Physiologic Basis of ECG Flashcards
Electrocardiogram
12 electrodes place on skin surface at various locations on torso
Each records voltage difference between itself and another electrode on the torso
Difference=deflection in the read out
No difference= No deflection in the read out
Electrocardiogram
12 electrodes place on skin surface at various locations on torso
Each records voltage difference between itself and another electrode on the torso
Difference=deflection in the read out
No difference= No deflection in the read out
Characteristics of the ECG
Illustrates changes of electrical activity of cardiac muscle tissue-depolarization or repolarization
“Measures” extracellular potential
Only causes deflection when: Part of the cardiac excitable tissue is at a different membrane potential than the rest of the heart. Current flow can occur between those regions
Characteristics of the ECG
Illustrates changes of electrical activity of cardiac muscle tissue-depolarization or repolarization
“Measures” extracellular potential
Only causes deflection when: Part of the cardiac excitable tissue is at a different membrane potential than the rest of the heart. Current flow can occur between those regions
P Wave
SA node causes atrial depolarize from right to left
Represents phase 0 of AP in atrial muscle
PR Interval
AV node delays signal
Beginning of P wave until beginning of QR. This is often referred to as the PQ interval, but since the Q wave may be absent the QR interval is very small
0.16 ms
QRS Complex
Ventricles depolarize generally from right to left, from apex to base, from interior to exterior
Represents phase 0 of AP in ventricular muscle
ST Segment
Action potential phase 2 delays repolarization of ventricles
End of QRS until beginning of T wave
All cell depolarized in ST segment, no difference so 0mv read
T wave
Ventricles repolarize generally from left to right, base to apex
Represents phase 3 (repol) of AP in ventricular muscle
Intervals and Segments
Segments-Represents the duration of a single event on the ECG
Intervals-Represent the duration of two or more events
QT Interval
Beginning of QR until end of T wave
Usually 0.35 ms
PR Segment
End of atrial depolarization until beginning of QRS
Voltages
QRS complex 1.0-1.5 mv from top of R wave to bottom of S wave
P wave= 0.1-0.3 mv
T wave= 0.2-0.3 mv
12 Leads
Each lead displays voltage differences in the myocardium from different perspectives by different electrodes
Standard (Bipolar) limb leads
- 1= right arm (RA) to left arm (LA)
- 2= RA to Left Leg (LL)
- 3= LA to LL
Augmented Limb Leads
-aVF=augmented vector, foot
(RA+LA) to LL
-aVR=augmented vector, right
(LL+LA) to RA
-aVL=augmented vector left
(LL+RA) to LA
Chest or precordial leads
- V1-6
- V1 and V2 mainly negative
- V4, V5, V6 mainly positive
Not assigned an axis
Instead, assigned to regions of the heart
Each leads acts as a positive electrode
Inferior=2, 3, aVF
Septal=V1 and V2
Anterior=V2, V3, and V4
Lateral= V4, V5, V6, 1 and aVL
AP spread in heart
As AP spreads through the heart-viewed from a different angle by each lead
If AP is spreading towards + lead of electrode set, a + deflection will be observed
Einthoven’s Triangle
Each limb has + and - polarity and voltage between
Mean Electrical Axis
Average direction of spreading AP ventricles
Helps determine morphology and conduction of heart
Uses average amplitude of QRS in Leads 1 and aVF
If general direction of AP spreads to upper left=left axis direction
If general direction of AP spreads to right=right axis deviation
