1. (D) Fetal arrhythmias occur in 1% to 2% of pregnancies and account for 10% to 20% of referrals to pediatric cardiologists. Of these arrhythmias, 80% to 90% are premature atrial contractions with only around 10% of the arrhythmias being sustained. SVT may limit diastolic filling time and result in hydrops or decreased ventricular function. Treatment is recommended for sustained arrhythmia (typically >50% SVT burden) or evidence of hydrops. Digoxin is usually first-line agent if no hydrops is present, with case series reports of 60% to 80% positive responders. It takes a relatively high digoxin level (typically around 2) to achieve adequate fetal transfer (0.6 fetal transfer rate). The fetal transfer rate decreases by around 50% in the presence of hydrops. Other agents used in the treatment of atrial flutter include flecainide, sotalol, and amiodarone. Both flecainide and sotalol have an excellent fetal transfer (0.8 to 1). These medications are used when SVT is refractory to digoxin or in the presence of hydrops. Amiodarone can also be used and may have a role in very hydropic infants, but the maternal transfer is poor (0.1 to 0.3). In the patient described, continued observation is not the best option as the fetus will likely develop hydrops (usually within 48 hours) if the tachycardia is sustained. At 32-week gestation on no therapy, immediate delivery is not indicated. Atenolol is very poorly transported across the placenta and is not typically used in the treatment of fetal SVT. Adenosine has a half-life of 3 to 5 seconds and would be metabolized prior to reaching the fetus. Administration of adenosine to the fetus via direct injection in the umbilical vein has been reported, but carries some risk to the fetus performing the injection and would only be used in extreme cases if medical management fails.

2. (A) Premature atrial contractions (PACs) are a benign finding and are present in many otherwise healthy adolescents. Although a very small percentage may have an atrial tachycardia, the finding of isolated premature atrial contractions does not require any restriction for athletic participation in a patient who is asymptomatic. In general, minimal workup is required for asymptomatic PACs and an echo is not typically required if the examination is normal. No treatment is required and the patient may participate in competitive sports without restrictions.

3. (B) The ECG shows a prolonged corrected QT interval (around 520 ms) consistent with a diagnosis of long QT syndrome (LQTS). β-Blockers are the first-line therapy for LQTS. β-Blockers have been shown to decrease the incidence of sudden cardiac death and syncope in LQTS, particularly in long QT syndrome type 1. Risk factors for sudden cardiac death in LQTS are length of the QT interval (with QT intervals over 500 being the highest risk) and previous episodes of syncope. A prophylactic ICD is not indicated in most cases of LQTS. In this 3 year old, placement of an ICD would be technically challenging and would have a high incidence of long-term complications. Given the lack of symptoms, a β-blocker would be a better first line of therapy. There is no indication for a routine electrophysiology study in LQTS as the diagnosis can be made based on the ECG. Amiodarone would be relatively contraindicated in LQTS as it prolongs the QT interval. A cardioversion is not indicated as the patient is in sinus rhythm.

4. (D) The rhythm strip (Figure 6.49) shows artifact mimicking a wide complex tachycardia. Any motion of the ECG leads during recording may create an artifact that may initially appear to be supraventricular or ventricular tachycardia. However, in this tracing, the narrow QRS complexes can be marched through the tracing (shown by the red arrows) and show an underlying sinus rhythm. The red star shows two very closely spaced deflections as the artifact fuses with the true QRS. It would not be physiologically possible to have two QRS complexes that are so closely coupled together as the ventricular myocardium needs time to repolarize prior to contracting again. With all of the evidence showing artifact, no therapy is necessary.

5. (B) A number of risk markers are used to assess the risk for sudden death in patients with hypertrophic cardiomyopathy, but these have changed over time and risk factors in children and adults are not always the same. Septal hypertrophy is a generally agreed-on risk factor for dangerous events (especially septal thickness >30 mm). While a family history of premature sudden death has been suggested as a risk factor, in a recent study in children, family history had no association with sudden cardiac death. Other risk factors include nonsustained ventricular tachycardia, unexplained (not neurally mediated) syncope, and a blood pressure decrease or inadequate increase during exercise testing. Late gadolinium enhancement on an MRI scan of the heart may also carry an increased risk of sudden death. The magnitude of left ventricular hypertrophy on ECG does not help in risk stratification.

6. (A) Amiodarone increases warfarin effect, digoxin and phenytoin levels, and class I antiarrhythmic toxicity. Digoxin is excreted primarily by the kidneys. Digoxin dose should be reduced when given in conjunction with amiodarone. There is no significant interaction between ACE inhibitors or furosemide with amiodarone.

