Incidence, clinical characteristics, electrophysiological characteristics and outcomes of patients with baseline PR prolongation undergoing radiofrequency ablation for Atrioventricular nodal reentrant tachycardia

Abstract

Aims and objectives

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common supraventricular tachycardia (SVT). Prolonged PR interval(>200 ms) on baseline electrocardiogram (ECG) is uncommon in such patients. The aim of the current study was to evaluate the incidence, clinical, electrophysiological characteristics, and outcomes of patients with baseline prolongation of PR interval undergoing radio-frequency ablation (RFA) for AVNRT.

Methods

Over 10 years, out of the total number of 1435 patients with diagnosed AVNRT, 16 patients had prolonged PR intervals at baseline. All underwent elective RFA. A retrospective analysis of clinical, and electrophysiological characteristics and outcomes was done. The PR interval and atria-ventricular block cycle length values were compared with those patients with a normal interval at baseline and had undergone a successful slow pathway modification for AVNRT.

Results

Out of 1435 patients with AVNRT, 16 (0.9 %) patients had baseline PR prolongation on ECG. The mean(+SD) age of the study population was 62.9 + 15.9 years. 10 (62.5 %) were males. The average PR interval was 264.2 + 24.1 ms. Slow fast AVNRT was seen in all. The anatomical site of success for ablation was the lower part of Koch's triangle in all patients. During ablation, a good sustained junctional rhythm was noted in all, with no AV (Atrioventricular) block or PR prolongation noted during ablation in any of the patients. PR interval decreased by more than 20 ms in 10 (62.5 %) patients. AVBCL (AV node block cycle length) increased on an average of 58.7 ms post-ablation. Only one patient developed AV block on follow-up.

Conclusion

A prolonged PR interval on baseline ECG is uncommon in patients with AVNRT. In these patients, slow pathway modification can be done safely and effectively. AVBCL (AV node block cycle length) increases immediately post-ablation. The risk of AV block though low persists on follow-up.

1. Introduction

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most common form of supraventricular tachycardia (SVT) [1]. A prolonged PR interval on baseline electrocardiogram (ECG) is uncommon. Slow pathway ablation/modification which is the recommended therapy for AVNRT [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11]] has been shown to be safe and effective in this subgroup of patients [[11], [12], [13]]. The incidence of post-procedure AV block during slow pathway ablation in patients with PR prolongation ranges from 0 to 9.3 % [5,14], as compared to the incidence of <1.5 % of atrioventricular block in the unselected population [2,6,15,16]. Patients with prolonged PR interval (>200 ms) have a higher probability of developing AV block after slow pathway modification [17]. Radio-frequency ablation (RFA) of the fast pathway in patients having first-degree AV block has a higher intra-procedural risk of complete AV block, but it is not associated with higher chances of having AV block during long-term follow-up [18,19,16,20]. We aimed to evaluate the safety and efficacy of slow pathway modification in patients with a first-degree AV block.

2. Materials and methods

Over 10 years and out of 2734 patients who underwent RFA, we retrospectively analysed 1435 patients with a diagnosis of AVNRT for the presence of baseline PR prolongation in the absence of any drug therapy or prior ablation. Patients having a baseline PR interval of more than 200 ms were included in the study.

Clinical characteristics, electrophysiology characteristics, the immediate and long-term outcomes were noted and analysed. The pre and post-ablation values were compared to an age and sex-matched population with a normal PR interval at baseline and who underwent successful electrophysiology study and radio frequency ablation for AVNRT.

All statistical studies were carried out using Apple Inc., Strobe Inc. – (Sprout Core). Quantitative variables (Wilcoxon signed-rank test) were expressed as the mean ± standard deviation and qualitative variables were expressed as a percentage (%). A comparison of parametric values between two groups was performed using the independent sample t-test. A nominal significance was taken as a two-tailed p-value <0.05.

3. Results

Out of 1435 patients, who had undergone AVNRT ablation, 16 (1.1 %) patients, who met the criteria for having baseline PR prolongation, were included in the study. The average age of these patients was 62.9 + 15.9 years. In contrast, the mean age in patients with AVNRT and normal PR interval was 46.3 + 11.3 years. Out of 16 patients, 10 (62.5 %) were males. The predominant clinical presentation was paroxysmal palpitations. One patient had incessant tachycardia. The average PR interval on baseline ECG was 264.3 + 24.1 ms. The AVBCL(AV node block cycle length) at baseline was 354.4 + 52.8 ms (Table 1).

