T-Peak to T-End Interval for Prediction of Positive Response to Ajmaline Challenge Test in Suspected Brugada Syndrome Patients

Background: Brugada syndrome (BrS) is diagnosed in patients with ST-segment elevation with coved-type morphology in the right precordial leads, occurring spontaneously or after provocative drugs. Due to electrocardiographic (ECG) inconsistency, provocative drugs, such as sodium-channel blockers, are useful for unmasking BrS. Ajmaline is superior to flecainide and procainamide to provoke BrS. Prolonged T-peak to T-end (TpTe) is associated with an increased risk of ventricular arrhythmia and sudden cardiac death in Brugada syndrome patients. Objective: This study aimed to investigate the predictive value of T-peak to T-end interval and corrected T-peak to T-end interval for predicting the positive response of the ajmaline challenge test in suspected Brugada syndrome patients. Methods: Patients who underwent the ajmaline test in our center were enrolled. Clinical characteristics and electrocardiographic parameters were analyzed, including TpTe, corrected TpTe, QT, corrected QT(QTc) interval, and S-wave duration, compared with the result of the ajmaline challenge test. Results: The study found that TpTe and corrected TpTe interval in suspected BrS patients were not significantly associated with a positive response to the ajmaline challenge test. Conclusions: The T-peak to T-end interval and corrected T-peak to T-end interval could not predict the positive response of the ajmaline challenge test in suspected Brugada syndrome patients.


Introduction
Brugada syndrome (BrS) is a genetic disorder that affects the cardiac ion channel, mainly the sodium channel. It increases the risk of life-threatening ventricular arrhythmia and sudden cardiac death in patients with a structurally normal heart. The prevalence of Brugada syndrome is 0.36% in Europe and the United States, 1.4% in Japan, and 1.8% in Thailand. The onset of symptoms typically occurs at a mean age of 41 ± 15 years [1,2].
Brugada syndrome is diagnosed in patients with ST-segment elevation with covedtype morphology in the right precordial leads, occurring spontaneously or after provocative drugs. Due to electrocardiographic (ECG) inconsistency, provocative drugs could unmask Brugada syndrome. Ajmaline was superior to flecainide and procainamide to provoke BrS [3]. Ajmaline is an intravenous class I antiarrhythmic drug with a potent sodium channel blocking property. The ajmaline challenge test's sensitivity and specificity are 80% and 94.4%, respectively [4,5]. In Thailand, the limited use of the ajmaline challenge test has restricted the diagnosis and the assessment of the true prevalence of the disease.
The cellular electrophysiological mechanism of the disease remains to be clarified. Prolonged T-peak to T-end (TpTe) interval is associated with an increased risk of ventricular arrhythmia and sudden cardiac death in Brugada syndrome patients [6]. We hypothesize that T-peak to T-end interval might predict the positivity of the sodium channel challenge test in suspected Brugada syndrome patients.
This study aims to identify whether T-peak to T-end interval and corrected T-peak to T-end interval could predict a positive response in the ajmaline challenge test in suspected Brugada syndrome patients. Furthermore, the electrocardiographic changes following the ajmaline challenge test in diagnosed Brugada syndrome patients, which might contribute to the knowledge of the ambiguous mechanism of Brugada syndrome, were also evaluated.

Study Population and Trial Design
This was a single-center analytical study. The study enrolled all patients aged at least 18 years with a familial screening of Brugada syndrome, unexplained syncope, or resuscitated sudden cardiac arrest (SCA) with suspicious Brugada pattern form. Patients underwent an indicative ajmaline challenge test in King Chulalongkorn Memorial Hospital, Thailand, between August 2018 and December 2021. The study excluded pregnancy, breastfeeding, history of myocardial infarction, allergy to ajmaline, liver disease or SGOT, SGPT above two times the upper limit of normal, concurrent use of tricyclic antidepressant, fluoxetine, lithium, antihistamine, cocaine, propofol, trifluoperazine, measured body temperature above 37.8 or below 36 Celsius, and hypokalemia. Written informed consent was acquired from all patients.

Ethics
The study protocol was approved by the Institutional Review Board of the Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand no. 503/64. No commercial organization was involved in the trial.

