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1 Institute for Cardiovascular Research, University of Leeds, Leeds LS2 9JT, UK 2 Department of Respiratory Medicine, Leeds Teaching Hospitals NHS Trust, Leeds LS1 3EX, UK 3 Department of Physiology, Medical School, Jordan University of Science and Technology, Irbid, Jordan
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(Received 25 August 2006;
accepted after revision 2 January 2007; first published online 4 January 2007)
Corresponding author V. L. Cooper: Institute for Cardiovascular Research, University of Leeds, Leeds LS2 9JT, UK. Email: victoria.cooper{at}leedsth.nhs.uk
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Introduction |
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We have recently examined the daytime variations in carotid baroreceptor control of blood pressure and vascular resistance in normal subjects and found that, at the earliest time studied (09.00 h), baroreflex sensitivity was at its highest and operating point at its lowest (Cooper et al. 2007). We suggest that this reflects a downwards resetting of the reflex during the night and that this may act to have an antihypertensive effect. We hypothesize that, owing to the intermittent episodes of asphyxia or repeated arousals, this nocturnal antihypertensive effect may be absent in patients with OSA. The present study, therefore, was undertaken to examine the daytime variations in baroreceptor function in OSA patients and, in particular, to determine whether the early morning resetting that we had seen in the normal subjects was absent in these patients.
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Methods |
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We studied 20 patients with obstructive sleep apnoea, all of whom were otherwise healthy. Patients had previously been diagnosed on the basis of respiratory variable studies using either the Embletta (Resmed Ltd, Sydney, NSW, Australia) or the Alice 4 system (Respironics Inc., Murrysville, PA, USA). Obstructive sleep apnoea was then confirmed by full polysomnographic sleep studies using the Alice 4 system, recording both respiratory and neurophysiological variables. Patient demographics are shown in Table 1. No patient was taking any medication with a cardiovascular action or was undergoing treatment with continuous positive airway pressure (CPAP) therapy. All were asked to refrain from smoking or drinking caffeine-containing drinks on the day of the experiment and all gave informed written consent. The study was approved by the Leeds Teaching Hospitals Research Ethics Committee, and all procedures conformed to the Declaration of Helsinki.
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The protocol was identical to that in our previous study in normal control subjects (Cooper et al. 2007). Briefly, baroreflex function was studied at 3 h intervals between 09.00 and 21.00 h except in six patients where 21.00 h recordings were not made. The timing of the first measurement was variable, i.e. some studies started later in the day and continued into the following day. Four subjects came to the laboratory from home and had their first recordings at 09.00 h. The other 16 subjects had their first recordings later in the day (four started at 12.00 h, one stared at 15.00 h and 11 started at 18.00 h). When the first recordings were not at 09.00 h, the 09.00 h and all subsequent recordings were taken the following day, after spending the night in hospital. In all cases, the 09.00 h recording occurred within 1–2 h of waking. Meal times were co-ordinated to occur after each recording so that patients had not eaten in the hour prior to a recording. Patients were asked to remain relatively inactive during the day, although they were allowed to do non-physical work, read or watch television between recordings. Patients were studied in a seated position and fitted with a three-lead ECG (Hewlett Packard 78325C, Boebringen, Germany). Finger arterial pressure was recorded continuously during each experiment using a finger photoplethysmographic device (Portapres, Model 2 TNO, Amsterdam, The Netherlands). Brachial arterial pressure was also determined, using a standard sphygmomanometer, taking diastolic pressure as Korotkof phase 5. This was done at least 5 min before the start and 5 min after the end of each series of baroreflex measurements and the values averaged to give the pressure readings for that series. The Portapres readings were verified using the sphygmomanometer readings and where there was a discrepancy in the readings of greater than 10% the finger cuff was repositioned. Brachial artery blood velocity was determined using a pulse wave Doppler system (T2-Dop, DWL Elektronische System GmbH, Sipplingen, Germany) with a 4 MHz probe positioned over the brachial artery at or near the antecubital fossa. The probe was adjusted to provide the strongest signal with the most acute angle of insonation and it was then clamped firmly in position. The probe position was noted and the angle of insonation measured, and these were kept as constant as possible for all subsequent measurements.
