CHAPTER 4 ECG Leads As discussed in Chapter 1, the heart produces electrical currents similar to the familiar dry cell battery. The strength or voltage of these currents and the way they are distributed throughout the body over time can be measured by a special recording instrument (sensor) such as an electrocardiograph. The body acts as a conductor of electricity. Therefore, recording electrodes placed some distance from the heart, such as on the wrists, ankles, or chest wall, are able to detect the voltages of cardiac currents conducted to these locations. The usual way of recording the electrical potentials (voltages) generated by the heart is with the 12 standard ECG leads (connections or derivations). The leads actually record and display the differences in voltages (potentials) between electrodes or electrode groups placed on the surface of the body. Taking an ECG is like recording an event, such as a baseball game, with an array of video cameras. Multiple video angles are necessary to capture the event completely. One view will not suffice. Similarly, each ECG lead (equivalent to a different video camera angle) records a different view of cardiac electrical activity. The use of multiple ECG leads is necessitated by the requirement to generate as full a picture of the three-dimensional electrical activity of the heart as possible. Fig. 4.1 shows the ECG patterns that are obtained when electrodes are placed at various points on the chest. Notice that each lead (equivalent to a different video angle) presents a different pattern. Fig. 4.2 is an ECG illustrating the 12 leads. The leads can be subdivided into two groups: the six limb (extremity) leads (shown in the left two columns) and the six chest (precordial) leads (shown in the right two columns). The six limb leads—I, II, III, aVR, aVL, and aVF— record voltage differences by means of electrodes placed on the extremities. They can be further divided Please go to expertconsult.inkling.com for additional online material for this chapter. into two subgroups based on their historical development: three standard bipolar limb leads (I, II, and III) and three augmented unipolar limb leads (aVR, aVL, and aVF). The six chest leads—V1, V2, V3, V4, V5, and V6— record voltage differences by means of electrodes placed at various positions on the chest wall. The 12 ECG leads or connections can also be viewed as 12 “channels.” However, in contrast to TV channels (which show different evens), the 12 ECG channels (leads) are all tuned to the same event (comprising the P–QRS–T cycle), with each lead viewing the event from a different angle. LIMB (EXTREMITY) LEADS Standard Limb Leads: I, II, and III The extremity leads are recorded first. In connecting a patient to a standard 12-lead electrocardiograph, electrodes are placed on the arms and legs. The right leg electrode functions solely as an electrical ground. As shown in Fig. 4.3, the arm electrodes are usually attached just above the wrist and the leg electrodes are attached above the ankles. The electrical voltages (electrical signals) generated by the working cells of the heart muscle are conducted through the torso to the extremities. Therefore, an electrode placed on the right wrist detects electrical voltages equivalent to those recorded below the right shoulder. Similarly, the voltages detected at the left wrist or anywhere else on the left arm are equivalent to those recorded below the left shoulder. Finally, voltages detected by the left leg electrode are comparable to those at the left thigh or near the groin. In clinical practice the electrodes are attached to the wrists and ankles simply for convenience. As mentioned, the limb leads consist of standard bipolar (I, II, and III) and augmented (aVR, aVL, and aVF) leads. The bipolar leads were so named historically because they record the differences in electrical voltage between two extremities.
Fig. 4.1 Chest leads give a multidimensional view of cardiac electrical activity. See Fig. 4.8 and Box 4.1 for exact electrode locations. I II III A B II aVR aVL aVF RA LA RL LL Cabl box e Fig. 4.3 Electrodes (usually disposable paste-on types) are attached to the body surface to take an ECG. The right leg (RL) electrode functions solely as a ground to prevent alternatingcurrent interference. LA, left arm; LL, left leg; RA, right arm.
