Arrhythmias: Bradycardia, Heart block, and Bundle branch block Flashcards
What are the two main types of arrhythmia?
Bradycardia <60bpm
Tachycardia >100bpm
Outline the complications of arrhythmias
Palpitations Chest pain Dizziness Syncope Heart failure Sudden death
Describe the function of the Sinoatrial node
A spindle-shaped structure located in the junction between the SVC and RA.
It is the natural cardiac pacemaker that automatically spontaneously depolarises, faster than other pacemaker cells. This initiates the electrical impulse that results in contraction of myocytes. The rate at which the SAN fires determines the heart rate.
Describe the sinoatrial node action potential
- HCN Na channels open, allowing slow depolarisation (known as the funny current)
- Upon reaching threshold, L-type Ca channels open, producing an action potential
- VG K channels open, allowing repolarisation
Explain the mechanisms of arrhythmia production
Accelerated automaticity: an area of myocardial cells depolarises faster than the SAN
Triggered activity: myocardial damage release electrical activity (after-depolarisations), that can reach threshold and produce an arrhythmia.
Re-entry: propagating AP travels retrograde to form a re-entry loop, which repeatedly excites the myocardium. This produces the majority of regular paroxysmal tachycardias.
Explain how the different types of bradycardia occur
Sinus bradycardia: failure of impulse formation
AV block: failure of impulse conduction from the atria to
the ventricles.
Name 4 causes of sinus bradycardia
Extrinsic factors:
- Hypothermia, hypothyroidism
- Cholestatic jaundice
- Raised intracranial pressure
- Drugs: Beta-blockers and other antiarrhythmics
- Neurally mediated syndromes: carotid sinus syndrome, vasovagal syncope, postural tachycardia syndrome
Intrinsic sinus node disease:
- Acute ischaemia and infarction of sinus node
- Chronic degeneration: fibrosis, sick sinus syndrome
- Cardiomyopathies
- Infilitrative disease: sarcoidosis, haemochromatosis
Define normal sinus rhythm
Pacemaker impulses arise from sinoatrial node
Regular rhythm at 60-100 bpm
Each QRS complex is preceded by a normal P wave
PR interval is constant
Narrow-complex QRS (<100 ms)
Define sick sinus syndrome
Abnormal sinus node function with resultant bradycardia and cardiac insufficiency
Define Carotid sinus syndrome
Bradycardia and syncope due to an exaggerated response to carotid sinus baroreceptor stimulation.
Occurs when rotating head.
Define Vasovagal syncope
Syncope resulting from autonomic failure due to physical and/or emotional stress. Causes bradycardia and/or vasodilation.
Define Postural tachycardia syndrome
Abnormal sudden and significant increases in heart rate produced by sitting or standing up. Failure of peripheral vasoconstriction causes dizziness and syncope.
What is the management of vasovagal syncope?
Avoidance of known triggers
Avoid prolonged upright position
Sitting/lying down and applying counter-pressure (pushing palms together, crossing legs) if attack occurs
Increased salt intake, compression hosiery
Fludrocortisone
Disopyramide (negative inotrope)
Differentiate the types of heart block
First-degree: PR interval prolongation (>220ms). All atrial activity is conducted to the ventricles.
Second-degree: Some P waves conduct, others don’t.
- Mobitz I: Progressive prolongation of PR interval, until a P wave fails to conduct.
- Mobitz II: Spontaneous dropping of QRS without progressive PR prolongation (usually wide >120ms).
- 2:1 or 3:1 block: Every 2nd/3rd P wave conducts.
Third-degree: All atrial activity fails to conduct to the ventricles.
Describe the pathophysiology of third-degree heart block
All atrial activity fails to conduct to the ventricles. Slower pacemaker cells become responsible for initiating ventricular contract to sustain life. These are known as spontaneous escape rhythms, and may be narrow- (originating from bundle of His) or broad-complex (originating from ventricular pacemakers).
