Lee D. Cady

The normal Q-T interval

Abstract 1. 1. The correlation between the Q-T and the R-R intervals was determined in 649 healthy men and 311 healthy women from 20 to 59 years of age, with consideration of constitutional and physiologic variables, and ten additional electrocardiographic items using an electronic computer. 2. 2. Of the numerous factors investigated, only age made a small, but statistically significant, contribution to the relationship between the Q-T and the R-R intervals. 3. 3. As best fit, logarithmic and linear regression equations were obtained. In actual application the difference between these two regression equations was so small that the simple linear regression equation is preferred. 4. 4. In comparison with six other formulas for the relationship between Q-T and R-R intervals, most commonly used in clinical application, the logarithmic CWS(1) and the linear CWS(2) regression equations gave the least discrepancies to the actual values in 9 subjects near the extremes of the wide scatter of Q-T versus R-R intervals. 5. 5. There was excellent agreement in the range of heart rate from 56 to 115 per minute between upper (97.5 per cent) and lower (2.5 per cent) normal limits determined from the percentile distribution and predicted from the linear regression equation. 6. 6. The normal limits of the Q-T interval are presented in five ranges of heart rate for convenient clinical application, and are believed to be more reliable than those previously suggested. The limitations of prediction of the Q-T interval from the heart rate are discussed.

Normal redundancy in chest leads as basis for recognition of minor abnormalities

Abstract The correlations between R amplitudes and between T amplitudes in Leads I, V 3 , V 4 , V 5 , and V 6 were calculated in 640 healthy men who ranged in age from 20 to 59 years. The correlations were found to be so high that R and T waves are predictable with high probability from two adjacent leads by means of regression equations. Thus, abnormality can be predicted as a break in the continuity of the QRS and T deflections across the chest even if any single lead is within normal limits. For easy clinical application, a set of nomographs was constructed.