Cardiac Rhythm Management
Articles Articles 2014 November

Chronic Ambulatory Monitoring: Results of a Large Single-Center Experience


Keck School of Medicine of USC, Los Angeles, CA

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ABSTRACT.  Introduction: Non-invasive, leadless monitoring patches capable of continuous long-term electrocardiogram (ECG) recording are now available for rhythm surveillance. This technology has the potential to greatly impact our understanding of arrhythmias occurring in the ambulatory setting. Methods: We reviewed data obtained from 524 consecutive patients referred to an academic electrophysiology practice and prescribed a leadless monitoring device. Patients were instructed to wear the device for up to 14 days and to activate a trigger button on the device when they experienced symptoms. Results: Overall, 99% of patients had some recorded arrhythmia. The most common was a ventricular premature beat (93%). The most common significant arrhythmia was supraventricular tachycardia in 231 patients (44%), followed by atrial fibrillation/flutter (AF) in 105 patients (20%), and non-sustained ventricular tachycardia in 79 patients (15%). Over one-third of initial arrhythmias were recorded after 48 h. The most common rhythm associated with patient triggered symptoms was normal sinus (50%). The most accurately reported symptomatic arrhythmia was AF; however, the majority of AF episodes (62%) were asymptomatic. Conclusion: Long-term ECG monitoring detected arrhythmias in all subjects, and a large percentage were detected after 48 h. Patient-reported symptoms do not correlate with arrhythmias, including AF, in half of all symptom recordings.

The authors report no conflicts of interest for the published content.
Manuscript received September 4, 2014, final version accepted September 25, 2014.

Address correspondence to: Leslie A. Saxon, MD, Division of Cardiovascular Medicine, Keck School of Medicine of USC, 1510 San Pablo Street, Suite 322, Los Angeles, CA 90033. E-mail:


Ambulatory electrocardiography (ECG) is the most widely accepted diagnostic method for known or suspected arrhythmia surveillance. Currently available wearable devices for non-invasive remote cardiac monitoring include the 24-h Holter monitor, 2- to 4-week patient-triggered event recorders, and mobile cardiac outpatient telemetry (MCOT). 2 Arrhythmia detection has proven to be challenging in the outpatient setting as most available methods of ambulatory ECG monitoring devices either rely on symptom-based recordings, are limited to a 24- to 48-h recording period, or require the patient to be within close proximity to an event transmitter. Further, patient compliance with these devices is limited by their bulky size and the presence of leads that must be removed before bathing. For these reasons, continuous monitoring is often not accomplished or monitoring time is curtailed. In this study, we examine an outpatient ambulatory cardiac rhythm-monitoring device, the Zio Patch (iRhythm Technologies Inc, San Francisco, CA) (Figure 1a). This device is capable of continuously recording a single-lead ECG for up to 14 days. We evaluated the indications for and diagnostic yield of this device in patients from a single center, an academic ambulatory electrophysiology practice.


Figure 1: 1a. Zio Patch (iRhythm Technologies Inc, San Francisco, CA) 1b. The device adheres to the pectoral region and uses a single vector to obtain continuous single-lead ECG data 1c. After monitoring is complete, the patient mails the device to a processing center where the data are analyzed using the manufacturer's algorithm and undergoes technical review, physician over-read, and report generation.


Figure 2: While most patients had their first recorded event during the first 48 h o fmonitoring, a large percentage (34%) continued to accumulate unique arrhythmias out to 12 days.


Figure 3: The total arrhythmia burden remained consistent throughout the monitoring period.


The Zio Patch is a single-use, leadless, non-invasive, water-resistant, continuously recording ambulatory cardiac rhythm monitor that is approved by the U.S. Food & Drug Administration (FDA) for up to 14 days of wear. The device adheres to the pectoral region and uses a single vector to obtain continuous single-lead ECG data (Figure 1b). The patch is equipped with an event button that patients may trigger when experiencing symptoms, highlighting the ECG recordings 45 s before and after activation. Patients are instructed to wear the device for up to 2 weeks and to press the event button during symptoms of an arrhythmia. After monitoring is complete, the patient mails the device to a processing center where the data are analyzed using the manufacturer's algorithm and undergoes technical review, physician over-read, and report generation (Figure 1c). An Internet-accessible report provides ECG and data analytics including rate variability, arrhythmia type, and frequency.

We reviewed the data from 524 consecutive patients referred to a five-physician, academic electrophysiology practice between May 28, 2010, and January 11, 2013, and prescribed the Zio Patch monitor. The ordering physician provided the indication for monitoring. All reports were additionally over-read to validate findings.

Arrhythmias were classified into two groups. Atrial and ventricular premature beats (APC, PVC), supraventricular tachycardia (SVT) <3 beats and non-sustained ventricular tachycardia (NSVT) <4 beats were classified as brief ectopy. Significant arrhythmias were defined as atrial fibrillation or atrial flutter (AF, these arrhythmias were grouped together)>4 beats, SVT >3 beats, NSVT >4 beats and < 30 s, sinus bradycardia <40 beats per min (bpm), or an atrial or ventricular pause longer than 3 s. Paroxysmal atrial fibrillation was defined as AF that did not account for 100% of recorded rhythm.

