BSI: Rapid Interpretation of EKG's (From book by Dale Dubin)

The electrocardiogram (EKG) records the electrical activity of the heart, providing a valuable, permanent record of its function. More specifically, an EKG records the electrical impulses that stimulate the heart to contract.

The medical profession used a "K" to replace the "C" of "cardio" for the abbreviation for an electrocardiogram to avoid confusion with an EEG (brain wave recording) because ECG and EEG sound alike. However, both EKG and ECG are correct terminology.

Muscle cells of the heart are charged or polarized in the resting state, but when they depolarize, this electrical stimulus causes their contraction.

In the resting state the cells of the heart (myocardial cells, or collectively, myocardium), are polarized. This means the inside of the cells are negatively charged.

In the strictest sense, a resting, polarized cell has negatively charged interior and a positively charged surface. For the sake of simplicity we will consider only the inside of the myocardial cell.

The interiors of the myocardial cells, which are usually negatively charged, become positively charged as the cells are stimulated to contract.

The electrical stimulation of the heart's muscle cells is called "depolarization" and causes them to contract.

Thus, a progressive wave of stimulation (depolarization) passes through the heart causing contraction of the myocardium.

This depolarization may be considered an advancing wave of positive charges within the cells.

The phase of repolarization (the recovery phase_ is when cells return to their original negative state after they have depolarized. Essentially, the myocardial cells are regaining the negative charge within each cell.

Repolarization is strictly an electrical phenomenon. The myocardial cells do not respond to repolarization.

As electrical activity passes through the heart, it can be detected by external skin sensors, also called electrodes, which record both depolarization AND repolarization.

Both depolarization and repolarization are strictly electrical phenomena.

As the positive wave of depolarization within the heart cells moves toward a positive (skin) electrode, there is a positive (upward) deflection recorded on the EKG.

Recall that an advancing wave of depolarization may be considered a moving wave of positive charges.

The SA Node ("Sinus-Atrial Node") begins the electrical impulse which spreads outward in wave fashion, stimulating both atria to contract at the same time. It is the heart's natural pacemaker, so its pacing activity is known as a "Sinus Rhythm."

The electrical stimulus originating in the SA Node proceeds away from the node in all directions. If the atria were a pool of water and a pebble dropped at the SA Node, an enlarging circular wave would spread out from the SA Node. This is the manner in which atrial depolarization is a spreading wave of positive charges within the myocardial cells.

The SA Node normally acts as pacemaker and sets the heart rate of 60 to 100 beats per minute. The SA Node is located within the upper-posterior wall of the right atrium.

The atrial stimulation is recorded as a P wave on an EKG. The P wave represents atrial depolarization (or stimulation) electrically.

From the SA Node, the electrical impulse of the heart reaches the AV Node (Atrio-Ventricular Node) where there is a brief pause, allowing blood to enter the ventricles.

The AV Node is the only electrical pathway between the atria and the ventricles.

Once the AV Node is stimulated, there is a pause before the impulse of depolarization can completely penetrate through the AV Node because the stimulus of depolarization slows within the AV Node. This pause allows blood from the atria to pass through the AV valves into the ventricles.

Although depolarization slows within the AV Node, this stimulus of depolarization proceeds rapidly down the His Bundle into the Left and Right Bundle Branches.

The His Bundle (also known as the Bundle of His), which extends down from the AV Node, divides into Right and Left Bundle Branches within the interventricular septum.

The electrical stimulus (from the Bundle Branches) continues on to be distributed via the terminal Purkinje Fibers to the ventricular myocardial cells, and it is the depolarization of the myocardial cells which produces a QRS complex on EKG tracing.

The QRS complex represents ONLY the depolarization of the myocardial cells of the ventricles.

The QRS complex on an EKG represents the beginning of ventricular contraction. The physical act of ventricular contraction actually lasts longer than the QRS complex, but we will consider the QRS complex to represent ventricular contraction. So the QRS complex represents depolarization of the ventricles, which causes ventricular contraction.

