Three-Dimensional Electromagnetic Catheter Technology:

Electroanatomical Mapping. Introduction : The difficult catheter orientation and navigation associated with conventional technology and mono-/multiplane fluoroscopy may complicate ablation procedures of atrial tachycardias. A new three-dimensional catheter technology for electroanatomical mapping of the right atrium and ablation of ectopic atrial tachycardia is described.
Methods and Results : A novel electromagnetic catheter-based mapping system was investigated for electroanatomical mapping of the entire right atrium in 12 patients. The system reconstructed three-dimensional maps from the multitude of endocardial sites that were sequentially mapped and color coded the individual activation times. The electrophysiologic information was superimposed on the geometry of the mapped area. The anatomical landmarks of the right atrium, i.e., the tricuspid annulus. mouth of the coronary sinus, ostia of the superior and inferior venae cavae, and right atrial appendage, could he depicted in all cases. The sinus node area and the preferential conduction along the crista terminalis could be delineated. In four patients with ectopic atrial tachycardia, the earliest endocardial activation could be identified with high spatial resolution as a "hot spot." After completion of the mapping procedure, the ablation catheter could be reliably renavigated to the site of origin, and ablation was successful with one or two impulses. In one patient with previous atrial septal repair, the activation map allowed the reconstruction of a long line of conduction block induced by the atriotoniy.
Conclusion : Three-dimensional electroanatomical mapping of the right atrium allowed detailed reconstruction of the chamber geometry and activation sequence. The sites of origin of ectopic atrial tachycardias could be identified precisely. The system allowed accurate renavigation to the site of earliest activation, thereby guiding successful ablation of the foci.     

Carto标测指导下射频导管消融左心耳房性心动过速

目的探讨三维电解剖Carto指导下标测消融源于左心耳部位房性心动过速(房速)的方法和可行性。方法结合电生理和空间信息,首先利用Carto系统建立左心房三维解剖结构。对3例起源于左心耳的房速进行Carto标测,根据Carto标测来确定最早激动点,并以此为靶点进行射频消融。同时分析心动过速时体表心电图的P波特点。结果电解剖标测证实3例房速均为局灶性房速,其最早激动点起源于左心耳,并向左心房前壁、房间隔和后下壁激动。左心耳放电成功消融3例房速。体表心电图分析显示房速时Ⅱ、Ⅲ、aVF和V1导联P波为正向,I、aVL导联为完全负向。结论三维电解剖标测可以清楚显示左心耳解剖结构以及源于其中的房速的激动顺序并有利于经导管进行射频消融。     

三维标测指导下射频导管消融右心耳起源的局灶性房性心动过速

目的 报道应用三维标测指导射频导管消融起源于右心耳的局灶性房性心动过速(房速),并初步探讨其临床及心电学特征.方法 共6例患者(男性4例,女性2例,年龄(43±19)岁]临床诊断为窄QRS心动过速,其中3例曾行常规射频消融失败,4例左心房内径明显扩大.经电生理检查证实为房速.术中行EnSite-NavX激动标测或者Carto电解剖标测以明确局灶性房速并指出最早激动大致范围.在局部做精细标测找到心房最早激动处,于心动过速时应用盐水灌注导管放电消融,能量30～40 W,温度43℃.即刻成功指标为心动过速终止并不再被诱发.结果 6例心动过速平均心动周期为(343±53)ms.三维激动标测结果显示房速呈右心耳部位点状扩布,并且整个右心房激动时间占心动周期的27%±8%.成功消融靶点局部A波较体表心电图P波提前(52±13)ms.消融后行右心房心耳造影确认消融导管位置.6例右心耳房速均成功消融且未有并发症发生.随访3个月其中1例复发心动过速,经再次标测证实为三尖瓣前侧部局灶性房速并且成功消融.左心房扩大者心房内径较术前显著缩小[(41±6)mm对(36±6)mm,P＜0.05].结论 局灶性房速可起源于右心耳并可以成功消融.三维标测有助于靶点定位及消融成功.     

