I'm attending a workshop on ecg in mysore medical College and they have asked us to atleast know the basics . Give me all the information about ecg from standard physiology books

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ECG — Basics from Standard Physiology Textbooks

Sources: Guyton & Hall Textbook of Medical Physiology, Ganong's Review of Medical Physiology (26th ed.), Costanzo Physiology (7th ed.)

1. What is an ECG?

The electrocardiogram (ECG or EKG) is a recording of the tiny electrical potential differences on the surface of the body that reflect the electrical activity of the heart. Body fluids are good conductors (the body acts as a "volume conductor"), so electrical currents generated by cardiac depolarization and repolarization spread all the way to the skin surface, where they can be detected by surface electrodes.

"When a cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body." — Guyton & Hall

2. Physiological Basis — Why Does the ECG Exist?

The ECG exists because the entire myocardium does not depolarize at once:

The atria depolarize before the ventricles
The ventricles depolarize in a specific sequence (endocardium → epicardium, apex → base)
The atria repolarize while the ventricles are still depolarizing
The ventricles repolarize in a sequence

This timing creates potential differences between different regions of the heart, which appear as waves on the ECG.

Key principle: When a depolarization wave moves toward an active electrode, it produces a positive (upward) deflection. When it moves away from the electrode, it produces a negative (downward) deflection.

3. The Cardiac Conduction System (What Generates the ECG)

Conducting system of the heart and corresponding ECG sequence — Ganong's Review of Medical Physiology

Ganong's Review of Medical Physiology — Conducting system of the heart (A) and sequence of cardiac excitation with the corresponding ECG below (B)

The impulse travels:

SA node (pacemaker, right atrium) → spreads across both atria → P wave
AV node (base of right atrium) → conduction slows here (built-in delay) → PR segment / isoelectric line
Bundle of His → right and left bundle branches → Purkinje fibers → ventricular muscle → QRS complex
Ventricular muscle repolarizes → T wave

4. Waveforms of the Normal ECG

ECG waveforms labeled with waves, segments and intervals — Ganong's Review of Medical Physiology

Ganong's Review of Medical Physiology — Standard names for waves, segments, and intervals of the ECG

Costanzo Physiology — The ECG measured from lead II

P Wave

Represents depolarization of the atria
Atrial repolarization is not seen on normal ECG — it is buried under the QRS complex
Duration reflects conduction time through the atria

PR Interval

Measured from the start of P wave to the start of QRS complex
Represents conduction time from atria → AV node → bundle of His → ventricles
Normal: 0.12–0.20 seconds (averages ~0.18 s at 70 bpm)
PR segment (isoelectric flat line between P and QRS) = AV node conduction time
Shortened by sympathetic stimulation; prolonged by parasympathetic stimulation

QRS Complex

Represents depolarization of the ventricles
Three components: Q (initial small downward deflection), R (tall upward), S (final downward)
Although ventricles are much larger than atria, QRS duration is similar to P wave duration because the His-Purkinje system conducts extremely fast
Normal QRS duration: up to 0.10 seconds
Atrial repolarization is buried in this complex

ST Segment

Isoelectric portion between end of QRS and start of T wave
Corresponds to the plateau phase of the ventricular action potential (all ventricular cells are depolarized — no current flows between them)
Should be on the isoelectric line (elevation or depression is clinically significant)

T Wave

Represents repolarization of the ventricles
Normally occurs 0.25–0.35 seconds after depolarization (Guyton)
T wave is upright in most leads because repolarization proceeds in the opposite direction to depolarization (epicardium repolarizes before endocardium)

QT Interval

From start of QRS to end of T wave
Represents entire ventricular action potential (depolarization + repolarization)
Normal: up to 0.43 seconds (corrected for heart rate)
Increases at slower heart rates; decreases at faster heart rates

U Wave

Small wave after the T wave; inconstant finding
May be due to ventricular myocytes with long action potentials (Ganong)

5. ECG Intervals — Summary Table

(From Ganong's Review of Medical Physiology, Table 29-2)

Interval	Average	Normal Range	Cardiac Event
PR interval	0.18 s	0.12–0.20 s	AV conduction (atria → ventricles)
QRS duration	0.08 s	up to 0.10 s	Ventricular depolarization
QT interval	0.40 s	up to 0.43 s	Ventricular action potential
ST segment	0.32 s	—	Plateau of ventricular AP

6. ECG Leads — The Recording System

An ECG lead is simply a pair of electrodes (or an electrode + reference point) that looks at the heart from a particular angle. The standard 12-lead ECG uses three sets of leads.

A. Standard Bipolar Limb Leads (Einthoven's Leads I, II, III)

These record the potential difference between two limbs:

Lead	Positive Electrode	Negative Electrode
Lead I	Left arm	Right arm
Lead II	Left leg	Right arm
Lead III	Left leg	Left arm

Einthoven's triangle: The heart sits in the center of an equilateral triangle formed by the three limb electrodes. The sum of voltages at the three points equals zero at all times. Einthoven's law: Lead II = Lead I + Lead III.

