Differentiating the QRS Morphology of Posterior Fascicular Ventricular Tachycardia From Right Bundle Branch Block and Left Anterior Hemiblock Aberrancy
Background Left posterior fascicular ventricular tachycardia (LPF-VT) is frequently misdiagnosed as supraventricular tachycardia with aberrant right bundle branch block (RBBB) and left anterior hemiblock (LAHB). The purpose of the present study was to define the morphological ECG characteristics of LPF-VT and attempt to differentiate it from RBBB and LAHB aberrancy.
Methods and Results A systematic Medline search was used to identify or locate ECG tracings from patients with LPF-VTs. ECGs with LPF-VT were also collected from patients who underwent ablation of this arrhythmia at the Tel Aviv and Sheba Medical Centers. These ECGs were compared with ECGs of consecutive patients with RBBB and LAHB and no obvious cardiac pathology by echocardiography. Overall, 183 ECGs of LPF-VT were compared with 61 ECGs showing RBBB and LAHB. Univariate analysis demonstrated differences in QRS axis, limb (I, aVr), and precordial (V1, V2, V6) ECG leads. On multivariate logistic regression analysis, LPF-VT was more often associated with atypical RBBB-like V1 morphology (odds ratio, 5.1; P=0.004), positive QRS in aVr (odds ratio, 19.2; P<0.001), V6 R/S ratio ≤1 (odds ratio, 6.7; P=0.01), and QRS ≤140 ms (odds ratio, 7.7; P<0.001). Using these 4 variables, a prediction model was developed that predicted LPF-VT with sensitivity and specificity of 82.1% and 78.3%, respectively. Patients with 3 of 4 positive variables had high probability of having LPF-VT, whereas patients with ≤1 positive variable always had RBBB plus LAHB.
Conclusions The morphological ECG characteristics of LPF-VT were defined, and a high accurate tool for correctly differentiating LPF-VT from RBBB and LAHB aberrancy was developed.
See Editorial by Moss and Scheinman
WHAT IS KNOWN
Left posterior fascicular ventricular tachycardia (LPF-VT)—the most common type of fascicular ventricular tachycardia—is frequently misdiagnosed as supraventricular tachycardia with aberrant right bundle branch block with left anterior hemiblock.
Known algorithms that were developed to differentiate structural ventricular tachycardia from supraventricular tachycardia with aberrancy do not apply to LPF-VT.
WHAT THE STUDY ADDS
The morphological ECG characteristics of LPF-VT were systematically defined and differentiated from right bundle branch block with left anterior hemiblock aberrancy.
A high accurate model for correctly differentiating LPF-VT from right bundle branch block with left anterior hemiblock aberrancy was developed.
The 4 parameters supporting the diagnosis of LPF-VT included in the model were (1) atypical right bundle branch block–like V1 morphology, (2) positive QRS in aVr, (3) V6 R/S ratio ≤1, and (4) QRS ≤140 ms.
Idiopathic verapamil-sensitive sustained ventricular tachycardia (VT) with a right bundle branch block (RBBB) and left axis deviation also called left posterior fascicular VT (LPF-VT) is the most common form of idiopathic left VT.1–5 The arrhythmia has been frequently misdiagnosed as supraventricular tachycardia (SVT) with RBBB and left anterior hemiblock (LAHB) aberration for several reasons: (1) it frequently occurs in young patients without obvious heart disease, (2) it is almost always well tolerated clinically, and (3) it is terminated by intravenous calcium channel blockers after usual progressive rate slowing.
Although the presence of atrioventricular dissociation, capture, or fusion beats should avoid the misdiagnosis of the arrhythmia,6 the ECG diagnosis may remain challenging when fusion or capture beats are absent, when a 1:1 atrioventricular relationship is present, or when P waves are not well visible. In fact, previous reports and case reports documented the frequent misdiagnosis of this arrhythmia7–11
There has been no systematic study comparing the ECG characteristics of LPF-VT with SVT associated with RBBB and LAHB. For example, in patients with LPF-VT, both atypical12 and typical13 RBBB patterns have been reported in V1. Also, as opposed to VT associated with structural heart disease and similarly to aberrancy, RS interval (the interval from the beginning of the precordial QRS to S nadir) has been found to be <80 ms in patients with LPF-VT14. The present study aims to establish QRS-based criteria that may differentiate LPF-VT from SVT with RBBB plus LAHB.
