Arrhythmias are traditionally categorized as supraventricular or ventricular based upon simple electrocardiographic (ECG) findings. Supraventricular arrhythmias originate from the atrium or atrioventricular (AV) node and are characterized by normal-appearing QRS complexes unless complicated by an aberrant ventricular condition. Ventricular arrhythmias originate below the AV node and are characterized by abnormal-appearing and prolonged QRS complexes. Disruptions in rhythm and rate occur in seven basic patterns: early beats, bigeminal beats, grouped beats, pauses, bradycardia, tachycardia, and chaotic rhythms. This section provides an approach to bradycardia and tachycardia and discusses the diagnosis and management of specific rhythm disorders.
Bradycardia (heart rate <60/min) may be asymptomatic or associated with symptoms of lightheadedness, syncope, exertional intolerance, dyspnea, and fatigue. It can result from disease in the sinus node, AV node, or His-Purkinje system, or from dysfunction of the autonomic system.
Diagnostic evaluation consists of a thorough history, physical examination, focused laboratory testing (electrolyte levels, thyroid function testing), and resting 12-lead ECG. It is important to identify severe or unstable conduction abnormalities that require urgent intervention. The evaluation should also include investigation for extrinsic and reversible causes of bradycardia, including ischemia, myocarditis, endocarditis, hypothyroidism, electrolyte disturbances, and medication use (especially β-blockers and digoxin). Clues from the history and physical examination may suggest Lyme disease, elevated intracranial pressure, or typhoid as other potential causes of bradycardia. Additional testing may include exercise treadmill testing to assess chronotropic competence and ambulatory ECG monitoring (see Diagnostic Testing in Cardiology).
In patients with nocturnal bradycardia or conduction disturbances, screening for symptoms of sleep apnea should be performed and, if the results are positive, followed by a formal sleep study. In patients with confirmed obstructive sleep apnea, treatment with nocturnal continuous positive airway pressure and weight loss is recommended.
Sinus bradycardia is defined as the presence of sinus rhythm with a heart rate below 60/min. Sinus bradycardia may be appropriate in trained athletes and during sleep. Inappropriate or pathologic sinus bradycardia is most commonly caused by sinus node dysfunction due to age-related myocardial fibrosis. Less commonly, sinus node dysfunction may result from right coronary ischemia; intracranial hypertension; postoperative scarring or fibrosis from cardiothoracic surgery; or infiltrative or inflammatory disorders, such as sarcoidosis. The most common extrinsic cause is use of medications, such as β-blockers or parasympathetic nervous system stimulants (for example, donepezil, neostigmine, and pyridostigmine).
AV block may be classified as first degree, second degree, or third degree. First-degree AV block is defined by a delay in AV conduction (PR interval >200 ms). In large cohort studies, first-degree AV block has been associated with an increased risk for atrial fibrillation and all-cause mortality.
In second-degree AV block, only some P waves conduct to the ventricles. Mobitz type 1 second-degree (Wenckebach) AV block is characterized electrocardiographically by a PR interval that progressively prolongs until a beat is dropped, resulting in grouped beating (Figure 10). Mobitz type 2 second-degree AV block is typified by intermittent nonconducted P waves with unchanging PR intervals (Figure 11). When 2:1 block is present, the Mobitz type cannot be determined. Mobitz type 2 AV block usually occurs below the AV node and has a higher risk for progression to complete heart block.
In third-degree AV block, also termed complete heart block, no P waves conduct to the ventricles. AV dissociation is observed on the ECG (Figure 12).
In patients with symptomatic bradycardia and hemodynamic distress, atropine should be administered. If atropine is ineffective, dopamine or epinephrine infusions can be given until transcutaneous pacing or a temporary pacing wire (preferred) can be placed. Temporary pacing is indicated in cases of hemodynamically unstable bradycardia or asystole.
In hemodynamically stable patients, reversible and extrinsic causes of bradycardia should always be addressed before more invasive measures, such as permanent pacing, are considered. Common indications for permanent pacing include the following:
High-degree AV block refers to the presence of more than one successive nonconducted P wave, resulting in several consecutive P waves without QRS complexes.
The various types of implanted cardiac electronic devices, their functions, and their general indications are reviewed in Table 16. Patients with left bundle branch block or right bundle branch block with or without a prolonged PR interval do not require permanent pacing because intraventricular conduction delays have a low risk for progressing to complete heart block (1%-3% per year).
Patients with tachycardia (heart rate >100/min) may be asymptomatic or experience tachypalpitations, a sensation of skipped beats, lightheadedness or dizziness, chest discomfort, dyspnea, exertional intolerance, fatigue, progressive heart failure, near-syncope, or syncope. In asymptomatic patients, tachycardia may be discovered incidentally during routine ECG, monitoring in the setting of hospitalization, or other medical care.
ECG documentation of tachycardia and correlation with symptoms is the key component of the diagnostic evaluation. After a thorough history and physical examination, a 12-lead ECG should be obtained in all patients with stable tachycardia. A 12-lead ECG recorded during symptoms, although often not possible to obtain, is far superior to most forms of ambulatory monitoring in terms of diagnostic value (see Diagnostic Testing in Cardiology for strategies for selecting an appropriate monitoring device). Thyroid function testing and echocardiography may be considered in selected patients with tachycardia.
Sinus tachycardia (sinus rhythm with a heart rate >100/min) is the most common tachycardia and is typically the result of physiologic demand or distress, including exercise, pain, fever, anemia, or anxiety. Diagnostic evaluation and treatment are guided by the underlying cause.
