14. Recognising Common ECG Abnormalities

Question 1

How is rate calculated if the rhythm on an ECG is regular or irregular?

Answer 1

Regular - divide 300 by the number of squares in the R-R interval.
Irregular - count the number of QRS complexes in 6 seconds (30 large squares) and multiply by 10.

Question 2

How long should the PR interval on an ECG be?

Answer 2

0.12-0.20 seconds, 3-5 small boxes.

Question 3

What would a longer PR interval suggest?

Answer 3

A delay in spread of depolarisation from AVN to bundle of His.

Question 4

What should the width of the QRS interval on an ECG be?

Answer 4

Less than 3 small squares

Question 5

What would a wider than normal QRS complex suggest?

Answer 5

Slow depolarisation through the Purkinje fibres.

Question 6

What four key things should be looked at on an ECG to interpret the rhythm?

Answer 6

If P waves are always preset.
If the PR interval is the appropriate length.
If there is always a P wave preceding a QRS complex and always a QRS complex following a P wave.
If the QRS complex is the appropriate width.

Question 7

What is the sinus rhythm in bradycardia and tachycardia?

Answer 7

Bradycardia 60 beats per minute.

Tachycardia 100 beats per minute.

Question 8

What is atrial fibrillation caused by?

Answer 8

Bits in the atrium begin to act as pacemakers so different bits of the atrium depolarise at different times. There is no P wave but just lots of little bits of electrical activity. Conduction to the ventricles is random and depends when depolarisation from the atria arrives in a non-refractory period. The atrium don’t have coordinated contraction.

Question 9

How does Mobitz I present on an ECG?

Answer 9

Progressively lengthening PR intervals. Eventually a QRS complex is dropped so the rhythm can reset. But the same thing happens again.

Question 10

How does Mobitz II present on an ECG?

Answer 10

There are regular PR intervals, but dropped QRS complexes.

Question 11

How does Mobitz III present on an ECG?

Answer 11

It is third degree heart block. There is no relationship between P wave and the QRS interval.

Question 12

What is first degree heart block?

Answer 12

The P wave is normal but there is slow conduction in the AV node and His bundle. This means the PR interval is prolonged. The QRS is normal though.

Question 13

What is 2nd degree heart block type I?

Answer 13

Aka Mobitz I. There is progressive lengthening of the PR interval until QRS complex is dropped to allow the AVN to recover and reset. The cycle begins again.

Question 14

What is 2nd degree heart block type II?

Answer 14

The PR interval is normal but then there is a sudden lack of conduction of a beat, where a QRS complex is dropped. There is a high risk of progression to complete heart block so a pacemaker is required.

Question 15

What is third degree heart block?

Answer 15

P waves have a normal rate but they do not conduct to the ventricles. The ventricular pacemaker takes over but the rhythm rate is very slow so the QRS complex is wide. The heart rate is too slow to maintain blood pressure and perfusion and pacemaker insertion is urgently required.

Question 16

What are ectopic foci?

Answer 16

Abnormal pacemaker sites within the heart that display automaticity. They’re normally suppressed by the higher rate of the SAN. They can occur in the atria or ventricles.

Question 17

What happens with ventricular ectopic beats?

Answer 17

There are ventricular ectopics that depolarise through muscle, not the fast Purkinje system. So there is slow depolarisation of the ventricles and a widen QRS complex with more of a sloping shape than normal.

Question 18

What is ventricular tachycardia defined as?

Answer 18

3 or more ventricular ectopics in a row.

Question 19

What is ventricular fibrillation?

Answer 19

Abnormal, chaotic, fast, ventricular depolarisation. It’s from impulses arising in numerous ectopic sites in ventricular muscle. There is no co-ordinated contraction.

Question 20

Which strip should be looked at on an ECG to asses rhythm?

Answer 20

The long rhythm strip.

Question 21

What is seen on an ECG of someone with ischaemic heart disease?

Answer 21

It is normal at rest, but changes are seen in exercise.

Question 22

What are the three ECG features of a fully evolved myocardial infarction?

Answer 22

Q waves from myocardial necrosis.
ST segment elevation from sub epicardial injury.
T wave inversion.

Question 23

What is classed as a pathological Q wave?

Answer 23

More than 0.04s wide and >2mm deep. It is in full thickness MI and remain after other changes resolve.

Question 24

What are the angles for the normal axis, right axis and left axis?

Answer 24

Normal axis -30 to +90 degrees.
Right axis more than +90 degrees.
Left axis less than -30 degrees.

Question 25

What is the significance of axis deviation?

Answer 25

Left axis - left ventricular hypertrophy and conduction blocks in anterior part of the left bundle branch.
Right axis - right ventricular deviation.

Question 26

In left axis deviation, which way do QRS complexes in lead I and III point?

Answer 26

Left axis deviation so they’re Leaving each other! Lead I is upright, lead III is inverted.

Question 27

In right axis deviation, which way do QRS complexes in lead I and III point?

Answer 27

Right axis deviation, the QRS complexes are Reaching each other. Lead I is inverted, lead III is upright.