Effects of procainamide and sotalol on restitution properties, dispersion of refractoriness, and ventricular fibrillation activation patterns in pigs

Ibutilide Reduces Ventricular Defibrillation Threshold and Organizes Ventricular Fibrillation Activation in Canine Heart Failure Model

PURPOSE

To compare the effects of class III antiarrhythmic agents (amiodarone vs. ibutilide) on ventricular fibrillation (VF) and hemodynamic status in a canine heart failure (HF) model.

METHODS

A total of 12 beagles were used to establish the HF model by rapid pacing for 4 consecutive weeks. These canines were randomly divided into two groups based on the administration of ibutilide and amiodarone. A 12 × 12 unipolar electrode plaque was used for ventricular epicardial mapping, and a 6-electrode plunge needle was inserted for ventricular transmural mapping. The restitution curve was estimated from activation recovery intervals (ARIs) by pacing from the plaque electrodes before and after drug administration. The defibrillation threshold (DFT) and VF activation patterns, including the activation rate, cycle length (VF-CL) and the transmural dispersion of the activation rate, were evaluated and the hemodynamic parameters were mearsured and compared before and after drug administration.

RESULTS

Compared to HF baseline, ibutilide administration has markedly decreased the DFT by 28% (18 ± 2 J vs. 13 ± 2.7 J, P < 0.01) without affecting the canine's hemodynamics (mean arterial pressure 91 ± 15 mmHg vs. 92 ± 17 mmHg, P > 0.05). Furthermore, VF activation pattern became more organized, and spontaneous termination was observed only after ibutilide administration. Conversely, amiodarone has significantly compromised the hemodynamic status (mean arterial pressure 92 ± 6.1 mmHg vs. 52 ± 11.6 mmHg, P < 0.05), but did not alter the DFT (17 ± 2.3 J vs. 16 ± 2.0 J, P > 0.05). Compared to pre-medication, both ibutilide and amiodarone have significantly prolonged the VERP (178 ± 9.6 ms vs. 208 ± 8.9 ms, P < 0.05; 185 ± 10.5 ms vs. 202 ± 7.5 ms, P < 0.05, respectively) and reduced the dispersion of refractoriness, the maximal slope of restitution curve, and the epicardial dispersion during pacing. Additionally, both drugs have significantly increased the VF-CL and reduced the transmural dispersion of the VF activation rate.

CONCLUSIONS

Ibutilide had potential antifibrillatory properties, which was shown by decreasing the DFT and organizing the VF activation in HF, and with no apparent impact on the hemodynamic status. In contrast, intravenous amiodarone administration demonstrated prominent negative effects on the hemodynamic status possibly by affecting the myocardial contractility before and after defibrillation but did not alter the DFT.

Effect of flunarizine on defibrillation outcomes and early refibrillation in a canine model of prolonged ventricular fibrillation

New Findings

What is the central question of this study?

Can successful electrical shock in combination with a delayed after‐depolarization (DAD) blocker suppress early refibrillation episodes following long duration ventricular fibrillation (LDVF)?

What is the main finding and its importance?

Flunarizine significantly reduced the activation of LDVF and early ventricular fibrillation (VF) recurrence following LDVF, suggesting that DADs potentially contribute to refibrillation in prolonged VF. Thus, DAD inhibition can be used as an adjunctive therapy for electrical defibrillation to treat prolonged VF and suppress refibrillation following LDVF.

Abstract

This study attempts to detect changes in the defibrillation threshold (DFT) at different stages of ventricular fibrillation (VF) (short duration VF, SDVF; long duration VF, LDVF) and during early refibrillation following successful defibrillation of LDVF by giving flunarizine, a blocker of delayed after‐depolarizations (DADs). Twelve beagles were divided into two groups (the control group, n = 6; and the flunarizine group, n = 6). Two 64‐electrode basket catheters were deployed into the left and the right ventricles for global endocardium mapping. The DFTs of SDVF and LDVF were determined at 20 s and 7 min, respectively, after VF induction in each group. Any refibrillation episodes were recorded within 15 min after the first successful defibrillation of LDVF. In the flunarizine group, the SDVF‐DFT values before and after the drug were not significantly different. The 7 min LDVF‐DFTs were markedly reduced by 26% (P < 0.05, the control group) and 38% (P < 0.01, the flunarizine group) compared to the 20 s SDVF‐DFTs within each group. The difference between SDVF‐DFT and LDVF‐DFT after flunarizine was larger than that in the control group (213 ± 65 vs. 120 ± 84 V, P < 0.05). The number of refibrillation episodes per animal (1.3 ± 1.0) following successful defibrillation of LDVF after flunarizine was 48% of that in controls (2.7 ± 2.0, P < 0.05). The effect of flunarizine on SDVF‐DFT and LDVF‐DFT indicates that the role of DADs in the defibrillation mechanism may differ as VF continues. Flunarizine significantly reduced early VF recurrence following LDVF, suggesting that DADs potentially contribute to refibrillation in a canine model of prolonged VF.

