How to read an ECG!

How to read an ECG in 10 steps. I would recommend checking the rate, rhythm, axis, and QRS-complex first.

If it is regular sinus rhythm + normal axis, then move onto Q-waves, ST-segment, and T-waves, and lastly the rest.

If it is regular sinus rhythm + abnormal axis/QRS, then consider ventricular arrythmias, RV-hypertrophy or BBB, and lastly the rest.

If it is not regular sinus rhythm check the rest of the ECG like P-waves, PR-interval, and QT-interval.

• Rhythm

The rhythm can either be:

• Regularly regular (regular sinus rhythm).
• Regularly irregular (seen mostly in 2nd degree heart block type 2).
• Irregularly irregular (seen mostly in atrial fibrillation.

How to check: See if the RR-interval is of equal length everywhere and if every R-wave is proceeded by a P-wave. Positive, positive, and negative in I, II, and aVR is sinus rhythm. You can also use this rule: P-wave upright in II, inverted in aVR, and every P-wave is followed by QRS-complex in II, then it is sinus rhythm.

• Rate

How to check: If rhythm is regular divide number of LARGE boxes between 2 QRS-complexes in 25 mm/s ECG by 300 or by 600 in 50 mm/s ECG. Usually the ECG-machine will calculate it for you. If the rhythm is irregular calculate the number of R-waves in 6 seconds (30 LARGE boxes) multiply by 10.

1 large square = 0,2 s

1 small square = 0,04 s

• Axis

Normal cardiac axis is -30 to +90 degrees. In ECG, there are 3 bipolar leads: I, II, and III; 3 augmented unipolar leads: aVR, aVL, and aVF; 6 precordial chest leads: V1-V6. The cardiac axis shows the major direction of the overall electrical activity of the heart which can be a useful tool to that can indicate various pathologies like hypertrophy.

How to check: 

First you find the general axis by checking if net QRS-complexes in lead I and aVF are positive or negative:

When you know the general axis you can figure out the specific axis by finding the lead that is biphasic and going perpendicular from that lead you will have 2 values (using the limb lead chart above). The value that is within the general axis is your specific axis.

Example: Lead I and lead aVF are positive and negative, respectively, and thus we have a general left axis deviation. The lead that is biphasic on the ECG is lead aVR. Therefore, a perpendicular line from aVR on the above diagram would hit either -60 or +120. Since -60 is within our general left axis deviation, then -60 must be our specific axis deviation.

Causes of LAD:

LAFB

LBBB

LV-hypertrophy

Ischemia (T-wave inversion, deep Q-waves, STEMI, or NSTEMI)

Causes of RAD:

LPFB

RV-Hypertrophy

Lateral

MI

PE

COPD

• P-wave

The P-wave indicates atrial depolarization and you can use it to deduce right or left atrial enlargement. Enlargement is different from hypertrophy. Enlargement is volume overload whereas hypertrophy is pressure overload.

How to check: If P-waves are present see the morphology in lead II and V1. The height of P-waves should be <2,5 mm in limb leads and <1,5 mm in precordial leads. The width should be <0,12 s.

Tall and pointy P-waves are called P-pulmonale indicating RIGHT ATRIAL ENGLARGMENT.

Short and bifid P-waves are called P-mitrale indicating LEFT ATRIAL ENLARGEMENT.

• PR-interval

The PR-interval indicates AV-node delay and thus should be horizontal.

How to check: Normal PR-interval is 0,12-0,2 sec (3-5 small squares).

Prolonged PR-interval = AV-blocks

An AV-block in the conduction system from SA to AV-node. There are different degrees and types of AV-block.

• 1st degree AV-block: PR-interval > 200 ms (5 small squares)
• 2nd degree AV-block.
• Mobitz 1 (Wenchebach): PR-interval > 200 ms + Progresively longer until drop
• Mobitz 2: PR-interval > 200 ms + Drop every now and then
• 2:1 block: May be Mobitz 1 or 2.
• Long PR-interval + normal QRS = Mobitz 1.
• Normal PR-interval + long QRS = Mobitz 2.
• 3rd degree AV-block: PR-interval > 200 ms + Constant PP-interval + Constant QQ-interval

In a 3rd degree AV-block the atria and ventricular are conducting independently of eachother. Note that P-waves might be buried within a QRS-complex.

Mnemonic for Mobitz 1: Longer, longer, longer drop. That is a Wenchebach!

Reduced PR-interval = WPW syndrome.

Depressed PR-interval = Pericariditis.

• Q-wave

How to check: Only pathological if >2 small squares deep.

Deep + wide Q-waves = MI

Only deep Q-waves = Ventricular hypertrophy

If you find Q-waves that are deep and wide, then the specific lead that has them can be used to deduce which heart muscle-wall is suffering from MI. For example: deep Q-waves in II, III, and aVF show inferior wall MI.

• QRS-complex

How to check: It is normally 0,08-0,12 s (2-3 small squares).

Wide QRS-complex = Ventricular arrythmias or Bundle Branch Blocks.

Ventricular arrythmias may be ectopic, tachycardia, or fibrillation.

Mnemonic for detecting RBBB: MarrooN (V1-V2 is an M and V5-V6 is a N).

Mnemonic for detecting LBBB: Villheim (V1-V2 is V and V5-V6 is M).

Criteria of LBBB: QRS-duration > 120 ms + absent Q-waves in I, aVL, V5, and V6 (lateral leads) + broad monomorphic S waves in V1 + broad monomorphic notched or slurred R waves in I and V6.

