ECG Complete Revision in One Shot | ECG Waves, Leads, Intervals, Arrhythmias & Interpretation

ECG — COMPLETE REVISION IN ONE SHOT

Electrocardiography • Waves • Intervals • Leads • Axis • Arrhythmias • Ischemia • MI • Systematic Interpretation

ECG: Electrocardiogram is a graphical recording of the electrical activity of the heart obtained from electrodes placed on the body surface.

1. LEARNING OBJECTIVES

• Understand the electrical conduction system of the heart.
• Identify P wave, QRS complex and T wave.
• Understand ECG paper, boxes and calibration.
• Identify all 12 ECG leads.
• Measure heart rate, rhythm and intervals.
• Determine cardiac axis.
• Recognize common arrhythmias.
• Recognize ischemia, injury and myocardial infarction patterns.
• Interpret an ECG systematically.

2. ELECTRICAL CONDUCTION SYSTEM OF THE HEART

The cardiac conduction system generates and transmits electrical impulses that coordinate atrial and ventricular contraction.

Sequence of conduction

SA Node → Atrial myocardium → AV Node → Bundle of His → Right & Left Bundle Branches → Purkinje Fibres → Ventricular myocardium

Structure	Main Function
SA node	Primary pacemaker; initiates atrial depolarization.
AV node	Delays conduction, allowing ventricular filling.
Bundle of His	Conducts impulse from AV node toward ventricles.
Bundle branches	Conduct impulse through interventricular septum.
Purkinje fibres	Rapidly distribute electrical impulse through ventricles.

3. ECG PAPER AND CALIBRATION

Standard ECG paper speed: 25 mm/second
Standard calibration: 10 mm = 1 mV

Small and large boxes

Measurement	Horizontal	Vertical
1 small box	0.04 sec	0.1 mV
1 large box = 5 small boxes	0.20 sec	0.5 mV
5 large boxes	1 second	—
30 large boxes	6 seconds	—

Memory:
1 small box = 0.04 sec
1 large box = 0.20 sec
5 large boxes = 1 sec

4. NORMAL ECG WAVEFORM

5. ECG WAVES

Wave	Represents	Normal characteristics
P wave	Atrial depolarization	Smooth, usually upright in lead II; ≤ 0.12 sec
QRS complex	Ventricular depolarization	Usually < 0.12 sec
T wave	Ventricular repolarization	Usually follows QRS; normally concordant with QRS in most leads
U wave	Small wave after T; mechanism incompletely understood	May be visible, especially in precordial leads

6. ECG INTERVALS AND SEGMENTS

Measurement	Normal / Important Range	Significance
PR interval	0.12–0.20 sec	Time from atrial depolarization to ventricular depolarization.
QRS duration	< 0.12 sec	Duration of ventricular depolarization.
QT interval	Rate dependent	Represents ventricular depolarization and repolarization.
ST segment	Normally near baseline	Important in assessment of myocardial ischemia/injury.

7. THE 12 ECG LEADS

A standard 12-lead ECG provides electrical views of the heart from different directions.

12 leads = 6 limb leads + 6 chest/precordial leads

Limb leads

Lead	Type	General view
I	Bipolar	Lateral
II	Bipolar	Inferior
III	Bipolar	Inferior
aVR	Augmented	Right-sided view
aVL	Augmented	Lateral
aVF	Augmented	Inferior

Precordial leads

Lead	Placement	General region
V1	4th intercostal space, right sternal border	Septal / right ventricular view
V2	4th intercostal space, left sternal border	Septal
V3	Between V2 and V4	Anterior
V4	5th intercostal space, mid-clavicular line	Anterior / apical
V5	Same horizontal level as V4, anterior axillary line	Lateral
V6	Same horizontal level as V4, mid-axillary line	Lateral

8. CORONARY TERRITORY — ECG LEAD ASSOCIATION

Region	Common leads	Commonly associated artery
Inferior	II, III, aVF	Usually RCA; sometimes LCx
Septal	V1–V2	LAD septal branches
Anterior	V3–V4	LAD
Lateral	I, aVL, V5–V6	LCx or diagonal branches of LAD

Important: The artery responsible for an ECG pattern can vary because coronary anatomy and dominance differ between individuals.

9. HEART RATE CALCULATION

Method 1 — 300 Rule

Heart Rate ≈ 300 ÷ number of large boxes between consecutive R waves

Large boxes	Heart rate
1	300/min
2	150/min
3	100/min
4	75/min
5	60/min
6	50/min

Method 2 — 1500 Rule

Heart Rate = 1500 ÷ number of small boxes between R waves

Irregular rhythm — 6-second method

Count QRS complexes in 6 seconds × 10 = approximate heart rate/min

10. NORMAL HEART RATE

Adult resting sinus rate: approximately 60–100 beats/minute.

