Lecture 7 Flashcards
Why is there a delay at the AV node?
Because the depolarisation from the SA node takes longer to spread across atrial muscle cells.
P wave
Atrial depolarisation
P-R interval
Time between beginning of atrial excitability and ventricular excitability.
How long is a normal P-R interval?
0.16 seconds
What is QRS?
Ventricular depolarisation. Atrial repolarisation occurs here too but is obscured.
Q-T interval
Contraction occurs here
How long is a normal Q-T interval
0.35 seconds
What shape should the S-T segment be?
A flat line- there is no change in electrical currents.
What is the T wave?
Ventricular repolarisation.
How would an ECG diagnose 1st degree heart block?
A longer P-R interval of about 0.28 seconds.
What can cause 1st degree heart block?
Verapamil or rheumatic heart disease
How can you see 2nd degree heart block on an ECG?
Dropped beat, not every P wave is followed by a QRS. 2:1 is where every second P wave has a QRS. 3:1 is where every 3rd P wave has a QRS.
What is an example of a 2nd degree heart block?
Atrial flutter
What would an ECG show for 3rd degree heart block?
Complete dissociation of P waves from QRS complexes. QRS still regular at 40 per minute due to auto rhythmicity of AV node.
What does 3rd degree heart block lead to?
Severe myocardial ischemia
What is bradycardia?
Slow heart rate below 60bpm
What is tachycardia?
Fast HR above 100bpm
Name a regular arrhythmia?
Flutter
Name an irregular arrythmia?
Fibrillation.
What does an ECG show for atrial fibrillation?
no P wave, narrow QRS complex.
Describe atrial fibrillation
No coordinated electrical activity in SA node. AV node fires occasionally.
What are the symptoms of atrial fibrillation?
Tiredness, shortness of breath, inability to exercise
Describe atrial flutter
Atria contract faster than normal, up to 300 times a minute. Atria and ventricle contract at different speeds. Can see second degree heart block.
What is seen on an ECG for ventricular tachycardia?
A not obvious P wave and abnormally fast heart rate.
What are the symptoms of ventricular tachycardia?
Chest pain, weak, breathless, lightheaded.
What is seen on an ECG with ventricular fibrillation?
No p wave, broad QRS.
What is a class 1 antiarrythmic drug?
Na+ channel blockers
What is class II?
Beta-blockers (anti-sympathetic)
What is class III?
K+ channel blockers
What is class IV?
Ca2+ channel blockers
What is class V?
Cardiac glycosides.
What do class Ia treat? Give an example
Atrial tachycardia or atrial fibrillation. e.g. quinidine. These are moderate blockers
What do class 1b treat and give an example?
Ventricular tachycardia or fibrillation. These are very weak blockers. E.g. lidocaine.
What do class 1c treat and give an example?
Life threatening ventricular fibrillation. e.g. flecainide
How do class II work?
Beta blockers that increase the time between each AP. they prolong phase 4.
Why are class III preferred drugs to use?
They only slow repolarisation rate, no risk in blocking depolarisation. e.g. amiodarone
What is HR set by?
The sinus rhythm of the SA node
What does the sympathetic system do to the heart?
Increases HR by innervating nodal tissue and ventricular muscle increases contractility. Positive ionotropic and chronotropic effects. Depolarisation of cell is quicker.
What is a class II drug?
Beta-blocker- propranolol.
What does the parasympathetic system do to the heart?
Right side nerves innervates the SA node, Left side to AV node. Has a negative chronotropic effect.
What neurotransmitter and mechanism is involved in the parasympathetic system?
Ach on M2 muscarinic receptors. causes hyperpolarisation which slows depolarisation. Calcium channels open more slowly. Gi has inhibitory effect on adenyl cyclase. G-beta and gamma open K+ channels.
