Slow antegrade excitation and delayed retrograde activation through an "inexcitable zone": A basis for arrhythmia formation in infarcted myocardium ex vivo.

INTRODUCTION: The electrophysiologic mechanism for rate-dependent PVBs associated with double potentials (DPs) was investigated in infarcted canine hearts using bipolar and intracellular microelectrode recordings. METHODS AND RESULTS: Dogs exhibiting rate-related ventricular ectopic beats (coupling interval, 390 ± 54 milliseconds) during sinus rhythm or atrial pacing were studied 4-5 days (N = 63) or 25 days (N = 16) following anterior descending coronary artery ligation. Sites of DP and rate-dependent arrhythmia formation were identified in vivo using bipolar recordings for subsequent ex vivo studies. Rate-dependent conduction delays with increasing duration isoelectric intervals representing very slow conduction were observed at sites of DP formation, frequently provoking both manifest and concealed reentry (non-stimulated beats) over a narrow range of paced cycle lengths. Both slow antegrade and retrograde activation across an inexcitable gap (reflection) were integral components of extrasystole formation. Retrograde reflection to a region of very slow conduction (mid-potential) during antegrade activation was routinely observed at 4-5 days (42 of 63 preparations, 67%) and 25 days (22 of 26 preparations, 85%) postcoronary artery ligation. Reflection and premature re-activation of the proximal site was then observed in 6 of 63 (9%), and 3 of 26 preparations (12%). CONCLUSION: The present experiments demonstrate DP formation and rate-dependent constant-coupled late epicardial premature beats in infarcted dog hearts. Microelectrode recordings at DP sites demonstrating prolonged isoelectric intervals display very slow conduction preceding distal activation and "reentrant" re-activation of more proximal sites, representing reflection as an arrhythmia mechanism in ischemically injured epicardium during subacute myocardial infarction.

Slow conduction through an arc of block: A basis for arrhythmia formation postmyocardial infarction.

INTRODUCTION: The electrophysiologic basis for characteristic rate-dependent, constant-late-coupled (390 + 54 milliseconds) premature ventricular beats (PVBs) present 4-5 days following coronary artery occlusion were examined in 108 anesthetized dogs. METHODS AND RESULTS: Fractionated/double potentials were observed in injured zone bipolar and composite electrograms at prolonged sinus cycle lengths (1,296 ± 396 milliseconds). At shorter cycle lengths, conduction of the delayed potential decremented, separating from the initial electrogram by a progressively prolonged isoelectric interval. With sufficient delay of the second potential following an isoelectric interval, a PVB was initiated. Both metastable and stable constant-coupled PVBs were associated with Wenckebach-like patterns of delayed activation following an isoelectric interval. Signal-averaging from the infarct border confirmed the presence of an isoelectric interval preceding the PVBs (N = 15). Pacing from the site of double potential formation accurately reproduced the surface ECG morphology (N = 15) of spontaneous PVBs. Closely-spaced epicardial mapping demonstrated delayed activation across an isoelectric interval representing "an arc of conduction block." Rate-dependent very slow antegrade conduction through a zone of apparent conduction block (N = 8) produced decremental activation delays until the delay was sufficient to excite epicardium distal to the original "arc of conduction block," resulting in PVB formation. CONCLUSION: The present experiments demonstrate double potential formation and rate-dependent constant-coupled late PVB formation in infarcted dog hearts. Electrode recordings demonstrate a prolonged isoelectric period preceding PVB formation consistent with very slow conduction (<70 mm/s) across a line of apparent conduction block and may represent a new mechanism of PVB formation following myocardial infarction.

Extra- and intracellular recordings from the avjunction: discerning the mechanisms for irregular ventricular responses during supraventricular arrhythmias.

