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Dysrhythmias and the electrical system of the heart.

February 4, 2013

When you start talking about heart rhythms and EKG’s, it’s easy to get really lost.  It’s like a maze filled with abbreviations and long words, like electrocardiogram which is abbreviated ECG or EKG.  It’s shortened two ways because ECG is from the english word and EKG is from the german electrokardiogram.

An electrocardiogram is a visual rendering of the electrical activity in the heart.  The individual cells of the heart (cardiac muscle fibers) coordinate with each other by means of electrical potentials created by movement of sodium and potassium ions to opposite sides of the cell membrane.  As one cell discharges its potential, the next cell is triggered to do so and this moves from cell to cell across the heart.

12_lead_generated_sinus_rhythm

Usually 12 leads are used for an EKG.  Each lead shows the changing electrical potential from a different direction.  The “V leads” show the best representation of the ventricles (the lower chambers of the heart).  The other 6 leads (I, II, III, AVR, AVL, AVF) show activity through the vertical plane as pictured below.  Altered perfusion of the heart muscle may be seen as abnormal electrical currents on the EKG.  Also the heart’s rhythm, that is the heart beats, can be monitored by EKG.

images

I found a lovely illustration of the electrical activity of the heart on the website of the National Heart, Lung, and Blood Institute.  There is a great animation showing the movement of the electrical impulse from the sino-atrial (SA) node to the atrio-ventricular (AV) node and down through the ventricles with the  associated movement of blood through the chambers of the heart.  The different sections of the waveform shown on the EKG are named, so you may hear terms such as P wave, QRS, and ST segment.  The video on the above website also illustrates these well.


heartbeat

Here is a still picture with labels.  The P wave represents depolarization of the atria.  That’s when they squeeze.  The QRS is depolarization of the ventricles.  They are much larger and therefore have a bigger wave on the tracing.  The T wave is when the ventricles are repolarizing (recovering).  There would also be a repolarization wave for the atria, but it essentially is swallowed up in the much larger QRS.  You may hear references to some of the intervals between events such as the PR or the ST.  This is the time in milliseconds between the beginning of one wave and and the next wave.  PR refers to how long between the start of depolarization of the atria and the start of depolarization of the ventricles.  ST segment is the space between the beginning of the S wave and the beginning of the T wave.

The heart’s rhythm is the way it is beating.  The normal heartbeat, called sinus rhythm, is a regular (evenly spaced) beat at the rate of 60 to 100 beats per minute.  These beats originate at the sinus node which is the white oval in the picture above showing the P wave.  This is the natural pacemaker of the heart.  The atrio-ventricular (AV) node is at the border of the right atrium and the right ventricle, just above the tricuspid valve.  The impulse pauses briefly there and then continues down into the ventricle.  The AV node controls the timing of the ventricular contraction to keep it coordinated with that of the atria.

Dysrhythmias (also called arrhythmias) occur when the heart beats differently than the way it is designed to.  They are usually named in reference to the part of the heart where they originate, such as atrial fibrillation, junctional tachycardia, ventricular tachycardia, etc.  Mayo Clinic’s page on Heart Arrhythmias provides detailed information on arrhythmia diagnosis and treatment.  I will write more about this topic soon and discuss common arrhythmias and how they affect people’s lives.

3 Comments leave one →
  1. Beverly permalink
    February 5, 2013 12:43 pm

    Excellent explanation! I could really follow what is happening. Could you discuss LBBB and Dyssynchrony when you return to this topic? You are so lucid!! Thanks, Beverly

    • February 5, 2013 2:34 pm

      Absolutely! Very important topic to cover. Also look for PAC’s and PVC’s sometime soon.

  2. March 7, 2013 2:37 pm

    Excellent write-up. Clear enough for the layman but also quite detailed.

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