Development of a magnetocardiography-based algorithm
for discrimination between ventricular arrhythmias
originating from the right ventricular outﬂow tract andthose originating from the aortic sinus cusp: A pilot study
Yoko Ito, MD,* Keisuke Shiga, B. Eng,† Kentaro Yoshida, MD,* Kuniomi Ogata, M. Eng,‡
Akihiko Kandori, PhD,‡ Takeshi Inaba, RDCS,* Yoko Nakazawa, MD,* Yukio Sekiguchi, MD,*
Hiroshi Tada, MD, FHRS,§ Kensuke Sekihara, PhD,† Kazutaka Aonuma, MD*
From the *Cardiovascular Division, Institute of Clinical Medicine, University of Tsukuba, Tsukuba, Japan,†
Department of Systems Design & Engineering, Tokyo Metropolitan University, Hachioji, Japan, ‡Central
Research Laboratory, Hitachi Ltd, Kokubunji, Japan, and §Cardiovascular Division, Institute of Clinical
Medicine, University of Fukui, Yoshida-gun, Japan.
BACKGROUND Although several reports address characteristic 12lead electrocardiographic ﬁndings of outﬂow tract ventriculararrhythmias (OT-VAs), the accuracy of electrocardiogram-basedalgorithms to predict the OT-VA origin is sometimes limited.
OBJECTIVE This study aimed to develop a magnetocardiography(MCG)–based algorithm using a novel adaptive spatial ﬁlter todifferentiate between VAs originating from the aortic sinus cusp
(ASC-VAs) and those originating from the right ventricular outﬂow
tract (RVOT-VAs).
METHODS This study comprised 51 patients with an OT-VA as thetarget of catheter ablation. An algorithm was developed bycorrelating MCG ﬁndings with the successful ablation site. The
arrhythmias were classiﬁed as RVOT-VAs or ASC-VAs. Three parameters were obtained from 3-dimensional MCG imaging: depth of theorigin of the OT-VA in the anteroposterior direction; distance
between the earliest atrial activation site, that is, sinus node, andthe origin of the OT-VA; and orientation of the arrhythmia
propagation at the QRS peak. The distance was indexed to thepatient’s body surface area (in mm/m2).
RESULTS Origins of ASC-VAs were signiﬁcantly deeper (81 ⫾ 6
mm/m2 vs 68 ⫾ 8 mm/m2; P o .01) and farther from the sinus node
(55 ⫾ 9 mm/m2 vs 41 ⫾ 9 mm/m2; P o .01) than those of RVOT-

Introduction
Radiofrequency (RF) catheter ablation has been established
as an effective and curative therapy for ventricular arrhythmias (VAs) originating from the outﬂow tract (OT-VAs) in
The ﬁrst 2 authors and the last 2 authors contributed equally to this work.
Address reprint requests and correspondence: Dr Kentaro Yoshida,
Cardiovascular Division, Institute of Clinical Medicine, University ofTsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8575, Japan. E-mailaddress: kentaroyo@nifty.com.

1547-5271/$-see front matter B 2014 Heart Rhythm Society. All rights reserved.

VAs. ASC-VA propagation had a tendency toward rightward axis.
Receiver operating characteristic analyses determined that thedepth of the origin was the most powerful predictor, with asensitivity of 90% and a speciﬁcity of 73% (area under the
curve ¼ 0.90; P o .01). Discriminant analysis combining all 3
parameters revealed the accuracy of the localization to be 94%.
CONCLUSION This MCG-based algorithm appeared to precisely
discriminate ASC-VAs from RVOT-VAs. Further investigation isrequired to validate the clinical value of this technique.
KEYWORDS Premature ventricular contraction; Right ventricular
outﬂow tract; Aortic sinus cusp; Magnetocardiography; Catheter
ablation; Spatial ﬁlter
ABBREVIATIONS ASC ¼ aortic sinus cusp; AUC ¼ area under the
curve; ECG ¼ electrocardiogram/electrocardiographic; MCG ¼
magnetocardiography/magnetocardiographic; OT-VA ¼ outﬂow
tract ventricular arrhythmia; RF ¼ radiofrequency; ROC ¼
receiver operating characteristic; RVOT ¼ right ventricular
outﬂow tract; TZ ¼ transitional zone; VA ¼ ventriculararrhythmia; VT ¼ ventricular tachycardia(Heart Rhythm 2014;11:1605–1612) I 2014 Heart Rhythm Society.
All rights reserved.

