HEART RHYTHM DISTURBANCES
The electrocardiogram during sinus rhythm and tachycardia in patients with Mahaim fibers
The importance of an "rS" pattern in lead III
Eduardo Back Sternick, MD*,*,
Carl Timmermans, MD ,
Eduardo Sosa, MD ,
Fernando E.S. Cruz, MD, FACC ,
Luz-Maria Rodriguez, MD, FACC,
Márcio Fagundes, MD,
Luiz M. Gerken, MD* and
Hein J.J. Wellens, MD, FACC
* Biocor Instituto, Belo Horizonte, Brazil
University Hospital, Maastricht, Maastricht, The Netherlands
Instituto do Coração, Faculdade de Medicina, Universidade de São Paulo, São Paulo, Brazil
Instituto Nacional de Cardiologia Laranjeiras, Ministério da Saúde, Rio de Janeiro, Brazil
Manuscript received April 12, 2004;
revised manuscript received May 28, 2004,
accepted July 14, 2004.
* Reprint requests and correspondence: Dr. Eduardo Back Sternick, rua Correias 281/301, Belo Horizonte, Minas Gerais, 30315-340 Brazil (Email: eduardosternick{at}aol.com).
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Abstract
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OBJECTIVES: The purpose of the study was to identify the electrocardiographic (ECG) characteristics of the Mahaim fiber.
BACKGROUND: Mahaim fibers are slowly conducting accessory pathways reaching into the right ventricle. They often play a role in tachycardias.
METHODS: We retrospectively analyzed 40 patients with Mahaim fibers. Five patients had associated Wolff-Parkinson-White syndrome and were excluded from the study. Two patients had a short atrioventricular decremental accessory pathwayand were also excluded. The remaining 33 patients had a tachycardia with anterograde conduction over a Mahaim fiber. Twenty were female. Their mean age was 24 ± 10 years.
RESULTS: The most common pattern of minimal preexcitation during sinus rhythm was an rS pattern in lead III. This was found in 20 patients. There was a match between the presence of rS in lead III during sinus rhythm and left axis deviation during tachycardia with anterograde conduction over the Mahaim fiber. After ablation, a different QRS pattern emerged in lead III, indicating the absence of conduction over the Mahaim fiber. To obtain information on the prevalence of an rS pattern in lead III in age-matched controls with palpitations and without structural heart disease, the 12-lead ECG of 200 young individuals were examined. An rS pattern in lead III was found in 6%.
CONCLUSIONS: A narrow QRS with an rS pattern in lead III during sinus rhythm in a patient with a history of palpitations should alert the physician to the possibility of a Mahaim fiber. During tachycardia, these patients typically show a left bundle branch block-like QRS complex with left axis deviation.
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Abbreviations and Acronyms
| | AH = atrio-His interval | | AV = atrioventricular | | ECG = electrocardiogram | | HV = His-ventricular interval | | LBBB = left bundle branch block | | RF = radiofrequency |
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Accessory pathways with long and decremental anterograde conduction have been the subject of extensive debate about their anatomic structure (1–4), location (5–7), related arrhythmias (8,9), electrophysiologic properties (10–12), ablative techniques (13,14), and automaticity (15). Less attention has been given to the 12-lead electrocardiogram (ECG), especially to the ECG during sinus rhythm. The latter is considered to be normal in the majority of patients with atriofascicular pathways and patients with long atrioventricular (AV) decrementally conducting accessory pathways. Minimal preexcitation is reported to occur from 0% to 30% (13,14,16,17), and apart from the absence of q waves in the left precordial leads (18), no specific QRS pattern has been described. The purpose of this article is to report on ECG findings in a large series of patients with the Mahaim fiber during sinus rhythm and tachycardia, with emphasis on a particular ECG pattern in lead III during sinus rhythm.
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Methods
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Study population.
We retrospectively analyzed 12-lead ECGs from 40 patients with anterograde conduction over accessory pathways with long conduction times and decremental properties during both sinus rhythm and tachycardia. Five patients also had anterogradely, rapidly conducting accessory pathways and were excluded from the study. Two patients with a short AV decremental pathway were also excluded from this series.
Definition of terms.
We used the eponym Mahaim fibers in this study as a synonym of accessory pathways with long and decremental properties with a long anatomic course to eitheratriofascicular pathways (n = 30) or AV pathways (n = 3).