INTRODUCTION: There has been a long-standing controversy regarding the mechanism(s) to explain the irregular ventricular response during atrial tachycardia (AT) or atrial fibrillation (AF) and where the site of block, if any, resides in the atrioventricular (AV) junction. METHODS: We studied 12 Langendorff preparations perfused with modified Tyrode's solution containing 5-10 mM diacetyl monoxime which suppressed contractility but allowed the use of intracellular action potential (AP) recordings. Octapolar catheters (2-mm rings, 2-mm spacing) were secured along the tricuspid annulus from the apex to the base of the triangle of Koch and along the anterior limbus of the fossa ovalis to record extracellular, slow pathway, fast pathway, His bundle (Hb) and AV nodal (AVN) extracellular potentials as well as intracellular action potentials. RESULTS: AT or AF induced by rapid atrial pacing showed a variety of irregular responses due to: (1) Wenckebach conduction showing decrement within the AVN and progressive diminution of extracellular AVN potentials (n = 5); (2) repetitive concealed conduction proximal to the AVN (n = 3); (3) ectopic beats arising within the AVN (n = 2); (4) ectopic beats arising at the Hb (n = 2). CONCLUSIONS: In this experimental preparation, extracellular and intracellular recordings provided presumptive evidence for the mechanisms causing the irregularities of the ventricular response such as repetitive concealed conduction, enhanced automaticity or electrotonically triggered activity. Also more definitive determinations of the site of block in the AV junction were also obtained.

Targeting proteasomes for cardioprotection.

The ubiquitin-proteasome system (UPS) plays a central role in intracellular protein degradation and regulates many cellular processes, including cell proliferation, inflammation, adaptation to stress, cell death, and the removal of damaged or misfolded proteins. Numerous studies have demonstrated that altered UPS function is involved in the pathogenesis of a wide range of cardiac diseases including hypertrophy and failure, myocardial ischemia, atherosclerosis, and diabetic cardiovascular disease. Impairment of proteasome function is a common feature of cardiac disease; however several studies have also demonstrated increased proteasome activity in models similar but not identical with those having decreased function. Recent studies have shown that use of proteasome inhibitors before or following production of the model of cardiac disease may confer cardioprotection under certain conditions.

Autonomic elements within the ligament of Marshall and inferior left ganglionated plexus mediate functions of the atrial neural network.

OBJECTIVES: We sought to systematically investigate the role of the ligament of Marshall (LOM) and inferior left ganglionated plexi (ILGP) in modulating electrophysiological functions. METHODS: The following structures were exposed in 36 dogs: (1) LOM, (2) superior left GP (SLGP) near the junction of left superior pulmonary vein (LSPV) and left atrium, (3) ILGP near the left inferior pulmonary vein-atrial junction, (4) anterior right GP (ARGP) near the sino-atrial node, and (5) inferior right GP (IRGP) at the junction of inferior vena cava and atria. High frequency stimulation (HFS; 0.6-8.0 V, 20 Hz, 0.1 msec in duration) was applied to the LOM, SLGP, ILGP, ARGP, IRGP, or vagosympathetic trunk. Ventricular rate (VR) during atrial fibrillation (AF) was compared before and after ablation of GP in different sequences. RESULTS: ARGP + ILGP ablation but not ARGP ablation alone eliminated the VR slowing response induced by LOM stimulation, suggesting that all the autonomic innervation from the LOM to AV node passes the ILGP. LOM ablation attenuated the VR slowing response caused by SLGP or left vagosympathetic stimulation, suggesting that LOM modulates the autonomic innervation between the AV node and the left vagosympathetic trunk or SLGP. ARGP attenuated while ARGP + ILGP ablation eliminated the VR slowing response induced by left vagosympathetic stimulation, suggesting that both ARGP and ILGP modulate the AV nodal innervation of the extrinsic and intrinsic cardiac autonomic nervous system (ANS). CONCLUSION: The LOM and ILGP function as the "integration centers" that modulate the autonomic interactions between extrinsic and intrinsic cardiac ANS on AV nodal function.

Antifibrillatory actions of cisatracurium: an atrial specific M2 receptor antagonist.