The study was approved by the Institutional Committee on Human Research at the Tel Aviv Sourasky Medical Center.
ECGs with LPF-VT were collected from 2 sources:
This comprised the ECGs of all patients who underwent radiofrequency ablation of LPF-VT at the Tel Aviv and Sheba Medical Centers between years 1992 and 2013.
Medline Search Group
A Medline search was used to locate ECGs of patients with LPF-VT. The systematic Medline review was conducted by 3 investigators (Y.M., M.R., and I.H.) with the typed search words idiopathic left VT or verapamil-sensitive VT. The search was limited to the years 1981 to 2013. In all identified articles, the abstract and if needed the entire article were examined to verify that they were suitable for the study. ECGs were collected if (1) the reported patients underwent electrophysiological study and radiofrequency ablation that confirmed the diagnosis of LPF-VT, (2) there was no evidence for organic heart disease, and (3) the ECGs were of satisfactory quality for accurate interpretation. Case reports but not review articles were included. Particular care was taken not to analyze same ECGs provided by same authors and published in different articles.
The control group included consecutive patients admitted to the Tel Aviv Medical Center who had 12-lead ECG showing RBBB and LAHB15 and no significant prior heart disease with echocardiogram showing normal left ventricular function, no significant ventricular hypertrophy, and no more than mild valvular heart disease. LAHB was defined when the QRS axis was ≤−30° and demonstrated a small r followed by a deep S wave in the inferior leads. Because patients in the control group were in sinus rhythm, we also collected all ECGs of consecutive patients without organic heart disease who had RBBB and LAHB and underwent electrophysiological study at our center between 2013 and 2016. In all included patients, atrial pacing with conduction at rates ≥100 beats per minute was achieved (atrial pacing control group).
The following parameters were analyzed:
General: QRS width, QRS axis, and heart rate.
V1: (a) typical RBBB was defined if R′ was taller than R. If there was no rsR′ pattern (ie, qR) or R was bigger than R′, V1 was defined as atypical; (b) we also analyzed whether there is a Q wave before typical RBBB pattern; (c) whether the S wave is below the isoelectric line; and (d) whether V1 is notched, defined as RsR′ pattern with small s and equal R and R′.
V2: (a) typical or atypical RBBB pattern and (b) the presence or absence of notched V2. Both were similarly defined as above (2a and 2d).
V6: (a) the R/S height ratio and (b) the width of R compared with S wave.
Lead I: the R/S relationship (both height and width).
aVr: (a) the presence of positive QRS (defined as R higher than S or R higher than Q) and (b) the presence or absence of identical QRS morphology in aVr and V1.
Deepest S in V1 to V6: in case that S wave was the deepest in several leads, all leads were taken into account.
Precordial leads RS time: defined as the time from the beginning of the QRS to the nadir of the first S wave in the precordial leads.14
All ECGs were examined separately by 2 experienced cardiologists (Y.M. and O.T.) blinded to each other. Thereafter, results were matched, and in case of disagreement, a consensus was reached by shared analysis or with the aid of a third experienced cardiologist (B.B.).
Interobserver agreement was measured using the κ statistics, the interpreted agreement was based on the cutoff values suggested by Landis and Koch.
Categorical variables are reported as frequencies and percentages, and continuous variables are reported as means and SD or medians (25th–75th percentile). Continuous variables were evaluated for normal distribution using histograms and Q-Q Plots. Categorical variables were compared using χ2 test or Fisher exact test and continuous variables by independent samples t test or by Mann–Whitney U test. Classification and regression tree and χ2 automatic interaction detection methods were used to divide the predictors into categories and to identify threshold values. Multivariate logistic regression was used to identify predictors to LPF-VT and to build the prediction model. The multivariate regression model included covariates, which were associated in the univariate analyses at a significance level of <0.2, and variables, which are a priori believed to be associated with the LPF-VT. Area under the receiver operating characteristic curve, discrimination slope, and Box-plot were used to evaluate how the model can discriminate between diagnoses. Hosmer–Lemeshow goodness-of-fit test was used to evaluate the regression model.
