Lecture 9: Introduction to Cardiac Electrocardiography (ECG) Flashcards
What produces extracellular and body surface electrograms?
Propagation of transmembrane action potential
What is an electrogram?
A recording of electrical potential difference
Shows the difference between the positive pole and negative pole
If positive lead is +1 mV and negative lead is -1mV then you have 1 – (-1) = +2mV
What is an electrode?
A contact with the body
What is a lead?
An arrangement of electrodes configured into positive and negative poles
A lead has a vector orientation
What are the factors that alter the amplitude of the vector of electrical activity?
- The mass of muscle generating the signal
Example: hypertrophy increases amplitude - Conduction velocity
- Degree of cancellation due to propagation in different directions simultaneous
If the orientation of the lead and the vector of electrical activity are parallel, what do you see on the electrogram?
The highest amplitude possible
If the orientation of the lead and the vector of electrical activity are perpendicular, what do you see on the electrogram?
No activity
How many electrodes make up the standard 12 leads of a standard ECG?
10 electrodes
How are leads I, II and III placed?
The Three “L” rule
Lead I = 1 L = Right to Left Arm = 0 and 180 degrees
Lead II = 2 L’s = Right Arm to Left Leg = 60 to -120 degrees
Lead III = 3 L’s = Left Arm to Left Leg = 120 to -60 degrees
What are the frontal leads?
Leads I, II and III
How are leads aVL, aVR, aVF oriented?
aVL = Left arm (positive pole) = -30 degrees to +150 degrees aVR = Right arm (positive pole) = -150 degrees to +30 degrees aVF = Left leg (positive pole) = +90 degrees to -90 degrees
What are the augmented frontal leads?
aVR, aVL, aVF
What does aVR stand for?
Augmented vector right
What are the precordial leads (chest leads)?
V1 – V6
What is the negative pole in the precordial leads?
The central terminal of Wilson
Created by connecting each of the three limb electrodes through 5000 Ohm resistors
It behaves as if it is located in the CENTER of the chest
What is the central terminal of Wilson?
The negative pole for all the precordial leads
Where are the precordial electrodes placed?
V1 = 4th intercostal space just right of the sternum V2 = 4th intercostal space just left of the sternum V3 = halfway between V2 and V4 V4 = 5th intercostal space in mid clavicular line V5 = Lateral to V4 in the anterior anxillary line V6 = Lateral to V4 in the mid axillary line
What is the ECG appearance of a normal sinus rhythm?
Upward P wave needs to be seen in V1, V2 and aVF
What is the order of the sinus rhythm events?
- SA node depol
- Atrial depol
- Atrial repol
- AV node depol
- Bundle of his depol
- Vent depol
- Vent. Pleateau
- Vent. Repol
What forms the P wave?
Atrial depolarization
What forms the QRS complex?
Ventricular depolarization
What forms the ST segment?
Ventricular plateau
What forms the T wave?
Ventricular repolarization
What sinus rhythm events cannot be seen in a normal sinus ECG?
- SA node impulse
- depolarization of the AV node
- repolarization of the atria
- depolarization in the His bundle and bundle branches
What is a Q wave?
Part of the QRS complex
An initial downward deflection BEFORE any R wave
Ventricular depol spreading from left to right, specifically left bundle to the septum!
As seen in the frontal leads
What causes the R wave in V1?
When electrical activity goes from left bundle to septum
What is a R wave?
Part of the QRS complex
The first UPWARD reflection
Can be present in the absence of Q
Ventricular depol spreading from right to left
What is a S wave?
Part of the QRS complex
A downward deflection AFTER an R