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Test your knowledge of the ECG waves, segments and intervals with these questions.

1. Which single statement regarding the p wave on the ECG is true?

A. It is normally between 120 and 150 ms in duration
B. It is positive in lead AVR
C. It represents atrial repolarisation
D. It is less than 2.5 mm in amplitude in the limb leads
E. It is negative in lead II

Answer: D. It is less than 2.5 mm in amplitude in the limb leads

The P wave is the first positive deflection on the ECG. It is a small smooth-contoured wave and represents atrial depolarisation. Atrial repolarisation is not visible as the amplitude is too small.

The normal P wave is:

• <120 ms in duration (3 ‘small squares’)
• <2.5 mm in amplitude in the limb leads
• <1.5 mm in amplitude in the chest leads
2. Which one of the following statements is true regarding Q waves on the ECG?

A. They can be a normal finding in lead V2
B. They can be a normal finding in lead III
C. They are pathological if greater than one third of the height of the subsequent R wave
D. They are pathological if greater than 0.02 seconds in duration
E. They are always an abnormal finding in lead V1

Q waves can be a normal finding in leads III and aVR. They are pathological if they are greater than half the height of the subsequent R wave or if they are greater than 0.04 seconds in duration.

3. Which single statement regarding the PR segment is true?

A. It commences at the start of the P wave
B. It ends at the end of the QRS complex
C. It represents the duration of the conduction of electrical impulses from the AV node to the bundle branches and Purkinje fibres
D. Deviation can occur under normal circumstances
E. It is the interval between ventricular depolarization and repolarisation

Answer: C. It represents the duration of the conduction of electrical impulses from the AV node to the bundle branches and Purkinje fibres

The PR segment commences at the endpoint of the P wave and ends at the start of the QRS complex. It represents the duration of the conduction of electrical impulses from the AV node to the bundle branches and Purkinje fibres. The PR segment is isoelectric under normal circumstances, but deviation can occur in the presence of pericarditis and atrial ischaemia.

The ECG segments © Medical Exam Prep

4. Which single statement regarding the ST segment is FALSE?

A. It commences at the J point
B. It is isoelectric under normal circumstances
C. It ends at the end of the T wave
D. The ventricles are contracted during the ST segment
E. The atria are relaxed during the ST segment

Answer: E. It ends at the end of the T wave

The ST segment commences at the end of the S wave (the J point) and ends at the beginning of the T wave. The ST segment is isoelectric under normal circumstances as the atria are relaxed and the ventricles contracted, and there is, therefore, no visible electrical activity.

The most important causes of ST segment deviation are myocardial ischaemia and infarction, with ischaemia causing ST depression and infarction causing ST elevation.

5. Which one of the following statements is true regarding the ECG waves, segments and intervals?

A. The P wave is 0.08-0.10 seconds in duration
B. The QRS complex is 0.12-0.20 seconds in duration
C. The corrected QT interval is normally less than 0.35 seconds
D. The corrected QT interval can be calculated by the square root of the RR interval divided by the QT interval
E. The PR segment is measured from the start of the P wave to the start of the QRS complex

Answer: A. The P wave is 0.08-0.10 seconds in duration

The P wave is 0.08-0.10 seconds in duration. The QRS complex is 0.06 – 0.10 seconds in duration.

The PR interval is measured from the start of the P wave to the start of the QRS complex. The PR segment represents the period of time from the end of the P wave to the start of the QRS complex.

The ST segment is measured from the end of the QRS complex to the start of the T wave. It is the interval between ventricular depolarisation and repolarisation.

The ST interval is measured from the end of the QRS complex to the end of the T wave.

The QT interval is measured from the start of the QRS complex to the end of the T wave. The corrected QT interval is normally less than 0.44 seconds. The corrected QT interval can be calculated by the QT interval divided by the square root of the RR interval.

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