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Baroreflex Sensitivity and the Blood Pressure Response To Journal of Human Hypertension (1999) 13, 185–190 1999 Stockton Press. All rights reserved 0950-9240/99 $12.00 http://www.stockton-press.co.uk/jhh ORIGINAL ARTICLE Baroreflex sensitivity and the blood pressure response to ␤-blockade X Chen, MO Hassan, JV Jones, P Sleight and JS Floras Department of Cardiovascular Medicine, University of Oxford, and Centre for Cardiovascular Research, University of Toronto, Canada -␤ .(؊0.08؍ The objective of this analysis was to determine whether P Ͻ 0.001), but not with its variability (r changes in baroreflex sensitivity (BRS) within 35 hyper- blockade attenuated the pressor response to exercise, tensive patients (25 M, 10 F, mean age 47 years) treated but there was a positive relationship between the effect with ␤-blockade as monotherapy relate to reductions in of ␤-blockade on BRS, and on the rise in systolic BP P Ͻ 0.001). Any dampening ;0.63 ؍ ambulatory blood pressure (BP) or its variability. BP during bicycling (r was recorded intra-arterially directly from the brachial effect of ␤-blockade on BP variability at rest in hyperten- artery before and during submaximal exercise. BRS was sive patients with the greatest increase in BRS may be determined by the phenylephrine injection technique. offset by increased pressor responses to physical MAP and its variability were determined for the awake activity such as exercise. Consequently, BP variability period of 24-h BP monitoring. Subjects were random- is unaffected, even though reductions in ambulatory BP ised to one of atenolol, metoprolol, pindolol, or propran- during chronic ␤-blockade are inversely related to olol, and restudied after a mean of 5 months. ␤-blockade changes in BRS. BP responses to ␤-blockade may be a increased BRS in 24 patients and decreased BRS in 11. function of the action of this class of drugs on BRS. -BRS increased from 6.53 ؎ 4.94 to 9.40 ؎ However, there is considerable variation, between sub ms/mm Hg (mean ؎ s.d.) (P Ͻ 0.01). Waking ambu- jects, in their effect on BRS. This may have implications 8.62 latory MAP decreased from 125.8 ؎ 15.8 to 106.4 ؎ 16.2 for other conditions, such as dilated cardiomyopathy, or mm Hg (P Ͻ 0.0001), but its variability did not change. following myocardial infarction, in which improvement Higher BRS after chronic ␤-blockade was associated in BRS is one mechanism by which ␤-adrenoceptor .؊0.55, blockade could improve survival؍ with a decrease in waking ambulatory MAP (r Keywords: ambulatory blood pressure; baroreflex sensitivity for heart rate; ␤-adrenoceptor blockade; blood pressure varia- bility; exercise; hypertension Introduction attenuated.9 However, there was considerable differ- ence between individuals in the effect of chronic ␤- There is considerable variation between hyperten- blockade on BP and on baroreflex sensitivity, sive patients in the blood pressure (BP) response to increasing in some, and decreasing in others. More- ␤-adrenoceptor blockade. Hypertensive patients also ␤ over, the functional implications of these changes in vary with respect to the effect of chronic -adreno- individual subjects, with respect to ambulatory BP ceptor blockade on baroreflex sensitivity (BRS) for 1–6 and its variability, have not been reported. heart rate. The purpose of the present analysis In a recent publication, Vesalainen et al2 con- was to determine if these two observations are trasted the effects of 4 weeks of monotherapy with related. metoprolol and ramipril on heart rate variability, We have previously reported that the arterial baro- baroreflex sensitivity, and ambulatory BP. Both reflex regulation of heart rate is impaired in patients 7 drugs caused comparable reductions in ambulatory with essential hypertension, and that this dimin- BP, but only metoprolol increased baroreflex sensi- ished arterial baroreceptor reflex is associated with tivity, and cardiac vagal activity, as assessed by increased variability of ambulatory BP and aug- heart rate, heart rate variability, and the high fre- mented BP responses to mental and physical activi- quency component of heart rate variability during 8 ␤ ties. After chronic -blockade using both cardiose- supine rest. Reductions in ambulatory BP correlated lective and non-selective drugs, mean arterial 9 significantly with increases in RR interval, its total ambulatory BP was significantly decreased, the variability, and high frequency variability. However, mean baroreflex sensitivity was significantly 1 there was no significant relationship between increased, and the pressor response to exercise was changes in baroreflex sensitivity and ambulatory BP in these patients. We undertook the present re-analysis of our orig- Correspondence: Dr John S Floras, Division of Cardiology, Room inal data set to determine whether improvement in 1614, Mount Sinai Hospital, 600 University Avenue, Toronto, arterial baroreflex sensitivity for heart rate, with Ontario, Canada M5G 1X5 ␤ Received 11 November 1998; revised and