The ventricular conduction system is composed of specialized nervous tissue which rapidly carries the electrical impulse (depolarization) away from the AV Node. The ventricular conduction system consists of the His Bundle, the Left and Right Bundle Branches terminating in the Purkinje Fibers. Depolarization moves much more rapidly through this specialized nervous tissue than is possible through the myocardial cells alone. The [rapid] passage of the stimulus down the ventricular conduction system does not record on EKG.

The Q wave is the first downward stroke of the QRS complex, and it is followed by the upward R wave. The Q wave is often not present.

If there is any upward deflection in a QRS complex that appears before a Q wave, it is not a Q wave, for by definition, the Q wave is the first wave of the QRS complex. The Q wave is always the first wave in the complex when it is present.

The first upward deflection of the QRS complex is the R wave.

Any downward stroke PRECEDED by an upward stroke is an S wave.

The complete QRS complex can be said to represent ventricular depolarization (and the initiation of ventricular contraction).

If there is no upward wave, we cannot determine whether the wave is a Q or an S, and therefore it is called a QS wave.

A QS wave is considered to be a Q wave when we look for Q's.

The T wave represents repolarization of the ventricles so they may be stimulated again.

Repolarization occurs so that the myocardial cells can regain the negative charge within each cell, so they can be depolarized again.

The ventricles have not physical response to repolarization. This is strictly an electrical phenomenon recorded on an EKG.

The atria also have a repolarization wave which is very small and usually lost within the QRS complex and therefore not seen under ordinary circumstances.

One cardiac cycle is represented by the P wave, the QRS complex, and T wave.

Physiologically a cardiac cycle represents atrial systole (atrial contraction) and ventricular systole (ventricular contraction), and the resting stage that follows.

The EKG is recorded on graph paper. The smallest divisions are one millimeter squares, being one millimeter long and one millimeter high.

There are five small squares between the heavy black lines.

The height and depth of a wave are measured in millimeters and this represents a measure of voltage.

Upward deflections are called "positive" deflections while downward deflections are called "negative" deflections on EKG's.

The horizontal axis represents time.

Each 1 mm x 1 mm box represents 0.04 (four hundredths) of a second.

Five boxes, which are found between two heavy black lines represent 0.2 seconds (2/10ths of a second).

The duration of any wave may be determined by measuring along the horizontal axis. For example, 0.12 seconds is represented by three small squares.

The vertical axis represents voltage.

The standard EKG is composed of 12 separate leads:

• 6 Limb Leads
• 6 Chest Leads

• Lead I
• Lead II
• Lead III
• Lead AVF
• Lead AVL
• Lead AVR

To obtain limb leads, electrodes are placed on the right and left arms and the left leg forming a triangle.

Each lead consists of a pair of electrodes, one is always positive and one is always negative, so they are sometimes called bipolar limb leads.

• The right arm is always negative.
• The left leg is always positive.
• The right arm is positive or negative depending on which lead we are talking about.

Lead I is horizontal with the outstretched arms. The right arm is always negative, making the left arm positive.

Lead II is diagonal and downward with the right arm, which always negative, and the left leg, which is always positive.

Lead III is diagonal and downward with the left leg, which is always positive, and the left arm, making the left arm negative.

Instead of using just two electrodes as in limb leads I, II and III, these limb leads use three electrodes (one on each arm). Two leads are always negative, and one is always positive.

The Doctor who introduced these "Augmented" limb leads discovered that in order to monitor a lead in this manner he had to amplify (augment) the voltage in the EKG machine to get a tracing of the same magnitude as leads I, II and III. He named the leads after which position obtains the positive lead.

Lead AVF (Augmented Voltage Foot) has the positive lead on the left foot. The right arm and left arm are channeled into a common (negative) ground.

Lead AVL (Augmented Voltage Left) is obtained in the same manner as AVF, except the positive electrode is on the Left arm.

