CARDIAC UNIT: ELECTROCARDIOGRAM

Question 1

Explain the timing and morphology of: P-Wave

Answer 1

Timing: The P-wave represents atrial depolarization, which occurs when the electrical impulse generated in the sinoatrial node (SA node) spreads across the atria, causing them to contract.

Morphology: The P-wave is typically small and rounded.

Question 2

Explain the timing and morphology of: PR interval

Answer 2

Timing: represents the time it takes for the electrical impulse to travel from the SA node through the atria, pause at the atrioventricular node (AV node), and then travel down the bundle of His and its branches to the ventricles.

Morphology: The PR interval is usually a flat line (isoelectric) with a duration of approximately 120-200 ms

Question 3

Explain the timing and morphology of: QRS complex

Answer 3

Timing: represents ventricular depolarization, which occurs as the electrical impulse rapidly spreads through the ventricles, causing them to contract.

Morphology: The QRS complex is typically a larger, more complex waveform consisting of a Q-wave, an R-wave, and an S-wave. The exact appearance and duration of the QRS complex can vary. <120 ms

Question 4

Explain the timing and morphology of: ST segment

Answer 4

Timing: represents the early phase ofventricular repolarization

Morphology: The ST segment is typically a flat, isoelectric line.

Question 5

Explain the timing and morphology of: T-wave

Answer 5

Timing: represents ventricular repolarization, which is the recovery of the ventricles as they prepare for the next contraction.

Morphology: The T-wave is usually a positive waveform, small and rounded, slightly larger than P-wave waveform

Question 6

Explain the timing and morphology of: QT interval

Answer 6

Timing: represents the total time it takes for ventricular depolarization and repolarization.

Morphology: The QT interval is measured from the beginning of the QRS complex to the end of the T-wave. Its duration varies with heart rate, and it can be prolonged in certain cardiac conditions. <360-440 ms

Question 7

What would cause a long PR interval?

Answer 7

Block at the atrioventricular node

Question 8

What would cause a short PQ interval?

Answer 8

Pre-excitation as seen in Wolff-Parkinson-White (WPW) syndrome

Question 9

Define: electrocardiogram

Answer 9

Electrocardiogram: measures the movement of positive charge, i.e. propagating depolarization

Question 10

Describe 3 ECG conventions:
- upwards deflection
- downward deflection
- recording electrode

Answer 10

Upwards deflection: movement of positive charge towards the electrode

Downward deflection: movement of positive charge away from the electrode

Recording electrode: represented by +

Question 11

Repolarization is represented by a positive upwards deflection (T wave) on an ECG. Why?

Answer 11

The endocardium gets excited before the epicardium but the epicardium action potential is shorter than endocardium.

Epicardium repolarizes first means that as we start to repolarize, the epicardium drops down to -90 mV although endocardium is still at 20mV

This creates a voltage gradient in the direction of the recording electrode which records it as a positive deflection.

Question 12

What are the advantages of ECG?

Answer 12

• simple and cheap

provides information about:
- anatomical orientation of the heart (its axis)
- chamber sizes (hypertrophy)
- arrhythmias and conduction blocks
- myocardial ischemia (degree and location)
- myocardial infarction (heart attack)
- congenital defects
- changes in cardiac function with time and therapy

Question 13

List all of the electrodes on a 12 lead ECG

Answer 13

10 electrodes total .

• 3 bipolar limb leads
• Unipolar leads: 3 augmented voltage limb leads; 6 precordial (“chest”) leads
• Reference electrode

Question 14

Discuss the position of the 3 bipolar limb leads

Answer 14

Lead 1: negative electrode goes on right arm, positive electrode on left arm

Lead 2: negative electrode on right arm, positive electrode on left leg

Lead 3: negative electrode on left arm, positive electrode on left leg

Question 15

Define: Eithoven’s triangle and Einthoven’s Law

Answer 15

Einthoven’s triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography, formed by the two shoulders and the pubis

Einthoven’s Law: 1+3 = 2

Einthoven’s Law explains that Lead II’s complex is equal to the sum of the corresponding complexes in Leads I and III and is given as II = I + III;

Question 16

Discuss the 3 augmented voltage unipolar limb leads

Answer 16

Unlike the bipolar limb leads (Lead I, Lead II, and Lead III), which record the voltage difference between two limb electrodes, unipolar limb leads measure the voltage at one limb electrode while referencing an “augmented” or “virtual” central point.

Not actually leads!

aVR: Right arm
aVL: Left arm
aVF: Left foot

Question 17

Discuss the precordial (“chest”) leads

Answer 17

Gives a transverse view of the heart

V1-V2: Septal
- V1: Placed in the fourth intercostal space just to the right of the sternum.
- V2: Placed in the fourth intercostal space just to the left of the sternum.
- V3-V4: anterior
- V3: Placed midway between V2 and V4.
- V4: Placed in the fifth intercostal space in the midclavicular line (around the level of the nipple).
- V5-V6: lateral
- V5: Placed horizontally in line with V4, at the anterior axillary line.
- V6: Placed horizontally in line with V5, at the midaxillary line.

Question 18

What does each ECG lead show?

Answer 18

Each lead shows a scalar (1-D) projection in that plane of the 3-D vector

Question 19

Define: mean electrical axis

Answer 19

average direction of electrical depolarization in the heart during the QRS complex of the cardiac cycle

Question 20

Describe the quadrant method to determine the mean electrical axis. List the pros and cons

Answer 20

Look at lead 1 and aVF - if upright, individual probably has a normal axis.

Normal axis: 0-90 degrees

Pro: easy and quick to determine if someone has a normal axis

Cons: not very accurate

Question 21

Describe the geometric method to determine the mean electrical axis. List the pros and cons

Answer 21

1. Measure magnitude of QRS
2. Mark on circle of axis, +2 units at 60 degrees (lead 2) and 1 unit at 30 degrees (negative direction on aVR)
3. Draw two perpendiculars
4. Connect center of the circle with intersection of two perpendiculars
5. Estimate axis of yellow arrow (about 95 degrees)

Pros: more accurate
Cons: need a protractor or ruler or computer

Question 22

Describe the isoelectric method to determine the mean electrical axis. List the pros and cons

Answer 22

1. Find the limb leads
2. Find the lead with the most biphasic QRS complex
3. Identify the lead at 90 degrees to this
4. Determine if the QRS is an upward or downward deflection

(see example from notes)

Question 23

Describe a right axis deviation and what can cause it

Answer 23

Right axis deviation: more electrical activity on the right side of the heart; can be determined if aVF is upright deflection and lead 1 is downward deflection
- If you are tall and thin your heart gets less squished by organs. Heart tends to hang more right.
- Common in infants too
- If you have hypertrophy of your right ventricle this can also cause a right axis deviation

Question 24

Describe a left axis deviation and what can cause it

Answer 24

Left axis deviation: more electrical activity on the left hand upward quadrant; lead 1 is upwards deflecting and aVF is downward deflecting

• Obesity or pregnancy: heart is being squashed by abdominal organs and pushed upward to the left
• Left ventricular hypertrophy

Question 25

Describe an extreme axis and what can cause it

Answer 25

Mean electrical axis is in the upper right quadrant; lead 1 and aVF have downward deflection

• Dextrocardia: heart is positioned on the right side of the chest instead of its normal position on the left side
• Cardiac pacemaker is placed on the bottom ventricle making the signal go int he other direction