ECG Flashcards
Which pathophysiology activities of the heart are represented by which part of the ECG?
P wave = Atrial depolarisation, diastole
Returns to baseline following depolarisation due to the conductance delay at the AV node
QRS complex - Ventricular depolarisation
T wave = repolarisation of the ventricles, immediately before ventricular diastole
U wave - Occasionally, an additional small deflection follows the T wave. This represents the late phases of ventricular repolarisation
P-Q = Time the signal has travelled from SAN to AVN
Q - This is the first downwards deflection following a P wave
- It represents depolarisation of the interventricular septum
R wave - The first upwards deflection, whether or not there is a Q wave
- Depolarisation of main mass of ventricles
S wave - Any downward deflection following the R wave is known as an S wave
- The last phase of ventricular depolarisation at base of the heart
- Atrial repolarisation also occurs at this time but the signal is obscured by
the large QRS complex
S-T = The plataeu in the myocardial AP, this is when the ventricles contract and pump blood
O
12 leads of ECg represent 12 electrical views of heart from 12 different angles
Attach 10 electrodes to body - 1 to each limb and 6 across body
6 chest limbs and 6 limb leads
10
Right leg - earth electrode
Where are the positions of ECG chest electrodes?
V1 = 4th ICS, 2cm to the right of the sternum
V2 = 4th ICS, 2cm from the let of the sternum
V3 = Midway between V1 and V4
V4 = 5th ICS, left midclavicular line
V5 = Same level as V4, left anterior axillary line
V6 = Same level as V4, left mid axillary line
How many limb leads and limb electrodes are there?
6 limb leads
4 limb electrodes
One on each limb, the right leg is an earth electrode though
Where are the limb leads, the + and - electrodes positioned?
They look at the heart in a vehicle plane and are obtained from 3 electrodes attached to the right arm, left arm, left leg and right leg (earth electrode)
The measurement of a voltage requires two poles, negative and positive.
Negative pole is the zero pole
Positive pole is point of view
Line connecting them = line of sight
Bipolar lead I: Positive electrode = LA, Negative electrode = RA
Bipolar lead II: Positive electrode = LL, Negative electrode = RA
Bipolar lead III: Positive electrode = LL, Negative electrode = LA
Unipolar lead aVR: Positive electrode = RA
Unipolar lead aVL: Positive electrode = LA
Unipolar lead aVF: Positive electrode = LL
The midpoint of the other two unipolar leads are used as the zero reference
Explain how the chest leads are read
Chest leads looks at heart in a horizontal plane
The corresponding chest electrodes serve as positive poles
The reference negative value is the same for all chest leads and is calculated as the average of inputs from the three limb electrodes
Depolarisation towards lead = positive deflection
Depolarisation away from the lead = negative deflection
Reverse is true for repolarisation
Therefore leads that look at the heart from different angles may have waves pointing in different directions
Describe the electrical activity and consequent conducting system of the heart
- The heart is made up of specialised cells
- These cells are able to generate action potentials
- The SAN sets the tone of the rhythm. It has the fastest rate so it’s called the sinus rhythm
- The wave of conduction travels downwards towards the AVN which conducts the impulse and causes a delay to allow atrial contraction and proper ventricular filling before allowing the impulse to be transmitted to The Bundle Of His.
- The Bundle of His is the only conducting path from atria to ventricles
- It conducts the impulse very rapidly to all subendocardial regions of the
ventricles via the right and left bundle branches. - These lie sub-endocardially in the interventricular septum
- This results in the depolarisation of the Purkinje fibres which are fine branches of the bundles of His
- This consequently results in rapid depolarisation of the myocytes
- Ventricles depolarise from endocardium to epicardium
- Enables coordinated contraction of atria and ventricles
How does the heart repolarise?
It happens in a reverse order
The last part of the ventricle to depolarise is the first to repolarise
What does an ECG do?
- Records changes on extracellular surface (outside of the cell) of cardiac myocytes during wave of depolarisation and repolarisation
- From surface of the body
- Using electrodes pasted on the skin
What would the depolarisation wave on an ECG look like if the depolarisation wave was coming directly towards the electrode and vice versa?
Tall positive complex
Deep negative complex
What would the repolarisation wave on an ECG look like if the depolarisation wave was coming directly towards the electrode and vice versa?
Negative complex
Positive complex
Which part of the ECG graph corresponds to the SAN depolarisation?
Nothing, it is a baseline observation
Insufficient signal to register on surface ECG
Which part of the ECG graph corresponds to atrial depolarisation?
The p wave
This is because:
- Spreads along atrial muscles fibres & internodal pathways
- Spreads throughout both right and left atria
• Direction: Downwards & to the left
Towards AV node
- Will produce a small upward deflection the p wave
- Upward because towards recording (+ve) electrode
Which part of the ECG graph corresponds to AVN conduction?
Which part of the hearts conduction system also contributes to the same part of the ECG?
- Signal is very small
- Isoelectric (flat line) segment 1 b
(The spread of depolarisation from the atrium to the ventricles also contributes to the isoelectric (flat line) segment
The presence of the AV node is vital:
- Fibrous ring between atria and ventricles
- No direct contact between atrial and ventricular myocytes
By referencing the ECG graph, describe the depolarisation of the myocardium of the IV septum
- First part to depolarise is Muscle in interventricular Septum
- Septum depolarises from left to right
- It produces a small downward deflection because moving obliquely away
- Termed a Q wave
- Usually called ‘q’ (lower case) because small