structurally normal hearts.1,2 Most of these arrhythmias
originate from the septal aspect of the right ventricular
outﬂow tract (RVOT)1,2 or the aortic sinus cusp (ASC).3,4
In reported case series, success rates of RF catheter ablationof left ventricular outﬂow tract ventricular tachycardia (VT)are lower than those of RVOT-VT, and left heart catheterization is associated with risks of thromboembolism andcoronary artery or aortic valve injury.5 Precise discriminationof ASC-VAs from RVOT-VAs before the ablation procedure
helps electrophysiologists to decide on necessary arterial or
http://dx.doi.org/10.1016/j.hrthm.2014.05.032

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Heart Rhythm, Vol 11, No 9, September 2014

venous punctures, to plan an appropriate ablation strategy,
and to inform patients about risks, beneﬁts, and success ratesof the procedures.
Although electrocardiogram (ECG)–based algorithms
have successfully been developed that can identify the origin
of VAs,3,4,6 it is possible that their accuracy is impaired byphysical characteristics of the patients,7 cardiac rotation,8and offset of lead placement.9 Therefore, neighboring
anatomic structures can result in ECG characteristics that
may be similar to those of RVOT-VAs.10
We developed a novel spatial ﬁlter to reconstruct a source
distribution from bioelectromagnetic data, which can provide a spatial resolution considerably higher than that ofconventional methods. The robustness of this method to thesource correlation was already validated in our computer
simulation and in our experiments using auditory-evokedmagnetoencephalographic data.11 The objectives of thisstudy were to apply this novel spatial ﬁlter to magnetocardiography (MCG), to calculate the coordinates of the VA
origin itself, and to demonstrate the ability of MCG todiscriminate ASC-VAs from RVOT-VAs.

Methods
Patients
This study comprised 51 patients (20 men and 31 women;mean age 49 ⫾ 15 years) who underwent successful catheter
ablation for symptomatic idiopathic VT or premature ventricular contractions originating from the outﬂow tracts
between January 2009 and October 2012. Patients withstructural heart disease or atrial arrhythmias were excluded
from the study. Surface ECGs of the OT-VAs showed left
bundle branch block morphology and inferior axis in all
patients. The origins of the VAs were identiﬁed in the RVOT
(n ¼ 41 [80%]) or the ASC (n ¼ 10 [20%]). There were no
signiﬁcant differences with regard to baseline patient characteristics (Table 1). All patients had a normal ECG duringsinus rhythm except for rotation of the R-wave transition,and no structural abnormalities were found by physical
Table 1

examination, echocardiography, and coronary angiography.
All antiarrhythmic drugs were discontinued for at least 5
half-lives of each drug before the procedure. All patientsgave written informed consent for their participation in thisstudy, and the study protocol was approved by the local
institutional review board.

Electrophysiological study and RF catheter ablation
Electrophysiological evaluation and catheter ablation were
performed as described previously.12 Brieﬂy, catheters wereintroduced into the His bundle region and right ventricularapex under ﬂuoroscopy via the right femoral vein. If theclinical arrhythmia did not occur spontaneously, intravenous
isoproterenol was administered to induce it. Activation
mapping and pace mapping were performed during theclinical arrhythmia. Mapping of the OT-VA was initiallystarted in the RVOT area. If suitable ablation sites were notfound in the RVOT, the ASC was mapped. A 7-F quadripolar catheter with a 4-mm distal electrode and a deﬂectable tip (Cordis Webster Inc, Baldwin Park, CA; EP
Technologies Inc, San Jose, CA) or a 7-F irrigated-tip
catheter (ThermoCool, Biosense Webster, Inc, Diamond
Bar, CA) was used for mapping and ablation. The RF energy
was delivered using a maximum power of 50 W and amaximum temperature of 551C with use of the non–irrigatedtip catheter as well as using a maximum power of 40 W and amaximum temperature of 421C with use of the irrigated-tip
catheter. The VA origin was deﬁned as the site where theearliest ventricular activation was recorded and/or a perfectpace map was obtained and VAs were successfully eliminated through ablation. After successful ablation, right atrial
CARTO mapping (Biosense Webster, Inc, Diamond Bar,CA) was performed to identify the location of the sinus nodein 5 patients for the validation of the accuracy of the MCG
measurements described below. Procedural success was
conﬁrmed by the absence of symptoms suggestive of atachycardia and of episodes of the OT-VA as documented onBaseline characteristics
Total

