Clinical Interpretations of ECG I

Question 1

Electrode

Answer 1

An electrode is a conductor through which electric current leaves the body.

Question 2

Lead

Answer 2

A lead is the electric potential between two defined electrodes (represented by the tracing).

Question 3

1 small square translated to *what *on the Y axis?

Answer 3

0.1mV

Question 4

One small square translates to what on the X axis?

Answer 4

0.04s

Question 5

What is the normal paper speed?

Answer 5

25mm/sex

Question 6

PR interval

Answer 6

Beginning of P to the beginning of QRS complex

Question 7

QRS interval

Answer 7

Beginning of Q to end of S

Question 8

QT interval

Answer 8

Beginning of QRS complex to end of T wave

Question 9

What do you check on an ECG?

Answer 9

1. Patients name, age and gender.
2. Date and time of trace.
3. Check paper speed and calibration.

Question 10

Describe the things we look at when analysing an ECG

Answer 10

1. Rate
2. Rhythm
3. Axis
4. P wave
5. PR interval
6. QRS complex
7. ST segment
8. T wave
9. (QT interval)
10. (U wave)

Question 11

How do you calaculate rate?

Answer 11

(i) R-R interval method

• 300 divided by the number of large squares between two consecutive R waves.

It can only be used if rhythm is regular.

(ii) Six second method

• The number of cardiac cycles in 6 seconds multiplied by 10

It can be used of rhythm is regular or irregular.

Question 12

Rate abnormalities

Answer 12

Tachycardias or tachyarrhythmias

• Faster than normal rate (>100bpm)

Bradycardias or bradyarrhythmias

• Slower than normal rate (<60 bpm)

Question 13

Rhythm

Answer 13

Regular

Sinus rhythm

• Every P wave must be followed by a QRS complex
• Every QRS complex must be preceded by a P wave

Other: eg. SVT, VT

Irregular

Regularly irregular

• e.g. second degree heart block, ventricular bigeminy/trigeminy etc.

Irregularly irregular

• e.g. AF

Question 14

Calculating axis via the two-lead method

Answer 14

*Examine QRS complexes wave in lead I and aVF. *

• If both predominanly positive, axis is normal.
• If I is positive, aVF is negative, left axis deviation.
• If I is negative, aVF is positive, right axis deviation.
• If both leads predominanly negative, extreme right axis deviation.

Question 15

Calculating axis via the three-lead method

Answer 15

Examine polarity of R wave in leads I, II and III.

• If all three are predominantly positive, axis is normal.
• If I is positive, II and III are negative, left axis deviation.
• If I is negative, II is biphasic or positive and III is positive, right axis deviation.
• If all three leads predominanly negative, extreme right axis deviation.

Question 16

P wave

Answer 16

Duration: 0.04-0.12 sec

Heigh: 1-3mm

Question 17

PR interval

Answer 17

0.12-0.20 sec

Question 18

QRS complex

Answer 18

Duration: 0.05 - 0.11 sec (approximately 1-3 small squares)

Q wave normal if:

• <25% R wave voltage

Rule of thumb:

• <10mm in limb leads
• <20mm in chest leads

Question 19

ST segment

Answer 19

Duration not important.

Morphology

• ST elevation
• ST depression

Question 20

T wave

Answer 20

Voltage (height)

• Should not exceed 50% to 1/3 preceding QRS complex

Generally same net deflection as preceding QRS complex.

Question 21

QT interval values

Answer 21

Varies between males and females.

Needs to be corrected for heart rate.

Normal QTc interval = 350-450ms

Question 22

U wave

Answer 22

Normal in children and young adults.

Prominent U waves:

• Hypokalaemia
• LVH
• Mitral valve prolapse