Inappropriate sinus tachycardia (IST) is a disorder characterized by an elevated resting heart rate, with exaggerated increases in heart rate with light activity. The sinus rate typically decreases during sleep, which can be documented with ambulatory ECG monitoring. IST frequently presents in women in their second to fourth decades and appears to be more common in health care professionals. Symptoms vary and can include palpitations, lightheadedness, syncope (or near-syncope), dyspnea, and fatigue. The diagnosis of IST is based on the exclusion of other causes of tachycardia, such as hyperthyroidism, anemia, pheochromocytoma, and structural heart disease. First-line therapy is removal of aggravating factors and exercise therapy. In patients with bothersome and persistent symptoms, pharmacologic therapy with β-blockers, calcium channel blockers, or ivabradine (in refractory cases) can be considered.
Postural orthostatic tachycardia syndrome (POTS) is another condition that often presents with tachycardia. POTS is a form of dysautonomia characterized by orthostatic intolerance and excessive tachycardia, particularly with standing. Diagnostic criteria for POTS include an increase in heart rate of 30/min or more, or an increase to greater than 120/min, within 10 minutes of standing. The diagnosis is often confirmed with tilt-table testing. Behavioral modification, compression stockings, exercise training, and increased fluid intake are important components of therapy. Medical therapy for POTS is highly variable and may include, but is not limited to, β-blockers, fludrocortisone, selective serotonin reuptake inhibitors, midodrine, and pyridostigmine.
Paroxysmal supraventricular tachycardias (SVTs), including atrioventricular nodal reentrant tachycardia (AVNRT), accessory pathway–mediated tachycardias, and atrial tachycardia, are frequently the cause of palpitations in younger persons. The accessory pathway may result from anterograde conduction, manifesting as a delta wave on ECG or retrograde conduction (so-called concealed accessory pathway). Management of these arrhythmias is discussed later in this chapter.
Older patients with palpitations are more likely to have atrial fibrillation, atrial flutter, or ventricular tachycardia (VT), often due to underlying cardiovascular disease. VT is often associated with hemodynamic compromise; however, some VT can be well tolerated, particularly in patients with normal ventricular function. Evidence of hemodynamic compromise, including syncope, may also be present in patients with atrial tachyarrhythmias. For further discussion of the clinical presentation and management of these conditions, refer to the Atrial Fibrillation, Atrial Flutter, and Ventricular Arrhythmias sections.
Antiarrhythmic agents are used to treat and suppress arrhythmias. These medications have traditionally been organized according to primary mechanism of action by using the Vaughan-Williams classification system (Table 17); however, most antiarrhythmic drugs exert their effects through several mechanisms.
Class I and class III antiarrhythmic drugs are the most effective antiarrhythmic drugs. Class IA agents are indicated for specific conditions (see Table 17). These medications have been associated with ventricular proarrhythmia, sudden death, and increased mortality in patients with coronary artery disease or structural heart disease. Class II agents (β-blockers) and class IV agents (nondihydropyridine calcium channel blockers) are commonly used to inhibit arrhythmia induction and AV conduction in patients with supraventricular or atrial arrhythmias. Class III agents sotalol and dofetilide are used to treat atrial and ventricular arrhythmias. Class III antiarrhythmic therapy should be initiated in an in-patient setting, with regular assessment of the corrected QT (QTc) interval. Ibutilide is an intravenous class III potassium channel blocker that is used for pharmacologic cardioversion of atrial fibrillation.
Amiodarone is among the most effective and commonly used antiarrhythmic drugs. This multichannel blocker is frequently used to treat patients with recurrent VT or atrial fibrillation. Amiodarone has no significant risk for proarrhythmia but is associated with thyroid, liver, lung, and eye toxicities, as well as neurologic side effects. Monitoring thyroid and liver function every 6 months is recommended in patients receiving amiodarone. Patients should also undergo annual pulmonary function testing and ophthalmologic examination. Amiodarone interacts with many drugs, including warfarin, statins, and digoxin. Dronedarone, another multichannel blocker, can be used in patients with intermittent atrial fibrillation and no overt heart failure.
Ranolazine, digoxin, and adenosine are excluded from the Vaughan-Williams classification. Ranolazine is used to treat angina and decreases the risk for atrial fibrillation and ventricular arrhythmias. Digoxin is a positive inotropic agent that also increases vagal activity, leading to a lower resting heart rate. It can be used for rate control in patients with atrial fibrillation. Adenosine is used in the acute treatment of arrhythmias to interrupt AV conduction and terminate SVT. Administering adenosine can also help in determining the type of arrhythmia.
Atrial fibrillation is defined by the presence of disorganized atrial activity with an irregularly irregular ventricular response on ECG (Figure 13). It is the most common sustained arrhythmia, affecting more than 33 million persons worldwide. Lifetime risk for atrial fibrillation is 25% in patients older than 40 years. Incidence is strongly associated with and increases with age. Accordingly, atrial fibrillation is particularly common in the elderly, occurring in 10% of persons older than 80 years. Atrial fibrillation is associated with an increased risk for adverse cardiac events, including a fivefold increased risk for stroke, as well as increased risk for heart failure and dementia. Among patients aged 55 years and older who have a cryptogenic ischemic neurologic event, such as a stroke or transient ischemic attack, occult intermittent atrial fibrillation is thought to be present in up to 25% of cases, and 30-day ambulatory ECG monitoring is indicated for detection. If 30-day ambulatory monitoring is inconclusive, implantation of a cardiac monitor (loop recorder) is reasonable to optimize detection of silent atrial fibrillation.
