The Miller Family Heart & Vascular Institute is well equipped for state-of-the-art clinical, basic, and translational research efforts. Research into the mechanisms underlying atrial fibrillation (AF) and new treatments for AF is being conducted at each of these levels. One of the fundamental characteristics of this effort is its collaborative nature. Studies typically involve clinicians, surgeons, and scientists, interacting on a frequent basis. This integration and coordination gives us the ability to overcome the traditional boundaries to translational research efforts.
Cleveland Clinic has modern and well equipped laboratory facilities for basic cardiac electrophysiology research. Research facilities include approximately 2000 square feet of laboratory space, immediately adjacent to the hospital, with state of the art equipment for doing a broad spectrum of electrophysiology research at multiple levels - from the isolated cardiac myocyte (heart muscle cell), to the isolated heart or cardiac tissue, to whole animal studies, and finally to collaborative clinical studies.
Key questions that we are addressing include:
Mechanisms of AF
In many patients, AF begins with short episodes, typically characterized as “palpitations” (a fluttering sensation in the chest), or “paroxysms.” Over time, there is a tendency for these episodes to become longer. Why does this happen? Once AF has been initiated, the atria undergo a process known as “remodeling.” AF-induced atrial remodeling causes both structural and electrical changes:
Structural changes:
As shown in the pictures below, individual muscle cells within the fibrillating atria tend to become elongated and sometimes wider.
Normal
Valvular AF
In addition the space between individual myocytes typically becomes more fibrotic, with fatty infiltration, and the atria is less able to contract. These changes make it more likely that blood will remain longer in the atrial appendage, increasing the possibility of clot formation that can cause strokes. (Learn more: Atrial Structural Remodeling)
Electrical changes:
Fibrillating atria tend to have more complicated patterns of electrical activity. This is due both to the increased fibrosis, and to intrinsic changes in the electrical activity in the atrial myocytes. Research at the Cleveland Clinic has helped to characterize the electrical remodeling process associated with long-standing AF. (Learn more: Atrial Electrical Remodeling)
The net result is that in patients with persistent AF, the atria are more able to sustain fibrillatory activity, due to the combined effects of both structural and electrical remodeling.
An interesting and important question is, “What causes the atrial remodeling to occur in the first place?” Studies are ongoing to understand this process, and may yield insights that help us to prevent atrial fibrillation from progressing in the future.
Back to Top
Atrial Electrical Remodeling
The work of the heart is dependent on intrinsic electrical activity. All cardiac myocytes have electrical activity. Every time the heart is stimulated to contract (see below), the individual myocytes (which are electrically connected to each other) undergo electrical activation (an action potential). The movie below shows the normal contractile response of isolated cardiac myocytes to electrical stimulation. This type of synchronized contraction underlies the pumping activity of the heart.
Figure 1
Figure 1 to the right shows representative action potentials (in response to electrical stimulation) recorded from a myocyte isolated from a patient with healthy atria in black, and from a patient with persistent AF in red. Note that the AF action potential is shorter and more triangular than the normal action potential. This change in action potential shape (electrical remodeling) is due to changes in the distribution and/or function of specialized protein molecules (ion channels and pumps) in the cell membrane that create:
Figure 2
- a normal gradient in ion concentrations between the inside and outside of the cell
- the ability to respond to electrical stimulation.
As shown in Figure 2 to the right, the net result of the electrical and structural changes is that the enlarged atria are more likely to sustain fibrillatory activity. Thus, AF can persist for a longer duration in the remodeled atria.
Several studies at the Cleveland Clinic* have contributed to our understanding of the impact of AF on action potentials and ion channels in the human atria.
These studies also help us to understand why AF persistent AF is more likely to reoccur after it has been terminated with an electrical cardioversion procedure. Ongoing studies are focused at understanding the mechanisms underlying this process. (Learn more: mechanisms of AER)
Back to Top
Mechanisms of Atrial Electrical Remodeling (AER)
- During a cardiac action potential, there are significant fluxes of ions through ion channels in the surface membrane of the cardiac cell. Sodium entry into the myocytes helps to propagate electrical activity, and calcium entry initiates contractile activity. Potassium ions leaving the cardiac myocyte help to reestablish the resting potential and the relaxation phase (diastole) between contractions (systole). (Ref 1)
- Experimental studies from several groups have shown that high rate electrical activation of the atria, whether by atrial fibrillation or by external pacing, makes it difficult for the individual myocytes to remove calcium ions that entered during a previous activation. Calcium ions have an important role in both the electrical and contractile activity of the heart. Thus, calcium overload is implicated as an early event in the electrical remodeling process. (References 2,3)
- If the high rate activity ends quickly, the atria can recover, pumping calcium back into intracellular storage compartments or out of the cardiac myocytes. However, if the high rate activity is prolonged, the calcium overload can have more long-lasting effects, such as the action potential changes shown above.