Carotid baroreceptor tests
These have been previously described (Cooper et al. 2004, 2005). Briefly, responses to loading/unloading of carotid sinus baroreceptors were assessed by applying suction or pressure to the neck overlying the sinuses in the following order: –40, –20, –10, +10, +20, +40 and +60 mmHg and then again in the reverse order so that each pressure application was made twice. To assess responses of vascular resistance (VR) and mean arterial pressure (MAP) the stimulus was applied for 20 s during spontaneous breathing. Cardiac responses (RR) were assessed separately during a 10 s held expiration with the stimulus being applied for the second 5 s. Pressure was restored to atmospheric between each step.
Data analysis
We assessed vascular resistance as MAP divided by mean blood flow velocity. The average values of VR and MAP for 12 s (three respiratory cycles) prior to the onset of stimulation were taken as the control values. We then calculated a moving average of VR and MAP for each 4 s during the stimulation (i.e. 0–4 s, 1–5 s, etc.). The lowest values of VR and MAP during neck suction and the highest values during neck pressure were compared with the control values obtained before each pressure application. Responses of VR were calculated as percentage changes from control and MAP responses as absolute changes from control values. The responses of RR interval were taken as the maximal changes during the stimulus from the pre-stimulus, three-beat average. Transmission of pressure from the collar to the carotid sinus was assumed to be 100%, and carotid sinus pressure was estimated as mean arterial pressure minus collar pressure.
Responses of the variables were plotted against estimated carotid sinus pressures and the data fitted with either a sigmoid function or a third-order polynomial, depending on which curve best fitted the data (GraphPad Prism v3.0, GraphPad Software Inc., San Diego, CA, USA). The first differentials of these curves were then calculated, and the maximal differentials, which correspond to the maximal slopes of the pressure–response curves, were taken as the measures of baroreflex sensitivity. The carotid sinus pressures corresponding to the maximal differentials were termed operating points, and these values were used to assess baroreflex resetting.
Unless otherwise stated, values are reported as means ± S.E.M. The data were tested for normality and the appropriate parametric or non-parametric statistical analysis was applied. Values for each time point were compared with the values at 09.00 h by means of repeated measures analysis of variance, and Dunnett's post hoc test was performed if P < 0.05 (GraphPad Instat, GraphPad Software Inc.).
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Results |
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Baroreflex sensitivity and operating point for the control of RR are shown in Fig. 2. There was considerable intersubject variability and no significant change in either baroreflex sensitivity or operating point throughout the day. This was consistent with previous results in control subjects (Cooper et al. 2007).
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Baroreflex sensitivity and operating point for the control of VR are shown in Fig. 3. Sensitivity was significantly lower (P < 0.05) at 09.00 h compared with 12.00 and 15.00 h. Operating point, however, showed no consistent change. We also determined the responses to neck suctions and neck pressures separately, to determine whether any observed effects resulted only from loading or unloading the baroreceptors. We found that any changes in reflex sensitivity occurred in both directions. That is, where responses to neck suctions were lower, so too were the responses to neck pressures.
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Figure 4 displays baroreflex sensitivity and operating point for the control of MAP. Baroreflex sensitivity showed no consistent change. Operating point tended to be higher at 09.00 h compared with later in the day, but this was only statistically significant when compared with 12.00 h (P < 0.05).
