The Electrocardiogram

Question 1

Lead I

Answer 1

Placed on the left hand.

If current travels towards Lead I, the deflection is upward.

If it travels away from Lead I, the deflection is downward.

If it travels perpindicular, there is no deflection.

Question 2

P wave

Answer 2

Atrial deflection. When the SA node fires and sends its signal to the AV node, also resulting in contraction of the atria.

Question 3

QRS complex

Answer 3

Represents firing of AV node, depolarization of septa, and depolarization of the ventricles (left dominant)

Question 4

Why does depolarization of the left ventricle dominate the ventricular electrical signal?

Answer 4

Because there is more muscle in the left ventricle! So consequently, there is more electrical activity, since all of those muscles are firing at once. In effect, the right ventricle signal is being hidden and the net vector is left.

Question 5

T wave

Answer 5

Repolarization of the ventricles. Also left dominant.

Question 6

P QRS T diagram

Answer 6

Question 7

Why is Q negative on Lead I?

Answer 7

Because Q represents depolarization of the interventricular septum, which is to the right of the left ventricle (especially given the bulge due to higher left ventricular hydrostatic pressure)

Question 8

x and y axis on an ECG

Answer 8

x = time

y = voltage

Question 9

Ca2+ resting potential

Answer 9

+123 mV

Question 10

Na+ resting potential

Answer 10

+67 mV

Question 11

K+ resting potential

Answer 11

-92 mV

Question 12

Depolarization potential

Answer 12

Usually -70 mV

(The potential necessary for fast sodium channels to open)

Question 13

Gating potential for voltage-gated potassium channels

Answer 13

+20 mV

Question 14

Depolarization stages

Answer 14

Question 15

V1 location

Answer 15

Right 4th intercostal space, on the sternal border

Question 16

V2 location

Answer 16

Left 4th intercostal space, on sternal border

Question 17

V4 location

Answer 17

Left 5th intercostal space, on midclavicular line

Question 18

V3 location

Answer 18

Halfway between V2 and V4

Question 19

V5 location

Answer 19

Left anterior axillary line, in the same axial plane as V4

Question 20

V6 location

Answer 20

Left midaxillary line, same axial plane as V4

Underneath the armpit

Question 21

Appendage leads

Answer 21

Two on the arms, in the same axial plane as V4 when the patient is supine, should be ~2/3 down the brachium

Two on the legs, same axial plane as one another

Question 22

Upward deflections are always going towards the ___ electrode.

Answer 22

Upward deflections are always going towards the + electrode.

Question 23

myocardial action potential durations are more prolonged in cells near the ___ than ___

Answer 23

myocardial action potential durations are more prolonged in cells near the inner endocardium (the first cells stimulated by Purkinje fibers) than in myocytes near the outer epicardium

Question 24

The cells close to the endocardium are the ____

Answer 24

The cells close to the endocardium are the first to depolarize but are the last to repolarize

Question 25

ECG placement diagram

Answer 25

Question 26

The right-leg electrode

Answer 26

The right-leg electrode **is not used for measurement but serves as an electrical ground.**

Question 27

aVR

Answer 27

Right arm + with respsect to other electrodes unipolar lead

Question 28

Unipolar leads

Answer 28

There is no single (−) pole; rather, the other limb electrodes are averaged to create a composite (−) reference. Ex: aVR, aVL, aVF

Question 29

aVF

Answer 29

Recorded by setting the left leg as the (+) pole, such that a positive deflection is recorded when forces are directed toward the feet.

Question 30

aVL

Answer 30

Lead aVL is selected when the left-arm electrode is made the (+) pole and it records an upward deflection when electrical activity is aimed in that direction

Question 31

Bipolar leads

Answer 31

Bipolar indicates that **one limb electrode is the (+) pole** and **another single electrode provides the (−) reference.**

Question 32

Limb lead diagram

Answer 32

Question 33

Axial reference system for limb leads

Answer 33

Question 34

The magnitude of the deflection, either upward or downward, relative to other deflections reflects . . .

