Tachyarrhythmia's Tachyarrhythmia's typically refer to isolated premature complexes (depolarizations) or to nonsustained and sustained forms of tachycardia originating from myocardial foci or reentrant circuits.
The standard definition of tachycardia is rhythm that produces a ventricular rate >100 beats/min.
This definition has some limitations in that atrial rates can exceed 100 beats/min despite a slow ventricular rate. Furthermore, ventricular rates may exceed the baseline sinus rate and be <100 beats/min but still represent an important "tachycardia" response, such as is observed with accelerated ventricular rhythms.
Tachyarrhythmias are broadly characterized as being a supraventricular tachycardia (SVT), defined as a tachycardia in which the driving circuit or focus originates, at least in part, in tissue above the level of the ventricle (i.e., sinus node, atria, AV node, or His bundle) or
a ventricular tachycardia (VT), defined as a tachycardia in which the driving circuit or focus solely originates in ventricular tissue or Purkinje fibers.
Because of differences in prognosis and management, the distinction between an SVT and VT is a critical distinction to make early in the acute management of a tachyarrhythmia.
In general, VT carries a much graver prognosis , usually implies the presence of significant heart disease, results in more profound hemodynamic compromise, and therefore requires immediate attention and measures to revert to sinus rhythm.
On the other hand, SVT is usually not lethal , often does not result in hemodynamic collapse, and therefore more conservative measures can be applied initially to convert to sinus rhythm.
Distinction between an SVT and VT
Is usually made on the basis of the electrocardiogram (ECG) obtained during tachycardia. It is important to obtain a 12-lead ECG during tachycardia if possible & to obtain a 12-lead (or at least multilead) rhythm strips during any intervention aimed at terminating the tachycardia, because this information is the best way to identify the specific arrhythmia at the bedside.
if the QRS is narrow (shorter than 120 milliseconds) during the tachycardia, then the ventricle is being activated via the normal His-Purkinje system & thus the origin of the tachycardia is supraventricular. Therefore, these tachycardias are also often referred to as narrow complex tachycardias . In contrast, whereas a wide QRS (120 milliseconds) during tachycardia suggests a ventricular tachycardia , there are common scenarios in which an SVT can produce a wide QRS complex; thus a more descriptive term, wide-complex tachycardia , is often used when the precise arrhythmia mechanism cannot be determined. For example, an SVT with a concurrent bundle branch block (BBB) or intraventricular conduction defect (IVCD) can produce wide-complex tachycardias despite a supraventricular origin. In addition, preexcited tachycardias (tachycardias in which the ventricle is activated in whole or in part over an accessory pathway) produce wide QRS complexes, despite their being supraventricular in origin.
Therefore, although a narrow complex tachycardia almost always makes the diagnosis of an SVT, a wide-complex tachycardia can be supraventricular or ventricular. The presence of fusion or capture beats and of AV dissociation are diagnostic of VT (discussed later, “Ventricular Tachycardia; Electrocardiographic Recognition”), but are often not present or are difficult to detect. Criteria have been developed based on the 12-lead ECG
Symptoms due toTachyarrhythmias
Tachyarrhythmias classically produce symptoms of palpitations or racing of the pulse.
For premature beats, skipping of the pulse or a pause may be experienced, and patients may even sense slowing of the heart rate. A more dramatic irregularity of the pulse will be experienced with chaotic rapid rhythms or tachyarrhythmias that originate in the atrium and conduct variably to the ventricles.
For very 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.
Hemodynamic collapse with the development of ventricular fibrillation (VF) can lead to sudden cardiac death (SCD) . SCD remains one of the principal causes of death in the adult population, thus emphasizing the importance of appropriate tachyarrhythmia prevention as well as management.