The effects of interleukin 17A on left stellate ganglion remodeling are mediated by neuroimmune communication in normal structural hearts

BACKGROUND

It is reported interleukin (IL)-17A, a classical proinflammatory cytokine, is implicated in neuroimmune-associated remodeling in neural plasticity and pathological conditions. However, the effect of IL-17A on left stellate ganglion (LSG) remodeling remains unclear.

OBJECTIVE

This study was performed to determine whether exogenous IL-17A promotes LSG remodeling and destabilize ventricular electrophysiological properties (EPs) in normal canines.

METHODS

24 beagles were randomly allocated into three groups. In the first group, animals were subjected to 0.1 ml phosphate buffer saline (PBS) microinjection of into LSG (n = 8), an equivalent IL-17A was administrated in the second group (n = 8), and an equivalent anti-IL-17A mAb plus IL-17A was administrated in the third group (n = 8). The ventricular EPs, neural function and activity of the LSG were determined at baseline and 30 min after administration. In the end, LSG tissues were collected.

RESULTS

Compared with the control group, the experimental group had a significantly shorter effective refractory period (ERP) and action potential duration (APD)₉₀, an increased ERP, APD₉₀, S_max dispersion, and APD alternans cycle length; and steepened APD restitution curves. In addition, IL-17A enhanced the neural function and activity of the LSG, upregulated the expressions of neuropeptides and proinflammatory cytokines and cells. And all these effects were attenuated by anti-IL-17A mAb. Importantly, IL-17 receptor A (IL-17R-A) was detected in sympathetic neurons in the LSG.

CONCLUSION

IL-17A promoted LSG remodeling by regulating the neural inflammation response. It did so by binding to IL-17R-A, resulting in unstable ventricular electrophysiology in normal structural hearts.

Effects of combination of sotalol and verapamil on initiation, maintenance, and termination of ventricular fibrillation in swine hearts

INTRODUCTION

Sotalol and verapamil alone reduce reentry incidence during ventricular fibrillation (VF). We tested whether the combination of these two drugs had a synergistic effect on initiation, maintenance, and termination of VF.

METHODS

Six open-chest pigs received intravenous sotalol (1.5 mg/kg) followed by verapamil (0.136 mg/kg). VF threshold (VFT) was determined by a burst pacing protocol. Two 20 seconds episodes of VF were recorded from a 21 × 24 unipolar electrode plaque on the lateral posterior left ventricular epicardium before and after each drug. VF activation patterns were quantified. The duration of long duration VF (LDVF) maintenance was compared to our previously published data.

RESULTS

Sotalol alone and combined with verapamil significantly increased the VFT from 12.3 ± 4.1 to 20.3 ± 7.1 and 26.7 ± 8.6 mA compared with baseline (P < .05). Sotalol decreased the number of wavefronts by 20%, VF activation rate by 17% and conduction velocity 11%, while the addition of verapamil neutralized these effects. Addition of verapamil to sotalol further decreased the fractionation incidence from 14% to 29% and multiplicity from 24% to 31% compared with baseline. The combination of the two drugs increased the VF cycle length, decreased synchronicity, increased regularity index and shortened the duration of LDVF maintenance compared with our previous data of verapamil alone or no drug. Synchronicity index was lower and regularity index was higher in animals in which VF spontaneously terminated earlier than 10 minutes than in animals in which VF terminated longer than 10 minutes.

CONCLUSION

The combination of sotalol and verapamil increased VFT but accelerated LDVF termination.

KATP channel inhibition blunts electromechanical decline during hypoxia in left ventricular working rabbit hearts

KEY POINTS

Heart function is critically dependent upon the balance of energy production and utilization. Sarcolemmal ATP-sensitive potassium channels (K_ATP channels) in cardiac myocytes adjust contractile function to compensate for the level of available energy. Understanding the activation of K_ATP channels in working myocardium during high-stress situations is crucial to the treatment of cardiovascular disease, especially ischaemic heart disease. Using a new optical mapping approach, we measured action potentials from the surface of excised contracting rabbit hearts to assess when sarcolemmal K_ATP channels were activated during physiologically relevant workloads and during gradual reductions in myocardial oxygenation. We demonstrate that left ventricular pressure is closely linked to K_ATP channel activation and that K_ATP channel inhibition with a low concentration of tolbutamide prevents electromechanical decline when oxygen availability is reduced. As a result, K_ATP channel inhibition probably exacerbates a mismatch between energy demand and energy production when myocardial oxygenation is low.

