Exploring the Electrophysiologic and Hemodynamic Effects of Cardiac Resynchronization Therapy: From Bench to Bedside and Vice Versa.

Cardiac resynchronization therapy (CRT) is an important therapy for heart failure patients with prolonged QRS duration. In patients with left bundle branch block the altered left ventricular electrical activation results in dyssynchronous, inefficient contraction of the left ventricle. CRT aims to reverse these changes and to improve cardiac function. This article explores the electrophysiologic and hemodynamic changes that occur during CRT in patient and animal studies. It also addresses how novel techniques, such as multipoint and endocardial pacing, can further improve the electromechanical response.

Cardiac activation-repolarization patterns and ion channel expression mapping in intact isolated normal human hearts.

BACKGROUND: The repolarization pattern of the human heart is unknown. OBJECTIVE: The purpose of this study was to perform a multisite analysis of the activation-repolarization patterns and mRNA expression patterns of ion channel subunits in isolated human hearts. METHODS: Hearts from 3 donors without reported cardiac disease were Langendorff perfused with the patient's own blood. A standard ECG was obtained before explantation. Up to 92 unipolar electrograms from 24 transmural needles were obtained during right atrial pacing. Local activation and repolarization times and activation-recovery intervals (ARI) were measured. The mRNA levels of subunits of the channels carrying the transient outward current and slow and rapid components of the delayed rectifier current were determined by quantitative reverse transcriptase polymerase chain reaction at up to 63 sites. RESULTS: The repolarization gradients in the 3 hearts were different and occurred along all axes without midmural late repolarization. A negative activation-repolarization relationship occurred along the epicardium, but this relationship was positive in the whole hearts. Coefficients of variation of mRNA levels (40%-80%) and of the Kv7.1 protein (alpha-subunit slow delayed rectifier channel) were larger than those of ARIs (7%-17%). The regional mRNA expression patterns were similar in the 3 hearts, unlike the ARI profiles. The expression level of individual mRNAs and of Kv7.1 did not correlate with local ARIs at the same sites. CONCLUSION: In the normal human heart, repolarization gradients encompass all axes, without late midmural repolarization. Last activated areas do not repolarize first as previously assumed. Gradients of mRNAs of single ion channel subunits and of ARIs do not correlate.

Percutaneous microembolization of the left coronary artery to model ischemic heart disease in rats.

Small animal models of myocardial infarction are used for a wide variety of research purposes, but common techniques for generating such models require thoracic surgeries that increase mortality risk and damage important structures, such as the pericardial sac. Here, we describe a technique for modeling myocardial infarction in rats by selective coronary microembolization, which has hitherto been described only in large animals. This technique selectively catheterizes the left coronary artery using a custom-made catheter that is introduced and precisely placed under fluoroscopic guidance. Microspheres are then injected through the catheter to cause embolization. This process creates multiple simultaneous micro-infarcts that resemble those from clinical embolization after a percutaneous coronary intervention. As this technique does not require thoracic surgery, a low attrition rate was expected and once it was optimized, this technique had a low mortality rate of just 14% during experimental application. This technique creates infarcts that appear small but are associated with transient ECG changes and a persistently lower ejection fraction after embolization. Microspheres are retained in the myocardial tissue and are visible by epifluorescent microscopy after histological staining and recognizable as a distinct speckle pattern in ultrasound images.

Prevention of adenosine A2A receptor activation diminishes beat-to-beat alternation in human atrial myocytes.

Atrial fibrillation (AF) has been associated with increased spontaneous calcium release from the sarcoplasmic reticulum and linked to increased adenosine A2A receptor (A2AR) expression and activation. Here we tested whether this may favor atrial arrhythmogenesis by promoting beat-to-beat alternation and irregularity. Patch-clamp and confocal calcium imaging was used to measure the beat-to-beat response of the calcium current and transient in human atrial myocytes. Responses were classified as uniform, alternating or irregular and stimulation of Gs-protein coupled receptors decreased the frequency where a uniform response could be maintained from 1.0 ± 0.1 to 0.6 ± 0.1 Hz; p < 0.01 for beta-adrenergic receptors and from 1.4 ± 0.1 to 0.5 ± 0.1 Hz; p < 0.05 for A2ARs. The latter was linked to increased spontaneous calcium release and after-depolarizations. Moreover, A2AR activation increased the fraction of non-uniformly responding cells in HL-1 myocyte cultures from 19 ± 3 to 51 ± 9 %; p < 0.02, and electrical mapping in perfused porcine atria revealed that adenosine induced electrical alternans at longer cycle lengths, doubled the fraction of electrodes showing alternation, and increased the amplitude of alternations. Importantly, protein kinase A inhibition increased the highest frequency where uniform responses could be maintained from 0.84 ± 0.12 to 1.86 ± 0.11 Hz; p < 0.001 and prevention of A2AR-activation with exogenous adenosine deaminase selectively increased the threshold from 0.8 ± 0.1 to 1.2 ± 0.1 Hz; p = 0.001 in myocytes from patients with AF. In conclusion, A2AR-activation promotes beat-to-beat irregularities in the calcium transient in human atrial myocytes, and prevention of A2AR activation may be a novel means to maintain uniform beat-to-beat responses at higher beating frequencies in patients with atrial fibrillation.

