In this new installment of EP Lab Digest’s EP 101 series, the authors present three brief case overviews, then ask readers to test their knowledge in EP by answering questions about each case. Case #1: A 74-year-old female with a history of hypertension and hyperlipidemia is undergoing an invasive electrophysiology procedure (Figures 1 and 2) for symptomatic recurrent palpitations. Question: Which of the following procedures is being performed on this patient? A. Cavo-tricuspid isthmus ablation for typical atrial flutter. B. AV nodal slow pathway modification. C. Epicardial ablation of a LV (left ventricular) focus of ventricular tachycardia. D. Left lateral accessory pathway ablation. The answer is B: the patient is undergoing an AV node slow pathway modification. The various catheters (A) ablation, (B) coronary sinus, (C) high right atrial quadripolar, (D) bundle of His, and (E) right ventricular quadripolar catheter are in their standard positions in the RAO and LAO views. During the first 20 seconds of radiofrequency ablation (at a temperature of 55 degrees, impedance 85 ohms and power 50 watts), the following tracing was obtained (Figures 3A and 3B). This case illustrates a standard five-catheter approach for AVNRT ablation. The ablation catheter “A” (note the larger distal catheter tip) is located in the region of the slow pathway of the AV node near the inferoposterior region of the Triangle of Koch. Slow pathway ablation is the preferred method for ablation of AVNRT because of a lower risk of inadvertant AV nodal damage. Option A was incorrect because the ablation catheter is not located in the cavotricuspid isthmus. There is also usually a right-sided “Halo” catheter. Option C was incorrect since the catheters are located in the right atrium and ventricle (relative to the CS catheter). Option D was also incorrect because the catheters are all right sided. Question: After careful review of the initial electrogram obtained (Figure 3A) during the procedure, what is the next best approach for management? A. Continue ablation. There is evidence of a junctional rhythm response, indicating an adequate ablation site. B. Stop ablation immediately and prepare to initiate backup pacing. C. Increase temperature to 75 degrees and observe for a slow increase in the impedance. D. Reposition the ablation catheter. There is no response to ablation. The answer is B: one should stop ablation immediately and prepare for backup pacing. AV block is a known complication of AVNRT slow pathway ablation, and is reported to occur in up to 2% of cases.1,2 The presence of junctional ectopy (first three beats in Figure 3A) is a highly sensitive finding, occurring in up to 90% of effective ablation sites.3 However, the presence of either a fast junctional rhythm or VA block during junctional ectopy are known predictors of complete AV block.4,5 Careful scrutiny of the intracardiac tracing is essential during ablation, even with the desired junctional ectopy response. Figure 3B illustrates the response of this patient immediately after the transient VA block (fourth beat in Figure 3B with no atrial signal in the high right atrium [hRA]) was noted and ablation was stopped. Note complete AV block (red arrows). Case #2: A 54-year-old male with a history of hypertension, diabetes and progressively worsening heart failure symptoms undergoes an invasive electrophysiology procedure (Figure 4). Question: Which of the following procedures is being performed on this patient? A. A left lateral accessory pathway ablation. B. An AV nodal slow pathway modification. C. Epicardial ablation of a focus for ventricular tachycardia. D. Cavo-tricuspid isthmus ablation for typical atrial flutter. The answer is D: the procedure being performed is cavo-tricuspid isthmus ablation for typical atrial flutter. The catheters (A) ablation, (B) Halo and (C) coronary sinus are in their standard positions in an LAO projection. After the introduction of catheters, the cycle length of his atrial rhythm is determined to be 240 msecs. The maneuver shown in his intracardiac tracing (Figure 5) is performed, and a measurement of A msecs (original cycle length) (shown in red arrow) is obtained. Option A was incorrect because with a post pacing interval > 290 msecs when pacing from the right atrium, the atrial flutter is unlikely to originate from the right side. Option B was incorrect because there is no ventricular pacing. The pacing artifact is from the ablation catheter located in the right atrium. Option C was also incorrect because it is not a criteria for entrainment. The entrainment criteria are: 1) Constant fusion during overdrive pacing except for the last paced beat, which is entrained not fused. 2) Progressive fusion during overdrive pacing. 