Errors in measurement of the P-R (P-Q) interval and QRS duration in the electrocardiogram

Abstract

1. 1. We have discovered errors in existing methods of measurement of the P-R interval and QRS duration in the electrocardiogram. 2. 2. These errors are usually the result of balancing of similar deflections of opposite direction in Leads I and III, producing isoelectric phases in Lead II. Lead II, therefore, is often the least reliable for the measurement of electrocardiographic time intervals. 3. 3. Such variations were found to occur in 3.8 per cent of 1000 consecutive cases. 4. 4. It is pointed out that the variations may be of considerable individual significance diagnostically, and may be of statistical importance from the standpoint of existing accepted values for normal limits of the P-R interval and QRS duration. 5. 5. A simple method of avoiding the errors is presented. In general, Lead III has been found to be the most reliable single lead for the measurement of time intervals.