ECG Learnings

Why are waves on the aVR lead deflected?

Because it is on the right arm, so waves of depolarisation move away from it, and thus will be downward deflections.

What are the dimensions of each small square?

• 1mm/0.1mV height

• 0.04 sec width

What are the dimensions of each large square?

• 5mm/0.5mV height

• 0.20 sec width

What are the characteristics of a normal P wave?

• height should not exceed 2.5mm

• width should not exceed 0.10 seconds (2.5 small squares)

What does the P wave represent?

Atrial depolarisation; the first part represents depolarisation of the right atria, and the second part represents depolarisation of the left atria.

What do high, tall, peaked P waves indicate?

Right atrial hypertrophy (P waves > 2.5mm)

What do wide notched p waves indicate?

Left atrial hypertrophy. P mitrale (in mitral stenosis) is a P-wave shaped like an M

What are the normal characteristics of a PR interval?

• Has a normal duration of 0.12-0.20s (3-5 small squares)

• It represents the time from the onset of atrial activation to the onset of ventricular activation

What are the normal characteristics of a QRS complex?

• Its duration is from 0.06 - 0.10 seconds (1.5-2.5 small squares). A duration of >0.12sec is abnormal

• Proceeding from V1-V6, the R waves get taller and the S waves get shorter

What are the normal characteristics of a Q wave?

• It is the first negative/downward deflection following the P waves. It represents septal depolarisation.

• Its amplitude should be less than 1/3 of the amplitude of the R wave in the same lead

• A deep or broad Q wave is normal

What are the normal characteristics of an R wave?

- Represents ventricular depolarisation

What are the normal characteristics of an S wave?

• Any downward deflection following the R wave

• Represents depolarisation of the Purkinje fibres

What are the normal characteristics of an ST segment?

• Corresponds to the plateau phase of ventricular depolarisation, and it should be on the baseline

What are the normal characteristics of the T wave?

• Represents ventricular repolarisation

• asymmetric with gradual incline and steeper decline

What are the normal characteristics of the QT interval?

• Measured from beginning of QRS to end of the T wave.

• It represents the interval from the beginning of depolarisation to the end of repolarisation (total ventricular activity)

• Usually between 0.33-0.42 seconds (8-10 small squares)

• Varies inversely with heart rate

Which part of the cardiac cycle does the P wave represent?

atrial systole

Which part of the cardiac cycle does the QRS complex represent?

ventricular systole

Which part of cardiac cycle does the T wave represent?

beginning of ventricular relaxation or diastole

Remember that:

1. An upward wave on the ECG represents a depolarisation wave which is moving towards a +ve electrode

2. A wave of depolarisation travelling away from a +ve electrode is downard deflection

3. the electrodes placed on the chest in all 6 leads are +ve

What are the steps for analysing any ECG?

1. Name, date, time

2. Technical errors

3. rate

4. rhythm

5. cardiac axis

6. P wave

7. PR interval

8. QRS complex

9. ST segment

10. T wave

11. U wave 

12. QT interval

What technical errors can affect an ECG?

• Misplacement of electrodes

• a patient not relaxed or shivering

• poor contact between the patient’s skin and the electrodes

How do we calculate heart rate from an ECG?

1. Count the number of small squares between 2 R waves (x)

2. Divide 1500 by this number (1500/x)

(OR - 300/big squares)

How do we calculate the heart rate of an irregular rhythm?

1. Count the number of big squares between 6 R waves (5 cycles)

2. Multiply 300×5.

3. Divide 300×5/the number of big squares between the 6 R waves.

How do we check the regularity of an ECG?

You need to measure R-R intervals. R-R intervals will be consistent in a regular rhythm, but an irregularly irregular rhythm has no pattern. 

How can we assess if there are rhythm abnormalities?

1. Look at distance between R waves

2. Check if there is a P wave

3. Check if each QRS complex is preceded by a P wave

4. Check if each P wave is followed by QRS

5. Measure the width of the QRS complex

What does the cardiac axis represent?

The average direction of the spread of depolarisation of the heart.

What is the normal cardiac axis?

between -30° and 90°

What is left axis deviation?

A cardiac axis smaller than -30°

What is right axis deviation

A cardiac axis greater than 90°

Explain the normal cardiac axis.

Usually, the path of depolarisation in the heart is from superior to inferior towards the left. This is due to the leftward orientation of the heart in the chest and because the left ventricle has more muscle mass than the right.

How can cardiac axis change?

It can deviate in some conditions when the electrical pathway goes a different way (e.g. - when disrupted by death of tissue in AMI), or if some parts of the heart are working harder than others.