Every one of the 33 remaining patients showed electrophysiologic criteria for a decrementally conducting bypass tract during atrial pacing, such as a progressive AH and A-V interval prolongation coupled with a decreasing HV interval leading to a greater degree of preexcitation with a left bundle branch block (LBBB)-like morphology (5,8,10).
There were 20 females and 13 males, with a mean age of 24 ± 10 years (range 8 to 52 years). All patients were referred for electrophysiologic assessment of a preexcited tachycardia. Preexcited AV node reentrant tachycardia, using a Mahaim fiber as a bystander, was present in one patient. One patient had atrial fibrillation with preexcited QRS complexes, and two patients were referred because of repetitive episodes of unsustained tachycardia caused by automaticity arising in the Mahaim fiber (Table 1) (9). Ebstein's disease was diagnosed in four patients. The atrial insertion of the Mahaim fiber was located by the recording of a discrete accessory pathway potential in 28 patients and in five patients by assessing the shortest AV interval during atrial pacing at different sites along the tricuspid annulus. All patients underwent successful surgical (n = 2) or radiofrequency (RF) catheter ablation (n = 31). The RF ablation was guided by discrete potentials at the tricuspid annulus (n = 28) and by right ventricular pace mapping in five patients.
We also analyzed the 12-lead ECG during sinus rhythm in 200 individuals with palpitations and without structural heart disease, matched for age and gender as a control group.
Recordings of the 12-lead surface ECG and intracardiac electrograms were made using the EP Tracer or MS System (CardioTek BV, Maastricht, The Netherlands).
Definitions of QRS patterns.
The ECGs were examined by two different observers with a magnified lens, and a third observer decided when there was a mismatch classification. The following QRS patterns were found: r, rS, RS, Rs, rsR', rsr', R, qR, QR, QS, qRs, and qRS. A very-low-voltage QRS complex (<0.3 mV) was depicted as small letters (r, rs, or rsr'). The QRS complex with a higher voltage was depicted according to the ratio between the positive (R, r) and negative waves (q, Q, S, and s). For example, an RS complex was defined by the presence of a QRS complex showing an initially positive deflection followed by a negative deflection of an even magnitude. Likewise, an Rs pattern means a QRS complex (>0.3 mV) with an initial positive deflection followed by a smaller negative one. A septal q wave was defined as a q wave in surface ECG leads I, aVL, and V6, with an amplitude <25% of the R wave and a width <0.04 s.
Statistical analysis.
Data are given as the mean value ± SD. The significance of differences (p < 0.05) between groups of clinical, ECG, or electrophysiologic parameters was assessed by the Student t test or Fisher exact test.
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Results
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Pre-ablation 12-lead ECG findings.
Minimal preexcitation, defined as subtle abnormalities suggesting the presence of preexcitation, with a QRS complex width within the normal range (<0.12 s), but with a short HV interval (<35 ms), was present during sinus rhythm in 24 patients (72%) (Fig. 1). The PR interval was not significantly different when comparing patients with (125 ± 21 ms) and without (132 ± 9 ms) minimal preexcitation (p = NS).
We found two patterns of the QRS complex (Fig. 2) during sinus rhythm: the most common one being an rS configuration in lead III. This was found in 20 patients. The other pattern in lead III—an rsR'—was found in two patients. In the presence of an rS pattern in lead III, no q wave was found in lead I in 15 patients (and in 8 patients in lead V6). Minimal preexcitation, as manifested by the absence of a q wave in lead I (without rS in lead III), was seen in only two patients (Patient #8 and #16) (Table 2).
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Figure 2 Five-lead electrocardiogram during sinus rhythm with the pre-ablation electrocardiogram showing the rS pattern in four cases and rsR' in one patient. The post-ablation electrocardiogram shows a clear change in QRS configuration.
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In three patients, minimal preexcitation was not always demonstrated, as documented by 12-lead ECGs taken on different days. Variability of minimal preexcitation on the same ECG was seen in two patients (Fig. 3).
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Figure 3 (A) ECG showing minimal preexcitation (rsR' pattern in lead III) only in the first three QRS complexes. (B) Electrocardiogram after intravenous verapamil shows sinus rhythm with AV conduction over the Mahaim fiber with a long PR interval and overt pre-excitation.
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Intracavitary signals, Mahaim potential recording, and right bundle and His bundle electrograms.
We found that the AM interval (i.e., atrium-proximal Mahaim potential interval) was always 20 to 40 ms shorter than the AH interval in patients with minimal preexcitation. In patients without preexcitation, the AH interval was shorter than the AM interval (Fig. 4).
