Unfortunately, many times cardiac arrhythmias are episodic in nature - they come and go without warning. In these cases, you may experience episodes of symptoms, often lasting just a few seconds. Recording a simple 12-second ECG at some random time in these cases is not likely to reveal the arrhythmia, so additional testing is necessary. But the basic principle remains the same: to diagnose a cardiac arrhythmia, the arrhythmia should be "captured" by recording an ECG while the arrhythmia is present.
Your Initial Evaluation
If your doctor thinks you may be having a cardiac arrhythmias, the first question he or she ought to consider is whether those arrhythmias are likely to be life-threatening. If you have had spells of unexplained, severe dizziness, or have had syncope (loss of consciousness) without any warning, and especially if you have underlying cardiac disease, your doctor should consider the possibility that you are having a potentially dangerous arrhythmia. Such arrhythmias might include ventricular tachycardia or heart block. If so, you should probably be placed in the hospital on a cardiac monitor until a firm diagnosis can be made (and, if necessary, effective treatment is instituted).
On the other hand, more typical symptoms such as palpitations, mild fatigue, or mild, transient dizziness, are unlikely to indicate a potentially life-threatening arrhythmia - and a more "routine" cardiac evaluation can be used. Generally, this is accomplished by attempting to "capture" an episode of your symptoms while you are wearing an ambulatory cardiac monitor, as you go about your daily routine.
Recording the Heart Rhythm During Symptoms
If your symptoms occur daily or almost daily, the best choice for diagnosing the cause may be to use the Holter monitor, a recording device which continuously records the heart rhythm for a 24 - 48 hour period of time. You may be asked to keep a careful diary, noting the precise times that episodes of symptoms occur. The diary can then be correlated with the rhythm recording to show whether symptoms are associated with a cardiac arrhythmia.
If episodes of symptoms occur less frequently, the best choice may be a long-term event recorder, which you can use for several weeks at a time. Some of these event recorders use a circular tape that continuously records only the last 30 seconds of your heart rhythm. When you experience an episode of symptoms, you press a button that freezes the recording. This recording can then be transmitted by telephone to an interpreting center. More advanced types of event recorders are now available that can store several very long examples of your heart rhythm, and in addition to recording "events" that you yourself notice, can automatically detect and record many cardiac arrhythmias, without your intervention.
If episodes of symptoms are so infrequent that even an event monitor used for up to a month is unlikely to capture one, there are small implantable heart rhythm recorders, placed under the skin near the collarbone, that can be used for very prolonged periods of time. It is rarely necessary to resort to such a device, however.
Interpreting the Recording
The purpose of recording the heart rhythm during an episode of symptoms is to try to correlate your symptoms with a recording of your ECG at the time the symptoms are occurring. Ideally, to make the diagnosis, the onset of symptoms will coincide with the onset of an arrhythmia, and the symptoms will resolve when the arrhythmia stops. If such a pattern is seen, it is almost certain that the arrhythmia is producing the symptoms.
Often, however, people will report symptoms at times when the heart rhythm turns out to be entirely normal; or conversely, an arrhythmia will be recorded at a time when no symptoms are present. Under these circumstances, it is likely that the symptoms you are experiencing are NOT due to an arrhythmia, and your doctor should begin considering alternative explanations for your symptoms.
The Electrophysiology Study (EPS)
As noted previously, however, if your symptoms are suggestive of a potentially dangerous cardiac arrhythmia, an outpatient evaluation is not a good idea. In such cases, oftentimes an electrophysiology study (EPS) can be used to elucidate the likelihood that a life-threatening arrhythmia is present. If so, the EPS can quickly direct your doctors toward the appropriate therapy.
Crawford MH, Bernstein SJ, Deedwania PC, et al. ACC/AHA guidelines for ambulatory electrocardiography: executive summary and recommendations. A report of the American College of Cardiology/American Heart Association task force on practice guidelines (committee to revise the guidelines for ambulatory electrocardiography). Circulation 1999; 100:886.