Lead AVL (Augmented Voltage Right) is similar to AVL, except the positive electrode is on the Right arm.

These augmented limb leads are sometimes called the unipolar limb leads, stressing the importance of the positive electrode.

Just FYI, the right foot sensor is never connected within the EKG machine for augmented leads, but it does have a sensor wire attached.

When all six limb leads are allowed to intersect and superimposed on each other, it creates a pin wheel with intersecting reference lines which like in a flat plan on the patient's chest, also called the frontal plane.

Your deductive mind will tell you that lead AVF is a mixture of leads II and III...just what the doctor who invented these leads was trying to accomplish. Therefore, lead AVF is a cross between (and oriented between) those two bipolar leads.

In essence, all of these limb leads allow us to view the electrical activity of the heart from different angles (or positions) to deduce many things from the EKG.

To obtain the six chest leads, a positive electrode is placed at six progressively different positions around the chest.

In all of the chest leads, the electrode sensor placed on the chest is considered positive.

The chest leads are numbered from V₁ to V₆ and move in successive steps from the patient's right to his left side. Notice how the chest leads cover the heart in its anatomical position within the chest.

Recall that because the electrode sensor for the chest leads is always positive, a depolarization wave moving toward a given chest sensor produces a positive (upward) deflection in that chest lead on the EKG tracing.

The chest leads are projected through the AV Node towards the patient's back, which is the negative end of each chest lead.

If leads V₁ through V₆ are assumed to be the spokes of a wheel, the center of the wheel is the AV Node.

Lead V₂ describes a straight line from the front to the back of the patient. The patient's back is negative.

The plane of the chest leads cuts the body into top and bottom halves and is called the horizontal plane.

When observing the chest leads from V₁ to V₆, one will notice gradual changes in all the waves (as the position of each lead changes).

Considering chest lead V₁, the QRS complex is normally negative (i.e. mainly below the baseline).

In lead V₆, the QRS complex is usually positive. This means that the positive wave of ventricular depolarization (represented by the QRS complex is moving toward the positive electrode of V₆.

Lead V₁ and V₂ are placed over the right side of the heart, while V₅ and V₆ are over the left side of the heart.

Lead V₃ and V₄ are located over the interventricular septum. The interventricular septum is a common wall shared by the right and left ventricle. In this area the His Bundle divides into Right and Left Bundle Branches.

Considering Lead V₃, the chest electrode is said to be positive.

• Rate
• Rhythm
• Axis
• Hypertrophy
• Infarction

60 to 100 beats per minute

The SA Node (Sinus Node) normally acts as pacemaker to set this normal rate in the heart.

Sinus Bradycardia is when the SA Node (Sinus Node) paces the heart at a rate slower than 60 beats per minute.

Sinus Bradycardia is present if a rate of less than one beat per second is produced by the SA Node.

Sinus Tachycardia is when the SA Node (Sinus Node) paces the heart at a rate of greater than 100 beats per minute. This would be a faster-than-normal rate.

Other areas of the heart have the ability to pace if the normal (SA Node) pacemaking mechanism fails. Because these focal centers of potential pacemaking activity originate in other areas of the heart, they are referred to as "ectopic" foci. An ectopic focus will assume pacing responsibility if it senses a failure of the SA node to pace.

Under normal conditions these ectopic foci of potential pacemakers are electrically quiet and do not function. That's why we call them "potential" pacemakers.

Depending on the location of the focus, the focus that resumes pacing responsibility has an inherent pacing rate that is typical for that spot. For example, the atrial ectopic focus has an inherent rate of 60 to 80 beats per minute.

However, in an emergency or certain pathological conditions, the ectopic foci in any area may suddenly discharge at a rapid rate. The rapid rate (150 to 250 beats per minute) is the same for foci in the atria, AV Junction and ventricles.

