The accuracy of the algorithm in localizing left-side or right-side septal APs by a positive or negative delta wave V1 was significantly high, giving the sensitivity of 100%, specificity of 89.3%, PPV of 81.3% and NPV of 100%.

The accuracy of the algorithm in localizing antero-septal or postero-septal APs by a positive or negative delta wave at V1 was significantly high, giving the sensitivity of 100%, specificity of 88.9%, PPV of 20% and NPV of 100%.

Table 3

LocationDelta wave polarity in inferior lead	Antero-septal	Postero-septal	Total
Positive delta wave in at least 2/3 inferior	1	4	5
Negative delta wave in at least 2/3 inferior	0	32	32
Total (n)	1	36	37

The accuracy of the algorithm in localizing mid-septal or non-mid-septal (antero-septal and postero-septal) APs by Qrs pattern (Qrs, qRs, qrS) or no Qrs pattern at inferior leads was significantly high, giving a sensitivity of 75%, specificity of 91.9%, PPV of 50% and NPV of 97.1%. Overall, the accuracy of the new ECG algorithm in localizing APs sites was high according to Table 1Table 2Table 3Table 4.

All patients (n=41) with septal APs required less procedure time with the mean value of 54.0 ± 24.3 minutes (ranging from 15 to 180 minutes) and less fluoroscopy time with the mean value of 7.4 ± 5.0 minutes (ranging from 2 to 20 minutes).

Table 5

Septal Accessory Pathway Site (n = 41)	Se (%)	Sp (%)	PPV (%)	NPV (%)
Sensitivity for accuracy for the Septal APs	87.8%
Left or right septal accessory pathways	98.3%	89.3%	81.3%	97.9%
Antero-septal or postero-septal accessory pathways	100%	88.9%	20%	100%
Mid-septal or no mid-septal accessory pathways	75%	91.9%	50%	97.1%
Mid-septal or right accessory pathways (antero-septal and right postero-septal)	75%	92.3%	75%	92.3%

Transition characteristics of the QRS complex on 12-lead ECG with septal location

Classified transition of the septal location was most common at V1, V2 lead (87.8%). The septal APs location with anatomic relation to the atrioventricular junction in relation to other cardiac structures, especially in the septal region’s transitional cell zone 5.

Some algorithms based on ECG have not been able to distinguish between antero-septal and right antero-lateral APs, and between postero-septal and postero-lateral (left or right). However, many studies showed that the transition of QRS complex could be used to predict locations of septal or free wall APs 1234.

Characterization of delta wave polarity in the V1 lead with left or right septal

Left septal APs commonly have a positive delta wave at V1 lead (100%), while right septal APs usually have a negative delta wave at V1 lead (89.3%). This is very useful to select an appropriate catheter approach such as in the vein or artery. Currently, all right septal APs (right-sided APs) are ablated using trans-venous atrial approach through the femoral vein, while left septal pathways (left-side APs) are ablated using retrograde arterial approach. If this approach fails, the pathway is ablated using antegrade transeptal approach 2.

Some ECG algorithms have been published to predict the location of left-sided or right-sided APs by positive or negative delta wave 1234. Besides, some other studies also diagnosed left or right-side APs by other ECG parameters such as R/S ratio < 0.5 or R/S > 0.5 in V1 lead 6, positive or negative QRS complex in V1 to diagnose left or right-side APs 7.

Characterization of delta wave polarity in at least 2/3 inferior lead (DII, DIII, AVF) with antero - septal or postero - septal

Using characteristic electrocardiogram, we observed strong positive delta waves in at least 2/3 inferior leads (II, III, aVF) in a patient with an antero-septal (100%). On the other hand, we observed a robust negative delta wave in at least 2/3 inferior leads (II, III, aVF) in patients with postero-septal APs (88.9%).

Successful ablation was performed at a location on the antero-septal annulus where a His bundle electrogram being recorded. The ablation was very anterior, thus was not in the vicinity of the compact AV node 3.

The posterior region is a triangular structure between the left ventricle and the right ventricle, located at the base of the Koch’s triangle, which can be mixed between left and right in pre-stimulus levels. The postero-septal region is located explicitly between the postero-superior process of the left ventricle and inferior wall or the right atrium. Many studies have shown that it is difficult in discriminating left or right-sided postero-septal APs; however, it has been suggested that a positive/negative delta wave in V1 can further differentiate between left/right sided postero-septal APs 189.

Some ECG algorithms have been published to predict the location of Antero- or postero- APs by positive or negative delta waves in the inferior lead as below 136. However, some studies only focused on certain locations of antero-septal and postero-septal APs 6.

Many studies have reported that the mid-septal is the only correct muscular septal area between the offset attachments of the mitral and tricuspid valves, while the previously named anterior and posterior septal regions are those regions that are anterior and posterior to the right septum. Mid-septal pathways are inserted into the Koch’s triangle region, close to the compact AV node. Accurate assessment of pathway slant and ablation long the ventricular insertion site by using cryo-energy may be required to minimize the risk of AV block when ablating pathways in this region 3.

D’Avila A (n=140) showed that the polarity of the QRS complex did not correctly localize midsepta1 APs. However, lead DIII showed a very similar QRS complex morphology in 5/10 patients with mid-septal APs. The negative QRS complex in lead DIII was characterized by a large “Q’ wave followed by a small “r” wave and an “s” wave. This “Qrs pattern” in lead DIII was highly specific for midsepta1 APs. However, the sensitivity was not high. Probably, D'Avila A only used tQrs pattern on DIII lead to localize mid-septal AP 7.

