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Elementary ECG (aka EKG)




What is the ECG?

The ECG is a medical device capable of recording the electrical activitity of the heart from electrodes placed on the skin in specific locations. Modern ECG's utilize 12 leads which will be described later in detail. Please note that the ECG in no way indicates contraction of any of the chambers of the heart as they can only be implied by the electrical activity of the heart.

What is the ECG used for in medicine today?

The ECG today is used to monitor the electrical workings of the heart. Physicians use this information to discover such things as heart rate, arrhythmias, myocardial infarctions, atrial enlargements, ventricular hypertrophies, and bundle branch blocks. Although it is not the purpose of this lesson to teach about these various abnormalities, a brief description of these will be described as the various ECG's are shown later.

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  1. Normal Conduction Pathway
  2. Recording of the ECG
  3. Vectors used in Determining Direction of Deflection
  4. Standards conventions when reading an ECG
  5. Elements of the ECG
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Normal conduction pathway:

SA node --> atrial muscle --> AV node --> bundle of His --> Left and Right Bundle Branches --> ventricular muscle

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Recording of the ECG:

Leads used:

Modern ECG's utilize 12 leads which are composed of 6 limb leads and 6 precordial leads.

(The lower case "a" in this notation refers to "augmented" in the sense that the person who develeped the augmented leads discovered that he had to augment or amplify the voltage in the EKG machine to get a tracing that would be of similar magnitude as leads I, II, and III.)

Furthermore each of the leads are bipolar in the sense that it requires two sensors which are on the skin to make a lead. If one connects a line between two sensors, one has a vector with the positive end being at one electrode and negative at the other.

The augmented leads utilize two electodes for a negative pole and one electrode to form the positive pole. The positioning for leads I, II, and III were first given by Einthoven as shown by this equalateral triangle called Einthoven's Triangle.

This diagram can be converted to the following (its utility will be seen later):

The augmented leads use one of the electrodes (i.e., either left foot, right arm, or left arm) as positive and the other two electrodes as negative (or common ground). This is shown as:

 Lead      Sensor used as Positive  
     Sensors used as negative
------------------   ----------------------------    
  aVL             Left Arm           L.
Foot and R. Arm
    aVR             Right Arm               L.
Foot and L. Arm
    aVF             Left Foot               L. Arm
and R. Arm

The resulting vectors appear as follows:

When the two vector types (i.e., the one generated with Leads I, II, and III and the one generated with aVL, aVR, and aVF) are placed on top of each other, the following is produced:

The lead markings indicate the positive pole of the lead. The significance of this will be seen later when trying to determine the axis of ECG's.

These measure the amplitude of cardiac electrical current in an anterior-posterior aspect with regard to the heart as opposed to the chest leads which record in the coronal plane. The following diagram shows the placement of these leads:

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Vectors used in Determining Direction of Deflection

The principle behind the way the ECG records the electrical impulse is quite simple. When the overall electrical current of the heart goes towards a particular lead, it registers a positive deflection. Those that go away from the lead register a negative deflection. Those which are at 90 degrees or perpendicular to the vector of the lead registers 0, i.e. is seen as an isoelectric line.

Those which are not in the direction of the vector but slightly off, e.g. at 60 degrees to the direction of the vector, then the amplitude of the deflection will be decreased.

               Example 1                         Example

        Lead I would read 1.              Lead I would show no

               Example 3            
              Example 4

   Lead I would read between 0 and 1.            Lead I would read

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Standards conventions when reading an ECG:

The rate of paper (i.e. of recording of the ECG) is 25 mm/s which results in:

The voltage recorded from the leads is also standardized on the paper where 1 mm = 0.1 mV (or between each individual block vertically) This results in:

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Elements of the ECG:

The individual components:

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Getting the Basics from the ECG

The basic things to read from the ECG is the following:

  1. Rate
  2. Rhythm
  3. Axis
  4. P wave morphology
  5. PR interval
  6. QRS complex morphology
  7. ST segment morphology
  8. T wave morphology
  9. U wave morphology
  10. QTc interval
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The basic way to interpret the rate is quite simple. You take the duration between two identical points of consecutive ECG waveforms such as the R-R duration. Take this duration and divide it into 60. The resulting equation would be:

Rate = 60/(R-R interval)

A quicker way to obtain an approximate rate is to go by the number of 5 small boxes (i.e., the size of one big box or the duration of 0.2 secs) that are in between the two identical points. For example, if the two points were 1 big box away, then the rate is approx 300 beats/min. The rest of the sequence would be as follows.