7. (C) This patient is at risk for having episodes of supraventricular tachycardia. The ECG tracing shows intermittent preexcitation (short PR interval and delta wave). In patients with Wolff-Parkinson-White (WPW) syndrome, the preexcitation can often be intermittent and picked up when monitoring for longer periods such as on a Holter monitor. Patients with WPW are at risk for having episodes of supraventricular tachycardia. Although it is possible that these are premature ventricular contractions, the PR interval is exactly the same on all of the beats with the wider QRS complex, making preexcitation much more likely. There is no evidence of an atrial arrhythmia.

8. (B) The ECG shows complete AV block. In combination with the classic rash with central clearing (erythema migrans with a bull’s eye rash), this patient likely has Lyme disease caused by the spirochete Borrelia burgdorferi. The infection is transmitted to humans by tick bites. In patients with Lyme disease, the incidence of cardiac involvement has been estimated to be 8% and usually occurs within a few weeks of the onset of the illness. The most common feature of Lyme carditis is atrioventricular (AV) block. The AV block usually resolves gradually with normalization of the PR interval in 1 to 2 weeks. Persistence of AV block requiring a pacemaker is unusual. Prompt treatment with antibiotics is the treatment of choice, but temporary pacing may be necessary if the heart rate is very slow. Treatment is usually with doxycycline, but cephalosporins and amoxicillin can also be used. Gentamicin and vancomycin are not typically used. IVIG is used for Kawasaki disease, which typically does not present with AV block. Myocarditis can also present with AV block, but the clinical picture is more consistent with Lyme disease.

9. (C) Neurocardiogenic syncope is very common in the teenage years. These patients may have a myoclonic jerk resembling a seizure or may actually have a seizure when syncopal. In the presence of a single episode and clear history suggesting neurocardiogenic syncope, no further workup may be necessary. Recommending liberalization of fluid intake may be adequate. A neurology referral is not necessary if history is strongly suggestive of neurocardiogenic syncope. Myoclonic jerks are occasionally seen with syncope and may be mistaken for seizures. The lack of a postictal phase goes against (but does not exclude) seizures. A 24-hour ambulatory monitor is unlikely to capture sporadic episodes. An implantable loop recorder is effective at ruling out arrhythmias in patients with syncope, but is typically used in patients with multiple episodes of syncope with no discernable cause and is not indicated for a single episode of syncope classic for neurocardiogenic syncope.

10. (D) The ECG shows left ventricular hypertrophy and T-wave inversion. T-wave inversion on an ECG may be a marker for abnormal ventricular myocardium. The ECG is most consistent with hypertrophic cardiomyopathy. T-wave inversion in the left precordial leads (V5 and V6) is almost always an abnormal finding. In addition, the T waves are inverted in leads I and aVL, which is also an abnormal finding. The forces are suggestive of increased left ventricular hypertrophy (notice that the ECG is half standard in the precordial leads). A complete workup including an echocardiogram to evaluate the coronary arteries is necessary before clearing the patient for sports.

11. (A) The Kearns-Sayre syndrome (characterized by its onset before the age of 20 years, chronic ophthalmoplegia, pigmentary retinal degeneration, and at least one of the following symptoms: ataxia, heart block, and high protein content in the cerebrospinal fluid) is a severe variant of chronic progressive external ophthalmoplegia with frequent rearrangements of the mitochondrial DNA (mtDNA). Patients typically present with a bundle branch block and prolonged QT interval that progress to complete heart block. Prophylactic pacemaker therapy is advisable in patients suffering from the Kearns-Sayre syndrome, who have bifascicular block on the precordial ECG as they may rapidly progress to complete AV block. Steroids or an electrophysiology study is of no benefit, and the risk for progression of conduction disease makes watchful waiting less advisable.

12. (B) The ECG demonstrates long QT syndrome (LQTS) with 2:1 AV block (more appropriately termed pseudo 2:1 AV block as the AV node has normal function). Note the prominent P waves in leads V1 and V2 that fall in the middle of the T wave and are not conducted. Because of the bradycardia and severity of the phenotype, long QT syndrome with 2:1 AVB has a poor prognosis with up to 50% mortality rate in infancy. Mutations of cardiac ion channel genes cause LQTS, manifesting as increased risk of ventricular tachycardia and sudden death. The prognosis is generally poor, but recent data has suggested more optimistic outcomes. β-Blocker therapy alone can have side effects including enhancing the AV block. Hence, β-blocker therapy along with pacemaker implantation is recommended, although a β-blocker may be added to decrease the chance of ventricular arrhythmias. There is no indication for isoproterenol in a stable patient with a reasonable underlying rate. Amiodarone may lengthen the QT interval and is relatively contraindicated in LQTS.

