The time constant τ of exponential pressure decay and degree of incomplete relaxation at mitral valve opening (MVO) and end diastole (ED) based on extrapolation of the exponential decay were assessed. Changes in LV diastolic distensibility were investigated using the LV transmural pressure-volume (PV) relation. LBBB caused prolongation of τ (P < 0.03) and increased the degree of incomplete relaxation during tachycardia at MVO (P < 0.001) and ED (P = 0.08) compared with normal electrical activation. This was associated with decreased diastolic distensibility seen as upward shift of the PV relation at MVO by 18.4 ± 7.0 versus 12.0 ± 5.0 mmHg, at ED by 9.8 ± 2.3 versus 4.7 ± 2.3 mmHg, and increased mean LAP to 11.4 ± 2.7 versus 8.5 ± 2.6 mmHg, all P < 0.006. CRT shifted the LV diastolic PV relation downwards during tachycardia, reducing LAP and LV diastolic pressures (P < 0.03). Tachycardia in LBBB reduced LV diastolic distensibility and increased LV diastolic pressures due to incomplete relaxation, whereas CRT normalized these effects. Clinical studies are needed to determine whether a similar mechanism contributes to dyspnea and exercise intolerance in LBBB and if effects of CRT are heart rate dependent.NEW & NOTEWORTHY Compared with normal electrical conduction, tachycardia in left bundle branch block resulted in incomplete relaxation during filling, particularly of the late activated left ventricular lateral wall. This further resulted in reduced left ventricular diastolic distensibility and elevated diastolic pressures and thus amplified the benefits of cardiac resynchronization therapy in this setting.

Detection of Regional Mechanical Activation of the Left Ventricular Myocardium Using High Frame Rate Ultrasound Imaging

We have investigated the feasibility of noninvasive mapping of mechanical activation patterns in the left ventricular (LV) myocardium using high frame rate ultrasound imaging for the purpose of detecting conduction abnormalities. Five anesthetized, open-chest dogs with implanted combined sonomicrometry and electromyography (EMG) crystals were studied. The animals were paced from the specified locations of the heart, while crystal and ultrasound data were acquired. Isochrone maps of the mechanical activation patterns were generated from the ultrasound data using a novel signal processing method called clutter filter wave imaging (CFWI). The isochrone maps showed the same mechanical activation pattern as the sonomicrometry crystals in 90% of the cases. For electrical activation, the activation sequences from ultrasound were the same in 92% of the cases. The coefficient of determination between the activation delay measured with EMG and ultrasound was R ² = 0.79 , indicating a strong correlation. These results indicate that high frame rate ultrasound imaging processed with CFWI has the potential to be a valuable tool for mechanical activation detection.

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.

Comprehensive MRI for the detection of subtle alterations in diastolic cardiac function in apoE/LDLR(-/-) mice with advanced atherosclerosis

ApoE/LDLR(-/-) mice represent a reliable model of atherosclerosis. However, it is not clear whether cardiac performance is impaired in this murine model of atherosclerosis. Here, we used MRI to characterize cardiac performance in vivo in apoE/LDLR(-/-) mice with advanced atherosclerosis. Six-month-old apoE/LDLR(-/-) mice and age-matched C57BL/6J mice (control) were examined using highly time-resolved cine-MRI [whole-chamber left ventricle (LV) imaging] and MR tagging (three slices: basal, mid-cavity and apical). Global and regional measures of cardiac function included LV volumes, kinetics, time-dependent parameters, strains and rotations. Histological analysis was performed using OMSB (orceine with Martius, Scarlet and Blue) and ORO (oil red-O) staining to demonstrate the presence of advanced coronary atherosclerosis. MR-tagging-based strain analysis in apoE/LDLR(-/-) mice revealed an increased frequency of radial and circumferential systolic stretch (25% and 50% of segments, respectively, p ≤ 0.012), increased radial post-systolic strain index (45% of segments, p = 0.009) and decreased LV untwisting rate (-30.3° (11.6°)/cycle, p = 0.004) when compared with control mice. Maximal strains and LV twist were unchanged. Most of the cine-MRI-based LV functional and anatomical parameters also remained unchanged in apoE/LDLR(-/-) mice, with only a lower filling rate, longer filling time, shorter isovolumetric contraction time and slower heart rate observed in comparison with control mice. The coronary arteries displayed severe atherosclerosis, as evidenced by histological analysis. Using comprehensive MRI methods, we have demonstrated that, despite severe coronary atherosclerosis in six-month-old apoE/LDLR(-/-) mice, cardiac performance including global parameters, twist and strains, was well preserved. Only subtle diastolic alterations, possibly of ischemic background, were uncovered. Copyright © 2016 John Wiley & Sons, Ltd.

