9 12 Lead Electrocardiogram Assessment

The three-dimensional view of the heart

If you think of the heart as a big electrical lightbulb, with electricity emanating in all directions, you may be able to see how placement of electrodes across the chest can offer us a range of views of the electrical signal. Much of 12 lead ECG interpretation is taking measurements of wave magnitude as well as the timing/duration of wave events. Take out a sharp pencil and some sharp eyes!

Basic waveform

Cardiac cycle and segments
Figure 1. The cardiac cycle and the various wave and segment measures.

Remember that the ECG paper has small boxes that are 1mm (0.1mv) in magnitude and 0.04 seconds in duration. So, by counting small boxes and converting to both magnitude and time, we can measure the characteristics of the cardiac cycle.

Rules of ECG waves

The ECG only graphs a voltage magnitude while the paper moves by, so what governs up or down in a lead? The electrical wave emanating from the heart will either travel towards the positive pole of the electrode, away or at an angle. So, the rules are:

 

  • A wave moving towards the positive pole of the lead will make positive deflection (upright wave)
  • A wave moving away from the positive pole of the lead will make a negative deflection (upside down wave)
  • A wave moving perpendicular (at a right angle) to the positive pole of the lead will make a biphasic (up and down) or flat wave.

Einthoven’s Triangle I, II, III. The original three leads. Frontal Plane View

Einthoven's triangle
Figure 2. Einthoven’s Triangle. Note positive and negative poles of Leads I, II and IIII. Electrodes placed either on the arm or chest for right arm (RA) left arm(LA) RL (right leg), which is a ground electrode) and LL (left leg).

 

Augmented leads: 3 more frontal plane views

With the advent of hardware and computer technology. Additional views were made possible by creating a null point at the center of the triangle and then measuring the voltages emanating to a point between each of the three leads. These “unipolar” (just one end has a charge) leads are known as the “augmented Voltage Leads”, and angle towards the right (aVR), left (aVL) and foot (aVF). This provides some added resolution of views, giving us 6 different leads, despite only using 4 electrodes!

augmented voltage leads
Figure 3. Augmented Voltage Leads. The center point is the “null” point, with each augmented lead having a single positive pole.

 

Precordial Leads or Chest Leads, or “V” leads. Transverse plane view wrapping around the heart

With the addition of 6 more unipolar leads across the chest, the complete 12 leads are created. The precordial, or chest leads are placed at specific points across the chest (see chapter 3 for information electrode placement).  All the electrodes are the positive pole of the circuit, so as the electrical depolarization wave travels out from the body, it may be traveling away from lead V1 (so V1 may be upside down) but traveling directly towards V5 (so it may be upright). So, you will see a range of looks to the ECG as you scan across V1 through V6.

Mason Likar electrode configuration
Figure 4. Standard (Mason Likar) ECG Configuration for Cardiac Stress Testing. Note the limb leads moved onto the torso. Chest leads are placed anterior (V1-V3) and then wrap around the lateral side (V4-V6) of the heart.

 

“Geography” of the 12 lead ECG

When you first look at a 12 lead ECG it can be a bit confusing. So many different looking waves. The first electrocardiographs were machines that rolled paper out and printed 3 leads at a time, before switching to the next 3 leads, and so on until all 12 leads were printed. This convention still holds for today, despite digital technology. So, if you look at the 12 Lead from left to right you will see Leads I, II and II are printed first. Keep in mind that the heart rhythm is continuous, so the first cardiac cycle you see in Lead I is the SAME cycle in lead II and II. It is just a view from a different perspective. The blue lines in the figure represent the switch to the next 3 leads (aVR, aVL, aVF) followed by the V leads. You may notice in some leads, waves are upside down, in others the P wave may be difficult to see and some measurements we take may be difficult.  Most often, Lead II is the clearest lead to use for many of the measurements you will learn in this chapter. This is because lead II is essentially the sum of lead I + II, so it gives us the largest waves to assess.  However, it may be the case that lead II is difficult to see. In that situation, simply use another lead.

Breakdown of area of the 12 lead
Figure 5. Blue lines demarcate the separation as the system graphs the leads three at a time. Note how each lead looks slightly different.

 

Location of heart according to ECG lead

Based on the positioning of the leads, we can obtain a three-dimensional view of the heart. If there is an issue in a particular aspect of the heart, it may only show up on the ECG in the leads that are facing that aspect. As such, we can gain information on the anatomy affected when a particular lead is indicating an issue. See the table below, it will come in handy!