As is well known, rates <60 bpm represent bradycardia and those >100 represent tachycardia. Whether they are sinus is dependent on other factors.

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Rhythm can be quite variable. Those that are possible include normal sinus rhythm (NSR), regular, and irregular.

NSR: indicates that the rate is between 60 and 100, inclusive, and that the P waves are identifiable and are of the same morphology throughout. However, NSR also can be irregular in the sense that the beat to beat rate can be slightly different. This is termed sinus arrhythmia, and it is due to the variation of heart rate with inspiratory (accelerated) and expiratory (slowing) phases of breathing. An example follows:

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Determining the axis is dependent on understanding the angles that each of the chest leads make in relation to each other. This diagram was shown before and can be seen again here:

When trying to determine the axis, the leads that are important are the limb leads. The basic principle behind finding the axis is that you must look for the lead whose QRS complex is closest to zero or equiphasic (i.e., when the areas under each component of the QRS are added up (with those above baseline being positive and that below baseline negative, you get an approximate negative or positive value or zero).

This axis represents the lead that is perpendicular to the main axis of the ECG (remember this from the vector discussion above). Then to find the lead which is closest to the axis of the ECG, it is necessary to know how the leads are in relation to each other. This was given in the picture immediately above.

With this lead (i.e. the lead that was perpendicular to the lead which was equiphasic), you have a general idea of what the axis is. Each lead has a positive and negative pole, and therefore after you look at this lead on the ECG and determine whether the QRS complex is overall negative or positive, you need simply look at this diagram to determine what the axis is (corresponding with the positive or negative pole of that lead).

One note with this method must be mentioned. Since it will be unusual that any of the lead's QRS complex will equal zero, it is only necessary to know which is closest to zero. Since it isn't equiphasic, that means that this lead is not perfectly perpendicular to the axis of the ECG. This will be helpful in that after you do the other steps of finding which lead is perpendicular to the "equiphasic lead", you use the original equiphasic lead to "skew" the axis toward it by 10 degrees.

An Example is necessary:

In this example, it is evident that aVF is the most equiphasic, however it is slightly positive. The lead which is perpendicular to avF is Lead I. Lead I's QRS is definitely positive meaning that the ECG's axis is close to 0 degrees (as can be seen on the picture with the axis). Now, remember that aVF was slightly positive. This "skews" the axis towards aVF's positive pole (i.e., clockwise) to a final axis determination of 10 degrees.

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P Wave Morphology

The P wave in general should not be more than 1 box wide or 1 box tall. If it exceeds these, it generally means that either or both atria is enlarged (hypertrophied).
The best lead to look at the P wave is V1. In lead V1, the following characteristics indicate pathology:
  1. Positive deflection greater than 1 box wide or 1 box in height --> right atrial hypertrophy
  2. Total P wave duration greater than 0.12 sec and negative deflection of P wave greater than 1 box wide or 1 box in depth --> left atrial hypertophy

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PR Interval

The PR interval indicates AV conduction time which is normally between 0.12 - 0.20 msec (3 - 5 boxes wide). If the PR interval is greater than 0.2 sec, then an AV block is present. There are several types of AV blocks:

A PR interval that is <0.12 sec (when associated with a prolonged QRS) should prompt evaluation for Wolff-Parkinson-White Syndrome (WPW).

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QRS Morphology

The QRS complex can be quite difficult to interpret. However, stepwise evaluation will make it easier. The things to consider are the following:

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ST Segment Morphology

The ST segment is important because it can show whether ischemia or infarct is present. In general, an ST segment depression indicates ischemia while elevation generally indicates infarction. When examining the ST segment, evaluate elevations or depressions 0.06 seconds after the J point (since the ST segment can at times be sloping). The location of the ST elevations on the ECG can help to identify a location of the infarct:

ST depression in V1 and V2 associated with large R waves may indicate Posterior Wall infarction (corresponding with Right Coronary Artery).

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T Wave Morphology

In general, T waves are in the same direction as the largest deflection of the QRS (normally the R wave). The following pathology can be associated with T wave changes:

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U Wave Morphology

The presence of U waves may indicate hypokalemia. See T waves also.

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QTc Interval

May be lengthened in the following: May be shortened in hypocalcemia.

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Other ECG Related Sites on the Net

EKG File Room
EKG Interpretation
Blanchard Valley's Virtual Classroom

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For comments: thomas.schimming@epfl.ch, Last updated: October 29, 1999