INTRODUCTION: Muscarinic receptor antagonists are proposed to prevent atrial fibrillation (AF), but also facilitate AV conduction, limiting clinical usefulness. METHODS: Cisatracurium, a neuromuscular blocker, was administered to anesthetized dogs (0.05-0.8 mg/kg IV) and was administered to superfused pulmonary vein (PV) tissues in vitro. RESULTS: Dose-dependent suppression of AF induced by premature atrial stimuli was observed under control conditions (n = 3), right vagus nerve stimulation (n = 7), and anterior right ganglionated plexus stimulation (n = 3). AF was prevented (P < 0.0001) concurrent with suppression of the decreased atrial MAP duration/ERP accompanying vagus nerve stimulation without altering AH intervals or sinus cycle length. Although atropine (0.001-0.016 mg/kg, n = 4) suppressed AF (P < 0.04) in association with suppression of atrial MAP shortening induced by vagus nerve stimulation, atropine also prevented sinus cycle length and AH interval prolongation with vagus nerve stimulation, and decreased AV effective and functional refractory periods. In vitro, both cisatracurium and atropine prevented (1) action potential shortening produced by acetylcholine administration and (2) action potential shortening and arrhythmia triggering within PV sleeves produced by local autonomic nerve stimulation, atropine producing competitive inhibition, and cisatracurium producing noncompetitive M(2) muscarinic receptor blockade. CONCLUSIONS: Cisatracurium demonstrates a dose-dependent (1) suppression of AF and atrial action potential shortening accompanying vagus nerve stimulation without facilitating sinus or atrioventricular nodal function and (2) noncompetitive blockade of action potential shortening and triggered firing induced in isolated PVs by local autonomic nerve stimulation. The data are consistent with allosteric binding of cisatracurium to the M(2) muscarinic receptor in canine atrium.

Inducibility of atrial and ventricular arrhythmias along the ligament of marshall: role of autonomic factors.

BACKGROUND: The mechanism(s) underlying atrial fibrillation (AF) initiation along the ligament of Marshall (LOM) remains controversial. OBJECTIVES: We sought to examine the role of the autonomic nervous system in arrhythmogenesis along the LOM. METHODS: In 31 anesthetized dogs, a left thoracotomy exposed the LOM. During atrial pacing, high-frequency stimulation (HFS: 200 Hz, 0.1 ms pulse width, 40 ms duration, 0.6-12 V) was delivered during atrial refractoriness to different sites of LOM (LOM(CS)= near coronary sinus; LOM(LIPV)= near left inferior pulmonary vein; LOM(LS-LIPV)= between LIPV and left superior pulmonary vein (LSPV); LOM(LSPV)= near LSPV). HFS was repeated after intravenous administration of esmolol (1 mg/kg; n = 9) or atropine (2 mg; n = 12). Norepinephrine (10(-7) M, 0.4 cc) was injected into LOM(LSPV) (n = 5). RESULTS: The median voltages for HFS to induce AF were 3.2 V, 3.2 V, 8.0 V*(,double dagger), and 12 V*(,double dagger) at LOM(CS), LOM(LIPV), LOM(LS-LIPV), and LOM(LSPV), respectively (*P < 0.01, compared with LOM(CS) and double dagger P < 0.01, compared with LOM(LIPV)). Esmolol or atropine markedly increased the threshold for AF induction. Ventricular tachycardias (VT) and accelerated junctional rhythm were induced in 8 of 12 and 6 of 12 dogs after atropine administration, respectively. Sustained VT occurred within minutes in 5 of 5 dogs receiving norepinephrine injection into the LOM(LSPV.) CONCLUSION: HFS induced AF along LOM with a gradient of stimulation thresholds from LOM(CS) (lowest) toward LOM(LSPV) (highest). This response was inhibited by esmolol or atropine. These data suggest an autonomic basis for AF initiation in LOM, and both sympathetic and parasympathetic neural elements play an important role in AF initiation. Hyperactivity of the sympathetic neural elements in LOM may be crucial in the initiation of ventricular tachyarrhythmias.

An acute model for atrial fibrillation arising from a peripheral atrial site: evidence for primary and secondary triggers.

BACKGROUND: We previously demonstrated that acetylcholine (Ach) injected into cardiac ganglionated plexi (GP) causes pulmonary vein (PV) ectopy initiating atrial fibrillation (AF). OBJECTIVE: To determine the effects of Ach applied at non-PV sites. METHODS: Overall, 54 dogs were anesthetized with Na-pentobarbital. A right and left thoracotomy allowed the placement of multielectrode catheters to record from the superior PVs, mid portion of the atrium and the atrial appendages (AA). A monophasic action potential (MAP) was recorded from the AA. Ach (1, 10, 100 mM) was applied sequentially to the AA. RESULTS: In 19 of 26 animals, Ach 100 mM on the right (n = 15) or left (n = 4) AA induced focal, sustained AF (>or=10 minutes) with rapid regular firing (cycle length = 37 +/- 7 ms) at the AA. A clamp with teeth placed across the AA caused arrest in the AA. However, AF was sustained only when PV sites adjacent to the GP manifested complex fractionated atrial electrograms (CFAE). Clamping the AA prior to Ach (100 mM) application resulted in focal AF arising at the PVs but not at the AA. When a clamp without teeth was applied prior to Ach application, no AF at either AA or PV site could be induced. CONCLUSION: Isolation of the focal AF at the AA (primary trigger) by clamping caused cessation of activity in the AA, but AF continued due to secondary triggers arising from PVs. The possible mechanism(s) responsible for these findings are discussed, and various ancillary experiments (n = 28) were added to help elucidate mechanisms.