Table 1. Heart Locations According to Leads

MI Leads Artery Side of Heart
Inferior II, III, aVF Right Coronary Right
Septal V1, V2 LAD Left
Anterior V3, V4 LAD Left
Lateral I, aVL, V5, V6 Left Circumflex Left

 

13 Points of Analysis of the 12 lead

These steps by themselves do not interpret the ECG, however they function as a checklist of measure to help determine anomalies with the ECG. Not all ECG anomalies can be determined, but these are the basic starting point of an otherwise extensive and complex process of interpretation. Let’s get our foundation started!

  1. Standardization. The ECG must be properly calibrated to that measures of magnitude (height) are accurate. Standardization is set at 10mm (10 small boxes or 2 large boxes). Paper speed is set at 25mm/s (1500mm/min)
standardization
Figure 6. Standardization. Do you see the rectangular, 10mm standardization mark on the left side of the image?

2. Heart Rate. There will actually be two heart rates to calculate. Atrial rate will be calculated using p waves and ventricular rate using the QRS complex. Try measuring in a lead where the p wave is more easily seen (i.e lead II). Often the 12 lead will provide a Lead II rhythm strip across the bottom of the ECG for this measure. For now, just use the figure above.

Atrial rate = ______  Ventricular Rate = _________

 

3. Rhythm. Determine whether it is a sinus rhythm (sinus bradycardia, NSR, sinus tach) or whether there is another type of rhythm present. Is there a p wave for every QRS? Are there more? Are they missing? Again, use the figure above.

Is it:   Sinus Bradycardia                 Normal sinus Rhythm                     Sinus Tachycardia

4. PR Interval. Measure the time from the beginning of the p wave to the beginning of the QRS. 0.12 – 0.20s is normal. (remember each small box is 0.04 sec so the interval should be 3-5 boxes). Time in seconds

PR interval
Figure 7. PR interval
PR interval = (#boxes x .04= ______s)

5. P wave size. Normally the P wave should not exceed 2.5mm in height and is usually less than 3 mm (.12s) wide in all leads. Tall or peaked P waves may be a sign of right atrial enlargement (RAE). Wide P waves may indicate left atrial enlargement (LAE). Height in mm, Time in seconds

P wave morphologies
Figure 8. P wave shapes or morphologies.

 

P wave size
Figure 9. P wave size

P wave height _____mm  P wave duration ________s

 

6. QRS width. Measure from the onset of the wave (in some cases there may not be a q wave, so from where the complex begins). Normally the QRS wave is 0.1 sec (2.5 boxes) or less in all leads. Time in seconds

QRS width
Figure 10. QRS interval.

In figure 10, measure from the beginning of the QRS to the end. In this example, the width is 2 small boxes, so 2 x 0.04= 0.08seconds.

QRS interval = _____________s

 

7. QT interval. a measure of the time between the start of the Q wave (or onset of complex) and the end of the T wave. In general, the QT interval represents electrical depolarization and repolarization of the left and right ventricles. A lengthened QT interval is a biomarker for ventricular arrhythmia risk. Normal intervals are dependent upon heart rate: Time in seconds.

QT interval
Figure 11. Identifying the QT interval. The onset is the beginning of the QRS complex and the end is the intersection of the T wave ending with the isoelectric line.

 

8. QRS Voltage.

The size of the heart muscle will influence the size or magnitude of the QRS complex. This means that both an athletic heart as well as a diseased heart could show large QRS voltage. Voltage can also be influenced by the amount of body fat (thin vs fat).

    1. Measure QRS voltage (in mm) S wave in V1 + R wave in V5
      1. Draw horizontal line at the PR interval and measure to end of wave
    2. Less than 35mm total is normal
    3. > 35mm indicative of left ventricular hypertrophy
QRS voltage
Figure 12. QRS voltage. Use ONLY the V leads. V1 and V5, you will measure the magnitude of the S wave in V1 plus the R wave in V5. Provide total voltage in mm. In this example V1 S wave = 6mm + V5 R wave= 9mm, total = 15mm.

 

9. QRS Electrical Axis. Determining the direction of the primary wave of ventricular depolarization is done using the limb leads. Note the positive and negative poles of the leads and the degrees of the axis.

QRS axis
Figure 13. Imagine the heart in the chest and a QRS depolarization wave spreading out from the chest. If we lay a grid over the chest, we can determine the angle or axis of this wave. Normal axis falls within a 0-to-90-degree range.

Steps to determine axis:

  1. Using only Leads I,II,II and aVR, aVL and aVF, find the smallest and most equiphasic lead,
  2. Identify the lead perpendicular, then is that perpendicular lead’s QRS upright or upside down?
  3. Using grid identify degrees for the depolarization wave

 

Lets try one:

QRS axis practice
Figure 14. Find the QRS axis
  1. Looking only at leads I,II,II, aVR,aVL, aVF (in figure 14), find the smallest and most equiphasic lead. Pick the best answer.
  2. Looking at the axis grid (figure 13, right image) find the lead that is perpendicular to the smallest and most equiphasic lead. Note the positive and negative poles of the lead
  3. Look at the perpendicular lead on the ECG above. Is the QRS upright (moving to positive pole) or negative?
  4. Identify degrees on the grid associated with the pole that the lead is moving towards.

