Ambulatory ECG monitoring and related investigations

What is an ambulatory ECG?

Ambulatory electrocardiography (AECG) is used to detect, characterise and document cardiac arrhythmias in clinical practice. As some arrhythmias are infrequent or may occur only during certain activities (eg, sleep or exercise), it is usual to record the electrical activity of the heart over a period of time, usually 24 hours.

A 24-hour cardiac monitor is also often termed Holter monitor after Norman Holter who invented cardiac monitoring in 1949. Nowadays monitoring may be done for 48 hours and even seven continuous days.

Intermittent recorders may also be used to provide brief records of recordings from a longer period of time. These recorders may have a memory loop to allow documentation of sudden change in rate or rhythm of the heart. Most modern pacemakers and implantable defibrillators can also be used to gather information about arrhythmias for retrieval.

Who is suitable for ambulatory ECG?

Ambulatory ECG monitoring is suitable for patients with symptoms which may be caused by arrhythmia (eg, palpitations, light-headedness or syncope):

• Patients should be able to record symptoms in a diary.

• Patients with symptoms occurring daily or almost daily, or those who have syncope without warning, should be evaluated with a 24-hour Holter monitor.

• Patients with symptoms occurring less frequently may be better evaluated using a patient-activated event recorder.

Newer cardiac monitors can also monitor heart rates with changes in activity and posture.

Ambulatory ECG equipment

The most commonly used method of extended ECG recording is a Holter monitor which uses a conventional tape recorder or solid-state storage system for acquiring ECG information that can then be reviewed. There are two commonly used types of AECG recorders:

Continuous recorders

• These recorders are typically used for 24 or 48 hours to record events which might reasonably be expected to occur within that timeframe, ie frequent, or at least once a day symptoms.

• The patient keeps a diary of symptoms and records the time on the Holter clock when the symptoms occur, for later correlation with ECG abnormalities.

• The ECG recording is in digital format which allows for accurate and speedy interpretation of the recording, some recorders even providing for 'online' analysis as required. Their use is limited by cost and reliance on computer software to analyse the results accurately (former limited storage capacity of digital data is rapidly being overcome).

Intermittent recorders

• These are generally for recording infrequent symptoms and are one of two types:
  

  • Event recorders, which store only a brief recording of ECG activity when activated by the patient in response to symptoms.

  • Loop recorders, which record the ECG in a continuous fashion but store only a brief record when activated by the patient.

• Both types of intermittent recorder may be worn by patients for periods of many weeks in order to capture infrequently occurring events.

• Newer loop recorders continuously record and erase so that data gathered from 1 to 4 minutes before and then 30 to 60 seconds after the device was activated can be retained.

• Recordings may often be transmitted via telephone/3G mobile/internet to a central point of analysis.

Indications for ambulatory ECG^{1 2 3}

• AECG may be used to assess patients in whom an arrhythmia is suspected, including:
  

  • Patients with syncope, near syncope or dizziness.

  • Patients with palpitations.

  • Patients who have had a cerebrovascular accident in whom paroxysmal atrial fibrillation (AF) or atrial flutter is suspected:
    

    • Continuous ECG monitoring of AF is useful to detect silent paroxysmal AF in patients without previously documented arrhythmic episodes, such as those with cryptogenic stroke.⁴

    • Early diagnosis enables earlier treatment for primary or secondary stroke prevention.

  • Patients with episodic chest pain, shortness of breath or fatigue with no other obvious cause.

• AECG may be used to assess the potential risk of developing an arrhythmia - for example:
  

  • Patients prior to discharge from hospital after a myocardial infarction.

  • Patients with congestive heart failure.

  • Patients with hypertrophic cardiomyopathy.

• AECG may be used to assess a patient's response to anti-arrhythmic treatment - eg, the rate of AF, or pro-arrhythmic responses to drugs.

• AECG may be used to assess the function of a pacemaker device or implantable cardioversion device.

• The newer AECG monitors (incorporating multichannels, flash cards, etc) may also be used as a tool for the detection of myocardial ischaemia, by measurement of S-T segment shifts - for example:
  

  • Patients with suspected variant angina.

  • In the evaluation of patients with chest pain, who are unable to exercise.

  • In pre-operative assessment for vascular surgery in patients who are unable to exercise.

Limitations of Holter recordings

• The sampling period is usually too short to allow capture of an infrequent arrhythmia.

• Although the period of observation could be extended, serial Holter monitor recordings are impractical and expensive.

Mobile cardiac outpatient telemetry

• This allows several days of ECG monitoring via a cellular-based transmission system.⁵

• In one study, mobile cardiac outpatient telemetry (MCOT) provided a significantly higher yield than standard cardiac loop recorders in patients with symptoms suggestive of a significant cardiac arrhythmia.⁶

• MCOT can detect asymptomatic clinically significant arrhythmias and is particularly useful to identify the cause of presyncope or syncope, even in patients with previously negative investigations.⁷