- Calcium overload can activate intracellular enzymes known as “proteases” that can degrade a variety of important cellular proteins. One protease that been shown to be elevated in the atria of patients with atrial fibrillation is calpain. (Reference 4)
- Mitochondria are responsible for energy production within the cardiac myocyte. Prolonged episodes of calcium overload also lead to mitochondrial abnormalities, and increased production of free radicals (oxidative stress). Several studies are underway at the Cleveland Clinic Foundation to study the relationships between oxidative stress and atrial fibrillation. (References 5,6)
- Longer episodes of high rate pacing or AF can result in changes in ion channel expression, presumably in an attempt to reduce the calcium overload, and preserve myocyte viability. (Reference 1)
- Persistent AF results in further changes in protein expression, loss of myofibrillar structure, and eventually myocyte death and replacement fibrosis (Reference 7)
Atrial Electrical Remodeling References:
- D. R. Van Wagoner and J. M. Nerbonne. Molecular basis of electrical remodeling in atrial fibrillation. J.Mol.Cell Cardiol. 32 (6):1101-1117, 2000. (PubMed citation)
- Goette, C. Honeycutt, and J. J. Langberg. Electrical remodeling in atrial fibrillation: time course and mechanisms. Circ. 94:2968-2974, 1996. (PubMed citation)
- R. G. Tieleman, C. De Langen, I. C. Van Gelder, P. J. de Kam, J. Grandjean, K. J. Bel, M. C. Wijffels, M. A. Allessie, and H. J. Crijns. Verapamil reduces tachycardia-induced electrical remodeling of the atria. Circ. 95 (7):1945-1953, 1997. (PubMed citation)
- Brundel BJ, Ausma J, van Gelder IC, Van der Want JJ, van Gilst WH, Crijns HJ, Henning RH. Activation of proteolysis by calpains and structural changes in human paroxysmal and persistent atrial fibrillation. Cardiovasc Res. 54(2):380-9, 2002. (PubMed citation)
- M. J. Mihm, F. Yu, C. A. Carnes, P. J. Reiser, P. M. McCarthy, D. R. Van Wagoner, and J. A. Bauer. Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation. Circ. 104 (2):174-180, 2001. (PubMed citation)
- C. A. Carnes, M. K. Chung, T. Nakayama, H. Nakayama, R. S. Baliga, S. Piao, A. Kanderian, S. Pavia, R. L. Hamlin, P. M. McCarthy, J. A. Bauer, and D. R. Van Wagoner. Ascorbate attenuates atrial pacing-induced peroxynitrite formation and electrical remodeling and decreases the incidence of postoperative atrial fibrillation. Circ.Res. 89 (6):E32-E38, 2001. (PubMed citation)
- V. L. Thijssen, J. Ausma, and M. Borgers. Structural remodelling during chronic atrial fibrillation: act of programmed cell survival. Cardiovasc.Res. 52 (1):14-24, 2001. (PubMed citation)
Back to Top
Atrial Structural Remodeling
The fibrillating atria are subjected to continuous, high rate electrical activity (with rates up to 500 per minute). This results in impaired atrial contractility, and the initiation of structural changes. At the macroscopic level, structural remodeling is frequently characterized by increased atrial fibrosis and fatty infiltration, both on the endocardial surface, and between muscle bundles.
The figure to the right shows an extreme example of AF-induced fibrosis. The normal left atrial appendage is very compliant and collapses when not filled with blood. This appendage had become a rigid structure, with the fibrosis keeping the appendage open even when empty. Thus, while muscular tissue surrounds the fibrous layer, the contractility of the tissue was significantly impaired by the mechanical restraints imposed by the fibrosis. This can promote blood stasis in the appendage and thus clot formation.
At the microscopic level, fibrosis can isolate muscle bundles. The sections in the figure below are stained to show fibrosis in blue and muscle bundles in red. The panel on the left was from a normal right atrial appendage with little fibrosis apparent. The panel on the right was from the atria of a heart transplant recipient with end stage heart failure and atrial fibrillation due to ischemic cardiomyopathy. It is evident that fibrosis can isolate muscle bundles, and that this can alter the pathway of electrical activation, creating a substrate that can promote the persistence of atrial fibrillation.
Healthy right atrial appendage with little fibrosis
Atrial appendage from patient with end stage heart failure and AF due to ischemic cardiomyopathy - note increased fibrosis
In addition to the impact of AF on atrial interstitial fibrosis, numerous subcellular changes are evident in the fibrillating atria. These include altered mitochondrial size and function, increased glycogen storage, loss of contractile elements, and myocyte hypertrophy. The AF-induced cellular changes have been compared to those changes that follow a myocardial infarction in the ventricle, in which the poorly contractile muscle is said to be in a “hibernating” state. This may represent an attempt of the atria to preserve myocyte viability, at the cost of decreased contractility. Studies are ongoing to evaluate the mechanisms underlying structural remodeling in the fibrillating human atria.
