Increased vulnerability to atrial and ventricular arrhythmias caused by different types of inhaled tobacco or marijuana products.

BACKGROUND: The emergence of a plethora of new tobacco products marketed as being less harmful than smoking, such as electronic cigarettes and heated tobacco products, and the increased popularity of recreational marijuana have raised concerns about the potential cardiovascular risk associated with their use. OBJECTIVE: The purpose of this study was to investigate whether the use of novel tobacco products or marijuana can cause the development of proarrhythmic substrate and eventually lead to arrhythmias. METHODS: Rats were exposed to smoke from tobacco, marijuana, or cannabinoid-depleted marijuana, to aerosol from electronic cigarettes or heated tobacco products, or to clean air once per day for 8 weeks, following by assays for blood pressure, cardiac function, ex vivo electrophysiology, and histochemistry. RESULTS: The rats exposed to tobacco or marijuana products exhibited progressively increased systolic blood pressure, decreased cardiac systolic function with chamber dilation, and reduced overall heart rate variability, relative to the clean air negative control group. Atrial fibrillation and ventricular tachycardia testing by ex vivo optical mapping revealed a significantly higher susceptibility to each, with a shortened effective refractory period and prolonged calcium transient duration. Histological analysis indicated that in all exposure conditions except for air, exposure to smoke or aerosol from tobacco or marijuana products caused severe fibrosis with decreased microvessel density and higher level of sympathetic nerve innervation. CONCLUSION: These pathophysiological results indicate that tobacco and marijuana products can induce arrhythmogenic substrates involved in cardiac electrical, structural, and neural remodeling, facilitating the development of arrhythmias.

Absence of natriuretic peptide clearance receptor attenuates TGF-ß1-induced selective atrial fibrosis and atrial fibrillation.

Aims: TGF-ß1 plays an important role in atrial fibrosis and atrial fibrillation (AF); previous studies have shown that the atria are more susceptible to TGF-ß1 mediated fibrosis than the ventricles. Natriuretic peptides (NPs) play an important role in cardiac remodelling and fibrosis, but the role of natriuretic peptide clearance (NPR-C) receptor is largely unknown. We investigated the role of NPR-C in modulating TGF-ß1 signalling in the atria. Methods and results: MHC-TGF-ß1 transgenic (TGF-ß1-Tx) mice, which develop isolated atrial fibrosis and AF, were cross-bred with NPR-C knock-out mice (NPR-C-KO). Transverse aortic constriction (TAC) was performed in wild type (Wt) and NPR-C knockout mice to study. Atrial fibrosis and AF inducibility in a pathophysiologic model. Electrophysiology, molecular, and histologic studies were performed in adult mice. siRNA was used to interrogate the interaction between TGF-ß1 and NP signalling pathways in isolated atrial and ventricular fibroblasts/myofibroblasts. NPR-C expression level was 17 ± 5.8-fold higher in the atria compared with the ventricle in Wt mice (P = 0.009). Cross-bred mice demonstrated markedly decreased pSmad2 and collagen expression, atrial fibrosis, and AF compared with TGF-ß1-Tx mice with intact NPR-C. There was a marked reduction in atrial fibrosis gene expression and AF inducibility in the NPR-C-KO-TAC mice compared with Wt-TAC. In isolated fibroblasts, knockdown of NPR-C resulted in a marked reduction of pSmad2 (56 ± 4% and 24 ± 14% reduction in atrial and ventricular fibroblasts, respectively) and collagen (76 ± 15% and 35 ± 23% reduction in atrial and ventricular fibroblasts/myofibroblasts, respectively) in response to TGF-ß1 stimulation. This effect was reversed by simultaneously knocking down NPR-A but not with simultaneous knock down of PKG-1. Conclusion: The differential response to TGF-ß1 stimulated fibrosis between the atria and ventricle are in part mediated by the abundance of NPR-C receptors in the atria.

The effects of remodeling with heart failure on mode of initiation of ventricular fibrillation and its spatiotemporal organization.