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To examine whether the level of blood pressure influenced the results, patients were divided into normotensive (NT) and hypertensive (HT) based on their average blood pressure readings over the day. Twelve patients had average blood pressures < 140/< 90 mmHg and were considered NT (mean ± S.D. and range of systolic pressures were 122 ± 11.1 and 98–134 mmHg). Seven had pressures > 140/> 90 mmHg and were designated as HT (mean ± S.D. and range of systolic pressures were 150 ± 8.1 and 141–166 mmHg). One patient, with an average systolic pressure of 138 mmHg, was excluded from this analysis because her blood pressure was considered borderline. Figures 5 and 6 show the baseline mean arterial pressure and vascular resistance, respectively. In HT patients, MAP tended to increase throughout the day and was significantly higher at 18.00 compared with 09.00 h. In contrast, in NT patients there was no significant change in MAP throughout the day. Vascular resistance tended to decrease throughout the day in both groups. We analysed the baroreflex data from these subgroups. The values of baroreflex sensitivity for the control of VR and operating point for the control of MAP are shown in Figs 7 and 8, respectively. In HT patients, baroreflex sensitivity showed the same trend as for the combined data. For example, sensitivity was lower at 09.00 h when compared with 12.00 h (P = 0.05) and 18.00 h (P < 0.05). Operating point for the control of MAP tended to be higher at 09.00 h compared with 12.00 h (P = 0.09) but was not different throughout the rest of the day. In the NT patients, baroreflex sensitivity for control of VR tended to be low and showed little variation throughout the day. Operating point for control of blood pressure tended to be higher at 09.00 h, significantly so (P < 0.05) compared with the value at 15.00 h. All other measures of baroreceptor control showed no significant change in either group. There was no significant difference in apnoea/hypopnoea index (AHI) between NT and HT groups.
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Discussion |
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Baroreflexes and hypertension
For many years it was thought that baroreflexes were only important in short-term (seconds to minutes) buffering of blood pressure changes and had no implications for the longer term control of arterial pressure. This is because baroreceptors are known to reset towards the prevailing MAP even when it is changed only for a short period (McCubbin et al. 1956; McMahon et al. 1996). However, there is an emerging body of evidence from experimental models of hypertension to suggest that baroreceptors may have a role in the long-term control of blood pressure (Thrasher, 2002; Barrett et al. 2003; Lohmeier et al. 2004). One possibility is that the control of vascular resistance may reset more slowly than that of heart rate (Thrasher, 2004). This is important because it is vascular resistance that is primarily responsible for determining the level of arterial blood pressure (Sundlöf & Wallin, 1978; Charkoudian et al. 2005).
Daytime variations in baroreflex function
We found no significant time-related variation in either sensitivity or operating point for baroreflex control of pulse interval. This is consistent with the results seen in control subjects (Cooper et al. 2007). Narkiewicz et al. (1998a) also found no impairment of baroreflex control of heart rate in patients with OSA compared with control subjects. In our study, we did not compare the absolute sensitivities or operating points in our patients with those in control subjects since they were not matched for age, which is known particularly to affect baroreflex sensitivity, or neck thickness, which would influence the transmission of the applied pressure to the carotid sinuses. We only compared the changes occurring over the day, and these showed no consistent change in either group
In contrast to the lack of consistent change in the control of heart rate, we found that in the OSA patients, baroreflex control of blood pressure and vascular resistance did vary during the day. The sensitivity for the control of vascular resistance was at its lowest at 09.00 h. This was directly opposite to that which we had observed in normal subjects (Cooper et al. 2007). We also found that the level of vascular resistance in the patients tended to be highest at 09.00 h, which could be explained by the low baroreflex sensitivity. Work by Hossman et al. (1980) and Panza et al. (1991) suggests that there is a decline in sympathetic vasoconstrictor activity at around midday, which is compatible with our findings of a decrease in vascular resistance at this time.
We compared the responses to increases in baroreceptor stimulation with those to decreases to assess whether there was any non-linearity of the reflex. It could be argued that changes in the baseline vascular resistance might themselves result in alteration of the responses. For example, the increased vascular resistance at 09.00 h may have resulted in reduced responsiveness to baroreflex deactivation owing to reduced reserve at this level of vasoconstriction. However, we found that the responses to both activation and deactivation of baroreceptors at 09.00 h were reduced compared with later in the day. This differs slightly from the findings of Narkiewicz et al. (1998a), who reported that responses of sympathetic nerve activity to hypotension induced by sodium nitroprusside were reduced at that time in patients with OSA, but the responses to hypertension induced by phenylephrine remained unchanged.