3) Localized conduction block to a site for one paced beat associated with interruption of the tachycardia, followed by activation of that site by the next paced beat from a different direction and with a shorter conduction time. 4) During pacing at two different rates during tachycardia, there is change in conduction time and electrocardiographic morphology at the electrode recording site (this is the equivalent of demonstrating progressive fusion — the second criterion — with intracardiac electrogram recordings). Question: Which of the following is a true statement regarding this case? A. The value for measurement “A” greater than 290 msecs indicates a likely right atrial origin of his tachycardia. B. There is evidence of ventricular pacing with intermittent capture. C. The last paced atrial beat is entrained and fused. D. The value “A” msecs minus the tachycardia cycle length (TCL) indicates pacing within the re-entrant circuit if the value is equal to or less than 30 msecs. The answer is D. Figure 5 shows the measurement of a post pacing interval (PPI) after pacing from the distal ablation catheter. PPI is calculated as the time between the last pacing stimulus that entrained the tachycardia and the next recorded stimulus from the pacing site. This value should usually be within 30 msecs of the tachycardia cycle length if pacing within a critical portion of the re-entrant circuit. It is a recognized method of differentiating left from right atrial re-entrant circuits.6 Entrainment,7 which may be either manifest or concealed, is the capture of the re-entrant circuit of a tachycardia without interrupting the tachycardia, so that upon cessation of pacing, the spontaneous re-entrant tachycardia is still present. During entrainment, the orthodromic wavefront from the pacing impulse resets the tachycardia to the pacing rate, while the antidromic wavefront either collides with the orthodromic wavefront of the previous beat (usual case) or is blocked by some other mechanism (refractoriness or another cause of block). Case #3: A 74-year-old male with a history of hypertension, hyperlipidemia, non-ischemic cardiomyopathy (left ventricular ejection fraction 15%) and NYHA functional class III heart failure symptoms presents for a right-sided biventricular ICD implantation. During device implantation, he had three non-sustained episodes of a tachycardia lasting less than 30 seconds each without symptoms or blood pressure changes. While recovering on the medical floor, he had an episode of sustained tachycardia with a blood pressure of 93/60 mmHg, and had the following EKG recorded (Figure 6). Immediately after administration of 6 mg of IV adenosine, the following tracing was obtained (Figure 7). Question: The most likely diagnosis is: A. Atrial flutter with 1:1 conduction. B. Atrial tachycardia. C. Wolff-Parkinson–White (WPW) syndrome. D. AV nodal re-entry tachycardia. E. Adenosine-sensitive ventricular tachycardia (VT). The answer is D. The patient most likely has an AV node re-entrant tachycardia. His baseline EKG had a QRS interval > 120 msecs (one of the criteria for his biventricular device implantation). Clues to this diagnosis include: • History of abrupt onset and termination during the biventricular ICD implantation procedure. • No definite visible p wave, but an rsr’ noted on lead V1. • Typical response to adenosine with a complete AV block that was masked by ventricular pacing until gradual restoration of sinus rhythm. Atrial flutter and atrial tachycardia will usually have visible atrial activity waves on the surface EKG and do not typically terminate with adenosine, although they can be slowed. There is no delta wave suggesting WPW. Adenosine-sensitive VT is unlikely, with no evidence of AV dissociation on the EKG and the earlier abrupt onset and terminations. Question: The next step in management for this patient is: A. Repeat administration of 12 mg of IV adenosine. B. Schedule an electrophysiology study with possible radiofrequency ablation the next day. C. Start a loading dose of digoxin prior to discharge, then follow up in the EP clinic. D. Reassure patient about the benign nature of the tachycardia and discharge home. E. Set VT zone in ICD with shock capability at 120 beats/minute. The answer is C: start a loading dose of digoxin prior to discharge, then follow up in the EP clinic. This patient has a common and readily treatable arrythmia of AV nodal re-entrant tachycardia. However, due to the proximity of the device implant, it will be prudent to wait at least two months to ensure optimal lead fixation and stability, therefore reducing the risk of lead dislodgment during catheter manipulation. Lowering the VT shock zone to detect at 120 bpm will lead to inappropriate shocks.