Focal atrial tachycardia originating from the left atrial appendage: electrocardiographic and electrophysiologic characterization and long-term outcomes of radiofrequency ablation

Introduction: This study sought to investigate electrophysiologic characteristics and radiofrequency ablation (RFA) in patients with focal atrial tachycardia (AT) arising from the left atrial appendage (LAA).
Methods: This study included seven patients undergoing RFA with focal AT. Activation mapping was performed during tachycardia to identify an earlier activation in the left atria and the LAA. The atrial appendage angiography was performed to identify the origin in the LAA before and after RFA.
Results: AT occurred spontaneously or was induced by isoproterenol infusion rather than programmed extrastimulation and burst atrial pacing in any patient. The tachycardia demonstrated a characteristic P-wave morphology and endocardial activation pattern. The P wave was highly positive in inferior leads in all patients. Lead V₁ showed upright or biphasic (±) component in all patients. Lead V₂–V₆ showed an isoelectric component in five patients or an upright component with low amplitude (<0.1 mV) in two patients. Earliest endocardial activity occurred at the distal coronary sinus (CS) ahead of P wave in all seven patients. Mean tachycardia cycle length was 381 ± 34 msec and the earliest endocardial activation at the successful RFA site occurred 42.3 ± 9.6 msec before the onset of P wave. RFA was acutely successful in all seven patients. Long-term success was achieved in seven of the seven over a mean follow-up of 24 ± 5 months.
Conclusions: The LAA is an uncommon site of origin for focal AT (3%). There were consistent P-wave morphology and endocardial activation associated with this type of AT. The LAA focal ablation is safe and effective. Long-term success was achieved with focal ablation in all patients.     

Successful surgical treatment of focal atrial tachycardia--a case report and review of the literature

Focal atrial tachycardia with frequencies up to 200/min in an 11-year-old girl had been resistant to drugs over a period of 5 years. An electrophysiological study demonstrated a left atrial ectopic tachycardia. Intraoperative epicardial mapping localized the automatic focus at the base of the left atrial appendage. Excision of the appendage and cryoablation of the adjacent area were performed using cardiopulmonary bypass. Histologic examination of the excised tissue showed no specific alterations aside from islets of fatty tissue. The girl has been in sinus rhythm and has shown no tachycardias during the 6 months following the operation. A review of the literature, covering 17 cases of right and 12 cases of left atrial focal tachycardia, indicates excision of the atrial tissue in combination with cryoablation to be the treatment of choice to ensure success.     

Ectopic atrial tachycardia: surgical therapy (case report and review of the literature)

Focal atrial tachycardia with frequencies up to 200/min in an 11-year-old girl had been resistant to drugs over a period of 5 years. An electrophysiological study demonstrated a left atrial ectopic tachycardia. Intraoperative epicardial mapping localized the automatic focus at the base of the left atrial appendage. Excision of the appendage and cryoablation of the adjacent area were performed using cardiopulmonary bypass. Histological examination of the excised tissue showed no specific alterations aside from islets of fatty tissue. The girl has been in sinus rhythm and has shown no tachycardias during the 6 months following the operation. Review of the literature covering 17 cases of right and 12 cases of left atrial focal tachycardia indicates excision of the atrial tissue in combination with cryoablation to be the treatment of choice to ensure success.     

Electrophysiological Characteristics of Localized Reentrant Atrial Tachycardia Occurring After Catheter Ablation of Long-Lasting Persistent Atrial Fibrillation

Background: Mapping of recurrent atrial tachycardia (AT) after extensive ablation for long-lasting persistent atrial fibrillation (AF) is complex. We sought to describe the electrophysiological characteristics of localized reentry occurring after ablation of long-lasting persistent AF.
Methods: Out of 70 patients undergoing catheter ablation of long-lasting persistent AF, 9 patients (13%, 55 ± 8 years, 8 males) in whom localized reentry was demonstrated in a repeat ablation were studied. Localized reentry was defined as reentry in which the circuit was localized to a small area and did not have a central obstacle. The mechanism of AT was determined by electroanatomical and entrainment mapping.
Results: Nine localized reentries with cycle length of 243 ± 41 ms were mapped in 9 patients. The location of AT was the left atrial appendage in 4 patients, anterior left atrium in 2, left septum in 2, and mitral isthmus in 1. In all ATs, a critical isthmus of <10 mm in width was identified in the vicinity of the prior linear lesions or ostia of isolated pulmonary veins. Ablation of the critical isthmus, which was characterized by continuous low-voltage activity (median voltage: 0.15 mV, mean duration: 117 ± 31 ms), terminated AT and rendered it noninducible. Additionally, ablation was performed for all of inducible ATs. At 11 ± 7 months after the procedure, 8 of 9 patients (89%) were free from any arrhythmias.
Conclusions: After ablation of long-lasting persistent AF, localized reentry may arise from a site in the vicinity of the prior ablation lesions. Ablation of the critical isthmus eliminates the arrhythmia.     

