Holter monitoring is ambulatory monitoring using a digital recorder attached to a belt. Two or three channels are usually recorded with an appropriate number of patch electrodes attached to the chest. Twelve channel Holter monitoring can also be done using standard lead placement . Holter monitoring is usually done when an arrhythmia is suspected, though ischemic ST segment deviation can also be picked up by Holter.
Dangerous signs to look for on a Holter tracing are life threatening bradyarrhythmias and tachyarrhythmias. Dangerous bradyarrhythmias are long pauses or episodes of complete heart block with low ventricular rate. Both these may present with recurrent syncope or dizziness.
Dangerous tachyarrhythmias are ventricular tachycardias and ventricular fibrillation. Complex ventricular ectopy and frequent ventricular ectopy may also be forerunners of ventricular tachycardia and ventricular fibrillation. Bidirectional ventricular ectopy suggest the possibility of bidirectional ventricular tachycardia as in catecholaminergic polymorphic ventricular tachycardia and Andersen syndrome. Digoxin toxicity is another cause of bidirectional ventricular tachycardia.
Bradycardia can also predispose to tachyarrhythmias. Complete heart block with QT prolongation can precipitate polymorphic ventricular tachycardia known as torsades des pointes. QT prolongation may be detected on Holter monitoring preceding episodes of torsades.
Leclercq JF et al analyzed Holter recordings of sudden death due to ventricular fibrillation in 62 cases . There were 13 deaths due to torsades des pointes in those without coronary artery disease. They were related to quinidine like drugs and or hypokalemia. The torsades were initiated by a long RR interval due to a post ectopic pause. A progressive decrease in mean heart rate during the preceding 3 hours was also noted. The ectopic beat initiating the ventricular tachycardia or fibrillation had a coupling interval which was shorter than the shortest value documented before that. So it was the prematurity of the ectopic beat which was ominous. The prematurity index which is the quotient obtained by dividing coupling interval with the preceding RR interval was lower in primary ventricular fibrillation than in ventricular tachycardia leading to ventricular fibrillation. In the last hour preceding the ventricular fibrillation, higher heart rate was also noted, reflecting an increased sympathetic tone.