March 31, 2025 — iRhythm Technologies, Inc. has announced results from two large real-world retrospective analyses presented at the American College of Cardiology (ACC) 2025 Scientific Sessions in Chicago. Drawing on data from more than 1.1 million patients who used iRhythm’s Zio long-term continuous monitoring (LTCM) ECG devices, these studies demonstrate that short-term (24–48-hour) monitoring, such as with Holter devices, fails to detect a significant proportion of actionable arrhythmias—even in patients reporting “daily symptoms” — and that Symptom–Rhythm Correlation (SRC) is notably low for most arrhythmias, underscoring that selection of monitoring duration based on the frequency of symptoms alone can lead to undetected (missed) actionable1 arrhythmias. Together, these findings highlight the benefits of Zio long-term continuous monitoring (LTCM)2 and the limitations in 24–48-hour Holter monitoring still prevalent in current clinical practices and payer policies.
Zio LTCM Daily Symptoms Study: Gaps in Short-Term Holter Monitoring
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64% Undetected in the First 48 Hours: Among daily-symptom patients—those with daily or greater symptom frequency— diagnosed with actionable arrhythmias, nearly two-thirds went undetected through two days monitoring—indicating that 24–48-hour monitoring, such as with Holter, would have failed to detect them.
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Higher Yield for Non-Daily Symptom Patients: Non-daily symptom patients—those with symptoms occurring with a frequency less than once per day—had an 80.9% arrhythmia yield versus 69.1% in daily-symptom patients, demonstrating that greater symptom frequency does not necessarily reflect increased arrhythmia burden.
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Mean Time to First Episode Exceeds 48 Hours: Across all arrhythmia types, the mean time to the first detected episode was greater than 48 hours—regardless of symptom frequency—underscoring the limitations of short-term monitoring.
Zio LTCM Symptom–Rhythm Correlation (SRC) Study: Symptoms Alone Are Unreliable
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Less Than 20% Correlation: In most arrhythmia types, fewer than one in five patients in the analysis documented a symptom coinciding with an arrhythmic episode. Symptom-rhythm correlation was higher for patients reporting daily vs. non-daily symptoms.
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AF Often Asymptomatic: Even for atrial fibrillation (AF)—the most commonly symptomatic arrhythmia—over half of all cases were asymptomatic.
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Serious Arrhythmias Frequently Not Correlated with Symptoms: Ventricular tachycardia, AV block, and significant pauses were frequently detected by Zio LTCM without patient-reported symptoms, suggesting that selection of monitor duration should not be based on symptom frequency alone, and that long-term continuous monitoring may offer advantages over other monitoring types with shorter duration or those which rely on patient triggered events to initiate recording.
“These findings challenge the long-held assumption that frequent symptoms justify short-duration monitoring,” said Mintu Turakhia, MD, iRhythm Chief Medical and Scientific Officer and EVP of Product Innovation. “They reinforce the limitations of Holter-duration monitoring and highlight the value of Zio long-term continuous monitoring up to 14 days. Once again, iRhythm’s real-world data are contributing evidence that can help guide both clinical practice and payer policy.”
Arrhythmias: A Growing Burden for Patients and Health Systems
Up to five percent of the general population experience arrhythmias,3 in which the heart may beat too quickly, too slowly, or sporadically. If left untreated, certain arrhythmias can damage the heart, brain, or other organs4 and increase the risk of stroke or death.5,6,7 Beyond these clinical concerns, the financial toll of undiagnosed arrhythmias is substantial. It’s estimated that undiagnosed atrial fibrillation alone costs the U.S. $3 billion per year,8 while heart failure costs could reach $70 billion by 2030.9 Taken together, these figures illustrate both the clinical urgency and health-economic rationale for long-term continuous monitoring.
