Answer to mystery EKG rhythm

Class, here is the result from the mystery EKG:  it is a normal sinus rhythm.  This means that the impulse started in the SA node and depolarized the atria.  The impulse was then held in the AV node to allow time for the ventricles to fill with blood while the atria contract.  This extra filling time for the ventricles increases the cardiac output by up to 30%.  After being held in the AV node, the impulse travels down the super highway of the bundle branches so that the ventricles depolarize simultaneously, resulting in an effective contraction.  And that's the way it's supposed to work!