Pacemakers in MRI for the Neuroradiologist.

Cardiac implantable electronic devices are frequently encountered in clinical practice in patients being screened for MR imaging examinations. Traditionally, the presence of these devices has been considered a contraindication to undergoing MR imaging. Growing evidence suggests that most of these patients can safely undergo an MR imaging examination if certain conditions are met. This document will review the relevant cardiac implantable electronic devices encountered in practice today, the background physics/technical factors related to scanning these devices, the multidisciplinary screening protocol used at our institution for scanning patients with implantable cardiac devices, and our experience in safely performing these examinations since 2010.

Relationship between serum potassium concentration and risk of recurrent ventricular tachycardia or ventricular fibrillation.

INTRODUCTION: Electrolyte abnormalities are considered a correctable cause of a life-threatening ventricular arrhythmia according to American Heart Association/American College of Cardiology Practice Guidelines, and ventricular tachycardia or ventricular fibrillation in the setting of an electrolyte abnormality is considered a class III indication for defibrillator implantation. However, there are little data to support this recommendation. The purpose of this study was to determine the risk of a recurrent sustained ventricular arrhythmia in patients with a low serum potassium concentration at the time of an initial episode of a sustained ventricular arrhythmia. METHODS AND RESULTS: One hundred sixty-nine consecutive patients who presented with a sustained ventricular arrhythmia and a serum potassium concentration determined on the day of the arrhythmia underwent defibrillator implantation. All patients had structural heart disease and left ventricular ejection fraction of 0.32+/-0.15. On the day of the index arrhythmia, 30% of the patients had a serum potassium concentration <3.5 or >5.0 mEq/L, including 7% who had a serum potassium concentration <3.0 or >6.0 mEq/L. For the entire cohort of patients, freedom from a recurrent sustained ventricular arrhythmia was 18% at 5 years and was not significantly different among patients with a serum potassium concentration <3.5 mEq/L (23%), between 3.5 and 5.0 mEq/L (16%), and >5.0 mEq/L (5%; P = 0.1). CONCLUSION: The results of the present study suggest that patients with structural heart disease and an abnormal serum potassium concentration at the time of an initial episode of sustained ventricular tachycardia or ventricular fibrillation are at high risk for a recurrent ventricular arrhythmia; therefore, implantable defibrillator therapy may be reasonable.

Differentiation of atypical atrioventricular node re-entrant tachycardia from orthodromic reciprocating tachycardia using a septal accessory pathway by the response to ventricular pacing.

OBJECTIVES: The purpose of this study was to determine whether the response to ventricular pacing during tachycardia is useful for differentiating atypical atrioventricular node re-entrant tachycardia (AVNRT) from orthodromic reciprocating tachycardia (ORT) using a septal accessory pathway. BACKGROUND: Although it is usually possible to differentiate atypical AVNRT from ORT using a septal accessory pathway, a definitive diagnosis is occasionally elusive. METHODS: In 30 patients with atypical AVNRT and 44 patients with ORT using a septal accessory pathway, the right ventricle was paced at a cycle length 10 to 40 ms shorter than the tachycardia cycle length (TCL). The ventriculo-atrial (VA) interval and TCL were measured just before pacing. The interval between the last pacing stimulus and the last entrained atrial depolarization (stimulus-atrial [S-A] interval) and the post-pacing interval (PPI) at the right ventricular apex were measured on cessation of ventricular pacing. RESULTS: All 30 patients with atypical AVNRT and none of the 44 patients with ORT using a septal accessory pathway had an S-A-VA interval >85 ms and PPI-TCL >115 ms. CONCLUSIONS: The S-A-VA interval and PPI-TCL are useful in distinguishing atypical AVNRT from ORT using a septal accessory pathway.

Effect of gender on atrial electrophysiologic changes induced by rapid atrial pacing and elevation of atrial pressure.

