Things you should know before you start in EP...

          The answers to the questions listed previously are included below.  If you were unable to answer these, it does not mean that you should never consider working in an EP Lab. It does mean that you should do some studying. These questions represent basic facts that every person who is good at EP should be able to answer without hesitation. If you had trouble with these questions, make sure to put forth the effort to learn as much as you can about cardiac anatomy and physiology.  This knowledge will become an invaluable tool in EP.  

1. What is the conductive connection between the right and left atria?  
Answer: Bachman’s Bundle - While there are several possible locations where electrical activation can pass between the atria, Bachman's Bundle is the primary conduit of electrical activity between the right and left atrium.

2. List the three inferior leads of the surface ECG.  
Answer:  II, III and aVF - All three of these leads have the positive pole located below the heart providing a good look at the verticle conduction axis. A strongly positive deflection in these leads represents high to low activation. This is why PVC's from the outflow tracks of the ventricles are distinguished by a positive QRS in leads II, II and aVF.

3. If a rhythm has a narrow QRS, variable conduction from A to V and has atrial activity that creates a saw tooth pattern on the baseline, what is the most likely arrhythmia? 
 Answer: Atrial Flutter - A narrow QRS complex suggests a supra-ventricular tachycardia.  Variable conduction between the atria and the ventricles is often due to atrial depolarizations occuring faster than the AV node can recover. While the AV node refractory time can vary from person to person, arrhythmias that show periodic block through the node will often have rates in excess of 200bpm. A rate this fast is suggestive of some type of flutter. The sawtooth pattern on the baseline is the final piece of the puzzel. This characteristic waveform occurs as the depolarization wavefront travels up the septum giving the negative deflection in the inferior leads followed by passage across the high posterior wall and then down the lateral wall giving the positive deflection in the inferior leads. By taking into consideration all the information provided, the answer clearly must be atrial flutter.

4. What three structures define Koch’s Triangle?
Answer: Coronary Sinus Ostium, Tricuspid Valve and the Tendon of Todaro - This region is important in cardiac conduction as it is the general location of the AV Node. The fast pathway runs anterior along the Tendon of Todaro while the slow pathway, which is the target for ablation in patients with AVNRT, lies posterior near the ticuspid valve annulus near the ostium of the coronary sinus.(1 )

5. What cell characteristic is demonstrated by spontaneous depolarization?  
Answer: Excitability - Cardiac cells all demonstrate the capability to generate an electrical signal. This signal is the basis of all cardiac activity and originates from ionic differences inside the cell when compared to the extra cellular spaces. Understanding the properties of cardiac conduction is essential to understand how arrhythmias develop.

6. What is the most likely source of a PVC that is strongly positive in lead II and has an R wave transition in V3? 
Answer:  RVOT - Learning to recognize characteristic configurations of both surface and intracardiac electrograms will give the EP practitioner a valuable tool to use in the lab.  As was described in the answer to question #2, outflow track arrhythmias show high to low activation as indicated by the positive deflaction of the inferior leads. The precordial leads, V1 - V6 will reflect the horizontal plane (2) and can be used to evaluate anterior / posterior activation. When an outflow track PVC originates in the RVOT, the transition from positive to negative in the V leads usually occurs in V3.  A transition ealier than V3 may be suggestive of a left ventricular outflow track origin.

7. Which surface leads are bipolar?  
Answer: Leads I, II and III - The original three leads established by Einthoven are the only bipolar leads on the surface twelve lead. The augmented leads and the precordial leads utilize a calculated negative reference (Wilson's Central Terminal) and are thus unipolar.

8. What chamber is the Ligament of Marshall located in?
Answer:  Left Atrium - The Ligament of Marshall lies in the left atrium in a mirror position to the crista terminalus of the right atrium. This ridge of tissue is significant in that it is the origin of many left atrial focal tachycardias.

9. The monitor shows a narrow QRS complex occurring at variable intervals with no clear P waves. What is the most likely arrhythmia?
Answer:  Atrial Fib - The narrow QRS complex suggests an atrial origin. The variable intervals between the QRS complexes suggests rapid activation with periodic block throught the AV node. This combined with the lack of clear P waves should immediately suggest atrial fib as the primary suspected rhythm.

10. What is indicated by a negative P wave in Lead I?
Answer:  Left to right activation - The positive electrode in lead I is located on the left shoulder. Analysis of the deflection of this lead will demonstrate either right to left activation by showing a positive deflection, or left to right activation as indicated by a negative deflection in this lead.

11. What structure lies posterior to the right ventricular outflow tract?
Answer:  Left Ventricular Outflow Track - When viewed from above, the LVOT lies posterior and to the left from the RVOT.

12. Which phase of the action potential represents depolarization?
Answer:  Phase 0 - This phase of the action potential is characterized by the rapid upslope of the recorded electrogram indicating rapid influx of positively charged sodium ions.(1)

13. What structure lies at the mouth of the coronary sinus?
Answer:  Thebesian Valve - This structure, along with the eustacian ridge, may cause difficulties in canulating the coronary sinus.

14. True or False: All surface leads are read as unipolar electrograms.
Answer:  True - Nine of the twelve leads ustilized in the surface cardiac electrogram are unipolar. The three bipolar leads contain positive and negative electrodes that are remote from each other. Lead I has the negative lead on the right shoulder while the positive lead is on the left shoulder. Lead II uses the left shoulder as negative and the left leg or hip as the positive electrode. Lead III uses the left shoulder as the negative electrode and the left leg as the positive electrode. The distance between the positivie and negative electrodes in these leads exceeds the local area of interest and are thus read as unipolar electrograms. If you wish to master reading cardiac electrograms, It is important to learn the distinction between unipolar, bipolar and remote bipolar recordings.

15. A 16 year old male presents with a wide QRS tachycardia with a rate of 180 beats per minute. What is the most likely arrhythmia?
Answer: Antidromic AVRT - Barring cardiac surgery for correction of a congenital defect, most 16 year old patients will have a relatively healthy heart.  The most common arrhythmia found in teenagers is Atrial-Ventricular Reentry Tachycardia. AVRT can be orthodromic or antidromic. Orthodromic AVRT conducts antegrade, or from the atrium to the ventricles, by way of the AV node through the right and left bundles. The signal returns to the atria via an accessory bypass tract. Antidromic AVRT conducts in an opposing direction using the bundles and the AV node as the retrograde limb of the circuit. Because antidromic tachycardias cause ventricular preexcitation of one ventricle before the other, they will display wide QRS configurations.