Ajmaline Challenge Test Protocol
Twelve-lead electrocardiography using a Philips PageWriter TC70 cardiograph was performed at baseline with leads V1, V2, and V3 placed at the fourth intercostal space (standard position) and the second and third intercostal spaces (Brugada lead position).
During continuous 12-lead ECG monitoring using a Philips PageWriter TC70 cardiograph, all patients were administered intravenous 1 mg/kg ajmaline in 10 min according to the protocol of the Brugada consensus conference. Defibrillator and isoproterenol were prompted for the treatment of ventricular arrhythmia. Intravenous ajmaline infusion was discontinued when Brugada type 1 morphology appeared in at least one right precordial lead, the occurrence of premature ventricular contractions, ventricular arrhythmias, prolongation of QRS duration more than 130 percent of baseline, or presence of higher degree atrioventricular block. Twelve-lead ECGs with leads V1, V2, and V3 in the standard and Brugada lead positions were performed at 1, 5, 10, and 15 min after the ajmaline challenge. After discontinuation of the ajmaline infusion, monitoring was continued for at least 60 min or until normalization of the ST segment. All the patients were observed for 4-6 h after the test.
A positive result included the conversion of type 2, 3 ECG of Brugada pattern to type 1 morphology (coved type) and a J-wave amplitude of more than 2 mm in leads V1, V2, and V3. Clinical characteristics and electrocardiographic parameters were analyzed.

Electrocardiographic Analysis
The T-peak to T-end interval, corrected T-peak to T-end interval (corrected using Bazett's formula), and S-wave duration were measured in leads II, III, aVF, V2, V5, and V6. The QT interval and corrected QT interval (corrected using Bazett's formula) were measured. Additional parameters measured were P-wave duration, PR interval, PQ interval, QRS duration, QT peak, T-segment elevation, RR interval, JT interval, terminal activation time (TAD), and ventricular activation time (VAT) in lead II. The values of each parameter were reported as the largest measurement evaluated in each derivation.
The electrocardiography was performed at ten times zoom with an amplitude of 10 mm/mV and a speed of 25 mm/s. Two independent physicians blinded to the clinical status measured ECGs. All measurements were performed using an electronic caliper, Phillips intelliSpace ECG Program. The intra-observer variability using the mean-centered coefficient of variation was 0.3%, and the inter-observer variability using interclass correlation was 0.86.

Statistical Analysis
Baseline clinical and electrocardiographic characteristics were presented as means and standard deviations for continuous variables and counts and percentages for categorical variables. Differences between the two groups were assessed using Fisher's exact test, independent t-test, and Mann-Whitney U test. A p-value less than 0.05 was considered statistically significant. Data were analyzed using SPSS version 24.3.

Electrocardiographic Parameters According to Ajmaline Challenge Test Result
The ajmaline challenge test showed a positive response in 12 out of 16 patients (75%). We found that the TpTe and corrected TpTe intervals in suspected BrS patients were not significantly associated with a positive response in the ajmaline challenge test (

Electrocardiographic Parameters at Baseline and End of the Test
After the ajmaline challenge test, the corrected QT interval and S-wave duration (lead II, V5) were significantly increased (QTc: Baseline 415.7 ± 29. 6

Electrocardiographic Changes following the Ajmaline Challenge Test in Diagnosed Brugada Syndrome Patients
In a total of 12 Brugada syndrome patients diagnosed with a positive ajmaline challenge test, corrected QT interval and S-wave duration in lead II were significantly increased following the ajmaline challenge test (QTc: Baseline 408.8 ± 26.3 ms vs. End of test 465.1 ± 38.2 ms, mean difference 56.2 ± 39.8 ms, p = 0.006, S: II Baseline 42.0 ± 14.1 ms vs. End of test 64.9 ± 34.0 ms, mean difference 22.9 ± 30.5 ms, p = 0.025) as shown in Table 4 and Figure 2.

Complications
Among the 16 performed ajmaline challenge tests, no serious adverse event was observed in the trial.