Autonomic mechanism to explain complex fractionated atrial electrograms (CFAE).

OBJECTIVE: To simulate complex fractionated atrial electrograms (CFAE) during sustained atrial fibrillation (AF) in experimental animals. BACKGROUND: The mechanism(s) underlying CFAE has not been fully elucidated. METHODS: Twenty-two dogs were subjected to a right and/or left thoracotomy. A gauze patch soaked with acetylcholine (ACh) was placed on the right atrial appendage (RAA) to induce sustained AF. During AF, varying concentrations of ACh (1, 10, 100 mM) were "painted" on the RA where electrograms showed regular organized activity. In another six dogs, anterior right ganglionated plexi (ARGP) near the sino-atrial node and inferior right GP (IRGP) at the junction of inferior vena cava and atria were sequentially ablated. In five dogs, ACh was injected into ARGP to induce CFAE. RESULTS: During sustained AF, local "painting" with ACh 1 mM and 10 mM induced intermittent CFAE in 1 of 11 and 10 of 11 dogs, respectively. With 100 mM ACh, all 11 showed CFAE (two intermittent, nine continuous). In six other dogs, continuous CFAE induced by topical application of 100 mM ACh were markedly attenuated by ARGP + IRGP ablation. In another five of five dogs, ACh injection into ARGP induced a gradient of CFAE with the continuous CFAE always occurring near the ARGP and CFAE also occurring at left pulmonary vein-atrial junctions. During ARGP ablation, AF was terminated in all five dogs immediately after regularization of the rotor-like electrograms or continuous CFAE. CONCLUSIONS: This study demonstrates an autonomic basis for CFAE formation, suggesting that graded hyperactive states of the autonomic nervous system (ANS) may induce various types of CFAE observed clinically.

Spontaneous pulmonary vein firing in man: relationship to tachycardia-pause early afterdepolarizations and triggered arrhythmia in canine pulmonary veins in vitro.

INTRODUCTION: Rapid firing originating within pulmonary veins (PVs) initiates atrial fibrillation (AF). The following studies were performed to evaluate spontaneous PV firing in patients with AF to distinguish focal versus reentrant mechanisms. METHODS: Intracardiac recordings were obtained in 18 patients demonstrating paroxysmal AF. Microelectrode (ME) recordings were obtained from superfused canine PV sleeves (N = 48). RESULTS: Spontaneous PV firing (566 +/- 16 bpm; 127 +/- 6 ms cycle length) giving rise to AF (52 episodes) was observed. Tachycardia-pause initiation was present in 132 of 200 episodes of rapid PV firing and 34 of 52 AF episodes. The pause cycle length preceding PV firing was 1,039 +/- 86 ms following tachycardia (420 +/- 40 ms cycle length). The remaining episodes were initiated following a 702 +/- 32 ms pause during sinus rhythm (588 +/- 63 ms). Spontaneous firing recorded with a multipolar mapping catheter did not detect electrical activity bridging the diastolic interval between the initial ectopic and preceding post-pause sinus beat. Tachycardia-pause initiated PV firing (138 +/- 7 ms coupling interval) in patients correlated with tachycardia-pause enhanced isometric force, early afterdepolarization (EAD) amplitude, and triggered firing within canine PVs. Rapid firing (1,172 +/- 134 bpm; 51 +/- 8 ms cycle length) following an abbreviated coupling interval (69 +/- 12 ms) was initiated in 13 of 18 canine PVs following tachycardia-pause pacing during norepinephrine + acetylcholine superfusion. Stimulation selectively activating local autonomic nerve terminals facilitated tachycardia-pause triggered firing in canine PVs (5 of 15 vs 0 of 15; P < 0.05). CONCLUSIONS: The studies demonstrate (1) tachycardia-pause initiation of rapid, short-coupled PV firing in AF patients and (2) tachycardia-pause facilitation of isometric force, EAD formation, and autonomic-dependent triggered firing within canine PVs, suggestive of a common arrhythmia mechanism.