There are three main Ectopic Foci (Potential Pacemakers) of the heart. They are:

• 1) Atrial Ectopic Focus: 60 to 80 beats per minute.
• 2) AV (Atrioventricular) Junctional Ectopic Focus: 40 to 60 beats per minute (also called an idiojunctional rythm, the Greek prefix idio- meaning "one's own.")
• [Note: the His Bundle is known as the AV Junction, so an ectopic focus in this area is called a Junctional ectopic focus. In other words, the AV Junction is the His Bundle.]
• 3) Ventricular Ectopic Focus: 20 to 40 beats per minute (also called idioventricular rhythm).

Note: The AV Node itself is named for its position between the Atria and the Ventricles (thus "AV"). The AV Node itself has no foci of potential pacemakers.

Observation alone can tell us the rate.

• 1) Look at the R waves.
• 2) Find an R wave that falls on a heavy black line.
• 3) Now, beginning with the next heavy line, count off in triplicates "300, 150, 100..." in succession.
• 4) And then the next triplicate: "75, 60, 50"

• Just FYI and NOT to memorize:
• There is a logical explanation for the unusual rate denominations of the heavy black lines. The unit (duration of time) between two heavy black lines is 1/300th of a minute. Recall that five black lines also represents 1/300th of a minute. Therefore, if a heart contracts 75 times/minute, there will be span equivalent to the distance between five heavy black lines between QRS complexes. This represents 4/300ths of a minute, or a rate of 75 beats per minute.

Recall that you can easily determine the rate down to 50 beats per minute by the afore-mentioned method. However, in bradycardia, there may be beats less than 50 beats per minute. So use the following method to find the heart rate:

• 1) At the top of EKG tracing there are small marks which signify "three second" intervals.
• 2) Taking two of the three second intervals, we have a six-second strip.
• 3) Count the number of complete cycles (R wave to R wave represent one cycle). With very slow rates there will be very few cycles.
• 4) The rate is obtained by multiplying the number of cycles in the six-second strip by 10. For example, if there were four and a half cycles in the six-second strip, the rate would be 45 beats per minute.

Arrhythmia literally means without rhythm. However, we use it to denote abnormal rhythm or breaks in the regularity of a normal rhythm.

To understand and diagnose arrhythmias, you must first be familiar with the normal electrophysiology of the heart (i.e, the normal pathway of electrical conduction).

This is the normal path of electrical stimulus in the heart and the corresponding EKG pattern:

• 1) SA Node (Sinus Node)
• 2) through the atria, causing the atria to contract [P wave]
• 3) AV Node
• 4) brief pause while the stimulus slowly penetrates through the AV Node [this produces a flat line between the P wave and QRS complex]
• 5) His Bundle
• 6) Left and Right Bundle Branches
• 7) Purkinje Fibers
• 8) myocardium depolarized to stimulate ventricular contraction [QRS complex] Note that atrial repolarization happens right around here but any sign of it is masked by the QRS complex.
• 9) ST segment [flat baseline occurring between the QRS complex and the T wave].
• 10) ventricular repolarization [T wave]

Note: The passage of depolarization through the AV Node and the ventricular conduction system in NOT recorded on EKG.

• Irregular Rhythms
• "Escape" and "Premature" Beats
• Rapid Ectopic Rhythms
• Heart Blocks

Sinus Arrhythmia (also called Irregular Rhythm): the pacemaking impulses originate in the SA Node, and therefore all P waves are identical, and in fact, the entire cycle of P-QRS-T waves of each cycle are usually identical and similar in size and shape. There is simply a continuous gradual rate change. This rate change is usually linked to inspiration and expiration.

Wandering Pacemaker is an irregular rhythm caused by pacing discharges from a variety of different atrial foci. It is characterized by P waves of varying shape since the pacemaking origin continuously changes location within the atria. The resulting rhythm is irregular and there is no consistent pattern.