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.

Ionic bases for electrical remodeling of the canine cardiac ventricle

Emerging evidence suggests that ventricular electrical remodeling (VER) is triggered by regional myocardial strain via mechanoelectrical feedback mechanisms; however, the ionic mechanisms underlying strain-induced VER are poorly understood. To determine its ionic basis, VER induced by altered electrical activation in dogs undergoing left ventricular pacing (n = 6) were compared with unpaced controls (n = 4). Action potential (AP) durations (APDs), ionic currents, and Ca(2+) transients were measured from canine epicardial myocytes isolated from early-activated (low strain) and late-activated (high strain) left ventricular regions. VER in the early-activated region was characterized by minimal APD prolongation, but marked attenuation of the AP phase 1 notch attributed to reduced transient outward K(+) current. In contrast, VER in the late-activated region was characterized by significant APD prolongation. Despite marked APD prolongation, there was surprisingly minimal change in ion channel densities but a twofold increase in diastolic Ca(2+). Computer simulations demonstrated that changes in sarcolemmal ion channel density could only account for attenuation of the AP notch observed in the early-activated region but failed to account for APD remodeling in the late-activated region. Furthermore, these simulations identified that cytosolic Ca(2+) accounted for APD prolongation in the late-activated region by enhancing forward-mode Na(+)/Ca(2+) exchanger activity, corroborated by increased Na(+)/Ca(2+) exchanger protein expression. Finally, assessment of skinned fibers after VER identified altered myofilament Ca(2+) sensitivity in late-activated regions to be associated with increased diastolic levels of Ca(2+). In conclusion, we identified two distinct ionic mechanisms that underlie VER: 1) strain-independent changes in early-activated regions due to remodeling of sarcolemmal ion channels with no changes in Ca(2+) handling and 2) a novel and unexpected mechanism for strain-induced VER in late-activated regions in the canine arising from remodeling of sarcomeric Ca(2+) handling rather than sarcolemmal ion channels.

Kinematics of cardiac growth: in vivo characterization of growth tensors and strains

Progressive growth and remodeling of the left ventricle are part of the natural history of chronic heart failure and strong clinical indicators for survival. Accompanied by changes in cardiac form and function, they manifest themselves in alterations of cardiac strains, fiber stretches, and muscle volume. Recent attempts to shed light on the mechanistic origin of heart failure utilize continuum theories of growth to predict the maladaptation of the heart in response to pressure or volume overload. However, despite a general consensus on the representation of growth through a second order tensor, the precise format of this growth tensor remains unknown. Here we show that infarct-induced cardiac dilation is associated with a chronic longitudinal growth, accompanied by a chronic thinning of the ventricular wall. In controlled in vivo experiments throughout a period of seven weeks, we found that the lateral left ventricular wall adjacent to the infarct grows longitudinally by more than 10%, thins by more than 25%, lengthens in fiber direction by more than 5%, and decreases its volume by more than 15%. Our results illustrate how a local loss of blood supply induces chronic alterations in structure and function in adjacent regions of the ventricular wall. We anticipate our findings to be the starting point for a series of in vivo studies to calibrate and validate constitutive models for cardiac growth. Ultimately, these models could be useful to guide the design of novel therapies, which allow us to control the progression of heart failure.