13. (B) The course of the AV node and His-Purkinje system in endocardial cushion defects passes through the central fibrous body beneath the crest of the VSD. It is therefore displaced posteriorly and inferiorly. In the frontal plane, the initial QRS vector forces are usually directed inferiorly to the right, and the QRS loop moves counterclockwise, superiorly and to the left resulting in left axis deviation. The mean QRS axis in the frontal plane ranges between -30 degrees and -180 degrees, with most axes directed between -30 degrees and -120 degrees.

14. (A) Negative predictors of long-term survival after tetralogy of Fallot repair include older age at operation, significant residual hemodynamic abnormalities after surgery, use of an outflow tract patch, a QRS duration >180 ms, elevated left ventricular end-diastolic pressure, and poor left ventricular function. The presence of premature ventricular contractions does not increase the risk of sudden death. Atrial tachycardia is found in 20% to 30% of patients in long-term follow-up and may predispose to sudden death. However, there is no correlation between sinus bradycardia and sudden death. Right bundle branch block and left anterior hemiblock (bifascicular block) were initially thought to be risk factors for long-term development of complete AV block, but this has not been proven to be true.

15. (A) The ECG shows a prolonged PR interval of around 400 ms. ECG evidence of PR interval prolongation is a minor Jones criteria for acute rheumatic fever. To make a diagnosis of acute rheumatic fever, you need two major or one major and two minor criteria. The clinical manifestations of acute rheumatic fever follow the inciting group A streptococcal infection after a period of latency of about 3 weeks. Rheumatic fever is a multisystem disease affecting primarily the heart, the joints, the brain, and the cutaneous and subcutaneous tissues. Carditis associated with acute rheumatic fever is seen in about 50% of the patients. Tachycardia is one of the early signs of myocarditis. Complete heart block is not usually seen in rheumatic carditis. Choreiform movements are rapid jerking movements of the hands, face, and feet and are characteristics of rheumatic fever. Kawasaki disease resulting in coronary artery aneurysms, Marfan syndrome resulting in a dilated ascending aorta, and juvenile rheumatoid arthritis do not typically give a prolonged PR interval. Systemic lupus erythematosus and scleroderma may also be associated with a prolonged PR interval. Glomerulonephritis is not associated with acute rheumatic fever and does not affect the PR interval. A bifid uvula is associated with Loeys-Dietz syndrome, which does not result in PR prolongation.

16. (E) Premature atrial beats are common in the fetus and the neonate and do not warrant any therapy. Blocked premature atrial contractions (PACs) may result in temporary decreases in the heart rate, but these are not concerning in the fetus with good ventricular function and no hydrops. Early delivery is not indicated. The incidence of premature beats detected in utero is approximately 2%, with less than 10% of these arrhythmias persisting in the newborn. In the fetus, PACs account for 80% to 90% of premature beats. In the newborn, premature ventricular contractions are recognized in approximately 30% of infants with extrasystoles.

17. (E) The ECG demonstrates findings consistent with Wolff-Parkinson-White (WPW) syndrome. Propranolol is frequently the first-line therapy for SVT in patients with WPW. Digoxin and verapamil are relatively contraindicated in the presence of WPW as it may increase the risk for ventricular fibrillation. Flecainide is a second-line agent if the patient is not responsive to β-blockers. Mexiletine is a class IB antiarrhythmic not indicated for SVT. Amiodarone has a very long half-life and takes several days to build up a therapeutic level orally. Therefore, a “pill in pocket” strategy of taking medications only after SVT starts will likely not be effective.

18. (C) A change in T-wave polarity from positive to negative in the right precordial leads describes the normal maturation of change in the ECG during the first week of life. The positive T wave in lead V1 in the first days of life most likely results from the early appearance of repolarization in the left ventricle and the late termination of depolarization in the right ventricle. An overall left ventricle to right ventricle sequence results, and this accounts for the upright T wave in lead V1 in the normal term infant. The axis will shift to the left, but not to less than 55 degrees. There are decreased right ventricular forces, so the R wave will typically get smaller in lead V1. The PR interval remains the same, although it will lengthen with age.