Implications of results for hypertension
It is possible that, in healthy normotensive subjects, resetting of the baroreflex stimulus–response curve to lower pressures may protect against hypertension. It is well known that blood pressure normally decreases during sleep (Weber et al. 1984), and our recent research has suggested that in the mornings, in normotensive control subjects, sensitivity is relatively high and operating point low (Cooper et al. 2007). Thus, the healthy normotensives would start each day not only with low blood pressure but with the baroreceptor reflex functioning, at least initially, to maintain the low pressure. In the OSA patients, however, this protective mechanism appears to be absent, with sensitivity tending to be low and operating point high at the start of each day. Then, during the day, subsequent events provoking increases in pressure would tend to increase pressure further and consequently might eventually lead to hypertension. The reason for the higher morning blood pressures in OSA patients may be related to the frequent nocturnal apnoeic events, which cause large increases in blood pressure and consequently interfere with the resetting process.
In both normotensive subjects and patients with essential hypertension, there has been shown to be a higher incidence of cardiovascular and cerebrovascular events in the early morning, shortly after awakening and arising (Muller et al. 1985; Panza et al. 1991; Weber, 2002; Kario et al. 2004), and a number of investigators have attributed this to decreased baroreflex sensitivity at that time (Kawano et al. 1995; Tochikubo et al. 1997; Nakazato et al. 1998). Our finding of a significantly higher operating point for MAP responses at 09.00 h in our patients suggests that they too could be at increased risk in the mornings. Interestingly, the higher morning operating point was more prominent in the normotensive patients than in the hypertensive patients. In normotensive patients, operating point was highest at 09.00 h but then fell and remained lower for the rest of the day, whereas in hypertensive patients, although operating point tended to be reduced at 12.00 h compared with 09.00 h, it tended to stay at a higher level throughout the day. Whether the raised operating point is the cause or effect of increased blood pressure is not clear. However, it may be that our normotensive patients are in the initial stages of altered blood pressure control. It is possible that the first effect is for blood pressure to be increased only in the mornings, but then subsequently it would remain higher throughout the day. We also found that sensitivity was low throughout the day in normotensive patients, whereas in hypertensive patients it was only reduced at 09.00 h and then increased. This was surprising because we would have expected that a lower sensitivity would be associated with an increased pressure. Baseline vascular resistance decreased during the day in a similar fashion in both groups, so this is unlikely to have been the cause of the observed variation.
Limitations of the study
Our study of baroreflex function was performed using the neck collar method, and this applies a stimulus only to carotid baroreceptors. Any resulting responses would rapidly be buffered by other reflexes, including aortic and coronary baroreceptors. We attempted to minimize this effect by determining only the early maximal changes. We felt that the advantages of the neck collar method outweighed its disadvantages. In particular, it is noninvasive and allows the full stimulus–response curve to be defined.
Another limitation to this study is that measurements of baroreflex function were made only over a 12 h daytime period, with no measurements being made during the night. However, because the nature of these tests requires the subject to be awake and co-operative and we wished them to continue their normal activities as far as possible, we did not attempt night-time studies. Furthermore, night-time changes in baroreflex function are thought to be related mainly to the degree of arousal (Bristow et al. 1969; Kasting et al. 1987) and, to avoid this, the patients were first studied between 1 and 2 h after waking.
One of the potential problems when comparing results from patients with OSA with those from normal subjects, as in our previous study (Cooper et al. 2007), is that OSA patients are almost always overweight, and this may well have influenced the results, partly because, owing to the thickness of the neck, the transmission of the applied pressure to the carotid sinus would be likely to be less. We assumed transmission to be 100%, but particularly in OSA patients it may have been considerably less. This would reduce the measured sensitivity, but it should not have affected the daytime changes, which were what we were actually studying.
