Atropine-induced sinus tachycardia protects against exercise-induced ventricular arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia

Abstract

Aims

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia syndrome characterized by exercise-induced ventricular arrhythmias, sudden death, and sinus bradycardia. Elevating supraventricular rates with pacing or atropine protects against catecholaminergic ventricular arrhythmias in a CPVT mouse model. We tested the hypothesis that increasing sinus heart rate (HR) with atropine prevents exercise-induced ventricular arrhythmias in CPVT patients.

Methods and results

We performed a prospective open-label trial of atropine prior to exercise in CPVT patients (clinicaltrials.gov NCT02927223). Subjects performed a baseline standard Bruce treadmill test on their usual medical regimen. After a 2-h recovery period, subjects performed a second exercise test after parasympathetic block with atropine (0.04 mg/kg intravenous). The primary outcome measure was the total number of ventricular ectopic beats during exercise. All six subjects (5 men, 22–57 years old) completed the study with no adverse events. Atropine increased resting sinus rate from median 52 b.p.m. (range 52–64) to 98 b.p.m. (84–119), P = 0.02. Peak HRs (149 b.p.m., range 136–181 vs. 149 b.p.m., range 127–182, P = 0.46) and exercise duration (612 s, range 544–733 vs. 584 s, range 543–742, P = 0.22) were not statistically different. All subjects had ventricular ectopy during the baseline exercise test. Atropine pre-treatment significantly decreased the median number of ventricular ectopic beats from 46 (6–192) to 0 (0–29), P = 0.026; ventricular ectopy was completely eliminated in 4/6 subjects.

Conclusion

Elevating sinus rates with atropine reduces or eliminates exercise-induced ventricular ectopy in patients with CPVT. Increasing supraventricular rates may represent a novel therapeutic strategy in CPVT.

What’s new?

• Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited arrhythmia syndrome characterized by exercise-induced ventricular arrhythmias and risk for sudden death in the young despite a structurally normal heart.

• Data from animal models suggest that elevating supraventricular rates prior to exercise protects against catecholaminergic ventricular arrhythmias in CPVT.

• In a prospective open-label trial, increasing the sinus heart rate with atropine prior to exercise significantly decreased the number of ventricular ectopic beats during exercise, with complete elimination in 4/6 CPVT patients.

• Elevating sinus rates prior to exercise may represent a novel therapeutic strategy in CPVT.

Introduction

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare genetic arrhythmia syndrome characterized by polymorphic ventricular tachycardia (VT) during exercise or stress in young patients with structurally normal hearts.¹ The diagnosis is often made during exercise treadmill testing, with a typical reproducible progression from sinus rhythm to isolated premature ventricular contractions (PVC’s) to bigeminy, then bidirectional couplets and non-sustained ventricular tachycardia (NSVT). Atrial arrhythmias and sinus node dysfunction manifest as mild resting bradycardia have also been noted as part of the phenotype of CPVT.^2,3

Catecholaminergic polymorphic ventricular tachycardia is usually caused by mutations in the gene encoding the sarcoplasmic reticulum (SR) Ca-release channel (ryanodine receptor 2 gene, RYR2),⁴ or in genes encoding the RYR2 binding proteins cardiac calsequestrin (Casq2)⁵ and calmodulin.⁶ Catecholaminergic polymorphic ventricular tachycardia mutations render RYR2 channels more active, promoting spontaneous SR Ca release during diastole that can trigger premature ventricular action potentials. Since beta-adrenergic stimulation increases RYR2 open probability and enhances Ca reuptake in the SR, catecholamines will exacerbate RYR2 hyperactivity. Hence, exercise or emotional stress, which are associated with sympathetic stimulation and parasympathetic withdrawal,⁷ are common triggers for ventricular arrhythmias in CPVT. Sympathetic stimulation with epinephrine at rest can also elicit ventricular arrhythmias in CPVT.⁸ Exercise combined with pharmacologic parasympathetic block might be expected to result in more severe ventricular arrhythmias, given the unopposed sympathetic stimulation during exercise,⁹ and the higher heart rate (HR), which independently promotes myocyte Ca loading. However, there are several lines of evidence suggesting that elevated supraventricular rates can suppress the exercise-induced VT seen in CPVT. First, bursts of atrial tachycardia have been observed to transiently suppress VT in CPVT.^10,11 Second, during exercise testing, after ventricular arrhythmias appear, up to one-third of patients who continue to exercise have suppression of VT as the sinus rate increases.¹² Third, we have previously shown in a robust mouse model of CPVT that accelerating supraventricular rates with either rapid atrial pacing or atropine can suppress bidirectional VT seen with isoproterenol challenge.¹² Here, we test the hypothesis that elevating sinus rates with atropine prior to exercise can reduce exercise-induced VT in humans with CPVT using a prospective open-label trial.

