The following are generalizations that apply to all species:

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  represents atrial depolarization (right to left direction) prior to atrial contraction


  the first negative wave preceding the R wave (not present in the base-apex image shown)


  the first positive wave in the QRS* complex


  the first negative wave following the R wave


  represents ventricular repolarization

  Dukes Physiology of Domestic Animals, Swenson and Reece, 1993

*    The QRS complex as a whole represents ventricular depolarization:  "the period of time required for the excitation process to spread the distance separating the end points of the Purkinje fibers from regions of the ventricular myocardium most remote to them...a measure of the degree of simultaneous activation of individual myocardial fibers."    (Smith, CR; Hamlin, RL; Crocker, HD.  "Comparative electrocardiography",  from the Annals of the New York Academy of Sciences.  HE Whipple, Editor; Vol 127, p 159, 1965.)


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