A 5- and 10-fold cross-validations were then performed for which the data set was randomly divided into 5 or 10 parts, respectively. Four or 9 parts were used to build the model from the start and the remaining one part was used for testing. This process was repeated x 5 or 10 times with a different part each time. Sensitivity, specificity, positive predictive value, negative predicted value, and accuracy were recorded.
Nomogram was used for visualization of the prediction model. A 2-tailed P<0.05 was considered statistically significant. Analyses were performed with IBM SPSS Statistics for Windows, version 22.0 (IBM Corp, Armonk, NY).
Thirty-nine patients (35 men), 27 from the Tel Aviv Medical Center and 12 from Sheba Medical Center who underwent LPF-VT ablation were included.
Medline Search Group
The search permitted retrieval of 709 citations. We identified 144 ECGs from 87 publications that were compatible with the inclusion criteria.
Overall, the study group (patient group plus Medline search group) consisted of 183 ECGs.
The control group that was used for ECG comparison with the study group consisted of 61 patients (47 men), that is, 1:3 ratio. These patients were in sinus rhythm and had ECG evidence of RBBB and LAHB.
A comparison between the study and control groups is presented in Table. As expected, patients in the control group were older and by definition had lower heart rate because most of them were in sinus rhythm. These 2 variables were not included in the multivariate logistic regression analysis and in the prediction model below. Further comparison of ECG findings demonstrated significant differences in QRS axis and width, as well as in specific ECG leads (V1, V2, V6, I, and aVr).
Descriptive Findings in Specific ECG Leads
A typical RBBB pattern was observed in 54% and 91.8% of the study and control groups, respectively (P<0.001). A small q wave before the QRS was present in 35% and 7% of the study and control groups, respectively (P<0.001). An S wave below the isoelectric line was noted in 21% and 72% of the study and control groups, respectively (P<0.001; Table).
An rS pattern with S deeper than R was observed in 88% and 59% of the study and control groups, respectively (P<0.001); however, an overlap between the groups was seen. S was wider than R in most cases of both groups (>90%).
The QRS was usually positive in the study group (94%); however, it was positive in only 51% of the control group (P<0.001).
Comparison Between the Medline Search and the Patients Groups With LPF-VT
To test the possibility of publication bias, demographic and ECG parameters of the patients from the 2 groups were compared. No significant differences were seen (Table I in the Data Supplement).
Comparison Between Women and Men
In 39 patients from the Medline search group, sex was not available. Comparison of ECG parameters between men (n=111) and women (n=33) with LPF-VT showed that men had a more negative axis (median of −90° compared with −70°; P=0.003), which was associated with a higher percentage of cases with positive QRS in aVr (98.8% versus 88%; P=0.04). Also the S wave in lead I tended to be more negative and broader when compared with the R wave. No significant differences were seen in other ECG parameters (Table II in the Data Supplement).
Prediction Model and Nomogram
Multivariate logistic regression was used to identify predictors of LPF-VT and to build a prediction model. Four variables were identified to be independently associated with the diagnosis of LPF-VT: positive QRS in aVr (odds ratio, 19.2; 95% confidence interval [CI], 4.3–86.5; P<0.001), QRS width ≤140 ms (odds ratio, 7.7; 95% CI, 2.9–20.3; P<0.001), R/S ratio in V6≤1 (odds ratio, 6.7; 95% CI, 1.6–28.5; P=0.01), and atypical V1 morphology (odds ratio, 5.1; 95% CI, 1.7–15.6; P=0.004). The probability of having LPF-VT can be calculated by using the following equation: P=1/(1+e−z), where P is the probability of having LPF-VT, Z=−5.219+1.901 (if V6 R/S ratio ≤1) +1.636 (if V1 is atypical) +2.04 (if QRS ≤140 ms) +2.958 (if QRS is positive in aVr). The logistic model showed good discrimination and calibration ability (area under the curve, 0.889; 95 CI, 0.836–0.943; Figure 1); discrimination slope 0.496, Hosmer–Lemeshow goodness-of-fit test, χ2=2.974; P=0.812. Figure 2 shows a nomogram based on the logistic model. Receiver operating characteristic (Figure 1B) analysis allowed identification of a threshold value of 0.59 for predicting LPF-VT with sensitivity and specificity of 82.1% and 78.3%, respectively. Both the 5- and 10-fold cross-validations presented the same sensitivity and specificity as the whole model with positive predictive value, 85.7%; negative predicted value, 73.4%; and accuracy, 80.6%. An example of applying the model to 2 patients in each group is presented in Figures 3 and 4.
