Intro To Ecg Flashcards
ECG Chest positions V1 – V6
To position the chest leads correctly, it’s important to be able to identify the angle of Louis. Place your fingers at the bottom of your throat and move them down until you feel a boney lump. From here, move your fingers to the right and feel a gap between the ribs (2nd intercostal space).
V1 – positioned in the 4th intercostal space, right sternal border.
V2 – positioned in the 4th intercostal space, left sternal border.
V3 – In-between V2 and V4
V4 – positioned in the 5th intercostal space, mid-clavicular line.
V5 – positioned on the same horizontal line as V4, anterior axillary line.
V6 – positioned on the same horizontal line with V4, mid axillary line.
V7 - left posterior axillary line same horizontal line with V4
V8 - left tip of scapula same horizontal line with V6
V9 - left paraspinal region, same horizontal line with V4
Limb Leads positions
RA – Right Arm. Red.
LA – Left Arm. Yellow.
LL – Left Leg. Green.
RL – Right Leg. Black (neutral).
The limb leads should be placed at least 15cm away from the heart. Ideally limb leads should be placed on the ankles and wrists.
Einthoven’s Triangle
Einthoven’s triangledescribes the relationship between the limb leads and electrodes. It is an imaginary formation of three limb leads in a triangle used inelectrocardiography, and is formed by the RA, LA and LL electrodes.The shape forms an inverted equilateral triangle with the heart at the center.
3 bipolar limb leads: Lead 1 looks at the voltage difference between the RA and LA. Lead 2 between RA and LL, and Lead III between LA and LL.
Then you have your 3 unipolar limb leads –aVr, aVl and aVf.
Lead polarities on ECG
With the use of these 10 electrodes – 12 leads can be derived.
Bipolar limb leads have a negative and positive electrode and measure the potentials between the two electrodes.
3 augmented unipolar leads have a single positive electrode on right and left arms and left leg.
6 unipolar chest leads – look at the heart from the front.
Cardiac Conduction System Components
1- The Sino-Atrial Node (pacemaker)
2- The Atrio-Ventricular Node
3- Bundle Of His
4 -Left Bundle Branch
5- Right Bundle Branch
6 -Purkinje Fibres
The sino-atrial node generates the electrical impulse that travels through heart causing contraction.
The electrical signal travels from cell to cell through the atria causing them to contract, then continues on to AV node.
AV node delays stimulus, before relaying it on to the ventricles.
Impulse is passed to the bundle of His and on to the right and left bundle branches.
Both bundles eventually terminate in individual purkinje fibres.
Contraction occurs in the ventricles.
Optimal haemodynamic function occurs.
What is an ECG?
A graphical representation of the heart’s electrical activity that occurs over time.
Cardiac depolarisation and repolarisation generates electrical currents which spread throughout the body - measured by electrodes placed on the body surface.
The ECG is recorded at a set speed, and the voltages are calibrated.
A standardised recording speed allows appreciation of heart rates and cardiac intervals and comparisons to be made between ECGs on different occasions.
Standard Settings
Time :- 25mm per second (Standard Paper Speed)
1 small square = 1mm = 0.04 seconds (40ms)
5 small squares (1 ‘big’ square) = 0.2 seconds (200ms)
Amplitude :- 10mm = 1mV (Standard Paper Amplitude)
1 small square = 1mm = 0.1mv
5 small squares (1 ‘big’ square) = 0.5mV
ECG is recorded using standard measures for amplitude of the electrical signal and for the speed at which the paper moves during recording. This allows appreciation of heart rates and cardiac intervals and comparisons to be made between ecgs on different occasions.
ECG Waveforms
The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles.
P Wave – Atrial depolarisation (atrial contraction)
PR Interval – Time between the onset of atrial depolarisation and the onset of ventricular depolarisation
QRS – Ventricular
depolarisation
(Ventricular contraction)
T Wave – Ventricular repolarisation
How to Calculate Heart Rate
Heart Rate = 1500
No. small sq between R-R interval
….Or count number of QRS complexes across 10 second rhythm strip and x 6
Normal ECG Intervals
Normal P wave :-
< 2.5mm/0.25mV in height (2.5 small squares)
0.08-0.10 seconds duration (2-2.5 small squares)
PR Interval:-
Between 0.12 - 0.2 secs (3-5 small squares)
QRS Complex :-
Between 0.08 - 0.12 secs (2-3 small squares)
QTc Interval :-
QTc is prolonged if > 440ms in men or > 460ms in women
The P wave
The P wave represents the depolarisation of the atria
The impulse originates from the SA node
Normal duration: 0.08 – 0.10 s (80 – 100ms)
Normal amplitude: < 0.25mV
The PR interval
The PR interval represents the length of time it takes for the signal to travel from the SA node to the AV node.
Represents the amount of time the signal is ‘held’ at the AV node.
Why is there a short delay at the AV node?
Normal PR interval: 0.12 – 0.2 s (120 – 200ms)
It is measured from the beginning of the P wave to the beginning of the QRS complex
The QRS complex
The QRS complex represents the depolarisation of the ventricles.
Normal duration: 0.08 – 0.12 s (80 – 120 ms).
The direction of the QRS depends upon the direction of the electrical current in relation to the position of the ECG electrode.
Current flow towards a lead = waveform deflects upwards
Current flow away from a lead = waveform deflects downwards
Current flow perpendicular to a lead = waveform deflects in both directions (biphasic)
- An initial negative wave
= Q wave - An initial positive wave
= R wave - A negative wave following a positive
= S wave
The QT interval
The QT interval represents the amount of time it takes the ventricles to depolarise and repolarise (recovery to the normal resting state).
The duration of the QT interval should vary depending on the heart rate. This is then corrected for heart and called the QTc.
The faster the heart rate, the shorter the QT interval should be.
QTc is prolonged if > 440ms in men or > 460ms in women
How to Measure QTc
QT Interval = Measure from the start of the QRS complex to the end of the T wave.
RR Interval = Measure between the two preceding consecutive R waves
QTc = QT/square root RR
The ST segment
The ST segment represents the transient period where no electrical impulses can travel through the myocardium.
It is measured from the end of the QRS complex to the start of the T wave.
It is useful in the diagnosis of myocardial infarction and ischaemia.
It should be no more than 1mm above or below the isoelectric line.
The T wave
The T wave represents ventricular repolarisation.
T wave shape and orientation varies from lead to lead.
There is no clearly defined range for T wave amplitude.
As a general guide, the T wave should be no more than half the size of the preceding QRS.
Sinus Rhythm
Sinus Rhythm :-
Atrial impulse originating from SA node.
P wave preceding each QRS.
HR between 60 - 100 bpm.
Regular rhythm.
Sinus bradycardia
HR <60bpm
Regular rhythm
P wave before each QRS, identical
PR interval .12-.20
QRS <.12
Sinus Tachycardia
HR >100bpm
Regular rhythm
P wave before each QRS, identical
PR interval .12-.20
QRS <.12
Sinus Arrhythmia
Common in children.
HR 60-100 bpm
Irregular rhythm
P wave before each QRS, identical
PR interval .12-.20
QRS <.12
