Session 8 Flashcards by Jamie Carruthers

What type of signal does a depolarisation moving towards an electrode give on an ECG?

Upward signal

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What type of signal does a depolarisation moving away from an electrode give on an ECG?

Downward signal

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What type of signal does a repolarisation moving towards an electrode give on an ECG?

Downward signal

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What type of signal does a repolarisation moving away from an electrode give on an ECG?

Upward signal

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What affects the amplitude of a signal on an ECG?

The amount of muscle depolarising

- How directly the signal is moving towards the electrode

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Describe the P wave on an ECG

Caused by atrial depolarisation

- Small upward deflection

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How long is the gap between a P wave and the Q wave on a normal ECG?

Around 120ms

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Describe the Q wave on an ECG

Caused by septal depolarisation spreading to the ventricle

- Small and very short downward deflection

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Describe the R wave on an ECG

Caused by the main ventricular depolarisation

- Large, relatively short upward deflection

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Describe the S wave on an ECG

Caused by end ventricular depolarisation

- Small downward deflection

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How long is ventricular contraction in a typical person and what is it represented by on an ECG?

Usually ~280ms

- Represented by the QRS complex on an ECG

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Describe the T wave on an ECG

Caused by ventricular repolarisation

- Medium upward deflection

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How does the QRS complex on an ECG change if the viewing electrode is moved around the heart?

Direction and amplitude of the complex changes

Large upward deflection slightly to the left of the septum
Deflection becomes smaller as the electrode moves anti-clockwise around the heart
No deflection at the atrioventricular septum as the QRS complex moves at a right angle to the electrode

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Which electrode in a 12 lead ECG is neutral?

Right lower limb lead

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What view of the heart do the limb leads provide?

Vertical view

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What view of the heart do the chest leads provide?

Horizontal view of the heart

From what angle does lead 1 view the heart?

0 degrees

From what angle does lead 2 view the heart?

+60 degrees

From what angle does lead VF view the heart?

+90 degrees

From what angle does lead 3 view the heart?

+120 degrees

From what angle does lead VR view the heart?

-150 degrees

From what angle does lead VL view the heart?

-30 degrees

How is a regular heart rate calculated from a rhythm strip?

300/(number of squares in the R-R interval)

How is an irregular heart rate calculated from a rhythm strip?

Using more than one R-R interval and multiplying by the number of beats used; best to use 30 beats if possible

What does a ventricular ectopic beat look like on an ECG?

- QRS complex which isn't preceded by a P wave - Wider than a normal QRS complex as the depolarisation doesn't originate from or flow down the normal conduction pathway

How does atrial fibrillation appear on an ECG?

- P wave absent - Irregular fibrillation waves present instead of P wave - No stimulus reaching AV node so other pacemakers must take over; heart rate may be reduced

How does ventricular fibrillation appear on an ECG?

- Uncoordinated ventricular contraction so no QRS complexes | - Completely irregular ECG with no clearly visible waves

What is a first degree heart block?

Communication problem between the atria and ventricles causing a conduction delay through the AV node

How does a first degree heart block appear on an ECG?

- P-R interval is elongated so is longer than 200ms | - All electrical signals eventually reach the ventricles

What is a type 1 second degree heart block?

Heart block where not all atrial beats get through to the ventricles

How does a type 2 heart block appear on an ECG?

- PR interval is erratic | - PR interval elongates until eventually a QRS complex is dropped and the system resets

What is a type 2 second degree heart block?

Heart block where sometimes electrical excitation fails to pass through the AV node or bundle of His

How does a type 2 second degree heart block appear on an ECG?

- Usually a constant PR interval | - Not all atrial contractions are followed by ventricular contractions

What is a complete, third degree heart block?

Heart block where no electrical conduction is conveyed to the ventricles

How does a third degree heart block appear on an ECG?

- No link between atrial and ventricular depolarisations | - Ventricles generate own electrical signal via ectopic pacemakers so ventricular depolarisations are usually slow

What is the effect of a bundle branch block on an ECG?

- Lengthens and changes the shape of the QRS complex | - Several variations depending on the location of the block

How does an action potential spread through the the heart?

- Action potential generated by pacemaker cells in the SAN - Electrical activity spreads over the surface of the heart to the AV node - Delay at the AV node of around 120ms - Excitation spreads from the AVN down the septum via the right and left bundle branches - Excitation spreads out from the apex of the heart over the ventricular myocardium from inside to outside until all ventricular cells are depolarised - Repolarisation starts from the outside of the heart to the inside