RVOT

ASC

Variable(n ¼ 51)(n ¼ 41)(n ¼ 10)P

Age (y)Sex: women/men
Body mass index (kg/m2)
BSA(m2)
Holter ECG
Patients with repetitive PVCs (43 beats)
Patients with sustained VT
PVC%
Echocardiographic parameters
LV ejection fraction (%)
LV diameter (mm)LV volume (mL)

49 ⫾ 15
31/20
23.4 ⫾ 3.3
1.7 ⫾ 0.2

48 ⫾ 15
23/18
23.1 ⫾ 3.3
1.7 ⫾ 0.17

53 ⫾ 17
8/2
24.6 ⫾ 3.6
1.6 ⫾ 0.13

.43
.28
.25
.14

24 (52)4 (9)20 ⫾ 1519 (53)2 (6)19 ⫾ 16

5 (50)
2 (20)
22 ⫾ 14

1.00
.20
.69

64 ⫾ 849 ⫾ 6113 ⫾ 33

64 ⫾ 849 ⫾ 6112 ⫾ 33

64 ⫾ 950 ⫾ 5115 ⫾ 34

.92
.54
.84

Values are presented as mean ⫾ SD and as n (%).
ASC ¼ aortic sinus cusp; BSA ¼ body surface area; ECG ¼ electrocardiogram; LV ¼ left ventricular; PVCs ¼ premature ventricular contractions; RVOT ¼ rightventricular outﬂow tract; VT ¼ ventricular tachycardia.

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MCG-Based Algorithm to Discriminate PVC Origins

1607

the 12-lead ECG or 24-hour Holter monitoring during thefollow-up period of 24 ⫾ 13 months.

minus the TZ score of the sinus beat, where the TZ score isgraded in 0.5-point increments according to the site of the TZ
in the precordial leads.6,8ECG-based algorithms
Surface 12-lead ECGs were recorded in all patients duringsinus rhythm and during the clinical arrhythmia at a paper
speed of 25 mm/s, with chest and limb leads placed instandard positions. The morphology of the sinus beats and
OT-VAs was analyzed on the same 12-lead ECG using
electronic calipers. We applied a recently reported ECGbased algorithm, the transitional zone (TZ) index, which isconsidered to more accurately discriminate RVOT-VAs
from ASC-VAs than do other previously reported algorithms.8 The TZ index is deﬁned as the TZ score of the VA

Spatial ﬁlter and MCG-based algorithms
All patients underwent MCG testing during sinus rhythm 1
day before ablation. The MCG methodology has been
described in detail previously.13,14 An MC-6400 MCG
system (Hitachi High-Technologies Corporation, Tokyo,
Japan) with 64 magnetic sensors was used. The magnetic
sensors were in an 8  8 matrix with a pitch of 25 mm and ameasurement area of 175  175 mm. The MCG signals fromeach patient were recorded in the resting state in theanteroposterior direction in a magnetically shielded room

MCG system
PVC

Si
Sinus
h th
rhythmMagnetic field [pT]Time [ms]Magnetic field
distribution

RENS spatial filter

Current arrow map

Sinus node

PVC

Distance

Depth

Estimation results

Discriminant analysis

Orientation ofcurrent arrow

ASC or RVOT

Figure 1 A: MC-6400 MCG system (Hitachi High-Technologies Corporation, Tokyo, Japan) with 64 magnetic sensors to measure the normal component of
the magnetic ﬁeld. B: MCG signals from the VA and normal sinus rhythm were averaged separately. These averaged signals were used to reconstruct 2
distinctive points by applying the RENS spatial ﬁlter algorithm. C: The location of the sinus node was obtained using an MCG ﬁeld map at the beginning of the P
wave of the normal sinus rhythm. The origin of the VA was obtained by reconstructing an MCG ﬁeld map at the beginning of the QRS complex in the averaged
VA signal. D: The 3 parameters were obtained: depth of the origin of the VA with respect to the sensor array, distance between the sinus node and the origin of
the VA, and orientation of the magnetic ﬁeld at the peak of the QRS complex. ASC ¼ aortic sinus cusp; MCG ¼ magnetocardiography; PVC ¼ premature
ventricular contraction; RENS ¼ resolution near-ﬁeld structure; RVOT ¼ right ventricular outﬂow tract; VA ¼ ventricular arrhythmia.

