A 63-year-old man is presented to the emergency department (ED) from a local clinic for evaluation of bradycardia. The patient reports that he is feeling well and is not experiencing any current symptoms, although he has been easily fatigued over the past few days. He states that he is not having chest discomfort, difficulty breathing, or palpitations. His medical history is significant for benign prostatic hypertrophy, diabetes mellitus, hypertension, and a remote history of syncope. En route to the ED, paramedics noted that he was initially tachycardic, with occasional ventricular pauses. Image courtesy of Wikipedia.
What abnormal finding is seen in this ECG?
A. Sinus arrhythmia
B. Atrial fibrillation
C. Atrial flutter
D. Nonconducted P waves
Answer, D. Nonconducted P waves
On physical examination, the patient is afebrile, with a heart rate of 38 beats/min, respiratory rate of 16 breaths/min, and blood pressure of 160/85 mm Hg. He is in no acute distress. His heart rhythm is bradycardic but regular, with a normal S1 and S2 and a III/VI systolic crescendo-decrescendo murmur that is loudest at the left sternal border. No rubs or gallops are appreciated, and the patient has no jugular venous distention. His lungs are clear, and his abdomen is soft and nontender.
The results of the patient's serum laboratory examinations are listed here.
Answer, B. Cardiomegaly
A repeat ECG is obtained while the patient is in the ED. Representative image courtesy of ECG WaveMaven.
What is this patient's diagnosis? (Hint: look closely at the P waves.)
A. Sinus bradycardia
B. Mobitz type II second-degree atrioventricular (AV) block
C. Mobitz type I second-degree AV block
D. Complete heart block
Answer: D. Complete heart block
This patient has third-degree, or complete, heart block, which is a disorder of the cardiac conduction system wherein atrial impulses are not conducted to the ventricles through the AV node. As a result, atrial and ventricular activity are completely dissociated.[1] As the ECG demonstrates, the P waves occur at a regular rate of 85-90 beats/min (red arrows); however, the QRS complexes occur independently of the P waves, at a rate of 30 beats/min (blue arrows). Although impulses are generated regularly by the sinoatrial node, a block somewhere in the conduction system prevents the atrial impulses from being conducted to the ventricles through the His bundle-Purkinje system. Representative image courtesy of ECG WaveMaven.
In complete heart block, the site of conduction dysfunction is most commonly below the His bundle in the Purkinje system (60%). It is less commonly found in the AV node (21%) or the His bundle (14%-18%).[2] A narrow QRS width (<0.12 sec; blue arrow) indicates a block proximal to the His bundle, unless underlying bundle branch block is also present. A widened QRS complex (red arrow) usually indicates a focus at or below the bundle of His. The ventricular rate can also help reveal the site of the escape pacemaker. Pacemakers in the His bundle usually produce rates of 40-60 beats/min, whereas idioventricular pacemakers characteristically produce rates < 40 beats/min and are often unstable.
Worldwide, the prevalence of third-degree AV block is 0.04%.[6] Complete heart block may be acquired or congenital. Children with congenital complete heart block are most often asymptomatic and occasionally require an implanted pacemaker. Acquired complete heart block can result from a variety of causes (nonpharmacologic causes are listed above). Congenital complete heart block usually occurs at the level of the AV node. Patients are relatively asymptomatic but may later develop symptoms because the heart rate cannot adjust for exertion. In the absence of major structural abnormalities, congenital heart block is often associated with maternal antibodies to SSA/Ro and SSB/La.[7]
Patients with complete heart block should be given oxygen with continuous cardiac monitoring. Two large-bore intravenous lines and transcutaneous pacing pads, as shown above, should be placed. The initial diagnostic workup should include a thorough history-taking and physical examination, determination of digoxin and electrolyte levels, and serial electrocardiography with tests of cardiac enzymes.
Treatment of third-degree heart block follows the American Heart Association's Advanced Cardiac Life Support algorithm for bradycardia.[8,9] If the individual has such symptoms as hypotension, altered mental status, chest pain, syncope, or other signs of shock, then treatment is warranted. Practitioners should provide basic treatment, including oxygen, airway maintenance and breathing assistance, ECG monitoring, and intravenous access, and prepare for transcutaneous pacing. Initially treat with atropine 0.5 mg intravenously (up to 3 mg maximum), with epinephrine, dopamine, and transcutaneous pacing as second-line agents if the patient does not respond to atropine.
When complete heart block is treated with permanent pacing, the prognosis is excellent. Complications related to pacemaker insertion are rare (< 1%). Ventricular arrhythmias from atropine or catecholamines may occur. Common complications include those related to line or transvenous pacemaker placement.[14,15]
Despite severe bradycardia, the patient in this case remained asymptomatic. A cardiologist was consulted, and the patient remained in stable condition while a temporary transvenous pacer wire was placed. He was admitted to the cardiac care unit for further diagnostic workup and implantation of a permanent pacemaker. The patient's renal failure and heart block were probably separate and unrelated problems, as there are no current studies in which renal failure in the absence of hyperkalemia or digitalis intoxication has been linked to complete heart block.
Author
Catherine A. Lynch, MD
Assistant Professor
Department of Surgery, Division of Emergency Medicine
Duke University Medical Center
Faculty, Duke Global Health Institute
Durham, North Carolina
Disclosure: Catherine A. Lynch, MD, has disclosed no relevant financial relationships.
Editor
Joseph U. Becker, MD
Co-Director, Medscape Reference Case Presentations
Chief Resident, Division of Emergency Medicine
Department of Surgery, Yale-New Haven Medical Center
New Haven, Connecticut
Disclosure: Joseph U. Becker, MD, has disclosed no relevant financial relationships.
Reviewers
John A. McPherson, MD
Assistant Professor of Medicine
Division of Cardiovascular Medicine
Vanderbilt University School of Medicine
Nashville, Tennessee
Disclosure: John A. McPherson, MD, has disclosed no relevant financial relationships.
Dissection, Aortic
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Strange Skin Patches With Syncope and Palpitations
Subtle EKG Syndromes in Children and Adults
A Worrisome ECG Demanding Timely Action
Worrisome Developments Following Heart Transplantation
