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Bài 1 (Giới Thiệu)

Warning : Use the following information at your own risk.  While accuracy is one my goals, there is always the possibility that some of the information could be wrong.  There could be typos.  I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within.  Remember, this is just some page on the internet.  (If you do find errors, please notify me by feedback.)




This set of pages is intended to help explain concepts of basic ECG interpretation.  If you are looking for advanced ECG topics or something from an authoritative source, you should probably look elsewhere. 

Electrocardiography is the practice of measuring and recording the electricity in the heart.  ECG stands for two different words : electrocardiogram andelectrocardiograph.  The abbreviation ECG is also interchangeable with EKG.  (EKG was the abbreviation for people who spelled "cardio" with a 'k'.)

Why should you care about the electrical activity in the heart?  Because dysrhythmias are common and often have important real-life consequences that are evident outside the hospital.

bullet When someone "drops dead", people often mistakenly attribute it to a heart attack.  Most of the times when a heart suddenly stops working, it is due to an arrhythmia known as ventricular fibrillation.  This may or may not have been caused by a heart attack. 
bullet If you have had the sensation of your heart "skipping a beat", it was likely due to what is called a premature complex.
bullet People who feel their heart suddenly start to "race" may be experiencing a paroxysmal supraventricular tachycardia
bullet Fainting is often due to one's brain telling his heart to slow down, causing a bradycardia
bullet The elderly often have an irregular pulse.  This is usually due to an arrhythmia called atrial fibrillation.


Do not be intimidated by the terminology.  Scientists and doctors often derive new  words from Greek and Latin rather than use simple words.  This is not done to make themselves seem smarter.  By using words that are not part of the public's everyday language,  they hope to convey a more precise meaning.  For example,  tachy-means fast and -cardia refers to heart.  If you wanted to create a word that defines a heart rate over 100 beats/min, it would be easier to gain acceptance with tachycardiathan with "fast-heart."  Also, the use of words derived from Greek and Latin allows scientists who speak different languages to use the same (or similar) terminology.

It's rare to see anything stated in its most simple English form. 


ECG : electrocardiogram

ECG is short for electrocardiogram.    Electro- refers to electricity, -cardio- refers to the heart, and  -gram refers to a recording.  Therefore, the electrocardiogram is a recording of the heart's electrical activity. 

Figure 1-1 : The classic lead II ECG rhythm




The term arrhythmia is more or less the same thing as dysrhythmia.  They both are used to signify any abnormal heart rhythm.  Is abnormal necessarily bad?  No.  It depends on the patient.  Most consider the normal range of resting heart rates to fall between 60 and 100 beats per minute (bpm).  Anything below 60 we consider too slow- this is called bradycardia.  When the heart rate is over 100, it is called tachycardia


ECG : electrocardiograph


   ECG (and EKG) is also used as an abbreviation for an electrocardiograph.  This usually refers to the machine that produces electrocardiograms.

It is rare to find a machine that is simply an electrocardiograph- most machines can do a number of things.  Portable ECG machines often double as defibrillators.  (A defibrillator is the machine with the paddles that famous television actors are always pressing against the chests of unknown actors and screaming "CLEAR!".  Yes, this happens in real life but it always seems to be less dramatic.)  These machines contain a electronic display screen for reading live (what I call "dynamic")  ECGs as well as a miniature printer for printing the rhythm out on the standard ECG graph paper.  Some possess the ability to fax the ECGs to the hospital.  Many machines also have the ability to act as transcutaneous ("across the skin") pacemakers.   


Einthoven's leads


In the late 1800's and the early 1900's, a man named Willem Einthoven pioneered the practice of electrocardiography.  He developed a machine that was sensitive enough to reliably measure electrical differences between two different parts of the body.  Plotting the values measured over time gave a picture of the electrical activity of the human heart.

To obtain a recording of the heart's electrical activity, the patient (or researcher) would stick a limb into a large jar filled with salt water.  He would then put another limb into a separate jar.  The machine that was connected to both jars would measure the electrical differences between the two limbs.  Einthoven and his contemporaries experimented with a variety of combinations of limbs and other regions of the body.  They found that putting the right hand in one jar and the left hand in the other would give a different result from a different combination (e.g. the right hand and the left leg).  Each of these combinations represented a different "view" of the heart's electrical activity.  They called each combination a lead, and noted that some leads gave a better view of the heart than other.  The scientists experimented with the different combinations of limbs and body parts and, after what probably resembled a high-risk Hokey-Pokey, Einthoven established three standard "limb leads."   