Right pulmonary vein potentials recorded from the posterior right atrial endocardium: human case report and validation in a porcine model

A 33-year-old woman underwent successful catheter ablation of an atrial tachycardia emanating from deep within a large right superior pulmonary vein (RSPV). A previous ablation attempt in this patient had failed, during which radiofrequency energy applications were made to the posterior right atrium only. The mistaken impression of a right atrial source for this tachycardia was due to RSPV potentials that were recorded during mapping in a region of the posterior right atrium contiguous to the vein ("contiguity region"). To further evaluate this, we performed activation mapping and radiofrequency ablation in atria of healthy pigs. Similar to the reported case, "biatrial" potentials were recorded from both left and right aspects of the contiguity region. Radiofrequency energy application altered only the potential emanating from the atrium in which the lesion was applied. Histologic analysis confirmed that the lesion was limited to this atrium. It is concluded that, due to their proximity, electrical activity in the RSPV may be recorded from certain areas of the posterior right atrium, and vice versa. In the reported case of left atrial tachycardia, this led to the mistaken impression of right atrial tachycardia.     

Identification of left atrial origin of ectopic tachycardia during right atrial mapping: analysis of double potentials at the posteromedial right atrium

INTRODUCTION: The high posteromedial right atrium is adjacent to the left atrium near the right superior pulmonary vein. We hypothesized that analysis of electrograms at this site could distinguish left from right atrial tachycardia. METHODS AND RESULTS: Atrial mapping was performed in 16 patients with left atrial origin ectopic tachycardia (11 patients with right superior pulmonary vein origin and 5 patients with other left atrial tachycardias). During left atrial tachycardia, earliest right atrial activation was recorded at the high posterior right atrium in 14 of 16 patients. At all of these 14 early sites, double potentials were recorded during tachycardia. The first potential was a far-field signal from left atrium as indicated by the following: (1) during sinus beats, the timing of the two potentials reversed such that the left atrial one was late; (2) ablation at the right atrial site did not decrease the amplitude of the first potential, but did decrease the amplitude of the second potential; and (3) the timing of activation at the adjacent left atrium agreed with that of the first potential. In the 11 right superior pulmonary vein tachycardias, the first potential was markedly earlier than the p wave onset, but in left atrial tachycardias with other origins it was later. In a control group of six patients with pacing to simulate right atrial tachycardia, double potentials were recorded in the posterior right atrium, but the timing of components did not reverse during sinus rhythm. CONCLUSION: For some left atrial ectopic tachycardias, particularly those originating from the right superior pulmonary vein, recognition of left versus right atrial origin can be accomplished during right atrial mapping by analysis of double potentials in the posteromedial right atrium.     

Ablation of atrial tachycardia originating from the vicinity of the atrioventricular node: significance of mapping both sides of the interatrial septum

OBJECTIVES: The purpose of the study was to examine the value of right- and left-sided mapping to identify the site of tachycardia origin. BACKGROUND: Focal atrial tachycardia may originate from the vicinity of the atrioventricular node from either side of the interatrial septum. METHODS: In 16 patients undergoing radiofrequency catheter ablation of perinodal atrial tachycardia, activation mapping of the right and left side of the interatrial septum was performed. RESULTS: Atrial tachycardia originated from the right side of the interatrial septum in 10 patients (group A) and from the left side in 6 patients (group B). On the right side, earliest atrial activity preceded the onset of the P-wave by 49 +/- 15 ms in group A and by 38 +/- 8 ms in group B (NS), and it preceded the signal recorded from the right atrial appendage by 59 +/- 19 ms in group A and by 60 +/- 13 ms in group B (NS). On the left side, earliest activity preceded the onset of the P-wave by 27 +/- 16 ms in group A and by 51 +/- 6 ms in group B (<0.01), and it preceded the signal obtained from the right atrial appendage by 38 +/- 19 ms in group A and by 73 +/- 9 ms in group B (<0.01). Atrial tachycardias were successfully eliminated in all patients without impairment of atrioventricular conduction. During follow-up, two patients had a recurrence of tachycardia. CONCLUSIONS: Mapping of only the right side cannot exclude a left-sided origin. Therefore, mapping of both sides of the interatrial septum is required prior to ablation of focal atrial tachycardia originating from the vicinity of the atrioventricular node.     