INTRODUCTION: The incidence of atrial fibrillation is greater in men than in women, but the reasons for this gender difference are unclear. The purpose of this study was to evaluate the effects of gender on the atrial electrophysiologic effects of rapid atrial pacing and an increase in atrial pressure. METHODS AND RESULTS: Right atrial pressure and effective refractory period (ERP) were measured during sinus rhythm and during atrial and simultaneous AV pacing at a cycle length of 300 msec in 10 premenopausal women, 11 postmenopausal women, and 24 men. The postmenopausal women were significantly older than the premenopausal women (61 +/- 8 years vs 34 +/- 10 years; P < 0.01). During sinus rhythm, mean atrial ERP in premenopausal women was shorter (211 +/- 19 msec) than in postmenopausal women and age-matched men (242 +/- 18 msec and 246 +/- 34 msec, respectively; P < 0.05). Atrial ERPs in all patients shortened significantly during atrial and simultaneous AV pacing. However, the degree of shortening during atrial pacing (43 +/- 8 msec vs 70 +/- 20 msec and 74 +/- 21 msec; P < 0.05) and during simultaneous AV pacing (48 +/- 16 msec vs 91 +/- 27 msec and 84 +/- 26 msec; P < 0.05) was significantly less in premenopausal women than in postmenopausal women or age-matched men. CONCLUSION: The results of this study demonstrate a significant gender difference in atrial electrophysiologic changes in response to rapid atrial pacing and an increase in atrial pressure. The effect of menopause on the observed changes suggests that the gender differences may be mediated by the effects of estrogen on atrial electrophysiologic properties.

Double potentials along the ablation line as a guide to radiofrequency ablation of typical atrial flutter.

OBJECTIVES: The purpose of this study was to determine the characteristics of double potentials (DPs) that are helpful in guiding ablation within the cavo-tricuspid isthmus. BACKGROUND: Double potentials have been considered a reliable criterion of cavo-tricuspid isthmus block in patients undergoing radiofrequency ablation of typical atrial flutter (AFL). However, the minimal degree of separation of the two components of DPs needed to indicate complete block has not been well defined. METHODS: Radiofrequency ablation was performed in 30 patients with isthmus-dependent AFL. Bipolar electrograms were recorded along the ablation line during proximal coronary sinus pacing at sites at which radiofrequency ablation resulted in incomplete or complete isthmus block. RESULTS: Double potentials were observed at 42% of recording sites when there was incomplete isthmus block, compared with 100% of recording sites when the block was complete. The mean intervals separating the two components of DPs were 65 +/- 21 ms and 135 +/- 30 ms during incomplete and complete block, respectively (p < 0.001). An interval separating the two components of DPs (DP(1-2) interval) <90 ms was always associated with a local gap, whereas a DP(1-2) interval > or =110 ms was always associated with local block. When the DP(1-2) interval was between 90 and 110 ms, an isoelectric segment within the DP and a negative polarity in the second component of the DP were helpful in indicating local isthmus block. A DP(1-2) interval > or =90 ms with a maximal variation of 15 ms along the entire ablation line was an indicator of complete block in the cavo-tricuspid isthmus. CONCLUSIONS: Detailed analysis of DPs is helpful in identifying gaps in the ablation line and in distinguishing complete from incomplete isthmus block in patients undergoing radiofrequency ablation of typical AFL.

Incidence and clinical significance of inducible atrial tachycardia in patients with atrioventricular nodal reentrant tachycardia.

INTRODUCTION: The purpose of this prospective study was to determine the prevalence and clinical significance of inducible atrial tachycardia in patients undergoing slow pathway ablation for AV nodal reentrant tachycardia who did not have clinically documented episodes of atrial tachycardia. METHODS AND RESULTS: Twenty-seven (15%) of 176 consecutive patients who underwent slow pathway ablation for AV nodal reentrant tachycardia were found to have inducible atrial tachycardia with a mean cycle length of 351+/-95 msec. The atrial tachycardia was sustained in 7 (26%) of 27 patients and was isoproterenol dependent in 20 patients (74%). The atrial tachycardia was not ablated or treated with medications, and the patients were followed for 9.7+/-5.8 months. Six (22%) of the 27 patients experienced recurrent palpitations during follow-up. In one patient each, the palpitations were found to be due to sustained atrial tachycardia, nonsustained atrial tachycardia, recurrence of AV nodal reentrant tachycardia, paroxysmal atrial fibrillation, sinus tachycardia, and frequent atrial premature depolarizations. Thus, only 2 (7%) of 27 patients with inducible atrial tachycardia later developed symptoms attributable to atrial tachycardia. CONCLUSION: Atrial tachycardia may be induced by atrial pacing in 15% of patients with AV nodal reentrant tachycardia. Because the vast majority of patients do not experience symptomatic atrial tachycardia during follow-up, treatment for atrial tachycardia should be deferred and limited to the occasional patient who later develops symptomatic atrial tachycardia.