Lead I, for example, records the difference in voltage between the left arm (LA) and right arm (RA) electrodes: Lead I LA RA =− Lead II records the difference between the left leg (LL) and right arm (RA) electrodes: Lead IIL =− LRA Lead III records the difference between the left leg (LL) and left arm (LA) electrodes: Lead III LL LA =− Consider what happens when the electrocardiograph records lead I. The LA electrode detects the electrical voltages of the heart that are transmitted to the left arm. The RA electrode detects the voltages transmitted to the right arm. Inside the electrocardiograph the RA voltages are subtracted from the LA voltages, and the difference appears at lead I. When lead II is recorded, a similar situation occurs between the voltages of LL and RA. When lead III is recorded, the same situation occurs between the voltages of LL and LA. Leads I, II, and III can be represented schematically in terms of a triangle, called Einthoven’s triangle after the Dutch physiologist/physicist (1860–1927) who invented the electrocardiograph. Historically, the f irst “generation” of ECGs consisted only of recordings from leads I, II, and III. Einthoven’s triangle (Fig. 4.4) shows the spatial orientation of the three
Einthoven’s Triangle RA I II III LL LA Fig. 4.4 Orientation of leads I, II, and III. Lead I records the difference in electrical potentials between the left arm and right arm. Lead II records it between the left leg and right arm. Lead III records it between the left leg and left arm. standard limb leads (I, II, and III). As you can see, lead I points horizontally. Its left pole (LA) is positive and its right pole (RA) is negative. Therefore, lead I = LA − RA. Lead II points diagonally downward. Its lower pole (LL) is positive and its upper pole (RA) is negative. Therefore, lead II = LL − RA. Lead III also points diagonally downward. Its lower pole (LL) is positive and its upper pole (LA) is negative. Therefore, lead III = LL − LA. Einthoven, of course, could have configured the leads differently. Because of the way he arranged them, the bipolar leads are related by the following simple equation: Lead I Lead III Lead II += In other words, add the voltage in lead I to that in lead III and you get the voltage in lead II.a You can test this equation by looking at Fig. 4.2. Add the voltage of the R wave in lead I (+9 mm) to the voltage of the R wave in lead III (+4 mm) and you get +13 mm, the voltage of the R wave in lead II. You can do the same with the voltages of the P waves and T waves. Einthoven’s equation is simply the result of the way the bipolar leads are recorded; that is, the LA is positive in lead I and negative in lead III and thus cancels out when the two leads are added: I = LA − RA III = LL − LA I + III = LL − RA = II Thus, in electrocardiography, one plus three equals two. In summary, leads I, II, and III are the standard (bipolar) limb leads, which historically were the first invented. These leads record the differences in electrical voltage among extremities. In Fig. 4.5, Einthoven’s triangle has been redrawn so that leads I, II, and III intersect at a common central point. This was done simply by sliding lead I downward, lead II rightward, and lead III leftward. The result is the triaxial diagram in Fig. 4.5B. This diagram, a useful way of representing the three aNote: this rule of thumb is only approximate. It can be made more precise when the three standard limb leads are recorded simultaneously, as they are with contemporary multichannel electrocardiographs. The exact rule is as follows: The voltage at the peak of the R wave (or at any point) in lead II equals the sum of the voltages in leads I and III at simultaneously occurring points (since the actual R wave peaks may not occur simultaneously).
AB Fig. 4.5 (A) Einthoven’s triangle. (B) The triangle is converted to a triaxial diagram by shifting leads I, II, and III so that they intersect at a common point.
bipolar leads, is employed in Chapter 6 to help measure the QRS axis. Augmented Limb Leads: aVR, aVL, and aVF Nine leads have been added to the original three bipolar extremity leads. In the 1930s, Dr. Frank N. Wilson and his colleagues at the University of Michigan invented the unipolar extremity leads and also introduced the six unipolar chest leads, V1 through V6. A short time later, Dr. Emanuel Goldberger invented the three augmented unipolar extremity leads: aVR, aVL, and aVF. The abbreviation a refers to augmented; V to voltage; and R, L, and F to right arm, left arm, and left foot (leg), respectively. Today 12 leads are routinely employed and consist of the six limb leads (I, II, III, aVR, aVL, and aVF) and the six precordial leads (V1 to V6). A so-called unipolar lead records the electrical voltages at one location relative to an electrode with close to zero potential rather than relative to the voltages at another single extremity, as in the case of the bipolar extremity leads.b The near-zero potential is obtained inside the electrocardiograph by joining the three extremity leads to a central terminal. Because the sum of the voltages of RA, LA, and LL equals zero, the central terminal has a zero voltage. The aVR, aVL, and aVF leads are derived in a slightly different way because the voltages aVF aVR aVL aVF aVL aVR Fig. 4.6 Triaxial lead diagram showing the relationship of the three augmented (unipolar) leads (aVR, aVL, and aVF). Notice that each lead is represented by an axis with a positive and negative pole. The term unipolar was used to mean that the leads record the voltage in one location relative to about zero potential, instead of relative to the voltage in one other extremity. bAlthough “unipolar leads” (like bipolar leads) are represented by axes with positive and negative poles, the historical term unipolar does not refer to these poles; rather it refers to the fact that unipolar leads record the voltage in one location relative to an electrodes (or set of electrodes) with close to zero potential. recorded by the electrocardiograph have been augmented 50% over the actual voltages detected at each extremity. This augmentation is also done electronically inside the electrocardiograph.c Just as Einthoven’s triangle represents the spatial orientation of the three standard limb leads, the diagram in Fig. 4.6 represents the spatial orientation of the three augmented extremity leads. Notice that each of these unipolar leads can also be represented by a line (axis) with a positive and negative pole. cAugmentation was developed to make the complexes more readable.
Because the diagram has three axes, it is also called a triaxial diagram. As would be expected, the positive pole of lead aVR, the right arm lead, points upward and to the patient’s right arm. The positive pole of lead aVL points upward and to the patient’s left arm. The positive pole of lead aVF points downward toward the patient’s left foot. Furthermore, just as leads I, II, and III are related by Einthoven’s equation, so leads aVR, aVL, and aVF are related: aVRaVL aVF ++=0 In other words, when the three augmented limb leads are recorded, their voltages should total zero. Thus, the sum of the P wave voltages is zero, the sum of the QRS voltages is zero, and the sum of the T wave voltages is zero. Using Fig. 4.2, test this equation by adding the QRS voltages in the three unipolar extremity leads (aVR, aVL, and aVF). You can scan leads aVR, aVL, and aVF rapidly when you first look at a mounted ECG from a single-channel ECG machine. If the sum of the waves in these three leads does not equal zero, the leads may have been mounted improperly. Orientation and Polarity of Leads The 12 ECG leads have two major features, which have already been described. They all have both a specific orientation and a specific polarity. Thus, the axis of lead I is oriented horizontally, and the axis of lead aVR is oriented diagonally, from the patient’s right to left. The orientation of the three standard (bipolar) leads is shown in represented Einthoven’s triangle (see Fig. 4.5), and the orientation of the three augmented (unipolar) extremity leads is diagrammed in Fig. 4.6. The second major feature of the ECG leads is their polarity, which means that these lead axes have a positive and a negative pole. The polarity and spatial orientation of the leads are discussed further in Chapters 5 and 6 when the normal ECG patterns seen in each lead are considered and the concept of electrical axis is explored. Do not be confused by the difference in meaning between ECG electrodes and ECG leads. An electrode is simply the paste-on disk or metal plate used to detect the electrical currents of the heart in any location. An ECG lead is the electrical connection that represents the differences in voltage detected by electrodes (or sets of electrodes). For example, lead
I records the differences in voltage detected by the left and right arm electrodes. Therefore, a lead is a means of recording the differences in cardiac voltages obtained by different electrodes. To avoid confusion, we should note that for electronic pacemakers, discussed in Chapter 22, the terms lead and electrode are used interchangeably. Relationship of Extremity Leads Einthoven’s triangle in Fig. 4.5 shows the relationship of the three standard limb leads (I, II, and III). Similarly, the triaxial (three-axis) diagram in Fig. 4.6 shows the relationship of the three augmented limb leads (aVR, aVL, and aVF). For convenience, these two diagrams can be combined so that the axes of all six limb leads intersect at a common point. The result is the hexaxial (six axis) lead diagram shown in Fig. 4.7. The hexaxial diagram shows the spatial orientation of the six extremity leads (I, II, III, aVR, aVL, and aVF). The exact relationships among the three augmented extremity leads and the three standard extremity leads can also be described mathematically. However, for present purposes, the following simple guidelines allow you to get an overall impression of the similarities between these two sets of leads. As you might expect by looking at the hexaxial diagram, the pattern in lead aVL usually resembles that in lead I. The positive poles of lead aVR and lead II, on the other hand, point in opposite directions. Therefore, the P–QRS–T pattern recorded by lead aVR is generally the reverse of that recorded by lead II: For example, when lead II shows a qR pattern: R q lead II shows an rS pattern: r S Finally, the pattern shown by lead aVF usually but not always resembles that shown by lead III. CHEST (PRECORDIAL) LEADS The chest leads (V1 to V6) show the electrical currents of the heart as detected by electrodes placed at
Fig. 4.7 (A) Triaxial diagram of the so-called bipolar leads (I, II, and III). (B) Triaxial diagram of the augmented limb leads (aVR, aVL, and aVF). (C) The two triaxial diagrams can be combined into a hexaxial diagram that shows the relationship of all six limb leads. The negative pole of each lead is now indicated by a dashed line.