左房房性心动过速三维电磁导管标测系统(Carto)标测与射频消融

目的探讨左房房性心动过速(房速)三维电磁导管标测系统(Carto)系统标测特点及射频消融价值.方法 9例左房房速患者,应用Carto系统标测左心房,实时重建左房三维电解剖图;根据电解剖图,判断房速类型局灶性或大折返性房速;于心房最早激动点处或折返环的关键峡部消融.结果 9例患者中共有10个房速.在冠状静脉窦(CS) 电极中、远端或近端均记录到相对提早A波;9个房速为局灶性房速,激动图显示最早激动点位于肺静脉口部(5个)、左房后壁(2个)、左心耳口部(1个)、左心耳体部(1个);1个为大折返性房速,折返经过右上肺静脉口部与卵圆窝之间关键峡部.8个局灶性房速在上述最早激动点处消融,均成功终止房速,1个左心耳体部房速消融失败;大折返性房速于关键峡部行线性消融,获成功;随访6～30个月,其中1例局灶性房速术后次日复发,再次消融成功;无并发症;成功病例手术时间为90～140 min,X线照射时间为8～16 min.结论本组结果提示,应用Carto系统标测左房房速,判断房速类型准确、快速;指导消融安全、有效,可减少X线照射时间,进一步提高消融成功率,特别是对于常规方法消融失败病例尤有帮助.     

Partial atrial standstill: a case report

Twelve-lead electrocardiograms revealed no atrial activity and a wide QRS escape rhythm at 38 beats/min in a 20-year-old man who presented with syncope. Doppler echocardiography documented the absence of A wave both in the tricuspid and mitral valve flow. The only mechanical activity was documented at the left atrial appendage. An electrophysiologic study demonstrated electrical inactivity in the right atrium and an atrial tachycardia in the left atrium. Atrial pacing with maximum output did not capture the atria. Our case represents an advanced stage of partial atrial standstill, with a mechanical and electrical atrial activity confined only to the left trial appendage. The patient remained asymptomatic after receiving a VVIR pacemaker and anticoagulation therapy.     

If in left human atrium: a potential contributor to atrial ectopy

OBJECTIVE: The left human atrium plays an important role in initiation of atrial fibrillation (AF) and the hyperpolarization activated cation current (I(f)) is a candidate for contributing to abnormal automaticity. However, electrophysiological data concerning I(f) are not available in this cardiac region and we therefore investigated I(f) in human left atrial tissue. METHODS: Human atrial myocytes were isolated from the left atrial appendage (LAA) and the left atrial wall (LAW) obtained from patients undergoing open heart surgery. I(f) was measured with the whole-cell patch-clamp technique. RESULTS: I(f) densities between -70 and -110 mV were found to be significantly higher in LAA than in LAW cells. Furthermore, in the group of LAA cells the half maximal activation potential (V(1/2)) was found to be less negative (V(1/2) of -84.3+/-1.9 mV, n=14/9) compared to LAW cells (V(1/2) of -97.8+/-2.1 mV, n=28/9). Beta-adrenergic receptor stimulation with isoproterenol (1 microM) caused an acceleration of current activation and a V(1/2) shift to more positive potentials in cells of both regions (LAA: 8.8+/-2.3 mV, n=6/4 and LAW: 8.9+/-2.6 mV, n=6/4). Simulations using a mathematical model of the human atrial myocyte demonstrated that I(f) was able to induce spontaneous activity in the model at a regular rhythm due to the interplay of I(f), Na(+)/Ca(2+) exchange current and Ca(2+) release of the sarcoplasmic reticulum (SR). CONCLUSIONS: Our study revealed the presence of I(f) in left atrial myocytes and showed that I(f) parameters depend on atrial region. I(f) current densities were sufficient to convert the mathematical model of a quiescent human atrial cell into a "pacemaker cell". These data support the hypothesis of I(f) as a contributor to abnormal automaticity in human atrial tissue.     

Impact of catheter ablation of the coronary sinus on paroxysmal or persistent atrial fibrillation