Number of Positive Criteria
The percentage of patients with 0 to 4 positive criteria is presented in Figure 2B. VT cases exhibited at least 2 positive criteria, whereas the likelihood for dealing with VT rather than with intraventricular aberrancy was higher when at least 3 criteria were positive.
Comparison and Application of the Model to the Atrial Pacing Control Group
The atrial pacing control group included 16 patients aged 81±6.4 years, 13 (81%) men, with mean heart rate during pacing of 129±31 beats per minute. These patients exhibited similar ECG findings as the nonpaced control group (data not shown). Also, only 2 (12.5%) had 3 positive criteria, whereas all other patients had ≤2 positive criteria (Figure I in the Data Supplement; P=0.67 when compared with the nonpaced control group). Of note, heart rate of this group was still lower compared with the study group (P<0.0001).
R/S ratio of ≤0.15 in lead V6 (21.9% of patients with VT), QRS width <120 ms (31% of patients with VT) and QRS axis <100° (16.7% of patients with VT) were only seen in patients with LPF-VT.
We found a substantial-to-almost perfect interobserver agreement (κ=0.803 for V2 and κ>0.9 for other parameters; V1, V6, aVr, and biggest S in the precordial leads).
The present study is the first to describe the morphological features of the QRS complex during LPF-VT. Also, it provides a prediction model based on 4 simple morphological criteria that may facilitate achievement of correct diagnosis of LPF-VT compared with SVT with RBBB plus LAHB.
Wellens et al16 demonstrated that ≈70% of RBBB type VTs associated with structural heart disease exhibit QRS duration >140 ms as opposed to SVT with RBBB. For wide QRS tachycardia with a left bundle branch block pattern, a cutoff of 160 ms was chosen above which VT is more likely. No cutoff was published for patients who had both RBBB and LAHB. In the present study that tested specifically RBBB and LAHB, QRS width was actually >140 ms in most control patients. Therefore, the cutoff suggested by Wellens might not be useful for differentiating RBBB plus LAHB-like VT from SVT associated with ventricular aberration. Of note, in our study, we found that as opposed to patients with RBBB and LAHB, most patients with LPF-VT have QRS width <140 ms
It is known that LPF-VT may have typical RBBB morphology in V1;6 however, the rate of typical versus atypical morphology in that lead was not previously studied. We found that 45.8% of patients with LPF-VT presented with atypical V1 morphology, suggesting that the exit point of the VT in these cases was probably not the left posterior fascicle but in the neighboring tissue. Other morphological features in V1 that differentiate LPF-VT from SVT with aberrancy include initial q wave preceding typical rsR′ pattern that supports VT and S wave below the isoelectric line that supports aberrancy. These 2 variables, however, were not statistically additive to the prediction model.
R/S ratio <1 supports VT when comparing VT and RBBB aberrancy.6,17 In RBBB aberration, the small S wave in V6 reflects delayed right ventricular activation as opposed to S wave in left ventricular VT that reflects depolarization of the left ventricle as it moves away from V6 electrode.18 Because the left ventricular mass is bigger than the right ventricular mass, the S wave in VT is deeper with R/S ratio <1. However, when LAHB is added to RBBB, the S wave in V6 becomes also deeper and, therefore, may result in greater overlap between VT and aberrancy. We found that R/S ratio <1 supports VT, and this criterion was included in the prediction model; however, only a ratio <0.15 was seen exclusively in patients with VT.
