12 Lead EKG Interpretation Part #2

Understanding 12 Lead EKG: Basic Cardiac Arrhythmias

In the second part of 12 lead EKG interpretation, we will look at simple heart arrhythmias that can be detected by 12 lead EKG or just a primary lead EKG strip. By basic I mean those rhythms that you can look at only one primary lead (usually lead 2) and tell what it is. There are other EKG abnormalities that needs you to look at more than one lead, the position of the heart (also called axis), specific segments, amplitude and voltage of the leads. But don’t let this make you run away from 12 lead EKG interpretation because it is quintessential you understand it. If reading this can make you save only 1 life, then it is all worth understanding 12 lead EKG interpretations. Needless to say, more lessons are coming about: –

  1. Understanding 12 ECG/EKG lead placement
  2. Understanding the circulation of the heart and which arteries when blocked caused AMI
  3. Understanding which leads on EKG are positive, negative or neutral in polarity.
  4. Understanding the normal axis of the heart and what deviation from normal axis mean (Have you ever seen some numbers at the top of EKG graph and you wonder what they mean? Just understanding those numbers points you to the problem area and when you look at specific waves, you are just confirming. This is a key to successful 12 lead EKG interpretation

The following are the common arrhythmias of the heart that you probably already know about. They are straight forward to diagnose and most of them has no emergent attention command

    1. Sinus Bradycardia:AKA Sinus Brady is a regular heart rhythm with less than 60 beats per minute. It should be noted that sinus bradycardia has regular beats. There must be P waves before every QRS at a ratio of 1:1, meaning per every 1 P wave, there is 1 QRS complex. The distance between P wave and the beginning of QRS complex (PR interval) must be between 0.12-0.20 seconds. Each small box in EKG measures 0.04 seconds, meaning a normal PR interval has 3-5 small boxes.  Got it? Are you sure? Soon we will find out because if you see a rhythm of 45 and call it sinus bradycardia before measuring PR interval, you could be misdiagnosing with junctional rhythms.
    2. Sinus tachycardia:AKA Sinus Tach is a regular rhythm with more than 100 beats is a minute but less than 150 beats. Why less than 150? If a rhythm has more than 150 beats, it is not possible to see the P-Wave as the beats succession is so rapid. This is what is called supraventricular tachycardia and recently renamed as narrow complex tachycardia. We will look into this soon. Sinus tachycardia must have a normal P-Wave before each QRS complex and PR interval must be the same. The distance from one R wave (top of QRS) to the other must be the same (R-R). If R-R wave is not regular, you might be looking at atrial fibrillation AKA A-Fib. Are you feeling like you might have misdiagnosed sinus Tachycardia already? Most likely not but keep reading
    3. Sinus Arrhythmia: What? Never heard of that! Sinus arrhythmia is an irregular heart rhythm usually seen in ICU or telemetry patients on continuous monitoring. The heart rate is variable depending on inspiration or expiration. With inspiration, the heart rate increases and with expiration, the heart rate decreases. All other parameters are normal like 1:1 ratio of P-wave and QRS complex, PR interval of 0.12-0.20 and QRS complex less than 0.10 seconds.
    4. Premature Atrial Complex (PAC):This is yet another rarely heart arrhythmia that is benign in nature. You never call a rhythm PAC but rather mention the underlying rhythm with PAC. Example, Sinus rhythm with PAC or Sinus Bradycardia with PAC’s. So what is it? There is an ectopic beat once in a while. The ectopic beat originates from atrial heart cells firing before they are supposed to. Except the ectopic beats, all other beats are normal with normal measurements. Patients may complain of feeling as if the heart stopped for a beat, something also reported in A-Fib
    5. Atrial Flutter: The so called saw tooth wave form is one of the easiest for many of us to diagnose. The atrial beats (firing) could be as many as 230-350 times in a minute but ventricular conduction may be normal (60-100 beats). These saw tooth like waves are called U-Waves. Aha! U-Waves. QRS is normal in morphology and measurements. Of course, there are no P-Waves as they have been replaced by U-waves.
    6. Atrial Fibrillation: A-fib is a silent killer FYI due to clots formation in the right atria. Nothing about A-Fib is regular. The rhythm is irregular meaning that there are variable R-R wave distances between each beat. The baseline may look like a wavy line with irregular U-Waves. It is an irregular irregularity in simpler terms. There are no P-waves but U waves. QRS is upright and normal in measurements (<0.1 second).
    7. Supra Ventricular Tachycardia (SVT) also called narrow complex tachycardia (NCT).This is a super fast sinus tachy is simpler terms. The heart is so fast, between 150-250 beats a minute. Do you know how it is treated? Ever heard of adenosine?

  1. Junctional Rhythms:There are 3 types of junctional rhythms namely Junctional escape(40-60 beats a minute, less than normal), accelerated junctional rhythm (61-99 beats, normal heart beats a minute) and junctional tachycardia (100-140 beats a minute). How do you know if it is a junctional rhythm?
    1. The heart rate is regular, meaning R-R is regular
    2. The P-wave is inverted instead of facing upright OR
    3. There is no P-Wave in front of every QRS complex
    4. The PR interval is larger than 1.20 seconds
  2. Ventricular Tachycardia:V-Tach (Code blue) is a life threatening emergency rhythm that must be intervened immediately or sudden death will occur. This monster is between 100-250 beats a minute with no P-waves. There is no QRS complex, meaning that the heart is NOT perfusing itself or other organs and cardiopulmonary resuscitation (CPR) is warranted. The waves in this rhythm are through to be bizarre looking QRS complexes. If you see this rhythm (monitor alarms will scream at you even before you see it if you have your alarms set and armed), immediately call for help, assess ABC’s and start CPR. Remember, this is a NON PERFUSINGrhythm and CPR is warranted unless the patient is awake and alert. Guess what, the patient cannot be awake and alert if they are in true V-Tach. If the monitor shows V-Tach and you see them moving, it could be a false rhythm caused by insecure leads or movement. If it is a true V-Tach, they will be unresponsive and you must start CPR NOW! Do you hear me? It is also a shockable rhythm per ACLS algorithms.
  3. Ventricular Fibrillation: Call Code blue and start CPR ASAP. Just like V-Tach, V-Fib is a NON-PERFUSING Rhythm. You cannot see any P-Waves, no heart rate, no QRS complex, just a wavy line.

In one of the future articles, I will talk about other rhythms that you may never see easily but can tell you that a life threatening emergency is likely. Example, Torsades de Pointes, which may be seen during long resuscitation due to low magnesium or calcium level. It can also be caused by Haldol IV-Push. (Always give haldol as I.M. and never as IV although you might say… we do this all the time and even doctors order it as I.V. Consider yourself warned!

In the next article, we will look at heart blocks, how to recognize them easily by just checking key areas and then we will go back to interpreting 12 Lead EKG. After heart blocks, we will look at how 12 Lead EKG leads are placed, what they mean, what side of the heart are they are looking at, and what lead correspond to what side of the heart. We will also look at the axes of the heart because understanding axes can accelerate your 12 lead EKG interpretation.


By replying here with comments, describe in as many words as you can: –

  1. A life threatening emergency you have ever taken part in. Tell us what happened and what was done to save the patient. Did the patient survive? Do not mention names or locations.
  2. Have you ever been in a code blue and how did it go? Did you participate? CPR? How did that go?
  3. There are other rhythms that are not mentioned here. Can you identify any?
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