Physiological Basis for ECG and Basic ECG Flashcards
The electrodes report voltage difference in:
Regions of either ventricles or atria, but not between them.
The average current flow:
Occurs with negativity toward the base of the heart and with positivity toward the apex.
P wave
SA nodes cause the atria to depolarize from right to left.
PR interval
AV node delays signal.
Beginning of P wave until the beginning of QR.
QRS complex
Ventricles depolarize generally from right to left, from apex to base, from interior to exterior.
ST segment
Action potential phase 2 delays repolarization of ventricles.
End of QRS until beginning of T wave.
T wave
Ventricles repolarize generally from left to right.
On ECG, 5 “large” boxes horizontally equals?
2 “large” boxes vertically?
1 sec
1.0 mv
QT interval
Beginning of QR until end of T wave.
PR segment
End of atrial depolarization until beginning of QRS.
Standard (bipolar) limb leads use:
- right arm to left arm.
- right arm to left leg.
- left arm to lower limb.
aVF (augmented vector, foot)
Right arm and left arm to lower limb.
aVR (augmented vector, right)
Lower limb and lower arm to right arm.
aVL (augmented vector left)
Lower limb and right arm to left arm.
Einthoven’s triangle
Each limb has + and - polarity and voltage between them.
Mean electrical axis
Average direction of spreading AP in ventricles.
Helps to determine morphology/condition of the heart.
Normal MEA
Both leads I and aVF are positive.
If lead I is negative:
Right axis deviation
aVF can be positive or negative
RV hypertrophy
If lead I is positive, but aVF is negative:
Left axis deviation.
LV hypertrophy.
Relationship of ECG to cardiac cycle
P wave
QRS complex
T wave
P wave begins after atrial contraction.
QRS complex begins before ventricular contraction.
T wave begins before ventricular relaxation.
