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To determine the difference between baroreflex sensitivity expressed in ms/mmHg (BRS) and in Hz/mmHg (BRSf) in relation to body growth parameters.
Two hundred one children and adolescents aged 10 to 21 years (mean age ± SD 14.7±3.1 years) were examined. This population was analyzed as a whole and subsequently was divided into two groups: group A consisted of normotensive, healthy control subjects (n=154); and group B consisted of children with systolic casual blood pressure on primary diagnosis greater than 140 mmHg, measured three times at least one week apart (n=47). These groups were divided into subgroups according to age. BRS and BRSf were determined by spectral analysis of blood pressure and pulse interval variability (5 min records by Finapres, metronome-controlled breathing at a frequency of 0.33 Hz). Body growth parameters (body height, weight and body mass index [BMI]) were assessed. The Spearman correlation coefficient was calculated between pairs of all parameters (age, pulse interval, BRS, BRSf, height, weight, BMI) for the whole group, for groups A and B, and for age-related subgroups.
BRS did not correlate with age; it correlated with pulse interval in the whole population, in groups A and B, and in age-related subgroups. BRS correlated significantly only with weight, height or BMI in the oldest group (age 17 to 19 years), but there was no correlation with groups A or B, or with the whole study population. BRSf correlated significantly (P<0.01) with age, and was pulse interval independent in group A and the age-related subgroups. BRSf correlated with weight, height and BMI in group A and in the whole population, but not in the age-related subgroups.
BRS (a mean pulse interval-related parameter) does not correlate with age. On the contrary, BRSf (which is mean pulse interval independent) decreases significantly with age. The significant relation between BRSf and parameters of body growth is related to age.
Arterial baroreflex is an important modulator of the autonomic influences on the heart; thus, baroreflex sensitivity has been used as a measure of the interaction between sympathetic and parasympathetic activities at the cardiac level (1).
Although many studies have been performed to explain the role of decreased baroreflex sensitivity in the development of hypertension in adults (2) and in patients after myocardial infarction (3), very little is known about the role of the baroreflex in children and adolescents. This is understandable because the first method by which baroreflex sensitivity was estimated was by the stress method (4), which is not used in children for ethical reason.
In our laboratory, we use a method of determining baroreflex sensitivity based on spectral analysis of the spontaneous variability of blood pressure and pulse intervals (5). Baroreflex sensitivity can be expressed in ms/mmHg (BRS) (6) or in Hz/mmHg (BRSf) (7).
We have applied these methods of BRS and BRSf determination in healthy children and adolescents. In 180 subjects (age 10 to 22 years) we did not find any correlation between BRS and age (8). This finding may have resulted from the large interindividual differences in the value of BRS: 16% of subjects had BRS of less than 5 ms/mmHg. Such low values of BRS have been found in hypertensive patients in other studies. We also proved that BRS depends on the mean pulse interval (8,9). On the contrary, BRSf, which is a mean pulse interval-independent index, decreased with age in children and adolescents (10,11), as does the compliance of arteries. BRSf in young healthy adults is a long term characteristic parameter; values obtained one year apart are correlated (12).
Two hundred one children and adolescents (mean age ± SD 14.7±3.1 years, range 10 to 21; 111 girls and 90 boys) participated in the study. This population was analyzed as a whole and subsequently was divided into two groups, A and B, as detailed in Table 1.
Indirect, continuous 5 min blood pressure readings from finger arteries (Finapres, Ohmeda, USA) were recorded in sitting, resting subjects, between 09:00 and 12:00. Recordings were taken during spontaneous and synchronized breathing with a metronome (0.33 Hz). The subjects were allowed to adjust the tidal volume according to their own comfort.
Beat to beat values of systolic pressure and of pulse intervals were measured for further analysis. The autocorrelation and cross-correlation functions (power spectra of the variations of pulse intervals and blood pressure) and cross-spectra (coherence and modulus between pulse intervals and systolic pressure) were calculated (5,6). The gain factor – that is, modulus H[f] of the transfer function between variations in systolic blood pressure and pulse intervals – was calculated in the frequency range [f], as follows:
where Gxy[f] corresponds to the cross-spectral density between systolic pressure and pulse intervals, and Gx[f] corresponds to the spectral density of systolic pressure. The value of the modulus at a frequency of 0.1 Hz was taken as the measure of baroreflex sensitivity. Baroreflex sensitivity was expressed in two dimensions: ms/mmHg (BRS) (5,6) and Hz/mmHg (BRSf) (7).
The Spearman correlation coefficient was calculated for pairs of all parameters (age, pulse interval, BRS, BRSf, height, weight and body mass index [BMI]) for the whole population, for groups A and B, and for age-related subgroups. Differences between groups were compared by means of the Wilcoxon test.
BRS correlated with pulse interval in the whole population, in groups A and B, and in age-related subgroups. It correlated significantly only with weight, height or BMI in the oldest subgroup of subjects (age 17 to 19 years), but there was no correlation with the other groups. The relation between BRS and body parameters was weak.
BRSf was pulse interval independent in the normotensive group (A) and in the age-related subgroups. It correlated with weight, height and BMI in group A and in the whole population, but not in the age-related subgroups.
Baroreflex sensitivity measure in ms/mmHg was dependent only on the pulse interval in all subjects and in age-related subgroups. BRS did not correlate with age. BRSf, which is mean pulse interval independent, decreased significantly with age. This is in agreement with our previous study (8,9).
BRSf correlated more strongly than BRS with body growth parameters (weight, height and BMI) in healthy children. In a previous study of the relation between baroreflex sensitivity and ambulatory blood pressure monitoring (13), we found a correlation between BRSf, and systolic ambulatory blood pressure and height. The height of children was also found in other studies (14,15) to be a factor in determining systolic blood pressure. Thus, BRSf may be a link between height and blood pressure.
The other body parameter, greater body weight, was attributable to the increased systolic blood pressure in group B. The impact of obesity on ambulatory blood pressure in children and adolescents is known (16). This effect may be explained by the secretion of leptin by adipose tissue, which increases sympathetic activity in obese patients with essential hypertension (17).
The importance of the link between baroreflex sensitivity and body growth parameters is not fully elucidated. Our results may be a first step in explaining this problem in the developing organism.
This work was supported by grant CEZ:J07/98:141100004.