Discussion
Our study is the first analytical study to demonstrate the practice of the ajmaline challenge test in suspected Brugada syndrome patients in Thailand. A prolonged T-peak to T-end interval was related to an increased risk of ventricular arrhythmia and sudden cardiac death, including appropriate ICD therapy [6][7][8]. We hypothesized that T-peak to T-end interval could predict a positive response in the ajmaline challenge test in patients with suspected Brugada syndrome. However, the main finding in our trial is that T-peak to T-end interval, corrected T-peak to T-end interval (including QT interval and corrected QT interval), and S-wave duration were not significantly related to positivity in the ajmaline challenge test. Consequently, the ajmaline challenge test remains the definite diagnostic test in clinical suspicion of the disease without reliable EKG markers for positive response.

Complications
Among the 16 performed ajmaline challenge tests, no serious adverse event was observed in the trial.

Discussion
Our study is the first analytical study to demonstrate the practice of the ajmaline challenge test in suspected Brugada syndrome patients in Thailand. A prolonged T-peak to T-end interval was related to an increased risk of ventricular arrhythmia and sudden cardiac death, including appropriate ICD therapy [6][7][8]. We hypothesized that T-peak to T-end interval could predict a positive response in the ajmaline challenge test in patients with suspected Brugada syndrome. However, the main finding in our trial is that T-peak to T-end interval, corrected T-peak to T-end interval (including QT interval and corrected QT interval), and S-wave duration were not significantly related to positivity in the ajmaline challenge test. Consequently, the ajmaline challenge test remains the definite diagnostic test in clinical suspicion of the disease without reliable EKG markers for positive response.
Our study demonstrated the safety of the ajmaline challenge test. There was no reported complication in this study, in which the incidence was lower than that of previous trials, reporting the development of symptomatic ventricular arrhythmias at 0.15-1.60 percent [9,10].
Resulting from the small population in this trial, this should be interpreted with caution. Therefore, we encourage the diagnostic use of the intravenous ajmaline challenge test under continuous ECG monitoring with prompted defibrillator and isoproterenol in clinically suspected patients in Thailand.
We also observed a significant change in many electrocardiographic parameters after ajmaline infusion: P-wave duration, PR interval, PQ interval, QRS duration, ventricular activation time, S-wave duration (leads II, V5), and corrected QT interval. The electrocardiographic change implies that ajmaline affects both depolarization and repolarization of the cardiac cycle. This result contradicts the concept that ajmaline affects only the sodium channels and notes the additional influence of ajmaline on potassium and calcium channels. A previous study investigated that ajmaline had multiple mechanisms of action, including an inhibitory effect on sodium channels (INa), L-type calcium channels (ICa-L), and transient outward potassium channels (Ito) in rat right-ventricular myocytes [11,12]. Ajmaline also inhibits calcium channels (ICa) and inwardly rectifying potassium channels (IK1) in guinea pig ventricular cardiomyocytes [11,12].
The pathophysiological mechanism behind Brugada syndrome is still debated. There are two main electrophysiological theories of the disease. The repolarization theory explains the transmural dispersion of repolarization between the endocardium and the epicardium of the right ventricle [13]. On the contrary, the depolarization theory focuses on structural abnormalities and delayed conduction of the right ventricle [14]. We detected an essential increase in the corrected QT interval, representing the repolarization of the cardiac cycle, and S-wave duration (lead II), representing the depolarization, following the ajmaline challenge test in diagnosed Brugada syndrome patients. The result might imply that Brugada syndrome influences both depolarization and repolarization.

Study Limitations
Even though this pilot study provided novel data about the ajmaline challenge test in Thailand, it remains a relatively small, single-center, nonrandomized analytical study. Hence, it may not be possible to extrapolate the results of our study to the broader population. Furthermore, the small size of the enrolled sample population limits statistical power to demonstrate the predictivity of T-peak to T-end interval and corrected T-peak to T-end interval. However, the absence of difference might be related to the minor differences in overall electrocardiographic parameters.

Conclusions
This study found that the T-peak to T-end interval and corrected T-peak to T-end interval could not predict positive response to the ajmaline challenge test in suspected Brugada syndrome patients. The EKG changes after ajmaline testing support the influence of both depolarization and repolarization on the Brugada syndrome patients.