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Table 2

Heart Rhythm, Vol 11, No 9, September 2014
Parameters related to catheter ablation
Total

RVOT

ASC

Variable(n ¼ 51)(n ¼ 41)(n ¼ 10)P

Procedure time (min)Irrigation catheter
Preceded electrogram (ms)Perfect or near-perfect map
Catheter ablation site
Duration of radiofrequency energy applications (min)124 ⫾ 49
19 (37)29 ⫾ 1247 (92)14 ⫾ 915 ⫾ 10119 ⫾ 43
15 (37)29 ⫾ 1238 (93)14 ⫾ 816 ⫾ 10152 ⫾ 63
4 (40)
31 ⫾ 149 (90)
11 ⫾ 1210 ⫾ 11

.06
1.00
.72
.60
.29
.12

Values are presented as mean ⫾ SD and as n (%).
ASC ¼ aortic sinus cusp; RVOT ¼ right ventricular outﬂow tract.

(Figure 1A). The sampling rate was 1 kHz, and the measurement period was 2 minutes. The MCG signals were passed
through a 0.1- to 50-Hz band-pass ﬁlter and a 50-Hz powerline noise ﬁlter. The outline of the proposed method ispresented in Figure 1. The MCG signals from the VA andnormal sinus rhythm were averaged separately. Theseaveraged signals were then used to reconstruct 2 distinctive
points by applying the resolution near-ﬁeld structure spatial
ﬁlter algorithm (Figure 1B).11 The location of the sinus node
was obtained using an MCG ﬁeld map at the beginning of the
P wave of the normal sinus rhythm. The origin of the VA was
obtained by reconstructing an MCG ﬁeld map at the
beginning of the QRS complex in the averaged VA signals(Figure 1C).
The ﬁrst parameter is the depth of the VA origin withrespect to the sensor array, and the second parameter is the
distance between the sinus node and the origin of the VA.
The ﬁrst and second parameters are corrected by eachpatient’s body surface area. The third parameter is obtained
as the orientation of the magnetic ﬁeld at the peak of the QRS
complex of the VA signal. The discriminant function was
obtained using these 3 variables from patients with a VA ofknown origin, and the analysis was applied to a data set
obtained from a patient with a VA of unknown origin(Figure 1D).

Statistical analysis
Continuous variables are expressed as mean ⫾ SD and werecompared using the Student t test. Categorical variables werecompared by using the χ2 test. A P value of o.05 was considered statistically signiﬁcant.
Discriminant analysis was based on the Mahalanobis
distance. To simultaneously compare groups using multiple
variables, it is insufﬁcient to consider the variables singly
because this ignores the correlations between them. To
measure the distance of a patient from the “multivariate
mean point” (centroid) of the RVOT and ASC groups in themultidimensional space, a statistically generalized distance isrequired. A suitable measure is the Mahalanobis distance.15
The Mahalanobis distance standardizes the scales, and so theresults are independent of the units of measurement of any
variable. Using 3 variables (depth, distance from the sinus
node, and orientation), the Mahalanobis distances from thecentroid of each group (RVOT or ASC) were calculated foreach patient. Herein, if one patient was closer to the centroidof the RVOT group compared to that of the ASC group, thearrhythmia origin of that patient was determined to be the
RVOT.16

Results
Catheter ablation
In 41 patients, the VA was eliminated by ablation in the
RVOT: in the septal aspect in 28 patients and in the free wallin 13 patients. The earliest ventricular activation recordedwith the ablation catheter preceded the onset of the QRS
complex by 29 ⫾ 12 ms. In the other 10 patients, the VA waseliminated by ablation in the ASC: in the left coronary cusp
in 8 patients and the right coronary cusp in 2 patients. The
earliest ventricular activation recorded with the ablation
catheter preceded the onset of the QRS complex by 31 ⫾14 ms. Comparisons of procedure-related parameters
between RVOT-VAs and ASC-VAs are summarized inTable 2. There was a trend toward longer procedure timein the ASC group vs the RVOT group (P ¼ .06).

