Phsyiological Bases of the ECG Flashcards
What does the ECG measure?
Extracellular potential.
Deflection is only caused when part of the cardiac excitable tissue is at a different membrane potential than the rest of the heart.
ECG tracing deflection characteristics
Average current flows with negativity toward the base of the heart and positivity toward the apex
What is the ecg path of depolarization in the atria?
From start of the P wave to the beginning of the Q wave
P wave
Represents phase 0 of AP in atrial muscle.
SA node causes atria to depolarize from right to left.
All cells are depolarized at end of P wave.
PR interval
AV node delays the signal
QRS complex
Represents phase 0 of AP in ventricles (ventricle depolarization)
ST segment
delay of repolarization of ventricles by phase 2, all cells are depolarized in ST segment (on isoelectric point)
T wave
represents phase 3 - ventricular repolarization
PR interval
- usually 0.16 ms
- beginning of P wave until beginning of QR
QT interval
- usually 0.35ms
- beginning of QR until end of T wave
PR segment
end of atrial depolarization until beginning of QRS
ST segment
end of QRS until beginning of T wave
Lead I
Right arm to Left leg
Lead II
Right arm to Left leg
Lead III
Left arm to Left leg
