2. Heart Rate and Rhythm Flashcards
What happens when the heart contracts?
The muscle thickens and ventricular cavity reduces in size to eject blood
What is activation of the heart controlled by?
Heart is an excitable tissue, activation is controlled by changes in membrane potential
What are the stages of the cardiac action potential?
Phase 0 - Rapid depolarisation Phase 1 - Partial repolarisation Phase 2 - Plateau Phase 3 - Repolarisation Phase 4 - Pacemaker potential
Describe Phase 0 - Rapid depolarisation.
Depolarisation occurs when membrane potential reaches threshold (-60mV) and is mediated by rapid sodium influx
Describe Phase 1 - Partial repolarisation.
Rapid sodium influx deactivation by closure of voltage-gated Na channels
Describe Phase 2 - Plateau.
Depolarisation maintained by slow calcium influx and initial fall in potassium efflux
Allows time for heart to eject blood and then refill (prevents AP firing at very high rates)
Describe Phase 3 - Repolarisation.
Slow calcium influx deactivation and increased potassium efflux resets membrane potential to resting potential
Describe Phase 4 - Pacemaker potential.
Gradual depolarisation of resting potential towards threshold by increasing sodium and calcium influx and decreasing potassium efflux
When membrane potential reaches threshold the next cardiac action potential will fire
Describe the pathway for depolarisation through cardiac conduction tissue.
- Sinoatrial node - main pacemaker in right atrium
- Depolarisation rushes across conducting tissue in atria to cause atrial contraction
- Depolarisation reaches AV node - conduction is slow to ensure atria have fully contracted to fill ventricles with blood
- Depolarisation conducted through His bundle, Left and Right bundles and then Purkinje fibres
- Rapid conduction through Purkinje fibres ensures ventricles contract as one to maximally eject blood
Where are pacemaker potentials found in cardiac tissue?
Why is this important?
- SA node (depolarises fastest = dominant pacemaker), AV node and Purkinje fibres
- If SA node fails still have pacemakers downstream to compensate
Where is rapid sodium influx not found in cardiac tissue?
What mediates depolarisation instead?
- SA node and AV node
- Slow calcium influx mediates depolarisation in nodal tissue
Where are long action potentials (plateaus) and refractory periods found in cardiac tissue?
- Purkinje fibres and ventricles depolarisation maintained by slow calcium influx
What are the 2 general mechanisms of arrhythmia?
- Abnormal impulse generation
2. Abnormal impulse propagation
What is triggered activity?
- When muscle is stimulated, get depolarisation and then shortly after get delayed after-depolarisation
- If 2 stimuli are applied, the delayed after-depolarisation increases in size
- Closer the 2 stimuli are, the larger the delayed after-depolarisation
- Certain point at which delayed after-depolarisation is large enough to reach threshold and triggers and additional depolarisation
What is increased automaticity?
- Due to biochemical upsets which cause patients to fire ectopic beats
- Normal rhythm is followed by a single irregular rhythm (ectopic beat)
What is re-entry?
- Normally, an impulse is propagated down myocardial tissue but cannot propagate back on itself due to tissue behind being refractory
- In damaged heart, can get abnormal impulse propagation
- If there is unidirectional block, impulse can propagate back on itself to depolarise tissue that has just repolarised due to time delay, causing another depolarisation
- Creates a re-entrance circuit leading to tachycardia
What is the normal delay between atrial depolarisation (P wave) and ventricular depolarisation (QRS complex)?
200m/s
Describe 1st degree heart block.
Delay between atrial depolarisation and ventricular depolarisation is >200m/s but all impulses get through
Describe 2nd degree heart block.
Intermitted dropped beat i.e. ventricular depolarisation does not follow atrial depolarisation as impulse is blocked
Describe 3rd degree heart block.
Atrial and ventricular depolarisation is totally dissociated - impulses do not reach ventricles due to complete heat block
Still get ventricular depolarisation due to AV node pacemaker potential but this is slower and therefore QRS complex is wider
Describe normal sinus rhythm.
