Arrhythmias
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Three type of heart arrhythmias
Atrial fibrillation Superventricular tachycardia Bradycardia
Atrial fibrillation
HB irregular and fast (350 to 600 beats per minute), not immediately life-threatening. Most common.
Superventricular tachycardia
Episodes of abnormally fast heart rate at rest. Ventricular rate generally around 100 to 200 beats per minute.
Bradycardia
Slow heart beat
Hierarchy of cardiac pacemakers
SA node - 60-100 beats/min AV node - 40-60 Bundle of His - 40-60 Purkinje Fibres - 20-40
Cardiac conduction pathway
SA node is the natural pacemaker it releases electrical stimulant at a regular rate, rate is dictated to the bodies needs. Electrical stimulus from the SA node reaches the AV node and is delayed briefly so all contraction atrial have enough time to pump all the blood into ventricles. AV valve closes and the atria begins to refill. Conductance passes through the AV node and bundle of His into the bundle branches and purkinje Fibres.
How does electrical propagation in the heart happen?
Propagation is from cell to cell. Depolarisation in cell decays electronically to neighbour, and depolarises it to its threshold. Gap junctions allow cell to cell passage of ions allowing the threshold of neighbouring cells to be met.
What ion channels are activated during spontaneous depolarisation of SA node (threshold -40 & amp; -30mV)
if (voltage-dependent) iT,Ca (voltage-dependent) iL,Ca (coltage-dependent)
What ion channels are activated during depolarisation?
iKr,Ks activate
What ion channels are activated during repolarisation?
iKAch (activated by Acetylcholine)
Ventricular action potential ion channel activation
Depolarisation iNa voltage-dependent iL,Ca voltage-dependent iNa time-dependent iL,Ca time-dependent Repolarisation iL,Ca and iNa/Ca iKr,Ks activates iK1 activates
A functional change in ion transporters can lead to?
Arrhythmias Na Ca exchanger Na-K pump
What is an arrhythmia?
A disturbance in the rate, rhythm or pattern with which the heart contracts
Causes of arrhythmias
Coronary artery disease leaving to myocardial ischemia or infarction. Structural changes that accompany heart failure Many drugs Electrolyte imbalance (K+, Na+, Ca2+) Congenital defects
What is fibrillation?
Quivering movement due to uncoordinated contraction of individual fibrils.
What can arrhythmias produce
Vascular stasis or reduce cardiac output to the point that adequate blood flow cannot be maintained to vital organs.
Arrhythmia classification based on anatomical origin
Superventricular - originates higher up the conduction route than the ventricle. Ventricular - originates in the ventricle.
How can ECG measure arrhythmias?
Measurement of change in the electrical field during propagation of cardiac action potential.
Arrhythmia classification based on mechanisms
Disturbance in conduction: Conduction block, re-entry Disturbance in impulse formation (Automaticity): Early delayed after depolarisation (EADs), delayed after depolarisation (DADs)
AV node block causes
Reduced ventricular rate = reduced CO
Bundle of His block causes
Asystole (no ventricular contraction) or Bradycardia (reduced heart rate.)
Purkinje fibre block causes
No change in heart rate but reduces cardiac output.
Why is the refractory period important?
Prevents tetany and prevents against re-entrant arrhythmias.
Wavelength equation
Wavelength = Conduction velocity x ERP