Cardiac arrhythmicity Flashcards
List the cardiac pacemakers in order of fastest to slowest
Sinoatrial node
Atrioventricular node
Bundle of His
Purkinje fibres
What is the pace of the SA node?
60-100 beats/min
What is the pace of the AV node?
40-60 beats/min
What is the pace of the bundle of His?
40-60 beats/min
What is the pace of the Purkinje fibres?
20-40 beats/min
What three tracts come off the SA node?
Anterior: Bachman’s bundle
Middle: Weckenbach’s
Posterior: Thorel’s
Describe an SA node action potential
4 0 3 4 Smooth
Describe a ventricular action potential
4 0 1 2 3 4 Pointy
How is a signal propagated in the heart?
From cell to cell
What does the heart form?
A functional syncytium
What allows the passage of ions between cells?
Gap junctions
What is the effect of the sympathetic nervous system on SA node action potential?
Sympathetic nervous system speeds up action potential
What is the effect of the parasympathetic innervation of the SA node?
Slows down action potentials
What is happening at phase 4 in the SA node?
Spontaneous depolarisation
Slow influx leakage of Na+
Slow leakage of Ca2+
What is happening at phase 0 in the SA node?
Depolarisation
Influx of Ca2+
Through L-type channels
What is happening at phase 3 in the SA node?
Repolarisation
Efflux of K+
No Ca2+ comes in
What is happening at phase 0 in the AV node?
Rapid depolarisation
Influx of Na+
What is happening at phase 1 in the AV node?
Partial repolarisation
Na+ stops coming in
K+ efflux
What is happening at phase 2 in the AV node?
Plateau phase
K+ efflux
Ca2+ influx
What is happening at phase 3 in the AV node?
Repolarisation
K+ efflux
No movement of Ca2+
What is happening at phase 4 in the AV node?
Ventricle diastole Pacemaker depolarisation Ions return to original location Via Na+/K+ATPase Na+/Ca2+ transporter Ca2+ ATP pump
What is the absolute refractory period?
A second action potential cannot be generated
What is the PQRST complex?
A heart beat seen on the ECG
What is P to R?
Atrial systole
Ventricle diastole
What is R to S?
Isovalemic contraction
AV valves close
Valves out of the heart are not open
What is S to T?
Ventricle systole
Ejecting blood
What is the T wave?
Isovalemic relaxation
Ventricles are relaxing but not filling with blood
What is T to P?
Ventricles filling with blood
What is R to T?
Ventricle contraction
Define arrhythmia
A disturbance in the rate, rhythm, or pattern with which the heart contracts
How many people in the UK have arrhythmias?
2 million
What causes arrhythmia?
Coronary artery disease leading to myocardial ischemia or infarction
Structural changes that accompany heart failure
Drugs
Electrolyte imbalance
Congenital defects
What are consequences of arrhythmias?
Produce vascular stasis (e.g. atrial fibrillation)
Reduce cardiac output
What is the primary classification of arrhythmias?
Anatomical origin
What are supraventricular arrhythmias?
Originate higher up the conduction route than the ventricle e.g. Sinus bradycardia Sinus tachycardia Atrial tachycardia Atrial fibrillation AV block Wolff-Parkinson-White syndrome
What are ventricular arrhythmias?
Originate in the ventricle e.g. Ventricular premature beats Salvos Bigemini Ventricular tachycardia Ventricular fibrillation Torsades de Points
What is an ECG
Electrocardiogram
Measures the change in electrical field during propagation of a cardiac action potential
What does the PR interval tell us?
Time of conduction through the AV node
What does the QT interval tell us?
Duration of ventricular action potential
What are the two classifications of arrhythmia based on mechanisms?
Disturbances in conduction
Disturbances in impulse formation
What is the most common conduction block?
Complete block of the forward movement of the action potential between the SA node and the ventricular myocardium
What would be the result of a block in the Weckenbach’s bundle?
Reduced ventricular rate
What would be the result of a block in the bundle of His?
Asystole (no ventricular contraction)
Bradycardia
What would be the result of a block in either branch of the purkinje system?
No change in heart rate but reduced cardiac output
What makes the heart more susceptible to re entry?
When conduction velocity is slow and the cardiac cells spend less time in a refractory state
What makes re entry less likely?
Long wavelength
What must happen for re entry to occur?
Presence of a unidirectional block within a conduction pathway
Critical timing
Refractory state of normal tissue that the wavefront encounters
What might be the cause of slow conduction velocity?
Ischemia
Are re entrant arrhythmias confined to one heart chamber?
No
What shape do reentrant arrhythmias form?
Spiral
What is ventricular tachycardia?
Ventricular rates generally around 100 to 200 beats per minute
What is ventricular fibrillation?
Life threatening
Many small waves propagate throughout the ventricles
What is atrial flutter?
Atrial rates of 250-350 beats per minute
Ventricular rate substantially slower
What is atrial fibrillation?
Rapid atrial rate (350-600 beats per minute) not immediately life threatening
What are the two kinds of disturbance in impulse formation?
Early delayed after depolarisations
Delayed after depolarisations
What does triggered activity mean?
Refers to a situation where heart tissue is stimulated once but results in the production of more than one conducted beats
What are early delayed after depolarisations?
Prolonged action potential duration is necessary for generation
Can contribute to initiation of arrhythmia syndrome, Torsades de pointes
What is Torsades de Pointes?
Drug induced blockade of K+ channel leads to prolonged action potential depolarisation
If net inward currents are larger than outward currents during phase 3 this causes an EAD
Causes multifocal ventricular tachycardia
What are delayed after depolarisations?
Cellular Ca2+ overload in the SR
Phasic release of Ca2+
Depolarising inward current
