SummaryBackground: Interatrial block (IAB; P waves ≥ 110 ms duration) has been reported in over 40% of unselected patients in sinus rhythm at widely separated general hospitals. It is thus of "pandemic" proportions. It should be better appreciated because it represents a large, baggy, poorly functional left atrium and is a forerunner of atrial fibrillation, atrial flutter, and other arrhythmias.Hypothesis: A search of all 12 leads will disclose the true prevalence of IAB in contrast to traditional reliance on lead II, as widely proposed in textbooks and other literature.Methods: In all, 500 consecutive unselected electrocardiograms (ECGs) were investigated using every lead and a magnifying graticule. For greater specificity, a minimal P duration of ≥ 120 ms was selected.Results: Thirty-one ECGs were discarded because of atrial arrhythmia, poor baseline, or undetectable P waves, leaving a base of 469 ECGs, the denominator for the results. A total of 182 patients had IAB, representing 38.8% of this series. The widest P wave was usually found in multiple leads (95.1% of patients). The widest P waves were found only in precordial leads in 59 patients and only in limb leads in 18 patients. "Traditional" lead II detected only 97 cases (53.3%), and IAB was found more frequently in leads V 3 and V4.Conclusions: Results confirm the pandemic frequency of IAB in one-third of hospitalized patients. Interpreters of ECGs should seek IAB in all 12 leads since reliance on lead II alone resulted in only 53.3% of the total cases. Its prevalence and serious implications with regard to patients' current and future status make this necessary.

No abstract