Lecture 2 Physiology of the Heart I Flashcards
How do cardiovascular drugs intervene with different aspects of physiology
Cardiovascular drugs interfere with normal physiology or pathophysiology where abnormal function has occurred as a result of disease
What happens during phase 0 of the cardiac action potential
Voltage-gated Na+ channels open and rapid Na+ influx depolarises the me and triggers opening of more Na+ channels creating a positive feedback loop and a rapidly rising membrane potential.
What happens during phase 1 of the cardiac action potential
Na+ channels close when the cell depolarises and the membrane potential peaks at around +30mV. At this point there is a deactivation of Na+ influx which results in a partial outward current and a small repolarisation
What happens during phase 2 of the cardiac action potential
Ca2+ entering through the now open voltage-gated Ca2+ channels prolongs depolarisation of the membrane potential results in the plateau phase. Plateau falls slightly because of some K+ leakage by most K+ channels remain closed until the end of the plateau. This is extremely important for the timing of the cardiac action potential
What happens during phase 3 of the cardiac action potential
Ca2+ channels close and Ca2+ is transported out of the cell. K+ channels open and rapid K+ efflux returns the membrane to its resting potential. This is the repolarisation phase of the cardiac action potential
How does the pacemaker potential differ to the cardiac action potential
The pacemaker potential is much shorter than the cardiac action potential due to the absence of a plateau phase. In addition the pacemaker potential has not resting membrane potential but instead is gradually depolarising due to slow Na+ influx
Which cells exhibit the cardiac action potential in the heart
Contractile cells such as cardiac myocytes
Which cells in the heart exhibit the pacemaker potential
Nodal and conducting tissue such as the AVN SAN Purkinje fibres
What ion movement mediates the rapid depolarisation in the pacemaker potential
Ca2+ influx
Explain what currents and ion movements occur in the pacemaker potential
In the pacemaker cells there is no resting membrane potential. Instead the membrane potential is gradually depolarising due to cation currents mediated by the funny channels (If). These funny currents bring the membrane potential to threshold where they activated voltage-gated Ca2+ channels. Opening of voltage-gated Ca2+ channels leads to rapid Ca2+ influx which mediates the rapid depolarisation phase of the pacemaker potential. At peak positive potential the Ca2+ channels close and K+ channels open. The fast K+ efflux mediates repolarisation of the pacemaker potential. Once repolarisation occurs the If channels are activated again due to the negative potential and so can mediate the gradual depolarisation once more.
Draw and explain the cardiac action potential
See completed diagram below
Draw and explain the pacemaker action potential
See completed diagram below
What is meant by atrioventricular delay
After the atria contract there is a slight delay before the ventricles do so. This is mediated by the AVN which delays passing the action potential down through the bundle of his to the ventricles. This allows the atria to finish contraction and maximises the amount of blood entering and thus is pumped by the ventricles
What are the two main mechanisms of arrhythmias
Abnormal impulse generation or abnormal impulse propagation
Give some examples of activity that can cause abnormal impulse generation in arrhythmias
Triggered activity and increase automaticity/ectopics
Give some examples of activity that can cause abnormal impulse propagation in arrhythmias
Damage to the heart muscle causing re-entrant activity and heart block
What is meant by triggered activity
This is where an action potential fires and after repolarisation there can be further depolarisation that leads to the threshold for further action potential firing
What causes triggered activity
Ca2+ overload in the cell
What can be the effects of triggered activity
Triggered activity can lead to ectopic beast ventricular tachycardia and subsequently ventricular fibrillation
