What do the P, QRS and T waves correspond with in the cardiac cycle?
What does the PR segment represent?
P wave= atrial systole
P-R: time taken from SA node to AV node
QRS complex = ventricular systole
T wave= repolarisation
What do upward (positive) and downward (negative) deflections on an ECG represent?
Downward (negative): depolarisation wave away from electrode
Upward (positive): depolarisation wave towards electrode
The electrical signal generated by an action potential reduces as the distance to the recording lead increases.
What does the signal amplitude represent?
Which ventricle normally predominates?
Represents myocardial mass (increased mass = more amplitude)
- Signal amplitude will therefore change in conditions affecting muscle mass
Left ventricle normally predominates as it usually has a larger myocardial mass (represented by normal QRS axis of -30-90°)
What is the QRS axis?
How is it determined?
What is a normal QRS axis?
The measure of ventricular electrical activity in the vertical plane.
Determined by observing the QRS complex in leads I, II, III, AVF, AVR, AVL:
- First look for lead whose deflections sum closest to 0 (equiphasic)
- The main QRS axis will be at right angles to this
- The dominant deflection in this lead (positive or negative) will indicate axis direction
What do leads I, II and III record?
What do leads AVF, AVR and AVL record?
I = difference in electrical potential between the right and left arms
II = difference in electrical potential between the right arm and left foot
III = difference in electrical potential between the left arm and left foot
AVF = the difference between the combined electrical signal between the two arms and the left foot.
AVR = the difference between the combined electrical signal between the left arm and left foot and right arm
AVL = the difference between the combined electrical signal between the right arm and left foot and the left arm
Where are the leads of a 12 lead ECG placed?
V1: 4th right intercostal space at sternal margin
V2: 4th left intercostal space at sternal margin
V3: Midway between V2 and V4
V4: 5th intercostal space in mid-clavicular line
V5: left anterior-axillary line, same horizontal plane as V4.
V6: left mid-axillary line on same horizontal plane as V4 and V5
Red: right arm
Yellow: left arm
Green: left leg
Black: right leg
What are 5 small squares equal to on an ECG?
1 small square: 0.04 seconds and 0.1 mV
5 small squares (1 large square): 0.2 seconds
5 large squares: 1 second
What is an indicator of sinus rhythm on an ecg?
p waves will be equidistant from regularly spaced QRS complexes
How can you calculate the heart rate on a ECG?
300/ number of large squares between QRS complexes
What are some common ECG abnormalities caused by changes in muscle bulk?
- Hypertrophy due to hypertension or stenotic valves
- Muscle destruction due to coronary artery disease or myocarditis
What are some common ECG abnormalities caused by electrical conduction?
- Ischaemic damage to conduction system
- Damage to sinus node
- Abherrant foci of generation
- Multiple foci of generation (atrial fibrillation)
What axis would indicate left axis deviation?
+90 - +180°
What axis would indicate right axid deviation?
-90 - 0°
What view of the heart do leads II, III and AVF represent?
What arteries are likely to be blocked if ST elevation is detected in these leads?
Right Coronary Artery
What view of the heart do leads I, AVL, V5 and V6 give?
What view of the heart do leads V1 and V2 give?
What view of the heart do leads V3 and V4 give?