Chapter 12 Electrocardiogram (ECG) Vector Analysis

Electrocardiographic Interpretation

• Changes in the pattern of impulse transmission in the heart can cause abnormal electrical potentials, altering the electrocardiogram(ECG) waves.
• Analyzing the contours of the waves in different ECG leads helps diagnose heart muscle abnormalities.

Vectorial Analysis of Electrocardiograms

• Understanding vectors and vectorial analysis is crucial for understanding how cardiac abnormalities impact ECG contours.
• A vector is an arrow pointing in the direction of electrical potential generated by current flow, with length proportional to voltage.

Resultant Vector in the Heart

• At any given instant electrical current flows between depolarized and non-depolarized areas inside and outside the heart
• More current flows downward from the base to apex of the ventricles.
• The instantaneous mean vector represents the summated generated potential and points from the base to the apex
• The greater the summation of current the larger the vector

Direction of a Vector

• A vector pointing horizontally to the person's left has a direction of 0 degrees.
• The scale rotates clockwise, with +90 degrees pointing straight down, +180 degrees from left to right, and -90 degrees (or +270) straight upward
• The mean QRS vector, averages +59 degrees in a normal heart
• Meaning the apex of the heart remains positive with respect to the base.

Axis for Standard Bipolar and Unipolar Limb Leads

• Each lead consists of a pair of electrodes on opposite sides of the heart, with the axis being the direction from negative to positive electrode.
• Lead I axis is 0 degrees (horizontal, positive to the left)
• Lead II axis is approximately +60 degrees (right arm to left leg).
• Lead III axis is about +120 degrees
• Lead aVR is +210 degrees
• Lead aVF is +90 degrees
• Lead aVL is -30 degrees.
• The hexagonal reference system shows the direction of all lead axes.

Vectorial Analysis of Potentials in Different Leads

• The voltage recorded in a lead depends on the projection of the heart's electrical vector (A) onto the lead's axis.
• A perpendicular line from the tip of vector A to the lead axis determines the projected vector (B).
• The arrow of the projected vector indicates the record direction in the ECG
• The length of B compared to A determines the instantaneous recorded voltage.
• If the vector is perpendicular, the voltage recorded is low
• When the heart vector is on the same axis as the lead, the entire voltage will be recorded

Bipolar Limb Leads

• Perpendicular lines from the tip of vector A to the axes of leads I, II, and III determine the projected vectors
• Each corresponds to the potential recorded and whether its positive or negative
• Augmented limb leads: a similar analysis can be used to determine potentials recorded in augmented limb leads.

Vectorial Analysis of the Normal Electrocardiogram

• First, the left endocardial surface of the septum depolarizes, represented by a short vector due to the small area (about 0.01 second).
• ECG voltages are low, with Voltage in lead II > I and III
• Next, depolarization spreads along the endocardial surfaces in ventricles (0.02 second)
• Producing a long vector and increasing the voltages in all ECG leads.
• As the outside of the heart apex becomes electronegative (0.035 second), the vector shortens, and voltages decrease.
• The vector axis shifts left as the left ventricle depolarizes slower than the right- ratio of lead I to lead III increases

Ventricular Muscle Mass

• With only a small part of the ventricular muscle polarized (0.05 seconds), the heart vector points toward the base of the left ventricle
• Voltages in leads II and III are negative, while lead I is positive
• After 0.06 seconds, the entire ventricular muscle is depolarized, there is no current flow, and the voltages in all leads become zero at the end of the QRS complexes.
• Sometimes the QRS complex has a negative depression at the beginning, called a Q wave.
• Major positive deflection is the R wave, and final negative deflection is the S wave.

Electrocardiogram During Ventricular Repolarization - T Wave

• The T wave in the ECG is caused by ventricular repolarization, occurring 0.15 to 0.35 seconds after depolarization.
• The outer surface of ventricles repolarizes first and the endocardial areas repolarize last due to high blood pressure reducing coronary blood flow in the endocardial areas
• Because the outer apical surfaces repolarize first, the overall ventricular vector points toward the apex
• Causes a positive T wave in all three bipolar limb leads, similarly to the polarity of the QRS complex

Repolarization Stages

• Repolarization Vector: begins weak because of the small area and becomes stronger, then decreases as remaining areas depolarize.
• The T wave in the ECG changes with the vector.

Atrial Depolarization - The P Wave

• Beginning in the sinus node and spreading in all directions causes original negativity to the point of entry of the the superior vena cava
• Because the vector generally remains in this direction, ECGs recorded in leads I, II, and III are usually positive and the record of atrial depolarization is the P wave.