Table 1.

Shows the baseline features like clinical, electrocardiogram and electrophysiological characteristics of patients enrolled in this study.

1	Total number of patients of AVNRT	1435
2	Patients with PR prolongation at baseline	16(1.1 %)
3	Male	10
4	Female	6
5	Age	62.9 + 15.9 years
6	Presentation paroxysmal palpitations	15
7	Presentation incessant tachycardia	1
8	PR interval at baseline(milliseconds)	264.3 + 24.1
9	AVBCL at baseline(milliseconds)	354.4 + 52.8
10	Type of AVNRT slow-fast	16
11	Site of ablation below superior lip of CS os	15
12	Site of ablation above superior lip of CS os	1
13	Junctional rhythm during ablation	16
14	AV block during ablation	0
15	PR interval post ablation(milliseconds)	229 + 33.5
16	AVBCL post ablation(milliseconds)	413.1 + 75.8
17	Dual AV nodal physiology post ablation/Echo beat	0
18	Number of patients in whom PR interval shortened (>20 ms) post ablation	12
19	Average decrease in PR interval (milliseconds)	36 + 35(-74 to 6)
20	Average increase in AVBCL post ablation (milliseconds)	58.8 (20–180)
21	Recurrence of tachycardia	0
22	AV block on follow up	1

Tachycardia induced in all these patients was a slow-fast AVNRT. Radiofrequency ablation was done with standard protocols and precautions. Junctional rhythm was noted in all patients during ablation. No AV block was noted in any of these patients during the procedure or immediate post-procedure. The successful anatomical site of ablation was at the lower Kochs triangle in all, except in one patient where the ablation was done superior to the coronary sinus ostium.

PR interval decreased from 264.3 + 24.1 ms to 229 + 33.5 ms post-ablation (Table 2, Table 3 and Fig. 1). By applying the Wilcoxon signed-rank rest, these changes in the value of pre-and post-ablation PR intervals were found to be significant (p = 0.001). There were two patients who had an increase in PR interval post-ablation (6 ms and 2 ms). Rest all patients had a decrease in PR interval (−4 ms to −74 ms). The decrease in PR interval from the pre-ablation value was more than 20 ms in 12 (75 %) patients. In 50 % of patients, the PR intervals remained more than 200 msec post-ablation.

Table 2.

Pre- and Post-ablation values of PR interval and AVBCL.Post ablation there was an average decrease in PR interval of 36 ms. Post ablation AVBCL point showed an average increase by 58 ms.

Patient	Pre ablation	Post ablation	Difference (pre-post)	Pre ablation AVBCL	Post ablation AVBCL	Difference (post-pre)
	PR interval in msec	PR interval in msec		in msec	in msec
1	272	268	4	380	450	70
2	240	210	30	280	300	20
3	240	216	24	350	400	50
4	278	204	74	380	420	40
5	280	201	79	360	410	50
6	216	192	24	320	500	180
7	308	289	19	350	450	100
8	264	270	−6	400	450	50
9	271	250	21	500	600	100
10	269	212	57	280	300	20
11	228	230	−2	300	320	20
12	262	200	62	320	380	60
13	260	198	62	360	400	40
14	240	220	20	380	450	70
15	270	210	60	360	380	20
16	330	290	40	350	400	50

Table 3.

Pre and post PR interval and AVBCL values in control group and study group.

	Pre ablation values control group	Pre ablation values study group	P value	Pre ablation values study group	Post ablation values in study group	P Value
PR interval (msec)	139.4 + 14.3	139.4 + 14.4	0.9921	264.3 + 24.1	229 + 33.5	p = 0.001
AVBCL(msec)	305.1 ± 41.7	302.8 ± 48.2	0.8089	354.4 + 52.8	413.1 + 75.8	p=<0.0001

Fig. 1.

A. Changes in PR interval and B. AVBCL, pre and post-ablation values in study group in 16 patients.

All patients having baseline PR prolongation had an increase in the AVBCL(Range 20–180 ms) post-ablation. The AVBCL value was significantly (p=<0.0001) increased from baseline 354.4 + 52.8 ms to 413.1 + 75.8 ms with an overall increase of 58.8 ms (Table 2, Table 3 and Fig. 1). No dual AV nodal physiology was seen in any of the patients post-ablation suggesting slow pathway ablation.