P wave polarity during pacing in pulmonary veins.

INTRODUCTION: Recent studies have demonstrated that premature depolarizations that trigger atrial fibrillation often arise in pulmonary veins. The purpose of this study was to evaluate whether P wave polarity is helpful in distinguishing which of the 4 pulmonary veins is the site of origin of a premature depolarization. METHODS AND RESULTS: In 28 patients without structural heart disease who underwent focal ablation of paroxysmal atrial fibrillation, P wave polarity on a 12-lead electrocardiogram (ECG) was analyzed during sinus rhythm, and during pacing at a cycle length of 500--600 ms in the high right atrium and within each of the 4 pulmonary veins. P waves were categorized as positive, negative, biphasic or isoelectric. A negative or biphasic P wave in lead I (sensitivity 85 %, specificity 71 %) or a positive P wave in V1 (sensitivity 85 %, specificity 89 %) were helpful in predicting a pulmonary venous site of origin as opposed to a right atrial site of origin. A positive P wave in lead II and III distinguished superior from inferior pulmonary veins (sensitivity 90 %, specificity 84 %). The sensitivity and specificity of negative or biphasic P waves in lead aVL in distinguishing a left from right pulmonary vein site of origin were 94 % and 42 %, respectively. CONCLUSIONS: Analysis of P waves polarity may be helpful in localizing the pulmonary vein that is the site of origin of a premature depolarization. Among the 12 ECG leads, I, II, III, aVL, and V1 are the most helpful in regionalizing premature depolarizations arising in the pulmonary veins.

Electrogram polarity and cavotricuspid isthmus block during ablation of typical atrial flutter.

INTRODUCTION: The atrial activation sequence around the tricuspid annulus has been used to assess whether complete block has been achieved across the cavotricuspid isthmus during radiofrequency ablation of typical atrial flutter. However, sometimes the atrial activation sequence does not clearly establish the presence or absence of complete block. The purpose of this study was to determine whether a change in the polarity of atrial electrograms recorded near the ablation line is an accurate indicator of complete isthmus block. METHODS AND RESULTS: Radiofrequency ablation was performed in 34 men and 10 women (age 60 +/- 13 years [mean +/- SD]) with isthmus-dependent, counterclockwise atrial flutter. Electrograms were recorded around the tricuspid annulus using a duodecapolar halo catheter. Electrograms recorded from two distal electrode pairs (E1 and E2) positioned just anterior to the ablation line were analyzed during atrial flutter and during coronary sinus pacing, before and after ablation. Complete isthmus block was verified by the presence of widely split double electrograms along the entire ablation line. Complete bidirectional isthmus block was achieved in 39 (89%) of 44 patients. Before ablation, the initial polarity of E1 and E2 was predominantly negative during atrial flutter and predominantly positive during coronary sinus pacing. During incomplete isthmus block, the electrogram polarity became reversed either only at E2, or at neither E1 nor E2. In every patient, the polarity of E1 and E2 became negative during coronary sinus pacing only after complete isthmus block was achieved. In 4 patients (10%), the atrial activation sequence recorded with the halo catheter was consistent with complete isthmus block, but the presence of incomplete block was accurately detected by inspection of the polarity of E1 and E2. CONCLUSION: Reversal of polarity in bipolar electrograms recorded just anterior to the line of isthmus block during coronary sinus pacing after ablation of atrial flutter is a simple, quick, and accurate indicator of complete isthmus block.

Prevalence of central venous occlusion in patients with chronic defibrillator leads.