Exploring the Electrophysiologic and Hemodynamic Effects of Cardiac Resynchronization Therapy: From Bench to Bedside and Vice Versa.

Cardiac resynchronization therapy (CRT) is an important therapy for heart failure patients with prolonged QRS duration. In patients with left bundle branch block the altered left ventricular electrical activation results in dyssynchronous, inefficient contraction of the left ventricle. CRT aims to reverse these changes and to improve cardiac function. This article explores the electrophysiologic and hemodynamic changes that occur during CRT in patient and animal studies. It also addresses how novel techniques, such as multipoint and endocardial pacing, can further improve the electromechanical response.

New electrocardiographic criteria to differentiate acute pericarditis and myocardial infarction.

OBJECTIVE: Transmural myocardial ischemia induces changes in QRS complex and QT interval duration but, theoretically, these changes might not occur in acute pericarditis provided that the injury is not transmural. This study aims to assess whether QRS and QT duration permit distinguishing acute pericarditis and acute transmural myocardial ischemia. METHODS: Clinical records and 12-lead electrocardiogram (ECG) at ×2 magnification were analyzed in 79 patients with acute pericarditis and in 71 with acute ST-segment elevation myocardial infarction (STEMI). RESULTS: ECG leads with maximal ST-segment elevation showed longer QRS complex and shorter QT interval than leads with isoelectric ST segment in patients with STEMI (QRS: 85.9 ± 13.6 ms vs 81.3 ± 10.4 ms, P = .01; QT: 364.4 ± 38.6 vs 370.9 ± 37.0 ms, P = .04), but not in patients with pericarditis (QRS: 81.5 ± 12.5 ms vs 81.0 ± 7.9 ms, P = .69; QT: 347.9 ± 32.4 vs 347.3 ± 35.1 ms, P = .83). QT interval dispersion among the 12-ECG leads was greater in STEMI than in patients with pericarditis (69.8 ± 20.8 ms vs 50.6 ± 20.2 ms, P <.001). The diagnostic yield of classical ECG criteria (PR deviation and J point level in lead aVR and the number of leads with ST-segment elevation, ST-segment depression, and PR-segment depression) increased significantly (P = .012) when the QRS and QT changes were added to the diagnostic algorithm. CONCLUSIONS: Patients with acute STEMI, but not those with acute pericarditis, show prolongation of QRS complex and shortening of QT interval in ECG leads with ST-segment elevation. These new findings may improve the differential diagnostic yield of the classical ECG criteria.

Electrophysiological changes in heart failure and their implications for arrhythmogenesis.

Heart failure is the final common pathway of various cardiac pathologies and is associated with sudden cardiac death, mostly caused by ventricular arrhythmias. In this paper we briefly review the electrophysiological remodeling and the alterations in intracellular calcium handling, and the resulting arrhythmogenic mechanisms associated with heart failure. Intercellular uncoupling and fibrosis are identified as a major arrhythmogenic factors. Diet and ventricular wall stretch are discussed as modulating factors. Finally, emphasis is placed on the hitherto poorly studied aspects of right ventricular failure. This article is part of a Special Issue entitled: Heart failure pathogenesis and emerging diagnostic and therapeutic interventions.

Changes in QRS duration and R-wave amplitude in electrocardiogram leads with ST segment elevation differentiate epicardial and transmural myocardial injury.