Atrial fibrillation is usually the result of long-standing risk factors, such as diabetes mellitus, obesity, hypertension, coronary artery disease, heart failure, and obstructive sleep apnea. Risk factor modification is recommended for these patients. The American Heart Association recommends a 10% reduction in weight, BMI of less than 27, and an increase in physical activity as reasonable goals. Bariatric surgery should be considered for appropriate candidates. Smoking cessation, blood pressure and blood glucose control, and management of obstructive sleep apnea also reduce risk. Moderate to heavy alcohol consumption has been shown to increase the risk for atrial fibrillation, and elimination of alcohol may reduce the risk for atrial fibrillation recurrence and total atrial fibrillation burden.
Atrial fibrillation may also be caused by reversible or acute physiologic insults, including cardiac surgery, pulmonary embolism, or hyperthyroidism. When there are no identified risk factors, a predisposing genetic background is often present.
Patients with atrial fibrillation may be asymptomatic or experience palpitations, lightheadedness or dizziness, dyspnea, exercise intolerance, chest pain, near-syncope, or, rarely, syncope. In some cases, atrial fibrillation can lead to hemodynamic compromise, especially in patients with advanced diastolic dysfunction or restrictive cardiomyopathy. Patients with atrial fibrillation uncommonly present with tachycardia-induced cardiomyopathy, characterized by asymptomatic left ventricular dysfunction or overt heart failure.
Atrial fibrillation is categorized according to its duration. Paroxysmal atrial fibrillation stops spontaneously within 7 days of onset, whereas persistent atrial fibrillation lasts for 7 days or more. Long-standing persistent atrial fibrillation is continuous, with a duration of more than 1 year.
Immediate cardioversion is indicated in patients with hypotension, acute myocardial ischemia, or decompensated heart failure. R wave synchronization during cardioversion is necessary to avoid an “R-on-T” event and provocation of ventricular fibrillation (VF). In patients with atrial fibrillation or atrial flutter of more than 48 hours' duration or unknown duration that require immediate cardioversion, anticoagulation should be initiated as soon as possible and continued for at least 4 weeks unless contraindicated. The decision about long-term anticoagulation therapy should be based on the thromboembolic risk profile and bleeding risk profile.
In stable patients, the primary goals of therapy are to prevent stroke, control heart rate, and minimize or eliminate symptoms. Upon diagnosis, reversible causes must be ruled out. All patients should undergo thyroid function testing to evaluate for hyperthyroidism. Patients with risk factors for or symptoms suggestive of sleep apnea should undergo testing (see MKSAP 18 Pulmonary and Critical Care Medicine). Echocardiography is indicated to evaluate for potential valvular or other structural heart disease. Echocardiography can also be used to assess left atrial size, which helps determine the severity of the underlying atrial myocardial dysfunction. Transesophageal echocardiography is reasonable to perform before elective (nonemergent) cardioversion to exclude the presence of left atrial thrombus or left atrial appendage thrombus if the patient has not received adequate anticoagulation therapy (3 weeks' duration) before the procedure.
In patients who are not undergoing cardioversion, intravenous anticoagulation is usually unnecessary; however, oral anticoagulation should be initiated if the patient has sufficient risk factors for stroke. The most common method of assessing stroke risk in nonvalvular atrial fibrillation is by calculating the CHA2DS2-VASc score (Table 18). Patients with valvular atrial fibrillation (mechanical prosthesis or moderate to severe mitral stenosis) require oral anticoagulation regardless of the presence or absence of other risk factors.
If cardioversion is planned, the duration of atrial fibrillation guides therapy. Patients with atrial fibrillation with a known duration of less than 48 hours have a low risk for thrombus formation and subsequent stroke, and preprocedural anticoagulation can be considered in patients who are otherwise at low risk for stroke. Preprocedural anticoagulation is reasonable as soon as possible before cardioversion for men with a CHA2DS2-VASc score of 2 or greater and for women with a score of 3 or greater followed by long-term anticoagulation therapy. In patients in whom the duration of atrial fibrillation is unclear or in whom atrial fibrillation has lasted longer than 48 hours, anticoagulation therapy for 3 weeks is required before cardioversion. In the absence of preprocedural anticoagulation, transesophageal echocardiography can be performed to exclude the presence of left atrial appendage thrombus and facilitate urgent cardioversion. Regardless of the duration or nature of atrial fibrillation, all patients who undergo cardioversion must receive anticoagulation therapy for at least 4 weeks following the procedure owing to an increased risk for thromboembolic events after sinus rhythm is restored.
Pharmacologic or electrical cardioversion should be pursued in patients with significant symptoms despite rate control. In patients without structural heart disease, class IC agents or ibutilide can be considered for pharmacologic cardioversion. Patients treated with ibutilide should be monitored on telemetry for a minimum of 6 hours or until the QTc returns to baseline, owing to a small risk for torsade de pointes.
Heart rate control is necessary in patients with rapid ventricular rates to improve cardiac function and alleviate symptoms. Acutely, the goal heart rate should be between 60/min and 110/min. Commonly used medications include AV nodal blockers, such as metoprolol or diltiazem. Intravenous or oral administration may be appropriate depending on a patient's symptoms. In patients with left ventricular dysfunction, calcium channel blockers should be avoided. Digoxin can be used as adjunctive therapy to improve rate control, especially in patients with heart failure.
Arterial thromboembolic events are the most serious complication of atrial fibrillation and can occur with any form (paroxysmal, persistent, or permanent). In patients with nonvalvular atrial fibrillation, the absolute risk for stroke is approximately 4% per year; however, the presence of comorbidities (such as heart failure, hypertension, diabetes, or vascular disease) can increase the risk 15- to 20-fold. Hypertension is associated with an increased risk for both atrial fibrillation and stroke; therefore, blood pressure control is critical in the management of atrial fibrillation.
