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THE ELECTROCARDIOGRAM - A record of the entire electrical activity occurring in the heart during one cardiac cycle. The EKG tracing is composed of three distinct deflections or waves, as well as, well defined intervals.

   1.  The P wave:

         a.  Has a duration of 0.08 seconds.

         b.  Corresponds to depolarization of atrial walls.

         c.  Atrial contraction occurs approximately 0.1 seconds after the P wave             begins.

    2.  The QRS wave:

          a.  Has a duration of 0.08 seconds.

          b.  Corresponds to depolarization of ventricular myocardium

    3.  The T wave:

          a.  Has a duration of 0.16 seconds.

           b.  Corresponds to ventricular repolarization.

    4.The PR interval (actually the PQ interval):

  a.  Has a duration of 0.16 seconds and extends from the             beginning of the P wave to the beginning of the QRS wave.

  b.  Corresponds to the period during which the atria depolarize,      contract and begin to relax (enter diastole).

5.  The QT interval:

   a.  Has a duration of 0.36 seconds and extends from the     beginning of the QRS wave through the T wave.

    b.  During this period the ventricles depolarize, repolarize,       contract and relax.

SUMMARY OF EVENTS OCCURING DURING THE CARDIAC CYCLE

STAGE I - Atrial and Ventricular Diastole

1. Both the upper and lower chambers are filling.

2. The atriaoventricular valves are open.

3. The semilunar valves are closed.

STAGE II - Atrial Systole; Ventricular Diastole

1. The atria are contracting forcing extra blood into the ventricles which become distended.

2.  Each ventricle now contains about 120 ml of blood.

3.  The atrioventricular valves are open.

4.  The semilunar valves are closed.

 

STAGE III - This stage is divided into two phases:

     A. Phase 1 -The Isovolumetric phase (Atrial Diastole; Ventricular Systole)

       1.   The atria have finished their contraction and are beginning to fill. The ventricles          are beginning to contract.

        2.  Due to the rising blood pressure in the ventricles, the atrioventricular valves close.           This action produces the first heart sound "lub".

        3.  The semilunar valves remain closed.

        4.  The volume of the blood in the ventricles remains the same during this phase             (isovolumetric).

 

     B. Phase 2 - The Ejection phase (Atrial Diastole; Ventricular Systole finishes)

          1.  The atria continue to fill.

          2.  The ventricles finish their systole. This creates a high enough pressure on the             blood in the ventricles to overcome the downward force of blood in the pulmonary             trunk and aorta. This opens the semilunars and allow about 70 ml of blood to be             ejected from each ventricle.

          3.  The atrioventricular valves remain closed.

    

 

STAGE IV - Atrial Diastole continues; Ventricular Diastole begins

           1.  The atria continue to fill.

2.  The ventricles enter diastole and begin to fill. This produces a decrease in    ventricular blood pressure. As a result, the blood in the pulmonary trunk and aorta begins to flow back down towards the ventricles. The cusps of the similunars fill with this blood and close the valve. This action produces the second heart sound "dup". The recoil of the descending blood against the cusps of the semilunars produces the slight, transient rise in aortic blood pressure called the "dicrotic notch".