ECG - Recognising Common Abnormalities

Question 1

Abnormal heart rhythms (arrhythmias) are due to what?

Answer 1

Abnormal impulse formation or abnormal conduction.

Question 2

Where do rhythms in the heart arise from?

Answer 2

Supraventricular rhythms arise from the SAN, the atrial musculature and the AVN. Ventricular rhythms arise from the ventricles.

Question 3

Normally, how is the heart rhythm conducted?

Answer 3

Supraventricular rhythms are conducted into and within the ventricles by the His-Purkinje system and there’s normal ventricular depolarisation with normal (narrow) QRS complexes.

Question 4

Where do ventricular rhythms come from and why do they result in different QRS complexes?

Answer 4

Ventricular rhythms come from a focus/foci in the ventricle and the conduction is not through the usual His-Purkinje system, so depolarisation takes longer resulting in a wide (>3 boxes) QRS, which can be bizarre - rhythms with different foci will have different shapes (could be ventricular premature beats/tachycardia/fibrillation).

Question 5

What is the difference in ECGs with different origins of the impulse and where should be looked at for interpretation?

Answer 5

The p wave and QRS complex will vary depending on the origin of the impulse, allowing diagnosis of the arrhythmia.
For interpretation, look at the ‘rhythm strip’ at the bottom of the 12 lead ECG (a long/10s recording of limb lead II - best for looking at p waves).

Question 6

What constitutes a normal sinus rhythm?

Answer 6

A regular rhythm, identical p waves before each QRS, a P-R interval of 0.12-0.2s, a QRS of <0.12s and a heart rate of 60-100bpm.

Question 7

What are sinus bradycardia and tachycardia?

Answer 7

Sinus rhythm with a rate<60bpm is sinus bradycardia and sinus rhythm with a rate>100bpm is sinus tachycardia.

Question 8

Atrial fibrillation:
Multiple atrial _____, chaotic impulses - atrial ____________ is chaotic, so no __ ______ on the ECG, just a wavy baseline - the atria quiver rather than __________. Impulses arrive at the AVN at a rapid, __________ rate and only some are conducted to the ventricles (at irregular intervals), when the AVN is not _________.

Answer 8

Fock
Depolarisation
P waves
Contract
Irregular
Refractory

Question 9

In atrial depolarisation, what is the action of the ventricles, what is the pulse/HR and what’s an associated risk?

Answer 9

In atrial depolarisation, the ventricles depolarise and contract as normal. The HR is irregularly irregular - atria is 350-650bpm and in the ventricles is slow to rapid. Stagnant blood in the auricles give thrombi to opportunity to form and perhaps embolism (leading to a stroke).

Question 10

What does ‘Heart Block’ refer to and what are the different types?

Answer 10

AV conduction blocks - delay/failure of conduction of impulses from the atria to the ventricles via the AVN and BoH.
3 types: 1st degree HB, 2nd degree HB (Mobitz types 1 and 2) and 3rd degree a.k.a. Complete heart block (CHB).

Question 11

In what type of heart block does a pacemaker in the ventricles take over (Ventricular escape rhythm)?

Answer 11

Complete/3rd degree heart block.

Question 12

What are the causes of heart block?

Answer 12

Degenerative changes or acute myocardial infarction.

Question 13

What does first degree heart block entail, in the heart and on an ECG?

Answer 13

The p wave is normal, but there is slow conduction to the AVN and BoH, so the P-R interval is prolonged (>5 small boxes). The QRS is normal. It is usually fairly benign, but may warn of worse heart block to come.

Question 14

Both Mobitz type 1 and Mobitz type 2 are types of second degree heart block, what’s the difference and which one has another name?

Answer 14

Mobitz type 1 a.k.a. Wenkebach phenomenon, involves progressive lengthening of the P-R interval, until 1 P isn’t conducted, allowing time for the AVN to recover, then the cycle repeats.
Mobitz type 2 involves abnormal P-R interval and then a sudden non-conduction of a beat (dropped QRS). This has a high risk of progression to CHB.

Question 15

What is happening to the heart in complete heart block?

Answer 15

Atrial depolarisation is normal, but the impulse is not conducted to the ventricle, so a ventricular pacemaker takes over (ventricular escape rhythm), but the rate is very slow (30-40bpm), often too low to maintain BP and perfusion (syncope, confusion etc).
Urgent pacemaker insertion is required.

Question 16

What is shown on an ECG during third degree heart block?

Answer 16

Usually wide QRS complexes. P-P intervals are constant at about 93bpm (depolarising normally) and R-R intervals are constant, but usually much slower at around 37bpm. No relationship between the P wave and the QRS complex, so P-R interval completely variable beat to beat.

Question 17

Ventricular ectopic beats:
An ectopic ______ in the ventricular muscle, the impulse is not spread via the _____ His-Purkinje system, so there’s much slower ___________ of the ventricle and a _______ QRS of a different shape to usual.

Answer 17

Focus
Fast
Depolarisation
Wide

Question 18

What is ventricular tachycardia?

