ECG rhythm recognition and conduction blocks — viva

Stem

Practice viva. The examiner hands you a series of ECG descriptions from patients on a cardiology ward and works through AV block, the level of block, bundle branch block and a wide QRS tachycardia. Answer for non-pregnant adults.

Branch A — Name the AV block

Examiner: The PR interval lengthens beat by beat until a P wave is not conducted. What is this, and how does it differ from Mobitz II?^[2]

Strong answer:

• This is second-degree Mobitz I (Wenckebach) block: ACC/AHA 2018 says Mobitz I block occurs after gradual PR prolongation and Mobitz II does not; both show group beating from dropped QRS complexes.^[2]
• ACC/AHA 2018: the AV node is more likely the site of block with Mobitz I block and a narrow QRS.^[2]
• ESC 2021: supranodal Mobitz I block has a benign course with a low risk of progression, whereas infranodal block, rare in this form, carries a high risk of progression to complete heart block, syncope and sudden death.^[1]

Follow-up: And if every second P wave is blocked?^[2]

• ACC/AHA 2018: 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]
• ACC/AHA 2018: an exercise treadmill test may help differentiate whether 2:1 block is Mobitz type I or II, or identify infranodal disease; exercise improves AV nodal conduction, while infranodal block will not resolve and will likely worsen as the sinus rate increases.^[2]
• 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.^[2]

Branch B — High-grade and complete block

Examiner: Define high-grade AV block. Do ESC and ACC/AHA agree?^[2][1]

Strong answer:

• ACC/AHA 2018: high-grade (high-degree, advanced) block is when two or more consecutive P waves at a normal rate are not conducted, without complete loss of AV conduction.^[2]
• ESC 2021 names advanced AV block also high-grade block, where the P:QRS ratio is 3:1 or higher.^[1]
• ACC/AHA 2018: third-degree block implies no conduction at all from atria to ventricles, may be paroxysmal or persistent, and is usually associated with a junctional or ventricular escape mechanism.^[2]

Follow-up: A patient in AF has a regular ventricular rate of 40 per minute. What do you conclude?^[2]

• ACC/AHA 2018: complete AV block may be imputed in AF when the ventricular response is slow (below 50 bpm) and regular, although a junctional rhythm with AV conduction abnormalities may be associated with the same finding.^[2]
• ESC 2021: pacing is indicated with an atrial arrhythmia (mainly AF) and permanent or paroxysmal third- or high-degree AV block irrespective of symptoms (Class I, Level C).^[1]

Branch C — Who needs a pacemaker?

Examiner: Which AV blocks need pacing even without symptoms?^[1][2]

Strong answer:

• 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]
• Its footnote: in asymptomatic narrow-QRS 2:1 block, pacing may be avoided if supra-Hisian block is clinically suspected (concomitant Wenckebach is observed and the block disappears with exercise) or demonstrated at EPS.^[1]
• ACC/AHA 2018: in acquired Mobitz II, high-grade or third-degree block not attributable to reversible or physiologic causes, permanent pacing is recommended regardless of symptoms (COR I, LOE B-NR).^[2]
• ACC/AHA 2018: in asymptomatic first-degree, Mobitz I or 2:1 block believed to be at the AV node, permanent pacing should not be performed (COR III: Harm, LOE C-LD).^[2]

Follow-up: And first-degree block with a PR of 340 ms?^[1]

• ESC 2021 calls the evidence weak that marked PR prolongation (300 ms or more), particularly when it persists or is prolonged during exercise, can lead to symptoms similar to pacemaker syndrome and/or that these can improve with pacing.^[1]
• ESC 2021: permanent pacing should be considered for persistent symptoms similar to those of pacemaker syndrome and clearly attributable to first-degree AV block (PR above 0.3 s) (Class IIa, Level C).^[1]
• ACC/AHA 2018: in marked first-degree or Mobitz I block with symptoms clearly attributable to the AV block, permanent pacing is reasonable (COR IIa, LOE C-LD).^[2]

Branch D — Bundle branch block

Examiner: Give me the criteria for complete RBBB.^[3]

Strong answer:

• 2009 AHA/ACCF/HRS, adults: QRS 120 ms or more; rsr′, rsR′ or rSR′ in V1 or V2; S wave of greater duration than R wave, or above 40 ms, in I and V6; normal R peak time in V5 and V6 but above 50 ms in V1.^[3]
• The first 3 criteria should be present; with a pure dominant R wave, with or without a notch, in V1, criterion 4 should be satisfied.^[3]

Follow-up: The next day the same patient shows LBBB. What does that mean?^[1][2]

• LBBB and RBBB morphologies on successive ECGs are alternating BBB (ESC 2021); ACC/AHA 2018 calls true alternating BBB evidence for significant infranodal disease, with a high likelihood of sudden complete heart block and a slow or absent escape rate.^[1][2]
• Pacing is indicated with or without symptoms (ESC 2021, Class I, Level C), and ACC/AHA 2018 recommends permanent pacing (COR I, LOE C-LD).^[1][2]

Branch E — Wide QRS tachycardia

Examiner: A haemodynamically stable patient has a regular wide QRS tachycardia and you are not sure of the mechanism. What is your working diagnosis, and which drug do you avoid?^[4]

Strong answer:

• ESC 2019: the default diagnosis should be VT until proven otherwise; reported proportions are 80% VT, 15% SVT with BBB aberration and 5% conduction over an accessory pathway.^[4]
• ESC 2019, among its rows for haemodynamically stable patients, says verapamil is not recommended in wide QRS tachycardia of unknown aetiology (Class III, Level B); ESC 2022 says i.v. verapamil is not recommended in broad QRS tachycardia of unknown mechanism (Class III, Level B).^[4][5]
• ESC 2019 says some drugs used for the diagnosis or treatment of SVT (for example verapamil) can cause severe haemodynamic deterioration in patients with a previously stable VT, so they should only be used when the diagnosis of SVT is fully established and secure.^[4]

Follow-up: Which ECG features would push you towards VT?^[4]

• ESC 2019 Table 9 includes AV dissociation, fusion or capture beats, negative chest lead concordance, absence of RS or RS above 100 ms in the precordial leads, an R wave peak time in lead II of 50 ms or more, and an axis of −90° to ±180°.^[4]
• ESC 2019 adds that several independent studies have found specificities of 40–80% and accuracies of about 75% for various ECG-based methods.^[4]