19. (A) Permanent junctional reciprocating tachycardia (PJRT) is an accessory pathway-mediated tachycardia due to a slowly conducting accessory pathway typically located in the right posterior septum. The position of the pathway creates P waves with a purely negative polarity in leads II, III, and aVF with a P-wave axis of -90 degrees. Because the accessory pathway conducts slowly, P waves are usually easily visible on the ECG. Because it is an accessory pathway-mediated tachycardia, there is a 1:1 relationship of the ventricles to the atria. PJRT tends to be an incessant form of tachycardia and may cause a cardiomyopathy. PJRT rates tend to be slower (150 to 200 bpm) than other accessory pathway-mediated tachycardias, which makes them more difficult to detect clinically.

20. (C) At K of 5.5 to 6.5 mEq/L, the T waves become tall and peaked. At serum K level above 6.6 mEq/L, QRS widening along with ST-segment elevation is noted. Above 8.5 mEq/L the P waves disappear. At ˜9 mEq/L, arrhythmias begin: AV block, ventricular tachycardia, and ventricular fibrillation. Q waves are indicative of infarction and are not typically seen with elevated potassium.

21. (E) Junctional ectopic tachycardia is the most common arrhythmia in the acute postoperative period following congenital heart surgery. It is a focal tachycardia with gradual warm-up and cool-down and rate variability. The P waves may be visible in the terminal portion of or shortly after the QRS complex or be completely dissociated. The rate may be constant or fluctuate with increases and decreases in the catecholamine state. The ECG shown demonstrates VA dissociation with the ventricular rate being faster than the atrial rate. As the ventricular rate is faster than the atrial rate, this excludes atrial flutter and ectopic atrial tachycardia as the cause. In a reentrant SVT using an accessory pathway, there should be a 1:1 relationship between the atria and ventricles. Although VA dissociation can be seen in ventricular tachycardia, the QRS complex is narrow, essentially excluding ventricular tachycardia as a cause.

22. (A) Neonatal thyroid abnormalities (hyper- or hypothyroidism) are the most common sequelae of maternal treatment with amiodarone. There is no significant impact to the neonatal liver, lungs, eyes, or kidneys.

23. (A) The ECG shown demonstrates left axis deviation (axis of -80 degrees). This is seen in patients with a primum atrial septal defect (ASD). The ECG also demonstrates right ventricular hypertrophy (qR in lead V1). Other types of ASDs (secundum, sinus venosus, and unroofed coronary sinus) typically show a normal to rightward axis, and older patients will frequently have an rSR′ (or qR) pattern in lead V1. A PFO is seen in 20% to 30% of the normal population and does not result in any changes on the ECG.

24. (D) Erythromycin is associated with prolongation of the QT interval. In patients with baseline-prolonged QT intervals, care should be taken to avoid administration of any medications that have been implicated in drug-induced torsades de pointes, including QT-prolonging antiarrhythmic drugs, tricyclic antidepressants, erythromycin, ondansetron, and chloral hydrate. The other drugs shown do not have a significant effect on the QT interval.

25. (C) Adenosine has a half-life of 2 to 10 seconds and is an excellent drug for acute termination of reentry SVT or diagnosis of atrial arrhythmias such as atrial flutter. DC cardioversion would be indicated in a hemodynamically unstable patient. Verapamil is relatively contraindicated in infants less than 1 year of age. Propranolol is used for long-term prevention of SVT and would not be a first-line IV treatment before adenosine is attempted. Amiodarone may be used when other first-line agents have failed or if the patient is unstable and there is around a 30% chance of significant hypotension with rapid IV administration. IV digoxin may be proarrhythmic and is not generally given as a first-line agent.

26. (B) DC cardioversion is at times used as a first-line therapy for neonatal atrial flutter (rapid atrial pacing using an esophageal catheter is preferred as first-line therapy by many). The recommended energy is 0.5 to 1 J/kg. However, frequently in neonates, a higher dose is required as the energy is not delivered as efficiently through small neonatal pads or patches. If energy delivery of 1 J/kg is unsuccessful, the energy should be increased and cardioversion reattempted. In a stable rhythm with a pulse, synchronized cardioversion is indicated, as unsynchronized cardioversion may result in a shock on a T wave inducing ventricular fibrillation. There is no indication to wait and repeat cardioversion as another 4 J shock is not likely to be successful. IV adenosine will only create temporary AV block and is not helpful in converting atrial flutter. It is important to differentiate failure to convert an arrhythmia and successful cardioversion with immediate reinitiation of the arrhythmia. If cardioversion is successful in restoring sinus rhythm but the tachycardia reinitiates shortly after cardioversion, it may be necessary to begin an antiarrhythmic agent like amiodarone. However, in this instance, where the cardioversion was not successful, the next most appropriate step would be increasing the energy dose.