What feature of triggered activity is seen on an ECG
The magnitude of after-depolarisations increases with multiple initial depolarisations
What is meant by increased automaticity
Usually a problem with the pacemaker cells whereby the fire impulses spontaneously. This can cause ectopic beats
What is meant by re-entrant activity
Re-entrant activity occurs as a result of damage to the cardiac muscle which causes a blockage of anterograde conduction. This results in the impulses travelling in the wrong direction around the heart muscle. Impulses become able to travel in the reverse direction through the region of damaged cardiac muscle. This in turn leads to associated circus movement
First degree heart block is benign T or F
T
What is seen in patients with first degree heart block
Increase in the PR interval due to electrical signals taking longer to get some the atria to the ventricles
First degree heart block is commonly seen in a certain group of people why is this
It is often seen in athletes where the high levels of training has caused changes at the level of the heart
What is often seen in patients with second degree heart block
Normal P wave corresponding to atrial depolarisation but the occasional absence of ventricular depolarisation and a missing QRS complex
What causes the features seen on the ECGs of patients with second degree heart block
The SAN node impulses reach the AVN but some of these impulses fail to generate ventricular depolarisation
What are the subtypes of second degree heart block and how do they differ
Mobitz type 1 is where patients have an increased PR interval and occasional failure to generate QRS complexes. Mobitz type 2 occurs where there is more frequent absence of the QRS complex and is much more serious
Which type of second degree heart block is also seen in some athletes and what is the name of this condition
Mobitz type 1 second degree heart block is often seen and is referred to as Wenckebach heart block
What is significant about the ratio of P waves to QRS complexes in patients with heart block
The higher the ratio of P:QRS the more severe the heart block
What P:QRS ratios are common in patients with Mobitz type 2 second degree heart block
2:1 or 3:1 or higher
Second degree heart block is often referred to as incomplete heart block T or F
T
Third degree heart block referred to as complete heart block is the most severe T or F
T
What happens in third degree heart block two possibilities
Either there are no signals being fired from the SAN or no signals reach the AVN
Why doesn’t third degree heart block result in death
Alternative pacemakers in the ventricles take over and set the rate of contraction
Why in third degree heart block is it common for there to be a mismatch in chamber contraction in the heart
The rate of beating in the ventricles due to the alternative pacemakers at a lower rate than the SAN. This causes incidents where the atria will be contracting against closed AV valves due to the existing presence of blood in the ventricles
In third degree heart block it is often seen that there is no consistent PR interval T or F
T
In third degree heart block what is the relationship between the P wave and the QRS complex
There is no relationship between the P wave and the QRS complex as different pacemakers are driving the activity of the atria and the ventricles
How can the origins of arrhythmias differ
Arrhythmias can be sinus (from the SAN) atrial nodal (from the AVN) or ventricular in origin
What does the following ECG show?
1st degree heart block
What does the following ECG show?
Increased automaticity/ectopic beats
What does the following ECG show?
Atrial fibrillation
What does the following ECG show?
Triggered activity
What does the following ECG show?
Sinus tachycardia
What does the following ECG show?
Normal sinus rhythm
What does the following ECG show?
Sinus bradycardia
What does the following ECG show?
Atrial tachycardia
What does the following ECG show?
2nd degree heart block
What does the following ECG show?
Ventricular tachycardia
What does the following ECG show?
Polymorphic ventricular tachcardia
What does the following ECG show?
3rd degree heart block
What does the following ECG show?
