The patient:  This ECG was obtained from a two-month-old girl who was a patient in the Emergency Department.  She had a fever due to a respiratory infection and was dehydrated. She was alert, active, and irritable.

The ECG:  There is a narrow-complex tachycardia at a rate of 194 bpm.  This is faster than the normal range for a two-month-old, which is about 80-160 bpm.  The intervals are all within normal range.  The frontal plane axis, at 145 degrees, is rightward, which is normal for this age. There are prominent, narrow Q waves in the inferior wall leads (II, III, and aVF) and in the left lateral leads (V4, V5, and V6).  There are no Q waves in the high lateral leads (I and aVL).  This is a normal pattern for this age group.   www.sciencedirect.com/science/article/pii/B9781416037743100280

The evaluation of this ECG must be preceded by a thorough evaluation of the patient.  SINUS TACHYCARDIA would be expected in the setting of fever, dehydration, hypoxia, pain or other discomfort. Should the rate fail to gradually return to a normal range after treatment, we would have to consider a reentrant supraventricular tachycardia. Reentrant tachycardias have a SUDDEN ONSET and SUDDEN TERMINATION.

Unfortunately, we do not have follow up on the patient.

This ECG was obtained from a 13-year-old boy who had previously been in NSR.  At that time, he was diagnosed with Wolff-Parkinson-White syndrome, and he had delta waves.  He now presents with a supraventricular tachycardia.  People with W-P-W have one or more accessory pathways which allow atrial impulses to bypass the AV node and “pre-excite” the ventricles.  An accessory pathway, along with the normal pathway through the AV node, can form a functional circuit, allowing conduction to proceed down one pathway and return up the other, in a rapidly repeating circular motion.  This causes paroxysmal supraventricular tachycardia. 

In W-P-W, the most common type of SVT is AV reciprocating tachycardia (AVRT), also called AV reentry tachycardia.  Conduction through the accessory pathway can be forward (anterograde) or backward (retrograde).  The most common type of reentrant conduction in W-P-W is ORTHODROMIC, which we see here.  The impulse conducts forward through the AV node, and backward over the accessory pathway.  There is no delta wave, as there is no pre-excitation of the ventricles. 

Features of orthodromic AVRT are:

Fast rate (usually approximately 140-250 beats per minute)    

Narrow QRS complexes (unless BBB or IVCD present)   

P waves retrograde, if seen (negative in II, positive in aVR and V1)  

ST depression and T wave inversion common. 

This ECG, taken from a nine-year-old girl, shows a regular rhythm with a narrow QRS and an unusual P wave axis. Unfortunately, we do not have any clinical information. 

Normally, P waves are positive in Leads I, II, and aVF and negative in aVR.  They can be biphasic in V1, but are usually positive in the rest of the precordial leads. . The P waves in this ECG are NEGATIVE in Leads I,II, III, aVF, and V3 through V6.  This indicates RETROGRADE conduction through the atria - the impulse starts low and continues in a backward fashion through the atria. This tells us that the rhythm originated in the AV junction or low atria.  The "junction" is usually defined as all of the complex AV node and the Bundle of His. 

The literature over the years has been very confusing about the exact location of the "junctional" pacemakers.  One commonly-accepted guideline was that a rhythm is "junctional" if there are retrograde P waves with a short PR interval, or a P wave that occurs within or after the QRS.  A rhythm with a retrograde P wave and a NORMAL PR interval is said to be "low atrial", indicating that the ectopic pacemaker involved was located in the low atrium, producing retrograde conduction through the atria and normal delay through the AV node.

While both of these scenarios are plausible, it probably is not possible to say with certainty where the actual pacemaker is just by looking at the surface ECG.  The AV node has been found to have pacemaking capability in all three of it's regions, and the Bundle of His is also able to produce ectopic impulses.  PR intervals vary greatly, especially in pediatric patients, and can be influenced by heart size and heart rate.  The retrograde conduction through the AV node toward the atria can occur over the fast or slow pathways. The "major" junctional pacemaker is thought to be in the proximal Bundle of His.  Junctional or low atrial ectopic rhythms can occur because they override the rate of the sinus rhythm, following the rule that "The fastest pacemaker controls the heart".  junctional rhythms can also occur as "escape" rhythms, only occurring because the sinus impulse has failed or been vlocked - often due to AV block.

Since the exact location of the ectopic pacemaker in this case cannot be determined without electrophysiology studies, it is important to evaluate the effect, if any, the rhythm is having on the patient. Since there is a P wave before every QRS, and the QRS complexes are narrow, it can be assumed that there will be no clinical effect on this patient.  In addition, the rate is within normal range, and that is also unlikely to produce any clinical effect.

The causes of ectopic rhythms are many, and range from completely benign to serious.  So, this child should be evaluated in light of her symptoms, history, and physical assessment.