27. (E) A wide QRS escape rhythm in a patient with complete heart block is a class I indication for a pacemaker. The class I recommendations for permanent pacing in children, adolescents, and patients with congenital heart disease are as follows:

1. Advanced second- or third-degree AV block associated with symptomatic bradycardia, ventricular dysfunction, or low cardiac output.

2. Sinus node dysfunction with correlation of symptoms during age-inappropriate bradycardia. The definition of bradycardia varies with the patient’s age and expected heart rate.

3. Postoperative advanced second- or third-degree AV block that is not expected to resolve or persists at least 7 days after cardiac surgery.

4. Congenital third-degree AV block with a wide QRS escape rhythm, complex ventricular ectopy, or ventricular dysfunction.

5. Congenital third-degree AV block in the infant with a ventricular rate less than 55 bpm or with congenital heart disease and a ventricular rate less than 70 bpm.

In this patient, a QRS duration of 130 ms constitutes a wide complex escape, which may be unstable, and therefore requires pacemaker placement. The atrial rate is not important in determining the need for a pacemaker. Although complex ventricular ectopy is an indication for pacemaker placement, rare PVCs would not meet this criterion. In a stable patient and in the absence of congenital heart disease, a heart rate of 60 bpm would not warrant immediate pacemaker placement.

28. (D) Long QT syndrome type 1, the most common form of long QT, presents as syncope or arrhythmias with exercise. The ECG in this patient clearly shows a prolonged QT interval. Genetic testing may reveal a cause in about 75% of patients with a high index of suspicion for long QT syndrome. An echocardiogram may be helpful in identifying hypertrophic cardiomyopathy but the ECG does not suggest hypertrophy. Magnetic resonance imaging may be helpful in identifying arrhythmogenic ventricular cardiomyopathy or myocarditis, but the ECG is consistent with long QT syndrome rather than either of these diagnoses. There are typically no abnormalities of the AH or HV intervals in patients with long QT syndrome and there is no suggestion of conduction disease of the AV node based on the ECG. Procainamide challenge may be helpful in bringing out Brugada syndrome, but there is no indication of Brugada syndrome on the ECG, and procainamide will further prolong the QT, which may precipitate ventricular arrhythmias.

29. (A) For SVT to occur, there need to be two pathways with differences in conduction properties and refractory periods separated by an area of nonconduction. Entrainment is a form of mapping reentry tachycardias, and triggered activity and increased automaticity are properties of focal tachycardias.

30. (D) One of the long-term complications that can manifest in patients who have had surgery for congenital heart disease is the development of heart block. This is more common in patients who had temporary AV block in the immediate postoperative period. Late development of AV block may play a role in sudden cardiac death. The presence of Mobitz type II second-degree heart block implies conduction system disease placing a patient at risk, and pacemaker placement is warranted even with a good underlying heart rate and no symptoms (class IIa). However, in the presence of Mobitz type I second-degree heart block (Wenckebach), pacemaker placement is not warranted unless the patient has a slow underlying rate and/or symptoms. Digoxin and amiodarone may worsen AV conduction. There is no indication for theophylline therapy for the treatment of AV block in the current era.

31. (C) Atrial flutter has traditionally been characterized as a macro reentrant arrhythmia with atrial rates between 240 and 400 bpm. The ECG usually demonstrates a regular rhythm, with P waves that can appear saw-toothed, also called flutter waves. Since the atrioventricular (AV) node cannot conduct at the same rate as the atrial activity, one commonly sees some form of conduction block, typically 2:1 or 4:1. This block may also be variable and cause atrial flutter to appear as an irregular rhythm. Atrial flutter is a common manifestation in patients who have undergone atrial surgery. DC cardioversion is the most effective way to terminate atrial flutter and would be indicated in a patient with hemodynamic instability. An alternative is atrial overdrive pacing that may be performed through atrial pacing wires in the postoperative setting. Temporary overdrive pacing can be an effective means of terminating reentry tachycardias such as atrial flutter and paroxysmal supraventricular tachycardia. Typically, the pacing rate is set at 10 to 20 bpm faster than the tachycardia rate. Progressively faster rates can be tried with multiple attempts although there is a risk of inducing atrial fibrillation. Adenosine and digoxin will block the ventricular response, but not typically terminate the arrhythmia. Correction of hypokalemia is unlikely to terminate the arrhythmia.

