2.6 Electrocardiogram

An electrocardiogram (ECG) is a recording of the electrical impulses that pass through the conduction system of the heart during the cardiac cycle. It is a valuable diagnostic tool used to detect heart abnormalities.

Definition

Electrocardiogram (ECG): An electrocardiogram is a recording of the electrical changes that occur in the heart during the cardiac cycle. These electrical changes result from depolarization and repolarization of cardiac muscle fibers and can be detected on the surface of the skin using an instrument called the electrocardiograph.

Depolarization: Depolarization is the electrical change that occurs when the inside of the cardiac muscle cell becomes more positive relative to the outside. It triggers contraction.

Repolarization: Repolarization is the electrical change that occurs when the cardiac muscle cell returns to its resting state (inside negative relative to outside).

Main Idea

• P wave — depolarization of the atria
• QRS complex — depolarization of the ventricles
• T wave — repolarization of the ventricles

The ECG is used to diagnose arrhythmias, conduction defects, heart damage, and other cardiac abnormalities.

Core Concept

• **ECG** = electrocardiogram
• Electrocardiograph = instrument used to record ECG
• P wave = atrial depolarization
• QRS complex = ventricular depolarization
• T wave = ventricular repolarization
• P-R interval = time from start of P wave to beginning of QRS complex
• S-T segment = period between ventricular depolarization and repolarization
• Uses = detect arrhythmias, conduction defects, heart damage, etc.

Detailed Explanation

Electrical Activity of the Heart

The electrical impulses that pass through the conduction system of the heart during the cardiac cycle can be recorded as an electrocardiogram (ECG).

• Electrical changes result from depolarization and repolarization of cardiac muscle fibers.
• These changes can be detected on the surface of the skin.
• An instrument called the electrocardiograph is used to record the ECG.

Principal Aspects of an ECG

The wave deflections, designated P, QRS, and T, are produced as specific events of the cardiac cycle occur.

• Depolarization of the atrial fibers of the SA node produces the P wave.
• The ventricles of the heart are in diastole during the expression of the P wave.
• The P wave represents excitation and occurs just prior to contraction of the atria.

• The P-R interval is the period of time from the start of the P wave to the beginning of the QRS complex.
• This interval indicates the amount of time required for the SA depolarization to reach the ventricles.
• Normal P-R interval: 0.12–0.20 sec.

• The QRS complex begins as a short downward deflection (Q).
• Continues as a sharp upward spike (R).
• Ends as a downward deflection (S).
• The QRS complex indicates the depolarization of the ventricles.
• During this interval, the ventricles are in systole and blood is being ejected from the heart.
• The QRS complex occurs just prior to ventricular contraction.
• Normal QRS duration: 0.08–0.10 sec.

S-T Segment: - The time duration known as the S-T segment represents the period between the completion of ventricular depolarization and initiation of repolarization.

• The T wave is produced by ventricular repolarization.
• The T wave occurs just before the ventricles relax.
• Normal QT interval: 0.4–0.43 sec.

J Point: - The J (junction) point is the junction between termination of the QRS complex and the beginning of the ST segment.

• The R-R interval is the time between two successive R waves.
• Normal R-R interval: 0.6–1.0 sec.

Normal ECG Values

Summary of ECG Waves

Uses of Electrocardiogram

An ECG is a painless test that measures heart electrical activity to show whether or not it is working normally.

Intermediate Level

ECG Waves and Intervals

ECG and Cardiac Cycle

Uses of ECG

Types of ECG

Formula / Mathematical Form

Heart Rate from ECG:

Heart Rate = 300Number of large boxes between R waves

Or:

Heart Rate = 1500Number of small boxes between R waves

Example: If there are 4 large boxes between R waves: Heart Rate = 3004 = 75 beats/min

Solved Example

Given: Number of large boxes between R waves = 4

Required: Heart rate

Formula: Heart Rate = 300Number of large boxes

Substitution: Heart Rate = 3004

Calculation: Heart Rate = 75 beats/min

Final Answer: The heart rate is 75 beats per minute.

Important Points

• 1. ECG records electrical activity of the heart.
• 2. Electrocardiograph is the instrument used.
• 3. P wave = atrial depolarization.
• 4. QRS complex = ventricular depolarization.
• 5. T wave = ventricular repolarization.
• 6. P-R interval = 0.12–0.20 sec.
• 7. QRS duration = 0.08–0.10 sec.
• 8. QT interval = 0.4–0.43 sec.
• 9. RR interval = 0.6–1.0 sec.
• 10. P wave occurs before atrial contraction.
• 11. QRS complex occurs before ventricular contraction.
• 12. T wave occurs before ventricular relaxation.
• 13. ECG is painless and non-invasive.
• 14. ECG detects arrhythmias, conduction defects, and heart damage.
• 15. ECG monitors drug effects and pacemaker function.

Remember

• - **ECG** = electrocardiogram
• - P wave = atrial depolarization
• - QRS complex = ventricular depolarization
• - T wave = ventricular repolarization
• - **P-R interval** = 0.12–0.20 sec
• - **QRS duration** = 0.08–0.10 sec
• - **QT interval** = 0.4–0.43 sec
• - **RR interval** = 0.6–1.0 sec
• - Uses = arrhythmias, conduction defects, heart damage
• - Painless = non-invasive test
• - Electrodes = placed on skin

Common Mistakes

• 1. Confusing P wave and T wave — P wave = atrial depolarization; T wave = ventricular repolarization.
• 2. Mixing up QRS complex and T wave — QRS = ventricular depolarization; T = ventricular repolarization.
• 3. Forgetting the normal values — P-R = 0.12–0.20 sec; QRS = 0.08–0.10 sec; QT = 0.4–0.43 sec.
• 4. Thinking the P wave represents ventricular activity — P wave represents atrial activity.
• 5. Confusing depolarization and repolarization — Depolarization = electrical activation; repolarization = recovery.
• 6. Believing the ECG is invasive — ECG is non-invasive.
• 7. Mixing up the intervals — P-R interval; QT interval; RR interval.
• 8. Thinking the T wave is atrial — T wave is ventricular.
• 9. Forgetting the J point — J point is the junction between QRS and ST segment.
• 10. Confusing the uses of ECG — ECG detects arrhythmias, conduction defects, heart damage, etc.

Quick Revision

An electrocardiogram (ECG) is a recording of the electrical impulses that pass through the conduction system of the heart during the cardiac cycle. The electrical changes result from depolarization and repolarization of cardiac muscle fibers and are detected on the skin using an electrocardiograph. The ECG shows characteristic waves: P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). The P-R interval is 0.12–0.20 sec; QRS duration is 0.08–0.10 sec; QT interval is 0.4–0.43 sec; RR interval is 0.6–1.0 sec. The ECG is used to detect arrhythmias, conduction defects, heart damage, and other cardiac abnormalities. It is a painless, non-invasive test.

60-Second Revision

• ECG = electrocardiogram
• P wave = atrial depolarization
• QRS complex = ventricular depolarization
• T wave = ventricular repolarization
• P-R interval = 0.12–0.20 sec
• QRS duration = 0.08–0.10 sec
• QT interval = 0.4–0.43 sec
• RR interval = 0.6–1.0 sec
• Uses = arrhythmias, conduction defects, heart damage
• Painless = non-invasive test
• Warning: P wave = atrial; QRS = ventricular; T wave = ventricular

NEXT TOPIC

• Chapter:
• 2. Blood Circulatory System of Man

• Topic:
• 2.7 BLOOD VESSELS

Source: