What Is Holter Monitoring?
Ambulatory Cardiac Monitoring
Holter monitoring is considered one of the types of longer-duration ambulatory monitoring as compared with the brief 10-second snapshots an electrocardiogram (ECG) provides. As such, recordings can continue uninterrupted for 24-48 hours. This on-going monitoring has two American Heart Association-sanctioned indications:
- Transient, unexplained palpitations
- Mysterious episodes of dizziness, passing out (syncope), or near-syncope
Aside from these official indications, vascular specialists also find it useful for the following:
- To assess the efficacy of treatment in patients with atrial fibrillation (AF) and observe for covert episodes of it
- To monitor cardiomyopathy patients (ventricular hypertrophy, congenital heart disease, etc.)
- For close surveillance of patients who had recently experienced acute coronary syndrome (ACS)
- To identify “silent” myocardial ischemia
The Holter monitor consists of an array of 3 external electrodes connected to a shoulder strap-carried monitor. The monitor is small enough to allow going about day-to-day tasks, which offers the advantage of identifying events that would ordinarily go hidden or unprovoked by bedrest or the sedentary non-activity other more cumbersome arrangements (e.g., 12-lead ECG) would create. The monitor is returned to the physician’s office for review of the data recorded during the 24-48 hours it was being collected.
Other Cardiac Monitors
When Holter monitoring indicates the need for continuous monitoring or when the 24-48 hours of data fail to identify on-going, intermittent complaints, longer monitoring is effected by implantable (under the skin) loop recorders, about the size of a small thumb drive, which can wirelessly stream the day’s data to the physician over the Internet every evening during sleep. Alternately, a small patch–the Zio patch–can be used. These small devices do not render the data a 3-lead Holter monitor can, but are suitable for continuous, unobtrusive cardiac surveillance; even with their limitations, they can provide helpful–even crucial–data.
Many normal fluctuations in cardiac activity can occur as a result of vagal stimulation, athleticism, medication side effects, or orthostatic- or respiratory-mediated blood pressure changes, and the continuous monitoring provided by Holter monitors and other devices can separate these normal variations from true cardiac pathology.
Diagnosis via Holter Monitoring
The 12-lead electrocardiogram (ECG) is the gold standard by which cardiac health (or the lack thereof) is confirmed. To understand its advantages, it is important to know the differences between electrodes and leads. An ECG with only 3 electrodes has only 3 leads, the leads referring to the “views” from a certain direction determined by an electrical “bridging” between two of the leads. When another 6 electrodes are added to the mix, it is possible for some electrodes to have associations with more than one other electrode, so the lead count increases to 12 in a “round-robin” of sorts. The result is a 360° view of the electrical goings-on of the heart from all directions.
The wave of electrical activity in the heart originates in the atria and travels down the heart’s electrical conduit; it is directional such that there is a synchrony of non-simultaneous contractions in the atria and ventricles to create a vector force outward from the heart, supplying the body with blood and perfusion. The different leads (or views) of an ECG can catch the direction of these electrical waves of stimulus from different viewpoints to create a picture of not only when–but where–difficulties or abnormalities in these waves occur. This identifies where arrhythmias may be originating, where ischemia has occurred, or whether infarction of heart tissue has already happened or is pending.
Data Accrued by a Holter Monitor
Although the data from the 3-electrode Holter monitor is more limited than the 12-lead ECG, it will still render data for a report that will include total heart beats, heart rates (average, minimum, and maximum), any premature supraventricular (atrial or junctional) or ventricular beats, arrhythmias, episodes of prolonged electrical “pauses,” ST segment changes on the ECG indicative of ischemia, and patient “marked” symptomatic episodes for comparing such symptoms with the associated cardiac recording at the time.
Other Types of Monitors
Another type of monitor is the Zio patch, a small, adhesive patch that has the advantage of being temporary, convenient, and yielding up to 14 days of data. Its disadvantage is that only one view (direction, or “lead”) is obtained, making it less thorough than the 3-lead Holter monitor. This limits its effectiveness in identifying ischemia, but it still can render important information on premature beats, arrhythmias, “pauses” in perceived cardiac activity, and patient “marked” episodes for correlation with the simultaneous cardiac electrical activity.