Atrial Fibrillation is caused by the continuous, rapid-firing of multiple foci in the atria. No single impulse depolarizes the atria completely, and only an occasional impulse gets through to the AV Node to stimulate the ventricles, producing an irregular ventricular QRS rhythm.

Since no single impulse depolarizes the atria, we cannot find any real P waves, only a rapid series of tiny, erratic spikes on EKG.

The ventricular rate may be rapid or slow, but it is always irregular in Atrial Fibrillation.

During a Sinus Rhythm, an unhealthy SA Node may fail to produce a pacing stimulus (Sinus Block), so this "pause" elicits a response from an impatient focus, producing an Escape Beat. Remember, "Escape" is the response of an ectopic focus to a pause of cardiac non-activity.

After such a pause of cardiac non-activity, an ectopic focus may "escape" (respond) by discharging an Escape Beat. The temporarily blocked SA Node eventually resumes pacing, but if SA Node pacing is arrested, then an ectopic focus will have to assume pacing responsibility.

Recall there are three main ectopic foci, and therefore, there are three potential pacemakers that could take over in the case of an escape beat and take over the heart rate at its inherent rate:

• 1) Atrial Ectopic Focus produces Atrial Escape Rhythm: 60 to 80 beats per minute. [P waves of slightly different appearance]
• 2) AV Junction Ectopic Focus produces Junctional Escape Rythm (Idiojunctional Rhythm): 40 to 60 beats per minute. [absent or inverted P waves]
• 3) Ventricular Ectopic Focus produces Ventricular Escape Rhythm (Idioventricular rhythm): 20 to 40 beats per minute. [large, drawn out QRS complexes; no P waves]

A Premature Beat originates in an ectopic focus which suddenly discharges, producing a beat which appears earlier than expected in the rhythm.

Again, because there are three main areas where Ectopic foci are found, there are three areas where a Premature Beat can originate:

• 1) Atrial Ectopic Focus produces a Premature Atrial Beat: produces an abnormal P wave earlier than expected. Remember, if it doesn't follow the same pathway of electrical conduction, it won't produce the same pattern on the EKG, so because the stimulus doesn't come from the SA Node, the P wave will look different than the other P waves. But eventually it should find the same pathway and the rest of the complex should look the same.
• 2) AV Junction Ectopic Focus produces a Premature Junctional Beat: Usually has a normal appearing QRS complex which occurs very early and is generally not preceded by a P wave. However, the AV Junction can send an impulse upward to stimulate the atria from below (called "Retrograde Conduction"). When this occurs, the backwards atrial depolarization may create an inverted P wave which can occur just before or just after the QRS, or this peculiar inverted P wave may be mixed in with the QRS complex.
• 3) Ventricular Ectopic Focus produces Premature Ventricular Contraction (PVC): produces a premature ventricular beat that is seen as a giant QRS complex, which is easily recognizable.

Depolarization of the PVC does not follow the usual ventricular conduction system pathway, and therefore the conduction is very slow (very wide QRS). There is also a compensatory pause after this wide QRS complex caused by the PVC. During normal ventricular contraction, the left and right ventricle depolarize simultaneously. As a result, depolarization going toward the left (left ventricle) is somewhat opposed by simultaneous depolarization going toward the right (right ventricle), and a relatively small QRS complex results. But a PVC originates in one ventricle, causing depolarization in one ventricle before the other. So the deflections of a PVC are very tall and deep (no simultaneous opposing depolarization from opposite sides) on the electrocardiogram. PVC's have greater deflections than normal QRS complexes.

Note that six or more PVC's is considered pathological. PVC's often indicate that the heart's own (coronary) blood supply is poor, so their appearance alerts us that something may be wrong.

Multifocal PVCs are produced by multiple ventricular ectopic foci. Each focus produces an identical appearing PVC every time it fires.

If a PVC falls on a T wave, it occurs during a vulnerable period and dangerous arrhythmias may result because it may cause the ventricles to beat rapidly.

