2:1 AV block with LBBB: finding the level of block — case discussion

Presentation

Practice case (not a real patient). A 71-year-old woman has had three weeks of exertional breathlessness and light-headedness. In clinic her blood pressure is 128/74 mmHg, she is alert, and she has no signs of heart failure; monitoring shows no ventricular arrhythmia. Her ECG shows sinus rhythm at 84 per minute with 2:1 AV block and a ventricular rate of 42 per minute.^[2] The QRS is 148 ms, with broad notched R waves in I, aVL, V5 and V6 and no q waves in I, V5 or V6.^[3] An ECG two years ago showed sinus rhythm with normal conduction, and she takes no rate-slowing drugs.

Step 1 — Is she stable?

Discussion:

• ANZCOR lists adverse signs that suggest a need for immediate treatment of bradyarrhythmia: systolic BP below 90 mmHg, heart rate below 40/min, ventricular arrhythmia and heart failure; she has none of these.^[9]
• ANZCOR markers of a potential risk of asystole are recent asystole, Mobitz II AV block, complete AV block (especially with broad QRS or an initial heart rate below 40/min) and ventricular standstill of more than 3 s; none is present yet.^[9]
• ANZCOR asks for a 12-lead ECG without delaying treatment; here it has been recorded.^[9]

Step 2 — Name what you see

Discussion:

• 2:1 AV block: ACC/AHA 2018 says that when only 2:1 block is present it cannot be classified as Mobitz I or II, so it is important to elucidate the level of block.^[2]
• The QRS meets the 2009 AHA/ACCF/HRS adult criteria shown for complete LBBB: a QRS of 120 ms or more, broad notched or slurred R waves in I, aVL, V5 and V6, and absent q waves in I, V5 and V6.^[3]
• ESC 2021 defines bifascicular block as LBBB, or RBBB with left anterior or posterior fascicular block.^[1]

Step 3 — Where is the block?

Discussion:

• ACC/AHA 2018: 2:1 block with bundle branch block is frequently assumed to be infranodal, but 15% to 20% of these patients can have block in the AV node, so the level still needs to be elucidated.^[2]
• ESC 2021: exercise testing is recommended in patients who experience symptoms suspicious of bradycardia during or immediately after exertion (Class I, Level C), and her symptoms are exertional; in intraventricular conduction disease or AV block of unknown level, exercise testing may be considered to expose infranodal block (Class IIb, Level C).^[1]
• ACC/AHA 2018: in patients with exercise-related symptoms suspicious for bradycardia or conduction disorders, or with 2:1 AV block of unknown level, exercise ECG testing is reasonable (COR IIa, LOE C-LD); she has exertional symptoms and 2:1 block of unknown level.^[2]

On a treadmill test her sinus rate rises to 120 per minute and conduction falls to 3:1.^[2][1]

• ACC/AHA 2018: exercise improves AV nodal conduction; if the baseline block is infranodal, it will not resolve and will likely worsen as the sinus rate increases.^[2]
• ESC 2021: tachycardia-related, exercise-induced second-degree and complete AV block has been shown to be located distal to the AV node and to predict progression to permanent block.^[1]
• A P:QRS ratio of 3:1 or higher is advanced, also named high-grade, AV block in ESC 2021.^[1]

Step 4 — Monitor, look for a cause and image the heart

Discussion:

• ACC/AHA 2018: in the evaluation of patients with documented or suspected bradycardia or conduction disorders, cardiac rhythm monitoring is useful to establish correlation between heart rate or conduction abnormalities with symptoms, with the type of monitor chosen based on the frequency and nature of symptoms and patient preferences (COR I, LOE B-NR).^[2]

• ESC 2021: when a reversible cause of bradycardia is clinically suspected, specific laboratory tests (for example thyroid function tests, Lyme titre, digitalis level, potassium, calcium and pH) are recommended to diagnose and treat these conditions, in addition to pre-implantation tests (complete blood counts, prothrombin time, partial thromboplastin time, serum creatinine and electrolytes) (Class I, Level C).^[1]

• ACC/AHA 2018: transthoracic echocardiography is recommended with newly identified LBBB, Mobitz II, high-grade or third-degree AV block, with or without apparent structural heart disease or coronary artery disease (COR I, LOE B-NR); her LBBB is new.^[2]

• ESC 2021: cardiac imaging is recommended in suspected or documented symptomatic bradycardia, to evaluate structural heart disease, determine LV systolic function and diagnose potential causes of conduction disturbances (Class I, Level C).^[1]

• ESC 2021: multimodality imaging (CMR, CT or PET) should be considered for myocardial tissue characterisation in the diagnosis of specific pathologies associated with conduction abnormalities needing pacemaker implantation, particularly in patients younger than 60 years (Class IIa, Level C); she is 71.^[1]

• Result: her tests show no reversible cause, and the echocardiogram is unremarkable.

Step 5 — Does she need a pacemaker?

Discussion:

• ESC 2021: pacing is indicated in sinus rhythm with permanent or paroxysmal third- or second-degree type 2, infranodal 2:1, or high-degree AV block, irrespective of symptoms (Class I, Level C).^[1]
• Her exercise-induced 3:1 block is advanced (high-grade) AV block in ESC 2021 terms; for this group, ESC 2021 text says that, in the absence of a reversible cause, patients should receive a pacemaker even in the absence of symptoms, and it says tachycardia-related, exercise-induced block has been shown to be located distal to the AV node.^[1]
• ESC 2021: pacing is not recommended when AV block is due to transient causes that can be corrected and prevented (Class III, Level C); none was found here.^[1]
• Pacing mode and device choice are covered in the bradycardia and pacing topic.

Learning points

• When only 2:1 block is present it cannot be classified as Mobitz I or II from the ECG; establish the level (ACC/AHA 2018).^[2]
• Exercise improves AV nodal conduction, while infranodal block will not resolve and will likely worsen as the sinus rate rises (ACC/AHA 2018).^[2]
• Transthoracic echocardiography is recommended with newly identified LBBB, Mobitz II, high-grade or third-degree AV block (ACC/AHA 2018, COR I, LOE B-NR).^[2]