Patient Directed Recording of a Bipolar Three-Lead Electrocardiogram using a Smartwatch with ECG Function

Summary

We describe a protocol for the patient-directed registration of a three lead bipolar electrocardiogram by a smartwatch that functions identically to the Einthoven leads from standard electrocardiograms. This enables patients to record electrocardiograms on their own immediately after the onset of symptoms.

Abstract

Cardiac arrhythmias and cardiovascular diseases are a major public health problem in developed countries. A major goal in preventive medicine is the reduction of cardiovascular death by early detection of atrial fibrillation (AF), which may cause stroke, or early detection of life-threatening myocardial ischemia in acute coronary syndrome. Detection of arrhythmia is often challenging if symptoms occur when patients have no chance for immediate electrocardiogram (ECG) diagnostic testing, or if the observation time period is short or an immediate visit to their doctor is not possible. Smartwatches and other wearable devices are able to record a single lead ECG recording, but a single lead ECG is often not sufficient for diagnosis of cardiovascular disorders. Even diagnosis of AF can be difficult with only information from a single lead bipolar ECG. Some smart devices use photoplethysmography for detection of cardiac rhythm, but this technique can only give indirect hints of the underlying cardiac rhythm, is prone to interferences, and cannot be used for detection of myocardial ischemia. A three-lead bipolar ECG like the Einthoven leads used in regular ECGs can add useful information concerning arrhythmia detection or even diagnosis of other cardiovascular diseases like ischemia. Therefore, we describe a protocol for the patient-directed recording of an Einthoven three-lead ECG using a smartwatch.

Introduction

Smartwatches or other so-called "wearable devices" show increasing popularity and a steeply rising daily use in Western countries. Nearly 80% of US-Americans own a smartphone and more than 10% have a smartwatch¹. Due to a photoplethysmographic sensor using LED-light and photodiodes, some smartwatches can record pulse frequency and irregularities¹. This feature enables the detection of arrhythmias, especially AF, with high diagnostic accuracy^2,3. For authentic ECG arrhythmia detection, portable, handheld, and wearable ECG devices have been developed to enable smartphone-assisted ECG recordings. Nevertheless, these devices allow patient-activated recording of electrocardiograms only if the patients’ compliance for carrying the ECG device is extremely high^4,5,6,7.

Thus, the optimal tool for a patient’s medical surveillance would be a smart device for daily use. Some last generation smartwatches enable a single-lead ECG recording comparable to bipolar lead Einthoven I from a standard 12-channel ECG using the backside of the watch as the positive and the crown as the negative electrode⁸. ECG recording is patient-controlled and activated if symptoms occur. Thereafter, an application creates a PDF document for further analysis by a healthcare professional. Nevertheless, using only a single-lead ECG for discrimination of P waves for diagnosis of sinus rhythm is sometimes insufficient⁹ for detection of the P wave and often multiple ECG leads are required⁵. In addition, multichannel ECG recording is mandatory for diagnosis of most acute or chronic structural heart diseases like myocardial infarction (MI), pulmonary embolism, or signs of acute heart failure.

More than 100 years ago, Einthoven developed a method for recording of a bipolar three- channel ECG¹⁰. This three-channel ECG offers the opportunity to identify the electrical heart axis and possibly the myocardial ischemia as well, especially in inferior regions of the myocardium¹¹. Therefore, in clinical daily practice bipolar Einthoven leads I-III are essential parts of the 12-lead ECG and enable heart rhythm determination or detection of myocardial ischemia.

Early diagnosis and especially early treatment of myocardial infarction has improved substantially during recent decades. Nevertheless, especially early after the onset of symptoms, many patients hesitate to contact professional help. Thus, first medical contact and initiation of adequate treatment is often delayed¹². Registration and transmission of a patient-directed ECG early after the onset of symptoms might accelerate specific treatment and thus enable a better patient outcome⁷. Until now, ischemia detection by smart devices is limited, because mainly single-lead (Einthoven I), or as in our study, maximal three-lead (Einthoven I-III) ECGs can be recorded, which only represent a limited area of the myocardium.

Several studies used patient-directed devices like portable ECG recorders, smartphones, and very recently smartwatches, for detection of AF in heart patients^1,2,5,9. The Apple Heart Study and the WATCH AF trial used the photoplethysmographic LED-light sensor of the smartwatch for detection of an irregular or variable pulse, which correlates with arrhythmia like AF^1,2. Insufficient signal quality was the limiting factor in these trials, leading to a high dropout rate². Another smartwatch trial used photoplethysmography for AF detection, but also showed reduced diagnostic accuracy compared to regular ECGs¹³.

The detection of AF by the registration of pulse irregularities is the limiting factor of photoplethysmography, because heartbeat variabilities due to extra systoles or sinus arrhythmia may also cause pulse irregularities. Thus, recording of an ECG by a smartphone or smartwatch may increase the sensitivity and specificity of arrhythmia detection. Several smartphone compatible devices can record a bipolar single-lead ECG simulating Einthoven lead I^5,9. In one study, a bipolar smartphone ECG device was used for AF screening⁹. In this trial, a small voltage of P waves in lead I led to incorrect AF determination, a limitation when only a single-lead ECG is available⁹. ECG devices for AF screening were also tested in hospitalized patients on cardiologic and geriatric wards⁵. Diagnostic accuracy of the automated algorithms was only suboptimal and additional 12-lead ECGs were often mandatory. Most of these devices have the limitation of only one ECG lead recording (Einthoven I), which is not always sufficient to ensure arrhythmia or repolarization detection.

