16 Origin of ECG

Question 1

Depolarization spreading towards the positive electrode produces a
________deflection.

Answer 1

positive/upward

Question 2

Depolarization spreading away from a positive electrode produces a
___________ deflection.

Answer 2

negative/downward

Question 3

Depolarization travelling at right angle/ perpendicular to the axis of
an ECG lead produces ________ deflection.

Answer 3

equiphasic or no

Question 4

Put the sequence of amplitude of the below leads in descending order.

aVF, Lead 1, Lead 2

Answer 4

Lead II > aVf > Lead 1

Question 5

Do chest leads or limb leads have stronger amplitude of ECG waves?

Answer 5

Chest leads

distance from heart is closer

Question 6

What do P wave, QRS complex, T wave and U wave represent?

Answer 6

P wave: atrial depolarization (up)
QRS complex: Ventricular depolarization (up)
T wave: Ventricular repolarization (up)
U wave: delayed repolarization of papillary muscles, Purkinje fibers
Seen in hypokalemia or hypercalcemia

(up wave after T wave)

Question 7

What is Ta wave?

Why it can’t be seen normally?

Answer 7

Atrial repolarization (down) after a P wave,
masked by QRS complex

Question 8

Why does T wave (Ventricular repolarization) goes up?

Answer 8

Duration of myocardial cells in epicardial later is shorter than in endocardial layer, thus the epicardial side repolarizes before the endocardial side.
Potential difference generated across ventrical wall during depolarization (QRS) and repolarization (T wave) is more or less the same > same direction

Question 9

Why do epicardial cells depolarize earlier than endocardial cells?

Answer 9

Difference in K+ channels.

Epicardial cells express KCNH2 K+ channels which are activated more rapidly to produce K+ current.

Question 10

How does the Q, R and S represented in the complex?

Answer 10

Q: First downward deflection
R: First upward deflection
S: Downward deflection after an R wave

Question 11

Describe the length of PR interval. (From where to where?)

Answer 11

P to beginning of QRS complex

Question 12

Describe the length of QT interval. (From where to where?)

Answer 12

beginning of QRS complex to the end of T wave

Question 13

The standard chart speed is 25mm/sec.

What long does a large box and a small box last?

Answer 13

Large box = 0.2 sec

Small box = 0.04

Question 14

During which interval can we identify heart block or AV block (1st, 2nd, 3rd degree)?
What is the normal range of that interval?

Answer 14

PR/PQ intervate

0.12 - 0.2 s (= one large box)

Question 15

What abnormalities can we observe in the QRS complex, if any?
What is the normal range of this interval?

Answer 15

Conduction abnormalities
like bundle branch block or through aberrant pathway

0.08-0.12s

Question 16

What does it mean by a normal QT interval?

Answer 16

0.3-0.44 s

usually does not exceed half of the RR interval

Question 17

What is the normal range of a RR interval? What is the use of a RR interval?

Answer 17

0.6 -1s

Measure of heart rate or ventricular rate

Question 18

During the PR/PQ interval, electrical activity is spread through the_________________system.The delay in conduction at the AV junction (0.1 s) allows the atria to
empty their contents into the ventricles (for ventricular filling)
before the latter start contracting.

Answer 18

AV node and His-Purkinje system

Question 19

What is a J point?

What is its function?

Answer 19

J point is where QRS complex ends and ST segment begins. It

helps in denoting ST segment elevation or depression.

Question 20

ST segment refers to the time from the end of ventricular
depolarization to the start of ventricular repolarization.

It is normally elevated/isoelectric/depressed.

Answer 20

Isoelectric

corresponding to the plateau
phase of the ventricular myocyte action potential.

Question 21

TP segment = RMP
should be an isoelectric line.
In myocardial infarction, how is the TP segment presented before and during depolarization?

Answer 21

Before depolarization: Depression of baseline (injured myocardium partially depolarized)

During depolarization, apparent ST elevation. (not because it is elevated, it is because the original baseline is decreased)

Question 22

Myocardial ischemia causes injured cells to become partially depolarized,
thus creating a dipole or potential. What does it do to the baseline (RMP)?

Answer 22

Shifted downwards