The Cardiac Cycle

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

Cardiac Cycle

• Occurs as illustrated by heart rate, indicated as beats per minute.
• A healthy human heart beats 72 times per minute, resulting in 72 cardiac cycles per minute.
• Comprises diastole, systole, and the intervening pause.
• Involves a complete contraction and relaxation of both the atria and ventricles.
• The cardiac cycle lasts approximately 0.8 seconds.
• Systole is the period of contraction when the heart pumps blood into circulation.
• Diastole is the period of relaxation when the chambers fill with blood.
• The human heart consists of four chambers.
• The two upper chambers are the left and right atria.
• The two lower chambers are the right and left ventricles.
• Atria are where blood comes to the heart.
• Ventricles are where blood leaves the heart.

Cardiac Cycle Phases

• Atrial Diastole: Chambers of the heart are calmed as the aortic valve and pulmonary artery close, and atrioventricular valves open, allowing chambers to relax.
• Atrial Systole: blood cells flow from atrium to ventricle, and the atrium contracts.
• As the atria contract, the pressure within the atrial chambers increases, forcing more blood flow across the 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, and the pulmonary artery and aortic valve close.
• Isovolumic Relaxation: No blood enters the ventricles, pressure decreases, ventricles stop contracting, and the aorta – 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 AV valves.

Electrocardiography (ECG or EKG)

• Electrical currents within the heart can be detected during a routine physical examination by monitoring electrodes attached to the skin, usually at the wrist, ankles, and six separate locations on the chest.
• Electrocardiogram is a graph of voltage versus time of the electrical activity of the heart.
• Electrodes (small, plastic patches that stick to the skin) are placed at certain spots on the chest, arms, and legs and connected to an ECG machine by lead wires.
• Readings from the electrodes collectively provide an accurate, comprehensive assessment of the electrical activity of the heart.
• Provides a composite tracing of all muscle impulses formed by myocardial cells.
• Records wave of change in voltage (potential) across sarcolemma.
• The wave originates in the SA node.
• It 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.
• A typical ECG tracing for one heart cycle has 3 principal deflections.
• A P wave above the baseline,
• A QRS complex that 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.
• Waves indicate depolarization and repolarization within specific regions of the heart.
• The P wave is generated when impulse originating in the SA node depolarizes the cells of atria.
• The QRS complex identifies the beginning of ventricles' depolarization.
• Simultaneously, the atria repolarize, though this signal is masked by ventricles' electrical activity.
• The T wave is a small, rounded peak that denotes ventricular repolarization.
• P-wave: depolarization of atria
• Atria begin contracting about 25 msec after the start of the P-wave.
• QRS-complex: ventricular depolarization
• Ventricles begin contracting shortly after the peak of the R wave.
• T-wave: ventricular repolarization

ECG Leads

• Electrodes, limb leads, chest (precordial) leads, and the 12-Lead ECG
• An electrode is a conductive pad that is attached to the skin and enables the recording of electrical currents.
• An ECG lead is a graphical description of the electrical activity of the heart created by analyzing several electrodes.
• The standard ECG which is referred to as a 12-lead ECG since it includes 12 leads, is obtained using 10 electrodes.
• The representation of the electrical activity of the heart is critically dependent on the position of the electrodes.
• The same electrical activity will look different on different electrode combinations.
• 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.
• The electrical potential difference is the difference in the electrical potential detected by two (or more) electrodes.
• Depolarization and repolarization generate electrical currents.
• The tissues and fluids surrounding the heart act as electrical conductors.
• By placing electrodes on the skin, it is possible to detect electrical currents.
• The electrocardiograph (ECG machine) compares, amplifies, and filters the electrical potential differences recorded by the electrodes.
• In a conventional 12-lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest.
• The overall magnitude of the heart's electrical potential is measured from twelve different angles ("leads") and recorded over a period (usually ten seconds).
• 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.
• In most leads, however, the reference is composed of a combination of two or three electrodes.
• A vector heading towards the exploring electrode yields a positive wave/deflection and vice versa.
• The electrical activity of the heart can be observed from the horizontal plane and the frontal plane.
• The ability of a lead to detect vectors in a certain plane depends on the relationship with the plane, depending on the placement of the exploring lead and the reference point.
• Consider a lead with one electrode placed on the head and the other electrode placed on the left foot; the angle of this lead would be vertical, from the head to the foot.
• The lead is angled in the frontal plane, and it will primarily detect vectors traveling in that plane.
• Now consider a lead with an electrode placed on the sternum and the other electrode placed on the back (on the same level).
• This lead will be angled from the back to the anterior chest wall, which is the horizontal plane.
• The lead will primarily record vectors traveling in that plane.
• Leads are broken down into three types: limb, augmented limb, and precordial or chest.
• The 12-lead ECG has a total of three limb leads and three augmented limb leads arranged like spokes of a wheel in the coronal plane (vertical), and six precordial leads or chest leads that lie on the perpendicular transverse plane (horizontal).

