Indications for Holter monitoring and what 24hr studies reveal
By Dr Harry Mond
OAM I MBBS I MD I FRACP I FCSANZ I FACC I FHRS I DDU
Ambulatory electrocardiographic (ECG) monitoring is a valuable investigation for patients with symptoms thought to be of cardiac origin.
There are a number of ways of performing ambulatory ECG monitoring:
Short-term 12 to 24 hour Holter monitoring.
Standard established equipment is now widely available. It is relatively inexpensive, non-invasive and if necessary can be performed more than once, although it has never been conclusively proven to be beneficial on consecutive days.
Medium-term One week to one month. Potentially longer usage.
The equipment involves simple patient activated event recorders, loop recorders which continually monitor patients and provide information of an event when activated and are able to provide ECG tracings for a fixed period immediately prior to activation. There are also longer term recorders (V-Patch) which may require no patient activation and transmit to a central station when the attached monitor recognises an arrhythmia. They may use novel electrodes which only require weekly replacement and allow the patient to shower. Many designs are becoming available. Will play a much larger role in the future as the technology improves. A major problem is unintentional artefact resulting in very frequent telemetric transmissions.
Long-term Two to three years
These are called “implantable loop recorders”. Available for many years but recent advances particularly in miniaturization, make this form of monitoring much more desirable. Now potentially can be implanted in a doctor’s office with minimal sedation or local anaesthesia. They too can be designed to transmit data remotely to a central office although this is energy expensive and generally the patient needs to attend a centre with a programmer to retrieve the stored data.
Major indications for ECG monitoring
Cardiac palpitations: By far the most common indication. Generally a Holter monitor is performed first even when symptoms are infrequent. Clues such as asymptomatic ectopic activity or short runs of an arrhythmia can be very helpful. A negative test in the presence of symptoms is diagnostic and often therapeutic. When a Holter is unhelpful, medium-term monitoring is indicated. There is rarely a need for an implantable device for diagnostic palpitations.
Unexplained syncope or dizzy turns: A very important group. If frequent such as daily dizziness, Holter monitoring is recommended and is very useful. Despite the low return, Holter monitoring is still performed in people with relatively infrequent episodes. Information such as heart block, pauses and rapid arrhythmias are occasionally documented without syncope. Medium-term automatic (non-patient activated) monitors will play an important role in the future. Long-term implantable loop recorders are important in this group and today is the major use of this invasive monitor.
Chest pain thought to be associated with palpitations: A rare indication for monitoring. Generally Holter monitoring is sufficient, but patient activated loop recorders are potentially useful.
Suspected slow heart rhythms: Generally a Holter monitor will suffice. This is preferable to all other monitoring as it provides a 24-hour window of the heart rate changes and can be partnered to symptoms. Occasionally needs to be done at infrequent intervals such as 6-monthly depending on the symptoms.
Congenital abnormalities associated with sudden cardiac death or re-entry tachyarrhythmias: An emerging important group where the indications are now being formulated. Holter monitoring is always performed on both symptomatic patients and the other family members. Very low return. May need to be repeated when being evaluated for pacemaker and implantable cardioverter-defibrillator therapy. Little or no role once these electronic devices are implanted as they all have electrogram monitoring capability. Occasional use of implantable loop recorders and medium-term monitors.
Abnormal ECGs such as high frequency of ectopic activity or evidence of high degree atrio-ventricular block: A very specialized and important group. Frequent ventricular ectopy may require electrophysiological ablative therapy. Such patients may require frequent Holter monitoring to record changes in the frequency of the ectopic activity either with pharmacological therapy or success of the ablation. No role for medium or long-term monitoring. Ongoing short-term Holter monitoring for high degree AV block is recommended if cardiac pacing is not undertaken.
Evaluate atrial fibrillation or atrial flutter “neuro-cardiology”. There is a very large emerging group of patients who require ECG monitoring. Atrial fibrillation is extremely frequent in all age groups, but particularly in the elderly and is commonly associated with embolic stroke. There is considerable therapeutic value and cost effectiveness in diagnosing the arrhythmia and commencing anticoagulation. There are many reasons to perform ECG monitoring to diagnose the arrhythmia in both patients who have had or not had a stroke. Most studies are short-term Holter monitoring, but there are emerging indications to perform both mid-term and long-term studies. Currently the use of implantable loop recorders is being evaluated following atrial fibrillation ablative therapy particularly in patients who cease anticoagulant therapy or those patients with cryptogenic stroke who may require anticoagulation.
