ECG theory and Practice Flashcards
What happens in sinus rhythm?
SA node conducts generates AP automatically that conduct rapidly through the atria causing atrial contraction
Relatively slowly through the AV node
-Rapidly through the bundle of His and the left and right bundle branches
- Rapidly through the purkinje fibers causing ventricular contraction
What is an ECG?
Recording of potential changes detected by electrodes
positioned in the body surface that allows the electrical activity of the heart to be monitored in a simple manner which is non invasive
How do potentials at the body surface occur?
Arise from currents that flow when the membrane potential of the myocardial tissue is changing (depolarization or repolarization)
What info can an ECG provide?
Rate Rhythm Chamber size Electrical axis of heart Assess for myocardial ischaemia and infarction
What does electrical activity within and between cardiac muscle cells cause?
Current flow within the heart
Current flow within the surrounding tissues
Potential differences between different sites on the body surface
What is a recording between potential differences at different sites of the body called?
Electrocardiogram (ECG) or EKG
Simplified physics of an ECG?
The AP propagating in sequence through the conducting system and muscle of the heart causes separation of charge or differences in potential between cardiac regions
Charges that are separate are called an electrical dipole which is a vector with components of magnitude and direction (eg atria to ventricle)
What does a dipole represent?
Electrical vector
Why is an electrical vector important in an ECG?
Clinically important as it allows the electrical axis of the heart to be estimated
Has components of magnitude and direction (orientation of arrow)
What is magnitude of an electrical dipole?
Determined by mass of cardiac muscle that is involved in generation of signal
What is direction of an electrical dipole determined by?
Overall activity of the heart at any instant in time and varies during cardiac cycle
What is an ECG lead?
Imaginary line (not the wire) between 2 or more electrodes
In a lead what do the two electrodes act as?
Recording electrode (red) Referencing electrode ( green)
What happens on the ECG when depolarization moves toward the recording electrode?
Generates an upward deflection
What happens on ECG when depolarization moves away from recording electrode?
Generates a downwards deflection
What is a flat line on ECG referred to as?
Isopotential
What do you call it when there is no movement on an ECG?
Isopotential
No movement towards or away from recording electrode
What does a 12 lead ECG comprise of?
3 standard limb leads - 1,2 and 3- termed bipolar
3 augmented voltage leads (aV) - aVR right, aVL left and aVF foot - termed unipolar
6 Chest leads V1-V6 (precordial leads)
What do the 12 leads provide?
Comprehensive picture of the heart on different planes and from different directions
What leads show the vertical (frontal) plane?
Leads 1, 2 and 3
aVR, aVL, and aVF
What leads show the horizontal plane?
V1- V6
Where is Lead 1?
RA (-ve)to LA (+ve)
Where is lead 2?
RA (-ve) to LL (+ve)
Where is lead 3?
LA (-ve) to RL (+ve)
What direction does lead 2 see the heart from?
Inferior
Describe what is seen for lead 2 and why?
Normally upward deflection (P wave)
Atrial depolarization spreads from the SA node inferiorly and to the left (depolarization is moving towards the recording electrode in lead 2
Which type of electrode is +ve?
Recording
How long should a P wave last?
0.12 seconds or less
What is the downward deflection preceeding an R wave called?
Q wave
What is an R wave?
A deflection upwards whether or not its following a Q wave
What is an S wave?
Downward deflection following an R wave
What does a QRS complex represent?
Ventricular activation or depolarization
Duration of a QRS complex?
0.1 s or less
What causes a small and narrow Q wave?
Ventricular depolarization starting in the interventricular septum and spreading from reft to right
What causes a tall and narrow R wave?
Free walls of ventricles depolarizing
What produces a small and narrow S wave?
Ventricles at the base of the heart depolarize
What does a T wave represent?
Ventricular repolarization
What is a T wave?
Upward deflection because the wave of REpolarization is spreading away from the recording electrode (-ve charge moving away from the recording electrode)
Full explanation and summary of lead 2?