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Figure 4 (Left) Patient no. 6 without preexcitation showing a shorter atrio-his interval (AH) than AM interval. (Right) Patient no. 2 showing an AM shorter than the AH interval, resulting in minimal preexcitation with an rS pattern in lead III.
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The ECG during tachycardia.
We analyzed 29 patients with a circusmovement tachycardia with anterograde conduction over the Mahaim fiber, 1 patient with AV node re-entrant tachycardia with bystander Mahaim conduction, 1 with atrial fibrillation with anterograde conduction over the Mahaim fiber, and 2 with automatic tachycardias arising in the Mahaim fiber. During circus movement tachycardia, the cycle length ranged from 430 to 250 ms. The QRS width during tachycardia (Table 2) varied from 120 to 140 ms. All patients had a monophasic R wave in lead I, and 30 of 33 patients had rS in lead V1 during tachycardia (3 patients had QS in V1).
Comparison between ECG during sinus rhythm and during tachycardia with anterograde conduction over the Mahaim fiber.
In all 20 patients with an rS pattern in lead III, we found a negative QRS complex in the same lead (either rS or QS pattern) during tachycardia. Also, the patient with atrial fibrillation showed a negative QRS complex. There were nine patients without the rS pattern in lead III during sinus rhythm, which showed left axis deviation during tachycardia. Three patients showed concordance between the absence of an rS pattern during sinus rhythm and their tachycardia QRS pattern (all three patients had an anterior Mahaim) (Fig. 5).
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Figure 5 Case 29: this patient had an anterior Mahaim fiber and an intermediate QRS axis (50°) during sinus rhythm and antidromic tachycardia. There is a minimal preexcitation but without the rS pattern in lead III.
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Post-ablation 12-lead ECG.
In the 24 patients showing minimal preexcitation in the 12-lead ECG, six patterns were observed in lead III during sinus rhythm after Mahaim ablation. The most common QRS configuration was the qR or QR pattern found in 18 patients, Rs in one patient, RS in one patient, rs in two patients, rsR' in one patient, and rsr' in one patient. Assessment of the left precordial leads after ablation showed that in only nine patients, the previous ECG pattern changed with the development of a small q wave, whereas the other patients showed the same pre-ablation QRS complex. Figure 2 gives examples when comparing the QRS before and after ablation.
Correlation between ECG findings and Mahaim fiber location.
The rS morphology in lead III was not seen in the three cases with an anteriorly located Mahaim or the two patients with a posterior Mahaim fiber. The distribution of the atrial end of the Mahaim fiber in the 20 patients with an rS pattern in lead III during sinus rhythm along the tricuspid annulus is depicted in Figure 6. It is of interest that the atrial end of the Mahaim fiber with an rS pattern in lead III can be found over a large area around the tricuspid annulus, from the anterolateral to the posterolateral and mid-septal region.
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Figure 6 Proportion of patients with an rS pattern in lead III during sinus rhythmin relation to the location of the atrial end of the Mahaim fiber along the tricuspid annulus. CS = coronary sinus; MS = mid-septal; RA = right anterior; RAL = right anterolateral; RL = right lateral; RP = right posterior; RPL = right posterolateral.
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Presence of an rS pattern in lead III during sinus rhythm in 200 matched controls.
We did a survey in 200 young individuals referred because of palpitations. Twelve (6%) of 200 matched controls (56% females; mean age 23 ± 12 years) without heart disease and without a history of palpitations showed an rS pattern in lead III during sinus rhythm (Fig. 7). However, all of them had a q wave in lead I (qR or qRs pattern).