Back to Top
Oxidative Stress and AF
What is Oxidative Stress?
- Is a generic term for biochemical modification of cells, tissues and lipids due to interactions with free radicals
- Interactions can increase, decrease, or alter the function of specific proteins, depending on the degree and type of modification
- Oxidative stress has a physiological role, as a key step in numerous signal transduction pathways and immune cell function
- Oxidative stress is also involved in pathological cardiovascular injury (atherosclerosis, heart failure, etc.)
What are the biologically relevant free radicals?
Major cellular oxidant species include:
- Nitric oxide (NO) - produced by a family of enzymes know as nitric oxide synthases (NOS). Production is increased in response to calcium overload.
- Superoxide - produced by xanthine oxidase and NADPH oxidase. Production is increased in response to Angiotensin II and by inflammatory responses.
- H2O2
- Hydroxyl radical
- Peroxynitrite (OONO-) - is formed by the interaction of nitric oxide (NO) and superoxide. Peroxynitrite can covalently modify a variety of cellular lipids and proteins.
What are the cellular defenses against oxidant?
- antioxidant enzymes (catalase, superoxide dismutase, peroxidases, etc.)
- antioxidant molecules (glutathione, vitamin C, vitamin E, etc.)
What is the Connection Between Oxidative Stress and AF?
- AF is associated with calcium overload. This can increase the production of nitric oxide (NO), and mitochondrial free radical production.
- AF is associated with neurohormonal activation, frequently leading to increased production of Angiotensin II (and superoxide).
- Our research (in collaboration with colleagues at the Ohio State University) has shown that, as a result of increased NO and/or superoxide production, protein nitration is increased during persistent AF, suggesting increased peroxynitrite formation. ( Learn more: Oxidative Stress and AF studies)
- This relationship suggests that, in some circumstances, antioxidants may help to prevent the cellular injury and electrical changes that normally accompany AF. Several studies have been completed to evaluate this hypothesis, and others are ongoing.
Back to Top
Oxidative Stress and AF studies
The observation that atrial tissue from patients with persistent AF is marked by signs of increased oxidative stress (ref 1) led us to hypothesize that treatments that either scavenge or prevent free radicals might alter the electrophysiological and/or structural remodeling processes associated with AF.
- In a recent study we have shown that the antioxidant ascorbate (vitamin C) can attenuate the electrical remodeling that accompanies rapid atrial pacing in an experimental model. In this study, the atrial tissue subjected to rapid atrial pacing showed direct evidence of increased oxidative stress (increased 3-nitrotyrosine formation), and ascorbate was able to minimize this effect. Further, supplemental ascorbate also helped to prevent tissue depletion of endogenous ascorbate. (Ref 2)
- Following cardiac surgery, many patients experience transient episodes of atrial fibrillation. This arrhythmia follows a time course very similar to that of the inflammatory response following cardiac surgery. In the same study (ref 2), we reported the results of a pilot study in which we evaluated the impact of supplemental ascorbate on the occurrence of atrial fibrillation following coronary artery bypass graft surgery. In the patients receiving supplemental ascorbate, 7/43 (16%) had post-operative atrial arrhythmias. In contrast, 15/43 (35%) experienced postoperative arrhythmias in an age- and gender matched control population. Thus, in this pilot study, supplemental ascorbate usage was associated with a 50% reduction in the incidence of postoperative atrial arrhythmia. In view of this promising result, a new study, being performed in a randomized, blinded, and placebo-controlled fashion is underway to better evaluate the statistical significance of the pilot study.
- Inflammation may also contribute to the persistence of atrial fibrillation in non-surgical patients. We recently observed that the levels of C-reactive protein (CRP), a sensitive marker of the systemic inflammatory state, are elevated in patients with atrial fibrillation, and that the degree of elevation was related to the persistence of atrial fibrillation. That is, CRP levels were elevated in patients with paroxysmal AF relative to the controls. However, the levels were even more elevated in patients with persistent AF. (Ref 3)
We conclude from these studies that oxidative stress may have an important role in the atrial pathologies associated both with rapid atrial rates, and with the inflammation-mediated postoperative arrhythmias.
A variety of follow-up studies are now underway to further elucidate the mechanisms by which oxidative stress and inflammation are involved in AF, as well as studies to evaluate novel and more effective treatment strategies.
