Intro to Electrocardiography

Question 1

Concerning the electric field generated during propagation of APs in heart, in which direction are the positive and negative poles compared to the direction of propagation?

Answer 1

The + end is in the direction of propagation, the - end is behind it.

Question 2

What does an upward deflection on ECG tell you? A negative one?

Answer 2

An upward deflection represents the part of the “propagation vector” parallel to the lead and pointing toward its positive electrode.
Negative deflections represents the “propagation vector” parallel to the lead and pointing toward its negative electrode.

Question 3

What are 3 factors that affect the amplitude of deflections seen on an ECG?

Answer 3

The mass of muscle generating the signal.
The conduction velocity (faster -> larger).
The degree of cancellation of electrical forces.

Question 4

What is Lead I connected to? What angle does it produce?

Answer 4

Right hand negative, left hand positive.

Angle: Straight horizontal, right to left, 0 degrees.

Question 5

What is Lead II connected to? What angle does it produce?

Answer 5

Right hand negative, left foot positive

Angle: +60 degree (“downward” from 0)

Question 6

What is Lead III connected to? What angle does it produce?

Answer 6

Left hand negative, left foot positive.

Angle: +120 degrees (30 degree to the right of pointing straight down toward feet)

Question 7

How is Lead aVR generated? What angle does it produce?

Answer 7

Left foot + left hand negative, right hand positive

Angle: -150 degrees (usefully, if you invert the deflections, you’ve got +30 degrees down from 0)

Question 8

How is Lead aVL generated? What angle does it produce?

Answer 8

Right hand + left foot negative, left hand positive.

Angle: - 30 degrees (30 degrees above 0)

Question 9

How is Lead aVF generated? What angle does it produce?

Answer 9

Right hand + left hand negative, left foot positive.

Angle: +90 degrees (pointing straight down toward feet)

Question 10

What forms the negative pole in chest (aka precordial) leads? Effect?

Answer 10

Left hand, right hand, and left foot tied together.

This makes the negative pole appear in the center of the chest cavity (“the Central Terminal of Wilson”)

Question 11

In the chest leads, how are 0 and +90 degrees defined? (which leads produce these vectors?)
(note that knowing the exact angles of these leads hasn’t been emphasized…)

Answer 11

0 degrees: pointing out from center of chest to the left midaxillary line - perpendicular to the saggital plane (approximately V6)
90 degrees: pointing straight forward out the center of the chest - perpendicular to the coronal plane (between V1 and V2)

Question 12

Where is V1 placed?

Answer 12

4th intercostal space, just to right of sternum.

Question 13

Where is V2 placed?

Answer 13

4th intercostal space, just to left of sternum.

Question 14

Where is V3 placed?

Answer 14

4th intercostal space, half way between V2 and V4.

Question 15

Where is V4 placed,

Answer 15

5th intercostal space, in the mid-clavicular line.

Question 16

Where is V5 placed?

Answer 16

Lateral to V4, in the anterior axillary line (basically, where the shoulder/armpit starts)
Note that the leads don’t follow the curves of the ribs, rather they go straight lateral, not upward.

Question 17

Where is V6 placed?

Answer 17

Lateral to V4 in the mid-axillary line

Question 18

What 4 events are visible on a normal sinus rhythm ECG? (how are they seen?)

Answer 18

Atrial depolarization (P wave)
Ventricular depolarization (QRS complex)
Ventricular plateau (ST segment)
Ventricular repolarization (T wave)

Question 19

In “QRS” terminology, what do Q, R, S, and R’ refer to?

Answer 19

Q wave: initial downward deflection.
R wave: first upward deflection
S wave: downward deflection after an R wave
R’ wave: a second upward deflection

Question 20

What’s a quick way to calculate HR (i.e. RR interval) by counting large boxes?

Answer 20

Divide 300 by number of large boxes between R waves:
1 box -> 300 bpm
2 boxes -> 150 bpm
3 boxes -> 100 bpm
etc.

Question 21

How should the P waves appear in Leads I, II, and III?

Answer 21

Upright

Question 22

What are the HR cut offs for bradycardia and tachycardia?

Answer 22

< 60 = bradycardia

> 100 = tachycardia

Question 23

How is the PR interval defined?
What is the normal range?
What is happening at this time?

Answer 23

Beginning of P wave to beginning of QRS complex (note that the Q wave is sometime absent).
Normal range is 120 to 200ms. (the slide that says normal PR is t confuse this with the PR segment, which from the end of the P wave to the beginning of the Q wave.

Question 24

How the QRS duration defined?
What is the normal range?
What is happening at this time?

Answer 24

Beginning of Q wave to end of S wave.
Normal is < 0.10 s, abnormal is > 0.12 s. (I guess in between those is borderline).
In the QRS complex, the ventricles are depolarizing.

Question 25

How is the QT interval defined?

What is normal QT?

Answer 25

Beginning of QRS to the END of the T wave.
Normal QT varies with HR.
At normal resting HRs (60-80bpm), it should be about 0.30 - 0.40s (but… I would look at the table)

Question 26

What does a PR interval > 0.20s suggest?

Answer 26

AV conduction delay.

Question 27

What does a QRS > 0.12s suggest? (ways this could happen?)

Answer 27

Slow ventricular depolarization.

Bundle branch block, “ventricular origin” -an infarct??, high extracellular K+, etc.

Question 28

What does a prolonged QT interval mean?

Answer 28

Prolonged ventricular action potentials. (more on this later…)

Question 29

What is the frontal plane QRS axis, and why is it useful to know?

Answer 29

Orientation of greatest net QRS depolarization.
An asymmetrical increase or decrease of electrical activity on one side of the heart produces an apparent change in the QRS axis, and can give you clues about fascicular block, infarctions, hypertrophy, etc.

Question 30

What’s the easiest way to determine the frontal plane QRS axis?

Answer 30

Find the isoelectric lead - that with the smallest QRS amplitude - which will be perpendicular to the QRS axis. Then use other leads to determine which quadrant the QRS axis will be in, based on whether the QRS deflection is positive or negative.

Question 31

What is the range for normal frontal plane QRS axis?

Answer 31

-30 degrees to +90 degrees (it’s a pretty big range)

Question 32

If the R wave deflection is positive, should the T wave deflection also be positive? Why or why not?

Answer 32

Yes, they should be in the same direction.

The ventricles repolarize in an order opposite to that in which they depolarize.

Question 33

What are the 3 degrees of AV block?

Answer 33

1st degree: AV delay - PR > 0.20s.
2nd degree: Intermittent AV block, with some P’s not followed by QRS.
3rd degree: Complete AV block - P waves and QRS are unrelated to each other.

Question 34

What happens the QRS interval in both right and left bundle branch block?

Answer 34

QRS interval elongation.

Question 35

What do you see in ECG of right bundle branch block?

Answer 35

There’s a delay in propagation to the right ventricle, so…
Early in the QRS, more electrical activity pointing to the left side of the heart.
Late in the QRS, more electrical activity pointing to the right (positive in Lead V1, negative in I and V6).

Question 36

What do you see in ECG of left bundle branch block?

Answer 36

There’s a delay in propagation to the left ventricle, so…
Early in the QRS, more electrical activity pointing to the right side of the heart.
Late in the QRS, more electricity pointing to the left side of the heart.
As the left ventricle is bigger, this makes the most QRS be a big, broad left-pointing deflection (positive in Leads I and V6, negative in V1).