The term tachyarrhythmias typically refers to nonsustained and sustained forms of tachycardia originating from myocardial foci or reentrant circuits. The standard definition of tachycardia is a rhythm that produces a ventricular rate >100 beats per minute. This definition has some limitations in that atrial rates can exceed 100 beats per minute despite a slow ventricular rate. Furthermore, ventricular rates may exceed the baseline sinus rate and be <100 beats per minute but still represent an important "tachycardia" response, as is observed with accelerated ventricular rhythms. Premature complexes (depolarizations) are considered under the category of tachyarrhythmias because they may cause arrhythmia-related symptoms and/or serve as triggering events for more sustained forms of tachycardia.
Symptoms Due to Tachyarrhythmias
Tachyarrhythmias classically produce symptoms of palpitations or racing of the pulse. With premature beats, skipping of the pulse or a pause may be experienced, and patients may even sense slowing of the heart rate or dizziness. A more dramatic irregularity of the pulse is experienced with chaotic rapid rhythms or tachyarrhythmias that originate in the atrium and conduct variably to the ventricles. With rapid tachyarrhythmias, hemodynamic compromise can occur, as can dizziness or syncope due to a decrease in cardiac output or breathlessness due to a marked increase in cardiac filling pressures. Occasionally, chest discomfort may be experienced that mimics symptoms of myocardial ischemia. The underlying cardiac condition typically dictates the severity of symptoms at any specific heart rate. Even patients with normal systolic left ventricular (LV) function may experience severe symptoms if diastolic compliance due to hypertrophy or valvular obstruction is present and a tachycardia develops. Hemodynamic collapse with the development of ventricular fibrillation (VF) can lead to sudden cardiac death (SCD) (Chap. 273).
Diagnostic Tests in Evaluating Tachyarrhythmias
In patients who present with nonlife-threatening symptoms such as palpitations or dizziness, electrocardiographic (ECG) confirmation of an arrhythmia with the development of recurrent symptoms is essential. A 24-hour Holter monitor should be considered only for patients with daily symptoms. For intermittent symptoms that are of prolonged duration, a patient-activated event monitor can be used to obtain the ECG information without the need for continuous ECG lead attachment and recordings. A patient-activated monitor with a continuously recorded memory loop ("loop recorder") can be used to document short-lived episodes and the onset of the arrhythmia. This is the preferred monitoring technique for symptomatic patients with less frequent arrhythmia events, but it requires continuous ECG recording. A monitor that automatically triggers to record a fast rhythm can be used to detect asymptomatic arrhythmias. Patients with infrequent, severe symptoms that cannot be identified by intermittent ECG monitoring may receive an implanted loop ECG monitor that provides more extended periods of monitoring and automatic arrhythmia detection (Fig. 233-1).
Spontaneous termination of atrial fibrillation at the time of a syncopal episode identified from implantable loop ECG recording.
In patients who present with more severe symptoms, such as syncope, outpatient monitoring may be insufficient. In patients with structural heart disease and syncope in whom there is suspicion of ventricular tachycardia (VT), hospitalization and diagnostic electrophysiologic testing are warranted, with strong consideration of an implantable cardioverter/defibrillator (ICD) device. The 12-lead ECG recorded in sinus rhythm should be assessed carefully in patients without structural heart disease for evidence of ST-segment elevation in leads V1 and V2 consistent with the Brugada syndrome, QT interval changes consistent with long or short QT syndromes, or a short PR interval and delta wave consistent with Wolff-Parkinson-White (WPW) syndrome. These ECG patterns identify a possible arrhythmogenic substrate that may cause intermittent life-threatening symptoms and warrant further evaluation and therapy. The individual syndromes are discussed in detail later in this chapter.
Monitoring for asymptomatic tachyarrhythmias is indicated in several specific situations. In patients with a suspected tachycardia-induced cardiomyopathy marked by chamber dilation and depression in systolic function, the demonstration of arrhythmia control is essential. Monitoring for asymptomatic ventricular premature complexes (VPCs) and nonsustained VT can be helpful in stratifying the risk of SCD in patients with depressed LV function after myocardial infarction (MI). Finally, in patients with asymptomatic atrial fibrillation (AF), anticoagulation treatment strategies depend on an accurate assessment of the presence of this arrhythmia. The duration of monitoring for asymptomatic arrhythmias may have to be extended to optimize detection capabilities.
A 12-lead ECG recording during the tachycardia can be an important diagnostic tool in identifying the mechanism and origin of a tachycardia to a degree not afforded by one- or two-lead ECG recordings. A 12-lead ECG of the tachyarrhythmia should be recorded and incorporated as a permanent part of the medical record whenever possible. For patients whose arrhythmias are provoked by exercise, an exercise test may provide an opportunity to obtain 12-lead ECG recordings of the arrhythmia and may obviate the need for more extended periods of monitoring.
Many paroxysmal supraventricular tachyarrhythmias are not associated with a significant risk of structural heart disease, and an evaluation for the presence of ischemic heart disease and cardiac function is required infrequently unless dictated by the severity or characteristics of the symptoms. However, in patients with focal or macroreentrant atrial tachycardias (ATs), atrial flutter (AFL), or AF, an evaluation of cardiac chamber size and function and of valve function is warranted. In patients with VT, an echocardiographic assessment of LV and right ventricular (RV) size and function should be the norm. Ventricular tachycardia that occurs in the setting of depressed LV function should raise the suspicion of advanced coronary artery disease (CAD). Ventricular tachycardia in the setting of isolated RV dilation should raise concern about the diagnosis of arrhythmogenic RV cardiomyopathy. Polymorphic VT in the absence of QT prolongation should always raise concern for a potentially unstable ischemic process that may need to be corrected to effect VT control.
Mechanisms of Tachyarrhythmias
Tachycardias are due to abnormalities of impulse formation and/or abnormalities of impulse propagation (Fig. 233-2).
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