Electrocardiography Basics

Electrocardiography (ECG)

• Records the electrical activity of the heart.
• Normal ECG consists of three main components: P wave, QRS complex, and T wave.
• P wave represents atrial depolarization preceding atrial contraction.
• QRS complex indicates ventricular depolarization preceding ventricular contraction.
• Both P wave and QRS complex are classified as depolarization waves.
• T wave (repolarization wave) reflects the recovery phase of the ventricles after depolarization.

Cellular Electrophysiology

• Cardiomyocytes, or cardiac cells, are electrically polarized during the resting state.
• Depolarization, also known as activation, is a primary electrical event in the heart where cells lose internal negativity.
• Phase 0 of action potential occurs when sodium (Na+) ions enter the cell, making it more positive and propagating action potential to neighboring cardiomyocytes.
• Repolarization occurs as potassium (K+) ions move out of the cell, restoring the cell's negative charge.
• Overall, during depolarization, the cell experiences a shift toward a more positive interior.

Conduction System of the Heart

• Impulse generation begins at the SA node, also known as the sinus node.
• Impulse travels to the atria through intraatrial tracts, causing a brief atrial contraction.
• After the atria, the impulse moves through internodal pathways to the AV node.
• From the AV node, the signal travels down the Bundle of His and branches into the right and left bundle branches in the ventricles.
• Conduction continues to the Purkinje fibers, stimulating contraction of the ventricles.

ECG Leads

• ECG leads measure the magnitude and direction of the heart's electrical impulses.
• Three types of leads: Limb leads, Augmented limb leads, and Precordial leads.

Limb Leads

• Bipolar leads consist of 2 electrodes at different sites, measuring the voltage difference between them.
• Unipolar leads measure the electrical potential at one site compared to a reference electrode at zero potential.
• Color coding for limb electrodes:
  • Right arm: Red
  • Left arm: Yellow
  • Left foot: Blue/Green
  • Right foot: Black

Augmented Unipolar Limb Leads

• Lead aVR: Records potential between the right arm and the average potential of the left arm and left foot.
• Lead aVL: Measures potential in the left arm relative to the average potential of the right arm and left leg.
• Lead aVF: Assesses potential in the left foot with reference to the average of the left arm and right arm.
• Augmentation of leads increases potential recording by 50% for improved clarity in ECG interpretation.

Precordial Lead Placements

• Precordial leads, commonly used in adults, include V1 through V6.
• V1 is located in the fourth intercostal space at the right parasternal area.
• V2 is positioned in the fourth intercostal space at the left parasternal area.
• V3 is placed between V2 and V4, completing the anterior view.
• V4 is found in the inframammary area, aligning with the heart's anterior aspect.
• V5 is situated in the midclavicular area at the same level as V4.
• V6 is positioned in the anterior axillary line, aligned with V4 and V5.
• These lead placements focus on the anterior portion of the heart.
• V1 and V2 specifically monitor the septal area of the left ventricle.
• V3 and V4 assess the anterior area of the heart.
• V5 and V6 are associated with the lateral area of the heart.

ECG Paper

• ECG paper consists of horizontal and vertical lines.
• Horizontal lines indicate duration in seconds or milliseconds.
• Vertical lines measure amplitude in millivolts.
• Standard speed for ECG tracing is 25 mm/sec.
• Standard amplitude is set at 10 mm/mV.
• The smallest square on ECG paper represents 1 mV (amplitude) and 0.04 seconds (duration).
• Five small squares equal 0.2 seconds, while 25 small squares equal 1 second.
• One millivolt corresponds to approximately 10 small squares vertically.

Waves, Segments, and Intervals

• The PQRST complex represents one complete cardiac cycle.
• The P wave indicates atrial depolarization, occurring before contraction.
• The QRS wave indicates ventricular depolarization just prior to ventricular contraction.
• The Q wave is the first negative deflection after the P wave.
• The R wave is the first positive reflection following the Q wave.
• The S wave is the first negative reflection occurring after the R wave.
• The T wave represents the process of ventricular repolarization.
• The P wave should last no longer than 0.12 seconds and reach a maximum amplitude of 0.25 mV (2.5 small squares).
• The PR interval measures the duration from the start of atrial depolarization to the onset of ventricular depolarization (0.12 – 0.20 seconds).
• The QRS interval measures the duration of ventricular depolarization (0.08 – 0.10 seconds).
• The ST segment reflects the time from the end of ventricular depolarization to the beginning of ventricular repolarization.
• The T wave records the process of ventricular repolarization.
• The vector direction for ventricular depolarization (and septal depolarization) is from left to right.
• Ventricular depolarization predominantly occurs in the left ventricle, with a leftward and inferior direction.

Cardiac Cycle Components

• P wave: Indicates atrial depolarization, which occurs before the atrium fills with blood.
• QRS complex: Represents ventricular depolarization and occurs before the contraction of the ventricles.
• T wave: Reflects ventricular repolarization, marking the end of the cardiac cycle.

ST Segment

• Measures time from the end of ventricular depolarization to the beginning of ventricular repolarization.
• Crucial for diagnosing heart attacks or myocardial infarction.

Phases of the Cardiac Cycle

• Systole: Starts at the QRS complex and lasts until the later half of the P wave, representing ventricular contraction.
• Diastole: Extends from the T wave up to the start of the QRS complex, indicating the relaxation phase of the heart.

This quiz covers the fundamental concepts of electrocardiography, including the recording of electrical activity in the heart. Key components such as the P wave, QRS complex, and T wave will be explored, along with their roles in depolarization and repolarization. Test your knowledge on how these waves contribute to heart function.