accepted 17 chronic -adrenoceptor blockade, will result in cor- December 1998 respondingly lower ambulatory BP, or BP varia- ␤-blockade, baroreflex sensitivity, and BP X Chen et al 186 bility, or attenuated BP responses to submaximal Statistics exercise in hypertensive patients. Because it is now ± recognised that 24-h ambulatory BP and BP varia- Data are presented as mean standard deviation. bility are more closely related to end-organ damage Values for baroreflex sensitivity were logarithmi- in hypertension than is the office BP measure- cally transformed to approximate a normal distri- ment,10–12 and that increased BP variability is an bution. Student’s t-test was used for paired data when comparing control versus treatment days. P independent adverse risk factor for greater vascular Ͻ and target organ damage,10,13 this question now has values 0.05 were considered statistically signifi- clinical relevance. cant. Results Subjects and methods Chronic ␤-adrenoceptor blockade increased barore- Details of the study protocol have been pub- flex sensitivity in 24 subjects, and decreased it in 11. lished.1,7–9 Observations in 35 subjects (25 males, 10 Patients whose BRS increased tended to be younger females) referred for the assessment of newly diag- (45.6 ± 12.4 vs 50.6 ± 10.9 years), and comprised nosed, untreated hypertension are reported. Sub- eight of the 10 women, but these age and sex distri- jects ranged from 16 to 69 years old, with a mean butions were not significantly different between the age of 47 (s.d. 12) years. Blood pressure was meas- two groups. The average ambulatory mean arterial ured on three or more clinic visits using a standard pressures (126 mm Hg) and heart rates (89 bpm) mercury sphygmomanometer. For the purpose of were identical in the two groups, and the number of this study, patients were considered hypertensive patients in whom BRS increased was similar for when their clinic BP was 140/90 mm Hg or greater, each drug. Mean values were significantly aug- if less than 40 years old, or at or above 160/95 mented, from 6.53 ± 4.94 msec/mm Hg before, to mm Hg, if 40 years or older. Subjects with secondary 9.40 ± 8.62 msec/mm Hg during ␤-adrenoceptor hypertension were excluded. The study protocol blockade (P Ͻ 0.01). Corresponding values for the was approved by the Hospital Ethics Committee and Log transformation of baroreflex sensitivity were informed, written consent was obtained from each 0.725 ± 0.276 and 0.831 ± 0.347 respectively (P Ͻ subject. 0.02). Paired data were available for analysis of ambulat- ory BP in 34 patients. As demonstrated in Table 1, ␤- adrenoceptor blockade reduced significantly MAP, Protocol from 125.8 ± 15.8 to 106.4 ± 16.2 mm Hg (P Ͻ ␤ Subjects were studied in the morning after a light 0.0001). -blockade had no effect on the variability ± ± breakfast at home, avoiding tea, coffee, or cigarettes. of MAP (14.8 4.1 mm Hg untreated vs 15.0 3.2 Arterial BP was recorded from a left brachial artery mm Hg on treatment). The coefficient of variation ± ± Ͻ cannula, and an adjacent antecubital vein was also increased, from 11.9 2.9% to 14.3 3.2% (P cannulated for injection of phenylephrine. ECG 0.002). (lead II) and intra-arterial BP were recorded continu- Increases in Log (BRS) were inversely related to ously for 24 h7,8 and digitised for computer analysis. reductions in ambulatory mean arterial pressure =− Ͻ Baroreflex sensitivity was measured during (r 0.55, P 0.001) (Figure 1). As illustrated in supine rest, using the phenylephrine method.7,8 Figure 2, there was no relationship between these Subjects performed a submaximal bicycle exercise changes in Log BRS and changes in the variability test,8,9 then left hospital to resume routine activities, of mean arterial pressure, as a result of treatment =− and kept a diary noting, in particular, times of sleep (r 0.08). and waking. Because the frequency response of the Paired data were available for analysis of BP dur- ␤ entire ambulatory recording and replay system falls ing exercise in 33 patients. Chronic -adrenoceptor off rapidly above 10 Hz,14 we used mean, rather than blockade attenuated significantly the increase in BP systolic, BP in these calculations. Average awake during bicycle exercise. Systolic BP increased by ± ± mean arterial pressure (MAP) was computed using 59.6 18.5 mm Hg before, and 42.5 20.2 mm Hg ␤ Ͻ all valid beats over this period. Blood pressure varia- after, -blockade (P 0.05), while diastolic BP rose ± ± bility was defined as the standard deviation of the by 17.7 9.4 mm Hg before and by 14.7 9.1 mm Hg ␤ Ͻ waking mean arterial pressures.7,8 After the first after -blockade (P 0.05). There was a positive lin- study, subjects were randomised to one of four ␤- ear relationship between increases in Log (BRS) on ␤ adrenergic receptor blocking drugs: atenolol (n=9), chronic -adrenoceptor blockade and changes in the metoprolol (n = 9), pindolol (n = 9), or slow-release systolic BP response to bicycle exercise (Figure 3, = Ͻ propranolol (n=8).
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