Stroke prevention with antithrombotic therapies is dependent on the patient's risk for stroke and risk for bleeding. Although several risk stratification scores are available, current guidelines recommend the use of the CHA2DS2-VASc score in patients with nonvalvular atrial fibrillation. Adjusted stroke rates and recommendations for antithrombotic therapy based on the CHA2DS2-VASc score are shown in Table 18. The 2019 American College of Cardiology/American Heart Association/Heart Rhythm Society focused update on atrial fibrillation recommends anticoagulation to prevent stroke in patients with nonvalvular atrial fibrillation who have a CHA2DS2-VASc score of 2 or greater in men or 3 or greater in women. The American College of Chest Physicians guideline on antithrombotic therapy for atrial fibrillation recommends anticoagulation for patients with one or more non-sex CHA2DS2-VASc stroke risk factors (score of ≥1 in men or ≥2 in women). Patients with valvular atrial fibrillation (moderate to severe mitral stenosis or mechanical valve replacement) should receive warfarin. Non–vitamin K antagonist oral anticoagulants (NOACs) are not approved for use in valvular atrial fibrillation. However, patients with atrial fibrillation and other valvular lesions (aortic valve disease, mitral regurgitation, tricuspid regurgitation, and mild mitral stenosis) are eligible for NOAC therapy.
Bleeding scores, such as the ATRIA, HAS-BLED, and ORBIT scores, may be used to identify patients with significant bleeding risk based on patient characteristics, including anemia, hypertension, labile INR, older age, kidney insufficiency, and treatment with antiplatelet medications. Patients with elevated risk (HAS-BLED score ≥3) should undergo more frequent and regular reviews or evaluation.
Several oral anticoagulants are available for stroke prevention in patients with atrial fibrillation. Vitamin K antagonism with dose-adjusted warfarin is an effective, low-cost therapy; however, warfarin has limitations, including the need for frequent monitoring and adjustment and numerous food and drug interactions. The safety and efficacy of warfarin therapy depend on the time the patient is in the therapeutic range (INR 2-3).
NOACs are recommended in preference to warfarin in NOAC-eligible patients with atrial fibrillation. Four NOACs are approved for the prevention of stroke in atrial fibrillation (Table 19). Dabigatran, an oral direct thrombin inhibitor, is superior to warfarin for the prevention of ischemic stroke and results in less intracranial bleeding. Patients taking dabigatran have a higher risk for gastrointestinal bleeding relative to warfarin and may experience dyspepsia. Rivaroxaban, a direct factor Xa inhibitor, is noninferior to warfarin in the prevention of stroke or systemic embolism and is associated with less intracranial and fatal bleeding. As with dabigatran, patients taking rivaroxaban have a higher risk for gastrointestinal bleeding compared with those taking warfarin. Apixaban, another oral factor Xa inhibitor, is superior to warfarin for the prevention of stroke and confers less risk for major bleeding, including intracranial bleeding. Edoxaban is noninferior to warfarin for stroke prevention and is associated with less major bleeding. Kidney and liver function should be evaluated before initiation of a NOAC and reevaluated at least annually. See MKSAP 18 Hematology and Oncology for recommendations regarding NOAC dosing in patients with chronic kidney or liver disease. All of the NOACs have shorter half-lives than warfarin; however, there are no quick, readily available serum assays to accurately determine anticoagulant activity. Reversal agents and antidotes continue to be developed for these agents. Andexanet alfa or 4-factor prothrombin complex concentrates are recommended for life-threatening bleeding due to rivaroxaban, apixaban, or edoxaban. Idarucizumab is a dabigatran-reversal agent available for emergency invasive or surgical procedures or in cases of uncontrolled or life-threatening bleeding. The management of warfarin overdose and warfarin-related bleeding is discussed in MKSAP 18 Hematology and Oncology.
In patients with atrial fibrillation undergoing percutaneous coronary intervention for acute coronary syndrome, both anticoagulant and antiplatelet therapies are necessary, and these patients with a CHA2DS2-VASc score of 2 or greater can be treated with “double therapy” (clopidogrel or ticagrelor plus warfarin, rivaroxaban, or dabigatran) rather than “triple therapy” (an oral anticoagulant, aspirin, and P2Y12 inhibitor) to reduce the risk for bleeding. In patients with atrial fibrillation and stable coronary artery disease, treatment with rivaroxaban alone is noninferior to rivaroxaban plus aspirin in the prevention of the composite end point of stroke, systemic embolization, myocardial infarction, need for revascularization, or death from any cause. Rivaroxaban monotherapy is also associated with significantly less bleeding.
Approximately 10% to 25% of patients with atrial fibrillation have contraindications to oral anticoagulation or discontinue therapy for various reasons, including bleeding events. In patients who are at moderate to high risk for stroke (CHA2DS2-VASc score ≥3), left atrial appendage occlusion to prevent stroke and systemic thromboembolism can be considered. Occlusion of the left atrial appendage can be achieved percutaneously with a self-expanding device that is implanted in the left atrial appendage or with surgical closure. Left atrial appendage occlusion has a lower risk for intracranial bleeding compared with dose-adjusted warfarin.
Studies have not demonstrated that sinus rhythm restoration is superior to rate control alone. Consequently, the decision to initiate a rate or rhythm control strategy is predominantly based on symptoms, patient age, and patient preference. Rate control can be used to manage asymptomatic patients, with a resting heart rate goal of less than 80/min. A goal of less than 110/min may be considered in select patients without left ventricular dysfunction. β-Blockers, calcium channel blockers, and digoxin can be used to control the ventricular rate in patients with atrial fibrillation. Combination therapy may be needed to adequately control heart rate. Aside from resting heart rate assessment, evaluation of the heart rate with activity, such as with a 6-minute walk test, stress test, or 24-hour ambulatory ECG monitoring, should be performed.