In vivo validation of longitudinal-circumferential area change ratio to estimate myofiber shortening in the heart

The aim of this paper was to validate area change ratio (%AC) against myofiber shortening (%λ(f)) in the heart in vivo. %AC is emerging as a mechanical index that may approximate %λ(f) by incorporating both circumferential and longitudinal shortening. However, the physiological significance of % AC remains unclear. We studied the time course of %AC in the anterior midleft ventricular wall of normal canine heart in vivo (n = 14) during atrial pacing over the entire cardiac cycle using transmurally implanted markers and biplane cineradiography (8 ms/frame). %AC was calculated as the myocardial area change relative to the elemental material area on the circumferential-longitudinal plane at the reference configuration (=end diastole). %AC was compared with %λ(f) that was determined from the transmural fiber orientation directly measured in the heart tissue. The time course of both %AC and %λ(f) was determined in the subepicardial, midwall, and subendocardial layers. The time course of %AC and %λ(f) was significantly different, and the difference was more pronounced towards the endocardium. %AC consistently overestimated %λ(f). The timing of the peak %AC was significantly delayed compared to that of the peak %λ(f). We conclude that %AC is significantly different from %λ(f) both in magnitude and timing in vivo. %AC overestimates %λ(f), and the overestimation is worse toward the endocardial layers. This may be a potentially important limitation when applying %AC to optimization and responder identification for cardiac resynchronization therapy.

Mechanism of prolonged electromechanical delay in late activated myocardium during left bundle branch block

During left bundle branch block (LBBB), electromechanical delay (EMD), defined as time from regional electrical activation (REA) to onset shortening, is prolonged in the late-activated left ventricular lateral wall compared with the septum. This leads to greater mechanical relative to electrical dyssynchrony. The aim of this study was to determine the mechanism of the prolonged EMD. We investigated this phenomenon in an experimental LBBB dog model (n = 7), in patients (n = 9) with biventricular pacing devices, in an in vitro papillary muscle study (n = 6), and a mathematical simulation model. Pressures, myocardial deformation, and REA were assessed. In the dogs, there was a greater mechanical than electrical delay (82 ± 12 vs. 54 ± 8 ms, P = 0.002) due to prolonged EMD in the lateral wall vs. septum (39 ± 8 vs.11 ± 9 ms, P = 0.002). The prolonged EMD in later activated myocardium could not be explained by increased excitation-contraction coupling time or increased pressure at the time of REA but was strongly related to dP/dt at the time of REA (r = 0.88). Results in humans were consistent with experimental findings. The papillary muscle study and mathematical model showed that EMD was prolonged at higher dP/dt because it took longer for the segment to generate active force at a rate superior to the load rise, which is a requirement for shortening. We conclude that, during LBBB, prolonged EMD in late-activated myocardium is caused by a higher dP/dt at the time of activation, resulting in aggravated mechanical relative to electrical dyssynchrony. These findings suggest that LV contractility may modify mechanical dyssynchrony.

Improvement in pump function with endocardial biventricular pacing increases with activation time at the left ventricular pacing site in failing canine hearts

Recently, attention has been focused on comparing left ventricular (LV) endocardial (ENDO) with epicardial (EPI) pacing for cardiac resynchronization therapy. However, the effects of ENDO and EPI lead placement at multiple sites have not been studied in failing hearts. We hypothesized that differences in the improvement of ventricular function due to ENDO vs. EPI pacing in dyssynchronous (DYSS) heart failure may depend on the position of the LV lead in relation to the original activation pattern. In six nonfailing and six failing dogs, electrical DYSS was created by atrioventricular sequential pacing of the right ventricular apex. ENDO was compared with EPI biventricular pacing at five LV sites. In failing hearts, increases in the maximum rate of LV pressure change (dP/dt; r = 0.64), ejection fraction (r = 0.49), and minimum dP/dt (r = 0.51), relative to DYSS, were positively correlated (P < 0.01) with activation time at the LV pacing site during ENDO but not EPI pacing. ENDO pacing at sites with longer activation delays led to greater improvements in hemodynamic parameters and was associated with an overall reduction in electrical DYSS compared with EPI pacing (P < 0.05). These findings were qualitatively similar for nonfailing hearts. Improvement in hemodynamic function increased with activation time at the LV pacing site during ENDO but not EPI pacing. At the anterolateral wall, end-systolic transmural function was greater with local ENDO compared with EPI pacing. ENDO pacing and intrinsic activation delay may have important implications for management of DYSS heart failure.

