ECG 1 and 4 - Theory and recognising normal/ abnormal ECGs Flashcards
What is the name for a recording of the potential differences between distant sites on the body surface?
Electrocardiogram (measured by an electrocardiograph)
What is the wave of depolarisation across the heart and what does this create?
Positive charge moving towards the ventricles leaving behind a negative chargeCreates an electric dipole (which generates an electrical field) - the dipole represents an electrical vector
In terms of leads, when is the measured potential difference greater?
When lead axis is parallel to the direction of the dipole- zero when perpendicular
What determines the magnitude of the electrical vector?Direction?
Magnitude = mass of cardiac muscleDirection = overall activity of the heart
Where does each of the standard limb leads (I, II, III) run?At what degree does each lead look at the heart?
I = RA (-) - LA (+) (0 degrees = 3 o’clock)II = RA (-) - LL (+) (+60 degrees = 5 o’clock)III = LA (-) - LL (+) (+120 degrees = 7 o’clock)RL = earthed
In terms of depolarisation, what causes an upwards deflection on the ECG and what causes a downwards deflection?
Upwards = deflection moving towards recording electrodeDownwards = deflection moving away from recording electrode
What causes the P wave? Duration?
Action potential upstroke through the atrial muscle 0.08 - 0.1 secs
What causes QRS complex? durationWhat causes each of the separate waves?
Action potential upstroke throughout the ventricular muscle
T wave cause?
Ventricular repolarisation moving in a direction opposite to that of depolarisation accounts for the usually observed upwards deflection
What causes the PR interval? Duration
Largely AV nodal delay (0.12 - 0.2 secs) - different from PR segment
What causes the ST segment?
Maintained ventricular depolarisation
What causes the TP interval?
Diastole
What are the augmented limb leads and where does each link?
aVR = RA (+) to [LA and LF] (-)aVL = LA to [RA and LF]aVF = LF to [RA and LA]
What cause the direction of the Q wave on an ECG?
Left to right depolarisation of the inter-ventricular septum moving slightly away from the recording electrode
What causes the direction of the R wave on an ECG?
Depolarisation of the main ventricular mass moving towards the recording electrode
What causes the direction of the S wave on an ECG?
Depolarisation of ventricles at the base of the heart moving away from the recording electrode
What causes the direction of the T wave on an ECG?
Ventricular repolarisation moving in a direction opposite to that of depolarisation
What is another name for the standard limb leads?What is another name for augmented limb leads?What is another name for chest (precordial) leads?
Bipolar limb leadsUnipolar limb leadsUnipolar leads (wilson’s leads)
What type of polarity do the augmented limb leads have?
Goldberger’s method: one +ve electrode, 2 others linked as -ve electrodes
How do the augmented limb leads differ in direction to the standard limb leads?
30 degrees away
What is the name given to the normal triangle that the standard limb leads are arranged in?
Einthoven’s triangle
How does the appearance of II and aVR compare?
aVR is practically an upside down version of II
What degrees are classified as a normal axis for the heart? (between)
-30 - +90
What range of degrees are classified as a left axis?
-30 to -90
What range of degrees are classified as a right axis?
+90 to 180
What range of degrees are classified as a far right axis?
-90 o 180
At what degree is aVL present at?
-30
At what degree is aVR present at?
-150
At what degree is aVF present at?
+90
Where is V1 placed?
Right sternal edge, 4th intercostal space
Where is V2 placed?
Left sternal edge, 4th intercostal space
Where is V3 placed?
Halfway between V2 and V4
Where is V4 placed?
Apex beat
Where is V5 placed?
Anterior axillary line
Where is V6 placed?
Mid-axillary line
What lead are the ECG waves best seen in?
Lead II
What happens during the ST segment?
Ventricles contract (systole)
What happens during the TP segment?
Ventricles relax (diastole)
What is the normal QT interval?
0.36-0.44 seconds at a heart rate of 60 (needs to be corrected for heart rate)
How many seconds does each small block represent when the paper speed = 25mm/sec?
0.04 seconds (and 1mm of amplitude)
Equation for calculating HR from ECG?
300/number of large squares between beats
What is the ECG strip?
Prolonged recording of one lead (usually lead II) which allows you to detect rhythm disturbances
How many wires does an ECG have?
10 (despite there being 12 leads)
What test should be done if stable angina is suspected?
An exercise ECG (ECG changes are usually absent during rest)
How to calculate the heart rate from an ECG if the heart rate is irregular?
Count the number of QRS complexes in 30 large squares and multiply by 10