A rhythm control strategy can improve quality of life in patients who continue to have symptoms despite adequate rate control. Because the long-term effects of rate control are unknown, rhythm control is often pursued in younger patients (aged <50 years) with minimal symptoms. Rhythm control may require cardioversion in addition to antiarrhythmic therapy. Antiarrhythmic drug selection is guided by the patient's comorbid conditions and safety considerations. Patients with infrequent atrial fibrillation who have no structural heart disease or conduction disease often benefit from a “pill-in-the-pocket” approach. With this strategy, patients take a class IC drug (flecainide or propafenone) at the onset of an episode of atrial fibrillation. These patients should be receiving β-blocker or calcium channel blocker therapy or should take one of these medications before taking the “pill in the pocket.” Pill-in-the-pocket therapy should be initiated in a monitored setting to ensure patient safety.
Catheter ablation with pulmonary vein isolation is an effective rhythm control therapy in patients with recurrent symptomatic atrial fibrillation despite antiarrhythmic drug therapy. Catheter ablation is most effective in patients without significant left atrial enlargement and multiple comorbid conditions. Seventy percent to 90% of patients with paroxysmal atrial fibrillation are symptom-free 1 year after the procedure; however, success rates vary. Complications include thromboembolism (0.5%-1% risk), tamponade, and vascular complications (such as insertion hematoma, pseudoaneurysm, arteriovenous fistula, and retroperitoneal bleeding). Longer-term complications, such as pulmonary vein stenosis, are uncommon.
AV node ablation is an option in patients with atrial fibrillation who have continued symptomatic tachycardia despite rate and rhythm control therapy. Therapeutic ablation of the AV node requires implantation of a permanent pacemaker. These patients remain in atrial fibrillation and still require anticoagulation.
Atrial flutter is an organized macro-reentrant tachycardia with discrete regular atrial activity on ECG, usually with a rate of 250/min to 300/min. Typical atrial flutter is characterized electrocardiographically by a sawtooth pattern with inverted flutter waves in leads II, III, and aVF and positive flutter waves in lead V1 (Figure 14). Typical atrial flutter is the result of counterclockwise reentry around the tricuspid annulus. In atypical flutter, the circuit can travel in a clockwise direction or can occur in other locations in the right and left atria. Atypical flutter may occur after ablation or after congenital or valvular cardiac surgery.
Management of atrial flutter is similar to atrial fibrillation management; however, a rhythm control strategy is favored in atrial flutter because rate control may be difficult and often requires high doses of more than one AV nodal blocker. Catheter ablation is the definitive treatment for typical atrial flutter, owing to a very high success rate (>95%) and low complication rate. Oral anticoagulation in patients with atrial flutter is approached in the same manner as in patients with atrial fibrillation.
SVTs are rapid heart rhythms that arise from the atrium or require conduction through the AV node. Atrial fibrillation and atrial flutter are technically SVTs, although the term generally pertains to paroxysmal SVTs. SVTs can affect all age groups but frequently occur in younger patients. Prevalence is higher in women than in men. SVTs usually occur in the absence of structural heart disease, although echocardiography should be performed to exclude underlying cardiac dysfunction or structural defects. Patients often have repeated episodes of tachycardia and may report palpitations, a sensation of pounding in the neck, fatigue, lightheadedness, chest discomfort, dyspnea, presyncope, and, less commonly, syncope.
The ECG typically demonstrates a narrow-complex tachycardia; however, wide QRS complexes (>120 ms) may be present in cases of bundle branch block, aberrancy, pacing, or anterograde accessory pathway conduction (antidromic tachycardia). SVTs may be classified electrocardiographically according to the relationship of the P wave and the QRS complex. Short-RP tachycardias (RP interval < PR interval) feature a P wave that closely follows the QRS complex, whereas long-RP tachycardias feature a P wave that is more than half the distance between the RR interval (RP interval > PR interval). Short-RP tachycardias include typical AVNRT, AVRT, and junctional tachycardia. Long-RP tachycardias include atypical AVNRT, sinus tachycardia, and atrial tachycardia.
Vagal maneuvers, including the Valsalva maneuver (bearing down), carotid sinus massage, or facial immersion in cold water, are first-line therapy to restore sinus rhythm in patients with SVT. Adenosine can be used to terminate SVT and simultaneously help diagnose its cause. Tachycardias that terminate with adenosine are typically AV node dependent (AVNRT and AVRT), whereas continued atrial activity (P waves) during AV block is consistent with atrial flutter or atrial tachycardia.
AVNRT accounts for two thirds of all cases of SVT, not including cases of atrial fibrillation and flutter. AVNRT is the result of a reentrant circuit within the AV node that uses both the fast and slow pathways. Typical AVNRT involves conduction down the slow pathway and back up to the atrium over the fast pathway (slow-fast). This conduction pattern results in a short RP interval with a retrograde P wave inscribed very close to the QRS complex (Figure 15). Atypical AVNRT occurs when the impulse travels down the fast pathway and returns to the atrium via the slow pathway (fast-slow), resulting in a long RP interval.
AVNRT may be terminated with vagal maneuvers or adenosine. AV nodal blocking therapy with β-blockers or calcium channel blockers is used to prevent recurrent AVNRT. In patients with recurrent AVNRT and patients who do not tolerate or prefer to avoid long-term medical therapy, catheter ablation should be considered. Catheter ablation of AVNRT has a high success rate, although it is associated with a 1% risk for injury to the AV node necessitating pacemaker implantation.