BACKGROUND: Many patients with previously implanted ventricular defibrillators are candidates for an upgrade to a device capable of atrial-ventricular sequential or multisite pacing. The prevalence of venous occlusion after placement of transvenous defibrillator leads is unknown. The purpose of this study was to determine the prevalence of central venous occlusion in asymptomatic patients with chronic transvenous defibrillator leads. METHODS: Thirty consecutive patients with a transvenous defibrillator lead underwent bilateral contrast venography of the cephalic, axillary, subclavian, and brachiocephalic veins as well as the superior vena cava before an elective defibrillator battery replacement. The mean time between transvenous defibrillator lead implantation and venography was 45 +/- 21 months. Sixteen patients had more than 1 lead in the same subclavian vein. No patient had clinical signs of venous occlusion. RESULTS: One (3%) patient had a complete occlusion of the subclavian vein, 1 (3%) patient had a 90% subclavian vein stenosis, 2 (7%) patients had a 75% to 89% subclavian stenosis, 11 (37%) patients had a 50% to 74% subclavian stenosis, and 15 (50%) patients had no subclavian stenosis. CONCLUSIONS: The low prevalence of subclavian vein occlusion or severe stenosis among defibrillator recipients found in this study suggests that the placement of additional transvenous leads in a patient who already has a ventricular defibrillator is feasible in a high percentage of patients (93%).

Physician interpretation of electrocardiographic artifact that mimics ventricular tachycardia.

BACKGROUND: Patients who are misdiagnosed with ventricular tachycardia because of electrocardiographic artifact may be subjected to unnecessary procedures. The purpose of this study was to determine how often electrocardiographic artifact is misdiagnosed as ventricular tachycardia. METHODS: Physicians (n = 766) were surveyed with a case simulation that included a two-lead electrocardiographic monitor tracing of artifact simulating a wide-complex tachycardia. RESULTS: The rhythm strip was not recognized as artifact by 52 of the 55 internists (94%), 128 of the 221 cardiologists (58%), and 186 of the 490 electrophysiologists (38%). One hundred fifty-six of the 181 electrophysiologists (88%), 67 of the 126 cardiologists (53%), and 14 of the 15 internists (31%) who misdiagnosed the rhythm as ventricular tachycardia recommended an invasive procedure for further evaluation or therapy. CONCLUSIONS: This physician survey suggests that electrocardiographic artifact that mimics ventricular tachycardia may frequently result in patients being subjected to unnecessary invasive cardiac procedures. Physicians should include artifact in their differential diagnosis of wide complex tachycardias to minimize unneeded procedures.

Role of transisthmus conduction intervals in predicting bidirectional block after ablation of typical atrial flutter.

INTRODUCTION: Complete bidirectional cavotricuspid isthmus block is the endpoint for ablation of typical atrial flutter. The purpose of this study was to determine whether the extent of prolongation of the transisthmus interval after ablation predicts complete bidirectional block. METHODS AND RESULTS: Fifty-seven consecutive patients underwent 60 ablation procedures for isthmus-dependent atrial flutter. The clockwise and counterclockwise transisthmus intervals were determined before and after ablation during pacing from the low lateral right atrium and the coronary sinus. Bidirectional block was achieved with ablation in 55 (96%) of 57 patients. The transisthmus intervals before ablation and after complete transisthmus block were 100.3 +/- 21.1 msec and 195.8 +/- 30.1 msec, respectively, in the clockwise direction (P < 0.0001), and 98.2 +/- 24.7 msec and 185.7 +/- 33.9 msec, respectively, in the counterclockwise direction (P < 0.0001). An increase in the transisthmus interval by > or = 50% in both directions after ablation predicted complete bidirectional block with 100% sensitivity and 80% specificity. The positive and negative predictive values were 89% and 100%, respectively. The diagnostic accuracy of a > or = 50% prolongation in the transisthmus interval was 92%. CONCLUSION: Prolongation of the transisthmus interval by > or = 50% in the clockwise and counterclockwise directions is associated with a high degree of diagnostic accuracy and an excellent negative predictive value in determining complete bidirectional transisthmus block. This may be a useful and simple adjunctive criterion for assessment of complete transisthmus conduction block.

Complete atrioventricular block after valvular heart surgery and the timing of pacemaker implantation.

The natural history of patients who developed complete atrioventricular block after valvular heart surgery was investigated to determine the optimal timing for pacemaker implantation. Patients who developed complete atrioventricular block within 24 hours after operation, which then persisted for > 48 hours, were unlikely to recover; such patients could potentially undergo earlier pacemaker implantation if otherwise ready for discharge.