Principles of the limb leads

• Leads I, II, III, aVF, aVL and aVR are all derived using three electrodes, which are placed on the right arm, the left arm and the left leg.
• Given the electrode placements, in relation to the heart, these leads primarily detect electrical activity in the frontal plane.
• Leads I, II and III compare electrical potential differences between two electrodes.
• Lead I compares the electrode on the left arm with the electrode on the right arm, of which the former is the exploring electrode.
• Lead I observe the heart "from the left" because its exploring electrode is placed on the left.
• Lead II compares the left leg with the right arm, with the leg electrode being the exploring electrode thus lead II observes the heart from an angle of 60°.
• Lead III compares the left leg with the left arm, with the leg electrode being the exploring one, observing the heart from an angle of 120°.
• Leads I, II and III are the original leads constructed by Wilhelm Einthoven where the 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, the same rule should apply, thus Lead I + Lead III = Lead II
• According to Einthoven's law
• Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode: I=LA−RA
• Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode: II=LL−RA
• Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode: III=LL−LA

Augmented Limb Leads

• Leads aVR, aVL, and aVF are the augmented limb leads and are derived from the same three electrodes as leads I, II, and III, but use Goldberger's central terminal as their negative pole.
• Goldberger's central terminal inputs from two limb electrodes, with a different combination for each augmented lead.
• The letter a stands for augmented, V for voltage and R is right arm, L is left arm and F is foot.
• In aVR the right arm is the exploring electrode, and 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.
• Since Goldberger's leads are composed of the same electrodes as Einthoven's leads, display a mathematical relation. aVL: Lead I – Lead III / 2, -aVR: Lead I + Lead II / 2, aVF: Lead II + Lead III / 2
• In lead aVF the electrode on the left leg serves as the exploring electrode and the reference is composed by computing the average of the arm electrodes.
• The average of the arm electrodes yields a reference directly north of the left leg electrode, thus any vector moving downwards in the chest should yield a positive wave in lead aVF.
• The angle by which lead aVF views the heart's electrical activity is 90°.
• Lead aVF “views the inferior wall of the left ventricle", where the same principles apply to lead aVR and lead aVL.
• II, aVF and III are called inferior (diaphragmal) limb leads and they primarily observe the inferior aspect of the left ventricle.
• aVL, I and aVR are called lateral limb leads and they primarily observe the lateral aspect of the left ventricle.
• Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system helping calcute the hearts electrical axis in the frontal plane.

Precordial Leads

• Lie in the transverse (horizontal) plane, 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, where unipolar precordial leads have been used to create bipolar precordial leads that explore the right to left axis in the horizontal plane.
• In the Cabrera system, the leads are placed in their anatomical order such that:
• V1-V2 ("septal leads"): primarily observes the ventricular septum but may occasionally display ECG changes originating from the right ventricle.
• V3-V4 ("anterior leads"): observes the anterior wall of the left ventricle.
• V5-V6 ("anterolateral leads"): observes the lateral wall of the left ventricle.
• None of the leads in the 12-lead ECG are adequate to detect vectors of the right ventricle.

Background grid

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

Why get an electrocardiogram?

• 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's function during a physical exam, which can be compared with future ECGs