Early myocardial dysfunction in streptozotocin-induced diabetic mice: a study using in vivo magnetic resonance imaging (MRI).

BACKGROUND: Diabetes is associated with a cardiomyopathy that is independent of coronary artery disease or hypertension. In the present study we used in vivo magnetic resonance imaging (MRI) and echocardiographic techniques to examine and characterize early changes in myocardial function in a mouse model of type 1 diabetes. METHODS: Diabetes was induced in 8-week old C57BL/6 mice with two intraperitoneal injections of streptozotocin. The blood glucose levels were maintained at 19-25 mmol/l using intermittent low dosages of long acting insulin glargine. MRI and echocardiography were performed at 4 weeks of diabetes (age of 12 weeks) in diabetic mice and age-matched controls. RESULTS: After 4 weeks of hyperglycemia one marker of mitochondrial function, NADH oxidase activity, was decreased to 50% of control animals. MRI studies of diabetic mice at 4 weeks demonstrated significant deficits in myocardial morphology and functionality including: a decreased left ventricular (LV) wall thickness, an increased LV end-systolic diameter and volume, a diminished LV ejection fraction and cardiac output, a decreased LV circumferential shortening, and decreased LV peak ejection and filling rates. M-mode echocardiographic and Doppler flow studies of diabetic mice at 4 weeks showed a decreased wall thickening and increased E/A ratio, supporting both systolic and diastolic dysfunction. CONCLUSION: Our study demonstrates that MRI interrogation can identify the onset of diabetic cardiomyopathy in mice with its impaired functional capacity and altered morphology. The MRI technique will lend itself to repetitive study of early changes in cardiac function in small animal models of diabetic cardiomyopathy.

Interactive atrial neural network: Determining the connections between ganglionated plexi.

BACKGROUND: The electrophysiologic functions of the intrinsic cardiac autonomic nervous system (ANS) are not well understood. OBJECTIVES: The purpose of this study was to investigate the functional interactions between ganglionated plexi within the intrinsic cardiac ANS. METHODS: The hearts of 21 dogs were exposed via right and/or left thoracotomy to expose the (1) anterior right ganglionated plexi near the caudal end of the sinoatrial node, (2) inferior right ganglionated plexi at the junction of inferior vena cava and atria, and (3) superior left ganglionated plexi near the junction of left superior pulmonary vein and left pulmonary artery. Ganglionated plexi were stimulated at 0.6 to 8.0 V (square waves, 20 Hz, 0.1-ms duration). Sinus rate, AH interval during atrial pacing, and ventricular rate during atrial fibrillation were compared before and after ganglionated plexi stimulation and after their ablation. RESULTS: Anterior right ganglionated plexi stimulation induced significant AH prolongation and slowing of ventricular rate and sinus rate. When inferior right ganglionated plexi was ablated, slowing of sinus rate by anterior right ganglionated plexi stimulation was unaltered, but inhibition of AV conduction was eliminated. Superior left ganglionated plexi stimulation induced similar effects on sinus and AV nodal function, and sinus rate slowing was markedly attenuated by anterior right ganglionated plexi ablation. Ablation of both anterior right ganglionated plexi and inferior right ganglionated plexi eliminated AV conduction inhibition but not sinus rate slowing by superior left ganglionated plexi stimulation. CONCLUSION: This study provides functional evidence for the interconnections between ganglionated plexi to modulate sinus and AV nodal function, supporting clinical evidence that interconnections within the intrinsic cardiac ANS are critical elements in identifying the targets for atrial fibrillation ablation.

Rapid and stable re-entry within the pulmonary vein as a mechanism initiating paroxysmal atrial fibrillation.

