Physiologic Basis for ECG - Steve Flashcards
What does the ECG measure?
Potential differences between recording electrodes generated by electrical currents projecting from the heart to body surface during depolarization and repolarization
What 3 things contribute to the reproducible rate and rythm observed on the ECG?
(1)Automaticity
Cyclic depolarization of autorhythmic cells
(2) Conduction System
Order and timing of electrical events
(3) Functional Syncytium
Current conducted cell-to-cell via gap junctions
Detail the following aspects of this ECG…
P wave = ?
QRS complex = ?
T wave = ?

P wave = atrial depolarization
QRS complex = ventricular depolarization (atria repolarizing simultaneously)
T Wave = Ventricular repolarization
Where does the P wave begin? Where does it go?
Begins at SA node, spreads towards AV node
Describe the QRS sequence of ventricular depolarization!
Depolarization starts at the interventricular septum (Q wave) and endocardial surfaces.
At the end of depolarization phase, current reverses, flows toward the outer walls of the ventricles near the base (S wave).
Average (vector) of current flow: Base à Apex (R wave)

Why does left ventricular electrical activity dominate the R wave?
What is the average vector of current in ventricular depolarization?
Because there are many more myocytes associated with the left ventricle vs. the right ventricle.
•Average vector of current is R à L and inferiorly oriented (Base à Apex)
Why is the normal Mean QRS Vector oriented in the Normal Quadrant in the frontal plane?
–Muscle mass of left ventricle
–Normal sequence and distribution of conduction system
–Anatomical position of the heart in the chest
If the first deflection is downward =
Q wave
The first upward deflection =
R wave
If there is a second upward deflection =
R’ wave
The first downward deflection following an upward deflection =
S Wave
If the first wave of the complex is an R wave, the following downward deflection =
–S wave, not Q wave
A downward deflection only =
–a Q wave if it is the first wave of the complex
If the entire configuration consists solely of one downward deflection =
QS wave
What are the arrows indicating?

S.A. Node
Atrial Muscle
AV node
Common Bundle
Bundle branches
purkinje fibers
ventricular muscle

What is the normal range for the vector of ventricular depolarization?
What range might you see in some individuals?
–Normally, between 0° to +90°
(May range from -30° to +110°, depending on source/person)
Name the waves

QS
R
Name the waves

RS
QR
Name the waves

QRS
RSR’
What is the average vector of the T wave?
•Average vector of current is L -> R and superiorly oriented (Apex -> Base)
What is the PR interval?

Atrial depolarization and AV nodal delay
What is the QT interval? How about the ST segment?

•QT interval = Ventricular
depolarization/repolarization
•ST segment = Ventricular contraction & ejection (isoelectric line)
- RR interval =
- TP interval =

- RR interval = Heart rate
- TP interval = Ventricular relaxation & filling
•Horizontal: Time
5 large squares = ?
–5 large squares = 1 second (5 × 0.2 seconds/large square)



