Differentiation based on the ECG-based algorithm
A cutoff value for the TZ index of o0 predicted an ASC
origin with 95% sensitivity and 80% speciﬁcity in this study(Table 3). The area under the curve (AUC) obtained byreceiver operating characteristic (ROC) analysis was 0.88.

Differentiation based on the MCG-based algorithm
Depth of the VA origin in the anteroposterior direction
The VA origin in the ASC group was signiﬁcantly deeper thanthat in the RVOT group (81 ⫾ 6 mm/m2 vs 68 ⫾ 8 mm/m2;
Table 3Differentiation based on the TZ index
Total

RVOT

ASC

Variable(n ¼ 51) (n ¼ 41) (n ¼ 10)

TZ score of the OT-VA
TZ score of the sinus
beat
TZ index

3.3 ⫾ 1.1 3.7 ⫾ 0.4
3.1 ⫾ 0.8 3.1 ⫾ 0.8P

1.8 ⫾ 1.6 .005
3.3 ⫾ 0.8 .55

0.2 ⫾ 1.3 0.6 ⫾ 0.9 1.5 ⫾ 1.6 .002

Values are presented as mean ⫾ SD.
ASC ¼ aortic sinus cusp; OT-VA ¼ outﬂow tract ventricular arrhythmia;
RVOT ¼ right ventricular outﬂow tract; TZ ¼ transitional zone.

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MCG-Based Algorithm to Discriminate PVC Origins

1609
48 mm/m2, with a sensitivity of 90% and a speciﬁcity of 78%(AUC ¼ 0.88; P o .01; Figure 3, orange line). The positive
predictive and negative predictive values were 97% and50%, respectively (Table 4). Discriminant analysis also
revealed that the cutoff distance had an accuracy of 78%(Table 5).
Orientation of VA propagation
Arrhythmia propagation was superior or rightward axis in
most patients with an ASC-VA, whereas it was variable andshowed no certain tendency in the RVOT group (Figures 2C
and 2D). ROC analyses determined the best cutoff value tobe 821, with a sensitivity of 80% and a speciﬁcity of 59%(AUC ¼ 0.73; P ¼ .03; Figure 3, blue line). The positive
predictive and negative predictive values were 92% and32%, respectively (Table 4). Discriminant analysis also
revealed that the cutoff angle had an accuracy of 61% for
localizing the ASC-VA from the RVOT-VA (Table 5).
Finally, a discriminate analysis that combined all 3
parameters could discriminate VA origins with a highaccuracy of 94% (Table 5). A representative case ispresented in Figure 4.

Validation of the MCG-based algorithm
The distance between the sinus node and the VA origin
measured by MCG mapping almost completely correlated withthat measured by CARTO mapping (Y ¼ 0.903X  0.652;
R ¼ .94; P o .01). However, the distance measured by
CARTO mapping was signiﬁcantly shorter than that determinedby MCG mapping (50 ⫾ 14 mm vs 62 ⫾ 15 mm; P ¼ .005).

Discussion
Major ﬁndings
Figure 2 Box plots of the 3 parameters in the RVOT and ASC groups and3-dimensional scatter plot. A: Depth of the VA origin B: Distance between
the sinus node and the origin of the VA C: Orientation of arrhythmia
propagation D: Three-dimensional scatter plot of the 3 parameters. ASC ¼
aortic sinus cusp; RVOT ¼ right ventricular outﬂow tract; VA ¼ ventriculararrhythmia.

P o .01; Figures 2A and 2D). ROC analyses determined thebest cutoff value to be 73 mm/m2, with a sensitivity of 90%
and a speciﬁcity of 73% (AUC ¼ 0.90; P o .01; Figure 3,
green line). The positive predictive and negative predictive
values were 97% and 41%, respectively (Table 4). Discriminant analysis also revealed that the cutoff distance had an
accuracy of 77% for differentiating an ASC-VA from a
RVOT-VA (Table 5).
Distance between the earliest atrial activation site, that is, thesinus node, and the origin of the VA
The distance between the sinus node and the VA origin in
the ASC group was longer than that in the RVOT group (55⫾ 9 mm/m2 vs 41 ⫾ 9 mm/m2; P o .01; Figures 2B and
2D). ROC analyses determined the best cutoff value to be