Rate	Term
< 60/min	Bradycardia
60–100/min	Normal adult resting range
> 100/min	Tachycardia

11. SYSTEMATIC ECG INTERPRETATION

RATE → RHYTHM → P WAVE → PR INTERVAL → QRS → AXIS → ST SEGMENT → T WAVE → QT → OVERALL INTERPRETATION

Step 1 — Rate

Determine whether the ventricular rate is slow, normal or fast.

Step 2 — Rhythm

Ask:

• Are R–R intervals regular?
• Are P waves present?
• Is there one P wave before each QRS?
• Is the PR interval consistent?

Step 3 — P wave

• Present or absent?
• One P for every QRS?
• Similar morphology?
• Normal duration?

Step 4 — PR interval

Normal: 0.12–0.20 seconds.

Step 5 — QRS

Normal QRS duration is generally 0.12 seconds. A widened QRS suggests abnormal ventricular conduction or a ventricular origin, although there are several causes of QRS widening.

Step 6 — Axis

Assess the overall direction of ventricular depolarization using the limb leads.

Step 7 — ST segment

Look for significant ST elevation or depression in anatomically related leads.

Step 8 — T wave

Assess polarity, symmetry, amplitude and morphology.

Step 9 — QT / QTc

Consider heart-rate correction when assessing QT duration.

12. CARDIAC AXIS — QUICK REVISION

The frontal-plane QRS axis represents the overall direction of ventricular depolarization.

Axis	Approximate range
Normal	−30° to +90°
Left axis deviation	Less than −30°
Right axis deviation	Greater than +90°
Extreme axis	Approximately −90° to ±180° depending on convention

Quick method: Lead I and aVF

Lead I	aVF	Interpretation
Positive	Positive	Normal quadrant
Positive	Negative	Consider leftward axis; inspect further
Negative	Positive	Rightward axis
Negative	Negative	Extreme axis

13. SINUS RHYTHM

A typical sinus rhythm has:

• Regular rhythm.
• Rate generally 60–100/min in adults at rest.
• Normal P waves.
• A P wave before each QRS.
• Consistent PR interval.
• Narrow QRS in most cases.

14. COMMON RHYTHM ABNORMALITIES

Sinus Bradycardia

• Sinus rhythm with rate below 60/min in an adult.
• P waves remain sinus in morphology.
• PR and QRS are generally normal.

Sinus Tachycardia

• Sinus rhythm with rate above 100/min.
• P wave precedes each QRS.
• Often occurs as a physiological response to exercise, fever, pain, hypovolemia, anxiety or other conditions.

Atrial Fibrillation

Typical ECG features:

• Irregularly irregular ventricular rhythm.
• No consistent discrete P waves.
• Fibrillatory baseline may be present.
• Ventricular rate may be fast, normal or slow.

Atrial Flutter

• Classically saw-tooth flutter waves.
• Often best seen in II, III and aVF.
• Atrial activity is usually rapid.
• AV conduction may be variable.

Supraventricular Tachycardia

• Usually rapid regular narrow-complex tachycardia.
• P waves may be hidden within QRS or T waves.
• Mechanism can involve re-entry above the ventricles.

Ventricular Tachycardia

• Usually a broad-complex tachycardia.
• Often regular and rapid.
• P waves may be absent, dissociated or difficult to identify.
• May cause hemodynamic instability.

Clinical emergency: A patient with a pulse and a patient without a pulse require different management pathways.

Ventricular Fibrillation

Chaotic ventricular electrical activity with no effective organized ventricular contraction. VF is a cardiac arrest rhythm.

15. HEART BLOCKS

First-Degree AV Block

ECG: PR interval > 0.20 sec and usually constant, with every P wave conducted to a QRS.

Second-Degree AV Block — Mobitz I

• Progressive PR prolongation.
• Then a QRS complex is dropped.
• Cycle repeats.

Second-Degree AV Block — Mobitz II

• PR interval remains relatively constant in conducted beats.
• Unexpected dropped QRS complexes occur.
• Can progress to higher-grade block.

Third-Degree AV Block

• Atrial and ventricular activity are independent.
• No consistent relationship between P waves and QRS complexes.
• Ventricular escape rhythm maintains ventricular activity.

16. BUNDLE BRANCH BLOCK — BASIC CONCEPT

A bundle branch block occurs when conduction through one of the ventricular bundle branches is delayed or interrupted.

Feature	Right BBB	Left BBB
QRS	Usually ≥ 0.12 sec when complete	Usually ≥ 0.12 sec when complete
V1	Often shows an RSR′ pattern	Often predominantly negative
Lateral leads	May show broad S waves	May show broad/notched R waves

17. ST SEGMENT AND T WAVE

ST Elevation

ST elevation can occur in acute myocardial injury but can also occur in other conditions, including benign early repolarization and pericarditis. Clinical context and appropriate diagnostic criteria are essential.

