The ECG - L3 Flashcards
What are 8 properties of an effective heart?
- Regular contractions at appropriate rate for metabolism
- Guaranteed time for ventricular filling after atrial & ventricular contractions
- Contraction duration long enough for physical movement of fluid
- Contractile strength sufficient to generate appropriate pressures
- Ventricular pressure directed towards exit valves
- Coordination of left & right, and atrial & ventricular contractions
- Matched volumes of emptying & filling
- All achieved by the electrical and mechanical properties of the heart and cardiac muscle
What does electrical excitation precede?
Electrical excitation precedes mechanical contraction
Exit valves?
Pulmonary and aortic valves - semilunar valves
Pacemaker activity cardiac auto rhytmic cells - do they have a resting potential?
These cells do not have a resting potential; rather, the membrane potential slowly depolarizes toward threshold (pacemaker potential).
What is the 1st half of the pacemaker potential a result of and what does this permit?
The first half of the pacemaker potential is 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 2nd half of the result?
The second half is the result of opening of T-type Ca2+ channels.
Once threshold is reach what is the rising phase of the a.p. a result of?
Opening of L-type Ca^2+ channels
What is the most important structural and functional feature about automatic cardiac cells
auto-rhythmic cells - under own control
Once threshold is reach what is the falling phase of the a.p. a result of?
The falling phase is the result of opening of Potassium ion channels - K^+ channels
Cardiac conducting system - 4 parts?
SA node, AV node, Bundle of His and Purkinje fibres
Another name for Sinoatrial node?
Cardiac pacemaker
Where is SA node located?
Located within right atrial wall at junction with superior vena cava
- Intrinsic rate of SA node?
- Conduction speed of SA node?
- Intrinsic rate of 70-80 A.P. per min
- Conduction speed of 0.05m/sec
Where is Atrioventricular Node node located?
Located above cardiac septum at junction of atria & ventricles
Explain pathway
Pathway: signal goes from SA node and passes across via the intra-atrial pathway so it goes across to right atrium
need to bring atria in sync and so pathway allows this to happen
Goes to the AV node first more rapidly
Intra-atrial pathway happens more slowly
From AV node the next site is the Bundle of His and this branches off to the left branch bundle and then the right branch bundle, and it makes its way down the ventricles and then it travels up around the ventricle through these Purkinje fibers.
Bundle of His location?
Location: left & right Branches run down ventricular septum to apex
Bundle of His: 1. intrinsic rate? 2. Conduction speed?
1.Intrinsic rate of 20-40 A.P. per min
2. Conduction speed of 1m/sec
Purkinje Fibres:
1. located?
2. Intrinsic rate?
3. Conduction speed?
- Located throughout ventricular myocardium from apex to base
- Intrinsic rate of 15-40 A.P. per min
- Conduction speed of 4m/sec
Electrical conduction in heart: 5 STEPS auto-rhythmic cell
- SA nodes depolarises
- Electrical activity goes rapidly to AV node via inter nodal pathways
- Depolarisation spreads more slowly across atria. Conduction slows through AV node
- Depolarisation moves rapidly through ventricular conducting system to the apex of the heart.
- Depolarisation wave spreads upward from the apex.
Auto-rhythmic -40mV threshold potential
exam Q must be able to explain all
AV nodal delay for how long?
100 msec
Cardiac Action Potential - contractile cells: more what you see in skeletal muscle
resting potential = -90mV
whenever neighbouring cardio myocyte or pacemaker cell triggers a response via an action potential it triggers a response in the contractile cell - triggered to depolarise towards -70mV threshold potential
sodium channels open and fast channels allow sodium current to come in
depolarisation - overshoot it is the period of AV delay
At this AV nodal delay, we need to bring back down to 0 mV, sodium channels close, potassium channels open and brings it to 0
Most Important is Plateau Phase
=> Plateau phase of action potential where it stays at 0mV for a period of time very important - need to give ventricles time to eject out blood
brought about by the activation of these long lasting Calcium channels
Ca current can come in more slowly
Once ventricles have carried out contraction, they are now ready to repolarise and we go towards the falling phase
refractory period gives adequate time for the cardiac muscle, the heart to complete out contraction
Otherwise we get tetanus and cannot get refill in the heart of blood
Need to facilitate the refill - very important
ECG:
1. P wave?
2. PR segment?
3. QRS Complex?
4. ST segment?
5. T wave?
6. TP 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
=> associated with lead 2
compare and contrast auto rhythmic cells and contractile cells EXAM Q
EXAM Q
Depolarisation/repolarisation events result in potential differences that are transmitted to the body surface in the direction of their propagation within the heart are detected where and what is it used for?
Detected by skin electrodes and we can get a recording for ECG
ECG is useful for ? 4 points
- 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)
ECG timing: in ms
1. P wave
2. PR segment
3. QRS complex
4. ST segment
5. T wave
- P wave: 80-100ms
- PR segment: 100-180 ms
- QRS complex: 80-100ms
- ST segment: 70-80ms
- T wave: ~200ms
SEE diagrams slides 17,18,19
do
ECG lead arrangements V1-V6 what do they do?
V1-V6: show electrical activity of the heart
What can ECG be helpful in showing?
Normalities or abnormalities in rate/rhythm of the heart
- Bradycardia?
- Tachycardia?
- Slow heart rate
- Fast heart rate
What cycle does QRST complex show?
QRST complex: whole heart cycle - whole contraction and relaxation of the heart
closer R waves come together
- What does ECG look like for ventricular fibrillation?
- What is ventricular fibrillation?
- No detectable wave form: shaking, quivering mode, heart is not working
- Type of irregular heart rhythm: arrhythmia - the lower heart chambers contract in a very rapid and uncoordinated manner. As a result, the heart doesn’t pump blood to the rest of the body.