P wave – atrial depolarization moving towards the recording electrode
Q wave – left to right depolarization of the interventricular septum moving slightly away from the recording electrode
R wave –depolarization of the main ventricular mass moving towards the recording electrode
S wave – depolarization of ventricles at the base of the heart moving away from the recording electrode
T wave – ventricular repolarization moving in a direction opposite to that of depolarization accounts for the usually observed upward deflection
When is the PR interval?
Start of the P wave to the start of the QRS complex
What does the PR interval show?
Reflects the time for the SA node impulse to reach the ventricles
What is the usual length of the PR interval?
0.12-0.2s
What is the PR interval strongly influenced by?
Delay in conduction through the AV node- diagnostically important
When is the ST segment?
End of the QRS complex to the start of the T wave
What does the ST interval look like?
Normally isoelectric
When is the QT interval?
Start of the QRS complex to the end of the T wave
What does the QT interval reflect?
The time taken for ventricular de and repolarization
Usual QT interval length?
- 44s in males
0. 46s in females
What does a prolongation of the QT interval indicate?
Predisposal to distrubances of cardiac rhythm
What is Goldbergers method?
One recording electrode, 2 others linked as referencing
- effectively positions the reference electrode in the center of the heart to which the recording electrodes ‘look’
What is hexaxial reference system?
6 views in total
views of the heart in frontal plane provided by standard and augmented leads
Describe the waves on the aVR lead?
Waves are negative
predominant vector is depolarization moving away from recording electrode
Describe the waves on lead 2?
Waves are positive
Predominant vector is depolarization moving towards the recording electrode
What type of leads are leads 1 and aVL?
Lateral leads
Which leads are inferior?
Lead 2, 3 and aVF
What do V1 & V2 look at?
Interventricular septum
What do leads V3 and V4 look at?
Anterior of the heart
What do V5& V6 look at?
Lateral aspect of heart (Left v)
What does an R wave look like on V1?
The first positive deflection in the QRS complex, the negative deflection immediately after in an S wave
What happens to R and S waves from V1-V6?
R wave gets progressively increased
S wave progressively decreases
Placement of V1?
4th ICS
Immediately right of sternum
Placement of V2?
4th ICS
Immediately left of sternum
Placement of V3?
Midway between V2 & V4
Placement of V4?
5th ICS
Mid-clavicular line
Placement of V5?
Same horizontal level as V4
Anterior axillary line
Placement of V6?
Same horizontal level as V4
Mid-axillary line
What does P wave represent?
Atrial depolarization
Length of P wave?
0.08-0,10 secs
QRS complex length and represents?
less than 0.1s
Ventricular depolarization
T wave represents?
Ventricular repolarization
What does ST segment represent?
Ventricles contracting (systole)
What represents diastole (ventricles relaxing)?
TP segment
What does PR interval represent?
Largely AV nodal delay
QT interval represents?
Depolarization and repolarization of ventricles - lengths needs to be corrected to heart rate
What is an ECG rhythm strip?
prolonged recording of one lead
usually lead 2
Allows you to determine HR and identify cardiac rhythm
Why do we need 12 leads?
12 leads look at heart from different directions which allows-
Determine the axis of the heart
Look for any ST segment or T wave changes across different areas of the heart - IHD
Look for any voltage criteria changes - crucial in diagnosing chamber hypertrophy
6 key steps for analyzing an ECG?
1) Confirm patients name and DOB
2) Check date and time ECG was taken
3) Check callibration of ECG paper
4) Determine axis
5) Workout rate and rhythm -
- electrical activity present
- Rhythm regular or irregular
- Heart rate
- P waves present
- PR interval
- Each P wave followed by QRS complex
- QRS duration normal?
6) Look at individual leads for voltage criteria changes or any ST segment r T wave changes
What does a normal ECG not exclude?
MI (can cause ST elevation though sometimes)
Intermittent rhythm disturbance - ABPM
Stable angina - if suspected do exercise ECG
