Choice of electrocardiographic leads for recording the earliest QRS onset in noninvasive measurements.

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Abstract
A significant error may be introduced in intervals measured from the onset of the QRS if an electrocardiographic lead that does not record the earliest deflection is used. To ascertain to what extent the commonly used leads can be relied on to show the earliest QRS onset, 100 normal subjects and 219 patients with heart disease were studied by means of simultaneous recording of three leads: a right precordial lead chosen to show an rS configuration, lead II, and another limb lead chosen to show a qR configuration. Lead II most frequently showed a delayed QRS onset--in 34% of normal subjects and 36% of the patients. In the other limb lead the initial QRS deflection was delayed in 24% of the normal subjects and 23% of the patients. The QRS onset in the right precordial lead was never delayed in the normal subjects; however, it was delayed in this lead in 6% of the patients. The delays in each of the leads ranged from 5-20 msec. We conclude that while a right precordial lead is by far the most reliable single lead that can be used for interval measurements, simultaneous recording of a right precordial lead and a limb lead assures that recording of the earliest QRS onset in all cases.