12 lead morphology

-The normal ST segment should not be flat. It should have an upward concavity sometimes referred to as a “take-off”.
-When an ST segment loses its concavity and becomes straight or upwardly convex, it can indicate acute myocardial infarction.
-You can draw an imaginary line between the J point and the apex of the T wave
-If the ST segment is below that line, then it’s upwardly concave.
-If it’s even with or above that line, then it’s “non-concave” (straight or upwardly convex) which is suspicious for acute myocardial infarction.

-an upwardly concave ST segment makes a “smiley face” (good) and an upwardly convex ST segment makes a “frowny face” (bad).
-The STE-mimics almost always present with upwardly concave ST segments and an absence of reciprocal changes.

-electrically speaking, leads I, II, and III form an equilateral triangle.
-He expressed this with Einthoven’s Law, which states:
I + (-II) + III = 0
-lead I is a dipole, with the negative electrode at the right arm (white electrode) and the positive electrode at the left arm (black electrode).
-lead III is a dipole with the negative electrode at the left arm (black electrode) and the positive electrode at the left leg (red electrode).
-lead II is a dipole with the negative electrode at the right arm (while electrode) and the positive electrode at the left leg (red electrode).

-The R wave is about 7 1/2 mm tall, and the S wave is about 2 1/2 mm deep. Subtract the S wave from the R wave, and you come up with 5 mm
-do the same thing for the signal in lead II.
-This is easier, because it’s essentially a monophasic QS complex. It’s about -10 mm.
-lead III has little nub of an R wave that is about 1 mm high, and the S wave is about 16 mm deep.
-Subtract the R wave from the S wave, and you get a complex that measures approximately -15 mm.
-plug these values into the equation for Einthoven’s Law.
I + (-II) + III = 0
5 + 10 -15 = 0
-when you plug in the measurements, you end up with an electrical value of zero.

-the first area to depolarize (1) is the interventricular septum which depolarizes in a left-to-right direction (responsible for the so-called septal Q waves in the lateral leads of a normal 12 lead ECG).
-Next, the area around the left and right ventricular apex (2) depolarizes from a endocardial-to-epicardial direction (inside-out)
-there are more arrows near the (2) on the left side of the heart.
-This is because the left ventricle is more massive than the right ventricle.
-It has to be more massive because it’s responsible for circulating blood to the entire body and back
-Finally, the lateral walls of the left and right ventricle depolarize (3) and last the high lateral wall of the left ventricle (4)

-the large block arrow superimposed over the top of the diagram is the heart’s mean electrical vector.
-That means if you averaged the millions of electrical vectors created as the ventricles depolarize in any given cardiac cycle, the average direction would be right-to-left, superior-to-inferior (for the normal heart).
-More heart cells depolarizing means a stronger signal that cancels out the signal coming from the right side of the heart, so the normal QRS axis runs from a right shoulder to left leg direction (very similar to lead II).

-When the heart’s mean electrical vector moves toward a positive electrode, you get an upright complex on the ECG in that lead.
-When the heart’s mean electrical vector moves away from a positive electrode, you get a negative complex on the ECG in that lead.
-When the heart’s mean electrical vector moves perpendicular to a positive electrode, you get a so-called equiphasic complex.
-It starts out positive (A) as the mean electrical vector approaches, but ends up negative (B) as the vector passes on by

-the red arrow is the heart’s mean electrical vector
-two vectors (or in this case leads) are equal as long as they are parallel and of the same intensity and polarity.
-Therefore, we can move the leads [...] to a point passing through the center of the heart, and they will be the same.

-for example, lead I sees the mean electrical vector like the diagram to the left.
-In other words, it sees the heart’s mean electrical vector relative to its own vector created by its negative and positive electrodes.

-leads II and III see the mean electrical vector relative to their own vectors.
Because this is true, we can take the three vectors (or sides) of Einthoven’s Triangle and make them intersect in the center.
-When leads I, II, and III are drawn this way (as they often are) the arrows and Roman numerals are placed in the position of the positive electrodes.

-aVR stands for augmented vector right,
-aVL stands for augmented vector left, and
-aVF stands for augmented vector foot.
They are called the augmented limb leads because they are augmented (or amplified) through a modification of Wilson’s Central Terminal (WCT).
-The modification was necessary because otherwise the complexes would have been too small
-leads aVR, aVL, and aVF are derived from the same 3 electrodes as leads I, II, and III.
-They just examine different vectors by combining two electrodes together to form the negative pole of each lead.

-the negative for lead aVR is the combination of the black (left arm) and red (left leg) electrodes.
-The positive electrode for lead aVR is the white electrode (right arm).
-This has the effect of making the vector for lead aVR point toward the right shoulder.
-For lead aVL, the negative is a combination of the white (right arm) and red (left leg) electrodes.
-The positive electrode for lead aVL is the black electrode (left arm).
-This has the effect of making the vector for lead aVL point toward the left shoulder.
-The negative for lead aVF is a combination of the white (right shoulder) and black (left arm) electrodes.
-The positive for lead aVF is the red (left leg) electrode.
-This has the effect of making the vector for lead aVF point toward the left leg (or foot).

