Electrical Properties of the Heart

Question 1

SA Node

Answer 1

Tissue Has most rapid rate of depolarization
Frequency of depolarization is most dependent on Ca++ inward diffusion
Modulated by sypathetic/parasympathetic by change in phase 4

Question 2

Atrial Muscle

Answer 2

Conduction from R to L via Bachmann’s Bundle
Generates P wave
Rapid conduction velocity due to Na+ diffusion into cells

Question 3

AV Node

Answer 3

Slows conduction to allow for ventricular filling
Ca++ dependent
Correlates with PR interval

Question 4

Fibrous Skeleton

Answer 4

Assures AV conduction is via specialized conduction system

Question 5

Bundle of His

Answer 5

Rapid conduction (2 m/s)
During phase 0, rapid inward Na+ diffusion

Question 6

Purkinje Fibers

Answer 6

Rapid conduction (4 m/s)
High density of Na+ channels
Co-ordinates ventricular myocardial contraction to be as simultaneous as possible

Question 7

PR Interval

Answer 7

Distance between beginning of P wave to beginning of QRS complex
Measures depolarization of the atria and firing of the SA Node

Question 8

QRS interval

Answer 8

Measures ventricular depolarization

Question 9

QT interval

Answer 9

Beginning of QRS complex to end of T-wave

Question 10

A current flowing to the right is directly parallel to a lead with its positive node on the right. Which way would an ECG mark this deflection?

Answer 10

Positive deflection up

Question 11

Left Axis deviation

Answer 11

Vectors sum to less than -30

i.e. rotated counter clockwise

Question 12

Right axis deviation

Answer 12

Vectors sum to greater than 110

i.e. rotated clockwise

Question 13

Einthoven’s Triangle is composed of which leads?

Answer 13

I, II, III

Question 14

Characteristics of ECG during Diastole

Answer 14

Net ionic flux is balanced
No current is generated
ECG is flat i.e. baseline

Question 15

Characteristics of ECG during Septal depolarization

Answer 15

Activation from LEFT to RIGHT

Question 16

Inferior Chest Leads

Answer 16

V4, V5, V6

Question 17

On the X-axis what is the value of a small box? what is the value of one big box?

Answer 17

Small box = 40 ms
Large box = 5 * 40 = 200 ms
5 Boxes make one second

Question 18

On the Y-axis what is the value of a small box? what is the value of one big box?

Answer 18

Little Box = 0.1 mV
Big Box = 0.5 mV
Two Big boxes = 1 mV

Question 19

Normal Intervals

Answer 19

PR = 120-200
QRS = 80-100
QT = 440
RR: 220 - Age

Question 20

Determining Rate

Answer 20

Count how many big boxes between QRS peak
300 -> 150 -> 100 -> 75 -> 60 -> 50
Count total number of peaks and multiply by 6

Question 21

Rhythm

Answer 21

Is there a P wave before every QRS complex

Question 22

Left Ventricular Hypertrophy

Answer 22

S in V1 + R in V5 or V6 sums to 3.5mV i.e. 7 Large squares

Question 23

For Atrial Enlargement, look at lead ____

Answer 23

Lead II (follows flow of electrical discharge)
Humped P wave = LAE
Increased Amplitude of P wave = RAE
Biphasic P wave = LAE
NB: THIS IS IN LEAD II

Question 24

First Degree AV block

Answer 24

PR > 200ms

Question 25

Second Degree: Type 1

Wenckebach

Answer 25

Progressive PR prolongation with eventual loss of conduction

i.e. blocked P wave that DOES NOT generate QRS complex

Question 26

Second Degree: Type 2

Mobitz 2

Answer 26

Unchanged PR with loss of conduction of P-wave

Question 27

Third Degree

Answer 27

No association between P-waves and QRS

Question 28

Which leads should one look at for Bundle Branch Block?

Answer 28

V1 and V6

Question 29

RBB

Answer 29

QRS > 120ms (3 small boxes)
M in V1
W in V6
MaRRoW
Introduces new late forces rightward and anteriorly

Question 30

LBB

Answer 30

QRS > 120ms
W in V1
M in V6
WilLLiaM
Introduces new late forces leftward and posteriorly