Did you get +60 degrees?

10. R Wave progression. Examine the chest leads only, with leads V1-V3 facing away from the primary depolarization wave (so small r but large S wave), while V5-V6 face the path of the wave. Therefore, early V leads should be QRS inverted. Normally there should be a transition from inverted QRS complexes to a transition in V3-V4 after largest R wave, next lead is typically smaller.

R wave progression
Figure 15. R wave progression. Don’t use the limb leads (crossed out). Note that the R wave is highlighted in brown for each of the V leads. Note the increase in size of the R wave. V4 has the largest R wave in this case, so the answer would be V4 as the lead for R progression. Note how R waves after V4 get smaller.

 

11. Q waves. Normally, Q waves are so small that one can barely measure the width or depth. Q waves that are 1/3 or more the size of the QRS complex are significant. However, Q waves are normal in leads III and aVR. Large Q waves can indicate an old (healed) myocardial infarction. In leads V1 and V2, the opposite, tall R waves would be present.

normal q wave
Figure 16a. Normal Q wave.

Notice how small the Q wave is above? Many leads will not even have a Q wave, while most have very small Q waves.

old MI
Figure 16b. Old Inferior Wall Myocardial Infarct. Note the Large Q waves in leads II, III and aVF. Using Table 1 in this chapter we can see that leads II,II and aVF represent the inferior wall of the heart and right coronary artery.

 

12. ST segment. The ST segment is measured from the “J” point to a point 0.08s (2 boxes) beyond. The level of this point is then compared to the level of the PR interval (the isoelectric line).

    1. Depression of the ST segment is indicative of ischemia
    2. The slope of the depression (upsloping, horizontal, down sloping) also indicates severity
    3. Elevation of the ST segment is indicative of Acute Myocardial infarction, or pericarditis
normal j point ST segment
Figure 17. ST segment. Note isoelectric line marked (brown). Compare the height to the mark .08s past the J point. Note how they align. Normal ST segment.

 

ST segment depression
Figure 18. Various morphologies of ST segment depression. Note how ST segment at 0.8s past J point is lower than the isoelectric line (dotted).

 

ST segment elevation
Figure 19. ST segment elevation. Note ST segment is higher than the isoelectric line. Can be indicative of myocardial infarction, pericarditis, or other heart injury.

 

13. T wave. Look in leads I, II,III, aVr, AvL, Avf only. T waves are normally upright and follow the direction of the QRS complex. In limb leads the direction of the T wave depends on the electrical axis. Abnormal T waves may be indicative of ventricular hypertrophy, ischemia, hyperventilation or even electrolyte imbalances. Identify the frontal plane leads to determine whether T waves are in the same direction as QRS.

Normal T waves
Figure 20. Normal T Wave. Note the T wave is oriented in the same direction as the QRS complex.

 

T wave inversion
Figure 21. T Wave inversion. Note the T wave is NOT in the same direction as the QRS, but rather is inverted.

 

Your Turn!

Try to use the following 12 lead ECG and complete the 13 steps :

 

Practice 12 L
Figure 22. Note: Lead II rhythm strip is across the bottom.

 

  1. Standardization: ____________
  2. Atrial rate:________ Ventricular rate:_________
  3. Rhythm_________________
  4. PR interval (in seconds):________________
  5. P wave height mm   ________               width   ___seconds
  6. QRS width seconds _____________
  7. QT interval seconds ______________
  8. QRS Voltage mm ___________
  9. QRS electrical Axis degrees ______________
  10. R wave progression Lead        __________________
  11. Abnormal Q waves (if present identify leads) ___________
  12. ST segment (if abnormal identify leads) _____________
  13. T wave (if abnormal identify leads) _________________

 

Chapter Sources

Marriott’s Practical Electrocardiography. 13th ed.Strauss, D,G and Schocken, D.D. Wolters Kluwer Pub. 2021.

Goldberger’s Clinical Electrocardiography: A simplified approach 10th ed.  Goldberger A., Goldberger, Z, Shvilkin A. Elsevier pub. ISBN-10 0323824757 2023.

Clinical Exercise Electrocardiography – Levine S., Coyne B., Colvin L. Jones & Bartlett Learning Pub. ISBN-10 1284034208 2015.

Life in the Fast Lane. https://litfl.com/ecg-exam-template/

 

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