different positions on the chest wall. The precordial leads used today are also considered as unipolar leads in that they measure the voltage in any one location relative to about zero potential (Box 4.1). The chest leads are recorded simply by means of electrodes at six designated locations on the chest wall (Fig. 4.8).d
Two additional points are worth mentioning here: 1. The fourth intercostal space can be located by placing your finger at the top of the sternum and moving it slowly downward. After you move your f inger down about 112 inches (40 mm), you can dSometimes, in special circumstances (e.g., a patient with suspected right ventricular infarction or congenital heart disease), additional leads are placed on the right side of the chest. For example, lead V3R is equivalent to lead V3, with the electrode placed to the right of the sternum.
Conventional Placement of ECG Chest Leads
Lead V1 is recorded with the electrode in the fourth intercostal space just to the right of the sternum.
Lead V2 is recorded with the electrode in the fourth intercostal space just to the left of the sternum.
• Lead V3 is recorded on a line midway between leads V2 and V4.
• Lead V4 is recorded in the mid-clavicular line in the fifth interspace.
• Lead V5 is recorded in the anterior axillary line at the same level as lead V4
.Lead V6 is recorded in the mid-axillary line at the same level as lead V4.
feel a slight horizontal ridge. This landmark is called the angle of Louis, which is located where the manubrium joins the body of the sternum (see Fig. 4.8). The second intercostal space is found just below and lateral to this point. Move down two more spaces. You are now in the fourth interspace and ready to place lead V4. 2. Accurate chest lead placement may be complicated by breast tissue. To ensure accuracy and consistency, remember the following. Place the electrode under the breast for leads V3 to V6. If, as often happens, the electrode is placed on the breast, electrical voltages from higher interspaces are recorded. Also, avoid using the nipples to locate the position of any of the chest lead electrodes, in men or women, because nipple location varies greatly in different persons.
The chest leads, like the six extremity leads, can be represented diagrammatically (Fig. 4.9). Like the other leads, each chest lead has a positive and negative pole. The positive pole of each chest lead points anteriorly, toward the front of the chest. The negative pole of each chest lead points posteriorly, toward the back (see the dashed lines in Fig. 4.9). The 12-Lead ECG: Frontal and Horizontal Plane Leads You may now be wondering why 12 leads are used in clinical electrocardiography. Why not 10 or 22
.Fig. 4.9 The positive poles of the chest leads point anteriorly, and the negative poles (dashed lines) point posteriorly
leads? The reason for exactly 12 leads is partly historical, a matter of the way the ECG has evolved over the years since Dr. Willem Einthoven’s original three extremity leads were developed around 1900. There is nothing sacred about the “electrocardiographer’s dozen.” In some situations, for example, additional leads are recorded by placing the chest electrode at different positions on the chest wall. Multiple leads are used for good reasons. The heart, after all, is a three-dimensional structure, and its electrical currents spread out in all directions across the body.