Methods

Subjects with a clinical diagnosis of CPVT who were ≥6 years of age, able to perform treadmill exercise and give written informed consent were eligible for this prospective open-label trial. Subjects were required to have had at least two clinically indicated prior treadmill exercise tests completed safely before inclusion in the study. Subjects were invited to enroll if they had residual ventricular ectopy on their most recent treadmill exercise test on current medical therapy, which was continued throughout the study. Exclusion criteria included contraindications to treadmill stress testing according to our Medical Center’s clinical protocol, pregnancy, and any clinically significant ongoing medical or surgical condition that might jeopardize the subject’s safety or interfere with the conduct of the study, in the judgement of the investigators.

After an overnight fast, subjects were admitted to the outpatient Clinical Research Center (CRC) and written informed consent was obtained. A peripheral intravenous (IV) catheter was placed in the arm and electrocardiogram electrodes were placed on the chest for exercise treadmill testing. Subjects then performed symptom-limited exercise treadmill testing using a standard Bruce protocol. The total number of ventricular ectopic beats and worst ventricular arrhythmia score (defined below) during exercise were quantified. After the baseline exercise test, subjects were monitored during a 2-h rest period. Subjects then underwent complete parasympathetic block with IV atropine (0.04 mg/kg given over 1 min, maximum 3 mg).¹³ After observing a stable increase in resting sinus HR, subjects then performed a second exercise treadmill test using the same protocol. After the atropine exercise test subjects were monitored for 4 h in the CRC. Figure 1 shows a flow diagram of the study procedures. This study was approved by the Vanderbilt University Medical Center Institutional Review Board and registered at ClinicalTrials.gov (NCT02927223). The study was supported in part by grant UL1 TR002243 from the National Center for Advancing Translational Sciences, National Institutes of Health. The funding source had no role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

Figure 1.

Flow diagram of study procedures. CPVT, catecholaminergic polymorphic ventricular tachycardia.

Data analysis

Heart rate was continually monitored throughout the exercise test. The total number of ventricular ectopic beats during exercise was considered the primary outcome. In addition, ventricular ectopy was quantified and scored as previously described¹⁴ on an ordinal scale of worst ventricular arrhythmia observed in which 0 = no ventricular ectopy; 1 = isolated PVC’s; 2 = PVC’s in a bigeminal pattern; 3 = ventricular couplets (2 consecutive beats); and 4 = non-sustained VT (≥3 consecutive beats). Continuous data are reported as medians with ranges and categorical data are reported as frequencies with percentages. All comparisons were evaluated using Wilcoxon signed rank test with a two-tailed P < 0.05 indicating statistical significance. Data were analysed using SPSS for Windows version 25.0 (SPSS Inc., Chicago, IL, USA).

