ECG Basics and Components

Study Notes

Electrocardiogram (ECG) Basics

An electrocardiogram (ECG) is a graphical representation of the heart's electrical activity over time.
It records the heart's electrical impulses as they travel through the heart, providing information about the heart's rhythm, rate, and structure.
The ECG is a non-invasive, painless procedure that involves placing electrodes on the skin of the chest, arms, and legs.
Each wave and segment on the ECG corresponds to specific events in the heart's electrical cycle.

Components of an ECG

P wave: Represents atrial depolarization (contraction of the atria).
PR interval: Represents the time from the start of atrial depolarization to the start of ventricular depolarization. This interval reflects the time it takes for the electrical impulse to travel through the atria and AV node.
QRS complex: Represents ventricular depolarization (contraction of the ventricles). It is a complex of three deflections: Q, R, and S.
ST segment: Represents the time between ventricular depolarization and repolarization; a flat line segment. The ST segment should remain isoelectric (at zero).
T wave: Represents ventricular repolarization (recovery of the ventricles).
QT interval: Represents the time it takes for the ventricles to depolarize and repolarize.
U wave: A small, often subtle wave following the T wave that is not fully understood but might be related to repolarization of the Purkinje fibers. Its clinical significance is debated.
P-R interval prolongation Indicates a delay in transmission of the stimulus from the atrium to the ventricles. Possible causes include AV nodal block.

ECG Interpretation

Interpretation involves analyzing the shape, size, and timing of the waves and intervals to detect abnormalities.
Heart rate: Calculated from the R-R intervals.
Heart rhythm: Regular or irregular. Identifying patterns like sinus arrhythmia or atrial fibrillation indicates arrhythmias.
Axis deviation: Determines the electrical direction of the heart's impulses. Can be normal or abnormal.
Chamber enlargement: Enlarged chambers (atria or ventricles) show changes in the size of the QRS complex.
Ischemia or infarction: Damage to specific parts of the heart results in changes to the ECG waves.
Hypertrophy: Thickening of the heart muscle results in increased QRS amplitude.
Conduction abnormalities: Problems with the electrical pathways in the heart result in altered PR intervals and QRS complexes.
ECG provides crucial information, but further tests are needed for a definitive diagnosis.

Normal ECG Characteristics

Normal sinus rhythm: Originates in the sinoatrial (SA) node with a consistent and regular rhythm. P waves precede each QRS complex, with a consistent PR interval.
Heart rate criteria are age-dependent and calculated from the rhythm.
Rate: Normally 60-100 bpm.
Rhythm: Regular.
Axis: Normal, typically between -30° and +90°.
P waves: Positive in leads I, II, and aVF.
PR interval: 0.12-0.20 seconds.
QRS duration: < 0.12 seconds.

Indications for an ECG

Patients experiencing chest pain, shortness of breath, palpitations, dizziness, or syncope.
To evaluate suspected cardiac conditions like myocardial infarction (MI), heart failure, or arrhythmias.
To monitor patients with known heart conditions (previous heart attack, risk factors).
Pre-operative evaluation for patients undergoing procedures with cardiac risks.
To diagnose or monitor the effects of electrolyte abnormalities or medication side effects on the heart.

Limitations of ECG

Doesn't directly visualize the heart's structure.
Doesn't always detect all cardiac abnormalities.
Cannot detect some arrhythmias or subtle conduction disturbances at specific times.
Interpretation requires experience and knowledge.

Equipment and procedure for ECG recording

Electrodes: Placed on specific anatomical sites for accurate measurement.
Electrocardiograph: Machine that amplifies and records the heart's electrical signals.
Gel: Ensures conductivity between electrodes and skin.
Leads: Connect electrodes to the electrocardiograph, producing a readable signal.
Paper or display: Shows the electrical signal pattern and waves (now typically a screen).

Description

This quiz explores the basics of electrocardiograms (ECG) and their key components, including the P wave, PR interval, QRS complex, and ST segment. Understand how ECGs measure the heart's electrical activity and the significance of each segment. Perfect for students of cardiology or anyone interested in heart health.