Ventricular fibrillation
What are the two different effects of arrhythmia on heart rate
Tachycardia or bradycardia
What signifies atrial tachycardia
Multiple P waves signifying multiple atrial depolarisations
What signifies a ventricular rhythm
Wide complexes
Ventricular tachycardia is often fatal T or F
T
What is seen in the ECG of patients with atrial fibrillation
No true P waves as the atria aren’t contracting
Atrial fibrillation patients will have an normal heart rhythm T or F
F – they will have an irregular heart rhythm due to an irregular ventricular response and fibrillation waves
What is the major risk with atrial fibrillation
Because the atria are fibrillating it’s possible that a thrombus can form in the atrium. This can break off into the circulation and cause stroke
What drugs are often given to patients in atrial fibrillation
Anti-coagulant drugs to prevent thrombus formation and possible strokes
Ventricular fibrillation is extremely fata T or F
T
How is ventricular fibrillation often treated
With electrical cardioversion (defibrillation)
What is the effect of sympathetic nervous system regulation of the pacemaker potential
Sympathetic nervous system stimulation of the pacemaker cells increases the slope of the prepotential phase of the pacemaker potential. This means that these cells reach action potential threshold quicker thus speeding up heart rate (positive chronotropic effects)
What are the mediators of the sympathetic nervous systems effects of heart rate
Noradrenaline acting on the β1 adrenoceptors which acts to increase cAMP levels
What is the downside of sympathetic nervous system stimulation of the heart
It increases automaticity which could then lead to arrhythmia
What is the effect of parasympathetic nervous system regulation of the pacemaker potential
Parasympathetic nervous system stimulation of the pacemaker cells decreases the slope of the prepotential phase of the pacemaker potential. This means that these cells reach action potential threshold slower thus slowing down heart rate (negative chronotropic effects)
What are the mediators of the parasympathetic nervous systems effects of heart rate
Vagus nerve releasing acetylcholine which acts on the M2 receptors in nodal and atrial tissue
What is the downside of parasympathetic nervous system stimulation of the heart
It decreases automaticity inhibits atrioventricular conduction and increases the PR interval
What are the class I anti-arrhythmic drugs
Na+ channel blockers
What are the class II anti-arrhythmic drugs
Β-adrenoceptor antagonists
What are the class III anti-arrhythmic drugs
Action potential prolonging agents
What are the class IV anti-arrhythmic drugs
Ca2+ channel blockers
Most class I anti-arrhythmic drugs are use-dependent blockers what is meant by this
They preferentially bind to open or refractory channels/active channels. This means that they actually block the channels causing tachycardia without a total inhibition of heart rate
How are class I anti-arrhythmic drugs sub-classified
There are subclassified depending on their effects on the duration of the cardiac action potential. Class 1A increased the duration class 1B slightly decrease the duration and class 1C have no effect on the duration of the cardiac action potential
Give an example of a class 1A anti-arrhythmic drug
Disopyramide quinidine procainamide
Give an example of a class 1B anti-arrhythmic drug
Lidocaine mexiletine
Give an example of a class 1C anti-arrhythmic drug
Flecainide propafenone
What is the downside to using class 1 anti-arrhythmic drugs
Because the electrical activity of the heart is intrinsically linked to the contractility of the heart these drugs impair heart function
What are the subclasses of class II anti-arrhythmic drugs
Non-selective or β1 selective antagonists
Give an example of a non-selective β blocker
Propranolol atenolol nadolol carvedilol
Give an example of a β1 selective blocker
Bisoprolol metoprolol
Class III anti-arrhythmic drugs supress arrhythmias and increase the duration of the cardiac action potential. Give an example of this type of drug
Amiodarone sotalol
Give an example of a Class IV anti-arrhythmic drug
Verapamil diltiazem
Digoxin is a non-classical anti-arrhythmic drug. Describe its how it acts
Digoxin inhibits the Na+/K+ATPase which leads to an increase in intracellular Na+. This Increase in intracellular Na+ leads to the reversal of the action of the Na+/Ca2+ exchanger which now brings Ca2+ into the cell. This increases intracellular Ca2+ increasing cardiac contractility. In addition digoxin also slows down heart rate through its effects on Na+ directly hence causing bradycardia
What are the downside to digoxin use
Digoxin has a narrow therapeutic window and causes a number of side effects such as nausea vomiting diarrhoea and confusion
What kind of compound is digoxin
Cardiac glycoside
Where is digoxin often used
Most commonly used in atrial fibrillation to reduce ventricular rate of response Its also used in severe heart failure due to its positive inotropic effect
What is QT prolongation what causes it and what are the effects
QT prolongation is where the interval from the start of the QRS complex to the end of the T wave is increased. Anti-arrhythmic drugs can sometimes lead to a prolongation of the QT interval which can lead to polymorphic ventricular tachycardia
Which two anti-arrhythmic drugs often cause polymorphic ventricular tachycardia
Amiodarone and sotalol
Why does amiodarone have so many side effects
It has a large volume of distribution meaning it becomes distrusted throughout the body
What are some of the side effects of amiodarone use
QT prolongation polymorphic ventricular tachycardia interstitial pneumonitis abnormal liver function hyperthyroidism/hypothyroidism sun sensitivity slate grey skin discolouration corneal microdeposits optic neuropathy
