The heart rate suddenly goes up to 170 beats per minute with wide QRS complexes and the blood pressure drops further. What are the diagnostic possibilities?
Tachycardia may be supraventricular or ventricular in ori-gin. Assuming p waves cannot be seen and the easy diagno-sis of an atrial tachycardia therefore excluded, it is imperative to differentiate supraventricular dysrhythmias, which are usually easily treated and relatively benign, from ventricular dysrhythmias, which are life-threatening and require immediate intervention.
Any wide-complex dysrhythmia (QRS>0.12 second) should generally be assumed to be ventricular tachycardia until proven otherwise. Further criteria suggestive of ven-tricular tachycardia are the presence of atrioventricular dissociation, fusion beats, left axis deviation, right bundle branch pattern with a QRS>0.14 second, or a left bundle branch pattern with a QRS>0.16 second.
Supraventricular rhythm diagnosis rests on the deter-mination of the rate, regularity, and the observation of p waves. Sinus tachycardia has identical-appearing p waves, a regular rate, and a fixed PR interval. The diagnosis of sinus tachycardia is less likely as the rate exceeds 150 beats per minute (bpm), especially in the elderly patient. Atrial fibrillation is an irregularly irregular rhythm without recognizable p waves, and the ventricular response often exceeds 150 bpm. Atrial flutter is more difficult to diagnose than atrial fibrillation. It often presents with a rate of exactly 150 bpm and a sawtooth baseline on ECG. Multifocal atrial tachycardia is a common rhythm in patients with under-lying pulmonary disease and presents with at least three different p wave morphologies preceding QRS complexes. Re-entry tachycardia has p waves hidden in the QRS com-plex and may often present with a rate of 150 bpm or greater. Administration of adenosine (6–12 mg intravenous push) will temporarily slow most supraventricular tachy-cardias. Although the dysrhythmia might quickly recur, diagnosis will be facilitated.