Repolarization of the Atria - the Atrial T Wave

• With no Purkinje system, it is much slower spread depolarization
• Sinus node becomes polarized long before the atria- area in the atria becomes repolarized first
• The vector is backward to that of depolarization as the sinus node becomes repolarized
• The atrial T wave follows 0.15 second after the P wave but is on the opposite side of the zero reference line- normally negative

Vectorcardiogram

• Current flows through the heart changing voltage (length) and direction of the vector
• Point 5 is zero reference point and vector negative end. While the heart is polarized, the vector is negative
• Depolarization: positive end of the vector move away from zero reference points

Ventricular Depolarization

• The vector extends down to apex and is weak because of the septums
• As more ventricular muscle is depolarized the vector gets slightly stronger
• Fully depolarized the vector returns to the zero point
• The QRS vectorcardiogram represents the elliptical figure created by the vector

Mean Electrical Axis

• Electrical potential direction in the heart during in ventricular depolarization is base to apex
• The Mean Electrical Axis of the Ventricles (normal at 59 degrees)
• Abnormal Mean Electrical Axis can be the result of pathological conditions

Determining the Electrical Axis

• Electrical Axis, Estimated: determined polarity during the heart cycle
• Determine Net Potential: if any recording is negative during that period the negative must be subtracted from the positive
• Plot axis on the respective leads
• Total Ventricular Mean electrical Vector, determined by perpendicular lines
• Mean Electrical Axis determination of the ventricles should be 59 degrees

Abnormal Ventricular Axis Deviation Conditions

• Mean Axis of the ventricle: 20 degrees to 100 degrees
• Axis Deviation can be caused by distribution of the Purkinje system, musculature differences or abnormal heart conditions
• Angulation of the Heart: Heart angulation can shift based off left or right based off pressure that increases or decreases the direction
• Hypertrophy: Axis shift will occur towards the Hypertrophied Ventricle because there is more muscle that will generate a greater electrical potential, it takes more time during the depolarization wave

Vectorial Analysis of Left Axis Deviation Resulting from Hypertrophy of the Left Ventricle

• Typical ECG will demonstrate a left axis deviation (mean axis is -15 degrees)
• Increased Muscle Mass: caused by axis deviation and a hypertensive event due to working against elevated systemic arterial pressure
• Similarly aortic and congenital situations can cause the left ventricle to hypertrophy

Bizzare Complexes and conditions

• Bundle Branch Block causes Axial Deviations in cases where the Lateral Walls don't depolarize at the same time
• Normally cardiac impulse is transmitted throughout the Purkinje system to Lateral Walls, if blocked an axis deviation can occur

Deviations

• Deviations in the ventricular conduction systems cause axis polarization to not occur
• Typically it lasts more than .09 seconds, certainly lasting for .12 seconds
• Bizarre patterns and rapid axis deviations can also cause triple peaks- especially if cardiac impulse is destructed from heart issue

Current of Injury

• The heart can remain partially/totally depolarized due to abnormalities, with current flowing between normal polarized areas (between heartbeats) or as abnormal negative current through electrodes
• Injured parts are typically negative emitting an negative charge
• Abnormalities such as mechanical trauma, infectious processes that damage the membrane can also cause injuries

Infarcted Area and QRS

• A lack of blood flow to the base will emit voltage during the TP cycle instead of a cardiac cycle
• Vector of this current of injury and the ECG create a record that deviates from the zero line and causes electrical impulses to electrodes to be generated

The J point as the zero for injury

• The electrode is used to determine currents caused by skin or ionic fluids
• the horizontal line is known as the zero in the voltage as injury currents are tested

Blood flow

• Lack of blood flow to the muscle depresses muscle
• Severe Ischemia or coronary occlusion with the injured vector can be viewed in ECGs such as V2 tests

Posterior Wall infraction

• Injury potential is only positive if there is a chest area of the heart.
• Analysis of 2-3 leads and vectorial analysis will help lead to which area of the heart damage would occur
• Use that the positive end points to the normal cardiac muscle and negative end points to the injured portion to assess the area

Progression and infartion

• Overtime post heart disease it will change depending on if collaterally the heart develops or not
• Q waves are the the cause of muscle mass from muscle lost in infraction sites

The heart and pectoral pain

• Agina Pectoris, chest pain can have increased currents when the heart is overloaded
• Overdoses of digitalis has the samm effect with the heart, typically not enough to repolarize diastole

Abnormal Wave

• T- waves, are normally positive and it effects the normal sequence of repolarization
• Slow Depolarization is known for have have long PR intervals
• Bundle Branch Block, changes what axis the heart travels on
• The T-wave and electrical axis is ofter opposite, with its position inversed across axis, where the polarity of the heart is reversed