Active contraction of cardiac muscle: in vivo characterization of mechanical activation sequences in the beating heart

Progressive alterations in cardiac wall strains are a classic hallmark of chronic heart failure. Accordingly, the objectives of this study are to establish a baseline characterization of cardiac strains throughout the cardiac cycle, to quantify temporal, regional, and transmural variations of active fiber contraction, and to identify pathways of mechanical activation in the healthy beating heart. To this end, we insert two sets of twelve radiopaque beads into the heart muscle of nine sheep; one in the anterior-basal and one in the lateral-equatorial left ventricular wall. During three consecutive heartbeats, we record the bead coordinates via biplane videofluoroscopy. From the resulting four-dimensional data sets, we calculate the temporally and transmurally varying Green-Lagrange strains in the anterior and lateral wall. To quantify active contraction, we project the strains onto the local muscle fiber directions. We observe that mechanical activation is initiated at the endocardium slightly after end diastole and progresses transmurally outward, reaching the epicardium slightly before end systole. Accordingly, fibers near the outer wall are in contraction for approximately half of the cardiac cycle while fibers near the inner wall are in contraction almost throughout the entire cardiac cycle. In summary, cardiac wall strains display significant temporal, regional, and transmural variations. Quantifying wall strain profiles might be of particular clinical significance when characterizing stages of left ventricular remodeling, but also of engineering relevance when designing new biomaterials of similar structure and function.

The role of the Frank-Starling law in the transduction of cellular work to whole organ pump function: a computational modeling analysis

We have developed a multi-scale biophysical electromechanics model of the rat left ventricle at room temperature. This model has been applied to investigate the relative roles of cellular scale length dependent regulators of tension generation on the transduction of work from the cell to whole organ pump function. Specifically, the role of the length dependent Ca(2+) sensitivity of tension (Ca(50)), filament overlap tension dependence, velocity dependence of tension, and tension dependent binding of Ca(2+) to Troponin C on metrics of efficient transduction of work and stress and strain homogeneity were predicted by performing simulations in the absence of each of these feedback mechanisms. The length dependent Ca(50) and the filament overlap, which make up the Frank-Starling Law, were found to be the two dominant regulators of the efficient transduction of work. Analyzing the fiber velocity field in the absence of the Frank-Starling mechanisms showed that the decreased efficiency in the transduction of work in the absence of filament overlap effects was caused by increased post systolic shortening, whereas the decreased efficiency in the absence of length dependent Ca(50) was caused by an inversion in the regional distribution of strain.

Changes in regional myocardial volume during the cardiac cycle: implications for transmural blood flow and cardiac structure

Although previous studies report a reduction in myocardial volume during systole, myocardial volume changes during the cardiac cycle have not been quantitatively analyzed with high spatiotemporal resolution. We studied the time course of myocardial volume in the anterior mid-left ventricular (LV) wall of normal canine heart in vivo (n = 14) during atrial or LV pacing using transmurally implanted markers and biplane cineradiography (8 ms/frame). During atrial pacing, there was a significant transmural gradient in maximum volume decrease (4.1, 6.8, and 10.3% at subepi, midwall, and subendo layer, respectively, P = 0.002). The rate of myocardial volume increase during diastole was 4.7 +/- 5.8, 6.8 +/- 6.1, and 10.8 +/- 7.7 ml.min(-1).g(-1), respectively, which is substantially larger than the average myocardial blood flow in the literature measured by the microsphere method (0.7-1.3 ml.min(-1).g(-1)). In the early activated region during LV pacing, myocardial volume began to decrease before the LV pressure upstroke. We conclude that the volume change is greater than would be estimated from the known average transmural blood flow. This implies the existence of blood-filled spaces within the myocardium, which could communicate with the ventricular lumen. Our data in the early activated region also suggest that myocardial volume change is caused not by the intramyocardial tissue pressure but by direct impingement of the contracting myocytes on the microvasculature.

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.