Cardiac Cycle and Heart Function - EASY

Cardiac Cycle

• Cardiac cycle refers to the sequence of events that occur when the heart beats.
• A healthy human heart beats 72 times per minute, resulting in 72 cardiac cycles per minute.
• The cardiac cycle comprises diastole, systole, and an intervening pause.
• Diastole is the period of time when the heart is in relaxation, and the chambers are filling with blood.
• Systole is the period of contraction that the heart undergoes while it pumps blood into circulation.
• The cardiac cycle involves the complete contraction and relaxation of both the atria and ventricles and takes approximately 0.8 seconds.
• The human heart consists of four chambers: left and right atria (upper chambers), and right and left ventricles (lower chambers).
• Atria are the structures where blood flows into the heart, while ventricles are where blood leaves the heart.

Cardiac Cycle Phases

• Atrial Diastole: Heart chambers relax; aortic valve and pulmonary artery close, atrioventricular valves open.
• Atrial Systole: Blood cells flow from atrium to ventricle; atria contract, increasing pressure and forcing blood through open atrioventricular valves.
• Isovolumic Contraction: Ventricles begin to contract, ventricular volume remains constant as all valves are closed.
• Ventricular Ejection: Ventricles contract and empty, pulmonary artery and aortic valve close.
• Isovolumic Relaxation: No blood enters the ventricles to relax, leading pressure to decrease; pressure in aorta and pulmonary artery causes aortic valve to close.
• Ventricular Filling: Ventricles relax, pressure difference between atria and ventricles causes blood to flow back through open A-V valves.

Electrocardiography (ECG or EKG)

• Electrical currents within the heart are detectable during a routine physical exam via monitoring electrodes attached to the skin at wrists, ankles, and six chest locations.
• Electrodes, which are small plastic patches, are placed on the chest, arms, and legs and connected to an electrocardiogram machine via lead wires.
• An electrocardiogram is a graph of voltage versus time that illustrates the heart's electrical activity and can provide an accurate, comprehensive assessment of the hearts electrical activity.
• The ECG records wave of changing voltage that originates in the SA node, radiates through both atria to the AV node, passes through the AV node and interventricular septum and stimulates Purkinje fibers in ventricular myocardium.

ECG Tracings

• It provides a composite tracing of all muscle impulses formed by myocardial cells.
• A typical ECG tracing for one heart cycle has three principal deflections (P wave, QRS complex, T wave).
• P Wave: is generated when impulse originates in the SA node depolarizes the cells of atria.
• The atria begin contracting.
• QRS Complex: identifies beginning of ventricles' depolarization and the end of atrial repolarization.
• The ventricles begin contracting.
• T Wave: A small rounded peak that denotes ventricular repolarization.

ECG Leads

• An electrode is a conductive pad attached to the skin for recording electrical currents.
• An ECG lead is a graphical display of the heart's electrical activity, analyzed using several electrodes.
• The standard ECG, referred to as a 12-lead ECG, uses 10 electrodes.
• Electrical potential differences are caused as electrical impulse travels through the heart.
• The electrical potential difference is identified as the difference in electric potential between two measurement points
• The electrocardiograph compares, amplifies, and filters electrical potential differences recorded by electrodes.

12-lead ECG

• In a conventional 12-lead ECG, ten electrodes are positioned on limbs and chest to measure the heart's electrical potential from twelve angles ("leads") over approximately ten seconds.
• Vector heading toward exploring electrode yields a positive wave/deflection and vice versa.

Anatomical Planes and ECG Leads

• Heart’s electrical activity is observable from horizontal and frontal planes.
• The ability of a lead to detect vectors in a certain plane is dependent on the placement of the exploring lead and the reference point.
• Frontal Plane example: A lead ( vertical from head to foot) detects vectors traveling primarily in the frontal plane.
• Horizontal plane example: A lead (from sternum to back) will record vectors traveling in the horizontal plane.
• Leads are broken down into three types: limb, augmented limb, and precordial or chest. The 12-lead ECG uses three limb leads and three augmented limb leads with six precordial/chest leads.

Limb Leads

• Leads I, II, III, aVF, aVL and aVR are derived using three electrodes on the right arm, left arm, and left leg; detect electrical activity in the frontal plane.
• These original leads were constructed by Wilhelm Einthoven and form a triangle in the chest.

The leads in the Einthoven triangle

• Lead I: compares the electrode on the left arm with the electrode on the right arm; observes the heart from the left.
• Lead II: compares the left leg with the right arm; observes the heart from an angle of 60°.
• Lead III: compares the left leg with the left arm; observes the heart from an angle of 120°.
• According to Kirchoff's law, the sum of all currents in a closed circuit must be zero.
• Einthoven's law - Lead I + Lead III = Lead II
• Lead I is the voltage between the LA (positive arm) electrode and RA (right arm)
• Lead II is the voltage between the LL (positive left leg) electrode and the right arm (RA) electrode.
• Lead III is the voltage between the LL (positive left leg) electrode and the left arm (LA) electrode.

Augmented Limb Leads

• Derived from the same three electrodes as leads I, II, and III, utilizing Goldberger's central terminal as their negative pole.
• In lead aVR the right arm is the exploring electrode, the reference is composed by averaging the left arm and left leg.
• In lead aVL, the left arm electrode is exploring, and the lead views the heart from -30°.
• In lead aVF, the exploring electrode is placed on the left leg, so this lead observes the heart directly from the south.
• Augmented limb leads display a mathematical relation, which are Goldberger's equations.
• a stands for augmented, V for voltage and R is right arm, L is left arm and F is foot.
• Lead aVF measures the hearts activity at 90 degrees
• The average of the arm electrode yields a reference directly north of the leg electrode.
• Electrodes aVF, II, and III: are called inferior (diaphragmal) limb leads and they primarily observe in the inferior aspect of the left ventricle.
• Electrodes aVL, I, and aVR: are called lateral limb leads and they primarily observe in the lateral aspect of the left ventricle.
• Augmented limb leads and leads I, II, and III make the hexaxial reference system.

Precordial Leads

• Transverse (horizontal) plane, perpendicular to other six leads; six precordial electrodes act as positive poles.
• The precordial leads act as the positive poles for the precordial leads V1, V2, V3, V4, V5 and V6
• Unipolar precordial leads may be used to create bipolar precordial leads that explore right-to-left axis in the horizontal plane.
• In the Cabrera system, the leads are placed in their anatomical order.
• V1-V2 (septal leads) primarily observes the ventricular septum, but may occasionally display ECG changes originating from the right ventricle.
• V3-V4 (anterior leads) primarily observes the anterior wall of the left ventricle.
• V5-V6 (anterolateral leads) primarily observes the lateral wall of the left ventricle.

Background Grid

• ECGs are normally printed on a grid, which is measured to certain standard values
• Represents time on the horizontal axis and voltage on the vertical axis.
• A small box, which is 1 mm is 0.1 mV x 0.04 sec.
• A large box which is 5mm is 0.5mV x 0.2 sec.

Conditions for ECG

• 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), endocarditis (inflammation or infection of one or more of the heart valves), or after heart surgery or cardiac catheterization
• How an implanted pacemaker is working
• How well certain heart medicines are working
• Baseline tracing of the heart