HV Interval:
The Unsung Hero in Myotonic Dystrophy Cardiac Risk
Patients with myotonic dystrophy type 1 (DM1) are a cardiac challenge, a ticking time bomb of conduction disease lurking beneath the surface.
The classic question for cardiologists has always been: who needs a pacemaker before the first syncope or sudden cardiac arrest?
For decades, we’ve leaned on the humble ECG, measuring PR intervals and QRS durations, hoping these simple electrical snapshots could guide prophylactic pacing.
But new data suggest it might be time to give the electrophysiological study (EPS)—and specifically the His-ventricular (HV) interval—its moment in the spotlight.
ECG vs EPS: David vs Goliath
Professional guidelines have long recommended pacing for asymptomatic DM1 patients if:
PR ≥ 240 ms, or
QRS ≥ 120 ms on ECG, or
HV interval ≥ 70 ms on EPS (class IIa indications).
The problem? Not every conduction delay visible on ECG predicts major bradyarrhythmic events (MBAEs), including sudden cardiac death, complete heart block, or resuscitated cardiac arrest. Some patients look fine on the surface—until they don’t.
In a multicenter French cohort study of 706 DM1 patients drawn from the DM1 Heart Registry, investigators compared ECG to EPS, head-to-head.
The results were eye-opening: ECG alone missed a large portion of high-risk patients, while HV interval provided a far more sensitive and accurate readout.
The Numbers That Matter
Among 706 adults with genetically confirmed DM1 (mean age 42, 51% male), 273 patients (38%) had HV ≥ 70 ms, while 232 (32%) met ECG criteria. Over a median follow-up of nearly six years, 99 patients (14%) experienced an MBAE.
Here’s where it gets interesting:
The HV interval was the only variable that consistently predicted MBAEs in multivariable models.
Compared with ECG, the HV criterion was not only more reliable (HR 2.89 vs 1.95) but also twice as sensitive (68% vs 35%).
It accurately reclassified 28.8% of patients who went on to have an MBAE.
Even better, lowering the HV threshold to 65 ms improved sensitivity to 90%, catching nearly all at-risk patients and improving net reclassification by 33.7%.
In short: HV isn’t just a number—it’s a crystal ball for anticipating the first big bradyarrhythmic event in DM1.
Why EPS Wins
The beauty of the HV interval lies in its direct measurement of His-Purkinje conduction—the critical highway between atria and ventricles. ECG gives you the façade: PR prolongation or QRS widening. EPS tells you whether the underlying conduction machinery is on the verge of collapse.
Think of it like this: ECG is a weather forecast, giving you the general sense of a storm brewing. EPS is the doppler radar, pinpointing exactly where the lightning strikes are most likely to hit.
Clinical Implications: Time to Rethink Risk Stratification
These findings suggest several practical takeaways for cardiologists managing DM1 patients:
Don’t rely on ECG alone. While PR and QRS are convenient and noninvasive, they miss a substantial portion of high-risk patients.
Consider EPS early. Measuring the HV interval can more accurately identify who will benefit from prophylactic pacing before a catastrophic event.
Adjust the threshold. Lowering the HV cut-off from 70 to 65 ms may capture additional high-risk patients without over-treating those at low risk.
Personalized pacing decisions. With better risk stratification, pacemakers can be deployed thoughtfully, reducing sudden death while avoiding unnecessary device implantation.
A New Standard for DM1?
Given these data, it may be time for professional societies to reconsider current guidelines, particularly the weight given to ECG versus EPS in DM1. While ECG remains indispensable for routine screening, HV interval measurement should perhaps move from “optional” to “essential” in any patient at risk for bradyarrhythmic events.
Moreover, this study highlights the broader principle that direct physiological measurement often trumps surface markers—especially in diseases like DM1, where conduction system disease can be silent until it’s too late.
The Bottom Line
For cardiologists, the message is clear:
When it comes to predicting major bradyarrhythmic events in DM1, the HV interval is the real MVP.
ECG gives you clues; HV interval gives you the map. And when lives hang in the balance, precision matters.
If you manage DM1 patients, this study argues for a lower threshold HV interval of 65 ms as a proactive trigger for prophylactic pacing. Doing so could prevent sudden cardiac death in a population where every millisecond of conduction counts.
Source Study: Electrocardiogram vs Electrophysiological Study and Major Conduction Delays in Myotonic Dystrophy Type 1
Authors: Nicolas Clementy, MD, PhD; Fabien Labombarda, MD, PhD; François Grolleau, MD, PhD, Vincent Algalarrondo, MD, PhD; Guillaume Bassez, MD, PhD; Henri-Marc Bécane, MD; Anthony Béhin, MD, Françoise Chapon, MD, PhD; Mohamed El Hachmi, PhD, Abdallah Fayssoil, MD, PhD, Bertrand Fontaine, MD, PhD Rodrigue Garcia, MD, Pascal Laforêt, MD, PhD Arnaud Lazarus, MD; Marion Masingue, MD; Armelle Magot, MD; Yann Pereon, MD, PhD; Vincent Probst, MD, PhD; Leslie Motté, MD; Malika Saadi, MD; Denis Duboc, MD, PhD; Tanya Stojkovic, MD; Raphaël Porcher, PhD; Karim Wahbi, MD, PhD