The control group consisted of age and sex-matched populations (Table 3) who had a normal PR interval and also underwent a successful ablation of typical AVNRT. The pre and post-ablation PR interval values were 139.4 + 14.3 ms and 139.4 + 14.4 ms respectively. The pre and post-ablation AVBCL values in this control group were 305.1 ± 41.7 ms and 302.8 ± 48.2 ms respectively. The change in these values was not significant when compared to the study group population (Table 3).

All patients were followed up for up to a period of 12 months. The first follow-up was an OPD visit at one month and then a telephonic follow-up at 12 months. One patient developed an asymptomatic AV block with narrow QRS with a good escape rate, during his follow-up after two months. In this patient, the pre-ablation PR interval was 330 ms, which decreased to 290 ms post-ablation, both of which were highest in the study group. Pre and Post AVBCL were 350 ms and 400 ms. Also, this patient was the only patient having a site of ablation at the superior to the border of CS ostium. No patient had a recurrence of tachycardia during a follow-up of 1year.

4. Discussion

AVNRT with baseline PR prolongation is uncommon. Slow pathway ablation has been considered to be safe and effective. We present our study in which out of a total of 1435 patients with diagnosed AVNRT, 16 (1.1 %) patients had a baseline PR prolongation and they underwent slow pathway ablation. The baseline increased PR interval in these patients can be related to a diseased fast pathway or a dual AV node physiology with a more dominant slow pathway conduction. The autonomic tone may also influence the PR interval in these patients. The patients presenting with a pre-existing prolonged PR interval were older (62.9 + 15.9 years) in our study as compared to rest of the patients with AVNRT and normal PR interval (46.3 + 11.3 years)(p-value <0.001). The higher age of patients having prolonged PR interval may suggest degenerative AV nodal disease [13]. However, only one study by J S Sra et al. [17] had a younger age group of patients (31 + 15 years).

AVNRT is more common in females [21,22]. In our study, a higher incidence of AVNRT with prolonged PR was seen in males (62.5 %) which is similar to the incidence seen in other studies [18]. A higher incidence of baseline PR prolongation in males may be related to the higher age of presentation in males.

All patients had an increase in the AVBCL value in our study. The increase in post-ablation AVBCL was also seen in other studies [13,21]. In our study 14 out of 16 patients had a decrease in the PR interval post-ablation, while in the remaining 2 patients the increase in PR interval was minimal(Table 2). AVBCL value in the slow pathway is at shorter CL when compared to the fast pathway and hence preferential conduction occurs through the slow pathway at baseline resulting in a longer PR interval and a lower AVBCL value at baseline. After ablation/modification of the slow pathway, the AVBCL value increases and there is a decrease in PR interval given that the conduction now occurs through the fast pathway. A loss of electrotonic modulation by the slow pathway over the fast pathway may also influence the AVBCL [23].

This increase in the value of AVBCL following ablation of the slow pathway, should have also been observed in the control group population. But, there was no significant difference in the pre and post ablation PR interval and AVBCL values in them. In many of these patients, isoprenaline was used pre-ablation for induction of tachycardia and that would have influenced these values.

The persistence of prolonged PR interval with higher AVBCL point post-ablation suggests the likelihood of a pre-existing diseased fast pathway.

During the procedure, no AV block was noted in our study. This suggests that ablation of the slow pathway is safe and effective for patients with AVNRT having baseline PR prolongation, as seen in various other studies [17,24,25]. The incidence of post-procedure AV block after slow pathway ablation in patients with PR prolongation ranges from 0 to 9.3 % [5,14]as compared to 0–5% in an unselected population [3]. Fast pathway modification can result in an immediate atrioventricular block, whose incidence was around 8–20 % [2,9,16,20].

During a follow-up of 1 year in all patients, there was no patient who had a recurrence of palpitations or clinical tachycardia. Late-onset asymptomatic AV block was seen in only one patient who had the highest values of baseline PR interval of the study and in whom the ablation was done superior to the CS os. Ablation at this site carries a higher risk of development of AV block as compared to lower sites due to its close proximity of the compact AV node [26]. Progressive fibrosis at the site of ablation or a progressive fast pathway disease could have led to the development of a complete heart block.

5. Conclusion

The association of AVNRT with a prolonged PR interval on the baseline is uncommon. Slow pathway ablation by radio-frequency ablation in these patients is safe and effective. Late-onset AV block may be seen in such patients and hence should be followed up regularly.

Source of support

This work was supported by U. N. Mehta Institute of Cardiology and Research Centre itself and received no specific grant from any funding agency, commercial or not for profit sectors.

Declaration of competing interest

No Conflict of interest.