A novel MCG-based algorithm using an originally developed spatial ﬁlter could precisely identify the side of VA
origin with a diagnostic accuracy of more than 90%. Deeper
origin, longer distance from the sinus node, and rightward
direction of arrhythmia propagation were observed in ASCVAs. These results may lead to a better understanding of the
anatomy related to OT-VAs and to optimal approaches forcatheter ablation. There was almost perfect correlation
between distances calculated from MCG signals and thosemeasured with a 3-dimensional mapping system, thus
supporting the accuracy of this MCG-based algorithm. To
our knowledge, this is the ﬁrst report demonstrating theusefulness of MCG to differentiate an ASC-VA from a
RVOT-VA in clinical practice.
Depth of the VA origin
In this study, the origins of VAs from the ASC weresigniﬁcantly deeper than those of VAs from the RVOT,
and this ﬁnding showed the most signiﬁcant difference
between the 2 VA types. Knowledge of the anatomic relation
between the RVOT and the ASC is essential for accurate
diagnosis of the VA origin. An anatomical study17 demonstrated that in a transverse section of the adult thorax at the1610Heart Rhythm, Vol 11, No 9, September 2014

Figure 3 ROC curve of the depth of the VA origin, the distance between the sinus node and the origin of the VA, and the orientation of arrhythmia
propagation. ROC ¼ receiver operating characteristic; VA ¼ ventricular arrhythmia.

level of the fourth costal cartilages, the septa lie at an angle ofapproximately 451 to the median plane, and the rightventricle therefore lies as much in front of the left ventricle
as it does to its right side. In the vertical section, as theterminal part of the RVOT ascends, it comes to lie increasingly more on the left side of the heart, and in this region, the2 outﬂow tracts are separated by the anterior wall of theaortic vestibule. Also, the mechanism behind certain ECGbased algorithms, which determine that larger R waves arefound in leads V1 and V2 of ASC-VAs, is considered to be
due to the posterior location of the aorta.18 These reports are
consistent with the results based on our MCG-based algorithm, in which the depth of the VA origin was the important
discriminator differentiating RVOT-VAs from ASC-VAs.
Distance between the sinus node and the origin of the VA
We measured the distance from the sinus node to the VA
origin because this parameter appeared to be less affected byphysical characteristics of the patients, such as body surface
area or body mass index, than did the depth of the VA origin.

As discussed above, the origin of ASC-VAs was farther fromthe sinus node compared with that of RVOT-VAs. The sinus
node is a lateral structure relative to the superior cavoatrial
junction and is remarkably constant in position,19 making thesinus node favorable as a base point for this parameter.
Orientation of arrhythmia propagation at the QRS peak
In this study, the activation pattern of VAs was signiﬁcantly
different in axis between RVOT-VAs and ASC-VAs. One of
the characteristics of the outﬂow tracts is known to be theircrossover relationship. The outﬂow tract undergoes rotation
during its development and remodeling. Rotation of the
myocardium at the base of the outﬂow tract is essential toachieve normal positioning of the great arteries. In thecoronal section of the adult thorax, the left ventricular
outﬂow tract extends upward from the apex of the chamberto the aortic valve. However, the sagittal section of the adultthorax can show a characteristic U-shaped form of the whole
right ventricle, indicating that the RVOT lies along theTable 5

Table 4 Parameters of magnetocardiography of ventriculararrhythmias
VariableSensitivity Speciﬁcity PPV NPV AUC P

Depth
90%
Distance
90%
Orientation 80%

73%
78%
59%

Accuracy

97% 45% 0.90 o.01 90%
97% 50% 0.88 o.01 80%
92% 32% 0.73
.03 80%AUC ¼ area under the curve; NPV ¼ negative predictive value; PPV ¼
positive predictive value.

Discriminant analysis

Variable

RVOT

ASC

Total

Depth
Distance
Orientation
Depth and distance
Depth and orientation
Distance and orientation
Combination of the 3 parameters

73%
78%
56%
88%
85%
88%
95%

90%
80%
80%
100%
80%
70%
90%

77%
78%
61%
90%
84%
84%
94%

ASC ¼ aortic sinus cusp; RVOT ¼ right ventricular outﬂow tract.