Oxidative Stress Related References:
- M. J. Mihm, F. Yu, C. A. Carnes, P. J. Reiser, P. M. McCarthy, D. R. Van Wagoner, and J. A. Bauer. Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation. Circ. 104 (2):174-180, 2001. (PubMed citation)
- C. A. Carnes, M. K. Chung, T. Nakayama, H. Nakayama, R. S. Baliga, S. Piao, A. Kanderian, S. Pavia, R. L. Hamlin, P. M. McCarthy, J. A. Bauer, and D. R. Van Wagoner. Ascorbate attenuates atrial pacing-induced peroxynitrite formation and electrical remodeling and decreases the incidence of postoperative atrial fibrillation. Circ.Res. 89 (6):E32-E38, 2001. (PubMed citation)
- M. K. Chung, D. O. Martin, O. Wazni, A. Kanderian, D. Sprecher, C. A. Carnes, J. A. Bauer, P. J. Tchou, M. Niebauer, A. Natale, and D. R. Van Wagoner. C-reactive protein elevation in patients with atrial arrhythmias: inflammatory mechanisms and persistence of atrial fibrillation. Circ. 104:2886-2991, 2001. (PubMed citation)
Back to Top
Rate control in Atrial Fibrillation
The Problem
The atria beat very rapidly and irregularly during AF. If each of the atrial beats were transmitted to the ventricles, ventricular fibrillation would be initiated. This seldom happens because of the filtering properties of the AV node, a small structure located between the atria and the ventricles. However, many impulses still manage to reach the ventricles and cause abnormally high ventricular rate, which if left untreated can lead to heart failure.
Conventional Therapy
Clinically, slowing of ventricular rate is now achieved either by prescribing drugs such as beta-adrenergic antagonists or calcium channel blockers, or by irreversible destruction (ablation) of the AV node by application of strong radiofrequency energy. By disrupting the electrical connection between the atria and the ventricles, AV nodal ablation frees the ventricles from high-rate atrial bombardment. However, left on their own, the ventricles beat so slowly that the resulting hemodynamic situation is even worse than during untreated AF, thus permanent ventricular pacemaker implantation is required. Ventricular pacing results in activation of ventricular contraction in a sequence opposite to the normal, i.e. “from the bottom to the top,” which may have long-term undesirable consequences.
Our Approach
We are analyzing the mechanisms by which the AV node fulfills its filtering role during AF and testing a new method for nondestructive modulation of this filtering. We hypothesized that by selectively stimulating vagal (parasympathetic) innervation of the AV node, conduction through the node would be slowed, and the conduction of the rapidly bombarding atrial impulses would be depressed within the node. This “selective AV nodal vagal stimulation” should yield a slower ventricular rate while the functional and anatomical integrity of the AV node would be preserved. In early studies, we have shown that the working hypothesis is valid.
In ongoing studies, we are evaluating the efficacy of this approach as a long-term therapy. Preliminary results, with 6 month results available, are quite encouraging. The procedures are well tolerated by the animals and no side effects have thus far been observed.
Relevance to AF patients
The proposed studies have specific importance for patients in which the AF is irreversible. This may include cases with drug resistance or those in which ablation or surgical intervention is impractical. In these cases vagally induced local depression of AV nodal conduction would provide the benefits of better hemodynamic function, due to the improved ventricular rate control and a normal anterograde sequence of ventricular contraction. We are confident that the results of these studies will provide the solid theoretical and practical basis that is needed in order to pursue the ultimate goal of discovering new methods for ventricular rate control during atrial fibrillation.
Ventricular rate control references:
- Zhang Y, Mowrey KA, Zhuang S, Wallick DW, Popovic ZB, Mazgalev TN. Optimal ventricular rate slowing during atrial fibrillation by feedback AV nodal-selective vagal stimulation. Am J Physiol Heart Circ Physiol. 2002 Mar;282(3):H1102-10. (PubMed abstract)
- Wallick DW, Zhang Y, Tabata T, Zhuang S, Mowrey kA, Watanabe J, Greenberg NL, Grimm RA, Mazgalev TN. Selective AV nodal vagal stimulation improves hemodynamics during acute atrial fibrillation in dogs. Am J Physiol Heart Circ Physiol. 2001 Oct;281(4):H1490-7. (PubMed abstract)
- Mazgalev TN, Garrigue S, Mowrey KA, Yamanouchi Y, Tchou PJ. Autonomic modification of the atrioventricular node during atrial fibrillation: role in the slowing of ventricular rate. Circulation. 1999 Jun 1;99(21):2806-14. (PubMed abstract)
Would you like more information? Contact: Todor Mazgalev, Ph.D.; e-mail: mazgalt@ccf.org.
Back to Top
*a new browser window will open with outside links. The inclusion of links to other web sites does not imply any endorsement of the material on the web sites or any association with their operators