Objectives: This study evaluated the impact of catheter ablation of the coronary sinus (CS) region during paroxysmal and persistent atrial fibrillation (AF).
Background: The CS musculature and connections have been implicated in the genesis of atrial arrhythmias.
Methods: Forty-five patients undergoing catheter ablation of AF were studied. The CS was targeted if AF persisted after ablation of pulmonary veins and selected left atrial tissue. CS ablation was commenced endocardially by dragging along the inferior paramitral left atrium. Ablation was continued from within the vessel (epicardial) if CS electrograms had cycle lengths shorter than that of the left atrial appendage.
RF energy was limited to 35 W endocardially and 25 W epicardially. The impact of ablation was evaluated on CS electrogram cycle length (CSCL) and activation sequence, atrial fibrillatory cycle length measured in the left atrial appendage (AFCL) and on perpetuation of AF.
Results: Endocardial ablation significantly prolonged CSCL by 17 ± 5 msec and organized the CS activation sequence (from 13% of patients before to 51% after ablation); subsequent epicardial ablation further increased local CSCL by 32 ± 27 msec (P < 0.001). AFCL prolonged significantly both during endocardial and epicardial ablation (median: 152 to 167 msec P = 0.03) and was associated with AF termination in 16 (35%) patients (46% of paroxysmal and 30% of persistent AF). AFCL prolongation ≥5 msec and/or AF termination was associated with more rapid activity in the CS region originally: P ≤ 0.04.
Conclusion: Catheter ablation targeting both the endocardial and epicardial aspects of the CS region significantly prolongs fibrillatory cycle length and terminates AF persisting after PV isolation in 35% of patients.     

Macroreentrant atrial tachycardia detouring the epicardium at the anterior wall of the left atrium

A 79‐year‐old woman with a history of pulmonary vein isolation for persistent atrial fibrillation was admitted for recurrence of atrial tachycardia, with a tachycardia cycle length of 236 milliseconds. The ultra–high‐resolution mapping system revealed that tachycardia circuit detouring the epicardium at the anterior wall scar and breaking through to the endocardium below the left atrial appendage. Radiofrequency energy was applied to this site, which successfully terminated the tachycardia. This case suggests that epicardial conduction could occur even at the left atrial anterior wall and identifies a variation in epicardial conduction around the left atrium, which could be a tachycardia circuit.     

Clinical significance of perfusion imaging of the left atrial wall and structures in its cavity by contrast echocardiography.

This study examined whether myocardial contrast echocardiography (MCE) can visualize left atrial appendage myocardial perfusion using transesophageal echocardiography (TEE) with intracoronary injection of sonicated albumin. We also evaluated blood flow into normal structures (i.e. muscular trabeculae) and abnormal masses (i.e. fresh thrombi and myxomas) within the left atrium by MCE. TEE images were obtained with a biplane or multiplane 5 MHz transducer in 16 patients without significant coronary artery occlusive disease. Left atrial appendage myocardium was divided into 4 segments in both the transverse and longitudinal planes, and contrast opacification of each segment during MCE was visually evaluated by 2 independent observers. Visual assessment of contrast opacification of prominent muscular trabeculae within the left atrial appendage (6 patients), and of left atrial or left atrial appendage thrombi (4 patients), was also performed. The ratio of background-subtracted peak videointensity from muscular trabeculae or thrombi versus left atrial appendage myocardium was determined as corrected peak videointensity. In 3 patients with myxomas, contrast opacification of the tumor was visually assessed. Ninety-six segments of left atrial appendage myocardium were visually analyzed. Contrast opacification of the left atrial appendage myocardium was identified in 92 of 96 segments (96%, 95% confidence interval 0.90-0.98) by Observer 1 and in 91 of 96 segments (95%, 95% confidence interval 0.88-0.98) by Observer 2. MCE also enhanced the imaging of left atrial appendage muscular trabeculae, but not of left atrial or left atrial appendage thrombi. Corrected peak videointensity from thrombi was significantly lower than that from muscular trabeculae (0.15 +/- 0.11 vs 0.95 +/- 0.18, p < 0.05). All myxomas were distinctly opacified by MCE. Transesophageal MCE with intracoronary injection of sonicated albumin can image left atrial appendage myocardial perfusion. MCE allows the evaluation of blood flow into normal structures and abnormal masses within the left atrium.     

Focal atrial tachycardia originating from the musculature of the coronary sinus

Focal atrial tachycardias originate predominantly from the right atrium along the crista terminalis and less commonly from the left atrium. Successful catheter ablation usually can be performed via an endocardial approach. We report the case of a 34-year-old patient in whom a focal atrial tachycardia was successfully ablated 4 cm within the coronary sinus after extensive mapping of the left atrial endocardium and coronary sinus using the three-dimensional CARTO mapping system. Rarely, atrial tachycardia can originate from the coronary sinus musculature and require ablation inside the coronary sinus.     

