Session 8 Flashcards
What type of signal does a depolarisation moving towards an electrode give on an ECG?
Upward signal
What type of signal does a depolarisation moving away from an electrode give on an ECG?
Downward signal
What type of signal does a repolarisation moving towards an electrode give on an ECG?
Downward signal
What type of signal does a repolarisation moving away from an electrode give on an ECG?
Upward signal
What affects the amplitude of a signal on an ECG?
- The amount of muscle depolarising
- How directly the signal is moving towards the electrode
Describe the P wave on an ECG
- Caused by atrial depolarisation
- Small upward deflection
How long is the gap between a P wave and the Q wave on a normal ECG?
Around 120ms
Describe the Q wave on an ECG
- Caused by septal depolarisation spreading to the ventricle
- Small and very short downward deflection
Describe the R wave on an ECG
- Caused by the main ventricular depolarisation
- Large, relatively short upward deflection
Describe the S wave on an ECG
- Caused by end ventricular depolarisation
- Small downward deflection
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
Describe the T wave on an ECG
- Caused by ventricular repolarisation
- Medium upward deflection
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
Which electrode in a 12 lead ECG is neutral?
Right lower limb lead
What view of the heart do the limb leads provide?
Vertical view
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