Paroxysmal means "sudden." It is used to describe Paroxysmal Tachycardia for example.

• Paroxysmal Tachycardia: 150 to 20 beats per min
• Flutter: 250 to 350 beats per min
• Fibrillation: 350 to 450 beats per min

Atrial Flutter originates in an atrial ectopic focus. P waves occur in rapid succession and each is identical to the next since only one ectopic focus is discharging.

Only the occasional atrial stimulus will penetrate through to the AV Node, so there are a few flutter waves in series before a QRS complex is seen.

The rate will be 250 to 350 beats per minute, although atrial flutter is usually diagnosed by appearance of waves rather than rate. Atrial flutter has been described as being "saw-tooth" in appearance. They waves fall in rapid succession and there is usually no flat baseline between them.

In examining EKGs, it may be difficult to distinguish between Tachycardia Heart Block and Atrial Flutter. The key difference is that in Atrial Flutter, there is no flat lines in between, whereas in heart block, you may have a few P waves that do not get through, but there should still be space in between those P waves.

Ventricular Flutter is produced by a single ventricular ectopic focus firing at the extremely rapid rate of 250 to 350 beats per min. This will exhibit as a smooth sine wave appearance on the EKG.

Note that Ventricular Flutter deteriorates into potentially deadly arrhythmias, such as Ventricular Fibrillation.

Atrial Fibrillation looks like totally erratic scribbles going up and down very fast.

Blood is a viscous fluid, and the ventricles can not be filled in ventricular fibrillation. For this reason, thre is no effective cardiac output in Ventricular Fibrillation when the heart beats at 350 to 450 beats per minute. Ventricular Fibrillation is a result of ventricular ectopic foci desperately trying to compensate. This is a cardiac emergency requiring cardio-pulmonary resuscitation and defibrillation.

If you do recognize any pattern of regularity, you are probably not dealing with Ventricular Fibrillation.

Atrial Fibrillation occurs when many ectopic foci in the atria fire rapidly, producing an excessively rapid series of tiny erratic spikes on the EKG (no P waves). Only a small portion of the atria is depolarized, so no one discharge is carried very far. The QRS response is not regular and may be rapid or slow.

Heart Blocks are electrical blocks which prevent the passage of electrical (depolarization) stimuli.

Note that when checking an EKG for heart blocks, keep in mind a patient may have more than one type of block.

Heart Blocks may originate in the SA Node, the AV Node, the His Bundle, the Bundle Branches, or in one of the two divisions of the Left Bundle Branch (Hemiblock).

An unhealthy Sinus Node (SA Node) may temporarily fail to pace for at least one cycle (Sinus Block), but then it resumes packing at the same rate and timing prior to the block.

However, a long enough pause may evoke an escape beat.

AV Block, when minimal, delays the impulse (from the atria) within the AV Node, making a longer than normal pause before stimulating the ventricles. More serious AV Blocks may totally stop some (or all) atrial stimuli from reaching the ventricles.

• 1) First Degree AV Block: The P-R interval is greater than 0.2 seconds (or one large square) on EKG.
• 2) Second Degree AV Block: The QRS complex is missing either after a series of prolonged P-R intervals or suddenly after a P interval.
• 3) Third Degree AV Block: When none of the atrial depolarizations can get to the ventricles, so the ventricles are paced independently. This is characterized by a series of P waves, followed by giant QRS complexes.

A second degree AV Block will have a P wave present before a missing QRS complex.

A Sinus Block will be missing an entire P-QRS-T cycle.

Bundle Branch Block is caused by a block (of depolarization) in the Right or in the Left Bundle Branch.

Normally the Right Bundle Branch quickly transmits the stimulus of depolarization to the right ventricle, and the Left Bundle Branch does the same to the left ventricle. This depolarization stimulus is transmitted to both ventricles at the same time (i.e., simultaneously).

In Bundle Branch Block one ventricle depolarizes slightly later than the other, causing two joined QRS complexes, also called "Bunny Ears."