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MCG-Based Algorithm to Discriminate PVC Origins

1611

RAO 35°ABL

LAO 45°

ABL

Figure 4 A: Representative 12-lead ECG from a patient with outﬂow tract ventricular arrhythmia. In this case, the TZ score of the arrhythmia was 2.5 and the
TZ score of the sinus beat was 2.5 (TZ index ¼ 0), suggesting that the VA origin is located in the RVOT. However, the depth calculated with the MCG-basedalgorithm was 92 mm/m2, the distance between the sinus node and the VA origin was 51 mm/m2, and the orientation was 331, suggesting that the VA origin is
located in the ASC (see also Figure 2D). B: Fluoroscopic views show the successful ablation site in the left coronary cusp. Another ablation catheter is located in
the RVOT for mapping. ABL ¼ ablation catheter, ASC ¼ aortic sinus cusp; ECG ¼ electrocardiogram; LAO ¼ left anterior oblique; MCG ¼
magnetocardiography; RAO ¼ right anterior oblique; RVOT ¼ right ventricular outﬂow tract; TZ ¼ transitional zone; VA ¼ ventricular arrhythmia.

cephalocaudal axis. This crossover arrangement suggeststhat the activation patterns of RVOT-VAs or ASC-VAs aredifferent from each other, and this could be a discriminatorfor determining the VA origin.

Validation of MCG measurements
It was advantageous that the distance between the VA origin
and the sinus node can be directly measured with a3-dimensional electroanatomic mapping system (CARTO)
during the procedure and can be used to validate the accuracyof MCG measurements. There was almost complete correlation between distances from the sinus node to the VA
origin measured with both the MCG-based algorithm and the
3-dimensional mapping system. However, the distancederived from CARTO mapping was signiﬁcantly shorter
than that derived from the MCG measurements. One possible
explanation is the difference in the patients’ volume status.
Fasting was required before ablation but not before MCG
measurement, which could result in smaller chamber size
during the ablation procedure. Another explanation is theeffect of the isoproterenol infusion used during the ablation
procedure to induce VAs. Isoproterenol increases cardiac
contractility and heart rate, which may affect the chamber
size and catheter contact with the endocardial wall.

Study limitations
Major limitations of this study were its small sample volume
and the absence of a validation cohort. Because the discriminant power may be overestimated in this retrospective
analysis, further prospective investigations to validate theability of the MCG-based algorithm are needed. For thesesame reasons, we could not clarify statistical superiority of
the MCG-based algorithm over the ECG-based algorithm.
Currently, the only noninvasive method for the identiﬁcationof arrhythmia origin in clinical practice is the ECG-basedalgorithm. The QRS morphology reﬂects arrhythmia propagation in the ventricles and indirectly estimates the arrhythmia origin, which is completely different from the MCGbased algorithm, which in turn directly identiﬁes thearrhythmia origin itself. We believe that this methodologicaldistinctiveness of MCG is of great interest to electrophysiologists.
With regard to discrimination of the side of VA origin—right or left—the diagnostic accuracy of the MCG-basedalgorithm appeared to be superior or noninferior to that of the
ECG-based algorithm. However, the ECG-based algorithm
can classify more detailed anatomical locations within the
RVOT, such as septum vs free wall and anterior vs posteriorwall. To make subdivision of the RVOT origin possible,
further development of a method and algorithm is needed,such as merging of MCG mapping with 3-dimensional
computed tomography.
Although VAs originating from the pulmonary artery or
non–left coronary cusp (right coronary cusp or junction
between the left and right coronary cusps) have beenreported, these arrhythmias were hardly observed in thepresent study. The ability of this algorithm to differentiate
such VAs remains to be evaluated.
The distance between the sinus node and the VA origin
was found to be a reliable and useful parameter in this study.

1612
However, this algorithm cannot be applied to patients whosebaseline rhythm is not sinus rhythm.

Conclusion
We developed an MCG-based algorithm to differentiate
RVOT-VAs from ASC-VAs with high sensitivity andspeciﬁcity. The deeper origin and rightward direction of
the activation pattern observed in ASC-VAs may lead us to abetter anatomical understanding of this type of arrhythmia as
well as to optimal catheter ablation of OT-VTs. Further
investigation is required to validate the clinical value of thistechnique and to clarify the added value of MCG over ECG.

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