Axis: The Science of Direction Case Studies

Home » MEDCLIN530 » EKG » 12-Lead EKG Confidence » II. Methodology: Measurements and their clinical significance » 6. Axis: The science of direction » Axis: The Science of Direction Case Studies
Headshot of Chris Anderson, MD · Clinical Education Director, Pediatrics
Chris Anderson
MD · Clinical Education Director, Pediatrics
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Headshot of Kevin Hodges, Vice Chair, Emergency Medicine
Kevin Hodges
Vice Chair, Emergency Medicine
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Table of Contents

Case 1: How leads determine direction

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How lead I separates left from right
For this example, let’s visualize ventricular electrical systole, which is represented by the QRS. To determine and visualize whether the QRS is pointing, right or left, as well as up or down, first examine lead I. Because lead I is positive, the QRS is visualized as pointing toward the patient’s left. This is not yet a complete description or picture.
How lead AVF tells up from down (careful!)

Because the QRS in lead I was positive, the QRS was visualized as pointing to the patient’s left side. To help complete the visualization, the observer needs to next determine whether the QRS force is toward the patient’s head or feet as well. The sensor that can distinguish up from down is called lead AVF. As lead AVF is positive, the observer visualizes the QRS as pointing toward the patient’s feet, that is, inferiorly. This can seem totally counterintuitive, so go over this page very carefully!

How lead AVF tells up from down (careful!)

Because the QRS in lead I was positive, the QRS was visualized as pointing to the patient’s left side. To help complete the visualization, the observer needs to next determine whether the QRS force is toward the patient’s head or feet as well. The sensor that can distinguish up from down is called lead AVF. As lead AVF is positive, the observer visualizes the QRS as pointing toward the patient’s feet, that is, inferiorly. This can seem totally counterintuitive, so go over this page very carefully!

How combining leads I and AVF determines a quadrant

Although the information from leads I and AVF was drawn on separate diagrams for purposes of illustration, drawing them on one diagram helps construct a more complete visualization. Looking at leads I and AVF together will narrow the direction to one of the four quadrants. In this example, the QRS axis lies below line I and to the left of line AVF, or in the lower left quadrant. (Mathematically, the axis is greater than 0° and less than positive 90°.) Remember that (a) any electrical event (P wave, QRS, T wave, anything else!) that is above the baseline in lead I can be visualized as pointing to the patient’s left, and (b) any electrical event that is above the baseline in lead AVF is pointing toward the patient’s feet, that is, inferiorly (see Figure 6.63).

Visualize more precisely within a quadrant

Using lead I and lead AVF will narrow down any force to one of the four quadrants on the diagram. In this example, because leads I and AVF are positive, the observer can visualize the QRS and ventricular depolarization as grossly pointing down and to the patient’s left. For greater accuracy, the axis can be narrowed down further to a multiple of 15°, as shown in Figure 6.25. To calculate the axis to multiples of 15°, additional information is needed from the other four one-dimensional sensors: leads II, III, AVR, and AVL. With the information from these leads, the observer can narrow the visualization down to one of the arrows.

Narrow down the direction with lead III

To determine the exact direction inside a quadrant, the observer can look at a lead outside the quadrant. In this example, leads III, AVR, and AVL lie outside the lower left quadrant. Again, each of the 12 EKG leads is so primitive it can “see” only in one dimension and can sense only if an electrical force is coming toward it or going away from it. Lead III gives the observer the perspective from the patient’s lower right side.

Combine the information—Leads I, AVF, and III

Combine the directional information from the three leads (I, AVF, and III) into one diagram.

  1. Lead I is positive.
  2. Lead AVF is positive.
  3. Lead III is positive.


Using 1, 2, and 3, from above, the visualized direction is greater than +30° and less than +90°. Since we are using multiples of 15, the axis must be +45°, +60°, or +75°.

Finalize the direction with lead AVL

To narrow down the exact direction inside a quadrant, the observer can again look at a lead outside the quadrant. In this example, AVR and AVL lie outside the lower left quadrant. Again, each of the 12 EKG leads is so primitive it can “see” only in one dimension and can sense only if an electrical force is coming toward it or going away from it. Lead AVL offers the observer the perspective from the patient’s left shoulder.

Summary and conclusion—step by step

We have determined that:

Lead Which told us
Lead I is positive
the QRS direction is to the patient’s left side
Lead AVF is positive
the QRS direction is to the patient’s feet
Lead III is positive
the QRS direction was >30°
Lead AVL is positive
the QRS direction was <60°, and so QED +45!

And all that information is equivalent to saying the QRS direction is +45!

Case 2: Another example of normal

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Let the observer construct another example of normal in which lead III is isoelectric, that is, neither obviously positive or negative.
Always begin with lead I and lead AVF

In this example, the QRS in lead I is positive, and so points to the patient’s left side. The QRS is positive in lead AVF (careful!). It so points toward the lead and, therefore, downward toward the patient’s feet (see Figure 6.63).