BACKGROUND: Acute transmural ischemia increases QRS duration and R-wave amplitude owing to depressed intramyocardial activation. Theoretically, when myocardial injury is confined to the epicardium, the intramyocardial activation is preserved without affecting QRS duration. OBJECTIVE: The purpose of this study was to distinguish epicardial from transmural myocardial injury based on the analysis of the QRS complex in leads with ST segment elevation. METHODS: Electrophysiological effects of epicardial injury induced by topical application (n = 7) or intrapericardial injection (n = 10) of potassium were assessed in pigs in local electrograms recorded with needles in the left ventricle and in the peripheral 12-lead electrocardiogram (ECG), respectively, and were compared with transmural injury induced by acute left anterior descending (LAD) occlusion in the same pig. RESULTS: Epicardial application of 50 mM potassium induced ST segment elevation in epicardial (0.2 ± 0.06 to 0.5 ± 0.09 mV; P <.05) but not in midmyocardial local electrograms (0.1 ± 0.07 to -0.1 ± 0.09 mV). Local midmyocardial activation times were not affected by epicardial applied potassium (182 ± 5.9 vs. 183 ± 5.8 ms) but increased significantly during acute LAD occlusion (246 ± 20.9 ms; P <.01). Intrapericardial injected potassium induced ST segment elevation on average in nine of 12 ECG leads but did not change QRS duration and R-wave amplitude. Acute LAD occlusion induced ST segment elevation (five of 12 leads) associated with increased QRS duration (69 ± 1.2 to 89 ± 3.6 ms; P <.001) and R-wave amplitude (0.1 ± 0.01 to 0.7 ± 0.09 mV; P <.001) in the ECG. CONCLUSION: Transmural but not epicardial myocardial injury delays intramural local activation and is associated with QRS prolongation and enlarged R-wave amplitude in leads with ST segment elevation. This differential ECG pattern may help to distinguish acute pericarditis (epicardial injury) from acute transmural ischemia in clinical practice.

The effect of enhanced gap junctional conductance on ventricular conduction in explanted hearts from patients with heart failure.

AIM: To investigate ventricular conduction and refractoriness before and after application of rotigaptide, an enhancer of gap junctional conductance, to explanted hearts of patients with heart failure (HF). METHODS AND RESULTS: In six explanted perfused hearts of patients with end-stage HF, activation/repolarization mapping was performed and refractory periods (RPs) and activation recovery intervals (ARIs) were measured before and after application of 50 nM rotigaptide. Rotigaptide caused a decrease of RP from 476 +/- 36 to 453 +/- 31 ms (P < 0.05), but did not change ARI-dispersion. During premature activation along the fibers rotigaptide decreased the minimal activation time (AT(min)) and maximal activation time (AT(max)) significantly from 35 +/- 12 to 24 +/- 9 and from 97 +/- 38 to 43 +/- 7 ms, respectively. Rotigaptide did not change AT(min) and AT(max) during activation perpendicular to the fiber direction. After application of rotigaptide conduction curves normalized in five/six recordings when activation was parallel, but destabilized in three/six hearts when activation was perpendicular to fiber direction. The destabilization was associated with local conduction delays rather than with facilitation of conduction. CONCLUSION: Rotigaptide applied to hearts of patients with end-stage HF shortened RPs normalized conduction curves and increased conduction parallel to fiber direction. However, in 50% of the hearts local slowing of conduction with destabilization of conduction (curves) occurs at sites close to the stimulation site, when activation is perpendicular to fiber direction.

Force frequency relationship of the human ventricle increases during early postnatal development.

Understanding developmental changes in contractility is critical to improving therapies for young cardiac patients. Isometric developed force was measured in human ventricular muscle strips from two age groups: newborns (<2 wk) and infants (3-14 mo) undergoing repair for congenital heart defects. Muscle strips were paced at several cycle lengths (CLs) to determine the force frequency response (FFR). Changes in Na/Ca exchanger (NCX), sarcoplasmic reticulum Ca-ATPase (SERCA), and phospholamban (PLB) were characterized. At CL 2000 ms, developed force was similar in the two groups. Decreasing CL increased developed force in the infant group to 131 +/- 8% (CL 1000 ms) and 157 +/- 18% (CL 500 ms) demonstrating a positive FFR. The FFR in the newborn group was flat. NCX mRNA and protein levels were significantly larger in the newborn than infant group whereas SERCA levels were unchanged. PLB mRNA levels and PLB/SERCA ratio increased with age. Immunostaining for NCX in isolated newborn cells showed peripheral staining. In infant cells, NCX was also found in T-tubules. SERCA staining was regular and striated in both groups. This study shows for the first time that the newborn human ventricle has a flat FFR, which increases with age and may be caused by developmental changes in calcium handling.

Transmural dispersion of refractoriness and conduction velocity is associated with heterogeneously reduced connexin43 in a rabbit model of heart failure.