-Einthoven was able to refer to leads I, II, and III as Einthoven’s Equilateral Triangle even though anatomically speaking, leads I, II, and III form a scalene triangle on the human body.
-For the exact same reasons, we can draw a mathematical representation of leads aVR, aVL, and aVF that looks symmetrical like the shape on the right.
this are the final 3 spokes of the hexaxial reference system!

lay the diagram of leads aVR, aVL, and aVF on top of the diagram created for leads I, II, and III.
-it will look like the diagram on the bottom.

-first, I cuts the body in half horizontally and lead aVF cuts the body in half vertically.
-Second, even though leads II, III, and aVF share the same positive electrode, they represent three separate vectors.
-This diagram should clearly demonstrate why we call them the “inferior” leads.
-It should also demonstrate why we call leads I and aVL the “high lateral” leads.
-You will notice that leads III and aVL are on opposite sides of the hexaxial reference system.
-That’s why they are two of the most reciprocal leads on the 12 lead ECG.

-You will notice that lead II cuts across the body in a “right shoulder-to-left leg” direction (white electrode to red electrode) which is the same direction as the heart’s normal axis.
-That’s probably why we were first taught to monitor lead II.
-It tends to show nice, upright P waves, QRS complexes, and T waves.

-When the heart’s mean electrical vector (or QRS axis) moves toward a positive electrode, you get an upright complex in that lead.
-When it moves away from a positive electrode, you get a negative complex in that lead. -When it moves perpendicular to a positive electrode, you get an equiphasic (and/or isoelectric) complex in that lead

-to use the hexaxial reference system, you find the most equiphasic (or isoelectric) lead in the frontal plane (first 6 leads of the 12 lead ECG) and look for the perpendicular lead on the hexaxial reference system.
-If lead III is perpendicular to lead aVR, then lead aVR is perpendicular to lead III.
-If you examine the hexaxial reference system, you will notice that leads I and aVF are perpendicular to each another. -Likewise, leads II and aVL are perpendicular.
-The diagram represents the layout of the first 6 leads of the 12 lead ECG in the standard format.
-notice that when we draw a line between the perpendicular leads, they crisscross in the center.

-lead I cuts the hexaxial reference system in half horizontally and lead aVF cuts the hexaxial reference system on half vertically.
-You can think of this as an x and y axis that divides the hexaxial reference system into quadrants.
-you can use leads I and aVF to place the heart’s electrical axis into one of the four quadrants.

-the normal QRS axis goes from a right shoulder-to-left leg direction in most patients.
-In other words, it tends to point down and to the left, or toward the left inferior quadrant of the hexaxial reference system, which ranges from 0 to +90 degrees.
-When the QRS axis in the frontal plane is in the normal quadrant, you will have positive QRS complexes in lead I and positive QRS complexes in lead aVF.

-When the QRS axis is 0 to -90 degrees, we call it a left axis deviation.
-This is the left superior quadrant of the hexaxial reference system.
-When the QRS axis is in the left superior quadrant, you will have positive QRS complexes in lead I and negative QRS complexes in lead aVF.
-The most common causes of pathological left axis deviation are left anterior fascicular block or Q waves from inferior wall myocardial infarction
-Some sources say that left ventricular hypertrophy pulls the axis to the left, and while this seems logical, in most cases patients with left ventricular hypertrophy have a normal QRS axis.
-Electrolyte derangements and ventricular rhythms may also present with a left axis deviation.
-Paced rhythms in particular should have a left axis deviation if the pacing lead is in the apex of the right ventricle.

-the QRS axis can be slightly into the left superior quadrant and still be considered normal.
-When the axis is between 0 and -30 degrees, it is sometimes referred to as a physiological (as opposed to pathological) left axis deviation.
-With a physiological left axis deviation, lead II is usually equiphasic
-remember that lead II is perpendicular to lead aVL and lead aVL points to -30 degrees on the hexaxial reference system

-If the QRS axis in the frontal plane is +90 to 180 degrees, it is a right axis deviation.
-This is the right inferior quadrant of the hexaxial reference system.
-With a right axis deviation, you will have negative QRS complexes in lead I and positive QRS complexes in lead aVF.
-A right axis deviation is usually abnormal.
-It might indicate pulmonary disease, right ventricular hypertrophy, Q waves from lateral wall myocardial infarction, left posterior fascicular block, electrolyte derangement, or tricyclic antidepressant overdose, or a ventricular rhythm.

-If the QRS axis is -90 to 180 degrees, something is very wrong (possibly your lead placement).
-This is the right superior quadrant of the hexaxial reference system, but in various publications it can be called an extreme right axis deviation, an indeterminate axis, or a right shoulder axis.
-It’s bad because it means the heart is depolarizing in the wrong direction.
-With an extreme right axis deviation, you will have negative QRS complexes in lead I and negative QRS complexes in lead aVF.

-the cheat sheet relies only on leads I, II, and III (although you can substitute lead aVF for lead III).
-This method works pretty good because, as we saw earlier, by looking for an equiphasic QRS complex in lead II we can distinguish between physiological and pathological left axis deviation.
-Remember, QRS complexes in lead III are allowed to be negative.
-However, negative QRS complexes in lead I or lead II are abnormal.