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.
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