AVRT is an accessory pathway (bypass tract)–mediated tachycardia that is often observed as preexcitation on ECG (Figure 16). Early ventricular activation over the accessory pathway causes shortening of the PR interval, and the initial part of the QRS complex is slurred because of premature ventricular depolarization in the myocardial tissue adjacent to the accessory pathway. In AVRT, the tachycardia can conduct anterograde over the AV node (orthodromic AVRT) or anterograde over the accessory pathway (antidromic AVRT). Orthodromic AVRT is the most common type of AVRT, accounting for more than 90% to 95% of cases. This type of AVRT has a narrow QRS complex, owing to conduction over the AV node and the His-Purkinje system. Antidromic AVRT is characterized by a wide, slurred QRS complex resulting from conduction over the bypass tract and activation of the ventricle without use of the specialized conduction system. Adenosine can be given to terminate orthodromic AVRT; however, adenosine and other AV nodal blockers are contraindicated in cases of antidromic AVRT.
Wolff-Parkinson-White (WPW) syndrome is defined by symptomatic AVRT with evidence of preexcitation on resting ECG (delta wave). It is often seen in patients with Ebstein anomaly. Atrial fibrillation occurs in up to one third of patients with WPW syndrome. Rapid conduction over an accessory pathway in atrial fibrillation can result in VF and sudden cardiac death (SCD), although this occurs in less than 1% of cases of WPW syndrome.
In patients with preexcitation on ECG, stress testing can effectively risk-stratify patients; patients in whom preexcitation is lost during exercise are generally at low risk for ventricular arrhythmia and SCD. Electrophysiology testing is also helpful in determining the risk for SCD and in localizing the pathway to facilitate catheter ablation. Catheter ablation is first-line therapy for patients with WPW syndrome and has a high success rate; however, ablation success is dictated by the location of the bypass tract. Antiarrhythmic therapy is considered second-line therapy. In patients with accessory pathways close to the AV node, antiarrhythmic drug therapy is particularly useful because catheter ablation carries an unacceptable risk for iatrogenic heart block.
In asymptomatic patients with preexcitation on ECG, management is controversial. Invasive testing is generally not required, unless the patient has a high-risk occupation, such as a commercial airline pilot.
Premature atrial contractions (PACs) are early isolated beats that arise from the atria. They are exceedingly common, and their frequency increases with age. During ambulatory ECG monitoring, only 1% of persons have no PACs. High PAC burden is associated with increased risk for atrial fibrillation. Symptomatic PACs are typically treated with β-blockers or calcium channel blockers.
Atrial tachycardia can arise in the presence or absence of structural heart disease. Drugs can cause atrial tachycardia; digoxin toxicity can cause paroxysmal atrial tachycardia with atrioventricular block. Stopping the causative drug, or initiating β-blocker or calcium channel blocker therapy, is first-line treatment for symptomatic atrial tachycardia. Second-line treatment consists of catheter ablation or antiarrhythmic drug therapy. Ablation success rates are generally lower in patients with atrial tachycardia than in patients with other SVTs.
Multifocal atrial tachycardia is typified by three or more P-wave morphologies and a heart rate greater than 100/min (Figure 17). It is usually seen in patients with end-stage COPD.
A wide-complex tachycardia is any tachycardia with a QRS complex of 120 ms or greater. Differential diagnoses include SVT with aberrancy, preexcited tachycardia (antidromic tachycardia), ventricular paced rhythm, and VT.
In adult patients with structural heart disease, 95% of wide-complex tachycardias are VT. Several important clinical and ECG features can distinguish VT from other conditions. Wide-complex tachycardias that are positive in lead aVR, have a QRS morphology that is concordant (all predominantly positive or negative) in the precordial leads, have QRS morphology other than typical right or left bundle branch block, and exhibit extreme axis deviation (“northwest” axis) are usually VT. AV dissociation, fusion beats, and capture beats are all highly suggestive of VT (Figure 18). When the origin of a wide-complex tachycardia cannot be determined, VT should be assumed until expert consultation can be obtained.
Premature ventricular contractions (PVCs) occur in up to 75% of healthy persons. Symptoms include palpitations or the perception of skipped beats. Forceful beats are caused by increased cardiac filling during the pause following the PVC. PVCs are more common in patients with hypertension, left ventricular hypertrophy, previous myocardial infarction, and other forms of structural heart disease, such as nonischemic cardiomyopathy.
In the absence of high-risk features (syncope, a family history of premature SCD, structural heart disease), reassurance is appropriate management. PVCs require treatment when symptoms are bothersome or frequent (>10% of all beats or 10,000 PVCs per day). Tachycardia-induced cardiomyopathy may result from frequent PVCs (see Heart Failure).
First-line treatment for PVC suppression is β-blocker or calcium channel blocker therapy. β-Blockers are preferred in patients with ventricular dysfunction. If PVCs persist despite β-blockade or calcium channel blockade, antiarrhythmic drug therapy may be used. The selection of an antiarrhythmic medication for PVC suppression depends on many factors, including kidney function and comorbid conditions. In young healthy patients without structural heart disease, class IC drugs are usually effective. Amiodarone is most commonly used in patients with structural heat disease, particularly heart failure. Catheter ablation should be considered in patients with continued frequent PVCs despite medical therapy, those who cannot tolerate medical therapy, and patients who develop PVC-related left ventricular dysfunction.
In ischemic and nonischemic cardiomyopathy, the presence of myocardial scar tissue facilitates a reentry circuit and the development of VT. VT is usually regular and monomorphic in patients with ventricular scarring. Sustained VT (≥30 seconds) can lead to hypotension, syncope, VF, and cardiac arrest; however, short episodes of VT (nonsustained) or slow sustained VT may be well tolerated or cause no symptoms.