Criteria of RBBB: QRS-duration > 120 ms + RSR' pattern or "bunny ears"/"M-shaped" in V1-V3 + Wide and slurred S-wave in lateral leads

Criteria of incomplete LBBB: All criteria of LBBB except the QRS-duration is < 120 ms

Sgarbossa Criteria: ST-elevation in positive QRS-complex in lead I + ST-depression in positive QRS-complex in lead V1-V3 + ST-elevation > 1 big-box in a lead with negative QRS-complex

Criteria of LAFB: Axis deviation between -45 and -60 (left axis deviation) + qR-complex in I and aVL + rS-complex in inferior leads + time from onset of QRS to the top of R-wave > 45 ms in aVL

LPF:

Based on the rhythm, rate, and length of QRS-complex you can conclude a set of diagnoses. The DDx of tachycardia:

1. Narrow + regular
• Sinus tachycardia
• Atrial flutter with 2:1 or 1:1
• SVT
2. Narrow + irregular
• A-fib
• Variable A-flutter
• M. A. T.
3. Wide + regular
• V. tach
• SVT w/ BBB
• Sinus tachycardia w/ BBB
• Antidromic WPW
4. Wide + irregular
• PMVT
• A-fib w/ WPW
• A-fib w/ BBB

The DDx of bradycardia:

1. Sinus bradycardia
2. AV-Block
3. Junctional rhythm
4. Ventricular rhythm

• QT-interval

How to check:

Normal QT-interval is less than ½ the preceeding RR-interval.

Long QT-interval = Torsades de Pointes

• ST-segment

How to check:

If you see ST-elevation use the following chart for DDx:

V1 + V2 = Septal wall MI

V3 + V4 = Anterior wall MI

I + aVL + V5 + V6 = Lateral wall MI

II + III + aVF = Inferior wall MI

ST-segment elevation is 1 mm in 2 CONTINOUS LEADS except V2 and V3 (2 mm in those leads).

ST-segment depression is 0,5 mm in 2 CONTINOUS LEADS.

DDx of ST-elevation:

STEMI

Benign early repolarization

Pericarditis

Vasospasm

PE

LV-aneurysm

LV-hypertrophy

LBBB

If they have upsloping ST-depression use De Winter T wave criteria which can indicate proximal LAD occlusion.

De Winter T wave criteria: Tall and peaked T-waves in precordial leads + upsloping ST-depression in precordial leads without ST-elevation in any precordial leads + reciprocal ST-elevation in aVR.

DDx of ST-depression:

NSTEMI

Posterior MI

LBBB

LVH w/ strain

Reciprocal changes

Digoxin toxicity

Note that sometimes a J-wave can occur right after the R-wave. DDx of J-wave:

Benign early repolarization

Hypothermia

Hypercalemia

Brugada syndroma

• T-wave

Should be upright in all leads except aVR + V1.

DDx of T-wave inversion:

LVH strain

Increased ICP

PE

BBB

Ischemia (Wellens-B)

Note that T-wave inversions are normal in V1-V2 and III

DDx of Hyperacute and broad T-waves:

Vasospasm (Prinzmetal angina)

Early STEMI

DDx of Hyperacute and narrow T-waves:

Hyperkalemia

Hypermagnesemia

Ischemia (De-Winters T-waves: proximal LAD occlusion)

DDx of Biphasic T-waves:

Ischemia (Wellens-A) if it is positive then negative

Hyperkalemia if it is negative then positive

DDx of Flat T-waves:

Ischemia

Hypokalemia

Also consider:

Sokolow-Lyon Criteria: Add the deepest S-wave in V1 or V2 + the tallest R-wave in V5 or V6. If the sum is >35 mm, then left ventricular hypertrophy is present.

Right ventricular hypertrophy: Right axis deviation is >100, R-wave is > S-wave in V1, and S-wave is > R-wave in V6.

Most common cause of RV-hypertrophy:

Congenital heart disease

Idiopathic pulmonary hypertension

Secondary pulmonary hypertension due to mitral valve disease, lung disease, or pulmonary embolism.

Physical signs of RV-hypertrophy:

Left parasternal heave

Displaced and diffused apex beat

Loud pulmonary second sound

Elevated jugular venous pressure

Flicking A-wave in jugular venous pulse if pulmonary hypertension is the cause

Treatment of RV-hypertrophy:

Anticoagulants

Diagnostic criterias:

• Chest pain + normal troponin + normal/abnormal ECG: Unstable angina
• Chest pain + high troponin + normal ECG: NSTEMI
• Chest pain + high troponin + abnormal ECG: STEMI

Other criterias:

• AVNRT: Regular narrow QRS-complex + >180 bpm. + unclear P-waves + may be pseudo-R-wave in V1 and pseudo-S-wave in inferior leads
• Orthodromic AVRT: Retrograde P-wave > 70 ms after QRS in V1.
• PSVT: Narrow QRS-complex + 150-240 bpm.
• Atrial fibrillation: No visible P-waves + Irregularly irregular QRS-complex

• Atrial flutter: Narrow complex tachycardia + Loss of isoelectric baseline + "saw-tooth" pattern of inferior leads. If ventricular rate is half of the atrial rate, then it is 2:1 - if a 3rd then 3:1 and if a 4th then 4:1 block. Regular atrial activity is at ~300 bpm.

• Bifascicular block (RBBB + LAH)
• Bifascicular block (RBBB + LPH):

Resources used:

• The Only EKG Book You’ll Ever Need (9 ed.) by Malcolm S. Thaler
• HOW TO READ AN ECG!! WITH ANIMATIONS by Intellect Medicos on YouTube
• AV Blocks (1st, 2nd, and 3rd Degree) by Dirty Medicine on YouTube
• https://www.skillstat.com/tools/ecg-simulator/