QRS Morphology in aVr
Most patients with LPF-VT had qR complex in aVr. Criteria from previous studies on QRS morphology in aVr in structural VT suggested that a qR complex during tachycardia supports the diagnosis of VT when the initial q wave has a duration >40 ms19,20; however, this criterion does not apply to patients with LPF-VT because initial septal ventricular activation is fast, and the q wave is narrower than 40 ms. We found that QRS positivity in aVr (seen in 94.2% of patients with LPF-VT compared with 50.8% in RBBB and LAHB aberrancy; P<0.001) helps in arrhythmia discrimination. Of note, in RBBB aberrancy in contrast to left bundle branch block pattern aberrancy, the QRS in aVr may be positive, nevertheless because more patients with LPF-VT had an axis that is shifted to the northwest, a positive QRS in aVr lead was seen more often in patients with LPF-VT.
Initial QRS Activation
Several criteria for differentiating SVT from VT refer to the initial ventricular activation, which may be slower in structural VT compared with SVT with aberration.18,19,21 Initial slower activation in structural VT is related to slow intra muscle-to-muscle conduction until propagation enters the His-Purkinje system. However, in LPF-VT, initial ventricular activation is fast, excluding the use of these criteria for arrhythmia discrimination. RS time >100 ms was shown to support structural VT22. However, as described by Andrade et al14 and shown also in our study, RS time in LPF-VT was <80 ms and is similar to the values in the presence of aberrancy.
The current model has lower sensitivity, specificity, and predictive values when compared with previous algorithms differentiating structural VT from SVT with aberrancy. Several reasons may account for this difference. First, our aim was to establish morphological criteria that may differentiate LPF-VT from RBBB plus LAHB, and, therefore, we did not include atrioventricular dissociation in the model in contrary to previous models.19,22 Andrade et al14 demonstrated in a small study that atrioventricular dissociation may be found in ≈70% of patients with LPF-VT. Thus, in case that atrioventricular dissociation will be looked at first before applying morphological criteria, we expect to achieve a significantly higher specificity for predicting LPF-VT. Second, previous algorithms analyzed patients with structural heart disease and VT. These patients may have greater difference from SVT with aberrancy because of their structural heart disease. Third, as stated by Brugada et al,22 most cases of SVT with RBBB do not have left axis. Thus, previous algorithms mostly compared structural VTs to RBBB without left axis, and, therefore, morphological differences were more obvious. For example, RBBB and LAHB are more associated with R/S V6 ratio <1 that may wrongly suggest VT, compared with RBBB aberrancy alone. Of note, discordant findings that may suggest the different diagnosis were also reported in structural VT versus aberrancy studies but their incidence is lower compared with LPF-VT and RBBB with LAHB aberrancy.22
Finally, women with LPF-VT were found to have lower rates of QRS positivity in aVr. Whether the ability to correctly define tachycardia mechanism is lower in women should be defined.
Single Criterion for Diagnosis
Several findings were found to be exclusively diagnostic of LPF-VT and, thus, may facilitate its diagnosis. These criteria which could be found in 20% to 30% of LPF-VT include QS patterns (or R/S ratio <0.15) in V6 and QRS axis <−100°. On the contrary, finding of only 0 or 1 criteria of the 4-model criteria excludes LPF-VT.
Established criteria that are traditionally used for VT versus SVT discrimination do not apply to LPF-VT and its main differential diagnosis, SVT with RBBB and LAHB. On one hand, criteria that support VT may be nonrelevant to LPF-VT diagnosis. These criteria include atypical RBBB pattern in V1, QRS duration >140 ms, RS time >100 ms, initial R wave in aVr or Q >40 ms, and initial-to-terminal ventricular activation ratio (Vi/Vt) >1. On the other hand, criteria that exclude SVT, such as QRS duration >140 ms, R/S ratio <1 in V6, and positive QRS in aVr have bigger overlap in cases of RBBB and LAHB compared with RBBB alone. Therefore, in the absence of atrioventricular dissociation or fusion/capture beats, achievement of correct diagnosis is difficult. Our model has 4 easy-to-implement criteria that can lead to the correct diagnosis. Of note, it should be stressed that the model does not have 100% sensitivity or specificity, and there are cases in which it is impossible to differentiate between the 2 diagnoses based on morphology criteria alone. A proposed algorithm for evaluating a patient with RBBB and LAHB-like tachycardia is presented in Figure 5.