Statistical analysis was performed using the SAS version 9.2 (SAS Institute Inc, Cary, NC). Continuous variables are expressed as mean±SD, and categorical variables are expressed as percentages. Student’s t tests were used to compare continuous variables, and χ2 or Fisher’s exact tests were used to compare categorical variables. Statistical significance was defined as p<0.05.


Patient characteristics are listed in Table 1 . The most common indication for monitoring was surveillance for any unspecified arrhythmia or palpitations (47%), followed by known or suspected AF (30%), syncope (8%), bradycardia surveillance (4%), tachyarrhythmia surveillance (5%), and chest pain (2%). Unspecified conduction delays, cerebrovascular events, and cardiomyopathies comprised the remaining indications for monitoring (4%). Antiarrhythmic drug use consisting of a β-blocker, amiodarone, and or calcium channel blocker was noted for 46% of patients.

Table 1: Baseline characteristics


Patients wore the patch for an average of 7 days (0.33–14, SD 2.6 days). An arrhythmia was detected in 99.5% of patients ( Table 2 ). Over the monitoring interval, the average minimum and maximum sinus heart rates were 47 bpm (14–74, SD±10) and 159 bpm (63–293, SD±32), respectively. The mean amount of PVCs recorded was 11,756 (SD 40,785, range 1–406,696). Premature atrial contraction (PAC) frequency averaged 5,750 beats during wear (SD 22,225, range 1–336,668).

Table 2: Patients wearing the patch for an average of 7 days had an arrhytmia detected in 99.5%


A total of 297 (57%) patients had significant arrhythmias. Many patients had multiple recorded events (mean, 2.7 episodes). Although most patients had their first recorded event during the first 48 h of monitoring, a large percentage (34%) continued to accumulate unique arrhythmias out to 12 days (Figure 2). The total arrhythmia burden remained consistent throughout the monitoring period (Figure 3).

Tachycardia events were prevalent, with the most common tachyarrhythmia being SVT (44%). Patients with SVT often had multiple events. Of note, if redefined to >30 s, there were significantly fewer patients with SVT episodes (24 vs. 231). Non-sustained VT episodes were fewer and shorter in duration ( Table 3 ). AF was identified in 105 (20%) patients and of these, 49 (47%) had paroxysmal AF ( Table 4 ). Among those with paroxysmal AF, the mean percentage of time in AF was 23%. Almost half (47%) of patients with paroxysmal AF had their first recorded episode after 24 h of wear. The mean monitoring period required to detect AF was 67 h (2.8 days).

Table 3: Non-sustained VT episodes were fewer and shorter in duration


Table 4: Atrial Fibrillation/flutter


Patient-triggered events were recorded in 310 patients (59%) with most patients (79%) triggering the event button more than once (range 1–298). Patient activation was most likely to be related to brief ectopy or normal sinus rhythm (NSR) ( Table 5 ). The most common rhythm correlating with patient activation was NSR in 94 patients (30%) followed by premature ventricular beats in 34 (11%).

Table 5: Associated rhythms during patient triggers


Patients with significant arrhythmias recorded on the monitoring device were more likely to be older (65 vs. 45, p<0.0001) and have a lower although normal EF (58% vs. 60%, p = 0.02). Females were more likely to have brief ectopy only ( Table 6 ).

Table 6: Significant arrhythmia versus brief ectopy


The majority of patients with significant arrhythmias did not report symptoms by triggering the event button ( Table 7 ). The most accurately reported symptomatic arrhythmia was paroxysmal AF, with 49% of patients activating the trigger button during ECG evidence of this rhythm. Patients with permanent AF less frequently documented symptoms (28%).

Table 7: Symptom documentation via trigger during significant arrhythmias


Patients pressing the trigger button during significant arrhythmias were older (61 vs. 48) compared to those triggering for brief ectopy or NSR. However, there was no difference in sex or EF between these groups ( Table 8 ).

Table 8: Comparison between patients using trigger during significant arrhythmia versus brief ectopy/NSR


Bradycardia with a ventricular rate less than 40 bpm was identified in 62 (12%) patients. None of these patients reported symptoms during bradycardia episodes. Fourteen patients had pauses longer than 3 s; the mean pause duration was 3.46 s (SD 0.41, 3.0–4.1 seconds). Only 2 (14%) patients documented symptoms within 45 s of the pause.

Of the 42 patients whose primary indication for wearing the Zio Patch was syncope, 22 (52%) had a recorded SVT, 6 (14%) had NSVT, 4 (18%) had bradycardia with a ventricular rate less than 40 bpm, and 1 (2%) had a pause >3 s. None of these patients recorded a syncopal event in their diary during these arrhythmias.