ABSTRACT

Sarcolemmal ATP-sensitive potassium channel (K_ATP channel) activation in isolated cells is generally understood, although the relationship between myocardial oxygenation and K_ATP activation in excised working rabbit hearts remains unknown. We optically mapped action potentials (APs) in excised rabbit hearts to test the hypothesis that hypoxic changes would be more severe in left ventricular (LV) working hearts (LWHs) than Langendorff (LANG) perfused hearts. We further hypothesized that K_ATP inhibition would prevent those changes. Optical APs were mapped when measuring LV developed pressure (LVDP), coronary flow rate and oxygen consumption in LANG and LWHs. Hearts were paced to increase workload and perfusate was deoxygenated to study the effects of myocardial hypoxia. A subset of hearts was perfused with 1 μm tolbutamide (TOLB) to identify the level of AP duration (APD) shortening attributed to K_ATP channel activation. During sinus rhythm, APD was shorter in LWHs compared to LANG hearts. APD in both LWHs and LANG hearts dropped steadily during deoxygenation. With TOLB, APDs in LWHs were longer at all workloads and APD reductions during deoxygenation were blunted in both LWHs and LANG hearts. At 50% perfusate oxygenation, APD and LVDP were significantly higher in LWHs perfused with TOLB (199 ± 16 ms; 92 ± 5.3 mmHg) than in LWHs without TOLB (109 ± 14 ms, P = 0.005; 65 ± 6.5 mmHg, P = 0.01). Our results indicate that K_ATP channels are activated to a greater extent in perfused hearts when the LV performs pressure-volume work. The results of the present study demonstrate the critical role of K_ATP channels in modulating myocardial function over a wide range of physiological conditions.

Effects of Dantrolene Treatment on Ventricular Electrophysiology and Arrhythmogenesis in Rats With Chronic β-Adrenergic Receptor Activation

Dantrolene, which is primarily used to treat malignant hyperthermia, has recently been suggested for the prevention of arrhythmogenesis in various animal models. In this study, the effects of dantrolene treatment on electrophysiological properties and ventricular arrhythmias (VAs) in rats with chronic β-adrenergic receptor (β-AR) activation were investigated. Rats were randomized to treatment with saline (control group), isoproterenol (ISO; ISO group), or ISO + dantrolene (ID group) for 2 weeks. An electrophysiological study was performed to assess action potential duration restitution (APDR) and induce action potential duration (APD) alternans or VA in vitro. The protein levels of Cav1.2, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), and ryanodine receptor 2 (RyR2) were detected by Western blot. Compared with the control group, chronic administration of ISO significantly increased APD, the maximum slope (Smax) of APDR curve, and the spatial dispersions of Smax and APD (all P < .01), and all effects were attenuated by dantrolene treatment (all P < .05). Additionally, chronic ISO administration significantly reduced the protein levels of SERCA2 and RyR2, but increased the Cav1.2 protein expression (all P < .05). However, compared with the ISO group, dantrolene treatment preserved SERCA2a and RyR2 protein levels and decreased Cav1.2 protein levels in the ID group (all P < .05). The intracellular Ca2+ ([Ca2+]i) levels measured by incubating isolated cardiomyocytes with Fluo-3/alveolar macrophages were significantly increased in the ISO group compared with the control group ( P < .01). Dantrolene treatment markedly reduced the rise of [Ca2+]i levels caused by chronic administration of ISO ( P < .05). Dantrolene treatment also prevented the reductions in the APD alternans and VA thresholds induced by chronic ISO stimulation (all P < .05). These data suggest that dantrolene stabilizes ventricular electrophysiological characteristics and increases the expression of key sarcoplasmic reticulum calcium cycling proteins to reduce vulnerability to VA in rats with chronic β-AR activation.