Evaluation with resting ECG, exercise treadmill testing (to provoke arrhythmias), and cardiac imaging (to identify structural heart disease) is indicated in all patients with VT. Patients with ischemic cardiomyopathy who present with VT should be considered for angiography and revascularization if appropriate. Cardiac magnetic resonance imaging clarifies the extent and pattern of myocardial scarring, which can be helpful in refining the cause of the cardiomyopathy and can assist in determining prognosis. For example, patients with a higher burden of myocardial scarring have higher risk for recurrent arrhythmia.
β-Blockers and ACE inhibitors reduce the risk for SCD in patients with prior myocardial infarction and cardiomyopathy. In those with recurrent VT despite β-blocker therapy, antiarrhythmic drug therapy with amiodarone may be considered. Catheter ablation should be considered in patients with recurrent VT despite medical therapy. ICD placement is indicated for secondary prevention of SCD in patients with structural heart disease or cardiomyopathy who have sustained VT/VF, provided that reversible causes have been excluded (such as acute coronary ischemia or cocaine ingestion).
Idiopathic VT (so-called normal heart VT) occurs in the absence of structural heart disease, typically arising from the outflow tracts, fascicles, and papillary muscles. Patients with idiopathic VT usually present with palpitations in the third to fifth decades of life. Episodes of syncope are uncommon. Arrhythmic events are often triggered by stress, emotion, or exercise. These tachycardias are responsive to adenosine.
Calcium channel blockers, especially verapamil, and β-blockers are first-line treatment for idiopathic VT. Catheter ablation can be considered if symptoms continue despite these therapies. ICDs are contraindicated in patients with hemodynamically stable idiopathic VT, owing to the benign prognosis and efficacy of other therapies.
Patients younger than age 40 years with unexplained SCD, unexplained near drowning, or recurrent exertional syncope, who do not have ischemic or other structural heart disease, should be evaluated for inherited arrhythmia syndromes. Additionally, unexplained premature death (age <35 years) or sudden death in a first-degree family member (age <40 years) should raise suspicion for an inherited arrhythmia syndrome and prompt referral to a cardiovascular specialist, with genetic counseling and genetic testing as indicated by clinical findings. The diagnosis of inherited arrhythmia syndromes can be complicated because of variable penetrance and expressivity of these disorders. Characteristic findings and treatments for these syndromes are reviewed in Table 20.
Long QT syndrome is among the most common inherited arrhythmias, affecting between 1 in 1000 and 1 in 5000 persons. Prolongation of the QTc interval has many causes, most of which are acquired, such as medication use, structural heart disease, and electrolyte abnormalities. Drugs that have been implicated in QT prolongation include antiarrhythmic agents, antibiotics (including some macrolides and fluoroquinolones), antipsychotic drugs, and antidepressants. A list of drugs categorized by their potential to cause QT prolongation is available at https://crediblemeds.org. The presence of a prolonged QTc interval (>440 ms in men and >460 ms in women) alone is insufficient to diagnose long QT syndrome. Diagnosis requires the presence of a QTc interval greater than 500 ms on repeated 12-lead ECGs accompanied by unexplained syncope or ventricular arrhythmia. Patients with a QTc interval greater than 500 ms are at greatest risk for SCD. β-Blockers are first-line therapy; however, patients with cardiac arrest or those who have recurrent events (syncope or VT) refractory to β-blocker therapy are candidates for ICD placement. These patients should not participate in competitive athletics.
Short QT syndrome is a rare and genetically heterogeneous disorder characterized by a short QT interval (QT <340 ms or QTc <350 ms). It is inherited in an autosomal dominant pattern. Patients can present with atrial and ventricular arrhythmias (including atrial fibrillation, polymorphic VT, and VF) and syncope. Patients with short QT syndrome are considered to be at very high risk for SCD; therefore, ICD placement is recommended in all patients.
Brugada syndrome is distinguished by right precordial ECG abnormalities, including ST-segment coving (concave or linear downsloping ST segment) in leads V1 to V3 with or without right bundle branch block, VF, and cardiac arrest (Figure 19). Brugada syndrome has an increased prevalence in men and persons of Asian descent. Arrhythmic events in patients with Brugada syndrome are more common at night during sleep. Abnormalities on ECG can be intermittent and may be elicited by fever or pharmacologic challenge with sodium channel blockade (such as with procainamide infusion). Patients with syncope or ventricular arrhythmia should undergo ICD implantation. Quinidine may be beneficial in patients with recurrent ventricular arrhythmias and/or ICD shocks.
Catecholaminergic polymorphic VT is a rare disorder characterized by intracellular calcium overload, polymorphic ventricular arrhythmias, and cardiac arrest. The arrhythmias are usually triggered by high-adrenergic states, including strong emotion and exercise. These arrhythmias can also be provoked with epinephrine infusion. β-Blocker therapy and ICD placement are treatments. Patients with catecholaminergic polymorphic VT should avoid exercise.
Early repolarization syndrome should be strongly suspected in patients with unexplained VF arrest, particularly when provoked during exercise. Early repolarization (J-point elevation) is a common and usually benign ECG finding; however, the presence of inferior and lateral early repolarization of more than 1 mm in a patient with VF and/or cardiac arrest should be considered early repolarization syndrome. ICD implantation is indicated in patients with VF or cardiac arrest.