Only one small case series of five patients demonstrated that a conventional 12 lead ECG is recordable by a conventional bipolar smartphone device after modification for unipolar lead recordings with ECG tabs and wires with alligator clips⁴. They showed ECG recordings with good signal quality, but the limiting factor is the need for device modifications that complicates patient-directed self-ECG recording.

In contrast, we performed the first study for recording an ECG with a smartwatch with the three bipolar Einthoven leads as a proof of concept in healthy subjects. We were able to show a high grade of consistency between the smartwatch leads and the Einthoven leads from a standard ECG using the following simple protocol.

Protocol

This study was performed according to the Declarations of Helsinki and approved by the Ethics Committee of the Aerztekammer Westfalen-Lippe (reference number 2019-456).

1. Study

1. Instruct subjects on how to use the smartwatch for proper ECG recording.

2. Recording of a standard 12-lead ECG by a common device

1. Use a common ECG device for standard ECG recording.
2. Adjust the paper running speed to 50 mm/s.
3. Perform the ECG recording after a 5 min resting period in a supine position.
4. Place the right arm electrode near the right shoulder.
5. Place the left arm electrode near the left shoulder.
6. Place the right leg electrode near the right ankle.
7. Place the left leg electrode near the left ankle.
8. Place the V1 electrode in the fourth intercostal space at the right parasternal line.
9. Place the V2 electrode in the fourth intercostal space at the left parasternal line.
10. Place the V3 electrode between V2 and V4.
11. Place the V4 electrode in the fifth intercostal space at the midclavicular line.
12. Place the V5 electrode in the fifth intercostal space at the anterior axillary line.
13. Place the V6 electrode in the fifth intercostal space at the mid-axillary line.
14. Record a standard 12-lead ECG with the standard ECG device.
    NOTE: The patient should not move during the ECG recording in order to prevent ECG artifacts.

3. Recording of Einthoven leads I-III by a smartwatch with ECG function

1. Record smartwatch ECGs directly after recording the standard ECGs.
2. Enable the application of the smartwatch for ECG recordings. A 30 s ECG will be recorded directly after proper skin contact with the smartwatch.
3. Record Einthoven I by placing the back of the smartwatch on the left wrist and the right index finger on the crown (Figure 1A).
4. Record Einthoven II by placing the back of the smartwatch on the left lower abdomen and the right index finger on the crown (Figure 1B).
5. Record Einthoven III by placing the back of the smartwatch on the left lower abdomen and the left index finger on the crown (Figure 1C).
   NOTE: The right and left index finger should not contact the skin of the left wrist or left lower abdomen for adequate ECG recording. The patient should not move during the ECG recording in order to prevent ECG artifacts.

4. Analysis of the ECGs

1. Recorded smartwatch ECGs are digitally stored using the smartphone app.
2. Use the "send PDF to your doctor" function to create a PDF document of every single smartwatch ECG lead. Print the digital smartwatch ECG on paper for comparison with the standard ECG on printed paper.
3. Classify all recorded smartwatch ECGs as of moderate signal quality if at least three consecutive QRS-complexes show noise-free signal quality and there are no artifacts in the isoelectric lines between QRS-complexes.
4. Classify smartwatch ECGs as of good signal quality if at least ten QRS-complexes show noise-free signal quality and there are no artifacts in isoelectric lines between QRS-complexes.

5. Statistical analysis

1. Perform statistical analysis using IBM SPSS Statistics.
   NOTE: Categorical variables are shown as absolute numbers and percentages. Continuous variables are presented as mean ± standard deviation. Differences of metric outcome variables were assessed by one way repeated analysis of variance (ANOVA) and paired t-test. In case of binary variables, the χ2-test was used.

Results

In a cohort of 100 healthy subjects (66 female) we investigated the feasibility of our smartwatch recording protocol. The subjects' characteristics are shown in Table 1. After a short tutorial all volunteers managed the ECG recording procedure with the smartwatch. All 300 smartwatch ECGs were useable for further analysis with at least adequate signal quality for diagnostics purposes. Of the total ECGs, 277 (92%) were of good quality and 23 (8%) of moderate signal quality. Three blinded cardiologists were...

Discussion

Smart devices like smartphones and smartwatches are increasingly used in daily life and medical care¹. These new devices and apps may have a significant impact on health awareness of the population, but their effective use needs to be tested in studies⁸. To the best of our knowledge, our group was the first to develop this method of single-lead ECG recordings corresponding to the conventional Einthoven ECG leads I-III using a smartwatch¹⁴.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Apple Watch Series 4	Apple	Smartwatch with bipolar ECG function
IBM SPSS Statistics	IBM	version 25 for Mac
MAC 5500	GE Healthcare	Standard 12 channel ECG device