How to handle an isoelectric lead

In this example, the positive QRS in lead I and positive QRS in lead AVF lets the observer visualize the direction as down and to the patient’s left side. (Mathematically, QRS direction is expressed as somewhere between 0° and 90°.) To narrow down the direction more precisely, look at another lead outside the lower left quadrant. Lead III or lead AVL would help. For this example, arbitrarily select lead III to examine first. The observer sees that the QRS in lead III is neither obviously positive or negative. This is called an isoelectric lead. When a lead is isoelectric, it provides a helpful and specific clue, because the true direction must be perpendicular to this lead.

Summary and conclusion—step by step

We have determined that:

Lead Which told us
Lead I is positive
the QRS direction is to the patient’s left side
Lead AVF is positive
the QRS direction is to the patient’s feet
Lead III is positive
the QRS direction was either −150° or +30°

And all that information is equivalent to just saying the QRS direction is +30! This is a normal QRS direction.

Case 3: Abnormal QRS direction in the frontal plane

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Begin at the beginning with lead I. In this example, the QRS in lead I is positive, so the QRS direction is to the patient’s left side. Mathematically, the QRS is somewhere between −90° and +90°.

Lead AVF is negative. This means the QRS direction is away from Lead AVF or upward. (Always be very careful thinking about lead AVF. It is very easy to make a careless mistake with this lead!) It is not normal for the QRS direction to be upward, so the observer needs to determine the direction more precisely. Although diagnostic possibilities include inferior infarction and left anterior hemiblock, do not worry about the diagnosis yet. These will be covered in depth in later chapters. Focus on learning to differentiate up from down and left from right (Figure 6.63)!

Combining the information from leads I and AVF, as was done in the previous examples, visualize the QRS direction to be leftward and upward.

The positive QRS in lead I and negative QRS in lead AVF let the observer visualize the direction to the patient's left side and upward. Mathematically, the QRS direction is somewhere between 0° and −90°. To narrow down the direction more precisely, look at another lead outside the left upward quadrant. Lead II or lead AVR would help. For this example, arbitrarily examine lead II. The QRS in lead II is negative. Because lead II records the QRS as negative, the QRS is going away from lead II.

Now combine the information from lead II with the information from leads I and AVF onto one diagram.

  • Using the information from lead I (a), the QRS direction is to the patient’s left side.
  • Using the information from lead AVF (b), the QRS direction is upward, superiorly, to the patient’s head.

 

(Mathematically, the direction is visualized as between 0° and −90°.) Lead II is negative, thus the observer can visualize the QRS as also pointing away from lead II. Now the direction is visualized as somewhere between −30° and −90°. The direction can be determined even more precisely by looking at another lead outside the upper left quadrant, namely lead AVR.

To improve visualization, add information from another lead outside the upper left quadrant, namely lead AVR. In this example, lead AVR is slightly more positive than negative. Therefore the QRS direction is headed toward lead AVR.

Summary and conclusion—step by step

We have determined that:

Lead Which told us
Lead I is positive
the QRS direction is leftward
Lead AVF is negative
the QRS direction is upward or superiorly
Lead II is negative
the QRS direction was <−30°
Lead AVR is positive
the QRS direction was <−60°, and so must be −75°!

And all that information is exactly equivalent to just saying the QRS direction is −75°! This QRS direction is abnormal.

Case 4: Use the frontal plane and the horizontal plane

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Examine this EKG, and visualize the QRS direction in three dimensions.

The frontal plane—step by step

We have determined that:

Lead Which told us
Lead I is positive
the QRS direction is leftward
Lead AVF is positive
the QRS direction is downward or inferiorly
Lead III is negative
the QRS direction was <+30°, and so the direction is +15°

And all that information is exactly equivalent to just saying the QRS direction is +15°! This QRS direction is normal.

Case 5: The P wave direction—Arm lead misplacement

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Always check the P wave direction on every EKG. An upside down P wave in lead I (Figures 6.64b and 6.65) suggests that the EKG was taken with the arm leads accidentally reversed on the patient’s arms. This results in a right-to-left reversal of direction of what the EKG sees, as is shown in Figure 6.65. Always check that the P wave direction is as expected to ensure that the EKG was taken correctly!

Case 6: The P wave direction—Dextrocardia

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A P direction that is rightward (positive in lead I) suggests dextrocardia, as well as possible arm lead reversal. Dextrocardia is a congenital condition in which the contents of the thorax are reversed in placement, resulting in a mirror image from normal. The difference is seen in the V leads. In arm lead reversal, the V leads appear normal. In dextrocardia, the V leads are on the opposite side of the chest from the heart. This results in the tell-tale decremental size of the QRS as one moves from lead V1 to lead V6.

Case 7: The P wave direction—Junctional rhythm

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Normally, the direction of atrial depolarization is from the sinus node to the AV node, which is from top to bottom. The observer sees this as a P wave direction that points downward and to the patient’s left side. In abnormal situations, the junction can start the rhythm and depolarize the atria from the AV node upward. This changes the atrial depolarization and P wave from downward to upward. An upward (relative to the patient) P wave direction (P wave is negative in leads II, III, and AVF) suggests junctional rhythm.

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