LPF-VT Versus Left Posterior Papillary Muscle VT
Another possible differential diagnosis of LPV-VT is posterior papillary muscle (PPM)-VT. Clinical differences that support LPF-VT as opposed to PPM-VT include presentation at a younger age with VT as opposed to premature ventricular complexes.23–26 Also, PPM-VT is not verapamil sensitive, and on electrophysiological study, focal mechanisms as opposed to reentry can be observed.26 Several studies tried to differentiate between these 2 arrhythmias based on ECG. Yamada et al23 suggested that the most reliable parameter that supports PPM-VT is QRS duration >160 ms that was observed in only 4.4% of ECGs in our LPF-VT patient cohort. Rivera et al24 reported a mean QRS duration of 142.2±9.2 among patients with PPM-VT, whereas QRS duration among our study group with LPF-VT was 127.5±18.6. Therefore, QRS is wider in patients with PPM-VT; however, an overlap between the groups exists. Another study25 suggested that spontaneous-variable QRS morphologies occur more often among patients with PPM-VT. Finally, Good et al13 reported that patients with LPF-VT had typical rsR′ pattern in V1 and discrete Q waves in leads 1 and aVL as opposed to PPM-VT. However, in our LPF-VT cohort, 54.2% had typical RBBB pattern in V1, 19.7% had a discrete q wave in lead 1, and 72% had q wave in aVL (this data was not shown above). Therefore, typical rsR′ pattern in V1 and discrete Q waves in leads 1 and aVL do support LPF-VT but their absence, especially in leads V1 and 1, does not rule it out.
A new type of fascicular VT that originates from the PPM was recently reported by Komatsu et al.26 When originating from the PPM, it was shown to have R/S ratio <1 in lead 1. Of note, we found in 33.5% (Table) of our LPF-VT cohort an R/S ratio <1 in lead 1. Therefore, an overlap probably exists between the ECG findings of LPF-VT and fascicular VT originating from the PPM.
To analyze a sufficient number of ECGs, we relied on published ECG data, which may lead to bias. However, we compared the ECGs of our patients and the Medline search group and did not find significant differences. Our control group patients were older and most were not in tachycardia. We chose this group as control because we could not find enough ECGs with this type of aberrancy in younger or tachycardia patients. To analyze the effect of heart rate, we used a second control group of patients that underwent atrial pacing. These patients did not show different ECG findings compared with the nonpaced controls. However, their number was limited, and heart rate during pacing was lower compared with patients with LPF-VT. Because the effect of heart rate and age on aberrancy was not fully tested, the results of our study will need validation. Nevertheless, all patients in the control group had echocardiography-proven structurally normal hearts. It should be emphasized that the results of the current study apply to LPF-VT, which accounts for ≈80% of fascicular VTs,27 whereas other types of fascicular VTs that involve the anterior or high septal fascicle were not analyzed in the current study. As with any model that includes cutoff values, when a given patient has measured parameter values that are close to the cutoff (as seen with the QRS width in Figure 4A), it may yield different results by different observers. However, in most ECGs, the QRS width was >5 ms different from the cutoff. Finally, this is a retrospective study, and its results should be tested prospectively.
Our study is the first to report the ECG characteristics of a large cohort of patients with LPF-VT. It gives a simple prediction model composed of 4 easy-to-implement criteria with a high likelihood of the correct diagnosis of LFP-VT if 3 or 4 of the criteria are fulfilled.
We thank Dr Tomer Ziv-Baran for statistical analysis support.
The Data Supplement is available at http://circep.ahajournals.org/lookup/suppl/doi:10.1161/CIRCEP.117.005074/-/DC1.
Circ Arrhythm Electrophysiol is available at http://circep.ahajournals.org.
- Received January 23, 2017.
- Accepted April 28, 2017.
- © 2017 American Heart Association, Inc.
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