To delineate the relative roles of genetic and environmental factors on physiological variations of blood pressure and heart rate, we performed 24-hour ambulatory blood pressure monitorings with simultaneous polygraphic sleep recordings in 28 monozygotic and 16 dizygotic healthy young male twin pairs investigated in a standardized physical and social environment. Blood pressure and heart rate were measured every 10 minutes for 24 hours. A best-fit curve based on the periodogram method was used to quantify changes in blood pressure and heart rate over the 24-hour span. Surprisingly, monozygotic twins as a group tended to have higher blood pressure values than dizygotic twins, and this difference reached the level of significance for daytime systolic blood pressure (P<.005). Although environmental influences largely controlled the mean levels and characteristics of the 24-hour systolic blood pressure variations, significant genetic effects were demonstrated for the mean levels and 24-hour patterns of diastolic blood pressure and heart rate. For both diastolic blood pressure and heart rate, the genetic effects concerned largely the same characteristics of the 24-hour profiles: the 24-hour mean, the daytime mean, the value of the evening acrophase, and the value of the major acrophase. Moreover, there was a strong genetic influence for the amplitude of the 24-hour rhythm of heart rate. 1 Twin studies, based on casual blood pressure recordings, have indicated that resting blood pressure levels are more concordant among monozygotic (MZ) than dizygotic (DZ) twins. This finding suggests that a significant component of the variability in blood pressure levels is of genetic origin. 25 However, casual blood pressure measurements also partially reflect the well-known reaction to the clinical setting usually referred to as the "white coat" effect. 6 A genetic contribution for cardiovascular reactivity, studied by various environmental stressors, has been demonstrated by several investigators, 7 " 10 but estimations of the relative importance of this genetic component have been variable across studies. Thus, at the present time, the relative importance of genetic and environmental factors in controlling resting blood pressure, resting heart rate, and cardiovascular reactivity in healthy humans remains to be determined.There are several approaches for the evaluation of the inheritance of blood pressure, including familial aggregation of blood pressure in adults and children, adoption studies, and twin studies. Generally, the estimates of genetic variance derived from twin studies are Correspondence to J.-P. Degaute, MD, Hypertension Clinic, H6pital Erasme, 808, route de Lennik, B-1070 Brussels, Belgium.higher than estimates derived from other study designs. This is probably due to the greater extent of shared environment in MZ twins, which gives rise to an upwardly biased estimate of genetic variance." Nevertheless, the twin model has been used extensively in medical research for more than half a century. 12 We have p...
