CARDIAC RHYTHMS                                                                                      

Sinus Tachycardia

Sinus Tachycardia Strip

This ECG is a bit problematic. There are a couple of reasons for this. First, the QRS complexes are wider than 0.12 seconds. This means you have to think about bundle branch blocks, aberrancy, or ventricular complexes as the source of the widened interval. Secondly, the T wave and the P waves run into each other to further complicate issues; however, with a P wave before each complex and a 1:1 ratio of P and QRS complexes, you can pretty well rule out ventricular complexes. A 12-lead ECG verified the fact that this was a patient with sinus tachycardia and a right bundle branch block.

Ectopic atrial tachycardia with block

Ectopic atrial tachycardia with block

The ECG strip above was taken in lead V1. Lead II was useless in this patient. This is an ectopic atrial tachycardia with a 2:1 AV block. The atrial and ventricular cadence is regular throughout the strip. The QRS complexes are normal morphology and there is some slight alteration in amplitude consistent with tachycardia-related electrical alternans. The blue dots represent P waves.

Third degree AVB

Third degree AVB strip

This strip shows a third-degree AV block with an underlying sinus rhythm and a junctional escape. Note how the first two QRS complexes are taller than the others. This is probably due to the fusion with the underlying P waves that just happen to fall at the same time. Using your calipers to map out a P-P interval width and then walking it back and forth across the strip is your best way to evaluate these rhythms and isolate the respective waves.

Ventricular tachycardia

Ventricular tachycardia

This strip shows a wide-complex tachycardia at about 190 BPM. There are some fusion beats present and obvious AV dissociation. Here is how you should approach the AV dissociation. Place the pins on your calipers over the two obvious P waves (third and fourth blue dots). Now, just walk your calipers in both directions, noting any irregularities in the complexes below. The presence of fusion of the P waves with the underlying complex will be apparent.

Atrial Flutter

Atrial Flutter

This rhythm strip is taken from lead II. The patient was in consistent 2:1 conduction. The key to making the diagnosis in this lead is that the presumed PR interval is fairly wide and the presumed "P" waves are inverted. Taking half of this "P-P" interval demonstrates another inverted "P" wave at the exact halfway point. The rate and regularity makes this atrial flutter and the "P" waves are actually F waves.

 Torsade de pointes

Torsade de pointes

This ECG shows a patient who is having a run of bigeminy. It is unclear from this strip if the bigeminy is composed of PJCs or PVCs, because we do not see the start of the complex. The patient then begins to show a very rapid, irregular rhythm. At this point, an argument can be made as to whether this is ventricular tachycardia or torsade de pointes. This is because the rate is very, very fast—greater than 300 BPM in some places. Additional leads demonstrated a more traditional torsade pattern that, as mentioned, is not clearly apparent on this strip.

Junctional rhythm

Junctional Rhythm

This strip shows a junctional rhythm at a rate of about 45 BPM. Notice the absence of P waves, the regularity of the rhythm, and the normal-looking supraventricular complexes.

 Mobitz I second-degree AVB

Mobitz I second-degree AVB

The rhythm strip shows a grouped rhythm with a conduction ratio of 3:2. There is progressive widening of the PR interval between the first and second complexes and then there is a dropped P wave. The cycle then begins all over again in a recurrent pattern. This has all the earmarks of a Mobitz I second-degree AV block. Note that you cannot make a comment about shortening R-R intervals in this case because there is only one R-R interval per group.

Accelerated Idioventricular

Accelerated Idioventricular

This rhythm strip shows wide complex ventricular complexes with marked ST-T wave abnormalities. The ST segment depression is flat and a bit troubling because it could be evidence of ischemia. They could also be an abnormal morphology simply due to their ventricular origins. Clinical correlation and a full 12-lead ECG would be helpful.

Sinus rhythm with first-degree AVB

Sinus rhythm with first-degree AVB

The strip shows prolonged PR intervals at about 0.28 seconds. The QRS complexes are small and relatively isoelectric, whereas the T waves are very prominent. The blue arrow is pointing at the T waves, and the pink arrow is pointing at the P waves. Note that the true PR interval may be slightly longer than 0.28 seconds because the P waves are partly buried in the previous T wave. This means that the early onset of the P waves, and the additional width of the PR interval, cannot be seen clearly.

Atrial fibrillation

Atrial fibrillation

This is one fast arrhythmia! It is a narrow-complex tachycardia above 200 BPM. The most important thing in the differential diagnosis is the irregularity of the rhythm. As a matter of fact, it is completely irregularly irregular. This is uncontrolled atrial fibrillation. The faster an Afib becomes, the greater the chance of an accessory pathway being involved. You always need to consider that possibility in any patient with Afib over 200 BPM.

 Sinus tachycardia with frequent PACs

Sinus tachycardia with frequent PACs

This poor patient is having one heck of an AMI with that much ST elevation! The underlying rhythm is sinus tachycardia associated with frequent PACs. The PACs are probably due to the myocardial irritability from the AMI, but other possibilities need to be ruled out. For example, drugs, hypoxemia, and congestive heart failure must be ruled out and treated accordingly. Remember, you need to treat the underlying cause of the PACs, not the PACs themselves, in this case.

Mobitz I second-degree AVB 

Mobitz I second-degree AVB

The strip shows a long strip of complexes with prolonged but constant PR intervals. In the middle of the strip, there is a blocked P wave (red arrow). The blue arrow points out the first PR interval after the pause. Notice that this PR interval is a bit narrower than the others, which occurs occasionally in Mobitz II blocks. The second PR interval after the pause already is back to the constant PR interval found throughout the strip.

 Multifocal atrial tachycardia

Multifocal atrial tachycardia


This is a great ECG because the P waves are so easy to identify. Note that the P waves vary in morphology and that the PR intervals are different as well. The rhythm is irregularly irregular. Putting it all together, it screams out: MAT! A couple of extra points: Note the differing QRS morphology. This is due to fusion in some cases, aberrancy in others. The varying heights of the QRS complex are due to electrical alternans, which is common in many tachyarrhythmias.

 Junctional tachycardia

Junctional tachycardia

This strip was taken in lead V1. In this example, we see a pseudo-R' wave clearly. It is called a pseudo-R' wave because there is a small "actual" or "true" R wave present at the start of the complex. Remember, the first positive complex after an S wave is known as the R' wave.

     At rates of 130-140 BPM the line that separates a junctional tach from an AVNRT blurs. Many times, it is impossible to distinguish between the two. However, it is important to remember that the pathophysiologic mechanisms for the two rhythms differ with junctional tach caused by an accelerated junctional pacemaker while AVNRT is typically caused by a microreentry circuit that forms in a dual pathway AV node. The clinical scenario is invaluable in differentiating between these two possibilities.