Verapamil reduces incidence of reentry during ventricular fibrillation in pigs

The characteristics of reentrant circuits during short duration ventricular fibrillation (SDVF; 20 s in duration) and the role of Ca(++) and rapid-activating delayed rectifier potassium currents during long duration ventricular fibrillation (LDVF; up to 10 min in duration) were investigated using verapamil and sotalol. Activation mapping of the LV epicardium with a 21 × 24 electrode plaque was performed in 12 open-chest pigs. Pigs were given either verapamil (0.136 mg/kg) or sotalol (1.5 mg/kg) and verapamil. Reentry patterns were quantified for SDVF, and, for LDVF, activation patterns were compared with our previously reported control LDVF data. Verapamil significantly increased conduction velocity around the reentrant core by 10% and reduced the reentrant cycle length by 15%, with a net reduction in reentry incidence of 70%. Sotolol had an opposite effect of decreasing the conduction velocity around the core by 6% but increasing the reentrant cycle length by 13%, with a net reduction of reentry incidence of 50%. After 200 s of VF, verapamil significantly slowed wavefront conduction velocity and activation rate compared with control data. Verapamil decreased the incidence of reentry in SDVF by accelerating conduction velocity to increase the likelihood of conduction block, possibly through increased sympathetic tone. The drug slowed activation rate and conduction velocity after 200 s of VF, suggesting that L-type Ca(++) channels remain active and may be important in the maintenance of LDVF. Sotalol in addition to verapamil caused no additional antiarrhythmic effect.

Renal sympathetic denervation modulates ventricular electrophysiology and has a protective effect on ischaemia-induced ventricular arrhythmia

Recently, a beneficial effect of renal sympathetic denervation (RSD) has been seen in patients with ventricular electrical storm. However, the effect of RSD on ventricular electrophysiology remains unclear. Thirty-three mongrel dogs were included in the present study. Renal sympathetic denervation was performed by radiofrequency ablation of the adventitial surface of the renal artery. In group 1 (n = 8), programmed stimulation was performed before and after RSD to determine the ventricular effective refractory period (ERP) and action potential duration (APD) restitution properties. The same parameters were measured in five other animals that underwent sham RSD to serve as controls. In group 2 (n = 10), acute myocardial ischaemia (AMI) was induced by ligating the proximal left anterior descending coronary artery after the performance of RSD, and the incidence of ventricular arrhythmia (VA) was calculated during 1 h of recording. In another 10 dogs (group 3), AMI was induced and VA was measured with sham RSD. In group 1, RSD significantly prolonged ventricular ERP and APD, reduced the maximal slope (Smax) of the restitution curve and suppressed APD alternans at each site. Renal sympathetic denervation also significantly decreased the spatial dispersion of ERP, APD and Smax. In the five control animals, no significant electrophysiological change was detected after sham RSD. The occurrence of spontaneous VA during 1 h of AMI in group 2 was significantly lower than that in group 3. These data suggest that RSD stabilizes ventricular electrophysiological properties in normal hearts and reduces the occurrence of VA in hearts experiencing AMI.

The effect of pinacidil on postshock activation and ventricular defibrillation threshold in canine hearts

AIM

To determine the postshock activation patterns with both successful and failed shocks in a canine model of ventricular fibrillation, and whether piniacidil, an early after-depolarization (EAD) inhibitor, altered the defibrillation threshold (DFT) and postshock activation patterns.

METHODS

In 6 beagles, a basket catheter with 64 unipolar electrodes was placed in the LV for global endocardial mapping, a monophasic action potential catheter was inserted into the LV apex, and a catheter with the negative electrode in the right ventricle and the positive electrode in the superior vena cava was inserted for defibrillation. The DFT, 90% action potential duration (APD(90)) and activation recovery interval (ARI) were evaluated before and after pinacidil administration (loading dosage 0.5 mg/kg and maintenance dosage 0.5 mg·kg(-1)·h(-1), iv). Electrical heterogeneities were defined with the dispersion of ARI. After successful and failed shocks with near-DFT strength, the earliest postshock activation patterns (focal or nonfocal endocardial activation), interval and location were detected.

RESULTS

Pinacidil significantly decreased APD(90) (from 178±16 ms to 168±18 ms) and ARI from (152±10 ms to 143±10 ms) at pacing cycle length of 300 ms. The drug significantly increased VF activation rate (from 10.0±1.9 Hz to 10.8±2.0 Hz). The drug did not affect the dispersion of ARI, neither it changed DFT (baseline: 480±110 V; pinacidil: 425±55 V, P>0.05). The earliest postshock activation arose locally on the LV apical endocardium before and after the drug treatment. Pinacidil significantly prolonged the postshock cycle length of cycles 2 to 5 for the successful episodes but not for the failed episodes.

CONCLUSION

Pinacidil increases the postshock cycle length suggesting that EAD may play a role in postshock activation, while it fails to alter DFT suggesting that EAD produced by shock does not determine a defibrillation success or failure.

Ascending-ramp biphasic waveform has a lower defibrillation threshold and releases less troponin I than a truncated exponential biphasic waveform

BACKGROUND

We tested the hypothesis that the shape of the shock waveform affects not only the defibrillation threshold but also the amount of cardiac damage.