The “loop” recorder is a small device–about the size of a thumb-drive–implanted under the skin with enough battery power to last a few years. Nightly it will send a report wirelessly to a bedside unit that will convey data to a patient’s doctor with a day’s worth of accrued data for evaluation.
While the patch and the implanted loop recorder are small and unobtrusive, their data is more limited than a full 12-lead ECG or the 3-lead Holter monitor, but they offer a last resort to catch pathology in action when even 48 hours of Holter monitoring fails to spot trouble. Patients cannot remain recumbent in an ECG lab for days, and the Holter monitor offers the next best chance to catch an occult, fleeting, or intermittent cardiac event as it happens, even with the limited “view” of one-lead surveillance. When the Holter comes up inconclusive, these other methods can extend surveillance by weeks or even years.
All of these methods allow patient “bookmarking” to tag the simultaneous electrical activity that occurs with suspicious symptoms they want to point out.
Holter Monitoring and Management
Management of cardiac events is dependent on the abnormalities identified via Holter or other monitoring methods of electrocardiography.
Syncope (passing out) and near-syncope (dizziness, light-headedness) are one of the official indications for Holter monitoring because they can indicate cardiac events such as those arising from arrhythmias or other cardiopulmonary disease. They are often accompanied by palpitations (the other official indication for Holter monitoring), nausea, visual blurriness, and unusual hearing effects.
For the most part, medical management of syncope, near-syncope, and palpitations that are discovered via Holter monitoring can be treated by replacing medication that has these side effects. There are antiarrhythmic drugs (beta blockers, etc.), but since these can create arrhythmic possibilities themselves, they are often begun in a hospital setting.
The following abnormalities can be addressed with an external pacemaker (temporary, outside of the body) or permanent (implanted):
- Bradycardia: contractions <40 beats per minute or sinus (autonomic pacemaker) electrical “pauses” > 3 seconds
- Atrioventricular heart block: In which the pacing atrial impulses are not passed successfully down the normal electrical conduit to effect synchronous cardiac activity and sustained, effective cardiac output
- Other electrical blocks within the ventricles that alter the normal pace making stimuli from the atria (sinoatrial “SA” node)
When there are serious, life-threatening arrhythmias (e.g., ventricular tachycardia), a “cardioverter-defibrillator” can be implanted under the skin that can administer an electrical shock to reboot the system when it senses a ventricular arrhythmia.
Many arrhythmias that are due to ectopic (abnormal or extra) pace making sites in the heart can be treated by destroying such areas with radio-frequency thermal ablation (destruction), targeted by imaging-directed catheters to the suspect site(s).
Holter Monitoring Prevention
Sudden cardiac death is often preventable, but the idea of prevention is not feasible unless a diagnosis is made, prompted by the occurrence of predisposing signs (ECG) or symptoms (syncope, palpitations). The 10-second burst tracings from even a 12-lead ECG can often miss transient episodes that portend poorly for a patient’s survival. Unless there is a great deal of luck or there is obvious, permanent heart disease, warning signals can be missed without the use of long-term monitoring equipment.
When life-threatening conditions lay dormant, only to arise and provoke sudden cardiac events or death, the Holter monitor offers an excellent opportunity to prevent such consequences. From information gleaned from the data accrued by long-term monitoring, management of current drugs and their side effects, addition of other drugs to mitigate symptoms, and implantations of pacemakers and cardioverters-defibrillators can extend a cardiac patient’s life indefinitely. When long-term monitoring identifies the presence of abnormal pacemakers, further isolation by a focused examination of repeat 12-lead electrocardiography can make possible pre-ablation strategies to destroy such tissue.
Serious cardiac events are often precluded by fleeting, intermittent events, such that when syncope, near-syncope, palpitations, or other uncomfortable symptomatology present, it is prudent to further explore such discomforting symptoms by either a better quality look (12-lead ECG) and a long-duration appraisal (Holter or other long-term monitoring) to prevent the natural progression of cardiac disease that can result in preventable sudden cardiac death.