The symptoms associated with a wide-complex tachycardia are typically caused by decreased cardiac output; they include orthostasis, hypotension, presyncope, syncope, dyspnea, and exercise limitation. Of note, monomorphic ventricular tachycardia can be asymptomatic, despite the widespread belief that sustained ventricular tachycardia always produces symptoms. Clinical symptomatology is, therefore, of limited use in the differentiation of ventricular tachycardia from supraventricular tachycardia.
Accurately diagnosing the underlying rhythm in a patient with a wide-complex tachycardia is critical for determining treatment and management, especially if the patient presents emergently and is hemodynamically unstable. This is of particular concern because the medications routinely used to treat supraventricular tachycardia can cause severe hemodynamic deterioration by inducing the relatively stable rhythm of ventricular tachycardia to degenerate into ventricular fibrillation, with subsequent cardiac arrest. In fact, misdiagnosis of ventricular tachycardia as supraventricular tachycardia with abnormal conduction in patients presenting with a wide-complex tachycardia is not uncommon, especially if the abnormal rhythm is hemodynamically tolerated.
In general, if the clinician is unsure, a wide-complex tachycardia should be presumed to be a ventricular tachycardia until the presence of supraventricular tachycardia can be definitively proven. A patient with a wide-complex tachycardia in an unstable condition should receive immediate electrical cardioversion. In patients with a stable ventricular tachycardia or with a wide-complex tachycardia of unclear origin, pharmacologic agents, including amiodarone, procainamide, or lidocaine, may be used in accordance with established advanced cardiovascular life support guidelines. If it is determined that a wide-complex tachycardia is a supraventricular tachycardia with abnormal conduction, a trial of vagal stimulation (carotid massage) or treatment with adenosine may be attempted.
Several studies have, by using various criteria or combinations of criteria, attempted to improve the diagnostic accuracy of differentiating ventricular tachycardia from supraventricular tachycardia in the evaluation of wide-complex tachycardia. Although no single algorithm is 100% sensitive and 100% specific, several characteristics and clues can be of use. One of the best-recognized systematic algorithms consists of four differentiating characteristics proposed by Brugada and colleagues, as follows:
If an RS complex cannot be identified in any precordial lead, ventricular tachycardia can be diagnosed with 100% specificity and 21% sensitivity.
If an RS complex is clearly distinguished in one or more precordial leads, the interval between the onset of the R wave and the deepest part of the S wave (RS interval) is measured (if RS complexes are present in several precordial leads, the longest RS interval is used). If the RS interval is > 100 ms, ventricular tachycardia can be diagnosed with 98% specificity and 66% sensitivity.
If the RS interval is < 100 ms, the presence or absence of AV dissociation must be determined. Evidence of AV dissociation is 100% specific and 82% sensitive for ventricular tachycardia; this is because AV dissociation does not occur in supraventricular tachycardia. AV dissociation is characterized by atrial activity that is completely independent of ventricular activity. Although the presence of AV dissociation establishes ventricular tachycardia as the etiology, its absence does not exclude the possibility of ventricular tachycardia with retrograde conduction of ventricular impulses through the AV node producing an atrial rhythm. This phenomenon, called "retrograde ventriculoatrial conduction," is easily misinterpreted as AV conduction because of the presence of P waves.
If the RS interval is < 100 ms, and if AV dissociation cannot clearly be demonstrated, the QRS morphology may be evaluated. Morphologic criteria suggestive of ventricular tachycardia are extensive and complex, and they should be evaluated in conjunction with a cardiologist, if necessary. This final step in evaluation moves the algorithm to 96.5% specific and 98.7% sensitive.
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