Bi-atrial mapping of atrial arrhythmias

Cardiac mapping of atrial activation was originally performed in animals during open chest preparations, using epicardial electrodes. The development of endocardial egg-shaped multiple electrodes provided detailed assessment of the minimum number of wavelengths required to sustain atrial fibrillation (AF), as well as the role of interatrial connections during AF. Subsequently, several studies on bi-atrial epicardial high-density mapping in animals and humans also reported on the importance of interatrial connections, as well as the specific characteristics of the left atrium as compared with the right atrium during chronic AF. Endocardial bi-atrial mapping studies using electrode catheters were reported using basket-shaped catheters carrying 64 electrodes. Animal studies suggested that septal activation was asynchronous and discordant, while a human study outlined the multiple origins of atrial ectopic beats following DC cardioversion in patients with chronic atrial fibrillation. The advent of non-fluoroscopic mapping systems significantly changed our approach to percutaneous endocardial mapping. Simultaneous bi-atrial studies using electroanatomic mapping were performed in sinus rhythm as well as in atrial flutter. These studies demonstrated the predominance of interatrial conduction over Bachmann's Bundle and the coronary sinus-left atrial connection during respectively, sinus rhythm and atrial flutter. Simultaneous bi-atrial non-contact mapping was initially performed during porcine studies and later in humans, demonstrating asynchronous and discordant septal activation both during sinus rhythm or left lateral atrial pacing. Preliminary studies from simultaneous bi-atrial non-contact mapping in humans in whom AF occurred spontaneously or was induced suggests three main types of atrial activation, consisting of left atrial drivers causing the right atrium to fibrillate following conduction over interatrial connections, the right atrium independently sustaining AF, even after pulmonary vein disconnection, and both atria fibrillating independently without activation over interatrial connections. Bi-atrial mapping has been essential for our understanding of normal and abnormal atrial activation, and ultimately may provide new approaches for ablation of atrial fibrillation.     

Magnetic Navigation and Catheter Ablation of Right Atrial Ectopic Tachycardia in the Presence of a Hemi-Azygos Continuation: A Magnetic Navigation Case Using 3D Electroanatomical Mapping

Introduction: We report on a 63-year-old female patient in whom an electrophysiologic study discovered a hemi-azygos continuation. Using the magnetic navigation system, remote-controlled ablation was performed in conjunction with the 3D electroanatomical mapping system.
Methods and Results: Failing the attempt to advance a diagnostic catheter from the femoral vein, a diagnostic catheter was advanced via the left subclavian vein into the coronary sinus. The soft magnetic catheter was positioned in the right atrium via the hemi-azygos vein, and 3D mapping demonstrated an ectopic atrial tachycardia. Successful ablation was performed entirely remote controlled. Fluoroscopy time was only 7.1 minutes, of which 45 seconds were required during remote navigation.
Conclusion: Remote-controlled catheter ablation using magnetic navigation in conjunction with the electroanatomical mapping system proved to be a valuable tool to perform successful ablation in the presence of a hemi-azygos continuation.     

Different distribution of abnormal endocardial electrograms within the right atrium in patients with sick sinus syndrome.

BACKGROUND--Prolonged and fractionated right atrial endocardial electrograms are characteristic of paroxysmal atrial fibrillation (idiopathic or associated with sick sinus syndrome). The distribution of these abnormal atrial electrograms within the right atrium and the way it is related to the likelihood that patients with sick sinus syndrome will develop paroxysmal atrial fibrillation was studied. METHODS--Endocardial catheter mapping of the right atrium during sinus rhythm was performed in 41 control patients with normal sinus node function and without paroxysmal atrial fibrillation, in 33 patients with sick sinus syndrome but without tachycardia, and in 27 patients with sick sinus syndrome and paroxysmal atrial fibrillation (group 3). The bipolar electrograms were recorded at 12 sites in the right atrium and an abnormal atrial electrogram was defined as lasting > or = 100 ms and/or showing eight or more fragmented deflections. RESULTS--1195 atrial endocardial electrograms were assessed and quantitatively measured. In patients with sick sinus syndrome and paroxysmal atrial fibrillation 54% of the abnormal atrial electrograms were recorded from the high right atrium, 28% from the mild right atrium, and 18% from the low right atrium. However, in patients with sick sinus syndrome without tachycardia 78% of the abnormal atrial electrograms were recorded from the high right atrium and 22% from the mid right atrium. No abnormal electrograms were recorded from the low right atrium. CONCLUSIONS--In patients with sick sinus syndrome without tachycardia abnormal atrial electrograms generally came from the high right atrium but in patients with sick sinus syndrome and paroxysmal atrial fibrillation abnormal atrial electrograms were more widely distributed in the right atrium. In patients with sick sinus syndrome the greater the extent of the compromised atrial muscle, the greater the likelihood that paroxysmal atrial fibrillation will develop.