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Discussion
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When accessory AV pathways have conduction times approaching that of the normal AV conduction system, little or no preexcitation may be present during sinus rhythm. The reported incidence of minimal preexcitation on the 12-lead ECG during sinus rhythm in patients with decrementally conducting accessory pathways is low. Bardy et al. (16) and Klein et al. (19)did not find it in any of their patients. McClelland et al. (13) reported that only one of their 26 patients displayed preexcitation on the 12-lead ECG. When we realized the prevalence of the rS pattern in lead III in our patients, we examined previous reports dealing with decrementally conducting bypass tracts. We did find the rS pattern in lead III on many ECGs considered as normal in cardiology journals (17,19,20) and textbooks (21,22). This suggests that the reported low figures of abnormal ECGs in patients with Mahaim bypass tracts is an underestimation. Some authors acknowledged the presence of minimal preexcitation in 25% to 50% of their patients (14,18). We found an incidence of 72% of minimal preexcitation, mainly in the presence of an rS pattern in lead III (60%). It should be stressed that in these patients, there is no classic delta wave. It is of interest that the rS pattern was found in patients with decremental accessory pathways having their atrial end over a very large area around the tricuspid annulus, from anterolateral to posterolateral, as well as in the only patient with a mid-septal location (Fig. 6). This supports a ventricular insertion in a small anterolateral area in the right ventricle in or close to the exit of the right bundle branch and also explains (when ventricular activation starts at this site) the absence of a q wave in lead I.
To validate the rS and rsR' as abnormal patterns in lead III due to preexcitation of a small region of the right ventricle, it was crucial to show a positive relationship between those patterns in lead III during sinus rhythm and left axis deviation during tachycardia with anterograde conduction over the Mahaim fiber (Fig. 1). All 20 patients with an rS in lead III had left axis deviation ( 0°) during tachycardia. Another important step in validation is to show a clear change in QRS complex configuration after ablation of the decremental accessory pathway. Figure 2 depicts most of the patterns of QRS that emerged after successful ablation of the Mahaim fiber.
The fact that nine patients did not show an rS pattern in lead III during sinus rhythm but an LBBB-like QRS with left axis deviation during tachycardia can be explained by impulse conduction over the Mahaim fiber during sinus rhythm slower than impulse conduction over the normal AV conduction system. We, like other authors (23), found day-to-day variability in the expression of minimal preexcitation. This is different from "intermittent" preexcitation that may occur in rapidly conducting accessory pathwayswith long anterograde refractory periods. Our patients with variable expression of preexcitation did not have long refractory periods of their accessory pathway. There is one case report of sudden death in a patient with similar findings (24). Conduction over Mahaim fibers can be so slow that no ventricular preexcitation occurs even during atrial pacing. Still, these so-called latent Mahaim's are capable of being involved in antidromic tachycardias (25).
Are all Mahaim fibers inserting close to or in the right bundle branch?.
Some Mahaim fibers are probably not inserting in that region. Our three patients with anterior Mahaim did not show an rS in lead III nor left axis deviation during tachycardia, suggesting that in those fibers, the ventricular insertion is not in the vicinity of the right bundle branch (6) (Fig. 5).
Septal q waves.
In our population, lead I was more sensitive for minimal preexcitation than lead V6. In patients with atriofascicular pathways inserting close to the apex, ventricular activation proceeds from an apical toward a basal direction, resulting in a q wave in lead V6. Minimal preexcitation due to left-sided accessory pathways can be better appreciated in lead V6, which has been shown to be more sensitive than leads I and aVL (26).
RS as a normal pattern in lead III.
It has been shown (27) that an rS pattern in lead III can be found in normal individuals. This may occur during posterior displacement of the apex leading to S waves in leads I, II, and III (S1S2S3 pattern [28]) and in counterclockwise rotation of the heart resulting in a qR in lead I and rS in lead III. However, in those situations, a normal q wave in lead I is likely to be present. In our survey of 200 ECGs from young individuals with palpitations, we found the rS pattern in lead III in 6%, but always associated with a q wave in lead I. No individual showed an rS pattern in lead III combined with the absence of a q wave in lead I, a pattern that seems specific for patients with a Mahaim fiber.
Study limitations.
Specificity of the rS pattern in the general population
The finding of an rS pattern in lead III in 60% of the patients with Mahaim fibers is significantly higher than its occurrence in young persons with palpitations (p < 0.0001).
Mahaim fibers comprise  3% of the overt accessory pathways (29). Based on the prevalence of accessory pathways in the general population (30) (0.2%), the prevalence of Mahaim fibers would be 0.5 to 1:10.000. The specificity of an rS pattern in lead III associated with the absence of a septal q wave will be close to 90% (if we assume one false positive in 1,000 individuals), albeit the sensitivity decreases to 45%.
Conclusions.
In young patients with tachycardias, the finding of a narrow QRS with an rS pattern in lead III during sinus rhythm should raise the suspicion of the presence of a Mahaim fiber, especially in those showing an absence of q wave in lead I.
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Acknowledgments
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We would like to thank Sávia Bueno, MD, for data collection.
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References
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Abstract
Methods