32. (B) Atrial fibrillation occurs in 10% to 15% of patients with hyperthyroidism. Thyroid hormone contributes to arrhythmogenic activity by altering the electrophysiologic characteristics of atrial myocytes by shortening the action potential duration and enhancing automaticity and triggered activity in the pulmonary vein cardiac tissue. Decreased voltages on an ECG can result from hypothyroidism, but not usually in hyperthyroidism. Hyperthyroidism may also be associated with a hyperdynamic state with an increased pulse pressure and hypercontractile function, although ejection fraction may decrease in the face of long-standing hyperthyroidism. Hyperthyroidism typically does not affect conduction and does not cause heart block.

33. (A) Surgery within the atrium can result in damage to the sinus node. The risk for sinus node dysfunction is directly related to the extent of surgery within the atrium. Patients after an atrial switch or Fontan procedure are at greatest risk for sinus node dysfunction. Lyme disease, maternal SLE, and myocarditis are more commonly associated with AV block. Patients with cardiomyopathy may present with atrial and ventricular arrhythmias.

34. (B) Sinus node dysfunction is the most common arrhythmia after sinus venosus ASD repair. SVASD differs from secundum atrial septal defect by its atrial septal location and its association with anomalous pulmonary venous connection. The SVASDs tend to have a higher incidence of sinus node dysfunction likely due to their proximity to the sinus node with the potential for direct damage or injury to the sinus node artery during repair. In one study, at follow-up after SVASD repair, 6% of patients had sinus node dysfunction, a permanent pacemaker, or both, and 14% of patients had atrial fibrillation.

35. (D) The Heart Rhythm Society and the British Pacing and Electrophysiology Group have developed a code to describe various pacing modes (NBG code). This is a series of letters used to describe how the pacemaker is programmed. The first position denotes the chamber(s) paced, and the second position denotes the chamber(s) sensed (A for atria, V for ventricles, and D for dual—both atria and ventricles). The third position indicates the response of the pacemaker to a sensed event (I for inhibit, T for track, and D for dual—both inhibit and track). In the inhibit mode, when the pacemaker senses an intrinsic cardiac event, it inhibits pacing. In this manner, it allows intrinsic cardiac events to happen without pacing. In the triggered or tracking mode, the pacemaker actively paces in response to a sensed event (e.g., senses an intrinsic atrial contraction, then paces the ventricle in response). A DDD pacemaker paces both atrium and ventricle, senses both atrium and ventricle, and both inhibits and tracks in response to a sensed event. As the heart rate of the patient is higher than the set rate limit of the pacemaker, the device is sensing the native atrial rate at a rate of 85 bpm and triggering the ventricle to pace at the same rate. If the patient were set AAI or VVI, it would pace only the atria or ventricles at the lower set rate. If a DDI setting was selected, the patient would sense and pace in both the atria and ventricles, but would not have the capability of tracking an atrial rate above the lower rate limit of the pacemaker as it is only set in the inhibit mode. The DDI mode is helpful in patients with atrial arrhythmias to avoid the pacemaker tracking rapid atrial rates with subsequent rapid pacing in the ventricles. In the DOO mode, the pacemaker would pace the atria and ventricle at the lower rate limit with no sensing (i.e., would pace both the atria and ventricles without regard to the intrinsic cardiac activity).

36. (B) Behavioral changes, depression, and mood swings are the most common side effects of β-blockers in children. Other less common side effects include lightheadedness, tiredness, headaches, nightmares, difficulty sleeping, heartburn, diarrhea, and constipation. Rarely, β-blockers can cause a rash. Hypoglycemia has been reported but is also rare. β-Blockers can exacerbate asthma. They should also be used with caution in patients with diabetes as they can block hypoglycemic symptoms in these patients. Amiodarone, rather than β-blockers, causes hypothyroidism.

37. (C) The ECG shows marked ventricular hypertrophy with QRS complexes going off the page as well as a short PR interval. There is also evidence of strain with ST-segment changes and T-wave inversion in the limb leads. Pompe disease is a glycogen storage disease (type II). The disease is linked to an inherited deficiency of the lysosomal enzyme acid α-glucosidase, which is responsible for the breakdown of glycogen to glucose. The result is intralysosomal accumulation of glycogen, primarily in muscle cells, that leads to a progressive loss of muscle function. It is one of the most severe and lethal form of hypertrophic cardiomyopathy, typically causing death within the first 2 years of life. Babies appear clinically well at birth, but within 6 months start developing hypotonia, severe cardiomegaly, hepatomegaly, poor weight gain, difficulty sucking, and an enlarged protruding tongue. The ECG classically shows a short PR interval and extremely large QRS voltages suggestive of left ventricular hypertrophy.