Einthoven's law: Upward deflections are recorded when the positive electrode becomes positive relative to the negative.

B. Augmented Unipolar Limb Leads (aVR, aVL, aVF)

In augmented leads, two limbs are connected to the negative terminal and the third limb is the positive (exploring) electrode:

Lead	Positive Electrode	"Looks At"
aVR	Right arm	Right shoulder / base of heart
aVL	Left arm	Left shoulder
aVF	Left foot	Inferior heart (diaphragmatic surface)

These increase amplitude by 50% compared to non-augmented unipolar leads. aVR is inverted relative to the others because it "looks away" from the main direction of depolarization.

Augmented limb lead recordings — Guyton & Hall

Guyton & Hall — Normal ECGs from the three augmented unipolar limb leads. Note aVR is inverted.

C. Precordial (Chest) Leads (V1–V6)

Six electrodes placed on the chest wall, each "looking" at the heart from in front:

Chest lead electrode positions on the thorax — Ganong's Review

Ganong's Review — Placement of precordial leads V1–V6 and augmented limb leads

Lead	Position
V1	4th intercostal space, right sternal border
V2	4th intercostal space, left sternal border
V3	Between V2 and V4
V4	5th intercostal space, mid-clavicular line
V5	Anterior axillary line, same level as V4
V6	Mid-axillary line, same level as V4–V5

Key patterns in precordial leads:

V1–V2: QRS mainly negative (electrode is near the base — electronegativity end during depolarization)
V4–V6: QRS mainly positive (electrode is near the apex — electropositivity end during depolarization)

Normal ECGs from the six precordial leads — Guyton & Hall

Guyton & Hall — Normal ECGs from leads V1 through V6. Note the transition from predominantly negative in V1 to predominantly positive in V4–V6.

7. ECG and the Cardiac Cycle — Timing Relationships

ECG Event	Mechanical Event
P wave	Atrial depolarization → atrial contraction follows
PR segment	AV nodal delay
QRS complex	Ventricular depolarization → ventricular systole begins
ST segment	All ventricles depolarized, plateau phase; no contraction difference
T wave	Ventricular repolarization → ventricular relaxation begins

"The QRS complex begins slightly before the onset of ventricular systole... The T wave occurs slightly before the end of ventricular contraction." — Guyton & Hall

8. Heart Rate from the ECG

Count the number of QRS complexes (R waves) per minute
At standard paper speed of 25 mm/s:
- Each large square = 0.2 s (5 mm)
- Each small square = 0.04 s (1 mm)
Quick method: Divide 300 by the number of large squares between two consecutive R waves
- e.g., 4 large squares between R–R → 300 ÷ 4 = 75 bpm

9. Electrical Axis of the Heart

The direction of the mean electrical vector of ventricular depolarization is the electrical axis. Normally it points downward and to the left (0° to +90°, with +60° as the average), because the left ventricle dominates and the apex points down-left.

Normal axis: 0° to +90°
Left axis deviation: 0° to −90° (left ventricular hypertrophy, left bundle branch block)
Right axis deviation: +90° to +180° (right ventricular hypertrophy, right bundle branch block)

10. Depolarization vs. Repolarization Waves — Why the T Wave is Upright

This is a classic exam point: the T wave is caused by repolarization, so why is it upright (same direction as the R wave)?

In depolarization, the wave travels endocardium → epicardium (inside → outside)
In repolarization, the wave travels epicardium → endocardium (outside → inside — reversed order)
Because both direction and sign are reversed, the net deflection is the same direction as the QRS
This produces an upright T wave that correlates with the QRS polarity

11. Clinical Significance of Key ECG Changes (Brief Overview)

Finding	Possible Cause
Prolonged PR interval (>0.20 s)	First-degree AV block
Wide QRS (>0.12 s)	Bundle branch block, ventricular ectopy
ST elevation	Myocardial infarction (STEMI), pericarditis
ST depression	Myocardial ischemia, posterior MI
Prolonged QT	Hypocalcemia, drugs, risk of torsades de pointes
Tall peaked T waves	Hyperkalemia
Inverted T waves	Ischemia, ventricular hypertrophy
Absent P waves + irregular rhythm	Atrial fibrillation

12. Quick Revision Summary

Feature	Description
P wave	Atrial depolarization
PR interval	AV conduction time (0.12–0.20 s)
QRS complex	Ventricular depolarization (<0.10 s)
ST segment	Ventricular plateau (isoelectric)
T wave	Ventricular repolarization (upright)
QT interval	Total ventricular electrical activity (<0.43 s)
12 leads	I, II, III + aVR, aVL, aVF + V1–V6
Normal axis	0° to +90°
Paper speed	25 mm/s; large square = 0.2 s
Heart rate	300 ÷ R-R in large squares

All content compiled from: Guyton and Hall Textbook of Medical Physiology (Chapters 11–13), Ganong's Review of Medical Physiology, 26th edition (Chapter 29), and Costanzo Physiology, 7th edition (Chapter 4).

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