The ECG - L3

Question 1

What are 8 properties of an effective heart?

Answer 1

1. Regular contractions at appropriate rate for metabolism
2. Guaranteed time for ventricular filling after atrial & ventricular contractions
3. Contraction duration long enough for physical movement of fluid
4. Contractile strength sufficient to generate appropriate pressures
5. Ventricular pressure directed towards exit valves
6. Coordination of left & right, and atrial & ventricular contractions
7. Matched volumes of emptying & filling
8. All achieved by the electrical and mechanical properties of the heart and cardiac muscle

Question 2

What does electrical excitation precede?

Answer 2

Electrical excitation precedes mechanical contraction

Question 3

Exit valves?

Answer 3

Pulmonary and aortic valves - semilunar valves

Question 4

Pacemaker activity cardiac auto rhytmic cells - do they have a resting potential?

Answer 4

These cells do not have a resting potential; rather, the membrane potential slowly depolarizes toward threshold (pacemaker potential).

Question 5

What is the 1st half of the pacemaker potential a result of and what does this permit?

Answer 5

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.

Question 6

What is the 2nd half of the result?

Answer 6

The second half is the result of opening of T-type Ca2+ channels.

Question 7

Once threshold is reach what is the rising phase of the a.p. a result of?

Answer 7

Opening of L-type Ca^2+ channels

Question 8

What is the most important structural and functional feature about automatic cardiac cells

Answer 8

auto-rhythmic cells - under own control

Question 9

Once threshold is reach what is the falling phase of the a.p. a result of?

Answer 9

The falling phase is the result of opening of Potassium ion channels - K^+ channels

Question 10

Cardiac conducting system - 4 parts?

Answer 10

SA node, AV node, Bundle of His and Purkinje fibres

Question 11

Another name for Sinoatrial node?

Answer 11

Cardiac pacemaker

Question 12

Where is SA node located?

Answer 12

Located within right atrial wall at junction with superior vena cava

Question 13

1. Intrinsic rate of SA node?
2. Conduction speed of SA node?

Answer 13

1. Intrinsic rate of 70-80 A.P. per min
2. Conduction speed of 0.05m/sec

Question 14

Where is Atrioventricular Node node located?

Answer 14

Located above cardiac septum at junction of atria & ventricles

Question 15

Explain pathway

Answer 15

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.

Question 16

Bundle of His location?

Answer 16

Location: left & right Branches run down ventricular septum to apex

Question 17

Bundle of His: 1. intrinsic rate? 2. Conduction speed?

Answer 17

1.Intrinsic rate of 20-40 A.P. per min
2. Conduction speed of 1m/sec

Question 18

Purkinje Fibres:
1. located?
2. Intrinsic rate?
3. Conduction speed?

Answer 18

1. Located throughout ventricular myocardium from apex to base
2. Intrinsic rate of 15-40 A.P. per min
3. Conduction speed of 4m/sec

Question 19

Electrical conduction in heart: 5 STEPS auto-rhythmic cell

Answer 19

1. SA nodes depolarises
2. Electrical activity goes rapidly to AV node via inter nodal pathways
3. 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.
   Auto-rhythmic -40mV threshold potential

exam Q must be able to explain all

Question 20

AV nodal delay for how long?

Answer 20

100 msec

Question 21

Cardiac Action Potential - contractile cells: more what you see in skeletal muscle

Answer 21

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

Question 22

ECG:
1. P wave?
2. PR segment?
3. QRS Complex?
4. ST segment?
5. T wave?
6. TP interval?

Answer 22

1. P wave = Atrial depolarisation
2. PR segment = AV nodal delay
3. QRS complex = Ventricular depolarisation (atria repolarising simultaneously)
4. ST segment = Time during which ventricles are contracting and emptying
5. T wave = Ventricular repolarisation
6. TP interval = Time during which ventricles are relaxing and filling

=> associated with lead 2

Question 23

compare and contrast auto rhythmic cells and contractile cells EXAM Q

Answer 23

EXAM Q

Question 24

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?

Answer 24

Detected by skin electrodes and we can get a recording for ECG

Question 25

ECG is useful for ? 4 points

Answer 25

1. Assessment of orientation of the heart
2. Localisation of areas that do not conduct electrical activity
   normally
3. Assessment of myocardial hypertrophy or atrophy (current
   amplitudes are proportional to electrically active tissue mass)
4. Accurate measurement of heart rate (60/ R-R interval)

Question 26

ECG timing: in ms
1. P wave
2. PR segment
3. QRS complex
4. ST segment
5. T wave

Answer 26

1. P wave: 80-100ms
2. PR segment: 100-180 ms
3. QRS complex: 80-100ms
4. ST segment: 70-80ms
5. T wave: ~200ms

Question 27

SEE diagrams slides 17,18,19

Answer 27

do

Question 28

ECG lead arrangements V1-V6 what do they do?

Answer 28

V1-V6: show electrical activity of the heart

Question 29

What can ECG be helpful in showing?

Answer 29

Normalities or abnormalities in rate/rhythm of the heart

Question 30

1. Bradycardia?
2. Tachycardia?

Answer 30

1. Slow heart rate
2. Fast heart rate

Question 31

What cycle does QRST complex show?

Answer 31

QRST complex: whole heart cycle - whole contraction and relaxation of the heart
closer R waves come together

Question 32

1. What does ECG look like for ventricular fibrillation?
2. What is ventricular fibrillation?

Answer 32

1. No detectable wave form: shaking, quivering mode, heart is not working
2. 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.