The effects of obesity on the autonomic nervous system are unclear. Obesity per se may also be linked to hypertension, with activation of the sympathetic nervous system through hyper-insulinaemia (Fletcher, 2000). Alvarez et al. (2002) found elevated muscle sympathetic nerve activity (MSNA) in subjects with elevated abdominal visceral fat compared with controls matched for age, total fat mass, or abdominal subcutaneous fat. However, they did not find any difference in responses of sympathetic nerve activity to blood pressure changes. The elevated MSNA in obese subjects is in contrast to the findings of Narkiewicz et al. (1998b), who reported that obesity alone, in the absence of OSA, was not accompanied by increased MSNA. The reasons for these differing results could be: (i) Narkiewicz et al. (1998b) did not control for the fat deposition, but defined obesity based on body mass index (BMI); and (ii) Alvarez et al. (2002) did not exclude the presence of OSA in their subjects. Thus we are uncertain of the importance of the BMI difference between our control subjects and patients in the present study.
Conclusions
We undertook this study to determine whether OSA causes alteration in the daytime reflex control of blood pressure. We have demonstrated that there are differences in the way blood pressure is controlled at different times of day, and this should be taken into account during any treatment or mechanistic study of the effects of OSA on blood pressure control. We also reasoned that in normal subjects, a night's rest, when blood pressure is known to be reduced (Bristow et al. 1969), would alter the characteristics of the reflex in a way that might favour a more prolonged lowering of blood pressure. Failure of this to occur, possibly as a result of the apnoeic episodes, might predispose to development of hypertension. The results have largely upheld this hypothesis, in that the lower operating point for blood pressure responses and the higher baroreflex sensitivity at 09.00 h compared with later in the day that we found in normal subjects were reversed in patients with OSA, so that baroreflex sensitivity was lower and operating point higher at 09.00 h. These alterations may contribute to the increased incidence of arterial hypertension associated with OSA.
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References |
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Barrett CJ, Ramchandra R, Guild SJ, Lala A, Budgett DM & Malpas SC (2003). What sets the long-term level of renal sympathetic nerve activity? Circ Res 92, 1330–1336.
Bixler EO, Vgontzas AN, Lin H-O, Have TT, Leiby BE, Vela-bueno A & Kales A (2000). Association of hypertension and sleep-disordered breathing. Arch Intern Med 160, 2289–2295.
Bristow JD, Honour AJ, Pickering TG & Sleight P (1969). Cardiovascular and respiratory changes during sleep in normal and hypertensive subjects. Cardiovasc Res 3, 476–485.
Carlson JT, Hedner JA, Ejnell H & Peterson L-E (1994). High prevalence of hypertension in sleep apnea patients independent of obesity. Am J Respir Crit Care Med 150, 72–77.[Abstract]
Charkoudian N, Joyner MJ, Johnson CP, Eisenbach JH, Dietz NM & Wallin BG (2005). Balance between cardiac output and sympathetic nerve activity in resting humans: role in arterial pressure regulation. J Physiol 568, 315–321.
Cooper VL, Bowker CM, Pearson SB, Elliott MW & Hainsworth R (2004). Effects of simulated obstructive sleep apnoea on the human carotid baroreceptor–vascular resistance reflex. J Physiol 557, 1055–1065.
Cooper VL, Elliott MW, Pearson SB, Taylor CM & Hainsworth R (2007). Daytime variability in carotid baroreflex function in healthy human subjects. Clin Autonomic Res, 26–33.
Cooper VL, Pearson SB, Bowker CM, Elliott MW & Hainsworth R (2005). Interaction of chemoreceptor and baroreceptor reflexes by hypoxia and hypercapnia – a mechanism for promoting hypertension in obstructive sleep apnoea. J Physiol 568, 677–687.
Fletcher EC (2000). Hypertension in patients with sleep apnoea, a combined effect? Thorax 55, 726–728.
Fletcher EC, Bao G & Li R (1999). Renin activity and blood pressure in response to chronic episodic hypoxia. Hypertension 34, 309–314.