Congestive Cardiomyopathy: Ambulatory ECG monitoring was infrequently used in this common disorder, unless the patient experienced palpitations. However, with studies recommending implantation of implantable cardioverter-defibrillators in appropriate patients, the routine use of short-term Holter monitoring has become very important. There is little or no role for medium or long-term monitoring in these patients.
Evaluate ventricular tachycardia: This is an uncommon group where ambulatory ECG monitoring is valuable to document the arrhythmia and the success or otherwise of therapeutic or ablative intervention. Mainly Holter monitoring, but there is a small role for medium and long-term monitoring and would depend on physician preference.
Evaluate pacemaker function in symptomatic patients and in particular to document high threshold exit block or unexplained tachyarrhythmias
Evaluate implantable cardioverter-defibrillator function particularly in symptomatic patients, where there may be over or undersensing of tachyarrhythmias: It is unusual to request ambulatory ECG monitoring for pacemaker and ICD patients and the interpretation of the data is very specialized. These implantable electronic devices have a number of test and programmable algorithms which can result in bizarre ECG appearances and thus cause considerable concern for the patient, referring physician and reporter. Nevertheless, the studies become important when symptoms persist post implantation or new ones emerge. Although the telemetric information from the device is usually diagnostic, there are on occasion concerns that require a short-term ambulatory ECG study. This may include arrhythmias, failure to pace or sense or an inappropriate pacing rate response with exercise. About 3% of Holter requests at CMS involve pacemaker patients, which can be a time expensive exercise.
The Value of 24-hour Holter Monitoring
It is hard to judge the usefulness of Holter monitoring as a cardiac investigation. The objective of the 24-hour clinical study is to document cardiac arrhythmias that are the cause of the patient’s symptoms. Unlike a 12-lead ECG which provides only a snapshot of arrhythmia information as well as evidence of cardiac conduction and structural abnormalities, the Holter monitor provides an arrhythmia analysis over a short time span. Prior to the availability of the Holter monitor, patients were required to be admitted to hospital for ECG monitoring at rest or with limited activity. Its wide availability obviated the need, therefore, of the expensive hospital admission except in exceptional circumstances. In general, the objectives of the investigation are well understood equally by physicians and general practitioners who understand the value of the results.
The results of the monitoring can be stratified into four groups of increasing arrhythmia importance. It must be remembered that accurate figures cannot be quoted as the interpretation of the results are subjective and dependent on the opinion of the reporter. The figures to be quoted are based on the findings of CMS, a large analysis and reporting company performing more than 1000 studies per week and covering a wide range of requests from hospitals, physicians, cardiologists and general practitioners.
Group 1: The results are essentially normal. Less than 5% of patients have a completely normal 24-hour recording without arrhythmia abnormalities irrespective of age. In almost every Holter study, atrial or ventricular ectopics are documented occasionally in clusters referred to as bigeminy, trigeminy, couplets, triplets or short atrial runs. These are as common as to be regarded “as within limits”. On occasion, ectopic activity causes symptoms and thus the monitor is diagnostic and reassurance becomes a therapeutic tool. Similarly, a patient with dramatic palpitations and a completely normal or near-normal study would be very diagnostic and helpful. Ironically, it is this group where Holter monitoring can be particularly useful and the results therapeutic. This group probably represents about 30% of all studies and is also the group which is least useful particularly when the patient fails to have symptoms during the recording period.
Group 2: In this group, there are modest numbers of abnormalities noted, which are probably of little consequence. These include more frequent atrial and ventricular ectopy up to about 1% of all beats and short runs of supraventricular tachycardia. Pauses less than 3-seconds and Wenckebach second degree atrioventricular block in the young particularly overnight are very common and the significance of these findings must be interpreted in the light of the severity of the symptoms and cardiological consultation may on occasion be required. About 25% of patients will record this level of abnormalities in their Holter monitors.
Group 3: In this group, the findings appear significant, but clinically are not regarded as severe. Again they must be interpreted in light of the clinical findings or symptoms. These include frequent atrial and ventricular ectopy, longer episodes of supraventricular tachyarrhythmias including atrial fibrillation, marked first degree atrio-ventricular block and ventricular triplets. About 25% of patients will record these abnormalities.
Group 4: Holter monitor results demonstrate severe abnormalities, where cardiological intervention, often urgently, is required. Findings include severe conduction abnormalities and prolonged rapid atrial and ventricular tachyarrhythmias. These represent about 20% of all studies.
In summary, ambulatory ECG monitoring has become a widespread cardiological tool for the investigation of syncope and palpitations. The 24 hour ambulatory Holter monitor is by far the most frequently used method although there are increasing indications and thus usage of medium term recorders and in particular implantable loop recorders.