PURPOSE: The effect of the heart failure substrate on the initiation of ventricular fibrillation (VF) and its resulting mechanism is not known. The objective of this study was to determine the effects of substrate on VF initiation and its spatiotemporal organization in the heart failure model. METHODS: Optical action potentials were recorded from LV wedge preparations either from structurally normal hearts (control, n = 11) or from congestive heart failure (CHF; n = 7), at the epicardial surface, endocardial surface which included a papillary muscle, and a transmural cross section. Action potential duration (APD(80)) was determined, and VF was initiated. A fast Fourier transform was calculated, and the dominant frequency (DF) was determined. RESULTS: The CHF group showed increased VF vulnerability (69 vs 26 %, p < 0.03), and every mapped surface showed an APD(80) gradient which included islands of higher APDs on the transmural surface (M cells) which was not observed in controls. VF in the CHF group was characterized by stable, discrete, high-DF areas that correlated to either foci or spiral waves located on the transmural surface at the site of the papillary muscle. Overall, the top 10 % of DFs correlated to an APD of 101 ms while the bottom 10 % of DFs correlated to an APD of 126 ms (p < 0.01). CONCLUSIONS: In the CHF model, APD gradients correlated with an increased vulnerability to VF, and the highest stable DFs were located on the transmural surface which was not seen in controls. This indicates that the CHF substrate creates unique APD and DF characteristics.

Molecular basis of selective atrial fibrosis due to overexpression of transforming growth factor-ß1.

AIMS: Animal studies show that transforming growth factor-ß1 (TGF-ß1) is an important mediator of atrial fibrosis and atrial fibrillation (AF). This study investigated the role of TGF-ß1 in human AF and the mechanism of atrial-selective fibrosis. METHODS AND RESULTS: Atrial specimens from 17 open heart surgery patients and left atrial and ventricular specimens from 17 explanted hearts were collected to assess the relationship between TGF-ß1, AF, and differential atrial vs. ventricular TGF-ß1 levels. A transgenic mouse model overexpressing active TGF-ß1 was used to study the mechanisms underlying the resultant atrial-selective fibrosis. Higher right atrial total TGF-ß1 levels (2.58 ± 0.16-fold, P < 0.0001) and active TGF-ß1 (3.7 ± 0.7-fold, P = 0.013) were observed in those that developed post-operative AF. Although no ventricular differences were observed, 11 explanted heart failure hearts exhibited higher atrial TGF-ß1 levels than 6 non-failing hearts (2.30 ± 0.87 fold higher, P < 0.001). In the transgenic mouse, TGF-ß1 receptor-1 kinase blockade resulted in decreased atrial expression of fibrosis-related genes. By RNA microarray analyses in that model, 80 genes in the atria and only 2 genes in the ventricle were differentially expressed. Although these mice atria, but not the ventricles, exhibited increased expression of fibrosis-related genes and phosphorylation of Smad2, there were no differences in TGF-ß1 receptor levels or Smads in the atria compared with the ventricles. CONCLUSIONS: TGF-ß1 mediates selective atrial fibrosis in AF that occurs via TGF-ß Receptor 1/2 and the classical Smad pathway. The differential atrial vs. ventricular fibrotic response occurs at the level of TGF-ß1 receptor binding or phosphorylation.

In vivo assessment of the electrophysiological integration and arrhythmogenic risk of myocardial cell transplantation strategies.

Cell replacement strategies are promising interventions aiming to improve myocardial performance. Yet, the electrophysiological impact of these approaches has not been elucidated. We assessed the electrophysiological consequences of grafting of two candidate cell types, that is, skeletal myoblasts and human embryonic stem cell-derived cardiomyocytes (hESC-CMs). The fluorescently labeled (DiO) candidate cells were grafted into the rat's left ventricular myocardium. Two weeks later, optical mapping was performed using the Langendorff-perfused rat heart preparation. Images were obtained with appropriate filters to delineate the heart's anatomy, to identify the DiO-labeled cells, and to associate this information with the voltage-mapping data (using the voltage-sensitive dye PGH-I). Histological examination revealed the lack of gap junctions between grafted skeletal myotubes and host cardiomyocytes. In contrast, positive Cx43 immunostaining was observed between donor and host cardiomyocytes in the hESC-CMs-transplanted hearts. Optical mapping demonstrated either normal conduction (four of six) or minimal conduction slowing (two of six) at the hESC-CMs engraftment sites. In contrast, marked slowing of conduction or conduction block was seen (seven of eight) at the myoblast transplantation sites. Ventricular arrhythmias could not be induced in the hESC-CM hearts following programmed electrical stimulation but were inducible in 50% of the myoblast-engrafted hearts. In summary, a unique method for assessment of the electrophysiological impact of myocardial cell therapy is presented. Our results demonstrate the ability of hESC-CMs to functionally integrate with host tissue. In contrast, transplantation of cells that do not form gap junctions (skeletal myoblats) led to localized conduction disturbances and to the generation of a proarrhythmogenic substrate.

Transmural characteristics of atrial fibrillation in canine models of structural and electrical atrial remodeling assessed by simultaneous epicardial and endocardial mapping.

