Concealed Accessory Pathway...huh?

As you all know Daylon has been diagnosed with Supraventricular Tachycardia (SVT). This means that in addition to normal pediatric visits, Daylon's first 6 to 12 months (hopefully no longer than that) will also include appointments to a pediatric cardiologist. The first cardiologist appointment was this past Friday.

Before diving into the details of how Daylon's first appointment went I'm guessing most of you might still be wondering what SVT is exactly. Unfortunately there seems to be a lack of material on SVT, at least for non-medical persons. General materials on cardiology are plentiful but documentation on specific cardiac studies, such as bradycardiac and tachycardiac arrhythmia conditions, have been hard to find. Daylon's cardiologist provided us with some materials that describe basic cardiology and arrhythmias, and the following is a crude overview of those materials as best as I can decipher and understand it.

The heart as a pump
The heart has four chambers; two receiving chambers on top and two pumping chambers on the bottom. For the heart to function as a good pump, the top chambers (the atria) must squeeze first, filling the bottom chambers with blood. The bottom chambers (the ventricles) then squeeze, pumping blood from the right side to the lungs to get oxygen and from the left side, out the aorta to the body.

The normal electrical system
The normal pacemaker of the heart is on the right side in the top chamber. It controls the normal heart rate automatically (faster when you run or are excited, slower when you sleep or rest). An electrical wave passes from the pacemaker (also called the sinus node) through the top chambers of the heart, much like the ripples from a pebble thrown in a puddle. This electrical wave makes the top chambers squeeze.

The only way for the electrical signal to get to the bottom of the heart to make the ventricles squeeze is through the normal conduction system. This conduction system is like a cable the runs from the atria to the ventricles and is called the atrioventricular node (AV node). This AV node cable has a delay mechanism in the upper part that slows the electrical signal. When the signal reaches the middle of the heart, the electrical wave spreads rapidly to the bottom chambers making them squeeze. Once that happens, there is no way (normally) for the electrical signal to get back up to the top part of the heart.

The normal sequence of electrical events, therefore, allows the normal top-delay-bottom squeeze of the heart muscle for normal heart pump function.

Abnormal heart rhythms
Abnormal heart rhythms can be too slow or too fast. Slow rhythms are called bradycardias. Fast rhythms are called tachycardias. Daylon's condition is of the tachycardia type so I won't write anymore on bradycardias.

There are many types of tachycardias. Some are only located in the upper chambers of the heart (for example, atrial tachycardia, atrial flutter), some are located only in the bottom chambers (ventricular tachycardia or VT) and many involve both the top and bottom chambers (supraventricular tachycardia or SVT).

Remember in a normal electrical system that the only way for the impulse to get from the top to the bottom of the heart is through the AV node. Sometimes, even if the heart is structurally normal (normal valves and chambers), there is a "short circuit" in the system. If you consider the AV node the normal electrical connection in the heart, these short circuits may be called "accessory" (extra) connections. They are tiny strands of muscle, smaller than a piece of hair, that bridge the areas where the atria meet the ventricles. This allows an electrical impulse to get from the top to the bottom through a route other than the AV node and can also conduct from bottom to top, allowing impulses to get back up to the top from the ventricles after a normal heart beat.

There are two main problems that accessory connections permit. The first involves conduction from top to bottom. If an abnormal atrial rhythm call atrial fibrillation occurs, the top of the heart beats very fast and irregularly (quivers like jello), the accessory connection can allow rapid conduction to the ventricles causing ventricular fibrillation. This is very rare, but very dangerous and life-threatening. Daylon does NOT have this sub-type of SVT. The second problem is with conduction form bottom to top. Normal conduction does not allow this to happen. Sometimes, the accessory connection does and the electrical signal "re-enters" the top chamber of the heart. Once that happens, the normal conductor doesn't care where the impulse comes from and it conducts again to the bottom chambers. A tachycardia "circuit" can be set-up, with a fast rhythm going top-bottom-top-bottom-top...until somthing makes it stop (vagal maneuvers or drugs such as Digoxin). This second sub-type of SVT is indeed what Daylon is most likely having episodes of.


Phew. Now, Daylon's appointment. He had both an electrocardiogram (EKG) and an echocardiogram (ultrasound). The EKG quickly told the cardiologist that Daylon does NOT have the Wolff-Parkinson-White (WPW) syndrome SVT. WPW is an SVT where if an abnormal conduction pathway runs between the atria and the ventricles, much like an accessory pathway, the electrical signal arrives at the ventricles sooner than normal. The cardiologist was quick to note that this SVT is not good at all so it was a relief to hear that Daylon's SVT is of the above mentioned non-life threatening accessory pathway SVT. The ultrasound indicated that Daylon does indeed have a murmur as well but it is benign. Another interesting bit that the ultrasound revealed is that Daylon's heart has a tiny, tiny hole called a patent foramen ovale. This too is benign and can actually be found in 25% of adults apparently. The ovale hole is a mechanism that helps the unborn baby and his or her heart pump normally while in the placenta. During the strenuous laboring and birthing process this hole seems to close up, but in plenty of cases this hole persists after birth and closes as the baby ages and grows. The cardiologist also noted that Daylon most likely had a brief SVT episode while being examined under the ultrasound.

A nurse weighed Daylon after the ultrasound. He is at 8 pounds 11 ounces, having gained almost a full pound in one week! Because of his growth and increased body weight Daylon's dosage of Digoxin has increased. After six months of Digoxin and with Daylon's continued body development the goal is to hopefully have the number of SVT episodes or occurrences as close to 0 as possible, if not 0. The other goal is that the condition will have been outgrown or in the process there of and that we can start weaning him off Digoxin medication. Note that the Digoxin medication (derived from the foxglove plant) keeps the heart in normal rhythm as much as possible and tries to keep the heart from using, in Daylon's case, the accessory pathway that causes the heart to race. Simply put, Digoxin keeps the number of SVT episodes to a minimum in hopes that as Daylon grows his heart will out-grow the SVT.

And finally tomorrow we are scheduled to take Daylon in to get fitted for a Holter device. This device provides for a continuous 24 hour period EKG reading. Though the cardiologist is almost positive Daylon has the accessory pathway SVT sub-type, the 24 hour reading will give the doctor a very accurate assessment of the SVT condition and will also indicate how well the current dosage of Digoxin is working. In one month we are scheduled for another visit to review the progress of Daylon's SVT and to review his body weight and dosage of Digoxin.