Basic Electrocardiography Flashcards
Why are ECGs important?
Because the electrical stimulus proceeds the physical occurrence e.g. ventricular contraction
1st heart sound is at QRS whilst 2nd one is at T wave ~
What does an ECG lead trace represent?
An aggregate of the entire cardiac current measured from a single point over time; each lead is unique to + identified by its position
What ions contribute to the electrical potential of the heart?
Na+: depolarization
K+: repolarization
What does each wave of a ECG trace represent?
P: atrial depolarisation
QRS complex: depolarization of ventricle + atrial repolarization
T: ventricular repolarization
How is cardiac electrical conduction measured?
- Electrical signal generated by an AP reduces as the distance to the recording lead increases
- Depolarisation wave toward electrode records upward/positive deflection
- Depolarisation wave away from electrode records downward/negative deflection
What is signal amplitude related to?
Myocardial mass
What and how do different factors affect signal amplitude?
- Small structures e.g. SA + AV node & bundle of His will not produce a measurable signal
- Left ventricular signal dominates right (normal QRS axis shows this; -30o-90o)
- Atrial signal smaller than ventricular
- Atrial repolarisation not visible
- Changes if muscle mass changes
What are the frontal ECG leads?
I: L + R arm
II: R arm + L foot
III: L arm + L foot
AVF: lead comes off of I + goes to L foot
AVL: lead comes off of II + goes to bony prominence of L arm on wrist
AVR: lead comes off of III + goes to bony prominence of R arm on wrist
How many ECG leads are there in total?
12 leads BUT 10 stickers
What ECG sticker represents the earth lead?
R foot
Where are the chest wall ECG leads placed?
V1: 4th R ICS at sternal margin
V2: 4th ICS at sternal margin
V3: Midway between V2 + V4
V4: 5th ICS in MCL
V5: L anterior axillary line on same horizontal plane as V4
V6: L mid-axillary line on same horizontal plane as V4 + V5
What timeframe do the different size squares on an ECG represent?
Small = 0.04 secs
Large = 0.2 secs
So 5 large squares = 1 sec
How do you calculate the heart rate from an ECG?
300/number of large blocks between QRS complexes
Count 30 big squares (6 seconds) + count no. of R waves within this x 10 = HR per minute (accounts for irregularity)
How can you tell if sinus rhythm is normal from an ECG?
P waves equidistant before every regularly spaced QRS
What is sinus rhythm?
Normal heart beat with respect to the heart rate and rhythm
Why do the ECG leads on the right have a smaller positive deflection than those on the left?
Further away from left ventricle (where majority of muscle mass is)
What is the QRS (electrical) axis a measure of?
The ventricular activity in the frontal (vertical) plane (normally directed leftward + downward)
How do you work out the QRS (electrical) axis?
Determined by observing the QRS complex in leads I, II, III, AVF, AVL + AVR:
- Look first for lead whos +ve & -ve deflections sum closest to 0 (equiphasic)
- Main QRS axis will be at R angles to this
- Dominant deflection in this lead (+ve/-ve) will indicate axis direction
What are the 2 commonest abnormalities of the QRS complex?
- Changes in muscle bulk e.g. hypertrophy due to hypertension or stenotic valve lesions OR muscle destruction due to CAD or myocarditis
- Abnormalities of electrical conduction e.g. damage to conduction system (often ischaemic), damage to sinus node OR abberant or multiple foci of generation (AF)
Where does a normal QRS axis lie?
-30o (+AVL) to +90o + AVF +ve
Where do deviated QRS axis lie?
LAD: (+AVF) -30o to -90o (-AVF) - I is +ve + AVF -ve
RAD: +90o (+AVF) to +180o (-I) - I is -ve + AVF is +ve
Where does a indeterminate QRS axis lie?
-180o to -30o
If there was a right sided QRS axis deviation, what could this imply?
Right ventricular hypertrophy
Where do the ECG leads face in terms of the heart itself?
Lateral: I, AVL, V5 + V6
Inferior: II, III + AVF
Anterior: V3 + V4
Septal: V1 + F2
AVR = anomaly
Which 3 things do you interpret from an ECG?
- Heart rate
- Heart rhythm
- QRS axis
If T waves are inverted, what can this mean?
Cardiac ischaemia
Previous infarct
If the ST segment is elevated or depressed, what can this mean?
Elevation: suggestion of infarction, high take off?
Depression: suggestive of cardiac ischaemia
How long is a normal PR interval?
0.12-0.2 secs (3-5 small squares)
How long is a normal QRS segment?
< 0.12 secs (< 3 small squares)
What does the Q-T interval represent?
Time for ventricular depolarization + repolarization; roughly estimates duration of ventricular AP - ranges from 0.2-0.4 secs
How will an increase in heart rate affect the Q-T interval?
Increased HR -> shortened ventricular APs -> decreased Q-T internal
Why is a decreased Q-T interval clinically important?
Need corrected QT = QTc
How is electrical current conducted through the heart?
Sinus node -> atrial muscle -> A-V node -> common bundle -> bundle branches -> purkinje fibres -> ventricular muscle
