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Depressed sympathetic and enhanced parasympathetic reflex responses of heart rate in arterial hypertension

Junqueira Jr., Luiz; Oliveira, Leonardo Capita; Pereira, Flávio; Jesus, Paulo César; Carvalho, Hervaldo
University of Brasilia Department of Internal Medicine, Division of Cardiology. Brasilia. Brazil

Abstract
Introduction
Objetive
Subject and Methods
Results

Abstract:
The characterization of the autonomic reflex responses of heart rate to several stimulus is of great pathopysiological and clinical importance in many cardiovascular conditions.
Objective: To evaluate in hipertensive subjects without treatment, the heart rate responses to sympathetic and parasympathetic stimulations.
Subjects and Methods: We studied 19 individuals (12 M, 7 F - 42 ± 9,1 (sd) years) with mild to moderate hypertension (145/95 - 190/110 mmHg) and 16 normal subjects (10 M, 6 F - 38.9 ± 11.3 years). RR intervals series (RRint) were sequentially obtained from the ECG registered (DII, 25 mm/s) during 5 minutes in supine position, before and during facial cooling induced by two bags containing water frozen (4-6oC), and after 2 minutes in orthostatic posture. The differences were compared by the Mann-Whitney test at a p £ 0.05.
Results: In supine position the medians (quartiles, ms) of the RRint individual means were similar (p = 0.23) in the control (997; 912 and 1138) and in hypertensive groups (938; 862 and 995). During the facial cooling the normotensives (999; 924 and 1155) and hipertensives (954; 933 and 1079) showed medians also similar (p = 0.73). Facial cooling induced median (quartiles) increments of the RRint equal to +0,67% (-1.03 and +2,38%) in the controls, and +2,10% (+1,01 and +5.69) in the hypertensives (p = 0.05). In orthostatic position the RRint median was also similar (p = 0.86) in the control (828; 780 and 900) and hipertensive (819; 786 and 917) groups, but the induced median decrement of the RRint was equal to -15.7% (-22.4 and -13.3%) in the control and -12.5% (-18.7 and -5.3%) in the hypertensive groups (p = 0.02).
Conclusion: The hypertensive group showed greater bradycardia to facial cooling and lesser tachycardia in orthostatic posture, what reflects improperly increased parasympathetic and blunted sympathetic responsiveness, probably due to the basal sympathetic hyperactivity associated to hypertension. (CNPq, FAPDF)

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Introduction: The reflex and tonic adaptive responses of the heart are critically dependent of the sympathetic and parasympathetic influences acting on the sinus node, in others excite-conducting structures and in atrial and ventricular musculature. One of the most important of these physiological responses of the heart are the reflex modifications of heart rate to several stimulus. The characterization of these responses is of great pathophysiological importance in many cardiovascular clinical conditions, since they can reflect autonomic dysfunction and can to explicate different clinical manifestations. In arterial hypertension, the reflex heart rate responses and the related autonomic functional status have not been fully characterized.

Objective: The aim of the study was to evaluate in hypertensive subjects without treatment, the heart rate responses following to a sympathetic stimulus represented by the active adoption of the orthostatic posture and to a parasympathetic activation induced by short-lasting facial cooling.

Subject and Methods: The study was conduced in 19 volunteers (12 males and 7 females - 42 ± 9,1 (sd) years) with established mild to moderate hypertension without treatment in occasion of the experiment (145/95 - 190/110 mmHg) and in 16 normal controls (10 males and 6 females - 38.9 ± 11.3 years). Ten hypertensive subjects which encountered under regular treatment were asked to suspend it since one week before the experimental evaluation. The study protocol was approved by the Institutional Committee of Ethics in Human Research and all subjects given their informed consent to participate. In the occasion of the experimental session, after clinical interrogatory and examination, the ECG was registered in DII (25 mm/s) during 5 minutes in the supine position, before and during facial cooling induced by two bags containing frozen water (4-6oC). Following some time for recuperation of the basal conditions, the ECG was newly registered for more 5 minutes in the orthostatic posture after 2 minutes of active standing up. For each situation the RR intervals (RRint) were manually measured on a beat-to-beat basis, using an appropriated rule at an error of precision of ± 1-2%. The individual average from each one of the three time series was calculated and the percent difference induced by each functional test obtained. The median with the upper and lower quartiles of the individual average values was calculated for each group. The differences were compared by the Mann-Whitney test at a significance level of 5% (p < 0.05).

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Results: In the supine position the median (extreme values) RRint of the individual mean intervals from the series obtained were similar (p = 0.18) in the control (998 ms; 707 and 1355 ms) and in hypertensive groups (938 ms; 803 and 1248 ms). During the facial cooling the normotensives (999 ms; 706 and 1439 ms) and hypertensives (954 ms; 825 and 1241 ms) showed medians of RRint also similar (p = 0.73). Facial cooling induced bradycardia in 63% of the controls and in 84% of the hipertensives (p = 0.76), being the respectives median variations (extreme values) of the RRint equal to +1.00% (-9.21 and +6.20%) and +2.50% (-5.41 and +16.44%) (p = 0.04). These results are indicated in Figure 1. The variation in control group was not statistically significant (p = 0.16), but the difference in hypertensive group showed a significant increment (p = 0.005), as illustrated in Figure 2. In the orthostatic position the median (extreme values) RRint was also similar (p = 0.78) in the control (828 ms; 632 and 1079 ms) and hypertensive (819 ms; 713 and 965 ms) groups. Adoption of the upright posture induced tachycardia in 100% of controls and in 95% of hipertensives (p = 0.90), being the respectives median (extreme values) RRint variations, -15.75% (-32.68 and -9.58%) and -12.63% (-25.54 and +2.72%) (p = 0.03). These observations are indicated in Figure 3. The variations were statistically significant in control (p < 0.001) and in hypertensive (p < 0.001) groups, as illustrated in Figure 4. Although a statistically significant difference was not observed in relation to the control group, a tendency for higher heart rate was noted in the hypertensive subjects in the supine position, during facial cooling and after standing up.

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Discussion and Conclusions: The hypertensive subjects showed a tendency for higher heart rate than controls in all the three experimental conditions. The same proportion of hipertensives and controls individuals showed bradycardia and tachycardia responses to the parasympathetic and sympathetic stimulus. However, the hypertensive group presented higher bradycardia during the facial cooling and lower tachycardia in the orthostatic posture, what reflects improperly increased parasympathetic and blunted sympathetic responsiveness. These disturbances are probably due to the sympathetic hyperactivity associated to hypertension, resulting in lesser heart rate increase to the orthostatic stress, and in higher heart rate decrease induced by the facial cooling.

(Supported by CNPq 52.0459/98, FAPDF 193.000.152/98)

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Questions, contributions and commentaries to the Authors: send an e-mail message (up to 15 lines, without attachments) to hbp-pcvc@pcvc.sminter.com.ar , written either in English, Spanish, or Portuguese.


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Bioengineering
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Update
Feb/19/2000


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