3. The ECG Flashcards
Do pacemaker cells have a resting potential?
No
What occurs instead of a resting potential for pacemaker cardiac cells?
The membrane potential slowly depolarises toward threshold (pacemaker potential).
What is the first half of the pacemaker potential the result of?
Simultaneous opening of unique funny channels, which permits inward Na+ current, and closure of K+ channels, reducing outward K+ current.
What is the second half of the pacemaker potential the result of?
Opening of T-type Ca2+ channels
Once the threshold is reached, what is the rising phase of the action potential the result of?
Opening of L-type Ca2+ channels, whereas the falling phase is the result of opening of K+ channels.
What is the cardiac pacemaker?
Sinoatrial Node (SA node)
Where is the SA node located?
Within right atrial wall at junction with superior vena cava
What is the intrinsic rate of the SA node?
70-80 A.P. per min
What is the conduction speed of the SA node?
0.05m/sec
Where is the atrioventricular node (AV node) located?
Above cardiac septum at junction of atria and ventricles.
What is the intrinsic rate of the AV node?
40-60 A.P. per min
What is the conduction speed of the AV node?
0.05m/sec
Where is the bundle of His located?
Left & right Branches run down ventricular septum to apex
What is the intrinsic rate of the bundle of His?
20-40 A.P. per min
What is the conduction speed of the bundle of His?
1m/sec
Where are the Purkinje fibres located?
Throughout ventricular myocardium from apex to base
What is the intrinsic rate of Purkinje fibres?
15-40 A.P. per min
What is the conduction speed of Purkinje fibre
4m/sec
Electrical conduction in heart steps
- SA node depolarises
- Electrical activity goes rapidly to AV node via internodal pathways
- Depolarisation spreads more slowly across atria. Conduction slows through AV node.
4. Depolarisation moves rapidly through ventricular conducting system to the apex of the heart.
5. Depolarisation wave spreads upward from the apex.
What are the two phases that cardiac contractile cell APs exhibit?
Prolonged plateau phase - accompanied by prolonged period of contraction.
What do the plateau phase and prolonged period of contraction allow for?
Ensures adequate ejection time
What is the prolonged plateau phase due to?
Activation of slow L-type Ca2+ channels
Is tetanus of cardiac muscle possible and why?
No due to refractory period
What is an ECG useful for? x 4
- Assessment of orientation of the heart
- Localisation of areas that do not conduct electrical activity normally
- Assessment of myocardial hypertrophy or atrophy (current amplitudes are proportional to electrically active tissue mass)
- Accurate measurement of heart rate (60/ R-R interval)
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 repolarisation
TP interval
Time during which ventricles are relaxing and filling
ECG diagram
P wave timing
80-100 ms
PR segment timing
100-180 ms
QRS complex timing
80-100 ms
ST segment
70-80 ms
T wave
~200 ms
ECG lead arrangements
ECG lead diagrams
ECG heart conditions diagram