BACKGROUND: Heterogeneity of repolarization and conduction is a potential source of arrhythmogenesis. In heart failure (HF), intercellular coupling is reduced and heterogeneities may become evident because of reduced intercellular coupling. OBJECTIVE: This study sought to investigate connexin43 (Cx43) expression, conduction velocity (CV), refractoriness and inducibility of arrhythmias at multiple sites of the left ventricle during HF. METHODS: HF was induced by pressure-volume overload in rabbits. Epicardial and intramural mapping was performed in isolated perfused hearts following programmed stimulation. Myocytes were enzymatically dissociated and studied using D-4-ANEPPS fluorescence. Western blotting and immunohistochemistry was performed to quantify heterogeneity of Cx43 expression. RESULTS: Cx43 was heterogeneously reduced in the midmyocardial, but not in the sub epicardium layer of the left ventricular free wall in HF compared to control rabbits. In HF, subepicardial and midmyocardial refractory periods (RPs) were increased compared to control rabbits (148 +/- 3 ms and 143 +/- 3 versus 131 +/- 2 and 129 +/- 2 ms, respectively, both P < 0.001). Also, transmural dispersion of RPs was larger in HF (30 +/- 4 ms) than in control rabbits (24 +/- 3 ms, P < 0.05). Intrinsic dispersion of action potential duration in isolated myocytes was similar in HF and control rabbits. Transmural CV was heterogeneous, although the mean CV was not different between groups. Arrhythmias were more easily inducible in HF, especially from midmyocardium. CONCLUSION: In HF, midmyocardial Cx43 expression is heterogeneously reduced. This is associated with increased transmural dispersion in refractoriness and conduction, and with increased arrhythmia inducibility.

Dopamine increases L-type calcium current more in newborn than adult rabbit cardiomyocytes via D1 and beta2 receptors.

Dopamine is used to treat heart failure, particularly after cardiac surgery in infants, but the mechanisms of action are unclear. We investigated differences in the effect of dopamine on L-type calcium current (I(Ca)) between newborn (NB, 1-4 days) and adult (AD, 3-4 mo) rabbit ventricular myocytes. Myocytes were enzymatically dissociated from NB and AD rabbit hearts. I(Ca) was recorded by using the whole cell patch-clamp technique. mRNA levels of cardiac dopamine receptor type 1 (D1), type 2 (D2), and beta-adrenergic receptors (beta-ARs) were measured by real-time RT-PCR. Dopamine (100 microM) increased I(Ca) more in NB (E(max) 87 +/- 10%) than in AD ventricular cells (E(max) 21 +/- 3%). Further investigation of this difference showed that mRNA levels of the D1 receptor were significantly higher in NB, and, with beta-AR blockade, dopamine increased I(Ca) more in NB than AD cells. Additionally, SKF-38393 (selective D1 receptor agonist) significantly increased I(Ca) by 55 +/- 4% in NB (P < 0.05, n = 4) and by 11 +/- 1% in AD (P < 0.05, n = 6). Dopamine in the presence of SCH-23390 (D1 receptor antagonist) increased I(Ca) in NB cells by 67 +/- 5% and by 22 +/- 2% in AD cells, suggesting a role for beta-AR stimulation. Selective blockade of beta(1)- or beta(2)-receptors (with block of D1 receptors) showed that the beta-AR action of dopamine in the NB was largely mediated via beta(2)-AR activation. Dopamine produces a larger increase in I(Ca) in NB cardiomyocytes compared with ADs. The mechanism of action is not only through beta(2)-ARs but also due to higher expression of cardiac D1 receptor in NB.

Larger cell size in rabbits with heart failure increases myocardial conduction velocity and QRS duration.

