Unstable tachycardias can be manifested by chest pain, shortness of breath, decreased urine output, mental status changes, or hypotension.
The assessment of a patient with a tachycardia requires a systematic approach. First, the physician must determine whether the patient is experiencing evidence of hemodynamic compromise as a result of the tachycardia, which is generally not seen until the heart rate is greater than 150 beats per minute.
Once a patient is noted to be hemodynamically unstable, the clinician then needs to assess the type of rhythm. This is important because the rhythm dictates what further management is needed.
Narrow complex tachycardias
Unstable Tachycardias like Narrow complex tachycardias are rhythms with QRS duration of less than 120 ms. There are three categories of narrow complex tachycardias: junctional tachycardia, paroxysmal supraventricular tachycardia, and atrial tachycardia.
In most cases of unstable narrow complex tachycardias (with the exception of junctional tachycardia) immediate synchronized cardioversion with 50 to 100 joules is warranted.
In the conscious patient, premedication with a sedative or analgesic, such as diazepam, midazolam, etomidate, propofol, fentanyl, or morphine, should be attempted.
Junctional tachycardia, which is rare and most frequently a sign of digitalis or theophylline toxicity, should not be cardioverted because it represents an escape rhythm.
An accelerated junctional rhythm is rarely faster than 120 beats per minute and should be treated with a beta-blocker or, if the underlying ejection fraction is not known, with amiodarone withdrawal and possibly treatment of the underlying cause (e.g., digoxin).
Wide complex tachycardias
Unstable Tachycardias like Wide complex tachycardias (QRS >120 ms) include narrow complex tachycardias with aberrant conduction, antidromic atrioventricular nodal reentry tachycardia in patients with Wolf-Parkinson-White syndrome, and ventricular tachycardia (monomorphic or polymorphic).
Immediate synchronized cardioversion with up to 360 joules (or 200 joules if biphasic defibrillation is used) should be attempted for all unstable wide complex tachycardia, with the exception of polymorphic ventricular tachycardia, which should be defibrillated with 360 joules.
American Heart Association in Collaboration with the International Liaison Committee on Resuscitation. Guidelines 2000 for cardiopulmonary resuscitation and emergency cardiovascular care. Part 6: Advanced cardiovascular life support: 7d: The tachycardia algorithms. Circulation 2000;102: 158.