BACKGROUND: Epicardial mapping has shown that atrial substrate may play a role in the characteristics of the resulting atrial fibrillation (AF). However, it is not known whether these differences also occur in 3 dimensions. OBJECTIVE: This study sought to examine the 3-dimensional characteristics of AF by simultaneously analyzing AF on the epicardial and endocardial surfaces. METHODS: Dogs were divided into 5 groups: congestive heart failure (CHF), rapid atrial pacing (RAP), mitral regurgitation (MR), control, and methylcholine. A noncontact mapping catheter (Ensite 3000 [Endocardial Solutions, Inc., St. Paul, Minnesota]) was placed in the left atrium (LA), and electrode plaques (240 unipoles) were placed over the epicardial surface. Several AF episodes of at least 30 s were recorded, and isopotential videos of activation and isochronal maps were constructed. In addition, each pair of matched electrograms were cross-correlated (XC) and analyzed with a fast Fourier transform (FFT). RESULTS: The RAP model was the only one with an AF mechanism of multiple wavelets in every dog on both surfaces. In addition, when individual signals were compared, the RAP model had the least amount of similarities between the recording surfaces, whereas the CHF model had the most as it had a higher percentage of signals with XC coefficients >0.8 and a higher percentage of signals with similar dominant frequencies (30 +/- 35% vs. 12 +/- 13% and 66 +/- 30% vs. 26 +/- 10%, P < .05). CONCLUSION: Although the RAP model had similar AF mechanisms in 3 dimensions, this did not correlate to transmural similarities. Focal mechanisms of AF may have a more uniform wavefront of activation, whereas models with mechanisms of multiple wavelets may have more 3-dimensional properties.

Embryonic inbreeding depression varies among populations and by mating system in Witheringia solanacea (Solanaceae).

PREMISE OF THE STUDY: Embryonic inbreeding depression is a key influence on mating system evolution and can be difficult to estimate in self-incompatible species. A pollen chase experiment was used to estimate the magnitude of embryonic inbreeding depression in Costa Rican Witheringia solanacea, a species polymorphic for self-incompatibility (SI). In a pollen chase experiment, bud self-pollinations are followed after anthesis by outcross pollinations, with a comparable pair of outcross pollinations used as a control. Lowered seed set for the self-precedence treatment indicates embryonic inbreeding depression. • METHODS: Embryonic inbreeding depression was assayed for self-compatible (SC) individuals and for SI plants from two populations that differ quantitatively in the onset and enzymatic activity of their SI response. Microsatellite markers were used to assay the selfing rate of a sample of surviving progeny from the prior self-pollination treatment. • KEY RESULTS: SC individuals showed no evidence of embryonic inbreeding depression. In SI plants, prior self-pollination reduced seed number by 28-70%, depending on population. Microsatellite genotyping revealed that embryonic inbreeding depression was even more severe than estimated by the phenotypic data: for mature fruits resulting from self-pollination precedence, the majority of the progeny were the result of outcross fertilization. • CONCLUSIONS: Lineage-specific purging of recessive lethals has accompanied the evolution of SC in this species. SI populations show contrasting levels of embryonic inbreeding depression, with nearly complete embryonic lethality upon selfing in the Monteverde population. In the face of high embryonic inbreeding depression, an increase in selfing rate can evidently occur only under severe pollen limitation.

Safety profiles and lesion size of different radiofrequency ablation technologies: a comparison of large tip, open and closed irrigation catheters.

INTRODUCTION: Different technologies have been developed for radiofrequency ablation (RFA), which include increasing electrode (tip) size and cooling the tip through irrigation either internally (closed-loop) with D5W or externally (open-loop) with saline. Although these catheters are widely used clinically, the propensity for adverse events and the lesion profiles of each of these catheter technologies have not been directly compared under a wide range of controlled conditions. METHODS AND RESULTS: Freshly excised canine thigh muscle was placed in a chamber filled with circulating, heparinized blood heated to 37 degrees C. Five different catheters were tested: 4 mm tip, 10 mm tip single thermistor, 10 mm tip multitemperature sensor, 4 mm closed-loop irrigated cooled-tip, and 4 mm open-loop irrigated cooled tip at several different contact and power settings. The catheter and tissue interface was continuously monitored with intracardiac echocardiography (echo) (Acuson). During the RFA, any bubbling generated from the tip and/or popping seen on echo was noted, and after each RFA, the catheter and lesion were examined for the presence of thrombus. For all of the catheters, complications correlated to the electrode tip temperature and power setting. All of the catheters experienced complications at any lesion size except for the open-irrigated catheter, which only had complications at the largest lesions. Overall, the cooled tip catheters experienced an at least sixfold greater odds of popping, bubbling, and impedance rises than the 4 mm, but the majority occurred at power levels greater than 20 W. The open-irrigated catheters created eccentric lesions that extended away from the tissue-catheter interface, in the direction of blood flow. In addition, it produced saline filled blisters at the lesion site in 16.7% of the burns. The 10 mm catheter had an at least twofold greater odds of thrombus, charring, and bubbling, but larger lesions than the 10 mm multitemperature sensor catheter. CONCLUSIONS: Catheter type, contact conditions, and power settings all play a role in lesion size and in the frequency of complications that occur during an RFA. Cooling the electrode tip, either internally or externally, does not prevent complications from occurring, especially at the higher power control settings. Adding more temperature sensors to the 10 mm seems to reduce the amount of complications that can occur.

