History and Physical Examination
In patients with Mobitz I second-degree atrioventricular (AV) block, symptoms may vary substantially, ranging from an absence of symptoms in well-trained athletes and persons without structural heart disease to recurrent syncope, presyncope, and bradycardia in patients with heart disease.
Most patients are asymptomatic. Patients may experience light-headedness, dizziness, or syncope, but these symptoms are uncommon. Patients may have chest pain if the heart block is related to myocarditis or ischemia. Patients may have a history of structural heart disease. The presence of absence of symptoms has important therapeutic implications.
In contrast to patients with Mobitz I AV block, those with Mobitz II AV block are more likely to experience light-headedness, dizziness, or syncope, though they may be asymptomatic as well. Patients may have chest pain if the heart block is related to myocarditis or ischemia.
Patients often have a regularly irregular heartbeat. Bradycardia may be present. Symptomatic patients may have signs of hypoperfusion, including hypotension.
Careful auscultation might show variations in the intensity of the first heart sound with variable AV conduction.
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Second-Degree Atrioventricular Block. Typical Mobitz I atrioventricular block with progressive prolongation of PR interval before blocked P wave. Pauses are always less than sum of 2 preceding beats because PR interval after pause always shortens.
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Second-Degree Atrioventricular Block. Mobitz II atrioventricular (AV) block with intermittent periods of 2:1 AV block. If only 2:1 block was seen in beginning of strip, site of block could not be localized with certainty; however, single dropped QRS complex at end of strip with constant PR interval indicates that this block is localized in one of the bundle branches.
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Second-Degree Atrioventricular Block. Variable-ratio Mobitz I atrioventricular block. Note marked PR-interval prolongation in first beat of each cycle. Maximum prolongation of PR interval takes place in second beat of cycle, with much smaller increments in subsequent beats. Also, notice that R-R interval actually shortens with each beat—paradox of shortening R-R interval when PR interval increases by diminishing increments.
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Second-Degree Atrioventricular Block. Sinus rhythm with Mobitz I second-degree 3:2 infranodal atrioventricular (AV) block and bifascicular block. Note that AH interval (indicative of AV nodal conduction) remains constant. HV interval (indicative of His-Purkinje conduction) increases from 65 msec (after first P wave) to 185 msec (after second P wave). Third P wave is followed a His bundle deflection (H) but no QRS complex. AV block occurs in His-Purkinje system below site of recording of His bundle potential. Note shorter PR interval after nonconducted P wave, typical of Mobitz I AV block. HRA = high right atrial electrogram; A = atrial deflection; HB = His bundle electrogram, proximal and distal; H = His bundle deflection; RV = right ventricular electrogram; T = time line, 50 msec.
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Second-Degree Atrioventricular Block. Representative 12-lead electrocardiogram in asymptomatic 78-year-old woman during recent noncardiac surgery. Patient was referred for implantation of permanent pacemaker with diagnosis of sinus tachycardia with 2:1 atrioventricular (AV) block and narrow QRS complex. As sinus rate slowed, 1:1 AV conduction resumed. Intracardiac recordings confirmed diagnosis of infra-Hisian 2:1 AV block.
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Second-Degree Atrioventricular Block. Electrocardiogram of patient with Mobitz I (Wenckebach) second-degree atrioventricular block.
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Second-Degree Atrioventricular Block. Electrocardiogram of patient with Mobitz II second-degree atrioventricular block.