The individual depolarization of each ventricle is still of normal duration. Because the ventricles do not fire simultaneously it produces a widened QRS appearance that we see on the EKG. The two out of sync QRS's are super-imposed on each other and it produces a widened QRS. The two peaks on this widened QRS, if visible, are named R and R.' (R' represents the late firing ventricle).

In Bundle Branch Block, the QRS is 3 small square or 0.12 seconds or greater.

In Left Bundle Branch Block, the left ventricle depolarizes late. In Right Bundle Branch Block, the right ventricle depolarizes late.

If there is a R, R' in Chest Leads of V₁ or V₂, then there is Right Bundle Branch Block. (Recall the anatomical position of the Chest leads).

If there is an R, R' in the left chest leads V₅ and V₆, then Left Bundle Branch Block is present and the R' represents delayed depolarization of the left ventricle.

Note: Blocks of either the two subdivisions of the Left Bundle Branch are referred to as Hemiblocks.

With Left Bundle Branch Block the left ventricle fires late, so the first portion of the QRS complex represents right ventricular activity. Therefore we cannot identify Q waves originating from the left ventricle, which signify infarction because they will be buried in the widened QRS complex.

AV Block

Bundle Branch Block

Axis refers to the direction of the movement of depolarization, which spreads throughout the heart to stimulate the myocardium to contract.

If the heart is displaced (for example, it could be "vertical" or "horizontal"), then the direction of depolarization is also displaced.

We can demonstrate the general direction of the movement of depolarization by using a vector (an arrow). The vector shows the directions in which depolarization is moving.

The "tail" of the arrow or vector (not the "head") is always the AV Node.

Remember that a vector represents both direction and magnitude of depolarization. Therefore, bigger vectors represent greater magnitude.

Recall that depolarization is an advancing wave of Na⁺ ions. The mean QRS Vector normally points downward and to the patient's left because this is the general direction of ventricular depolarization.

For our purposes, the answer is yes....depolarization and contraction can be said to occur at the same time. (In reality, however, the contraction lasts a little longer).

Once depolarization is beyond the AV Node, the ventricular conductions system conducts this stimulus to the ventricles with great speed. In this way, ventricular depolarization begins at the endocardial lining of the ventricles and proceeds through the thickness of the ventricular wall in all areas at about the same time.

The Purkinje fibers transmit depolarization to the myocardial cells just beneath the endocardium that lines both ventricles; this occurs so fast that depolarization begins at the general level of the endocardium in all areas at about the same time.

Depolarization of the ventricles generally proceeds from the endocardial lining to the outside (epicardial) surface through the full thickness of the ventricular wall in all areas at once.

In myocardial infarction there is necrotic (dead) area of the heart that has lost its coronary (the heart's own blood) supply and does not depolarize.

The unopposed vectors from the other side draw the Mean QRS Vector away from the infarct. In other words, the infarcted area cannot depolarize, and therefore it has no vectors because it is electrically dead. Since there is no depolarization (and no vectors) in the infarcted area, the vectors from the opposite side are unopposed, so the Mean QRS Vector tends to point away from the infarct.

• Range of Normal Axis: QRS Vector points downward and to the patient's left (in the 0 to +90 degree range).
• Ventricular Hypertrophy: QRS Vector points toward ventricular hypertrophy.
• Myocardial Infarction: QRS Vector points away from myocardial infarction.

Employ these tools whenever you examine a 12 lead EKG.

To determine the Axis, we need to examine two leads in particular: Lead I and Lead AVF.

• Steps to determine Axis from Lead I:
• 1) Visualize a large sphere surrounding the heart. The AV Node is the center of the sphere.
• 2) Examine Lead I (Recall it is a horizontal plane through the heart with a negative right side and a positive left side).
• 3) Recall that an advancing wave of depolarization may be considered a moving wave of positive charges. Therefore, when "like goes to like," or when positive charges go towards the positive end of the sphere, you will see an upward deflection on the EKG.
• 4) If the QRS complex is positive in Lead I, the Mean QRS Vector is pointing somewhere into the patient's left half (the positive side). If the QRS complex is negative, the QRS Vector is pointing somewhere towards the right half (the negative side).

