A 12 lead EKG should be obtained to evaluate the rhythm.  In so far as it may alter treatment, any co-existing rhythm disturbance that may have precipitated<unk>  tachycardia should be ascertained such as
Digitalis toxicity.
ST elevation MI
The rate is >60 beats per minute
The rate is generally regular
The QRS complex is narrow
Retrograde p waves may be present due to retrograde conduction from the AV node. The p waves will be inverted in leads II and III.
The p wave may be buried within the QRS complex and may not be discernable
AV nodal reentry tachycardia (AVNRT) should be excluded.
Shown below is an EKG depicting<unk>  tachycardia (rate about 115/min), dissociated from a slightly slower sinus tachycardia (rate about 107/min) producing one form of double tachycardia; pairs of ventricular capture (C) beats (5th, 6th, 19th, and 20th beats); see laddergram.
Image obtained courtesy of Jason E. Roediger, CCT,CRAT
Junctional tachycardia electrocardiogram
Atrial flutter electrocardiogram
AVNRT electrocardiogram
Pulseless ventricular tachycardia electrocardiogram
Junctional bradycardia electrocardiogram
Premature atrial contraction electrocardiogram
Ventricular tachycardia electrocardiogram
Ventricular fibrillation electrocardiogram