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ECG Basics: Sinus Tachycardia

Wed, 06/10/2015 - 20:53 -- Dawn

This is a good teaching strip on many levels.  At the BASIC level, we see a strip that clearly meets all the criteria for sinus tachycardia:  a regular rhythm over 100/min. with P waves that look normal and all look alike.  The rate is 110 per minute.  The PR interval is just at the upper limits of normal at .20 second, or 200 ms.  The QRS complex is within normal limits, but slightly wide at .10 seconds.

This strip is good for teaching rate determination by several different methods.  It is helpful that QRS complexes 1, 5, and 10 fall on the dark lines of the paper.

This is a Lead II rhythm strip, and it is helpful to show students that not ALL Lead II strips produce an upright QRS complex.  Of course, correct lead placement should be confirmed.  In this particular case, the patient had suffered an anterior - septal wall M.I., and has a left anterior hemiblock, also called left anterior fascicular block.  This shifts the frontal plane axis to the left, causing Lead II to have a negative QRS.  Axis can't be accurately determined from one lead, but axis shift explains the negative QRS in this strip.

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Comments

Jerry W. Jones MD FACEP's picture

Thanks, Dawn, for such a great teaching website and for making the ECG examples available to those of us who teach.

While I don't teach what sinus tachycardia is during my advanced classes (the participants are already intermediate to advanced level), I do take the time to reaquaint them with what actually happens during a mild sinus tach.

Ventricular systole occurs during repolarization which lasts from the J point to the end of the T wave (NOT during the QRS complex which occurs too rapidly for ventricular contraction). As the heart rate speeds up, ventricular systole moves closer and closer to atrial systole which occurs shortly after the onset of the P wave. The T-P interval represents diastole - the time that the ventricles are filled with blood PASSIVELY from the atria. This represents anywhere from 2/3 to 3/4 of the blood that the ventricles will pump during the next beat. Atrial systole - the contraction of the atria - adds 1/4 to 1/3 of the total ventricular load (the "atrial kick").

As the heart rate picks up, diastole becomes shorter and shorter and there is less and less time for passive ventricular filling. This can be very challenging for patients with low cardiopulmonary reserves. As the P waves get even closer to the T wave (or even within the T wave if there happens to be a little first degree AV block), the atria find themslves contracting against closed AV valves. As I tell my students, that atrial blood has to go somewhere and suddenly forward is no longer an option. Blood from the right atrium will back up into the superior and inferior vena cavae, sending waves into the liver and also into the neck veins ("cannon A waves"). Blood from the left atrium is sent back into the pulmonary circulation and the lungs. Imagine what this will do to patients who already have CHF or COPD!

Another thing to remember about sinus tachycardia is that the shorter R-R intervals decrease the refractory period of the ventricular conduction system (His-Purkinje system) but will prolong the refractory period of the AV node. If the tachycardia is due to autonomic influence ("adrenaline surge"), then the refractory period of the AV node will actually tend to decrease - offsetting its natural tendency to prolong.

So, as you can see, for some patients even a mild sinus tachycardia is NOT the benign rhythm that it is for most of us.

Since it is so extremely rare for a sinus tachycardia to be a primary dysrhythmia (there are such things as idiopathic persistent sinus tachycardia and a reentrant sinus tachycardia), treatment remains the treatment of the underlying condition that is resulting in the increased rate. This usually involves giving the patients oxygen, fluids or pain or anxiolytic meds.

Thanks again for a great teaching website.

Jerry W. Jones MD FACEP FAAEM
https://www.medicusofhouston.com
Twitter: @jwjmd

Submitted by Dawn on

Thanks, JWJ (Dr Jerry Jones, MD).  Your discussion of sinus tachycardia is presented in a way that is interesting to both beginners and advanced students.  I hope the readers of the ECG Guru who are teachers will take this as an example for their own lectures.   I envy your advanced students!

 

 

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