Effects of atrial fibrillation substrate and spatiotemporal organization on atrial defibrillation thresholds.

BACKGROUND: We previously showed that canine models of atrial fibrillation (AF) have different substrates (either structural or electrical) that lead to differences in AF characteristics. OBJECTIVE: The purpose of this study was to determine whether the differences in AF characteristics also would lead to differences in atrial defibrillation thresholds (ADFTs). METHODS: Dogs were divided into five groups: control; MR-mitral regurgitation for 5 weeks; CHF-congestive heart failure for 4 weeks; RAP-rapid atrial pacing for 6 weeks; and METH-acetyl-beta-methylcholine acutely administered. A cross-sectional area of the left atrium was calculated, and AF was induced with rapid atrial pacing. Biphasic shocks with a pulse width of 3/3 ms were delivered through specially constructed shocking catheters with a surface area of 3.7 cm(2) that were placed in the right and left atria. An up-down-up protocol was used to determine the 50% ADFT threshold (ADFT(50)). A wide-bipole AF signal was digitally filtered, and a fast Fourier transform was calculated over a 2-second window every 1 second. The dominant frequency was determined, and the organization index was calculated as the ratio of the area under the dominant peak and its harmonics to the total area of the spectrum. RESULTS: For left atrial size, the CHF and MR groups had a significantly larger atria than did control. ADFT(50) for control, MR, CHF, RAP, and METH groups were 160 +/- 30 V, 120 +/- 50 V, 132 +/- 20 V, 668 +/- 205 V, and 593 +/- 128 V, respectively (analysis of variance, P <.0001). Dominant frequencies were significantly higher and organization indexes significantly lower in the RAP and METH models compared with the other models. CONCLUSION: RAP and METH canine models had a significantly higher ADFT(50) compared with the other AF models. The increase in ADFT(50) in these models corresponded with higher global dominant frequencies and lower measured organization indexes.

Structural atrial remodeling alters the substrate and spatiotemporal organization of atrial fibrillation: a comparison in canine models of structural and electrical atrial remodeling.

Several animal models of atrial fibrillation (AF) have been developed that demonstrate either atrial structural remodeling or atrial electrical remodeling, but the characteristics and spatiotemporal organization of the AF between the models have not been compared. Thirty-nine dogs were divided into five groups: rapid atrial pacing (RAP), chronic mitral regurgitation (MR), congestive heart failure (CHF), methylcholine (Meth), and control. Right and left atria (RA and LA, respectively) were simultaneously mapped during episodes of AF in each animal using high-density (240 electrodes) epicardial arrays. Multiple 30-s AF epochs were recorded in each dog. Fast Fourier transform was calculated every 1 s over a sliding 2-s window, and dominant frequency (DF) was determined. Stable, discrete, high-frequency areas were seen in none of the RAP or control dogs, four of nine MR dogs, four of six CHF dogs, and seven of nine Meth dogs in either the RA or LA or both. Average DFs in the Meth model were significantly greater than in all other models in both LA and RA except LA DFs in the RAP model. The RAP model was the only one with a consistent LA-to-RA DF gradient (9.5 +/- 0.2 vs. 8.3 +/- 0.3 Hz, P < 0.00005). The Meth model had a higher spatial and temporal variance of DFs and lower measured organization levels compared with the other AF models, and it was the only model to show a linear relationship between the highest DF and dispersion (R(2) = 0.86). These data indicate that structural remodeling of atria (models known to have predominantly altered conduction) leads to an AF characterized by a stable high-frequency area, whereas electrical remodeling of atria (models known to have predominantly shortened refractoriness without significant conduction abnormalities) leads to an AF characterized by multiple high-frequency areas and multiple wavelets.

Mechanisms of ventricular fibrillation in canine models of congestive heart failure and ischemia assessed by in vivo noncontact mapping.