OBJECTIVES: We investigated the hypothesis that re-entrant pulmonary vein (PV) tachycardias may serve as a mechanism for initiating and sustaining paroxysmal atrial fibrillation (PAF). BACKGROUND: The mechanisms of rapid repetitive discharges from the PV initiating PAF remain incompletely understood. Pulmonary vein myocardial sleeves appear to provide a favorable substrate for re-entry formation. METHODS: The electrophysiologic properties of canine PV sleeves were investigated using a combination of high-resolution optical mapping (n = 5) and extracellular bipolar and intracellular microelectrode recordings (n = 56) in a superfused PV preparation. RESULTS: From the left atrium to distal PV, there was progressive shortening of the action potential (AP) duration, reduction in AP and bipolar electrogram amplitude, and depolarization of resting membrane potentials. Sustained PV tachycardias were induced exclusively in the presence of acetylcholine (10(-7) to 10(-6) mol/l, n = 12). Sustained PV tachycardias were rapid (mean cycle length = 93 +/- 15 ms), regular, and capable of induction, termination, and resetting by single extrastimuli. Re-entry as the mechanism underlying PV tachycardias was confirmed by optical mapping (n = 5). Acetylcholine also reduced the slope of the AP restitution curve and suppressed AP alternans (n = 6). Importantly, PV tachycardias exhibited 1:1 conduction into the atrium at short cycle lengths (<100 ms), emphasizing the potential role of re-entrant PV tachycardia in atrial fibrillation. CONCLUSIONS: Pulmonary veins provide a favorable substrate for re-entry formation. Heterogeneity of the electrophysiologic properties and marked abbreviation of action potential duration and refractoriness by acetylcholine combine to produce rapid and stable re-entrant PV tachycardias. Elevated parasympathetic tone and re-entrant PV tachycardia may serve as a mechanism underlying the perpetuation of PAF.

The neural basis of atrial fibrillation.

This review addresses recent basic and clinical studies which suggest that targeting autonomic nerves and ganglia on the heart can result in suppression of atrial fibrillation (AF) with less damage to myocardium than the presently employed procedure which involves extensive pulmonary vein (PV) isolation from the rest of the left atrium. CLINICAL STUDIES: Clinical electrophysiologists in 1998 discovered that the majority of patients with paroxysmal form of AF, resistant to drugs and cardioversion, had focal, ectopic firing arising from the myocardial sleeves covering the PVs. They developed a strategy which called for inducing radiofrequency lesions which would supposedly isolate the PVs from the atria thereby curing this form of AF. To date this strategy has had limited success (70-85%). A new approach relies on targeting the ganglionated plexi (GP) at the entrances of the PVs. Several clinical reports provide evidence that this new approach can increase the success rate for radiofrequency ablation of paroxysmal AF (91-99%). BASIC STUDIES: Experimental investigations in animal studies, both in vivo and in vitro, have accumulated evidence for a mechanistic basis for the ablation of GP to terminate paroxysmal AF. Specifically, release of the neurotransmitter, acetylcholine, from these GP causes shortening of atrial and PV sleeve refractoriness. In addition, the concomitant release of adrenergic neurotransmitters mobilizes excess calcium intracellularly leading to early afterdepolarizations and triggered firing particularly in PV cells. We conclude that hyperactivity of these local cardiac GP play a critical role in initiating the paroxysmal form of AF resistant to drugs and cardioversion. Targeting the GP for ablation can substantially increase the success rate for terminating AF in these patients.

Triggered firing in pulmonary veins initiated by in vitro autonomic nerve stimulation.

