ECG Flashcards
What is the direction of spread of depolarisation? Repolarisation?
- Endocardium to epicardium
* Epicardium to endocardium
What is an ECG?
Recording of potential changes, detected by electrodes on body surface
What do potentials on body surface arise from?
Currents that flow when membrane potential of myocardial tissue changes (e.g. depolarisation, repolarisation)
What cardiac tissues generate sufficient current to be detected on body surface?
Large masses of cardiac tissue e.g. atria, ventricles
Why does left ventricle generate greater potential than right ventricle?
Greater mass of cardiac tissue
What info does ECg provide? (4)
- Cardiac rate
- Cardiac rhythm
- Chamber size
- Electrical axis of heart
What is ECG the main test for? (2)
Myocardial ischaemia and infarction
What does electrical activity within and between cardiac myocytes cause? (2)
- Current flow within the heart
* Current flow in surrounding tissues
What is the difference between electrocardiograph and electrocardiogram?
- Electrocardiograph is the recording of potential differences on body surface
- Elecetrocardiogram is visual output electrocardiograph produces
What are separations of charge called?
Electrical dipole
What is a vector?
Separation of charge (dipole) that has a particular direction and magnitude
Which direction does vector travel in heart?
From atria to ventricles
Why is the magnitude (length) of vector clinically important?
Allows electrical axis of heart to be estimated
What is magnitude of electrical vector determined by?
Mass of cardiac muscle involved in generation of signal
What is direction of electrical vector determined by?
Overall activity of heart
What are the 2 types of electrode? (2)
- Recording electrode (‘seeing electrode’)
* Reference electrode
What is seen when depolarisation moves towards recording electrode?
Generates upward deflection on ECG
What is seen when depolarisation moves away from recording electrode?
Generates downward deflection on ECG
What is lead axis?
Imaginary lines between electrodes
What happens if their is no movement towards or away from recording electrode?
No deflection on ECG - isopotential
What does 12 lead ECG comprise? (3)
- 3 standard limb leads (I, II, III)
- 3 augmented voltage (aV) leads (aVR, aVL, aVF)
- 6 chest/precordial leads (V1-V6)
What are standard limb leads termed? Augmented voltage leads?
- Bipolar
* Unipolar
What plane do standard limb leads and augmented voltage leads provide a view of? Precordial leads?
- Vertical (coronal)
* Horizontal (transverse)
What is the recording electrode [ X ] in each limb lead?
- Lead I: RA to [LA]
- Lead II: RA to [LL]
- Lead III: LA to [LL]
What lead is often used as the rhythm strip?
Limb lead II
Which direction does lead II view the heart from?
Inferior
Describe the appearance of the P wave in lead II
- Atrial depolarisation spreads from SA node inferiorly and to the left
- Depolarisation moves towards recording electrode in lead II producing upward deflection in ECG (P wave)
How long does P wave normally last?
Less than 0.120 s (120 ms)
What is a negative deflection preceding an R wave called?
Q wave
What is a positive deflection called, irrespective of whether it is preceded by a Q wave?
R wave
What is a negative deflection following an R wave called?
S wave
How long does QRS complex normally last?
0.1 s (100 ms) or less
Why is Q wave negative in lead II?
Ventricular depolarization starts in interventricular septum and spreads from left to right meaning it moves slightly away from recording electrode
Why is R wave positive in lead II?
Depolarization of main ventricular mass moves towards recording electrode
Why is S wave negative in lead II?
Final point in ventricular depolarisation occurs at base of heart meaning positive charge moves from apex to base, away from recording electrode
Why is T wave positive deflection in lead II?
Wave of repolarisation (negative charge) moves away from recording electrode (i.e. the same as positive charge moving towards it)
What is the PR interval?
From start of P wave to start of QRS complex - reflects time taken for SA nod impulse to reach ventricles
What is diagnostically important about the PR interval?
Influenced by delay in conduction through the AV node
What is the ST segment?
From the end of the QRS complex to the start of the T wave
What feature of the ST segment would indicate pathology?
- It is normally isoelectric
* Elevation or depression would indicate pathology
What is the QT interval?
From start of QRS complex to end of T wave - reflects time for ventricular depolarisation and repolarisation
What are the normal durations for QT interval in males and females? (2)
- 0.44 s (440 ms) in males
* 0.46 s (460 ms) in females
What does prolongation of QT interval do?
Predisposes to disturbances of cardiac rhythm (may be caused by drugs)
What is the hexaxial reference system?
Six views in total (3 standard + 3 augmented limb leads)
Are aVR waves positive or negative? Why?
Negative - predominant vector is depolarisation moving away from recording electrode (inverted compared to lead II)
Of the hexaxial reference system, what are the lateral leads?
Leads I and aVL (view heart from the left via recording electrode on left arm)
Of the hexaxial reference system, what are the inferior leads?
Leads II, III and aVF (recording electrode eon left foot and views heart from inferior direction)
Which view of the heart do precordial leads (V1 - V6) provide?
Horizontal plane
Where do the different chest leads ‘look’?
- V1 and V2 come from the right and look at intraventricular septum
- V3 and V4 look at anterior of heart
- V5 and V6 look at left ventricle of heart
Describe the appearances of the waves of precordial leads
- First positive deflection is R wave (no Q wave) and negative deflection that follows is the S wave
- R wave progressively increases from V1 to 6 whilst S wave progressively decreases
What are the positions of precordial chest leads?
- V1 - 4th intercostal space, immediately right sternum
- V2 - 4th intercostal space, immediately left of sternum
- V3 - mid-way between V2 and V4
- V4 - 5th intercostal space, mid-clavicular line
- V5 - same horizontal level as V4, anterior axillary line
- V6 - same horizontal level as V4, mid-axillary line
What happens in ST segment? TP segment?
- ST segment - ventricles contract (systole)
* TP segment - ventricles relax (diastole)
How long does P wave last? QRS complex? PR interval? QT interval?
- P wave - 0.08-0.1 sec
- QRS complex - <0.1 sec (<3 small squares)
- PR interval - 0.12 - 0.2 sec (no bigger than 1 large square)
- QT interval - 0.36 - 0.44 sec
What is normal calibration of ECG trace?
- Paper speed 25mm/sec
* 10mm/1mV reference pulse
What does one large box represent? One small box?
- Large box - 0.2 seconds and 5mm amplitude
* Small box - 0.04 seconds and 1mm amplitude
How is heart rate calculated from ECG paper?
300/number of large squares between R-R interval
What is ECG rhythm strip?
Prolonged reading of one lead (normally lead II) that allows you to detect rhythm disturbance
Why are 12 leads necessary? (3)
- Determine axis of heart in thorax
- Look for any ST segment or T wave changes in specific regions of heart (e.g. in ischaemic heart disease)
- Look for voltage criteria changes e.g. in chamber hypertrophy
What are the steps to analysing an ECG? (6)
1) Verify patient details (name, DOB)
2) Check date and time ECG was taken
3) Check calibration of ECG paper
4) Determine axis
5) Work out the rhythm (7 questions)
6) Look at individual leads for voltage criteria or changes in ST or T waves
What questions are asked to determine rhythm from rhythm strip? (7)
- Is electrical activity present?
- Is the rhythm regular or irregular?
- What is the heart rate?
- P-waves present?
- What is the PR interval?
- Is each P-Wave followed by a QRS complex?
- Is the QRS duration normal?
What conditions does a normal ECG not exclude? (3)
- Myocardial infarction
- Intermittent rhythm disturbance
- Stable angina ( ST changes absent during rest so should do exercise ECG)