BACKGROUND: Patients with heart failure (HF) have an increased QRS duration, usually attributed to decreased conduction velocity (CV) due to ionic remodeling but which may alternatively result from increased heart size or cellular uncoupling. We investigated the relationship between QRS width, heart size, intercellular coupling, and CV in a rabbit model of moderate HF and in computer simulations. METHODS AND RESULTS: HF was induced by pressure-volume overload. Heart weight (21.1+/-0.5 versus 10.2+/-0.4 g, mean+/-SEM; P<0.01) and QRS duration (58+/-1 versus 50+/-1 ms; P<0.01) were increased in HF versus control. Longitudinal CV (thetaL; 79+/-2 versus 67+/-4 cm/s; P<0.01) and transversal subepicardial CV (thetaT; 43+/-2 versus 37+/-2 cm/s; P<0.05) were higher in HF than in controls. Transmural CV (thetaTM) was unchanged (25+/-2 versus 24+/-1 cm/s; P=NS). Patch-clamp experiments demonstrated that sodium current was unchanged in HF versus control. Immunohistochemical experiments revealed that connexin43 content was reduced in midmyocardium but unchanged in subepicardium. Myocyte dimensions were increased in HF by approximately 30%. Simulated strands of mammalian ventricular cells (Luo-Rudy dynamic model) revealed increased thetaL and thetaT with increased myocyte size; however, increased CV could not compensate for increased strand size of longitudinally coupled cells, and consequently, total activation time was longer. CONCLUSIONS: Increased myocyte size combined with the observed expression pattern of connexin43 yields increased thetaL and thetaT and unchanged thetaTM in our nonischemic model of HF. A hypertrophied left ventricle together with insufficiently increased thetaL and unaltered thetaTM results in a prolonged QRS duration.

Conduction slowing by the gap junctional uncoupler carbenoxolone.

BACKGROUND: Cellular electrical coupling is essential for normal propagation of the cardiac action potential, whereas reduced electrical coupling is associated with arrhythmias. Known cellular uncoupling agents have severe side effects on membrane ionic currents. We investigated the effect of carbenoxolone on cellular electrical coupling, membrane ionic currents, and atrial and ventricular conduction. METHODS AND RESULTS: In isolated rabbit left ventricular and right atrial myocytes, carbenoxolone (50 micromol/l) had no effect on action potential characteristics. Calcium, potassium, and sodium currents remained unchanged. Dual current clamp experiments on poorly coupled cell pairs revealed a 21+/-3% decrease in coupling conductance by carbenoxolone (mean+/-S.E.M., n=4, p<0.05). High-density activation mapping was performed in intact rabbit atrium and ventricle during Langendorff perfusion of the heart. The amplitude of the Laplacian of the electrograms, a measure of coupling current in intact hearts, decreased from 1.45+/-0.66 to 0.75+/-0.51 microA/mm(3) (mean+/-SD, n=32, p<0.05) after 15 min of carbenoxolone. Carbenoxolone reversibly decreased longitudinal and transversal conduction velocity from 66+/-15 to 49+/-16 cm/s and from 50+/-14 to 35+/-15 cm/s in ventricle, respectively (mean+/-SD, n=5, both p<0.05). In atrium, longitudinal and transversal conduction velocity decreased from 80+/-29 to 60+/-16 cm/s and from 49+/-10 to 38+/-10 cm/s (mean+/-SD, n=8, both p<0.05). CONCLUSIONS: Carbenoxolone-induced uncoupling causes atrial and ventricular conduction slowing without affecting cardiac membrane currents. Activation delay is larger in poorly coupled cells.

Remodeling of gap junctions in mouse hearts hypertrophied by forced retinoic acid signaling.

BACKGROUND: Beta-MHC-hRARalpha transgenic mice express a constitutively active (truncated) form of the human retinoic acid receptor which triggers development of dilated cardiomyopathy. In those hearts, we studied expression of gap junction proteins in relation to electrical impulse propagation. METHODS AND RESULTS: As compared to wildtype mice, hearts of 4-6 month old mice with 7-12 inserted hRARalpha copies are marked by an increased heart weight/body weight- and heart weight/tibia length ratio. 3-extremity lead ECGs revealed prolongation of the Q-j interval suggesting delayed ventricular activation. Mapping of electrical activity of epi- and endocardial left ventricular free wall revealed activation delay, increased heterogeneity in conduction and regional conduction block. Ventricular tachycardias did not occur spontaneously nor could be induced by ventricular pacing. Immunohistochemical analysis showed profound and heterogeneous redistribution and down-regulation of the gap junction protein connexin43 (Cx43) in the left ventricular free wall. Here, hRARalpha expression induced re-expression of the hypertrophic markers alpha-skeletal actin and beta-MHC, and in 3 out of 10 severely affected mice, re-expression of Cx40. Concomitant with changes in expression/distribution of Cx43, changes in expression and distribution of beta-catenin and N-cadherin (two other intercalated disk associated proteins) were observed. CONCLUSIONS: Beta-MHC-hRARalpha transgenic hearts show heterogeneous re-expression of (early) sarcomeric genes while expression of connexin43, N-cadherin and beta-catenin is down-regulated. We postulate that the resulting aberrant ventricular activation does not trigger development of lethal arrhythmias due to the small size of remaining healthy ventricular tissue where the transgene is not expressed.