BACKGROUND: Rapid firing within pulmonary vein sleeves frequently initiates atrial fibrillation. The role of the autonomic nervous system in facilitating spontaneous firing is unknown. OBJECTIVES: The purpose of this study was to determine if autonomic nerve stimulation within canine atrium and pulmonary vein sleeves initiates arrhythmia formation. METHODS: Extracellular bipolar and intracellular microelectrode recordings were obtained from isolated superfused canine pulmonary veins (N = 28) and right atrium (N = 5) during local autonomic nerve stimulation. RESULTS: Autonomic nerve stimulation decreased pulmonary vein sleeve action potential duration (APD90 = 160 +/- 17 to 92 +/- 24 ms; P < .01) and initiated rapid (782 +/- 158 bpm) firing from early afterdepolarizations in 22 of 28 pulmonary vein preparations. The initial spontaneous beat had a coupling interval of 97 +/- 26 ms. Failure to induce arrhythmia was associated with a failure to shorten APD90 (151 +/- 18 to 142 +/- 8 ms; P = .39). Muscarinic receptor blockade (atropine: 3.2 x 10(-8) M) prevented APD90 shortening in 8 of 8 preparations and suppressed firing in 6 of 8 preparations, whereas beta1-adrenergic receptor blockade (atenolol: 3.2 x 10(-8) M) suppressed firing in 8 of 8 preparations. Suppression of the Ca transient with ryanodine (10(-5) M) completely suppressed firing in 6 of 6 preparations. Inhibition of forward Na/Ca exchange by a transient increase in [Ca+2]o completely suppressed firing in 4 of 6 preparations. The same stimulus trains produce atropine-suppressed APD90 shortening in superfused right atrial free wall but fail to produce triggered arrhythmia. CONCLUSIONS: The data demonstrate triggered firing within canine pulmonary veins with combined parasympathetic and sympathetic nerve stimulation. Both an enhanced Ca transient and increased Na/Ca exchange may be required for arrhythmia formation.

Characteristics of a charged-coupled-device-based optical mapping system for the study of cardiac arrhythmias.

We develop an optical fluorescent mapping system that is able to record the action potential wavefront propagation within cardiac tissue samples with high spatial and temporal resolutions. The system's main component, the fluorescence acquisition device (customized CCD camera), offers a high spatial resolution of 128 x 128 pixels, with 12-bit digitization and a frame rate of 490 frames/s. The system is designed and implemented to image an area of approximately 20 x 20 mm at its minimum object distance of 140 mm, corresponding to a spatial resolution of approximately 3 line pairs/mm. Experiments using this system with di-4-ANEPPS-stained canine cardiac tissues with stimulated action potentials through external electrodes result in successful mappings of the distribution and propagation of the action potential wavefronts, showing the system's sensitivity to the change in fluorescence intensity in regions of action potentials. These data demonstrate this optical mapping system as a powerful device in the study of cardiac arrhythmia mechanisms.

Delineation of AV conduction pathways by selective surgical transection: effects on antegrade and retrograde transmission.

INTRODUCTION: The role for transitional cells as determinants of AH and HA conduction was examined in the superfused rabbit AV junction. METHODS: Bipolar electrodes and microelectrodes were used to record antegrade A-H and retrograde H-A activation, before and after transection of the transitional cell input to the compact AV node. RESULTS: During pacing from the high right atrium, inferior to the coronary sinus os, beneath the fossa ovalis, or on the anterior limbus, AV Wenckebach block (WB) was mediated by identical transitional cells grouped in close apposition to the compact AV node. Paced WB cycle lengths were shorter from the high right atrium (196+/-12 msec) and inferior to the coronary sinus os (195+/-8 msec) versus the fossa ovalis (217+/-9 msec) or anterior limbus (206+/-11 msec). With His bundle pacing, retrograde HA WB (211+/-17 msec) was observed within the N cell region within the compact AV node. After transection of posterior and superior transitional cell input to the compact AV node, the antegrade AH WB cycle length was prolonged (245+/-18 msec), with an increased WB incidence within the NH region (compact AV node)(5% to 41%; p=0.014). The incidence of retrograde HA WB determined within the NH region was increased (30% to 88%), with a decrease in the stimulus-fast pathway conduction time (98+/-7 to 49+/-6 msec; p<0.01). CONCLUSIONS: The data demonstrate (1) a common transitional cell population determining AH WB, independent of atrial stimulation site, and (2) a plasticity of transitional cell-compact AV node connections, with rapid AH and HA conduction favored by removal of posterior/superior AV nodal input.

Electrical stimulation to identify neural elements on the heart: their role in atrial fibrillation.

