ECG pt.1

Question 1

Importance of ECG

Answer 1

• Most important test for interpretation of the cardiac rhythm, conduction system abnormalities, and the detection of myocardial ischemia
• Great value in the evaluation of other types of cardiac abnormalities including valvular heart disease, cardiomyopathy, pericarditis, and hypertensive disease
• Can be used to monitor drug treatment (specifically antiarrhythmic therapy) and to detect metabolic disturbances

Question 2

What is an ECG?

Answer 2

• A plot of voltage on the vertical axis against time on the horizontal axis
• Reflects the net electrical activity at a given moment

Question 3

How does an ECG work?

Answer 3

• Electrodes are connected to a galvanometer that records a potential difference
• The needle (or pen) of the ECG is deflected a given distance depending upon the voltage measured
• At every beat, the heart is depolarized to trigger its contraction. This electrical activity is transmitted throughout the body and can be picked up on the skin
• This is the principle behind the ECG

Question 4

What is an ECG recorded on? Characteristics of it

Answer 4

• Recorded on special graph paper that is divided into 1 mm2 grid-like boxes
• Has large boxes (heavy lines) of 0.5 cm sides with each large box having smaller boxes on the horizontal axis and 5 smaller boxes on the vertical axis

Question 5

What does each large box represent on the horizontal axis?

Answer 5

Represents 0.2 seconds at a typical paper speed of 25mm/sec

Question 6

ECG paper speed

Answer 6

• Ordinarily 25 mm/sec
• On occasion, the paper speed is increased to 50 mm/sec to better define waveforms

Question 7

What does each small box represent on the horizontal axis?

Answer 7

0.04 seconds at a typical paper speed of 25mm sec

Question 8

What does each box represent on the vertical axis?

Answer 8

• Each small box (1mm) represents 0.1mv, large box (5mm) 0.5mv, two large boxes (10mm) 1 mv
• On occasion, particularly when the waveforms are small, double standard is used (20 mm equals 1 mv). When the waveforms are very large, half standard may be used (5 mm equals 1 mv)

Question 9

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Answer 9

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Question 10

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Answer 10

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Question 11

What does the ECG measure on the X axis?

Answer 11

Measures the height (amplitude) of a given wave or deflection

Question 12

What would each box represent on y axis if the paper speed is increased to 50mm/sec?

Answer 12

Each large box is only 0.10 sec and each small box is only 0.02 sec

Question 13

How would the heart rate appear on ECG with a speed of 50mm/sec?

Answer 13

The heart rate appears to be one-half of what is recorded at 25 mm/sec

Question 14

ECG intervals in 50 mm/sec vs 25 mm/sec

Answer 14

All of the ECG intervals are twice as long as normal on a 50 mm/sec paper speed

Question 15

Where can we find the paper speed and voltage used in an ECG?

Answer 15

Paper speed and voltage are usually printed on the bottom of the ECG for reference

Question 16

What is a normal ECG composed of?

Answer 16

Several different waveforms that represent electrical events during each cardiac cycle in various parts of the heart

Question 17

What waveforms do we find on an ECG?

Answer 17

• ECG waves are labeled alphabetically starting with the P wave, followed by the QRS complex and the ST-T-U complex (ST segment, T wave, and U wave)
• The J (junction) point is the junction between the end of the QRS and the beginning of the ST segment, the point where the QRS complex joins the ST segment

Question 18

What does the P wave represent?

Answer 18

Atrial depolarization

Question 19

What is is the P wave and how is the P wave seen on a normal ECG?at

Answer 19

Represents atrial depolarization and is seen as an initial low amplitude deflection preceding the QRS complex that is positive in most leads

Question 20

Duration and amplitude of P wave?

Answer 20

Generally <0.12 sec (three small boxes) and the amplitude <0.25 mv (2.5 small boxes)

Question 21

What represents atrial repolarization and when does it occur?

Answer 21

• Atrial repolarization sequence (Atrial ST and T wave phases) occurs just before, simultaneously, and just after depolarization of the ventricular myocardium

Question 22

Atrial T wave presence on ECG

Answer 22

The atrial “T wave” itself is usually hidden by the QRS complex and not observed on the routine ECG

Question 23

What is the PR interval and what does it represent?

Answer 23

• Time between the first deflection of the P wave and the first deflection of the QRS complex
• It includes time for atrial depolarization (the P wave) and conduction through the AV node and the His-Purkinje system (which constitute the PR segment)

Question 24

What does QRS wave complex represent?

Answer 24

Ventricular depolarization

Question 25

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Answer 25

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Question 26

How to read QRS complex?

Answer 26

The rule is: if the wave immediately after the P wave is an upward deflection, it is an R wave; if it is a downward deflection, it is a Q wave

Question 27

What does the Q wave represent?

Answer 27

Correspond to depolarization of the interventricular septum

Question 28

What does the R wave represent?

Answer 28

Depolarization of the main mass of the ventricles –hence it is the largest wave

Question 29

What does the S wave represent?

Answer 29

Signifies the final depolarization of the ventricles

Question 30

What is the ST segment and what does it represent?

Answer 30

• The time between the end of the QRS complex and the start of the T wave
• Occurs after ventricular depolarization has ended and before repolarization has begun
• It is a time of electrocardiographic silence reflecting the period of zero potential between ventricular depolarization and repolarization

Question 31

What does the T wave represent?

Answer 31

T waves represent ventricular repolarization (atrial repolarization is obscured by the large QRS complex)

Question 32

What should every ECG reading start with?

Answer 32

Every ECG reading should start with an assessment of the rate, rhythm, and axis

Question 33

What is the PR segment also known as?

Answer 33

PR interval