Author + information
- Received April 12, 2004
- Revision received May 28, 2004
- Accepted July 14, 2004
- Published online October 19, 2004.
- 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†
- ↵*Reprint requests and correspondence:
Dr. Eduardo Back Sternick, rua Correias 281/301, Belo Horizonte, Minas Gerais, 30315-340 Brazil
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 rSpattern in lead III. This was found in 20 patients. There was a match between the presence of rSin 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 rSpattern 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 rSpattern in lead III was found in 6%.
Conclusions A narrow QRS with an rSpattern 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.
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 qwaves 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.
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 RScomplex 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 Rspattern means a QRS complex (>0.3 mV) with an initial positive deflection followed by a smaller negative one. A septal qwave was defined as a qwave in surface ECG leads I, aVL, and V6, with an amplitude <25% of the Rwave and a width <0.04 s.
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 ttest or Fisher exact test.
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 rSconfiguration 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 rSpattern in lead III, no qwave was found in lead I in 15 patients (and in 8 patients in lead V6). Minimal preexcitation, as manifested by the absence of a qwave in lead I (without rSin lead III), was seen in only two patients (Patient #8 and #16) (Table 2).
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).
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).
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 Rwave in lead I, and 30 of 33 patients had rSin lead V1during tachycardia (3 patients had QSin V1).
Comparison between ECG during sinus rhythm and during tachycardia with anterograde conduction over the Mahaim fiber
In all 20 patients with an rSpattern in lead III, we found a negative QRS complex in the same lead (either rSor QSpattern) during tachycardia. Also, the patient with atrial fibrillation showed a negative QRS complex. There were nine patients without the rSpattern in lead III during sinus rhythm, which showed left axis deviation during tachycardia. Three patients showed concordance between the absence of an rSpattern during sinus rhythm and their tachycardia QRS pattern (all three patients had an anterior Mahaim) (Fig. 5).
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 qRor QRpattern found in 18 patients, Rsin one patient, RSin one patient, rsin 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 qwave, whereas the other patients showed the same pre-ablation QRS complex. Figure 2gives examples when comparing the QRS before and after ablation.
Correlation between ECG findings and Mahaim fiber location
The rSmorphology 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 rSpattern 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 rSpattern in lead III can be found over a large area around the tricuspid annulus, from the anterolateral to the posterolateral and mid-septal region.
Presence of an rSpattern 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 rSpattern in lead III during sinus rhythm (Fig. 7).However, all of them had a qwave in lead I (qRor qRspattern).
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 rSpattern in lead III in our patients, we examined previous reports dealing with decrementally conducting bypass tracts. We did find the rSpattern 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 rSpattern in lead III (60%). It should be stressed that in these patients, there is no classic delta wave. It is of interest that the rSpattern 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 qwave in lead I.
To validate the rSand 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 rSin 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 2depicts most of the patterns of QRS that emerged after successful ablation of the Mahaim fiber.
The fact that nine patients did not show an rSpattern 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 rSin 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).
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 qwave 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).
RSas a normal pattern in lead III
It has been shown (27) that an rSpattern in lead III can be found in normal individuals. This may occur during posterior displacement of the apex leading to Swaves in leads I, II, and III (S1S2S3pattern ) and in counterclockwise rotation of the heart resulting in a qRin lead I and rSin lead III. However, in those situations, a normal qwave in lead I is likely to be present. In our survey of 200 ECGs from young individuals with palpitations, we found the rSpattern in lead III in 6%, but always associated with a qwave in lead I. No individual showed an rSpattern in lead III combined with the absence of a qwave in lead I, a pattern that seems specific for patients with a Mahaim fiber.
Specificity of the rSpattern in the general population
The finding of an rSpattern 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 rSpattern in lead III associated with the absence of a septal qwave will be close to 90% (if we assume one false positive in 1,000 individuals), albeit the sensitivity decreases to 45%.
In young patients with tachycardias, the finding of a narrow QRS with an rSpattern in lead III during sinus rhythm should raise the suspicion of the presence of a Mahaim fiber, especially in those showing an absence of qwave in lead I.
We would like to thank Sávia Bueno, MD, for data collection.
- Abbreviations and acronyms
- atrio-His interval
- His-ventricular interval
- left bundle branch block
- Received April 12, 2004.
- Revision received May 28, 2004.
- Accepted July 14, 2004.
- American College of Cardiology Foundation
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