Lead nameNegative electrode (i.e. "start")Positive electrode (i.e. "stop")
Lead I Right arm Left arm
Lead II Right arm Left leg
Lead III Left arm Left leg

Fortunately, they no longer use jars filled with salt water for ECGs.  Today, we connect the electrocardiograph to the patient by small electrodes, the little button-like devices that stick to the skin.

ECG beginners are often confused over the use of the word lead.  Lead (rhymes with speed) in electrocardiography actually has two meanings.

  1. The "view" through which you are measuring the heart.  This was the definition that originally specified which two limbs were being used. 
  2. The wires that connect the patient to the machine.  I will try not to use lead in this sense.  Instead, I will probably use the words cable or wire.


Let's look at the first definition.  What do I mean by "view"?  The movement of electricity is three dimensional, but the ECG machine only measures the electricity in one dimension.  Imagine drawing a picture of someone on a bike.  The picture would differ depending on whether you draw it from the side, from in front, from behind, from above, etc.  The shape of an ECG also depends on what view is being used, i.e. where the electrodes are attached to the body. Only the parts of voltage that are parallel to the line of view are measured. 



3 Lead ECG


Most simple ECG machines are capable of recording at least three leads.  The three common leads are called the limb leads.  Perhaps the most commonly used lead isLead II.  Lead II is the "view" from the patient's right arm to his left leg.  It is often times the best lead to use for interpreting the heart's rhythm.  Unless otherwise stated, all of the ECGs on this site will be Lead II. 

The three limb leads are often shown by a diagram called "Einthoven's triangle."  Figure 1-2 shows a representation of this.  Often times, the electrodes are not placed on the limb but rather on almost-equivalent parts of the torso.  In the diagram, you can see the right arm electrode placed instead on the right shoulder.  The other two electrodes are placed in their almost-equivalent torso regions as well.  (Note that the color of the cables in the diagram is the common U.S. color standard.  Europe uses a different system of color-coding.)       


Figure 1-2 : Einthoven's triangle represents the three limb leads.

12 Lead ECG

The term lead in "a 12 lead ECG" refers to the "view" meaning of the word.  The typical 12-lead ECG setup can show 12 different leads but only has 10 wires. 

We won't go very much into 12 lead ECG interpretation.  The basics of it is that it contains the three that are common with the basic ECG machine : I, II, III.  It also includes three more leads that are similar to the limb leads : aVR, aVL, and aVF.  It adds 6 leads on to this : V1, V2, V3, V4, V5, and V6. 

The electrocardiograms produced by a 12 lead machine are used for more than just simple rhythm interpretation; they are also used to help diagnose a variety of conditions.  For this reason, electrodes must be placed exactly as specified.  An electrode placed between the wrong ribs can result in a completely different diagnosis.  Different hospitals/regions may have different protocols for electrode placement; always defer to your local guidelines. 



Figure 1-3 : A 12 lead ECG printout - my heart's electrical activity from 12 different points of view.


In figure 1-3, compare leads II and III.  Although they both represent the exact same electrical activity, they are from different points of view.  In most cases as well as this one, lead II gives the best overall view for interpreting arrhythmias.    


Twelve lead ECG machines usually have some type of software that will attempt to interpret the data and diagnose the problem.  One should not rely on these results as they are often inaccurate.  When compared with humans, computers are superior in their ability to perform complex calculations.  On the other hand, a computer's ability to sense basic patterns usually falls behind that of humans.  Consider a computer and the average six-year-old child.  Let them both analyze two different photographs : one of a puppy and one of a kitten.  The child will easily pick out which one is which.  The computer, on the other hand, would need to have been programmed with an incredibly complicated algorithm for mapping out the different characteristics of puppies and kittens as they appear in different breeds and through different viewing angles.  Although algorithms for ECG interpretation are probably much easier to program than the one previously mentioned, they are not yet competitive with humans.  This, of course, will probably change in the near future.