Classified QRS complex morphology (Qrs, qRs, qrS) of mid-septal locations was commonly found at inferior lead (DII, DIII, aVF), which was found in 5/6 patients with mid-septal APs (83.3%). This “Qrs pattern” in at least 1/3 inferior lead was able to distinguish mid-septal APs from other septal locations. Among other septal locations (60 patients), only eight patients with postero-septal APs (13.3%) were found to have this QRS complex morphology.

Figure 2

Besides, 8 out of 59 patients (13.6%) diagnosed with an abnormal posterior septum had abnormal signs in mid-septal regions indicated by imaging results. That may be because two areas have similar anatomical features. Given the anatomical feature of the right mid-septal region, which is located between the anterior septum and posterior septum, forming Koch’s triangle, incorrect images can be identified by technicians.

We developed a new algorithm using some simple ECG parameters such as transition QRS complex at V1V2 or after V1V2 lead to localize septal or lateral APs, positive/negative delta wave at V1 lead to localize left or right side APs, positive/negative delta waves in at least 2/3 inferior to localize Antero- or postero- APs, and Qrs pattern (QRS, qRs, qrS) in at least 1/3 inferior lead (DII, DIII, aVF) to localize mid-septal APs (Figure 3).

Then, this algorithm was prospectively tested in 41 patients to compare with the locations of APs identified by successful ablation by RF, which was used to calculate sensitivity, specificity, positive predictive value and negative predictive value of the diagnosed algorithm in predicting the locations of APs (Table 1Table 2Table 3Table 4Table 5). For the left or right side APs, by using positive/negative delta waves at V1 lead (Table 1); Chang CE (1995) achieved Se of 94.4% and Sp of 87.5% 8. Similarly, for septal or lateral APs by using transition QRS complex at V1V2 or after V1V2 lead (Table 2); Dar M (2008) achieved Se of 97% and Sp of 95% 4. For the antero or postero APs by positive/negative delta waves in inferior (Table 3), Dar M achieved Se and Sp from 85 to 100% 4. This "Qrs pattern" in the inferior lead (DII, DIII, aVF) was highly sensitive (75%) and specific (91.9%) for mid-septa1 locations (Table 4) 7.

Figure 3

There are three out of 36 cases showed specific signals of mid-septal regions (Table 4). However, they were actually in the posterior septum, which can be explained by the fact that these two regions are close to each other, which have same physiological features and some transitional muscles are running through these two regions. Indeed, the results of successful radiofrequency ablation indicated that the three patients had APs which were very close to the mid-septal areas. The diagnostic results confirmed our hypothesis. We suspected the signals are indicating posterior septum localization because there were some signals from posterior regions: a negative delta wave in at least 2/3 inferior lead (DII, DIII, aVF), specifically in the case having a positive delta wave in the V1 lead.

Table 6

Authors	Accessory pathway location	Mean Proceduretime (minute)	Mean Fluoroscopy time (minute)
Calkins H. (1992) 9	WPW syndrome (n = 250)	134 ± 75	47 ± 33
Dong TV. (2006) 10	Right posteroseptal (n = 26)	99.8 ± 48.0	23.2 ± 12.8
		Antero-septal & mid-septal (n=10)	106.7 ± 56.8	22.9 ± 16.4
Haghjoo M. (2008) 11	Posteroseptal (n=94)	-	40.0 ± 13.0
Our research (2016 - 2017)	Septal APs (n = 41)	54.0 ± 24.3	7.4 ± 5.0

Above parameters of 12-lead ECG was able to predict successful ablation sites, which significantly shortened the procedure time and fluoroscopy time, and enhanced the fluoroscopy safety (Table 6) 12.

The study significantly improved patients’ hospital experiences. To be specific, the research helped reduce X-ray exposure to decrease the side-effects for both doctors and patients from radiologic techniques. Also, the study provided a fast method to identify root causes of arrhythmia as well as to localize APs, and to forecast any accident during RFCA, which decreased patients treatment deration, and increased medical staffs’ performance.

We developed and validated a new algorithm to localize septal APs. We found that the left septal APs mostly have a positive delta wave at V1 lead (92%) and right septal APs mostly have a negative delta wave V1 lead (87.5%). The classified transition of septal location is most common at V1, V2 lead (89.2%). Antero-septal APs mostly have a positive delta wave at least 2/3 inferior lead (100%), and postero-septal APs mostly have a negative delta wave at least 2/3 inferior lead (92.6%). Mid-septal APs commonly have a negative QRS with specific morphology (Qrs, qRs, qrS) at least 1/3 inferior lead (83.3%).

The new algorithm was proved to be highly accurate and sensitive (from 83.3% to 100%), with high specificity (75% to 100%). The mean procedure time and fluoroscopy were significantly shortened and safer when using the above parameters to predict the location of septal APs.

The authors declare no potential conflict of interest.

Si CD designed the study, collected data, conducted the statistical analysis and wrote the manuscript. Khanh PQ and Dong TV are revised the manuscript during the review process.

Vietnam University of Military Medicine & Vietnam National Heart Institute, Bachmai Hospital.

AP: Accessory pathway

APs: Accessory pathways

AV: Atrial Node

ECG: Electrocardiogram

RFCA: Radiofrequency catheter ablation

WPW: Wolff-Parkinson-White