38. (B) Patients with congenitally corrected transposition of the great arteries (ccTGA) are at risk for development of complete AV block. Because of the displacement of the AV node and the abnormal course of conduction tissue that runs very superficially, there is an increased risk for development of complete AV block. Approximately 10% of the patients may present with heart block. Spontaneous complete heart block occurs at a rate of up to 2% per year in this population. These patients also have an increased risk for Wolff-Parkinson-White. The atria are not generally dilated in the absence of AV valve regurgitation and patients are not at a high risk of atrial flutter or sinus node dysfunction. Junctional tachycardia and TdP are also rare in ccTGA.

39. (B) By delivering premature atrial stimuli (S₂ at 320, 310, and 300 ms) after a pacing train (S₁ at 600 ms), there should be a small decremental change in the AH interval. However, if there is a large change (>50 ms) in the AH interval with a 10 ms change in the premature atrial stimulus (S₂), this increase in the AH interval is called an “AH jump” and suggests dual AV node pathways (i.e., both a fast pathway and a slow pathway) that may be a substrate for AV nodal reentry tachycardia. If the S₂ captures the atrium but does not conduct to the ventricles, then the AV node effective refractory period has been reached. If the S₂ fails to capture the atrium, the atrial muscle effective refractory period has been reached. Wenckebach is seen during atrial pacing and not during an extrastimulus protocol. The lengthening of the AV conduction time represented by the AH interval is a normal finding (decremental property of the AV node) and does not indicate AV node conduction system disease.

40. (D) The intracardiac ECG shows a drive train of pacing (S₁) followed by a prematurely paced ventricular stimulus (S₂). The paced beat is delivered creating a pacing spike (excluding failure of output). However, this beat does not capture the ventricle (there is a pacing spike but no resulting QRS on the surface ECG) and therefore represents the ventricular muscle ERP. If the premature stimulus captured the ventricle beat but then blocked going to the atrium (ventricular signal on the ventricular catheter and a QRS on the surface ECG) but did not conduct to the atrium, this would represent the AV node retrograde ERP. There is normal atrial activation and no evidence of an accessory pathway. VA Wenckebach occurs during ventricular pacing and not during a ventricular extrastimulus protocol.

41. (C) The ECG demonstrates pacing in the VVI mode. The rate is 75 bpm. The P waves have no relationship to the QRS complex (are dissociated), indicating that there is no sensing of the atrium, eliminating the possibility of DDD or VDD pacing. There are no atrial pacing spikes eliminating the possibility of DOO (asynchronous dual chamber) pacing. There is pacing in the ventricle, so AAI is not a possibility. The pacemaker could be pacing VOO or VVI (since no intrinsic beats are seen on the tracing), but VOO was not a choice and therefore VVI is the correct answer.

42. (A) The surface tracings show a wide complex tachycardia. The atrial electrical signals (shown in the coronary sinus tracings) have no relationship to (are dissociated from) the ventricular electrical signals (shown on the bottom two ventricular electrogram tracings). As the atrial rate is slower than the ventricular rate, the tachycardia is originating distal to the AV node and is therefore a ventricular tachycardia. As the atrial rate is slower than the ventricular rate, this cannot be an atrial tachycardia. In an accessory pathway-mediated tachycardia, there is a 1:1 relationship between the atria and ventricles as the atria, AV node, ventricles, and accessory pathway are all obligatory parts of the circuit. In AV node reentry tachycardia, the AV relationship is also typically 1:1. As the patient is in a wide complex rhythm with a rapid ventricular rate, although there is VA dissociation, there is no evidence of antegrade complete AV block (to make this diagnosis, the atrial rate must be typically faster than the ventricular rate).

43. (B) This patient has evidence of right ventricular hypertrophy (qR in lead V1) and ST-segment elevation consistent with ischemia in V1 and V2. This is consistent with right ventricular ischemia. This can be seen in patients with pulmonary atresia with intact ventricular septum and right ventricle-dependent coronary circulation. The high pressure in the right ventricle creates sinusoidal connections between the right ventricle and coronary artery circulation that may predispose the patient to ischemia. Tetralogy of Fallot and truncus arteriosus can result in RVH, but not typically ischemia. Total anomalous pulmonary venous return may result in RVH and right atrial enlargement, but also does not give signs of ischemia.

