Objective: To compare the QT/RR relation in healthy subjects in order to investigate the differences in optimum heart rate correction of the QT interval. Methods: 50 healthy volunteers (25 women, mean age 33.6 (9.5) years, range 19-59 years) took part. Each subject underwent serial 12 lead electrocardiographic monitoring over 24 hours with a 10 second ECG obtained every two minutes. QT intervals and heart rates were measured automatically. In each subject, the QT/RR relation was modelled using six generic regressions, including a linear model (QT = β + α × RR), a hyperbolic model (QT = β + α/RR), and a parabolic model (QT = β × RR α ). For each model, the parallelism and identity of the regression lines in separate subjects were statistically tested. Results: The patterns of the QT/RR relation were very different among subjects. Regardless of the generic form of the regression model, highly significant differences were found not only between the regression lines but also between their slopes. For instance, with the linear model, the individual slope (parameter α) of any subject differed highly significantly (p < 0.000001) from the linear slope of no fewer than 21 (median 32) other subjects. The linear regression line of 20 subjects differed significantly (p < 0.000001) from the linear regression lines of each other subject. Conversion of the QT/RR regressions to QTc heart rate correction also showed substantial intersubject differences. Optimisation of the formula QTc = QT/RR α led to individual values of α ranging from 0.234 to 0.486. Conclusion: The QT/RR relation exhibits a very substantial intersubject variability in healthy volunteers. The hypothesis underlying each prospective heart rate correction formula that a "physiological" QT/RR relation exists that can be mathematically described and applied to all people is incorrect. Any general heart rate correction formula can be used only for very approximate clinical assessment of the QTc interval over a narrow window of resting heart rates. For detailed precise studies of the QTc interval (for example, drug induced QT interval prolongation), the individual QT/RR relation has to be taken into account.T he QT interval adapts to changes in heart rate, which makes it difficult to compare the QT interval recorded at different heart rates. To allow such a comparison, the concept of the heart rate corrected QTc interval was developed and many formulas have been proposed to describe the QT interval heart rate adaptation. Bazett's formula 1 is both the most frequently used and the most criticised. 2-4In principle, every heart rate correction formula assumes that a mathematical form exists to describe the physiological QT/RR relation. Such a form may be converted into a formula that normalises a measured QT interval to that which would be associated with a "standard" heart rate, for example, of 60 beats/min. Most studies that have proposed a heart rate correction formula relied on QT and RR interval data obtained from healthy volunteers, different groups of patients, o...

Abstract-We investigated whether renal function and microalbuminuria are independent predictors and whether any interaction exists between them, regarding future cardiovascular disease in hypertensive patients (nϭ10 881) followed for 4.5 years. The primary end points (PEs) were fatal and nonfatal myocardial infarction and stroke and other cardiovascular deaths. Creatinine and glomerular filtration rate (GFR), estimated using the formulas of the Modification of Diet in Renal Disease study group and Cockroft and Gault and in a subsample (nϭ4929) of microalbuminuria and interaction terms of microalbuminuria and renal function, were related to the risk of the PE using Cox proportional hazards model after full adjustment. Increased creatinine (PϽ0.001), decreased GFR from Cockroft and Gault (Pϭ0.001), and decreased GFR from the Modification of Diet in Renal Disease study group (Pϭ0.001) were all independent risk factors for the PE. Stepwise exclusion of patients with the poorest renal function excluded the possibility that the relationship between decreasing renal function and the PE was driven only by patients with severely impaired renal function. Microalbuminuria and all 3 of the indices of renal function predicted the PE independent of each other. There was a significant interaction between microalbuminuria and GFR from Cockroft and Gault (Pϭ0.040) in prediction of the PE. Both renal function and microalbuminuria add independent prognostic information regarding cardiovascular risk in hypertensive patients. The cardiovascular risk associated with microalbuminuria increases with a decline in GFR, as demonstrated by a significant interaction between microalbuminuria and GFR from Cockroft and Gault. Because estimation of the total cardiovascular risk is essential for the aggressiveness of risk factor interventions, simultaneous inclusion of GFR and microalbuminuria in global cardiovascular risk assessment is essential.

P-wave duration, P area, and PR interval show a significant circadian variation in healthy subjects. The relations between P area/RR,PR/ RR, and P duration/RR also demonstrate a significant diurnal pattern.