Implications for Clinical Care and Payer Policy
While 24-48-hour Holter monitoring is widely used in current clinical practice and historically supported by payer policies—especially for patients reporting daily symptoms—these new findings indicate that 64% of daily-symptom patients with actionable arrhythmias remain undetected following the first 48 hours of monitoring, which could lead to missed diagnoses and delayed care. In contrast, Zio LTCM provides uninterrupted, continuous monitoring for up to 14 days, enabling more accurate and timely detection of actionable arrhythmias. The Cardiac Ambulatory Monitor EvaLuation of Outcomes and Time to Events (CAMELOT) study, published in the American Heart Journal, further demonstrated that Zio LTCM service had the highest yield of specified arrhythmia diagnosis and the lowest likelihood of repeat testing compared to all other monitoring services.10,11,12,13 As healthcare systems increasingly adopt value-based care models, extending monitoring beyond 48 hours can improve patient outcomes, reduce missed diagnoses, and help contain healthcare resource utilization.
New Data
These new data build on iRhythm’s comprehensive clinical evidence program, encompassing more than 125 original research manuscripts,14 insights derived from over 2 billion hours of curated heartbeat data15 and more than 10 million patient reports posted since the company’s inception—underscoring the company’s ongoing commitment to expanding evidence that supports improved patient outcomes.
1 Actionable Arrhythmias defined as Atrial Fibrillation ≥30 sec, Supraventricular Tachycardia ≥90 bpm & ≥30s, Ventricular Tachycardia ≥100 bpm & ≥4 beats, any Ventricular Fibrillation, Pause ≥3 sec, and/or Atrioventricular Block (any 2nd Degree or Complete Heart Block).
2 The Zio monitor is a prescription-only, single-use ECG monitor that continuously records data for up to 14 days. It is indicated for use on patients who may be asymptomatic or who may suffer from transient symptoms such as palpitations, shortness of breath, dizziness, lightheadedness, pre-syncope, syncope, fatigue, or anxiety.
3 Desai et al. Arrhythmias. In: StatPearls. Treasure Island (FL): StatPearls Publishing; June 5, 2023. https://pubmed.ncbi.nlm.nih.gov/32644349/
4 National Heart, Lung, and Blood Institute. Arrhythmias - What Is an Arrhythmia? www.nhlbi.nih.gov. Published March 24, 2022. Accessed April 25, 2024. https://www.nhlbi.nih.gov/health/arrhythmias
5 Ataklte et al. Meta-analysis of ventricular premature complexes and their relation to cardiac mortality in general populations. The American Journal of Cardiology. 2013;112(8):1263-1270. doi:10.1016/j.amjcard.2013.05.065
6 Lin et al. Long-term outcome of non-sustained ventricular tachycardia in structurally normal hearts. PLOS ONE. 2016;11(8). doi:10.1371/journal.pone.0160181
7 Wolf et al. Atrial fibrillation as an independent risk factor for stroke: The Framingham Study. Stroke. 1991;22(8):983-988. doi:10.1161/01.str.22.8.983
8 Turakhia et al. Economic Burden of Undiagnosed Nonvalvular Atrial Fibrillation in the United States. The American Journal of Cardiology. 2015;116(5):733-739. doi:https://doi.org/10.1016/j.amjcard.2015.05.045
9 Heidenreich et al. Forecasting the Impact of Heart Failure in the United States: A Policy Statement From the American Heart Association. Circulation: Heart Failure. 2013;6(3):606-619. doi:https://doi.org/10.1161/hhf.0b013e318291329a
10 Reynolds et al. Comparative effectiveness and healthcare utilization for ambulatory cardiac monitoring strategies in Medicare beneficiaries. Am Heart J. 2024;269:25–34. https://doi.org/10.1016/j.ahj.2023.12.002
11 A specified arrhythmia refers to an arrhythmia encounter diagnosis as per Hierarchical Condition Categories (HCC) 96.
12 Based on previous generation Zio XT device data. Zio monitor utilizes the same operating principles and ECG algorithm. Additional data on file.
13 Zio LTCM service refers to Zio XT and Zio monitor service.
14 Data on file. iRhythm Technologies, 2025.
15 Data on file. iRhythm Technologies, 2024.
May 10, 2024 
