Physiologic Basis of ECG

Question 1

Electrocardiogram

Answer 1

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

Question 2

Electrocardiogram

Answer 2

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

Question 3

Characteristics of the ECG

Answer 3

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

Question 4

Characteristics of the ECG

Answer 4

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

Question 5

P Wave

Answer 5

SA node causes atrial depolarize from right to left

Represents phase 0 of AP in atrial muscle

Question 6

PR Interval

Answer 6

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

Question 7

QRS Complex

Answer 7

Ventricles depolarize generally from right to left, from apex to base, from interior to exterior

Represents phase 0 of AP in ventricular muscle

Question 8

ST Segment

Answer 8

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

Question 9

T wave

Answer 9

Ventricles repolarize generally from left to right, base to apex

Represents phase 3 (repol) of AP in ventricular muscle

Question 10

Intervals and Segments

Answer 10

Segments-Represents the duration of a single event on the ECG

Intervals-Represent the duration of two or more events

Question 11

QT Interval

Answer 11

Beginning of QR until end of T wave

Usually 0.35 ms

Question 12

PR Segment

Answer 12

End of atrial depolarization until beginning of QRS

Question 13

Voltages

Answer 13

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

Question 14

12 Leads

Answer 14

Each lead displays voltage differences in the myocardium from different perspectives by different electrodes

Question 15

Standard (Bipolar) limb leads

Answer 15

• 1= right arm (RA) to left arm (LA)
• 2= RA to Left Leg (LL)
• 3= LA to LL

Question 16

Augmented Limb Leads

Answer 16

-aVF=augmented vector, foot
(RA+LA) to LL

-aVR=augmented vector, right
(LL+LA) to RA

-aVL=augmented vector left
(LL+RA) to LA

Question 17

Chest or precordial leads

Answer 17

• 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

Question 18

AP spread in heart

Answer 18

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

Question 19

Einthoven’s Triangle

Answer 19

Each limb has + and - polarity and voltage between

Question 20

Mean Electrical Axis

Answer 20

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