The optimism of Sir Thomas Lewis and the cautions ofDr. Nadas regarding the electrocardiogram (ECG) remainvalid even in this day of sophisticated echocardiography,Doppler flow analysis, and magnetic resonance imaging.

Although it must be admitted that fine details of cardiacanatomy are now best evaluated with these modern techniques, the ECG is not (and never will be) obsolete. It isstill the quickest, safest, and least expensive diagnostic toolin cardiology and is unparalleled in its ability to registerarrhythmias and conduction defects. With proper interpretation, the ECG also offers a useful reflection of cardiacposition, chamber enlargement, myocardial damage, andcertain metabolic disorders. It has clearly proven its worthafter more than a century of continuous clinical use.

This chapter is intended as a review of electrocardiographyas it applies to the pediatric patient. Rather than simplycatalogue a litany of rules for ECG interpretation, we haveexpanded the discussion here to encompass the basic cellular events underlying cardiac electrical activity, along witha survey of invasive and noninvasive techniques used forin-depth analysis of cardiac rhythm and conduction patterns.

This information is intended not only to clarify the originof ECG signals recorded from the body surface but also toserve as an introduction to the broader topic of cardiacarrhythmias that will be addressed further in Chapter 29 ofthis text.