EXPERIMENTAL STUDIES: Anesthetized dogs were subjected to a right then left thoracotomy. Two modes of electrical stimulation were used to activate ganglionated plexi (GP) on the epicardium of the atria: (1) Near the base of each pulmonary vein (PV), trains of high frequency stimuli (HFS) were coupled to each atrial paced beat so as to fall within the refractory period to achieve nerve stimulation without atrial excitation; and (2) Continuous HFS was applied via plaque electrodes sutured to epicardial fat pads (containing a GP) near the right superior (RS) and left superior (LS) PVs. The chest was then closed. An ablation catheter, inserted percutaneously, was positioned fluoroscopically in the right atrium across from the epicardial plaque electrode near the RSPV. Transeptal puncture was used to place an ablation catheter at the LSPV-left atrial junction. HFS applied to each of the epicardial fat pads induced atrial fibrillation (AF) and also caused high grade AV block due to a strong parasympathetic effect on the AV node. Radiofrequency ablation from the right and left atrial endocardium abolished the vagal response to HFS delivered to the plaque electrodes on the fat pads close to the RSPV and LSPV, respectively. CLINICAL STUDIES: Sixty (60) patients with paroxysmal or persistent AF underwent PV antrum isolation (27 patients) or PV antrum isolation plus left atrial GP ablation (33 patients). Endocardial HFS at the border of the PV antra near the 4 GPs produced AF and high grade AV block (vagal response) during AF. RFA at these sites abolished the vagal response. Testing in a small number of patients with very short follow-up suggests that adding GP ablation to PV antrum isolation may increase ablation success (absence of AF recurrence) from 70% to 91%. CONCLUSIONS: These basic and clinical studies suggest that localized cardiac autonomic ganglia (GPs) may play a critical role in the initiation and maintenance of AF.

Encapsulation of a plasminogen activator speeds reperfusion, lessens infarct and reduces blood loss in a canine model of coronary artery thrombosis.

In the present study, a polymer-encapsulated plasminogen activator was investigated as an alternative to restore blood flow more effectively than free plasminogen activator. While current fibrinolytic agents have limited efficacy, attributable to delayed onset of sustained reperfusion and bleeding complications, encapsulated plasminogen activators have shown promise in addressing these shortcomings. A polymer-encapsulated plasminogen activator could offer an effective formulation with a prolonged shelf-life. In this study, coronary artery thrombosis was produced in the anesthetized dog by the injection of thrombin + whole blood, and then one of five randomly selected formulations was administered intravenously: saline, blank microcapsules, free streptokinase (FREE SK), streptokinase and blank microcapsules (FREE SK + BLANK), or streptokinase entrapped in polymer microcapsules (MESK). MESK significantly accelerated the time to reperfusion compared to FREE SK or FREE SK + BLANK. Additionally, substantial reductions were observed in residual clot mass, infarct mass, reocclusion episodes, fibrinogen depletion and blood loss with MESK compared to FREE SK. The results of this study demonstrate that MESK accelerates thrombolysis and the restoration of blood flow compared to identical dosages of FREE SK while also reducing systemic fibrinogenolysis and blood loss. Microencapsulation may produce an improved dosage form for restoring arterial blood flow and reducing bleeding complications with thrombolytic therapy.

Accelerated thrombolysis in a rabbit model of carotid artery thrombosis with liposome-encapsulated and microencapsulated streptokinase.

The present study compares the efficacy of two formulations of encapsulated streptokinase to streptokinase in a rabbit model of carotid artery thrombosis. Arterial thrombosis followed the injection of thrombin mixed with autologous whole blood into a carotid artery of New Zealand white rabbits. Thirty minutes after the confirmation of an occlusive thrombus, one of four streptokinase formulations was infused at a dosage of 6000 IU/kg into the jugular vein. Free streptokinase (FREE SK) was compared to identical dosages of streptokinase encapsulated in a liposome (LESK), streptokinase entrapped in a water-soluble polymer (MESK), and free streptokinase admixed with blank microparticles (FREE SK + BLANK). Carotid arterial blood flow was determined by pulsed Doppler flowmetry to confirm clot formation and reperfusion. Two hours after drug infusion, the rabbits were killed and the residual thrombus mass was determined. Compared to FREE SK (74.5 +/- 16.9 min; mean +/- SEM), LESK demonstrated significantly reduced reperfusion times (19.3 +/- 4.6 min) while MESK exhibited even greater improvement (7.3 +/- 1.6 min). FREE SK + BLANK showed no statistical improvement versus FREE SK. LESK and MESK also resulted in reduced residual clot mass and greater return of arterial blood flow. These studies suggest that encapsulation of streptokinase offers a potential method of improved fibrinolytic treatment for patients with clot-based disorders. MESK performed slightly better than LESK with improved production and storage characteristics.