• Steps to determine Axis from Lead AVF:
• 1) Visualize a large sphere surrounding the heart. The AV Node is the center of the sphere.
• 2) Examine Lead AVF (Recall it is a vertical line through the heart with a positive top half and a negative lower half).
• 3) Recall that an advancing wave of depolarization may be considered a moving wave of positive charges. Therefore, when "like goes to like," or when positive charges go towards the positive end of the sphere, you will see an upward deflection on the EKG.
• 4) If the QRS complex is positive in Lead AVF, the Mean QRS Vector is pointing somewhere into the patient's bottom half (the positive side). If the QRS complex is negative, the QRS Vector is pointing somewhere towards the upper half (the negative side).

• Normal Axis
• Lead I: QRS will be positive
• Lead AVF: QRS will be positive
• *two thumbs up!

• Right Axis Deviation
• Lead I: QRS will be negative
• Lead AVF: QRS will be positive

• Left Axis Deviation
• Lead I: QRS will be positive
• Lead AVF: QRS will be negative

• Extreme Right Axis Deviation (rare)
• Lead I: QRS will be negative
• Lead AVF: QRS will be negative

• Lead I: QRS will be positive
• Lead AVF: QRS will be positive

Therefore, two thumbs up from both leads means we have a normal axis.

Axis Rotation can be found by examining the 6 chest leads.

If there is rightward rotation of the heart, the "normal" QRS complex will be favored to V₁ (the chest lead on the right side of the chest).

If there is leftward rotation of the heart, the "normal" QRS complex will be favored to V₆ (the chest lead on the left side of the chest).

Otherwise, in a normal axis, the "normal" QRS complex will be in the middle around V₃ and V₄.

Axis deviation can be found from the frontal plane (Leads I, II, III, AVF, AVR, AVL)

Axis rotation can be found from the horizontal plane (Chest Leads V₁ through V₆)

Hypertrophy is an anatomical enlargement. Hypertrophy can effect the direction of depolarization, just as the axis can. For example, with hypertrophy (enlargement) of one ventricle, the greater depolarization activity of the hypertrophied side displaces the mean QRS vector toward the hypertrophied side. In other words, the Mean QRS Vector deviates toward the ventricle that is hypertrophied (enlarged).

Hypertrophy of a chamber of the heart means that the muscular wall of that chamber has dilated and thickened beyond normal thickness.

Since the P wave represents the depolarization and contraction of both atria, we examine the P wave for evidence of atrial enlargement.

With Atrial hypertrophy, the P wave is usually diphasic (both positive and negative).

Lead V₁ is directly over the atria, so the P wave in V₁ is the best source to check for atrial enlargement.

• Ischemia
• Injury
• Necrosis

Note: Not all three of these need to be present to diagnose a myocardial infarction.

Ischemia (decreased blood supply) is characterized by inverted T waves.

Injury indicates the acuteness of an infarct. Elevation of the ST segment denotes injury and is sometimes called the "current of injury."

If there is ST elevation, this indicates that the infarction is acute. ST elevation alone can indicate an infarction.

Any significant ST depression indicates compromised coronary blood flow until proven otherwise. Therefore, a flat ST depression is another indication of infarction.

The Q wave indicates necrosis and makes the diagnosis of infarction. For instance, the diagnosis of myocardial infarction is usually based on the presence of significant Q waves produced by an area of necrosis in the wall of the left ventricle.

A significant Q wave is at least one small square wide (0.04 seconds) or one third of the entire QRS amplitude.

Note you can scan all 12 leads EXCEPT Lead AVR to check for significant Q waves.