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. 

This rhythm strip offers two leads taken at the same time, Lead II and Lead V1.  The Lead II strip may not look "typical" to a beginning student, because the sinus beats are very small and biphasic.  This is due to an axis shift, which cannot be evaluated without more leads.

One of the best teaching opportunities in this strip is the concept of "underlying rhythm" with ectopy.  The underlying rhythm here is sinus.  But there are sinus P waves which are hidden, making the sinus rate twice what it appears to be.  The P waves are invisible in the Lead II strip, with baseline artifact making them even harder to see.  But in V1, we are able to find them at the end of the PVCs' T waves.  The sinus rhythm is a bit irregular toward the end of the strip.  There are probably many things a more advanced practitioner could say about this strip, but it usually requires more than one or two leads to do a complete evaluation.  For your basic student, it is a good example of sinus rhythm with ventricular bigeminy.

This patient has an underlying atrial fibrillation with complete heart block and an idioventricular escape rhythm.  She was treated successfully with a permanent implanted pacemaker.

This strip offers something interesting for both your basic-level students and for your more advanced students.  First, it is a good example of sinus rhythm with a premature beat.  The PR interval was measured by the machine at .21 sec (218 ms).    The premature beat is supraventricular - that is, it is not a PVC.  Because of the slightly long PRI in this strip, it's P wave COULD be buried in the preceding T wave.  That would make this a premature atrial contraction (PAC).  

For discussion with your more advanced students, the P wave could, instead, be retrograde, and occurring during the QRS or slightly after it.  That would make the premature beat junctional, or an atrial echo beat. The origin of the premature beat is mostly academic - there is likely no clinical need to determine the origin.  

In looking for clues as to the origin of the premature beat, we would scrutinize the premature beats for "hidden" P waves.  Upright and before the premature beat would indicate a PAC.  Negative P waves before, during, or after the premature QRS would indicate PJCs.  In this strip, the T waves just before the premature beats are slightly deeper than the other T waves.  This could indicate atrial "echo", or reciprocal beats, which requires the presence of dual junctional pathways, in which the impulse turns around, reenters the atria, and causes a new beat.  It can be helpful to look at multiple leads (the more the better) in your search for P waves.  For a look at this patient's 12-lead ECG, go to this link.  

The P wave of a premature beat often penetrates the SA node and "resets" it, causing the next normal beat to occur after a "normal" R-to-R interval from the premature beat. This fact can help us find "hidden" P waves, as well.

Another interesting feature of this strip for your students who are interpreting 12-Lead ECGs, is that this ECG shows the criteria for left ventricular hypertrophy.  See the link above for the 12-lead and discussion.