Elementary ECG (aka EKG)

TABLE OF CONTENTS

1. BASIC QUESTIONS
2. BASIC BACKGROUND
3. GETTING THE BASICS FROM THE ECG
4. OTHER ECG RELATED SITES ON THE NET

BASIC QUESTIONS

What is the ECG?

What is the ECG used for in medicine today?

BASIC BACKGROUND

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

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.

• Limb leads are: I, II, III, aVR, aVL, and aVF.

(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

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.

• Precordial leads are: V1, V2, V3, V4, V5, and V6.

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
2

        Lead I would read 1.              Lead I would show no
deflection

               Example 3            
              Example 4

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

Standards conventions when reading an ECG:

• 1 mm = 0.04 sec (or each individual block)
• 5 mm = 0.2 sec (or between 2 dark vertical lines)

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:

• 5 mm = 0.5 mV (or between 2 dark horizontal lines)
• 10 mm = 1.0 mv (this is how it is usually marked on the ECG's)

Elements of the ECG:

• P wave: represents depolarization of both atria
  
  
• PR interval:
  1. normal duration: 0.12-2.0 seconds (3-4 horizontal boxes). This is measured from the onset of the P wave to the onset of the QRS complex regardless if the initial wave is a Q or R wave.
  2. represents: atria to venricular conduction time (through His bundle)
  
  
• QRS complex
  1. normal duration: 0.08-0.12 seconds (2-3 horizontal boxes)
  2. naming convention:

  Q wave: first downstroke of the QRS complex. This wave is not always present. However, if there is an upward deflection (i.e. R wave) noted before a "Q wave" occurs, then that Q wave is actually an S wave because Q waves must be before any R wave by definition. A monophasic negative QRS complex is called QS.
  
  R wave: first upward deflection of the QRS complex. Upward deflections occurring after an S wave are noted by a "prime mark" such as R'
  
  S wave: the first downward deflection occurring after the R wave.
  

  Multiple variations of the QRS complex are presented here:
  
  
  

• ST segment represents: this segment is important in identifying pathology such as myocardial infarctions (elevations) and ischemia (depressions). In normal situations, it serves as the isoelectric line from which to measure the amplitudes of the other waveforms.
• J Point: this is the junction between the QRS complex and the ST segment.
• QTc interval
  

  QTc = QT + 1.75 (ventricular rate - 60)
  

  The QT interval represents the duration of activation and recovery of the ventricular muscle. This duration varies inversely with the heart rate. Since this is the case, the QT is not used, but rather the corrected QT is.
  

  The normal QTc is approx 0.41 seconds. It tends to be slightly longer for females and increases slightly with age.

• T wave represents: ventricular repolarization
• U wave (not shown)
  This upright wave, when present, follows the T wave. What it represents is not certain.

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

Rate

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.

• 1 big box = 300 beats/min (duration = 0.2 sec)
• 2 big boxes = 150 beats/min (duration = 0.4 sec)
• 3 big boxes = 100 beats/min (duration = 0.6 sec)
• 4 big boxes = 75 beats/min (duration = 0.8 sec)
• 5 big boxes = 60 beats/min (duration = 1.0 sec)

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

Axis

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:
ECG from EMBBS

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

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

PR Interval

• 1st degree AV Block: PR>0.20 sec.
• 2nd degree AV Block: 2 types:
  1. Type I (Mobitz I or Wenckeback): increasing PR interval until a QRS complex is dropped. It is usually benign.
  2. Type 2 (Mobitz II): QRS dropped without any progressive increase in PR interval (i.e., PR interval is constant but still >0.20 sec). Here's an example.
• 3rd degree AV Block: atria and ventricles are electrically dissociated. Therefore, P waves and QRS complexes will occur independent of each other. As always, use the QRS complexes to determine heart rate. Here's an example.

QRS Morphology

• Duration: should be 0.08 - 0.10 sec (2 - 2.5 boxes). If duration is prolonged, then the presence of bundle branch blocks and WPW (if PR interval is also abnormally shortened in duration).
  
• Presence of Q waves: can indicate presence of infarct if present in V1, V2, and V3. A Q wave in III and aVR is normal. A Q wave is significant if it is greater than 1 box wide or greater than 1/3 the amplitude of the QRS complex.

ST Segment Morphology

• Anterior Wall Infarct (corresponding to Left Anterior Descending Artery): V1, V2
• Lateral Wall Infarct (Circumflex Artery): V3, V4
• Inferior Wall Infarct (can be combination of Circumflex or Right Coronary Artery): V5, V6, I, aVL

T Wave Morphology

• Ischemia: when T waves are in an opposite direction (inverted), it may indicate that ischemia is present, especially when it occurs in a pattern as previously described for ST segment changes.
• Hyperkalemia: associated with tall peaked T waves, flat P waves, and wide QRS complexes
• Hypokalemia: associated with flat T waves, U waves, U waves taller than T waves

U Wave Morphology

QTc Interval

• Quinidine Toxicity
• Hypocalcemia

Other ECG Related Sites on the Net