Answer 34

The magnitude of the deflection, either upward or downward, relative to other deflections reflects **how parallel the electrical force is to the axis of the lead being examined**

Question 35

Precordial leads in relation to ventricles

Answer 35

Question 36

The first region to depolarize is . . .

Answer 36

The first region to depolarize is **the midportion of the interventricular septum on the left side**

Question 37

Current vectors of a standard myocardial depolarization

Answer 37

Question 38

Standard depolarization as seen from different precordial leads

Answer 38

Question 39

Horizontal axis of an ECG

Answer 39

The horizontal axis represents time. Because the standard recording speed is 25 mm/sec, **each 1 mm division represents 0.04 sec** and **each heavy line (5 mm) represents 0.2 sec.**

Question 41

ECG machines routinely inscribe a ____ at the beginning or end of each 12-lead tracing to document the voltage calibration of the machine.

Answer 41

ECG machines routinely inscribe a **1.0-mV vertical signal** at the beginning or end of each 12-lead tracing to document the voltage calibration of the machine.

Question 42

Each 1-mm vertical box on the ECG paper represents \_\_\_

Answer 42

each 1-mm vertical box on the ECG paper represents **0.1 mV** So the calibration signal records a 10-mm deflection

Question 43

Sinus rhythm

Answer 43

Normal cardiac rhythm: 1. every P wave is followed by a QRS 2. every QRS is preceded by a P wave 3. the P wave is upright in leads I, II, and III 4. the PR interval is greater than 0.12 sec

Question 44

normal sinus rhythm

Answer 44

If the heart rate in sinus rhythm is between 60 and 100 bpm and the rhythm is sinus

Question 45

sinus bradycardia

Answer 45

If the heart rate is less than 60 bpm and rhythm is sinus

Question 46

sinus tachycardia

Answer 46

If the heart rate is greater than 100 bpm and the rhythm is sinus

Question 47

Calculating HR from an ECG

Answer 47

Question 48

When measuring irregular heart rates on ECG, it is most convenient to . . .

Answer 48

. . . count the # of QRS complexes between the first and third markers on the strip (6 seconds) and multiply by 10.

Question 49

When measuring regular heart rates on ECG, it is most accurate to . . .

Answer 49

. . . divide 1,500 over the number of mm between beats (1,500 comes from 25 mm/sec and 60 sec/min) **1500 / (mm between QRS)**

Question 50

corrected QT interval

Answer 50

Since **QT varies with HR**, this should be corrected for by **dividing the measured QT by the square root of the R—R interval**.

Question 51

Angle of a normal QRS

Answer 51

Question 52

Leads __ and __ are best used to classify a normal vs abnormal current vector

Answer 52

Leads **I and II** are best used to classify a normal vs abnormal current vector

Question 53

isoelectric complex

Answer 53

**When the upward and downard deflections of QRS are of equal magnitude** When an ECG limb lead inscribes an isoelectric QRS complex, it indicates **that the mean electrical axis of the ventricles is perpendicular to that particular lead.**

Question 54

How to calculate QRS axis

Answer 54

1. **Inspect limb leads I and II.** If the QRS is **primarily upward in both, then the axis is normal** and you are done. **If not, then proceed** to the next step 2. **Inspect the six limb leads** and determine which one contains the **QRS that is most isoelectric**. The **mean axis is perpendicular to that lead.** 3. **Inspect the lead that is perpendicular to the lead containing the isoelectric complex.** If the **QRS** in that perpendicular lead is **primarily upward**, then the mean axis points to the **(+) pole** of that lead. If **primarily negative**, then the mean QRS points to the **(−) pole of that lead.**

Question 55

Bundle Branch Block summary

Answer 55

Question 56

Normal vs path Q waves

Answer 56

Question 57

Left lateral wall infarction on ECG

Answer 57

Question 58

Conditions detectable on ECG

Answer 58