Atrial depolarisation (P wave) followed by AV node delay (200m/s) followed by ventricular depolarisation (QRS complex) followed by ventricular repolarisation (T wave)
Describe sinus bradycardia.
Origin of rhythm is still sinus (P wave present)
Increased interval between QRS complexes slows heart rate (Bradycardia)
Describe sinus tachycardia.
Origin of rhythm is still sinus (P wave present)
Decreased interval between QRS complexes accelerates heart rate (Tachycardia)
Describe atrial tachycardia.
Atria contracting very rapidly, but due to AV node delay not all impulses get through to ventricles
Decreased interval between QRS complexes accelerates heart rate (Tachycardia)
Describe ventricular tachycardia.
Impulse comes from ventricles, as this is not propagated through fast channels QRS complexes are wider
Decreased interval between QRS complexes accelerates heart rate (Tachycardia)
Describe atrial fibrillation.
- Atria do not contract in an organised way so there is no atrial output
- No true P waves only fibrillation waves
- Depolarisations reach AV node but only some get through to ventricles leading to irregular heart rhythm
- Makes heart inefficient as lose atrial output and irregular heart rate
What is a risk in atrial fibrillation patients?
What are these patients therefore given?
- If atria stand still develop atrial thrombus, which can break of and cause stroke
- Anti-coagulants
Describe ventricular fibrillation.
- Wide QRS complexes (Ventricular rhythm)
- No cardiac output (Sudden death)
- No pattern of ventricular rhythm
What can save someone’s life in ventricular fibrillation?
Defibrillation restores normal ventricular rhythm
What is the intrinsic heart regulated by?
Intrinsic heart rate is continuously suppressed by vagal tone
What is the effects of sympathetic stimulation of the heart?
What mediates this effect?
How does it cause this effect?
- Increased heart rate (positive chronotropic) and force of contraction (positive inotropic)
- Increases automaticity (increases risk of arrhythmias)
- Mediated by Adrenaline (from adrenal glands) and Noradrenaline (from sympathetic neurones) at B1-adrenoceptors
- Increases the slope of pacemaker potential, therefore threshold is reached quicker and heart rate increases
What is the effects of parasympathetic stimulation of the heart?
What mediates this effect?
How does it cause this effect?
- Decreases heart rate (negative chronotropic)
- Decreases automaticity (decreases risk of arrhythmias)
- Mediated by Acetylcholine (from parasympathetic neurones) at M2 muscarinic receptors
- Decreases slope of pacemaker potential, thereofore threshold is reached slower and heart rate decreases
What are the 4 classes of drugs in the Vaughan Williams classification?
- Na+ channel blockers
- B-adrenoceptor antagonists
- Prolong AP
- Ca2+ channel blockers
How do Na+ channel blockers treat arrhythmias?
- Use dependent i.e. drugs are specific to refractory-state Na+ channels but do not affect resting Na+ channels
- During tachycardia, there is increased AP firing, meaning increased Na+ channel activation - when these become refractory blockers will bind to channel and inhibit AP firing
Describe the mechanism of action of Digoxin.
- Digoxin is a cardiac glycoside that inhibits the Na+/K+ pump
- Normally, Na+/K+ pump maintains a low [Na+]i and high [Na+]o which creates a driving force for inward Na+ movement
- Na+/Ca2+ pump uses this driving force to extrude Ca2+ from the cell
- When inhibit Na+/K+ pump, this decreases the driving force for Na+ movement therefore decreases Na+/Ca2+ pump activity
- Leads to increased [Ca2+]i
What are the main effects of Digoxin?
- Bradycardia (increased vagal tone)
- Slowing of atrioventricular conduction (increased vagal tone)
- Increased ectopic activity (pro-arrhythmic)
- Increased force of contraction (increased [Ca2+]i)
What is significant about Digoxin’s narrow therapeutic range?
- Need to achieve a certain level to see beneficial effects, but this is close to level that causes adverse effects
What is Digoxin used in?
- Atrial fibrillation to decrease ventricular response rate by increasing AV node delay
- Severe heart failure as increases force of contraction
What can be caused by drugs that delay repolarisation and therefore prolong QT interval?
Ventricular polymorphic tachycardia - lethal
