12- Lead ECG Interpretation Overview

Question 1

1. Explain how cardiac electrical activity results in positive and negative deflections on an electrocardiogram.

Answer 1

electrical conduction through the heart muscle can be measured via surface electrodes

positive waves signify electrical current that traveling in the same vector as the lead and the opposite is true for negative waves

note observing membrane equipotential does not indicate polarized or depolarized

Question 2

2. Describe each of the 12 ECG leads placement.

Answer 2

three frontal leads are placed, one each on the R and L arms and one on the leg

the horizontal/ precordial leads are placed one on either side of the arm pit and then 4 more, each down one intercostal space each, heading towards the axilla

Question 3

2. Describe each of the 12 ECG leads differentiation of standard limb leads.

Answer 3

I from left arm to right arm
II from left arm to left leg
III from right arm to left leg

Question 4

3. Describe the normal T wave, ST segment and Q wave.

Answer 4

P wave is atrial depolarization
PR interval time taken for conduction from SA through the His-Purkinje system
QRS occurs when ventricles depolarize (120ms)
ST segment from full ventricular depolarization until repolarization
T wave is ventricular depolarization
QT interval total time that some part of the ventricle is depolarized

Question 5

2. Describe each of the 12 ECG leads differentiation of augmented leads.

Answer 5

measured by comparing one lead with the electrical average from the other two standard limb leads (simulates a lead placed at the center of the chest) with the following vectors that measure electrical activity in the frontal plane

aVF- toward feet
aVR towards right shoulder
aVL towards left shoulder

Question 6

2. Describe each of the 12 ECG leads differentiation of 6 precordial leads.

Answer 6

precordial leads see the spread of activation between anterior and posterior surfaces of the heart (unipolar with reference electrode calculated by combining the 3 limb leads

Question 7

Name 4 important characteristics of an EKG to observe on your systematic approach?

Answer 7

rate
rhythm
conduction
axis

Question 8

What is the rule of 300?

Answer 8

a way to calculate HR on a standard speed trace, divide 300 by the number of big boxes between QRS complexes to calculate the rate

above 100 is tachycardia, below 60 is bradycardia

Question 9

What is sinus rhythm and how can you reliable determine if a rhythym is sinus?

Answer 9

originating from the SA node, P wave before every QRS, P wave in same direction as QRS complex

in lead II, Pwave should be positive and in V1 Pwave should be bi-phasic + then -

Question 10

Suggest a way to calculate the HR when a patient has an irregular heart beat.

Answer 10

standard length EKG is 10s, so count the number of QRS complexes across a horizontal line and then multiply by 6

Question 11

Give 3 ways to describe the origin of a rhythm, distinguishing normal origins with abnormal.

Answer 11

sinus, ectopic or multifocal

Question 12

4. Give the normal intervals of PR, QRS and QT segments.

Answer 12

PR <0.2 sec, less than one large box
QRS 0.08-0.12 sec (2-3 small boxes)
QT 450 ms in women, 460ms in women or half the R-R interval

Question 13

Give the 3 basic types of AV conduction blocks.

Answer 13

1st deg. PR interval is fixed and lengthen >0.2
2nd deg PR changed, QRS dropped
3rd deg PR and QRS completely dissociated

Question 14

Differentiate between 2nd deg. Mobitz type 2 and Mobitz type 1.

Answer 14

Mobitz 1 PR gradually lengthened then QRS dropped (AKA Wenkebach)
Mobitz 2 PR fixed but dropped QRS randomly occurs

Question 15

4c. Recognize the presence of left ventricular hypertrophy

Answer 15

many different criteria, none very accurate

sum of the S wave voltage in V1 of V2 plus the R voltage of V5 or V6 should be less than 35 mm

Question 16

5. Utilize the vector orientation of the leads to calculate the average QRS vector (axis).

Answer 16

the net direction of spread of excitation through the ventricles is characterized by the QRS axis, normally represented by a vector down and left because left ventricle usually has most muscle mass

x-axis formed by lead I and Y axis by aVF or any perpendicular leads

if both leads I and II are positive, the net QRS must be within normal range

Question 17

Give the normal intervals for PR, QRS, QTc

Answer 17

PR 120-200 (three little boxes- one big box)
QRS 60-120 (less than 3 little boxes)
QTc <470 (one half the R-R interval)

Question 18

What is the corrected QT?

Answer 18

normal QT interval varies with heart rate, as HR increase, QT decreases because factors that speed up heart rate also shorten cardiac twitch

corrected QT is normalized to a HR of 60 bpm
QTc = QT/sqrt (R-R)

Question 19

5. What conditions might cause a shift in QRS axis?

Answer 19

ventricular hypertrophy
myocardial infarction (shift away from dead tissue)
conduction delay in one of the fascicles (shift toward the last portion to depolarize)

conditions that change the heart’s position in the chest: pregnancy, abdominal obesity, ascietes, scholiasts, COPD, pneumothorax