Since its origin as "pulsus alternans" described by Traube in 1872, the definition of alternans has evolved into a term encompassing multiple physiologic and pathologic phenomena that, although united by the term cardiac alternans, diverge widely with respect to etiology, mechanism and clinical significance.
In an effort to standardize terminology and criteria for clinical electrocardiography, and as a follow-up of its work on definitions of terms related to cardiac rhythm, an Ad Hoc Working Group established by the World Health Organization and the International Society and Federation of Cardiology reviewed criteria for the diagnosis of conduction disturbances and pre-excitation. Recommendations resulting from these discussions are summarized for the diagnosis of complete and incomplete right and left bundle branch block, left anterior and left posterior fascicular block, nonspecific intraventricular block, Wolff-Parkinson-White syndrome and related pre-excitation patterns. Criteria for intraatrial conduction disturbances are also briefly reviewed. The criteria are described in clinical terms. A concise description of the criteria using formal Boolean logic is given in the Appendix. For the incorporation into computer electrocardiographic analysis programs, the limits of some interval measurements may need to be adjusted.
While a number of electrolytes play a role in the genesis of the transmembrane action potential (AP), the changes in the action potential most clearly related to arrhythmias are dependent to a large extent on K+. Potassium gradient is a major determinant of the magnitude of transmembrane resting potential (TRP), and secondarily the rate of rise (dV/dt) of phase 0, and consequently the speed of conduction. The cell membrane conductance for K+, or a decrease therein, is most likely the major determinant of spontaneous slow depolarization during phase 4. Thus K+ has a pronounced effect on both conduction and automaticity. Furthermore, these electrophysiologic properties are altered within levels of K+ encountered in clinical medicine, a situation which, with rare exceptions, is not seen with Ca + Mg +, or Na These latter ions affect the action potenby guest on June 15, 2015 http://circ.ahajournals.org/ Downloaded from *Modified from Fisch C Greenspan K.49 Circl/anon, Volume XLVII, February 1973 409 by guest on June 15, 2015 http://circ.ahajournals.org/ Downloaded from AN, SMARTZ C: Digitalis toxicity and hypomagnesiema.
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