For over four decades, the Holter monitor has been the most widely used technology for continuous ECG recording. 1,3 Although its utility is well established in the literature, a significant limitation is that these devices can only store 48 h of recorded cardiac rhythms. In addition, the inability of these devices to tolerate water exposure prevents truly continuous cardiac rhythm recording. In a novel study published in the Annals of Internal Medicine in 1990, Bass et al had patients wear three subsequent Holter monitors for a total of 72 h to better evaluate cardiogenic syncope. 4 Of the 95 patients studied, 26 had ECG abnormalities, and 12 were not detected within the first 24 h of ambulatory monitoring, supporting the observation that longer monitoring intervals are needed to detect culprit arrhythmias.

Although FDA approved for 14 days of wear, there is insufficient knowledge regarding the ideal length of time needed to detect arrhythmias in patients in whom one is suspected or in symptomatic patients. The largest study to date was recently published by Turakhia et al 5 and included data obtained from 26,751 patients in a large hospital system who wore Zio Patches. This large cross-sectional study detected an arrhythmia in 61% of patients, with only 70% of these rhythms occurring by 48 h. Interestingly, over 90% of initial recorded arrhythmias were captured by day 5. Rosenberg et al instructed patients with paroxysmal AF to wear both a Zio Patch and Holter monitor for the first 24 h and to continue wearing the Zio Patch for up to 14 days. 2 During the initial 24 h, the investigators found excellent reproducibility between the Zio Patch and the Holter monitor. However, in the subsequent monitoring period, the Zio Patch identified an additional 18 patients with paroxysmal AF and clinically significant arrhythmias that prompted a change in management in another 28% of subjects. In our study, significant arrhythmias accumulated over 14 days and 34% of patients had their first clinically significant arrhythmia recorded after 48 h of wear. Both aforementioned studies and ours support the improved detection rate of longer term arrhythmia detection using the leadless 14-day patch.

Event and loop recorders are long-term monitors that record during predefined significant arrhythmia events and when patients trigger an event button. However, they do not perform continuous recording. Event recorders are reported to be more effective than Holter monitors in patients with intermittent palpitations, syncope, and presyncope. 3 However, numerous studies have documented the low negative predictive value of monitoring arrhythmias based on symptoms alone, with some studies suggesting symptomatic correlation with ECG evidence of arrhythmias in <5% of patients. 2,4,7 This is especially true in the identification of paroxysmal AF. The low diagnostic yield of a symptoms-based approach for identifying paroxysmal AF has been demonstrated in numerous studies. 2,3,6,7,8 Ziegler et al demonstrated a significant increase in the sensitivity of identifying AF when using continuous recording devices compared to intermittent or symptom-based monitoring. 7 Our data support these findings, with a majority of patients failing to report symptoms associated with ECG evidence of paroxysmal AF. In addition, a large number of patients triggered symptoms of an arrhythmia during NSR. These findings support the hypothesis that patient activation of ECG recording is often largely inaccurate and thus emphasizes the utility of continuous monitoring for arrhythmia detection.

Unlike an event recorder, the Zio Patch is limited by its inability to transmit ECG information to a health-care provider in real time over a wireless connection. The time delay between patch receipt and report availability is significant. Newer data collected in patients with cardiac rhythm management devices indicate that remote transmission of daily rhythm events can significantly benefit patients compared to patients with devices that do not transmit daily diagnostic information. 12,13 These data suggest that the immediate notification of arrhythmia events is important, as is the enablement of patient notification to the care provider for significant symptoms.

The ability of a long-term, continuous recording device to detect and quantify asymptomatic arrhythmias is important. Previous studies have shown that a high burden of both symptomatic and asymptomatic PVCs can adversely affect ventricular function. 14,15 Our finding that half of all AF episodes were asymptomatic has been reported after ablation for known AF, and it is well established that any AF burden imparts risk of subsequent AF and thromboembolic risk. 16 Our findings in this highly selected population referred for arrhythmia management suggest that further study is needed to determine if routine screening of higher risk populations for AF should be carried out. One recent study showed that AF screening, using a smart phone-enabled wireless 30-s ECG recording could identify patients at risk and was cost effective. 17

In summary, our study demonstrates a high rate of patient compliance with long-term, continuous ECG monitors and a significant improvement in arrhythmia detection with >48 h of wear. In addition, there was a high incidence of arrhythmia burden and a poor correlation between symptoms and significant arrhythmia events. The ability to detect and quantify asymptomatic arrhythmias is an advantage of long-term, continuous monitoring devices. The role of this technology is currently being evaluated and has great potential to broaden our understanding of ambulatory arrhythmias and their subsequent impact on patients.


This was a highly selected population referred for arrhythmia consultation and management, and the high yield of arrhythmia detection may not be applicable to a broader population. The lack of follow-up data relating monitoring findings to clinical events limits the conclusions that can be drawn from the high arrhythmia detection rate.


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