Heart Blocks: Week 2 — Basics, Types, and Management
Overview of Heart Block Classification and Terminology
Heart blocks refer to delays or interruptions in the electrical conduction pathway that coordinates the heartbeat.
Key conduction pathway: atria -> AV node -> bundle of His -> bundle branches -> Purkinje fibers.
The PR interval measures the time for an impulse to travel from the atria to the ventricles; changes in this interval help classify blocks.
Major categories discussed: first-degree AV block, second-degree AV block (Mobitz I/Wenckebach and Mobitz II), third-degree (complete) AV block, and bundle branch blocks.
Important clinical notes:
Some blocks have multiple names (e.g., Mobitz I = Wenckebach; Mobitz II = second-degree type II).
Not all blocks require treatment; decisions depend on rhythm, heart rate, perfusion, symptoms, and underlying cause (e.g., MI, drug toxicity).
New bundle branch blocks may indicate a new myocardial infarction (MI).
First-Degree AV Block
Definition: delay of electrical impulses between the atria and the bundle of His.
Key features:
PR interval is greater than PR > 0.20 \,\text{s}.
PR interval is consistent (no variability from beat to beat).
The QRS complex is usually normal in width.
Rhythm strip appearance: prolonged but stable PR interval with every P wave followed by a QRS.
Clinical significance: usually not treated; may indicate myocardial damage.
Common causes: myocardial infarction (MI) and drug toxicity.
Management and assessment:
Usually observe and monitor.
Carefully assess the patient for underlying myocardium damage or contributing factors.
Second-Degree AV Block Mobitz I (Wenckebach) – Also Known As Mobitz I or Mobitz type I
Names: Mobitz I, Mobitz type I, Wenckebach.
Common term heard: Wenckebach.
Rhythm characteristics:
PR interval gradually lengthens with each beat until a QRS is dropped.
After a dropped QRS, the cycle restarts with a shorter PR interval and repeats.
Example pattern described: PR intervals such as 2.4, 0.30 s, then a QRS drops; then PR returns to 0.20–0.24 s and repeats with another drop.
Mnemonic description (from the transcript):
"Longer, longer, longer, drop, then you have a Wenckebach."
Rhythm features: PR gradually lengthens; occasional dropped QRS; P waves are regular, and some P waves are blocked (QRS is dropped).
Causes and significance:
Usually due to AV nodal delay; can be related to MI, drug effects, or increased vagal tone.
Generally less dangerous than Mobitz II but can progress.
Management:
If heart rate is stable, monitor, oxygen as needed.
Obtain a 12-lead ECG.
IV fluids as needed.
Consider transcutaneous pacing if bradycardia is symptomatic or unstable.
Atropine can be tried for bradycardia; epinephrine if atropine does not achieve adequate rate.
Dopamine infusion if hypotensive.
Second-Degree AV Block Mobitz II (Second-Degree Type II)
Name: Mobitz II (second-degree AV block type II).
Rhythm characteristics:
Intermittent dropped beats occur.
The PR interval of conducted (non-dropped) beats is constant.
P waves are regular; R waves are irregular due to dropped beats.
The overall pattern may cause the R-R intervals to appear irregular because QRS drops occur unpredictably.
Pathophysiology: interruption of conduction near or below the AV node (often below the AV node or at the His-Purkinje system).
Clinical significance: potentially life-threatening; can progress to complete heart block.
Severity indicators: ventricular rate can be less than in severe cases.
Etiology: MI, structural heart disease, or drug toxicity.
Management:
Oxygen as needed.
Obtain a 12-lead ECG.
IV fluids.
If the heart rate is very low (e.g., ventricular rate around ), consider a transvenous pacer or transcutaneous pacemaker.
Epinephrine is generally used for third-degree block (not as a primary treatment for Mobitz II);
atropine may be less effective in Mobitz II.Dopamine drip if hypotensive.
Third-Degree AV Block (Complete Heart Block)
Definition: complete AV dissociation; atria and ventricles depolarize independently.
Rhythm characteristics:
P waves are regular (P-P interval is constant).
R waves are regular (R-R interval is constant).
