Download PBL- Case 1: Cardiac Arrhythmias Pre

PBL- Case 1: Cardiac Arrhythmias
Pre-Meeting Questions
1. Describe how the interaction between the anatomy & physiology of the cardiac conduction
system results in generation of the heart beat
Myocardial cells have 2 functions:
- contraction (myocytes- majority)
- initation & conduction of electrical impulses
o form ‘conducting system of the heart’, modified myocytes
o Generate impulses which are conducted to the myocytes contraction
 ‘Excitation-coupling’
 Rate of impulse/force of contraction modified by ANS & stretch.
Conduction of the heart:
Sinus node:
o
Lies b/w junction SVC/RA
o
Generates impulses automatically via
spontaneous depolarisation of its membrane (quicker than
other cardiac cells- ‘pacemaker’)
o
Relies mostly on influx of Na+ ions into the
cells
o
Rate of depolarisation modulated by
autonomic tone, stretch, temperature, hypoxia, blood pH
and hormonal influences i.e 5HT, T3
Atrial & ventricular myocyte APs
o
Sinus node depolarisation of atrial &
ventricular myocytes
o
Resting membrane of these cells relies on
flow of K+ into cells (Na & Ca channels closed)
o
Arrival of adjacent APs triggers opening of
voltage-gated self-inactivating Na+ channels sharp
-
depolarisation spike.
o This is followed by a partial repolarisation.
o Plateau phase (unique to myocytes): result of calcium influx- sustained plateau prevents early
reactivation of the myocytes & determines the strength of contraction
AV node, Bundle of His, Purkinje fibres
o Depolarisation continues AV node (lies beneath right atrial endocardium w/i lower inte-atrial
septum).
o AV node continues as BoH, which conducts the cardiac impulse rapidly towards the ventricle
o BoH divides at crest of interventricular septum 
 Right bundle branch: continues down the right side of the iv septum to the apex.
 It then radiates & divides to form Purkinje network, which spreads t/o the
subendocardial surface of the right ventricle
 Left bundle branch: short structure, fans out into many strands on the left side of the iv
septum
 Strands can be grouped into anterior hemi-bundle: supps Purkinje network of
anterior & superior surfaces of left ventricle
o Posterior hemi-bundle: supps inferior & posterior surfaces
o AV impulse conduction is slow & dependent on slow trans-membrane Ca flow
o In atria, ventricles and His-Purkinje system, conduction is rapid b/c of APs generated by rapid Na
diffusion
In adults, the normal resting heart rate is 60-80 bpm. Higher in kids
(ECG: P wave- SA node; PR interval- AV node/Bundles; QRS complex- Purkinje fibres/ventricular
myocardium; T wave- ventricular repolarisation).
2. What are the incidence and prevalence in the community of the most common cardiac
arrhythmias, including atrial fibbrilation, atrial flutter, supra-ventricular tachycardia and
ventricular tachycardia?
Atrial fib: common, 5-10% patients >65 y/o (also occurs paroxysmally in younger patients)
Atrial flutter:
Supra-ventricular tachycardia: Most common in children, 20-60% ped patients present in 1st year of
life
Ventricular tachycardia:
3. Discuss the significance of age and underlying cardiac pathology in relation to their impact
on the risk of occurrence of specific arrhythmias
High prevalence of CAD, CHF and valvular disease and calcification (common in older patients) puts
them at higher risk of atrial fibrillation.
Cardiac valvular stenosis or regurgitation caused by either rheumatic or age related degenerative
changes increases left atrial pressure and results in the enlargement of the left atrium and/or the left
ventricle. These changes to cardiac structure and function activate sympathetic nerves and modulate
hormones that control ion channels, which might explain the high prevalence of atrial fibrillation in
older patients with heart failure and valvular dysfunction.
4. What investigations are routinely used to identify/diagnose common arrhythmias?
-ECG: Look for signs of IHD, AF, short PR interval (WPW syndrome), long QT inteval (metabolic
imbalance, drugs, congenital), U waves (hypokalaemia)
- 24 hr ECG monitoring
- exercise ECG
- FBC, U & E, glucose, Ca, Mg, TSH
- Echo: structural heart disease? (i.e. mitral stenosis)