CVS 12 - Interpreting ECG's (Part 1) Flashcards
Give an overview of the cardiac conducting system + how electrical activity is spread throughout the heart.
1) Initiated at the SAN (junction of RA + SVC), depolarisation of LA + RA.
2) Hits AVN in inter-atrial septum near tricuspid valve, AP delayed.
3) From AV node down bundle of His, through the annulus fibrosus.
4) Bundle of His separates in the interventricular septum into the LBB + RBB
5) RBB + LBB terminate in purkinje fibres, which continue to conduct depolarisation wave through the ventricles.
What does an ECG do?
How is it set up
Why is it called the “12-lead” ECG?
- Measures changes in electrical potential (mV) over successive areas of myocardium.
- 10 electrodes, 4 on limbs, 6 on chests (limb + chest leads).
- Gives 12 views of the heart, therefore is “12-lead”
Where do limb leads l,ll & lll run from and to?
What kind of electrodes are limb lead l, ll + lll?
1 = RA to LA (LA is positive electrode) 2 = RA to LL (LL is positive electrode) 3 = LA to LL (LL is positive electrode)
- Remember, RL electrode is grounding electrode, not used for any leads/views
- They are bipolar electrodes (negative + positive)
Where are the augmented limb leads located?
What kind of electrodes do these leads have?
aVR = Right arm aVL = Left arm aVF = Left leg
- Only have a positive electrode (unipolar)
What defines whether the electrodes cause a positive or negative deflection in the ECG?
- Cardiac view from a lead is from the perspective of the positive electrode. Therefore …
1) Depolarisation travelling towards positive node = positive deflection.
2) Repolarisation travelling towards positive node = negative deflection
What type of electrodes are the precordial (chest) leads?
- Unipolar - the other electrode is average of the limb electrodes and positions in the middle of the chest (reference lead).
How does direction of the depolarisation wave affect the shape of the deflection?
- Depolarisation travelling directly towards the (+) electrode causes tall-upright deflection
- Wave travelling obliquely towards electrode causes smaller upright deflection
- Wave travelling at 90 degrees to electrode causes no complex
- Depolarisation travelling directly away from electrode causes deep negative deflection.
How is SAN depolarisation represented on the ECG + why?
How is Atrial depolarisation represented on the ECG + why? (How long does this last?)
How is AV node delay represented on the ECG + why?
1) Nothing registers as the signal is insufficient
2) Depolarisation travelling down and to the left through atrial muscle fibres + internodal pathways towards the (+) electrode causing a small upward deflection (P-wave/ 80-100ms).
3) Signal is very small so see isoelectric line (flat) after the P-wave. Length of this line is important (changed in pathology).
How is bundle of his spread of DP from atria to ventricles represented on the ECG + why? (how long should this last?)
How is depolarisation of interventricular septum represented on the ECG + why?
1) Conduction through bundle of his gives an isoelectric (flat) line. 120-200ms from start of atrial DP to start of ventricular DP.
2) Interventricular septum DP’s first from left to right. DP is moving obliquely away causing small downward deflection (Q-wave). So Q-wave = DP of interventricular septum.
How is depolarisation of the apex + ventricular walls represented on the ECG + why?
How is depolarisation of the base of the ventricles represented on the ECG + why?
How is ventricular repolarisation represented on the ECG + why?
1) Depolarisation travelling directly towards electrode. Large muscle mass so produced large-upward deflection (R-wave).
2) DP travelling obliquely away so produced small negative deflection (S-wave) as DP spreads to base of ventricles. Full QRS = 80-120ms.
3) Repolarisation moving away (not directly) from positive electrode so produces small positive deflection (T-wave)
Give a summary of what produces the P wave, QRS complex, + T wave in the ECG.
P-wave = Atrial depolarisation NOT atrial contraction QRS = Depolarisation of ventricles NOT ventricular contraction T-wave = Repolarisation of ventricles NOT relaxation of ventricles.
- Contraction of atria immediately follows P-wave
- Contraction of ventricles immediately follows QRS complex
- Relaxation of ventricles immediately follows T-wave
What aspect of the heart do limb leads l + aVL look at + what kind of problems are they good for diagnosing?
What aspect of the heart do limb leads ll, lll + aVF look at + what kind of problems are they good for diagnosing?
1) l + aVL look at the left side of the heart, best at looking for problems in the lateral wall of the left ventricle - e.g.: necrosis due to left coronary artery occlusion.
2) ll, lll + aVF look at the inferior surface of the heart, best for detecting problems on the inferior surface, e.g.: necrosis due to right coronary artery occlusion.
What aspects of the heart do the 6 precordial chest leads look at?
- All view the heart in the horizontal plane
- V1 + V2 = RV + septal leads
- V3+V4 = apex + anterior wall of RV + LV
- V5+V6 = LV
Which ECG leads look at:
1) Inferior surface of ventricles
2) Septum + anterior surface of ventricles
3) Right ventricle + septum
4) Apex + anterior surface of ventricles
5) Lateral surface of ventricles
1) ll, lll + aVF
2) V1-V4
3) V1, V2 + aVR
4) V3 + V4
5) l, aVL, V5 + V6
How do you calculate HR from an ECG when heart rhythm is normal?
- 1 small square = 1mm = 0.04 seconds
- 1 large square = 5mm = 0.20 seconds
- 300 small squares = 1 minute
- Do 300/Number Large Squares between adjacent R-R waves.
- E.g.: 300/4 = 75bpm.
