Conduction Study

          One of the most important areas within the conduction system of the heart is the HIS bundle region. Under normal circumstances, all the signals from the top of the heart must pass through the HIS bundle to reach the ventricles. Damage to this area can cause high levels of heart block and subsequently require a pacemaker implant.

          While there are no real pacing protocols designed to test the function of the HIS bundle, it is routinely tested during EP studies. Whenever you are pacing the atrium you are, in essence, testing the HIS bundle region. Since any atrial signal that conducts to the ventricles must pass through this area, you are testing the HIS every time you pace the atrium. All that you need to do to evaluate the function of this portion of the heart is keep your eyes open and watch for signals that don't make it down to the ventricles. When you see an atrial signal that does not conduct to the ventricles, you need to determine what stopped it. As there are only a few reasons why a signal from the atrium doesn't make it down to the ventricle, figuring this out is a simple process. It only requires two easy questions.

1) Did the atrium depolarize? This question assumes that you were pacing the atrium at the time the blocked signal occurred. To answer this question, look at the signals on the right atrial channels. Find your pacing artifact and look to see if there is and atrial signal just after the pacing signal. If there is no signal, the atrium did not depolarize. If there is an atrial signal, proceed to the next question.

2) Did the signal pass through the AV node? If the signal was blocked because the AV node was refractory, you will not see anything beyond the atrial signal. This is most easily determined by looking at the A signal on the HIS channels. If there is an A signal present without a HIS or V signal, then the tissue of the AV node was refractory and could not carry the signal to the ventricles. If, however, there is a HIS signal, then the AV node did conduct the signal and the block occurred below the HIS. This is a direct indicator of a high possibility of advanced heart block. When the signal blocks below the HIS, start making plans for a permanent pacemaker.

          The block below the HIS is demonstrated in this next image [Block Below the HIS]. How's that for an appropriate title! This was recorded during atrial induction with a single extra stimulus. (If you are uncertain what atrial induction with a single extra stimulus refers to, please see the Basic EP section on induction.) Two consecutive atrial signals on the HIS channel are indicated by the blue A. The HIS signals following each of the atrial beats and are indicated with the green H. The red V show that one of the atrial signals conducted down to the ventricles, but the second one did not. While this happens frequently in atrial induction, the important thing to note here is that the signal that did not conduct down to the V did not block until after the HIS. This is called a non-physiologic block, or one that was not caused by the normal physiology of the AV node. This indicates that the tissue below the HIS region has been damaged. There is a very high possibility that higher grades of heart block will develop in this patient. Depending on how often this occurs during a study, the physician may choose to implant a permanent pacemaker before the patient leaves the lab.

          Once we are done testing the sinus node, it is time to test the AV node. The purpose of this test is to determine the point at which signals begin to block at the AV node, or the Wenckebach cycle length. We may also continue the test until we find the point at which every other signal is blocked at the AV node, or the 2:1 cycle length. The results of this test will give you valuable information regarding the patient's conduction system.

          When Wenckebach testing is started, the right atrium is paced at a cycle length above the rate of the patient's intrinsic rhythm. During the pacing train, the response is recorded and 1:1 conduction is verified. The pacing cycle length is decreased and the process is repeated. This continues until a Wenckebach pattern is noted on the monitor. The point where this consistently occurs is documented. The pacing cycle length can be decreased further to find the 2:1 conduction point.

          This first image, [Antegrade WBach], demonstrates how the wenckebach cycle length appears during an EP study. In this figure, the right atrium is being paced on the "RA Dst" channel. The second set of numbers on the screen (as read from the top of the image), reads 560ms. This is the pacing interval. By focusing on surface lead V1 and the RA Dst channels, we can see the wenckebach develop.

          The first ventricular complex on V1 was generated by a pacing spike that is not visible on the page. The next pacing signal is seen on the RA Dst channel immediately after the first ventricular complex. This is marked with a blue "A". The arrow from the A extends to the ventricular complex (the red "V"), that is generated by the atrial pacing signal. A second atrial pacing signal is seen occurring at the same time as the second ventricular complex. The arrow from this second atrial pacing signal shows that it does not conduct a ventricular beat. It points, instead, to the "?". After the ?, the next occurrence we see on either of these channels is another atrial pacing signal on the RA Dst channel. This signal does conduct a ventricular complex, as does the fourth atrial paced beat.

          The second atrial paced signal did not conduct to the ventricles because the AV node was refractory. If you measure the next two conducted atrial paced signals, you will see the A to V internal is increasing in the normal wenckebach pattern.  If we continued to decrease the pacing cycle length, we would reach the point where every other signal was not conducted. The is the point where the antegrade 2:1 block occurs. The image [Antegrade 2 to 1 Block] shows 2:1 block occurring at a paced cycle length of 240ms. Every other A does not conduct a subsequent V signal.