Lecture 3 -ECG Flashcards
What are all cells in the heart?
Both contractile and able to produce their own action potential
Pacemaker cells
Cells of the sinoatrial node Produce lots of AP (~60-100/min) Very little contraction
Myocytes
Strongly contracting cells of heart walls, especially the ventricles
Action potential Initiated at SA node where does it first spread to?
Throughout both atria
What happens in AV node?
Action potential can spread from atria to ventricles
What does ventricular conduction system consist of?
Bundle of His Purkinje fibres
What is the ECG?
Graphic record of direction and magnitude of electrical activity generated by depolarisation and repolarization of atria and ventricles of heart
What is membrane potential?
The voltage change across the membrane
What are the 3 distinct wave forms of ECG?
P wave. QRS complex T wave
When does the P wave occur?
When the wave of depolarisation spreads across the atria
Why is the P wave smaller than QRS complex?
Atria have a smaller muscle mass than Ventricles and generate less electrical activity
When does the ECG remain at base line?
When no net current flow is occurring in heart muscle
P wave
Atrial Depolarisation
PR segment
AV nodal delay
QRS complex
Ventricular depolarisation (Atria repolarising simultaneously)
ST segment
Time during which ventricles are contracting and emptying
T wave
Ventricular repolarization
TP segment
Time during which ventricles are relaxing and filling
What is “Blocks”?
Areas of abnormal (or absent) conduction
What is Reentries?
Areas where the conduction turn back on itself
What is the effect of sympathetic stimulation on heart?
Controls heart action in emergency or exercise situations Require greater blood flow - “revs up” heart Speed up depolarisation so that threshold is reached more rapidly The rate of depolarisation increase due to greater inward movement of Na+ and Ca2+ though If and T type ca2+ channel More frequent action potential - increase heart rate Speed up the spread of AP throughout the specialised conduction pathway Increase contractile strength so that the heart beat more forcefully and squeeze out more blood Improve it’s effectiveness as a pump by increasing HR Decreasing delay between atrial and ventricular contraction Decreasing conduction time throughout heart - increase force of contraction Speeding up relaxation process so that more time is available for filling
What is the effect of parasympathetic stimulation of heart?
ACH suppresses sympathetic activist by inhibiting the release of norepinephrine from neighbouring sympathetic nerve endings Increase potassium permeability of the pacemaker cells in SA node ( the rate at which AP are initiated is reduced) Decreases the AV nodes excitability (prolongs the transmission of impulses to the ventricles) Atrial contraction is weakened Has little effect on ventricular contraction
Sympathetic stimulation
Various efferent fibres
Parasympathetic stimulation
Vague nerve that affect SA node and AV node
Atrial fibrillation
The ventricles won’t fill properly
Ventricular tachycardia
Ventricles going faster than they can be refilled
Ventricular fibrillation
Defibrillator required
Second degree AV block
Can miss whole beats
AV node signals lost
First degree AV block
Longer PR interval
Held in AV node
Premature ventricular complex
Direction of the ECG wave is dependent on the direction of depolarisation wave
The ventricles are still contracting
Third degree AV block
AV node no longer transmitting
Ventricles independently contracting
Stimulated by their own myogenic contraction or purkinje fibre
