ECG pt.2 Flashcards
How many electrodes are used in ECG?
Standard ECG is done by putting 10 electrodes, 6 on the chest (horizontal/transverse plane) and 4 on the extremities (Vertical plane) which allows recording of the ECG signal on 12 leads
Where are the electrodes placed on the chest?
V1: Placed in the 4th right intercostal space at the sternal border (parasternal line)
V2: Placed in the 4th left intercostal space at the sternal border (parasternal line)
V3: Placed halfway between V2 and V4
V4:Placed in the 5th left intercostal space in the midclavicular line
V5: Placed in the anterior axillary line (AAL) at the same horizontal plane as V4 (just more lateral)
V6: Placed in the midaxillary line (MAL) at the same horizontal plane as V4
Name of peripheral leads and where are they positioned
Lead 1/D1 = right arm and left arm (the axis is directed toward the positive electrode)
* Lead 2/D2 = right arm and left foot
* Lead 3/D3 = left arm and left foot
* AVL = left arm
* AVR = right arm
* AVF = foot
In fact, the active electrodes of the peripheral recording
are only 3 (right arm, left arm and left foot)
Axes of peripheral leads
Based on an orthogonal system of axes (axis is directed towards positive electrode):
Lead D1 corresponds to an axis of 0° (x axis, horizontal)
Lead D2 corresponds to an axis of 60°
Lead D3 corresponds to an axis of 120°
aVR: 210°
aVL: 330° (or -30°)
aVF: 90° (perpendicular to D1)
Orthogonal system of axes
Axis system in which the lines of the leads pass through the electrical center of the heart
What does the signal direction on an ECG depend on?
- What we record in each lead at any time depends on
the projection of the ECG signal on each lead - If the ECG signal generated by the heart is projected on the positive part of the lead we have a positive wave = directed upward on the recording of the ECG
- If the ECG signal generated by the heart is projected on the negative part of the lead we will have a negative wave = directed downward on ECG recording
Difference electrical measurement (plane) between leads on the chest and leads on the extremities
These leads record the projections of the cardiac electrical activity on a horizontal plan
whereas the peripheral leads record it on a frontal plane.
What are the leads on the chest called?
Precordial lead
Types of peripheral leads
Peripheral (limb) bipolar leads and peripheral (limb) unipolar leads
How many electrodes does each lead use?
- First 3 leads use 2 electrodes, 1 negative and 1 positive making them bipolar leads
- Last 3 leads only use positive electrodes, making them unipolar leads
Describe a wave on ECG formed by depolarization heading towards a positive electrode
Upward spike
Describe a wave on ECG formed by depolarization heading away from a positive electrode
Downward spike
Describe a wave on ECG formed by repolarization heading away from a positive electrode
Upward spike
When would see a flat line on an ECG
- No net movement of electrical activity
- Electrical activity moving perpendicular to the axis
Why would see a downwards spike in Q wave in V2?
- Left bundle of his depolarizes the interventricular septum (axis is thus moving to the left and slightly upward, so kind of a superolateral axis moving to the left)
- This makes the axis of activation move slightly away from the positive electrode and thus the downward spike
Why is the ST segment flat?
All of the ventricle is already depolarized so there is no net movement of electrical charges happening at this point, thus the flat line
P wave amplitude direction with different leads
The P wave must be positive meaning directed upwards in D1, D2 and aVF
* Negative in lead aVR
* Positive in all precordial leads apart from V1 which is usually biphasic (small
initial positive wave followed by a small negative wave)
QRS complex wave amplitude direction with different leads?
- Mainly positive in ⇢ leads D1, D2, aVF
Negative ⇢ in aVR - A progressive increase from V1 to V6 of the positive wave (R wave)
- A progressive reduction from V1 to V6 of the negative wave (S wave
- Therefore:
In V1-V2 ⇢ predominance of S waves (negative)
In V5-V6 ⇢ predominance of R waves (positive)
In V3-V4 ⇢ we can have intermediate aspects (since we have a progressive increase of R wave and a progressive decrease of the S wave)
QRS complex wave amplitude direction with different leads?
- Mainly positive in ⇢ leads D1, D2, aVF
Negative ⇢ in aVR - A progressive increase from V1 to V6 of the positive wave (R wave)
- A progressive reduction from V1 to V6 of the negative wave (S wave
- Therefore:
In V1-V2 ⇢ predominance of S waves (negative)
In V5-V6 ⇢ predominance of R waves (positive)
In V3-V4 ⇢ we can have intermediate aspects (since we have a progressive increase of R wave
and a progressive decrease of the S wave)
T wave amplitude direction with different leads
Always positive in ⇢ D1, D2, aVF
Always negative in ⇢ aVR
Always positive in ⇢ V3,V4,V5,V6
Negative in ⇢ V1
Normal P wave height and width
Generally <0.12 sec (three small boxes) and the amplitude <0.25 mv (2.5 small boxes)
Normal PR interval duration
0.12 to 0.20 s (3 - 5 small squares)
QRS interval duration
0.06 to 0.10 sec (1.5 to 2.5 small boxes) and is not influenced by heart rate
What does the R wave represent?
It represents depolarization of the left ventricular myocardium. Right ventricular depolarization is obscured because the left ventricular myocardial mass is much greater than that of the right ventricle
