ECG - MEDIUM

• Cardiac cycle refers to the sequence of events that take place when the heart beats.

Cardiac Cycle

• The occurrence of a cardiac cycle is illustrated by a heart rate, indicated as beats per minute.
• A healthy human heart beats 72 times per minute and completes 72 cardiac cycles per minute.
• The cardiac cycle comprises the diastole, the systole, and the intervening pause.
• It includes a complete contraction and relaxation of both the atria and ventricles, lasting about 0.8 seconds.
• Systole involves the period of contraction when the heart pumps blood into circulation.
• Diastole involves the period of relaxation as the heart chambers fill with blood.
• The human heart consists of four chambers.
• Two upper chambers are the left and right atria, while the lower two are the right and left ventricles.
• Atria are structures where blood comes to the heart, and ventricles are where blood leaves.

Cardiac Cycle Phases

• Atrial Diastole: Chambers of the heart are calm. The aortic valve and pulmonary artery close while atrioventricular valves open, allowing the chambers to relax.
• Atrial Systole: Blood cells flow from the atrium to the ventricle as the atrium contracts.
• As the atria contract, pressure increases, forcing more blood flow across open atrioventricular (AV) valves, leading to a rapid flow of blood into the ventricles.
• Isovolumic Contraction: Ventricles begin to contract, but ventricular volume does not change because all valves are closed.
• Ventricular Ejection: Ventricles contract and empty, causing the pulmonary artery and aortic valve close.
• Isovolumic Relaxation: No blood enters the ventricles, causing pressure to decrease, and ventricles stop contracting and relax. Due to the pressure in the aorta, the pulmonary artery and aortic valve close.
• Ventricular Filling Stage: Ventricles relax, and the pressure difference between atria and ventricles causes blood to flow back into the ventricles through open A-V valves.

Electrocardiography (ECG or EKG)

• Electrical currents are detected during a routine physical via electrodes attached to the skin, typically at the wrist, ankles, and six separate locations on the chest.

• Electrocardiogram is a graph of voltage versus time representing electrical activity of the heart.

• Electrodes (small, plastic patches that stick to the skin) are put on spots on the chest, arms, and legs, connected to an ECG machine by lead wires.

• Comparisons of readings from the different electrodes provide an accurate and comprehensive assessment of the heart's electrical activity.

• An ECG provides a composite tracing of all muscle impulses formed by myocardial cells.

• It records changes in voltage (potential) across the sarcolemma that:

  • Originates in the SA node.
  • Radiates through both atria to the AV node.
  • Passes through the AV node and interventricular septum to the heart apex.
  • Stimulates Purkinje fibers in ventricular myocardium.

• An typical ECG tracing features 3 principal deflections for one heart cycle.

  • A P wave above the baseline
  • A QRS complex: it begins (Q) and ends (S) with small downward deflection from the baseline and a large deflection (R) above the baseline
  • A T wave above the baseline

Electrocardiogram Waves

• These waves are indicators of depolarization and repolarization in specific heart regions.
  • I. P wave: Generated when impulse starts in the SA node and depolarizes the cells of atria.
  • II. QRS complex: Identifies start of ventricles' depolarization; Atria repolarize at the same time
  • III. T wave: small, rounded peak denoting ventricular repolarization.
• P-wave shows depolarization of atria that begins contracting about 25 msec after the start of the P-wave
• QRS-complex is ventricular depolarization where ventricles start contracting shortly after the peak of the R wave
• T-wave is ventricular repolarization

ECG Leads and Electrical Potentials

• Electrode: A conductive pad attached to the skin to record electrical currents.
• ECG lead: A graphical description of the electrical activity of the heart, analyzed with multiple electrodes.
• Standard ECG: Also known as a 12-lead ECG since it includes 12 leads - it's obtained using 10 electrodes.
• A combination of electrodes for recording is called a LEAD.
• Electrical potential differences arise as the electrical impulse travels through the heart.
• Electric potential difference is defined as a difference in electric potential between two measurement points.
• This means that the electrical potential difference is the difference in the electrical potential detected by two (or more) electrodes.
• Depolarization and repolarization generate electrical currents.
• Tissues and fluids surrounding the heart act as electrical conductors.
• Electrodes placed on the skin detect these electrical currents.
• The electrocardiograph (ECG machine) compares, amplifies, and filters electrical potential differences recorded by the electrodes.
• In a conventional 12-lead ECG, ten electrodes are placed on the limbs and chest surface; The heart's electrical potential is measured from twelve angles ("leads").
• Every lead represents differences in electrical potentials measured in two points in space. The simplest leads are composed using only two electrodes.
• The electrocardiograph defines one electrode as an exploring (positive) and the other as a reference (negative) electrode.