Results

A total of six subjects (five men) from five families were enrolled and all completed the study without any adverse events (see Table 1). All had an unambiguous clinical diagnosis of CPVT based on reproducible bidirectional/polymorphic VT on exercise treadmill test (5) or reproducible polymorphic couplets combined with a family history of CPVT and a known RYR2 founder mutation (G357S).¹⁵ Subject #2 is the maternal uncle of Subject #3. Four subjects were on nadolol which continued throughout the study. The two subjects not on nadolol were one woman who was intolerant of nadolol and had undergone left cardiac sympathetic denervation (LCSD) and one asymptomatic 45-year-old man who was diagnosed on family screening. He is genotype and phenotype positive (reproducible bidirectional VT on treadmill) but has chosen not to take nadolol as advised, as he has never had syncope despite remaining physically active. One subject with a history of cardiac arrest had an implantable cardioverter-defibrillator, and two had undergone LCSD (the woman mentioned above and a 26-year-old man with residual couplets on maximally tolerated nadolol prior to LCSD).

Table 1.

Demographic characteristics of study cohort

Patient no	Age (years)	Sex	Clinical history	Mutation	Therapy
1	22	Male	Cardiac arrest Reproducible bidirectional VT on treadmill Normal echocardiogram	None	Nadolol 120 mg daily ICD
2	45	Male	Asymptomatic Family history of CPVT Reproducible bidirectional VT on treadmill	RYR2 R15P	None
3	26	Male	Syncope Reproducible bidirectional VT on treadmill	RYR2 R15P	Nadolol 80 mg daily LCSD
4	57	Male	Family history of sudden death (son at 14 years of age) Reproducible polymorphic VT on treadmill with normal echocardiogram and coronary angiogram	None	Nadolol 80 mg daily
5	33	Male	Asymptomatic Family history of CPVT Bidirectional couplets on treadmill	RYR2 G357S	Nadolol 20 mg daily
6	44	Female	Asymptomatic Family history of CPVT Reproducible polymorphic VT on treadmill	RYR2 Q3742P	LCSD

CPVT, catecholaminergic polymorphic ventricular tachycardia; ICD, implantable cardioverter-defibrillator; LCSD, left cardiac sympathetic denervation; RYR2, ryanodine receptor 2 gene; VT, ventricular tachycardia.

The baseline exercise test was performed on current medical therapy and all subjects had ventricular ectopy with a median of 46 (range 6–192) total ventricular ectopic beats. The worst degree of ventricular arrhythmia observed was NSVT in three, bigeminy in one, and isolated PVC’s in two. After atropine, the pre-exercise resting HR increased from a median of 52 b.p.m. (range 52–64) to 98 b.p.m. (range 84–119), P = 0.028. There was no ectopy observed after administering atropine at rest. Comparing the baseline and atropine exercise tests, peak HR did not differ (149 b.p.m., range 136–181 vs. 149 b.p.m., range 127–182, P = 0.46) and exercise times were also similar (612 s, range 544–733 vs. 584 s, range 543–742 s, P = 0.22). Systolic blood pressure did not differ between the baseline and atropine exercise tests at rest (114 mmHg, range 90–130 vs. 115 mmHg, range 94–140, P = 0.684) or during exercise (140 mmHg, range 120–160 vs. 140 mmHg, range 120–180, P = 0.45). During the atropine exercise test, four subjects had no ventricular ectopy at all, one had a single PVC, and one had 29 total ventricular ectopic beats with NSVT as the worst ventricular arrhythmia score. The total number of ventricular ectopic beats decreased significantly from a median of 46 (range 6–192) on the baseline exercise test to median 0 (range 0–29) on the atropine exercise test, P = 0.026. The worst ventricular score observed during exercise trended towards a reduction (median 3, range 1–4 vs. median 0, range 0–2; P = 0.066). The results of individual level exercise tests are shown in Table 2 and Figure 2.

Table 2.

Exercise test data

Patient no	Baseline						Atropine
	Rest HR	Peak HR	Number of ectopic beats	Worst ventricular arrhythmia	HR at onset of ectopy	Exercise time (s)	Rest HR	Peak HR	Number of ectopic beats	Worst ventricular arrhythmia	HR at onset of ectopy	Exercise time (s)
1	52	136	141	NSVT	111	580	119	127	0	None	NA	580
2	52	181	192	NSVT	160	622	114	182	29	NSVT	166	633
3	52	145	7	PVC’s	134	733	95	142	1	PVC	131	732
4	64	138	6	NSVT	133	602	84	151	0	None	NA	580
5	52	154	85	Bigeminy	131	682	98	148	0	None	NA	587
6	58	153	6	PVC’s	151	544	97	149	0	None	NA	543

HR, heart rate; NA, not applicable; NSVT, non-sustained ventricular tachycardia; PVC, premature ventricular contraction.