No relationship between P waves and QRS complexes; PR intervals vary and are not predictive of conduction to the ventricles.
It is common to see more P waves than QRS complexes.
Pathophysiology: the electrical impulse is completely blocked at or beyond the AV node; atria and ventricles pace themselves separately.
Clinical significance: life-threatening if the ventricular rate is too slow to maintain perfusion.
Etiology: MI, severe heart disease, or drug toxicity.
Management:
Oxygen as needed.
Obtain a 12-lead ECG.
IV fluids.
If the ventricles are too slow (e.g., very slow rate), use pacing strategies:
Transvenous pacer or transcutaneous pacer as needed.
Epinephrine is used for third-degree block because atropine generally does not work well here (it may increase SA node activity but won’t fix the AV block and QRS may not increase).
Dopamine infusion if hypotensive.
Bundle Branch Blocks (BBB)
Definition: delay or block in conduction through the bundle of His and/or its branches leading to a widened QRS complex.
Rhythm characteristics:
QRS duration greater than QRS > 0.12\,\text{s}.
QRS may appear broad; sometimes described as having "rabbit ears" on the waveform.
Implications:
There is a delay in impulse conduction through the bundle branches.
May see a widened QRS complex and distinctive morphology.
Management and significance:
There is typically no treatment for an isolated bundle branch block.
However, recognizing a new bundle branch block can indicate a recent or new MI.
Mnemonic and Memory Aids
Poem to remember blocks (as shared in the transcript):
"r if the r is far from the p, then you have a first degree. Longer, longer, longer drop, then you have a Wikibach. If some p's don't get through, then you have a Mobitz two. If p's and q's don't agree, then you have a third degree. For me, it's easier to remember in these rhyme words. Sometimes it clicks in my brain what it is I'm looking for."
Use this rhyme to recall:
First degree: long PR but constant.
Wenckebach (Mobitz I): progressively lengthening PR until a dropped beat.
Mobitz II: fixed PR with dropped beats.
Third degree: complete AV dissociation (P waves and QRS independent).
Practical Management: What the Video Emphasized
General initial steps for non-normal blocks:
Oxygen as needed.
12-lead ECG.
IV access with IV fluids as indicated.
If bradycardia or low perfusion is evident:
Transcutaneous pacing or transvenous pacing may be used.
Epinephrine is particularly highlighted for third-degree block because atropine often does not produce an adequate response.
Dopamine drip can be used if hypotension persists.
For bundle branch blocks:
No specific treatment unless there is symptomatic bradycardia or an evolving MI indicated by a new BBB.
Clinical Significance, Real-World Relevance, and Connections
Understanding these blocks helps in assessing cause and severity in patients with chest pain, MI, syncope, or hypotension.
The PR interval, P-P regularity, and R-R regularity are key to distinguishing blocks.
The progression risk:
Mobitz I can progress sometimes to higher-degree blocks but is often more transient.
Mobitz II has a higher risk of progressing to third-degree block and usually warrants closer monitoring and consideration of pacing.
Third-degree block is a medical emergency if symptomatic due to risk of poor perfusion; pacing is often required.
Etiologies (MI, structural heart disease, drug toxicity) emphasize the overlap between rhythm disturbances and underlying pathology.
Practical implications include decisions about pacing, use of atropine, epinephrine, and dopamine, and recognizing new BBB as a potential MI marker.
Summary and Key Takeaways
First-degree AV block: prolonged but constant PR interval (> PR > 0.20\,\text{s}); usually not treated but warrants assessment for myocardial damage.
Mobitz I (Wenckebach): progressively lengthening PR until a dropped QRS; generally monitored and managed with supportive measures if symptomatic.
Mobitz II: fixed PR with dropped beats; can be life-threatening and progress to complete block; often requires pacing and careful hemodynamic management.
Third-degree (complete) AV block: complete AV dissociation; independent atrial and ventricular activity; emergency management and pacing are often required.
Bundle branch blocks: QRS width > ; no direct treatment if isolated, but new blocks may signal an MI.
Mnemonics from the lecture can aid recall of block types and relationships between P waves and QRS complexes.
Always assess perfusion status, obtain ECG, and tailor treatment to the rhythm type and patient stability.