Interatrial block: pandemic prevalence concealed by anachronistic electrocardiographic standards

Interatrial block (IAB; P waves 2 > or =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.

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.

In all, 500 consecutive unselected electrocardiograms (ECGs) were investigated using every lead and a magnifying graticule. For greater specificity, a minimal P duration of > or =120 ms was selected.

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 V3 and V4.

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.