28. Conductive disturbances. Treatment of conductive disturbances.
Covers the conduction system of the heart and how slowing or blocking of the impulse at the sinus node, AV node, His bundle or bundle branches produces bradyarrhythmias or abnormal QRS complexes. It then sets out a classification table, the causes, and the ECG forms of sinus node dysfunction and AV block, including a degree-by-degree table with rhythm strips. Bundle branch and fascicular blocks follow, with a comparison table, a memory aid, and a note on long QT. It ends with diagnosis, acute and permanent pacing, and prognosis.
Topic 28 of 139 in the MU Sofia syllabus for Internal Diseases and Therapy (Medicine, Years 4 and 5). The free AceMed note covers it in 10 sections, about 1,564 words (15 min of reading), with 16 flashcards.
In this note
Definition and the conduction system
Classification
Etiology
Sinus node dysfunction (sick sinus syndrome)
Atrioventricular block
Bundle branch and fascicular blocks
Long QT (disorder of repolarisation)
Diagnosis
Treatment
Prognosis
Definition and the conduction system
Conduction disturbances: slowed or blocked spread of the cardiac impulse at the level of the sinoatrial (SA) node, the atrioventricular (AV) node, the His bundle or the bundle branches and fascicles. They cause bradyarrhythmias (sinus node dysfunction, AV block) or an abnormal QRS complex on the electrocardiogram (ECG) (bundle branch and fascicular blocks).
SA node (high right atrium) fires at 60–100/min → atria → AV node (physiological delay; PR interval normally 120–200 ms) → His bundle → right bundle branch (one fascicle) and left bundle branch (left anterior and left posterior fascicles) → Purkinje fibres.
Escape rhythms when a higher level fails: AV junction 40–60/min with a narrow QRS; ventricles 20–40/min with a broad QRS (slower and less reliable).
Blood supply: the SA node artery comes from the right coronary artery (RCA) in about 60%; the AV node from the RCA (posterior descending artery) in most people — hence AV block in inferior myocardial infarction (MI).
Image: “Electrical conduction system of the heart” by Madhero88, CC BY-SA 3.0, via Wikimedia Commons.
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