Anatomical Planes and Lead Placements

• Electrical activity of the heart is observed from the horizontal and frontal planes.
• Detection of vectors in a certain plane depends on the angle of the lead.
  • This angle depends on placement of the exploring lead and the reference point.
• A lead with leads on the head and the left foot would be angled in the frontal plane and detect vertical vectors.
• A lead with leads on the sternum and on the back. This lead will be angled from the back to the anterior chest wall, which is the horizontal plane.
• Leads are broken down into three types: limb, augmented limb, and precordial or chest.
• The 12-lead ECG has:
  • Three limb leads
  • Three augmented limb leads arranged like spokes of a wheel in the coronal plane (vertical).
  • Six precordial or chest leads on the perpendicular transverse plane (horizontal).
• Principles of the Limb Leads.Leads I, II, III, aVF, aVL and aVR uses three electrodes placed on the right arm, the left arm and the left leg in order to detect electrical activity in the frontal plane.
• Leads I, II and III show compare electrical potential differences between two leads, for example, - Lead I : compares electrode of the left are versus that of right arm
  • Lead II: Left leg compares to that of right arm
  • Lead III: The left leg Electrode compares to that of the left arm

Wilhelm Einthoven, Kirchhoff's Law, Einthoven's Law, and Lead Voltage

• Leads I, II and III are original leads constructed by Wilhelm Einthoven. Spatial organization of these leads forms a triangle in the chest.
• According to Kirchhoff's law, the sum of all currents in a closed circuit must be zero. Since Einthoven's triangle can be viewed as a circuit applies to it. Thus emerges Einthoven's law :
  • Lead I + Lead III = Lead II
• Voltage is calculated as:
  • I = LA - RA
  • II = LL - RA
  • III = LL - LA

Augmented limb leads (aVR, aVL, and aVF)

• Derived from same three electrodes as leads I, II, and III using Goldberger's central terminal as their negative pole.
• Letter 'a' stands for augmented, V for voltage, R for right arm, L for left arm, and F for foot.
  • In aVR, the right arm is the exploring electrode, with the reference composed by averaging the left arm and left leg.
  • In lead aVL, the left arm electrode is exploring, viewing the heart ≈ −30°.
  • In lead aVF, the exploring electrode is the left leg.
  • Since Goldberger's leads are composed of the same electrodes that Einthoven's display Goldberger's mathematical relationship:
    • aVL: (Lead I – Lead III)/2
    • (-aVR): (Lead I + Lead II)/2
    • aVF: (Lead II + Lead III)/2
• The electrode on the left leg serves as the exploring electrode while the reference is composed of the average of the arm electrodes. The angle by which aVF views the heart’s electrical activity is 90°.
• Clinically, the aVF viewed as "views the inferior wall of the left ventricle". The principals apply to lead aVR/ aVL:
• II, aVF and III are known as interior or diaphragmal limb leads that are located on the left ventricle.
• aVL., I, and aVR are lateral limb leads located on the lateral side of the ventricles.

Hexaxial references

• In conjunction with leads I, II, and III, leads aVR, aVL, and aVF complete the hexaxial reference system. The system helps calculate the heart's electrical axis in the frontal plane.

Precordial Leads and the Background Grid

• Precordial leads lie in the transverse (horizontal) plane, which is perpendicular to the other six leads.
• The six precordial electrodes act as the positive poles for the six corresponding precordial leads: (V1, V2, V3, V4, V5, and V6). Wilson's central terminal is used as the negative pole.
• Precordial and Unipolar leads can be used to generate bipolar precordial leads that explore the horizontal plane (left to right axis).
• In the Cabrera system, the leads are placed in anatomical order, i.e.,
  • V1-V2 ("septal leads"): primarily observe the ventricular septum but may display ECG originating from the right ventricle.
  • V3-V4 ("anterior leads"): observe the anterior wall of the left ventricle.
  • V5-V6 ("anterolateral leads"): observe the lateral wall of the left ventricle.
• Note that none of the leads in the 12-lead ECG can precisely detect vectors in the ventricals
• ECGs are normally printed on a grid, with the horizontal axis representing time and the vertical axis representing voltage.

Background Grid

• ECGs are normally printed on a grid. The horizontal axis represents time, and the vertical axis represents voltage. The standard values on this grid are shown in the adjacent image at 25mm/sec:
  • A small box is 1 mm × 1 mm and represents 0.1 mV × 0.04 seconds.
  • A large box is 5 mm x 5 mm and represents 0.5 mV × 0.20 seconds.

Application

• Why ECG Needed:
  • Chest pain
  • Severe tiredness (fatigue), shortness of breath, dizziness, or fainting
  • Irregular heartbeats
  • Overall health of the heart before procedures, such as surgery;
  • After treatment for a heart attack (myocardial infarction)
• How implanted pacemaker is working
• How certain heart medicines are working
• Baseline tracing of the heart's function during a physical exam, which can be compared with future ECGs