Sinus Bradycardia

• Defined as sinus rhythm with a rate < 60 bpm
• Could be normal in adults or children, particularly when at rest or asleep or athletic.
• Pathophysiologic causes include sinoatrial node disease (e.g., from MI or toxins)

Signs and Symptoms

• Often asymptomatic
• May present as weakness, fatigue, lightheadedness, presyncope or syncope

Investigation

• 12-lead ECG

Management

• If asymptomatic → no treatment required but may consider ETT, TSH and/or Holter monitoring
• If symptomatic
  • Haemodynamically unstable → IV atropine 0.5 mg, repeat every 3-5 minutes for a maximum total of 3 mg → transcutaneous pacing if unresponsive to atropine
  • Haemodynamically stable → treat underlying cause (MI, toxicity, hypothyroidism) → if not responsive to treatment → permanent pacemaker

First-Degree AV Block

• PR prolongation > 200 ms with constant PR interval
• Due to delayed or slowed AV conduction
• Can be due to increased vagal tone (physiological) or iatrogenic (e.g. β-blocker, digoxin, rate-limiting calcium channel blocker)

Signs and Symptoms

• Often asymptomatic

Investigation

• 12-lead ECG

Management

• Often not required

Second-Degree AV Block: Mobitz Type I

• Conduction disorder where some P waves fail to conduct ventricular depolarization
• Progressive PR prolongation
• Also known as Wenckebach block
• Can be idiopathic or secondary to MI, cardiomyopathy, myocarditis, medications or increased vagal tone

Signs and Symptoms

• Often asymptomatic
• When the degree of AV block is high, there may be symptoms associated with reduced cardiac output e.g., dizziness, lightheadedness, presyncope or syncope
• Signs of underlying heart disease: MI, myocarditis, or cardiomyopathy

Investigation

• 12-Lead ECG

Management

• Asymptomatic: no immediate treatment required + treatment of the underlying cause if applicable + ECG monitoring every 6-12 months
• Symptomatic
  • Haemodynamically unstable → IV atropine 0.5 mg push (repeat every 3-5 minutes for a maximum total of 3 mg) → if unresponsive to atropine transcutaneous pacing
  • Haemodynamically stable → monitoring ECG with transcutaneous pacing pads and prompt treatment if there is deterioration
  • Long-term management: treat reversible cause, if none identified then permanent pacemaker (dual chamber DDD)

Second-Degree AV Block: Mobitz Type II

• Conduction disorder where some P waves fail to conduct ventricular depolarization
• Common causes: idiopathic, pathological (MI, myocarditis, cardiomyopathy) or iatrogenic (β-blocker, calcium channel blocker, digoxin, adenosine, antiarrhythmic drugs), increased vagal tone
• The area of AV block is often infranodal

Signs and Symptoms

• Often presents with symptoms of a reduced cardiac output → dizziness, lightheadedness, presyncope or syncope

Investigation

• 12-Lead ECG

Management

• Haemodynamically stable → continuous monitoring with transcutaneous pacing pads in place as Mobitz type II often progresses to a complete heart block
• Haemodynamically unstable → β-adrenegic agonist (dobutamine, dopamine or isoproterenol) → transcutaneous pacing if not responsive to medical treatments
• NB: atropine is often avoided as the Mobitz II block is often infranodal → increasing SA node firing will not improve conduction
• Once patient stable → treat underlying cause if found. If no reversible cause found → dual chamber DDD pacemaker implantation

Third Degree or Complete AV Block

• No atrial activities reach the ventricles due to AV nodal or infranodal conduction block
• Common causes similar to second-degree AV blocks e.g. MI, myocarditis, cardiomyopathy or medications

Signs and Symptoms

• Often presents with symptoms of a reduced cardiac output → dizziness, lightheadedness, presyncope or syncope

Investigation

• 12-lead ECG

Management

• Haemodynamically unstable: IV atropine 1 mg STAT + transcutaneous pacing
• Haemodynamically stable: continuous monitoring with transcutaneous pacing pads in place
• Long-term management: treatment of reversible cause → permanent pacemaker implantation if no reversible cause identified

Reference: UpToDate – Atrioventricular Conduction Disease