Lecture 5: Physiological Basis of the ECG Flashcards
How many electrodes in an electrocardiogram; how many readings generated?
- 9 electrodes
- Generate 12 readings
What is produced on an ECG if there is a difference in voltage between two electrodes?
A deflection
What is the ECG measuring?
Extracellular potential
What 2 factors are necessary to cause a deflection?
1) Part of the cardiac excitable tissue is at a different membrane potential than the rest of the heart
2) Current flow can occurs between those two regions
What will occur if only the atria and ventricles are of differing potentials?
Does not a cause a deflection
Why does the ECG not report differences between the atria and ventricles?
Due to the fibrous cartilage between the atria and ventricles
Which direction does lead I look at the heart from?
Left to right (left arm (+) to right arm (-))
SA node causes atria to depolarize from _____ to _____?
Right to Left
What is occurring during the PR interval?
AV node is delaying the signal
AP phase 2 delays repolarization of ventricles, and is seen as what segment?
ST segment
Ventricles repolarize from ______ to ______; seen as which wave?
Left to right; T wave
Which phase of the atrial AP is represented by the P wave?
Phase 0
Why is no signal seen at the end of the P wave?
All of the atrial cells are effectively the same polarity, all depolarized, no voltage difference.
Which phase of the ventricular AP is represented by the QRS complex?
Phase 0
Which phase of the ventricular AP is represented by the T wave?
Phase 3 or repolarization
What does a segment represent on the ECG?
Duration of single event
What does an interval represent on the ECG?
Duration of several events
What is the PR segment?
End of atrial depolarization until beginning QRS
What is the ST segment?
End of QRS until beginning of T wave
What is the PR interval?
Beginning of P wave until beginning of QRS
What is the QT interval?
Beginning of QRS until end of T wave
What would cause a prolonged QT interval?
A prolonged phase 2, caused by Ca2+ and K+(b).
When determining rate what does the time between the dark lines (bolder) represent?
.2 sec
When determining rate what does the time between the light lines represent?
.04 sec
Why use 12 leads?
Each lead displays voltage differences in the myocardium seen from different perspectives by different electrodes
Which limbs are involved in an aVF?
(RA+LA) to LL (+)
Which limbs are involved in aVR?
(LL+LA) to RA (+)
Which limbs are involved in aVL?
(LL+RA) to LA
How many vectors for chest or precordial leads?
V1-V6
What makes up Einthoven’s Triangle?
Lead I: RA (-) to LA (+)
Lead II: RA (-) to LL (+)
Lead III: LA (-) to LL (+)
How do augmented limb leads work?
One of the limb leads is assigned a positive polarity and the remaining 2 electrodes are averaged and assigned a negative polarity
Chest or precordial leads are not assigned an axis, they are instead assigned?
To a certain region of heart and each acts as a positive electrode
Which leads predominantly represent the inferior region of the heart?
II, III, aVF
Which leads predominantly represent the septal region of the heart?
V1 and V2
Which leads predominantly represent the anterior region of the heart?
V2, V3, and V4
Which leads predominantly represent the lateral region of the heart?
I, aVL, V4, V5 and V6
What is the mean electrical axis?
Average direction of spreading AP in ventricles
What is used for determining the mean electrical axis?
Average amplitude of QRS in I and aVF
If both lead I and aVF are positive what does this signify?
A normal mean electrical axis
If lead I is negative, what does this signify?
A right axis deviation, regardless if aVF is either positive or negative.
If lead I is positive, but aVF is negative what does this signify?
A left axis deviation
The P wave begins ________contraction of the atria.
Before
QRS begins ______ ventricular contraction
Before
T wave begins ________ ventricular relaxation
Before