What can left axis deviation suggest?

• inferior MI

• left ventricular hypertrophy

• loss of electrical activity in the right ventricle

What can right axis deviation suggest?

• right heart strain

• chronic lung disease

• high lateral MI

• RVH

What easy way is there to suggest that the electrical axis of the heart is normal?

If QRS is +ve in L1, aVF, L2

Steps for calculating the cardiac axis

1. Look at QRS in L1. Is it +ve or -ve?

2. Look at QRS in aVF. Is it +ve or -ve?

3. Find out which quadrant both L1 or aVF fall.

What if both LI and aVF QRSs fall in lower inner quadrant? (South-East, NEE)

normal axis

What if both LI and aVF QRSs fall in the lower outer quadrant? (South-West)

right-axis deviation

What if both LI and aVF QRSs fall in the upper outer quadrant (north-west)

No man’s land

The upper inner quadrant (NEE)?

Normal axis or Left-Axis Deviation

What do we need to do if the LI and aVF QRSs fall in the upper inner quadrant (NEE?)

If between 0-30°, the cardiac axis can be either normal or LAD (if between 30 and -90°)

Here, you need to also look at QRS in L2. If QRS +ve in L2 it is normal axis. If it is -ve in L2 it is LAD.

How to assess P wave in ECG analysis?

1. Look at L2

2. Is it absent or present

3. Look at the shape

What abnormalities of P wave are there?

• Absence — can be AF

• Peaked/Tall >2.5mm — RAH

• Broad/Bifid > 0.10sec — LAH

How to assess PR wave in ECG analysis?

Measure the PR interval; should be between 0.12-0.20seconds (3-5 small squares).

Abnormalities suggest delays in conducting system

1st degree AV block?

• PR interval is longer than normal, but every P-wave is followed by a QRS complex

2nd degree AV block?

• characterised by an occasional drop of a QRS complex

2nd degree AV Block Type I

• PR interval gradually lengthens until for one beat, AV conduction does not occur at all

2nd degree AV block Type II

• A sudden AV block results in the sudden absence of QRS complex (no gradual lengthening of the PR interval)

3rd degree AV block

The AV node and the bundle branches do not let any supraventricular impulse to pass through them (atrial impulses do not reach ventricles)

Why are Q waves downward?

Because they represent septal depolarisation, which occurs from left to right.

How to analyse ECG Q waves

Almost absent in all leads.

A significant Q wave (one small square wide) might indicate necrosis/MI

How to analyse QRS complexes

• Measure QRS duration (usually 0.06-0.10 seconds).

• If duration >0.12sec:

  • delay in intraventricular conduction

  • ventricular tachycardia

  • pre-excitation of the ventricles

How to assess LVH?

• Add the height of the deepest S wave in V1 or V2 to the height of the tallest R wave in V5 or V6.

• If sum ≥ 35mm, this is LVH

How to assess RVH?

• Add the height of R in V1 to the height of S in Vt

• If sum ≥ 10mm, this is RVH

What happens in LVH?

More electrical activity will be moving to the leads oriented to the left ventricle (V5, V6, aVL), and away from the leads oriented away from the left ventricle (V1, V2).

• Thus leads V5, V6, aVL have tall R waves while leads V1, V2 have deep S waves.

What happens in Bundle Branch Blocks?

The electrical signal takes longer to pass throughout ventricles and accordingly, the QRS complex widens (> 0.12sec), and the R wave is M shaped.

How to assess RBBB

QRS complex > 0.12sec, and the M shaped R wave appears in V1 and V2 (right leads)

How to assess LBBB

QRS complex > 0.12sec, and the M shaped R appears in V5 and V6 (left leads)

Remember that a normal 12 lead ECG does not rle out an AMI. T/F?

True - History, Physical Exam and serum cardiac markers should all be considered

How to assess Myocardial Infarction

• Initially may be tall T waves, however, this may be short-lived.

• ST-elevation in the leads present at area of injury, within the first few hours after symptoms

• ST-depression in the opposite leads from where the ST elevation is seen.

• T waves may invert hours after the infarct

What do tall T waves signify?

Cardiac ischemia (initial)

What do flat, inverted T waves signify?

ischemia

What do wide/deep Q waves only without ST elevation signify?

OLD MI (2 days, 20 years… can’t do anything to save that part of the heart)

What do wide/deep Q waves signify?

necrosis

What does atrial flutter look like?

Saw tooth appearance, due to re-entry within the atria

What does Ventricular fibrillation look like?

• irregularly irregular rhythm, no P waves, wide QRS complexes.

Which part of the ECG represents potassium?

The T wave (ventricular repolarisation)