Intro to the ECG Flashcards
Describe the three leads involved in Einthoven’s triangle.
Lead I points horizontally towards the left side of the heart and is considered 0 degrees. Therefore the positive electrode is on the left arm and the negative is on the right arm.
Lead II points to +60 degrees, somewhat towards the left leg. Therefore the positive electrode is on the left leg and the negative is on the right arm.
Lead III points to +120 degrees, somewhat towards the right leg. The positive electrode is on the left leg and the negative is on the left arm.
What does the P wave represent physiologically?
Atrial depolarization
Why doesn’t atrial repolarization show up on an ECG?
It happens during ventricular depolarization and is therefore masked by the large mass of the ventricles’ activity.
What physiologic process does the PR interval represent?
The time from the beginning of atrial depolarization to the beginning of ventricular depolarization.
What physiologic process does the PR segment represent?
Starts at the end of atrial depolarization and ends at the beginning of ventricular depolarization - represents the time it takes for the current to pass through the AV node.
What physiologic process does the QRS complex represent?
Ventricular depolarization
What physiologic process does the ST segment represent?
The plateau phase (Ca2+ channels and K+ channels open) of ventricular depolarization.
What physiologic process does the T wave represent?
Ventricular repolarization
What physiologic process does the QT interval represent?
The duration of the entire ventricular action potential (depolarization + repolarization)
What are the augmented leads?
Three additional frontal plane leads: aVR, aVF, aVL
Describe the positioning of the three augmented leads.
aVF: +90 degrees. Negative electrode grounds the left arm and right arm and positive electrode is the left leg.
aVR: -150 degrees. Negative electrode grounds the left arm and left leg and positive electrode is the right arm.
aVL: -30 degrees. Negative electrode grounds the right arm and left leg and the positive electrode is the left arm.
What are the precordial leads?
Six additional leads (V1 - V6) placed radially around the ribs in a transverse plane with the center being the middle of the heart.
What are the four steps of the geometric method of determining the mean QRS axis?
- Determine the algebraic sum of the QRS complex from two leads.
- Transfer the values to a graph.
- Draw a perpendicular line at the tips of each arrow on the graph.
- The intersection gives you the mean QRS axis.
How is the inspection method of determining the mean QRS axis performed?
Find the lead that shows the least deflection. This is the lead that is perpendicular to the ventricular depolarization. Then, think about which lead is perpendicular to the lead that showed no deflection, and look at the QRS complex - if QRS is positive then the mean QRS points towards that lead’s positive electrode. If negative then the mean QRS points away from that lead’s positive electrode.