Recall that the ECG leads were described as being like video cameras by which the electrical activity of the heart can be viewed from different locations. To a certain extent, the more points that are recorded, the more accurate the representation of the heart’s electrical activity. The importance of multiple leads is illustrated in the diagnosis of myocardial infarction (MI). An MI typically affects one localized portion of either the anterior or inferior portion of the left ventricle. The ECG changes produced by an anterior MI are usually best shown by the chest leads, which are close to and face the injured anterior surface of the heart. The changes seen with an inferior MI usually appear only in leads such as II, III, and aVF, which face the injured inferior surface of the heart (see Chapters 9 and 10). The 12 leads therefore provide a three-dimensional view of the electrical activity of the heart. Specifically, the six limb leads (I, II, III, aVR, aVL, and aVF) record electrical voltages transmitted onto the frontal plane of the body (Fig. 4.10). (In contrast, the six precordial leads record voltages transmitted onto the horizontal plane.) For example, if you walk up to and face a large window (being careful to stop!), the panel is parallel to the frontal plane of your body. Similarly, heart voltages directed upward and downward and to the right and left are recorded by the frontal plane leads. The six chest leads (V1 through V6) record heart voltages transmitted onto the horizontal plane of the body (Fig. 4.11). The horizontal plane (figuratively) bisects your body into an upper and a lower half. Similarly, the chest leads record heart voltages directed anteriorly (front) and posteriorly (back), and to the right and left. The 12 ECG leads are therefore divided into two sets: the six extremity leads (three unipolar and three bipolar), which record voltages on the frontal plane of the body, and the six chest (precordial) leads, which record voltages on the horizontal plane. Together these 12 leads provide a three-dimensional dynamic representation of atrial and ventricular depolarization and repolarization.e eModifications of the standard 12-lead ECG system have been developed for special purposes. For instance, the Mason–Likar system and its variants are widely employed during exercise testing. To reduce noise due to muscle movement, the extremity electrodes are placed near the shoulder areas and in the lower abdomen. These changes may produce subtle but important alterations when comparing modified ECGs with standard ones using the wrist and
Fig. 4.10 Spatial relationships of the six limb leads, which record electrical voltages transmitted onto the frontal plane of the body.
CARDIAC MONITORS AND MONITOR LEADS Bedside Cardiac Monitors Up to now, only the standard 12-lead ECG has been considered. However, it is not always necessary or feasible to record a full 12-lead ECG. For example, many patients require continuous monitoring for a prolonged period. In such cases, special cardiac monitors are used to give a continuous beat-to-beat record of cardiac activity, usually from a single monitor lead. Continuous ECG monitors of this type are ubiquitous in emergency departments, intensive care units, operating rooms, and postoperative care units, and a variety of other inpatient settings.
Fig. 4.12 is a rhythm strip recorded from a monitor lead obtained by means of three disk electrodes on the chest wall. As shown in Fig. 4.13, one electrode (the positive one) is usually placed in the V1 position. The other two are placed near the
Fig. 4.11 Spatial relationships of the six chest leads, which record electrical voltages transmitted onto the horizontal plane.
right and left shoulders. One serves as the negative electrode and the other as the ground. When the location of the electrodes on the chest wall is varied, the resultant ECG patterns also vary. In addition, if the polarity of the electrodes changes Electrode placement 1 G 2 3 5 4 Fig. 4.13 Monitor lead. A chest electrode (+) is placed at the lead V1 position (between the fourth and fifth ribs on the right side of the sternum). The negative (–) electrode is placed near the right shoulder. A ground electrode (G) is placed near the left shoulder. This lead is therefore a modified V1. Another configuration is to place the negative electrode near the left shoulder and the ground electrode near the right shoulder.
Fig. 4.12 (A and B) Rhythm strips from a cardiac monitor taken moments apart but showing exactly opposite patterns because the polarity of the electrodes was reversed in the lower strip (B).
(e.g., the negative electrode is connected to the V1 position and the positive electrode to the right shoulder), the ECG shows a completely opposite pattern (see Fig. 4.12).Ambulatory ECG Technology: Holter Monitors and Event RecordersThe cardiac monitors just described are useful in patients primarily confined to a bed or chair. Some-times, however, the ECG needs to be recorded, usually to evaluate arrhythmias, in ambulatory patients over longer periods (Box 4.2). A special portable system designed in the mid-20th century by physicist Norman “Jeff” Holter records the continuous ECG of patients as they go about their daily activities (Box 4.3). The concept and the practical implementa-tion of recording ECGs with portable systems was a technological breakthrough that helped usher in the era of modern cardiac electrophysiology.Most of the Holter monitors currently in use consist of electrodes placed on the chest wall and lower abdomen interfaced with a special digital, portable ECG recorder. The patient