A 70 year old female with HIV, diabetes, hypertension, COPD/asthma, and CKD3 presents to the emergency department with a complaint of worsening shortness of breath. She believes her shortness of breath is typical of her COPD exacerbation but notes new, intermittent left-sided chest pressure. The initial lab results are significant only for a BNP of 545. The vitals are notable for bradycardia with a rate in the 40s. The patient's initial EKG (Figure 1).
Interpretation: Rate: 43 bpm; Rhythm: sinus bradycardia with second degree 2:1 block and RBBB (RSR’ in V1-V2 and wide S wave in V4-V6) Axis: left axis deviation (I: pos, II: neg., aVF: neg) Intervals: PR: ~200m–wide ; QRS: 143ms–wide; QTc: 469–increased; P-Waves: present, 2 for every QRS; QRS Complex: wide complex ST Segment/T-waves: no ST elevations or depressions, TWI in V2 These EKG findings are consistent with a fixed 2:1 2nd degree AV block.
Cardiology was consulted and recommended admission for observation and stress testing. During admission, the patient converted to a complete third degree block on repeat EKGs ultimately requiring PPM placement.
Discussion:
The electrical conduction of the heart begins in the SA node, is then conducted through the atrium to the AV node, then down through the bundle of His, followed by distribution through the right and left bundle branches, and finally through the purkinje fibers to left and right ventricles. Conduction disruption at the level of the AV node or Bundle of His causes AV blockade where there is either a delay or failed transmission of impulse from atria to ventricles. AV conduction can be assessed by evaluating the relationship between the P wave and the QRS complex, specifically the distance between the P wave and QRS complex known as PR interval. AV blocks can be congenital or acquired. Congenital causes include structural heart diseases (AV canal defects, transposition of great arteries, tetralogy of Fallot), autoimmune conditions such as SLE, or through maternal viral infection. Acquired causes include myocardial infarction/ischemia, electrolyte abnormalities, myocarditis, Lyme disease, and medications/toxins [1].
AV blocks can be classified into three main types based on severity. First degree blocks can be described as a delayed conduction where every atrial impulse is transmitted to ventricles with a prolonged PR interval. On EKG, there is constant 1:1 association between P waves and QRS complex with constant prolonged PR interval (>200ms). Second-degree AV blocks are subdivided into Mobitz type I and type II. Mobitz type I or Wenckebach occurs at the level of AV node and results in progressive lengthening of the refractory period of AV nodal conduction until a beat is dropped/not conducted from atria to ventricles. On EKG, P waves occur at a consistent rate <100bpm with gradual prolongation of the PR interval until a p-wave is conducted without a subsequent QRS complex (a “dropped beat”). Mobitz type II, involves the His-purkinje system (below the AV node) and is most commonly caused by structural damage of the conducting system (fibrosis, ischemia) and indicates a more serious underlying pathology and higher risk to progress to complete block. On EKG, P waves occur at a consistent rate <100 bpm with consistent PR intervals and non-conducted P waves/QRS complexes that occur at unpredictable intervals. Fixed rate blocks, on the other hand, have non-conducted P waves at predictable intervals. For instance, when every other P wave is non-conducted it is difficult to differentiate between Mobitz I versus Mobitz II; these blocks are classified by their inherent conduction rate and in this case, the block is referred to as a 2:1 AV block. Third degree AV blocks or complete heart block involves total failure of conduction between the atria and ventricles, causing atrial and ventricular rhythms to conduct independently. On EKG, P waves occur at a regular, fast rate and QRS complexes also occur at a regular but slower rate in total dissociation from the other [1].
The patient's EKG in this case demonstrates a 2nd degree fixed AV block with a conduction rate of 2:1, this is two p-waves for every QRS. The atrial rate is approximately 90 bpm. The ventricular rate is 43 bpm. As highlighted in red arrows below, the non-conducted P waves can be seen superimposed on the T waves. The non-conducted p waves also appear in a predictable fashion.
Figure 2. Index EKG with labeled p-waves demonstrating a second degree heart block
Q: How do you differentiate between Wenckebach or Mobitz II AV blocks? Usually conduction of at least two consecutive P waves is required to differentiate between Wenckebach vs Mobitz II – otherwise you’re unable to assess PR interval progression. QRS width can sometimes provide clues to help differentiate. Wenckebach is more likely to produce narrow QRS as blockade occurs at the level of the AV node. These fixed blocks improve with atropine and have a more benign prognosis. Mobitz II are more likely to produce wide QRS as blockade occurs below the AV node and if in the setting of preexisting LBBB or LAFB. These fixed blocks worsens with atropine and more likely to progress to complete heart block or asystole. However, it is important to note that in 25% cases of Mobitz II the QRS is narrow due to the blockade occurring at the level of the bundle of His. Therefore, the ONLY way to be certain is by admitting the patient on telemetry and obtaining serial EKGs to observe PR intervals [2]. In this case, the QRS was prolonged which could support the diagnosis of a fixed 2:1 Mobitz type II AV block. Additionally, during the hospital course, serial EKGs progressed to complete 3rd degree AV block which further supports that the initial EKG was likely fixed 2:1 Mobitz type II AV block.
Take away points:
- Fixed 2nd degree AV blocks present with non-conducted P waves at predictable fixed ratios of P:QRS.
- QRS width can sometimes help differentiate between Mobitz I (narrow) versus Mobitz II( wide) in fixed AV blocks to guide prognosis and assess risk, however it is not always reliable.
- Ultimately it can be very difficult to distinguish whether fixed blocks are Mobitz I or Mobitz II therefore patients should be admitted for telemetry and serial EKGs with cardiology on board.
References:
1. Kashou, A. H. (2024, February 12). Atrioventricular Block. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK459147/
2. Burns, E., Cadogan, M., & Cadogan, E. B. and M. (2024, October 8). AV block: 2nd degree, “fixed ratio” blocks. Life in the Fast Lane • LITFL. https://litfl.com/av-block-2nd-degree-fixed-ratio-blocks/