This strip was taken from a patient at rest. It shows a regular tachycardia with a slightly-widened QRS complex at about .10 seconds duration. It is somewhat difficult to evaluate the baseline for P waves or flutter waves. We ALWAYS recommend multi-lead assessment for such evaluation. The P waves (or flutter waves) here have a sharp point, and can be easily "marched out", with a rate of about 300 per minute.
Whenever the ventricular rate is near 150/min., we should always consider the possibility of atrial flutter with 2:1 conduction. Since atrial flutter results in atrial depolarization at around 250 - 350 per minute, conducting every other P wave results in a rate of about 150. It can masquerade as sinus tach, but a patient with sinus tach at such a fast rate would probably have an obvious cause for a rapid heart rate, such as hypovolemia, drug overdose, or exertion. This rhythm could also be mistaken for atrial tachycardia or other forms of supraventricular tachycardia (SVT, PSVT, AVNRT, etc.). Multiple leads can more easily uncover the flutter waves running continuously "behind" and "through" the QRS complexes.
There is one beat that is obviously different from the others. This beat is about the same width as the other QRS complexes, but is opposite in direction. This probably represents aberrant conduction, possibly a hemiblock that occurs only in this beat. Careful measurement will show that this QRS is very slightly early, while the others are all very regular. The slight width of all the QRS complexes suggests that there is a conduction delay, which cannot be diagnosed on one strip with no patient history available.
There are other differential diagnoses, such as ventricular tachycardia with a captured sinus beat. We welcome discussion of this interesting strip.