Figure 2.

Heart rate (blue lines, left Y-axis) and ventricular ectopy (red lines, right Y-axis) are plotted against exercise time (in seconds on X-axis) throughout the baseline and atropine exercise tests in six CPVT subjects. All had ventricular ectopy during the baseline test which was eliminated (4) or reduced (2) when pre-treated with atropine to raise the sinus heart rate. Examples of ECG tracings from subject 1 (2 min into Stage 3) are shown in the bottom right. At baseline, there are couplets and non-sustained VT, after atropine only sinus rhythm. CPVT, catecholaminergic polymorphic ventricular tachycardia; ECG, electrocardiogram.

Discussion

The major finding of this study is that in humans with CPVT, elevating the sinus HR with atropine prior to exercise was associated with elimination or significant reduction in ventricular ectopic beats during exercise. This aligns with our previous work showing that increasing the supraventricular rate with atropine or atrial pacing can suppress bidirectional VT seen during isoproterenol challenge in a mouse model of CPVT.¹² This is also consistent with previous clinical observations that bursts of atrial tachycardia can temporarily suppress VT in human CPVT patients.^10,11 The observation that elevated sinus HRs can suppress VT in CPVT is somewhat counter-intuitive. Sympathetic stimulation during exercise, the classic trigger for ventricular ectopy in CPVT, is inherently accompanied by an increase in sinus HR. Parasympathetic block with atropine during exercise could be expected to result in increased ventricular arrhythmias, due to unopposed sympathetic stimulation. An increase in HR without sympathetic stimulation, achieved with atrial pacing at rest, does not result in ventricular ectopy in CPVT patients.¹⁶ Here, we show that increasing the sinus rate with parasympathetic block suppresses VT in CPVT, even in the setting of sympathetic stimulation that accompanies exercise.

Sinus bradycardia is part of the phenotype of CPVT,² and studies in mouse models have shown that intrinsic HR (during double autonomic block with propranolol and atropine) is lower in CASQ2 knockout mice compared with wild-type.¹⁷ Fibrosis in the sinoatrial node has also been demonstrated in the CASQ2 knockout mouse¹⁸ and relative sinus bradycardia may be a risk factor for ventricular arrhythmias in CPVT.¹⁹ We recently reported that CPVT patients with a blunted HR response to exercise had worse ventricular arrhythmias during exercise and more frequently presented with syncope or cardiac arrest than those with normal HR response to exercise.²⁰ Although beta-blockers have an unquestionable protective role in CPVT, it has been noted that the onset of ventricular arrhythmias during exercise occurs at a lower HR on beta-blockers compared with untreated patients.²¹ We also observed that the HR at onset of ventricular ectopy was highest in the two subjects not on beta-blockers during the baseline exercise test. The incomplete protection from beta-blockers in CPVT is well documented, with approximately one-third of treated patients experiencing subsequent events.²² It is possible that the HR lowering side-effect of beta-blockers is pro-arrhythmic, despite the antiarrhythmic anti-adrenergic effect in CPVT.

Four of the six subjects in our study were on chronic beta-blocker therapy and a fifth not on beta-blocker had undergone LCSD, so most would be expected to have a somewhat blunted sympathetic response to exercise. We did not discontinue chronic beta-blocker in these clinically affected CPVT patients for the purposes of this study. One subject (#2) was studied in the drug-free and LCSD-free state. This individual is a healthy triathlete who was diagnosed with CPVT based on family screening. He has had multiple exercise treadmill tests showing reproducible bidirectional VT and carries the familial RYR2 mutation but has chosen to remain off nadolol against medical advice given his lack of lifetime syncope. This is the one subject who had residual NSVT after atropine. The only other subject (#3) with any residual ectopy (a single PVC) after atropine is related to Subject #2 and they share the same RYR2 mutation. It is possible that the response to atropine may be mutation-specific, but larger studies are required to make this conclusion. Nevertheless, both subjects with residual ectopy had a significant reduction in the number of ectopic beats (192–29 and 7–1, respectively). It appears that elevating sinus rates with atropine is protective in CPVT in the presence or absence of beta-blocker therapy, as well as in patients with LCSD.