METHODS AND RESULTS

Defibrillation thresholds were determined for 11 waveforms-3 ascending-ramp waveforms, 3 descending-ramp waveforms, 3 rectilinear first-phase biphasic waveforms, a Gurvich waveform, and a truncated exponential biphasic waveform-in 6 pigs with electrodes in the right ventricular apex and superior vena cava. The ascending, descending, and rectilinear waveforms had 4-, 8-, and 16-millisecond first phases and a 3.5-millisecond rectilinear second phase that was half the voltage of the first phase. The exponential biphasic waveform had a 60% first-phase and a 50% second-phase tilt. In a second study, we attempted to defibrillate after 10 seconds of ventricular fibrillation with a single ≈30-J shock (6 pigs successfully defibrillated with 8-millisecond ascending, 8-millisecond rectilinear, and truncated exponential biphasic waveforms). Troponin I blood levels were determined before and 2 to 10 hours after the shock. The lowest-energy defibrillation threshold was for the 8-milliseconds ascending ramp (14.6±7.3 J [mean±SD]), which was significantly less than for the truncated exponential (19.6±6.3 J). Six hours after shock, troponin I was significantly less for the ascending-ramp waveform (0.80±0.54 ng/mL) than for the truncated exponential (1.92±0.47 ng/mL) or the rectilinear waveform (1.17±0.45 ng/mL).

CONCLUSIONS

The ascending ramp has a significantly lower defibrillation threshold and at ≈30 J causes 58% less troponin I release than the truncated exponential biphasic shock. Therefore, the shock waveform affects both the defibrillation threshold and the amount of cardiac damage.

The effects of acute amiodarone on short- and long-duration ventricular defibrillation threshold in canines

INTRODUCTION

Some studies have shown that the defibrillation threshold (DFT) differs between short-duration ventricular fibrillation (SDVF, <1 minute) and long-duration ventricular fibrillation (LDVF > 1 minute). The goal of this study is to evaluate the effects of acute intravenous amiodarone on SDVF-DFT and LDVF-DFT and its possible mechanism as well.

METHODS

Twelve open-chest dogs were randomly divided into 2 groups (control group, normal saline, 10 mL·kg·h maintenance, n = 6; amiodarone group, loading dose 10 mg/kg over 10 minutes, maintenance dose 5 mg·kg·h, n = 6). VF was electrically induced and recorded with a 12 × 12 unipolar electrode plaque (2-mm spacing) sutured on the left ventricular epicardium and a plunge needle (6 unipolar electrode) inserted in the left ventricular apex. The DFTs of SDVF and LDVF were determined 20 seconds and 3 minutes after VF induction, respectively. Restitution was estimated from activation recovery intervals during pacing from the plaque and plunge needle electrodes. The activation rate was estimated by Fast Fourier Transform analysis of VF at same electrodes. The VF cycle length was defined as the reciprocal of the activation rate. The epicardial and transmural dispersion of the maximal slope of the restitution curve and VF cycle length of SDVF and LDVF were calculated, respectively.

RESULTS

: In controls, LDVF-DFT was higher than SDVF-DFT (645 ± 61 vs. 520 ± 63 V, P < 0.01). Amiodarone did not significantly alter SDVF-DFTs (496 ± 49 vs. 552 ± 69 V, P > 0.05) but decreased LDVF-DFT by 31% (P < 0.01). Compared with control, amiodarone significantly reduced the maximum slope of the restitution curve (1.12 ± 0.35 vs. 0.81 ± 0.25, P = 0.03) and its epicardial dispersion (0.32 ± 0.07 vs. 0.24 ± 0.04,coefficient of variation, P = 0.017). Amiodarone significantly increased the SDVF-CL (92 ± 8 vs. 99 ± 10 milliseconds, P < 0.01) and epicardial dispersion 0.14 ± 0.06 vs. 0.18 ± 0.05, P < 0.01. Amiodarone did not change the LDVF-CL (228 ± 12 vs. 226 ± 10 milliseconds, P > 0.05) or epicardial dispersion (0.17 ± 0.03 vs. 0.15 ± 0.02, P > 0.05) compared with control. However, the drug significantly decreased the transmural dispersion of LDVF-CL (68 ± 28 vs. 39 ± 14 milliseconds, P < 0.01) without changing the transmural dispersion of SDVF-CL (29 ± 22 vs. 32 ± 30 milliseconds, P > 0.05).

CONCLUSIONS

Acute amiodarone significantly decreased the LDVF-DFT. The decreased transmural dispersion of LDVF-CL by amiodarone might contribute to the decreased LDVF-DFT.