ST Depression

ST depression may occur with myocardial ischemia, reciprocal changes, ventricular hypertrophy, medication effects and other conditions.

T-wave inversion

T-wave inversion can occur with ischemia and several non-ischemic conditions. Its significance depends on the lead distribution and clinical context.

18. MYOCARDIAL INFARCTION — ECG REVISION

ECG Region	Leads
Inferior	II, III, aVF
Anterior	V3, V4
Septal	V1, V2
Lateral	I, aVL, V5, V6

ECG findings must be interpreted with symptoms, serial ECGs, cardiac biomarkers and the overall clinical picture. Do not diagnose acute MI from a single ECG feature alone.

19. RECIPROCAL CHANGES

Reciprocal ST-segment changes are abnormalities appearing in leads that view the opposite side of an affected myocardial region.

Primary region	Possible reciprocal leads
Inferior	I and aVL
Lateral	Inferior leads may show reciprocal changes

20. ECG ARTIFACTS

Artifact	Possible cause	Correction
Baseline wandering	Movement, respiration, poor electrode contact	Improve electrode contact and minimize movement.
Muscle artifact	Patient shivering or muscle tension	Relax muscles; keep patient warm.
Electrical interference	Nearby electrical equipment	Check cables, equipment and grounding.
Loose electrode	Poor skin preparation or adhesive failure	Replace/reposition electrode.

21. 12-LEAD ECG ELECTRODE PLACEMENT — QUICK GUIDE

22. QUICK NORMAL VALUES

Parameter	Typical normal value
Heart rate	60–100/min at rest in adults
P-wave duration	≤ 0.12 sec
PR interval	0.12–0.20 sec
QRS duration	< 0.12 sec
QT	Rate dependent; assess QTc
Standard speed	25 mm/sec
Standard gain	10 mm/mV

23. ECG INTERPRETATION CHECKLIST

1. Patient & ECG quality: Confirm patient, date/time and adequate tracing.
2. Rate: Calculate ventricular rate.
3. Rhythm: Regular or irregular?
4. P waves: Present? Normal? Relationship to QRS?
5. PR interval: Measure and assess.
6. QRS: Narrow or wide?
7. Axis: Normal, leftward or rightward?
8. R-wave progression: Assess across V1–V6.
9. Q waves: Look for pathologic patterns.
10. ST segment: Elevation or depression?
11. T waves: Inversion, hyperacute appearance or other abnormalities?
12. QT/QTc: Assess duration.
13. Compare: Look for a previous ECG when available.
14. Clinical correlation: Interpret ECG together with symptoms, examination and other investigations.

24. ONE-MINUTE ECG REVISION

P = Atrial Depolarization

QRS = Ventricular Depolarization

T = Ventricular Repolarization

PR = 0.12–0.20 sec

QRS = < 0.12 sec

25 mm/sec = Standard Paper Speed

10 mm/mV = Standard Calibration

II, III, aVF = Inferior

I, aVL, V5, V6 = Lateral

V1–V2 = Septal

V3–V4 = Anterior

25. EXAMINATION PEARLS

• PR prolonged + every QRS conducted → Think first-degree AV block.
• Progressive PR prolongation + dropped QRS → Think Mobitz I.
• Constant PR + suddenly dropped QRS → Think Mobitz II.
• AV dissociation → Think complete heart block.
• Irregularly irregular rhythm + absent consistent P waves → Think atrial fibrillation.
• Saw-tooth atrial activity → Think atrial flutter.
• Wide-complex tachycardia → Consider ventricular tachycardia, while remembering other causes of a wide complex.
• Chaotic ventricular activity with no effective pulse → Ventricular fibrillation.
• ST/T abnormalities → Always interpret with clinical context and lead distribution.

26. FINAL ECG INTERPRETATION TEMPLATE

Rate: ______ bpm

Rhythm: Regular / Irregular

P waves: Present / Absent / Abnormal

PR interval: ______ sec

QRS: ______ sec; Narrow / Wide

Axis: Normal / Left / Right / Extreme

R-wave progression: __________________

ST segment: __________________

T waves: __________________

QT/QTc: __________________

Overall interpretation: _________________________________________

27. GOLDEN RULE FOR ECG INTERPRETATION

Never interpret an ECG by looking at only one wave.

Use a systematic approach:

RATE → RHYTHM → P → PR → QRS → AXIS → R-WAVE PROGRESSION → ST → T → QT/QTc → CLINICAL CORRELATION

Always consider the patient's symptoms, vital signs, previous ECG and other clinical investigations.

ECG COMPLETE REVISION

Concepts • Waves • Intervals • Leads • Axis • Arrhythmias • Blocks • Ischemia • MI • Systematic Interpretation

For educational and revision purposes.