Electrocardiogram Flashcards
Describe how excitation spreads through the conducting system and myocardium
- SA node depolarises
- Impulse spreads through atria
- AV node delays transmission to allow atrium to contract before ventricle
- Spreads to ventricle via Bundle of His
Fibrous ring located where atria and ventricular muscles are attached so conduction can only spread through Bundle of His - Signal spreads rapidly down right and left bundle branches and purkinje system
- Myocytes of interventricular septum are depolarised first
- Apex, RV and LV free walls are depolarised next
- Base of the ventricles are the last to be depolarised
- Repolarisation of the ventricles happen in the reverse order
State rules governing the sign of the signal recorded by a positive recording electrode when depolarisation and repolarisation spreads towards and away from that electrode
If wave of depolarisation moves from negative to positive electrode, it will form a positive complex (positive deflection)
A wave of repolarisation towards positive electrode will form a negative complex
Identify the electrical event at waves P, Q, R, S, T on an ECG
SA node depolarisation unseen on ECG
Insufficient signal to register on surface ECG
Atrial depolarisation causes P wave
Spreads along atrial muscle fibres and internodal pathways
Spreads downwards and to the left towards AV node
Delay at AV node creates flat line (isoelectric segment)
Allows time for atrial contraction to fill ventricle
Depolarisation can only travel through fibrous rings via bundle of His - contributes to isoelectric segment
Depolarisation of myocardium in interventricular septum causes Q wave
Septum depolarises from left to right to produce a downward deflection
Depolarisation of apex and free ventricular wall causes R wave
Produces a large upward deflection as depolarisation moving directly towards electrode
Large because large muscle mass - more electrical activity
Left ventricular hypertrophy causes a taller R wave
End of depolarisation towards ventricle base causes S wave
Depolarisation spreads upwards to the base of the ventricles
Ventricular repolarisation produces T wave
Begins on the epicardial surface
Negative wave moves the opposite direction
Where do you place the 4 electrodes on the limbs
Red lead on right arm
Yellow lead on left arm
Green lead on left leg
Black lead on right leg
State the direction the 6 limb leads look at
Lead 1 - horizontal left
Lead 2 - inferior through apex
Lead 3 - inferior from left arm
aVR, aVL, aVF look at respective direction
State what the 6 chest leads look at
Leads V1 and V2 face the right ventricle and septum
Leads V3 and V4 face the apex and anterior wall of ventricles
Leads V5 and V6 face the left ventricle - left lateral direction
What do the small and large squares represent on an ECG
ECG moves at 25mm/sec
5 large squares is one second, 1 small square is 0.04sec (40ms)
300 large squares is one minute
How do you calculate heart rate on an ECG
300/number of boxes between R wave
If heart rate is irregular, calculate heart rate by counting number of QRS complexes in 6 seconds (30 boxes), then multiply by 10
What are the axis of ECG
Voltage on y-axis, time on x-axis
State the normal PR, QRS and QT intervals
PR interval 3-5 small boxes (0.12-0.20 seconds)
Prolonged is longer than 1 large box
QRS interval < 3 small boxes (<0.12 seconds)
QT interval - length of repolarisation
Varies with heart rate - faster heart rate = shorter QT interval
Upper limit = 11 - 12 small boxes (<0.45s for males and < 0.47s for females)
Determine if ECG shows sinus rhythm
Sinus rhythm should have P waves, normal PR interval and a normal QRS complex after every P wave
Sinus rhythm varies with respiration
Breathing deeply will increase heart rate
Normal sinus rhythm takes into account normal heart rate as well as rhythm
Sinus rhythm with rate < 60 bpm is called sinus bradycardia
Sinus rhythm with rate > 100bpm is called sinus tachycardia