can then be monitored over a sustained, continuous period (typically 24 hours; sometimes up to 48 hours). Two ECG leads are usually recorded. The digital recording can be played back, and the P–QRS–T complexes are displayed on a special screen for analysis and annotation. The recording can also be digitally archived, and selected sections can be printed out. The patient (or family member) provides a diary to record any symptoms.A 24–48-hr Holter monitoring period (Box 4.3) is most useful for: (1) the detection or exclusion of arrhythmias associated with symptoms (especially palpitations, dizziness, or near-syncope) occurring BOX 4.2 Major Types of Ambulatory ECG Monitors• Holter monitors• External event monitors• Basic event monitor (no loop memory)• External loop recorders (ELRs)• Mobile cardiac outpatient telemetry (MCOT)• External patch recorders• Implantable loop recorders (ILRs)• Implantable pacemakers and cardioverter–defibrillators (ICDs)
several times a day; (2) assessing ventricular rate-control in atrial fibrillation during activities of daily living; and (3) detection of ST changes during chest discomfort or in the diagnosis of “silent ischemia.”Limitations of conventional Holter monitors in diagnosing the cause of intermittent symptoms or syncope, which are unlikely to be captured in a 24-48-hr monitoring period, have led to the ongoing development and widespread use of several additional classes of ECG monitors (Box 4.2): external event recorders, including patch devices, mobile (real-time) cardiac outpatient telemetry systems (MCOTs), and implanted loop recorders (ILRs).External loop recorders (ELRs) are widely prescribed for arrhythmia analysis and detection. These devices are designed with replaceable electrodes so that patients can be monitored for prolonged periods (up to 2–4 weeks or longer) as they go about their usual activities. The ECG is continuously recorded (via a “looping mechanism”) that allows for auto-matic erasure unless the patient (or companion)
3 Some Advantages and Disadvantages of 24-Hour Holter MonitorsAdvantages• Detecting very frequent, symptomatic arrhythmias or seeing if frequent symptoms (e.g., palpitations) have an arrhythmic correlate.• Providing very accurate assessment of rate control in established atrial fibrillation.• Detecting ST segment deviations with “silent” ischemia or more rarely in making the diagnosis of Prinzmetal’s angina (see Chapter 9).• Detecting nocturnal arrhythmias (e.g., bradycardias or atrial fibrillation with sleep apnea).• Detecting sustained monitoring during real-world strenuous activity (e.g., certain types of “in the field” sports, especially when a graded exercise test may be of limited use).Disadvantages• Cannot capture clinically important but intermittent arrhythmias that occur less frequently than every day/other day. Such transient arrhythmias are not uncommon.• Cannot exclude life-threatening events with a “negative” study: i.e., one with no index symptoms and/or no significant arrhythmias.
presses an event button or an auto-event trigger is electronically triggered. When patients experience a symptom (e.g., lightheadedness, palpitations, chest discomfort), they can push a record button so that the ECG obtained around the time of the symptom is stored. The saved ECG also includes a continuous rhythm strip just (e.g., 45 sec) before the button was pressed, as well as a recording after the event mark (e.g., 15 sec). The stored ECGs can be transmitted by phone to an analysis station for immediate diagnosis. Current event recorders also have automatic settings that will record heart rates above or below preset values even if the patient is asymptomatic. Event recorders can also be used to monitor the ECG for asymptomatic drug effects and potentially important toxicities (e.g., excessive prolongation of the QT/QTc interval with drugs such as sotalol, quinidine, or dofetilide) or to detect other potentially proarrhythmic effects (see Chapters 16, 20, and 21) of drugs. Ambulatory monitoring has been extended to include MCOT technology. These recorders provide continuous home recording. The ECG is transmitted via wireless technology to an analysis center if the ECG rhythm exceeds predesignated rate thresholds or meets automated atrial fibrillation criteria (auto
trigger mode) or if the patient pushes a button because of symptoms (patient-trigger option). The patient’s physician can then be immediately notified of the findings. In some cases, life-threatening arrhythmias (e.g., intermittent complete heart block or sustained ventricular tachycardia) may occur so rarely that they cannot be readily detected by any of the usual ambulatory devices. In such cases, a small monitor can be surgically inserted under the skin of the upper chest (insertable/implantable loop recorder [ILR]) such that the device records the ECG and saves recordings when prompted by the patient (or family member if the patient faints, for example) or when activated by an automated algorithm. Such ILR devices may be used for up to 2 years. Patch-based monitoring devices are increasingly being used in contemporary practice. These adhesive, “lead-less” recorders present an alternative to traditional ELRs, especially for 2–14 days monitoring periods. Finally, as discussed in Chapter 21, implantable pacemakers and cardioverter–defibrillators (ICDs) have arrhythmia monitoring, detection, and storage capabilities. Advances in wireless transmission, algorithm development, smartphones and recording technology are likely to accelerate progress in external and implantable ambulatory monitoring in coming years.