What is the underlying mechanism responsible for the protective effect of elevating sinus rates with atropine in CPVT patients? Based on our current understanding of CPVT pathophysiology, ventricular ectopy is caused by spontaneous Ca release from intracellular SR Ca stores in the ventricular myocardium or specialized conduction system.²³ Unlike physiological SR Ca release during a sinus beat, spontaneous Ca release occurs during diastole when SR Ca load reaches a critical threshold. Catecholaminergic polymorphic ventricular tachycardia mutations lower this spontaneous SR Ca release threshold, especially during conditions of high SR Ca load (e.g., catecholamine stimulation). Since SR Ca refilling occurs only during diastole, longer diastolic intervals will promote higher SR Ca loading and hence spontaneous SR Ca release that triggers ventricular ectopy. Thus, one possible explanation for the protective effect by atropine is that accelerating sinus rates will shorten the diastolic interval sufficiently to prevent spontaneous SR Ca release prior to the next sinus beat, which will empty the SR prior to reaching the spontaneous release threshold. The concept that shortening diastolic intervals prevents spontaneous Ca release has been confirmed experimentally in cardiomyocytes isolated from CPVT mouse model.¹² Taken together, the sinus node dysfunction in CPVT patients likely represents both a risk factor and potential therapeutic target.¹⁹ Chronic therapy with an oral chronotropic drug such as theophylline, which increases HR in sick sinus syndrome,²⁴ is intriguing but potentially dangerous as both atrial and ventricular arrhythmias have been associated with therapeutic serum concentrations.²⁵ Agents with alpha-blocking activity have been shown to reduce exercise-induced arrhythmias in a mouse model of CPVT and could be considered in patients.²⁶ Any potential protective effect from direct alpha-block might be enhanced by the reflex tachycardia.

Study limitations

The number of CPVT patients studied here is relatively small, and conclusions should be drawn with appropriate caution. It should be noted that atropine may have HR independent effects that could contribute to suppression of VT. For example, studies of ventricular cardiomyocytes isolated from a mouse CPVT model have suggested that muscarinic receptor stimulation directly promotes spontaneous SR Ca release.¹⁷ Hence, block of muscarinic receptors in ventricular cardiomyocytes could also contribute to the protective effect of atropine in CPVT. Atropine also directly inhibits phosphodiesterase type 4 (PDE4), increasing contractility and HR independent of muscarinic antagonism, although these effects may be counteracted by the use of beta-blockers in most of our subjects.²⁷ We do not propose atropine (plasma half-life of 2–4 h) as a therapeutic agent in CPVT, rather are using it here to test the hypothesis that elevated sinus rates will suppress VT observed during exercise. Atrial pacing during exercise would be another complementary way to test this hypothesis, but none of our CPVT patients had atrial leads, so we have not yet tested this hypothesis.

Conclusions

Elevating sinus HRs with atropine prior to exercise eliminates or severely reduces ventricular ectopy observed during exercise in CPVT patients. Taken together with data from animal studies and other clinical observations, elevated supraventricular rates (sinus tachycardia, atrial tachycardia) appear to be protective in CPVT. Atrial pacing during exercise may be a potential therapeutic strategy in CPVT but would need testing in human subjects.

Funding

This work was supported in part by funds from the U.S. National Institutes of Health, National Center for Advancing Translational Sciences [UL1 TR002243], National Heart Lung and Blood Institute [R35HL144980 to B.C.K], and the Leducq Foundation [18CVD05]. M.B.S. was supported by K23 HL127704.

Conflict of interest: none declared.