Chp. 11: Electrocardiography

Question 1

Equivalent Dipole Theory

Answer 1

The heart can be approximated to a dipole; the force that is generated has a direction and magnitude and can be represented by a vector

Question 2

When is the ECG wave of maximum amplitude?

Answer 2

When the cardiac depolarization vector is parallel to the lead

Question 3

Einthoven’s Triangle

Answer 3

Electrodes are placed on the right forelimb, left forelimb, and left hindlimb. The three potential differences created by this arrangement are leads I, II, and III.

Question 4

Lead I

Answer 4

Measures potential difference between right and left forelimb with left considered positive

Question 5

Lead II

Answer 5

Measures potential difference between right forelimb and left hindlimb with hindlimb considered positive

Question 6

Lead III

Answer 6

Measures potential difference between left forelimb and left hindlimb with hindlimb considered positive

Question 7

How are unipolar leads created?

Answer 7

By comparing the three electrodes to a reference voltage created by the monitor (“Wilson’s central terminal”).

aVL (left forelimb), aVR (right forelimb), aVF (left hindlimb)

Question 8

What plane do the precordial leads provide information on?

Answer 8

The transverse, rather than frontal, plane

Question 9

Why is a multilead ECG of less value in horses?

Answer 9

Due to extensive branching of the Purkinje system in the ventricles

Question 10

Base-apex lead

Answer 10

Lead I setting

RA over jugular furrow or manubrium
LA over left apex of heart at olecranon
LL on either side of neck

Question 11

Wave vs. Segment vs. Interval

Answer 11

“Wave” represents an electrical event in the myocardium and is a deflection above or below baseline

“Segment” is a line joining two waves

“Interval” is the time between two events

Question 12

P wave

Answer 12

Atrial depolarization

Question 13

PQ (or PR) interval

Answer 13

Time for wave of depolarization to propagate from atria through the ventricular conduction system

Question 14

QRS complex

Answer 14

Three waves representing ventricular depolarization

Question 15

T wave

Answer 15

Ventricular repolarization

Question 16

ST segment

Answer 16

Segment between end of QRS (J point) and T wave, corresponding to ventricular systole

Question 17

J wave and U wave

Answer 17

Early and late ventricular repolarization, respectively

J wave in normal heart or with hypothermia, hypercalcemia, or myocardial ischemia

U wave with bradycardia or hypokalemia

Question 18

50 vs. 25 mm/s paper speed

Answer 18

50mm/s is a faster paper speed where each small box (1mm) equals 0.02s

25mm/s is a slower paper speed where each small box (1mm) represents 0.04s

Question 19

ECG heart rate calculation

Answer 19

Simplest: Count number of R waves in 3s and multiply by 20

More accurate: Count R-R interval, then multiply by 0.02 or 0.04 depending on paper speed, then divide by 60 for bpm

Question 20

Systematic approach to ECG evaluation

Answer 20

1) Determine HR
2) Determine if ventricular and atrial rates match (P-P interval and R-R interval match)
3) P for every QRS
4) QRS for every P
5) Are QRS complexes supra ventricular or ventricular

Question 21

Mean Electrical Axis

Answer 21

Direction and orientation of ventricular depolarization; refers solely to the QRS complex.

Normally to the LEFT

Question 22

Lead I MEA assessment

Answer 22

If the net deflection of QRS in lead I is positive, MEA lies to the left. If net deflection is negative, MEA lies to the right.

Question 23

In what species is the QRS complex deflection largely negative in Lead II?

Answer 23

Horses, pigs, ruminants

Question 24

Respiratory Sinus Arrhythmia

Answer 24

Normal in dogs.

Rate accelerates during inspiration and decelerates during expiration.

Question 25

Wandering Pacemaker

Answer 25

Changes in site of impulse origin within sinus node and/or changes in exit pathway from sinus node

Question 26

Arrhythmias in normal, resting horses

Answer 26

Sinus arrhythmia, sinus block, sinus arrest, sinus bradycardia, wandering pacemaker, first and second-degree AV block

Question 27

Bandpass filter

Answer 27

Filter on ECG that allows signal frequencies within a specific range to be displayed while removing electrical activity above and below this range

Question 28

What inhalant sensitizes the heart to certain arrhythmias?

Answer 28

Halothane