Hypertrophic cardiomyopathy or arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) can often present as SCD in young persons. Hypertrophic cardiomyopathy and arrhythmic risk stratification are discussed in Myocardial Disease. Most patients with ARVC/D present between puberty and young adulthood; however, it can also occur in older age. Patients with ARVC/D usually present with frequent ventricular ectopy and/or monomorphic VT, although severe cases can present with heart failure. The diagnosis is established by ECG abnormalities, family history, the presence of arrhythmias, and structural abnormalities of the right ventricle. Cardiac magnetic resonance imaging can demonstrate enlargement (segments of poorly contracting heart muscle), focal aneurysms, and wall motion abnormalities in the right ventricle (hypokinesis). ARVC/D is usually progressive, and patients with ARVC/D should abstain from vigorous exercise. Patients with ARVC/D and cardiac arrest or risk factors (nonsustained VT, inducible VT) should be offered ICD implantation. β-Blockers are first-line therapy for ventricular arrhythmia, although antiarrhythmic therapy with sotalol or amiodarone or catheter ablation is often required for recurrent VT.
SCD is defined as an instantaneous fatal event or collapse within 1 hour of symptom onset in an apparently healthy person. In patients in whom death was unwitnessed, SCD is considered to have occurred if the patient was known to be alive and well within the past 24 hours. VT and VF are the most common causes of SCD.
In the United States, more than 350,000 episodes of SCD occur each year. The annual risk for SCD is 1:1000 in the general population. The highest incidence occurs in patients with pre-existing structural heart disease, although left ventricular function is normal in most patients experiencing SCD. Risk factors include heart failure, diminished left ventricular function, previous myocardial infarction, unexplained syncope, left ventricular hypertrophy, nonsustained ventricular arrhythmia, chronic kidney disease, and sleep apnea. It is important to distinguish between myocardial infarction and SCD when a family history of cardiac disease is obtained.
Cardiac arrest necessitates immediate cardiopulmonary resuscitation (CPR) and advanced cardiac life support. Basic life support guidelines emphasize the importance of immediate, rapid, and sustained chest compressions in caring for individuals with cardiac arrest. Following activation of the emergency medical system and request for an automated external defibrillator, the patient's pulse should be checked immediately. Chest compressions should be initiated if no definite pulse is detected within 10 seconds. High-quality CPR (30 compressions and 2 breaths per cycle if no advanced airway is present) includes adequate depression of the lower sternum at a rate of at least 100 compressions per minute with adequate time for chest recoil. Interruptions in chest compressions should be minimized. Once an airway has been secured, breaths should be delivered at a rate of 1 breath per 6 seconds (10/min) to avoid breath stacking and increased thoracic pressure, which impedes cardiac output. Defibrillation should occur as soon as possible in patients with a shockable rhythm because time to defibrillation is an important determinant of the likelihood of survival to hospital discharge.
According to the 2020 American Heart Association advanced life support guidelines, the presence or absence of a shockable rhythm guides management after CPR has been initiated. In patients with asystole or pulseless electrical activity, CPR is continued with reassessment of rhythm status for a shockable rhythm every 2 minutes. Epinephrine should be administered intravenously to increase coronary perfusion. Vasopressin provides no advantages over epinephrine. Likewise, atropine should not be used for the treatment of asystole or pulseless electrical activity arrest. Any reversible causes (such as tamponade) should be identified and treated.
Patients with VT/VF should be shocked, followed by immediate resumption of CPR and reassessment of the rhythm in 2 minutes. Epinephrine should be administered after the second shock and every 3 to 5 minutes thereafter. Amiodarone should be given as a bolus if VT/VF continues despite three shocks and epinephrine administration. A second dose of amiodarone can be given if VT/VF persists.
Patients with suspected opioid overdose should receive naloxone, but emergency response systems should be requested immediately and should not be dependent on an observed response to the naloxone.
ICDs have demonstrated efficacy in the primary and secondary prevention of SCD. Patients with sustained ventricular arrhythmias (>30 seconds) or cardiac arrest without a reversible cause have a class I recommendation for secondary prevention ICD placement. Patients with heart failure who meet specific criteria should undergo ICD placement for primary prevention (see Heart Failure). Patients with heart failure and interventricular conduction defects (predominantly left bundle branch block) often benefit from cardiac resynchronization therapy or cardiac resynchronization therapy in combination with a defibrillator.
Patients with ICDs need to avoid strenuous upper extremity exercises, including weight lifting, because of concern for lead stress and subsequent fracture. Inappropriate detection of VT/VF and shocks can result from electromagnetic interference; therefore, patients need to avoid devices that pose risks, such as arc welding equipment and high-voltage machinery. Patients with ICDs who are undergoing invasive procedures or surgery should be evaluated by their electrophysiologist for device reprogramming recommendations.
In the past, ICDs were implanted almost exclusively by using a transvenous approach. New techniques allow for implantation of defibrillators in the lateral chest at the midaxillary line adjacent to the heart with tunneling of the lead under the skin next to the sternum. Subcutaneous defibrillators have several advantages, including reduced risk for device infection.
Device infections have many different forms, ranging from pocket infections to endocarditis. Most infections are due to Staphylococcus epidermidis and Staphylococcus aureus. Symptoms of cardiac device infection include fever, chills, malaise, lassitude, and failure to thrive, particularly in the elderly.
Physical examination of the pocket may reveal erythema, swelling, drainage, or wound dehiscence. In patients suspected of having device infection, several blood cultures, an erythrocyte sedimentation rate, and a C-reactive protein level should be obtained. A transesophageal echocardiogram should be obtained to evaluate for intracardiac or lead vegetations. Aspirating the device pocket is never indicated because this can damage the leads or introduce infection in a sterile or uninfected pocket. PET-CT can also identify infection of the device pocket or leads if other testing is inconclusive.
Treatment of cardiac device infection includes complete extraction of all hardware, debridement of the pocket, sustained antibiotic therapy, and reimplantation at a new location after infection has been eradicated.