44. (A) The ECG demonstrates atrial sensing followed by ventricular tracking and resultant ventricular pacing. This is a dual-chamber pacemaker and the pacing mode is either DDD or VDD. The VDD mode is capable of atrial sensing but not pacing in the atrium. Most patients have both an atrial and a ventricular lead placed with a dual-chamber pacemaker. However, there are leads that have the capability in a single lead to sense both the atrium and the ventricle as well as pace the ventricle achieving AV synchrony using a single lead set VDD. If the pacemaker was set VVI or VOO, it would pace at the lower rate limit of 60, not the rate of 80 seen in the tracing. If the pacemaker were set DOO (pacing in the atria and ventricles without sensing), both atrial and ventricular pacing spikes would be present in the tracing and would pace through the tracing without any regard to exiting P waves or QRS complexes.

45. (D) The ECG shows complete AV block. AV conduction block is a complication in 1% to 3% of surgical operations for congenital heart disease. Unless treated with an implanted pacemaker, postoperative complete heart block is associated with 28% to 50% mortality, and permanent pacemaker implantation is a class I indication for surgically induced complete AV block regardless of the ventricular escape rate and condition of the patient. Postoperative heart block often proves to be transient, typically resolving within 7 to 10 days of onset, and it is prudent to wait at least 7 days to determine whether AV nodal conduction will return. In a stable patient with a reasonable escape rate, there is no indication for epinephrine or isoproterenol.

46. (C) Mobitz type I second-degree AV block, also referred to as Wenckebach periodicity, is characterized by progressive PR prolongation, usually owing to changes in the AH interval (reflecting AV node delay), with eventual failure of conduction to the ventricle. In contrast, type II AV block refers to abrupt failure of AV conduction of one or more atrial impulses without prior PR prolongation. Type II block usually occurs below the AV node. In addition to the absence of progressive PR prolongation before block, type II block may abruptly progress to third-degree block with an inadequate escape rhythm and is therefore an indication for a pacemaker, even in the absence of symptoms. Sinus bradycardia, Wenckebach, and a prolonged PR interval are not indications for a pacemaker in an asymptomatic patient, but would be an indication if symptoms correlate with the rhythm. Left bundle branch block by itself is not an indication for pacemaker placement, regardless of QRS duration.

47. (B) The ECG shows bidirectional ventricular tachycardia with the QRS changing in every other beat during the VT. This tachycardia is most consistent with CPVT. Features of CPVT are a catecholamine-driven (typically exercise or emotion) ventricular tachyarrhythmias, a typical pattern of bidirectional ventricular tachycardia during exercise or emotion with a normal resting ECG and a structurally normal heart. CPVT is a genetic disease related most commonly to mutations in the cardiac ryanodine receptor gene (RYR2) or calsequestrin 2 gene (CASQ2). Long QT syndrome and Brugada syndrome arrhythmias are typically triggered by an early afterdepolarization with a PVC appearing at the end of the T wave and the classic arrhythmia is torsades. Arrhythmogenic right ventricular cardiomyopathy typically produces VT from the right ventricle and hypertrophic cardiomyopathy patients are at an increased risk of VT, but bidirectional VT is the hallmark feature of CPVT.

48. (E) The findings on this ECG are consistent with intermittent capture of ventricular lead. There are pacing spikes with no capture, ventricular escape beats at a slow rate, and intermittent capture of the ventricle by the pacemaker. This is most consistent with a partial fracture of the ventricular lead. The lead impedance will often be out of the normal range when interrogated. This patient will need to be admitted and have the output of the pacemaker adjusted. If this is not successful or there are other indicators of lead malfunction, the pacing lead will need to be replaced. Undersensing is the inability of the pacemaker to sense spontaneous myocardial depolarization and results in paced complexes in the presence of the heart’s intrinsic rhythm. This leads to inappropriate pacing complexes after native QRS beats. Oversensing refers to the pacemaker sensing artifacts and hence not pacing when indicated. The pacemaker is continually pacing in the tracing without inhibiting (withholding pacing), so oversensing is not present. ERI is the point in the pacemaker battery life where replacement is needed, but the pacemaker will continue to operate and will still result in ventricular capture. At the end of life when the battery voltage is extremely low, the pacemaker may not be able to generate enough energy to capture the heart which could result in a similar picture to the ECG shown. Placing a magnet over a pacemaker typically causes it to pace asynchronously, but not lose capture. Typically when a pacemaker performs a testing feature, it will not allow loss of capture for even a single beat.