Term
| List the 5 parts of the heart excitation travels through |
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Definition
1. Atria
2. AVN
3. Bundle of His
4. Bundle branches
5. Purkinje fibres |
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Term
| Describe excitation of the atria |
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Definition
| The AP spreads rapidly away from the SAN throughout the atrial muscle at a rate of 1ms via two pathways and reaches the AVN 70ms after initiation. |
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Term
| What are the two pathways the atria are excited by |
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Definition
1. from cell to cell via intercalated discs
2. via a specialised conduction pathway: 'the internodal tract'; this takes the AP directly to the AVN located above the tricuspid valve |
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Term
| What does atria excitation lead to? |
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Definition
| contraction of the atria so the remaining blood in the atria will be forced into the ventricles |
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Term
| How are the atria and ventricles connected? |
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Definition
physically not electrically.
the AVN is the only (indirect) electrical connection between them. |
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Term
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Definition
Very slow (0.05ms)
The AVN acts to delay the AP from the atria to the ventricles by about 100ms. This delay is very important as it allows the atria to contract and the ventricles to fill with blood before ventricular excitation |
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Term
| Describe conduction of the bundle of His |
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Definition
| AP passes down the bundle of His from the AVN rapidly (5ms-1) |
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Term
| Describe conduction of the left and right bundle branches |
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Definition
The bundle of His splits into the left and right branches which extend down the septum to the apex of the heart.
Rapid conduction of 5ms-1 |
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Term
| Describe conduction of the purkinje fibres |
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Definition
| Bundle branches divide into the urkinje fibres which excite the ventricular muscle. AP spreads throughout the rest of the ventricular muscle via cell to cell copling (rate of 1ms-1) from epicardium to endocardium (inwards). |
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Term
| How does blood leave the heart? |
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Definition
| the apex contracts first to push blood up and out of the heart |
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Term
| Give the conduction velocities of cardiac tissue |
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Definition
Atrial muscle.....................1ms-1
AVN...................................0.05ms-1
Bundle of His and bundle branches......5ms-1
Purkinje fibres...........5ms-1
Ventricular muscle........1ms-1 |
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Term
| ___ of blood in the ventricles is caused by atria contraction, the rest is by passive filling. |
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Definition
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Term
| How is an ECG measured? (equipment) |
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Definition
12 separate leads
3 bipolar (so 6 altogether)
6 unipolar electrodes |
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Term
| How is Einthoven's triangle created? |
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Definition
3 bipolar leads are attached to the left arm, right arm and left leg.
This creates a triangle with the heart in the centre.
The 6 unipolar leads are arranged on the chest wall.
The combination of all 12 leads allows clinicians to observe the electrical activity of the heart from different perspectives |
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Term
| What two circumstances produce a positive deflection? |
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Definition
When depolarisation travels towards the positive electrode.
When repolarisation travels away from the positive electrode. |
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Term
| What two circumstances produce a negative deflection? |
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Definition
When depolarisation travels away from the positive electrode.
When repolarisation travels towards from the positive electrode. |
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Term
| What is measured between the two electrodes when the whole tissue is depolarised? |
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Definition
| There is no potential difference because the whole tissue is depolarised. |
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Term
| What does the Lead II ECG look at? |
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Definition
electrical activity int he heart from a longitudinal aspect.
the ECG will reflect thespread of the AP throughout the heart |
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Term
| What does the Lead II ECG consist of? |
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Definition
1. P wave
2. QRS wave
3. T wave |
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Term
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Definition
atrial depolarisation (AP spreads from SAN throughout atria)
80ms |
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Term
| Why does the ECG return to its baseline once the atria are fully depolarised? |
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Definition
| there is no net movement of charges across the membrane - there is no potential difference across the membrane. |
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Term
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Definition
a delay as the AP is held up at the AVN
following excitation, the atria contract. |
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Term
| What causes the Q deflection? |
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Definition
spread of the AP across the septum of the heart from LV to RV
vector for this spread of excitation is no longer towards the positive electrode and so the deflection goes obliquely downwards. |
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Term
| What causes the QRS complex and explain its deflection |
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Definition
depolarisation spreads rapidly down the two bundle branches to the apex of the heart.
the vector is again towards the positive electrode resulting in a positive deflection from Q to R. |
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Term
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Definition
| Depolarisation proceeds up the walls of the heart (towards the base of the heart) via Purkinje fibres. The vector of depolarisation is moving away from the +ve electrode giving downward deflection on the ECG trace. |
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Term
| What else occurs during this time and which ventricle dominates? |
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Definition
Atrial repolarisation (but it obscured by the larger signal arising from the ventricles).
The larger left ventricle. |
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Term
| How long does the QRS last? |
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Definition
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Term
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Definition
| Ventricles have fully depolarised so there is no movement of electrical signals and so the ECG returns to its baseline. |
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Term
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Definition
ventricular repolarisation from epicardium to endocardium
the base repolarises first then the apex last. |
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Term
| Explain the T wave deflection |
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Definition
The main repolarisation signal (epi to endo) is moving away from the positive electrode therefore a positive deflection
wave lasts about 160ms |
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Term
| Compare and explain the differences between the T and P wave |
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Definition
| Repolarisation is slower than depolarisation and less unidirectional - therefore the T wave is broader and has a lower amplitude than the QRS complex |
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Term
| What does the P-Q interval represent and how long does it last? |
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Definition
the time between atrial excitation and ventricular excitation
includes atrial contraction and AVN conduction
160ms |
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Term
| How can changes in the P-Q interval be used in diagnosis? |
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Definition
| prolongation = damage to atrial conduction pathways or AVN |
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Term
| What does the S-T interval represent and how long does it last? |
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Definition
| represents the time when the ventricles are fully depolarised during the plateau of the ventricular AP |
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Term
| How can changes in the S-T interval be used in diagnosis? |
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Definition
S-T segment is:
- elevated in acute myocardial infarction
- depressed when the heart is ischaemic (restriction in blood supply to tissues, causing a shortage of oxygen and glucose needed for cellular metabolism (to keep tissue alive)) |
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Term
| How are most cases of elevated S-T myocardial infarction treated? |
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Definition
| thrombolysis or percutaneous coronary intervention |
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Term
| What does the Q-T interval represent and how long does it last? |
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Definition
This is the period from the beginning of ventricular depolarisatin to the end of repolarisation
Ventricular contraction takes place
400ms |
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Term
| How can changes in the Q-T interval be used in diagnosis? |
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Definition
| It can be lengthened by myocardial damage, coronary ischaemia or conduction abnormalities as well as in genetic syndromes. |
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Term
| What is the R-R interval? |
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Definition
| the time between successive heart beats |
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Term
| How can heart rate be determined using an ECG? |
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Definition
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Term
| Describe the normal sinus rhythm |
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Definition
| 1:1 ratio between P, QRD and T waves |
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Term
| Describe (and draw) a Junctional Rhythm. How does the heart respond? |
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Definition
SA node non-functional so P waves are completely absent.
Heart is now using the AVN as its pacemaker (40-60bpm) |
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Term
| Describe (and draw) a second degree heart block. |
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Definition
| Some P waves are not conducted through the AVN so there are two P waves per cycle |
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Term
| Describe (and draw) a ventricular fibrillation |
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Definition
| chaotic ECG as seen ina cute heart attacks; the ventricles no longer pump blood and death occurs |
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Term
| Describe (and draw) a surpaventricular arrhythmia |
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Definition
two pacemakers
inverted P waves from secondary pacemaker |
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