Answer 18

A run of 3 of more consecutive ventricular ectopics is defined as VT - broad complex tachycardia. Persistent VT is a dangerous rhythm needing urgent treatment. Carries a high risk of ventricular fibrillation.

Question 19

What is ventricular fibrillation, how is it different to atrial fibrillation and what needs to be done about it?

Answer 19

Abnormal, chaotic fast ventricular depolarisation with impulses from numerous ectopic sites in the ventricular muscle. There is no coordinated contraction and the ventricles just quiver, so no CO leads to cardiac arrest.
Unlike atrial fibrillation, there’s no pulse or heart beat. It requires CPR and immediate Defibrillation to restore rhythm.

Question 20

ECG changes of ischaemia and MI are a result of reduced perfusion of the myocardium, which doesn’t affect all parts of the heart. Changes are seen in leads facing the affected area, so you need to look at the PQRST in all 12. Where are they all looking?

Answer 20

I, aVL, V5 and V6 = lateral.
II, III and aVF = inferior.
V1 and V2 = septal.
V3 and V4 = anterior.
aVR = right upper side of the heart.

Question 21

What role does atherosclerosis play in ischaemia and MI?

Answer 21

Reduced myocardial perfusion may be due to coronary atherosclerosis.

Question 22

Which are of the heart is most vulnerable to ischaemia, when and why?

Answer 22

Major coronary arteries lie on the epicardial surface, so the subendocardial muscle is the furthest away and most vulnerable. The flow happens during diastole and if this is short (high HR from exercise etc), there is less time for the blood to flow.

Question 23

When a subendocardial area of the heart becomes ischaemic, what do leads facing the affected area show?

Answer 23

ST segment depression, , T wave inversion due to an abnormal current during repolarisation. Ischaemic ECG changes may only be seen during exercise, but if there is a severe reduction in the lumen, ischaemic changes may be seen at rest.

Question 24

A week after a non ST elevation myocardial infarction (NSTEMI), what will be seen on an ECG?

Answer 24

ST segment and T wave normal, but no Q waves.

Question 25

What is a ST elevation myocardial infarction (STEMI) caused by?

Answer 25

Due to a complete occlusion of the lumen by a thrombus. The muscle injury extends full thickness from endocardium to epicardium. With an epicardial injury, there’s ST segment elevation in the leads facing the area, because of abnormal currents in repolarisation. If perfusion isn’t reestablished, muscle necrosis will follow.

Question 26

What ECG changes are seen with a STEMI?

Answer 26

It is normal, then there’s ST elevation, then ST elevation with a low R wave and a Q wave appears (hours), then there’s T wave inversion and the Q wave is deeper (1-2 days), the ST normalises and the T wave is still inverted (days), the ST and T wave normalise, but the Q wave persists (weeks).

Question 27

Q wave development signals muscle necrosis, what constitutes a pathological Q wave?

Answer 27

Pathological Q waves are > 1 small square/0.04s wide, > 2 small squares deep or the depth is 1/4 of the height of the subsequent R wave.

Question 28

Extracellular fluid potassium levels affect the resting membrane potential. Briefly, how does it change it in hypo and hyperkalaemia?

Answer 28

In hyperkalaemia, the resting membrane potential is less negative and for hypokalaemia, the resting membrane potential is more negative (hyperpolarised).

Question 29

With hyperkalaemia, some voltage operated Na+ channels are inactivated, so the heart becomes less excitable as the hyperkalaemia worsens. What are the changes seen at 7,8,9 and 10mmol/L (4-5 is normal)?

Answer 29

At 7mmol/L, there are conduction problems and a high T wave.
At 8mmol/L, there is a prolonged P-R interval and a depressed ST segment.
At 9mmol/L, p waves are absent and the atria are at a standstill and there’s intraventricular block.
At 10mmol/L, there’s ventricular fibrillation.

Question 30

When there’s worsening hypokalaemia, what difference can eventually be seen on an ECG (after 3.5mmol/L low T wave, 3 add a high U wave and 2.5 add a low ST)?

Answer 30

Eventually ST depression and a prominent U wave after the T.

Question 31

What should be included when reporting an ECG?

Answer 31

Rhthym, rate, P-R interval, QRS interval, QT interval, p wave (LA/RV enlargement), describe QRS, ST segment, T wave and axis.

Question 32

What is the cardiac axis?

Answer 32

The average direction of spread of ventricular depolarisation.

Question 33

What is the normal cardiac axis?

Answer 33

Downwards and to the left: -30 to +90 egress (0 is lead I, to the left).

Question 34

What is left axis deviation and why does it happen?

Answer 34

The overall direction of depolarisation is upwards and to the left (more negative than 30 degrees). Associated with conduction block of the anterior branch of the left bundle, inferior MI, LV hypertrophy.

Question 35

What is right axis deviation and why does it happen?

Answer 35

The overall direction of ventricular depolarisation is downwards and to the right (more positive than 90 degrees). Associated with Rav hypertrophy.

Question 36

Where can left and right axis deviation be spotted on an ECG?

Answer 36

For left axis deviation, the QRS is positive in lead I and inverted in aVF.
For right axis deviation, the QRS in lead I is inverted and upright in lead III (or aVF).