BACKGROUND: Much of the research performed studying the mechanism of ventricular fibrillation (VF) has been in normal ventricles rather than under a pathological condition predisposing to VF. We hypothesized that different ventricular substrates would alter the mechanism and characteristics of VF. METHODS AND RESULTS: Three groups of dogs were studied: (1) control (n=8), (2) pacing-induced congestive heart failure (n=7), and (3) acute ischemia produced by 30 minutes of mid left anterior descending artery ligation (n=5). A noncontact mapping catheter (Ensite 3000, ESI) was placed via transseptal into the left ventricle (LV), along with an electrophysiology catheter. A multielectrode basket catheter (EP Technologies) was placed in the right ventricle, along with an electrophysiology catheter. Several episodes of VF were recorded in each animal. In addition to constructing isopotential and isochronal maps of the VF episodes, signals underwent frequency domain analysis as a fast Fourier transform was performed over a 2-second window every 1 second. From the fast Fourier transform, the dominant frequency was determined, and the organization was calculated. In control dogs, meandering, reentrant spiral wave activity was the main feature of the VF. The congestive heart failure group showed evidence of a stable rotor (n=3), evidence of a focal source (n=3), or no evidence of a driver in the LV (n=1). The ischemic group showed evidence of an initial focal mechanism that transitioned into reentry. In the control and ischemic groups, the LV always had higher dominant frequencies than the right ventricle. CONCLUSIONS: Different ventricular substrates produced by the different animal models altered the characteristics of VF. Thus, different mechanisms of VF may be present in the LV, depending on the animal model.

Left atrial dilatation resulting from chronic mitral regurgitation decreases spatiotemporal organization of atrial fibrillation in left atrium.

Atrial conduction properties have been shown to differ among animal atrial fibrillation (AF) models of rapid atrial pacing (RAP), chronic mitral regurgitation (MR), and control. We hypothesized that these conduction differences would continue with the onset of AF, which would affect AF spatiotemporal organization, resulting in model-specific characteristics of AF. With frequency domain analysis of electrograms acquired from high-density optical mapping, AF from the right (RA) and left (LA) atrium in animals with RAP and MR were compared with control animals. At follow-up, the hearts were excised and perfused, and optical action potentials were recorded from a 2 x 2-cm area each of the RA and LA free wall with a 16 x 16 photodiode array. AF was induced with extra stimuli, several 2.4-s AF episodes were recorded in each dog, and a fast Fourier transform was calculated. The dominant frequency (DF) was determined, and the organization (organization index, OI) was calculated as the ratio of the area under the dominant peak and its harmonics to the total area of the spectrum. All possible pairs of electrograms for each episode were cross-correlated. LA AF in the chronic MR model showed an increase in the highest DF, the number of DF domains, and in frequency gradient compared with AF in control or RAP models. In addition, there was a decrease in OI and in the correlation coefficients in the LA of the MR model. These results suggest that the AF substrate in the MR model may be different from that of control or RAP models.

Tissue structure and connexin expression of canine pulmonary veins.

OBJECTIVE: Rapid electrical activity in pulmonary veins (PVs) has been proposed as a mechanism for focal atrial fibrillation. The way in which the myocardial sleeve inside PVs can form a substrate for focal activity is not well understood. Therefore, we have studied tissue structure and connexin distribution at the veno-atrial transition in the dog. METHODS: In adult mongrel dogs, the anatomy of the PV area was studied. Tissue structure in individual veins was assessed in formalin fixed sections using Masson's Trichrome staining. Gap junction protein distribution was examined using antibodies against connexin40 (Cx40) and connexin43 (Cx43). The ultrastructure of myocytes in myocardial sleeves was studied using electron microscopy. RESULTS: Individual PVs in the dog had a gross morphology similar to that observed in the human, with myocardial sleeves extending into the veins for 4-20 mm. In all veins examined, myocytes in myocardial sleeves had a normal atrial morphology and anti-desmin staining pattern. Cx43 was expressed throughout the sleeve at levels comparable to normal atrial myocardium. By contrast, Cx40 expression was lower in myocardial sleeves than in the rest of the left atrium. Myocytes in the sleeve, which were ultrastructurally similar to normal atrial myocytes, were predominantly organized in a circumferential pattern. CONCLUSIONS: PVs in the dog and various canine models of heart disease will be a suitable model for (patho)physiology of the veno-atrial transition. Myocytes in myocardial sleeves are similar to normal atrial myocytes. The circumferential orientation of these myocytes may provide a substrate for rapid circular reentry.