Fletcher EC, Lesske J, Culman J, Miller CC & Unger T (1992). Sympathetic denervation blocks blood pressure elevation in episodic hypoxia. Hypertension 20, 612–619.
Hossmann V, Fitzgerald GA & Dollery CT (1980). Circadian rhythm of baroreflex reactivity and adrenergic vascular response. Cardiovasc Res 14, 125–129.[Medline]
Kario K, Pickering TG, Hoshide S, Eguchi K, Ishikawa J, Morinari M, Hoshide Y & Shimada K (2004). Morning blood pressure surge and hypertensive cerebrovascular disease. Am J Hypertens 17, 668–675.[CrossRef][Medline]
Kasting GA, Eckberg DL, Fritsch JM & Birkett CL (1987). Continuous resetting of the human carotid baroreceptor-cardiac reflex. Am J Physiol Regul Integr Comp Physiol 252, R732–R736.
Kawano Y, Tochikubo O, Miyajima E & Ishii M (1995). Circadian variation in baroreflex sensitivity evaluated beat-to-beat hemodynamic change in patients with essential hypertension. J Cardiol 26, 159–165.[Medline]
Lohmeier TE, Irwin ED, Rossing MA, Serdar DJ & Kieval RS (2004). Prolonged activation of the baroreflex produces sustained hypotension. Hypertension 43, 306–311.
McCubbin JW, Green JH & Page IH (1956). Baroreceptor function in chronic renal hypertension. Circ Res 4, 205–210.[Medline]
McMahon NC, Drinkhill MJ & Hainsworth R (1996). Reflex vascular responses to stimulation of carotid, aortic and coronary baroreceptors in the anaesthetized dog. Exp Physiol 81, 397–408.[Abstract]
Mancia G, Parati G, Pomidossi G, Casadei R, Di Rienzo M & Zanchetti A (1986). Arterial baroreflexes and blood pressure and heart rate variabilities in humans. Hypertension 8, 147–153.
Morgan BJ, Crabtree DC, Palta M & Skatrud JB (1995). Combined hypoxia and hypercapnia evokes long-lasting sympathetic activation in humans. J Appl Physiol 79, 205–213.
Muller JE, Stone PH, Turi ZG, Rutherford JD, Czeisler CA, Parker Cet al. (1985). The MILIS Study Group. Circadian variation in the frequency of onset of acute myocardial infarction. N Engl J Med 313, 1315–1322.[Abstract]
Nakazato T, Shikama T, Toma S, Nakajima Y & Masuda Y (1998). Nocturnal variation in human sympathetic baroreflex sensitivity. J Auton Nerv Syst 70, 32–37.[CrossRef][Medline]
Narkiewicz K, Pesek CA, Kato M, Phillips BG, Davison DE & Somers VR (1998a). Baroreflex control of sympathetic nerve activity and heart rate in obstructive sleep apnea. Hypertension 32, 1039–1043.
Narkiewicz K, van de Borne PJH, Cooley RL, Dyken ME & Domers VK (1998b). Sympathetic activity in obese subjects with and without obstructive sleep apnea. Circulation 98, 772–776.
Panza JA, Epstein SE & Quyyumi AA (1991). Circadian variation in vascular tone and its relation to 
-sympathetic vasoconstrictor activity. N Engl J Med 325, 986–990.[Abstract]
Peppard P, Young T, Palta M & Skatrud J (2000). Prospective study of the association between sleep-disordered breathing and hypertension. N Engl J Med 342, 1378–1384.
Sica AL, Greenberg HE, Ruggiero DA & Scharf SM (2000). Chronic intermittent hypoxia: a model of sympathetic activation in the rat. Respir Physiol 121, 173–184.[CrossRef][Medline]
Sundlöf G & Wallin BG (1978). Human muscle nerve sympathetic activity at rest. Relationship to blood pressure and age. J Physiol 274, 621–637.
Thrasher TN (2002). Unloading arterial baroreceptors causes neurogenic hypertension. Am J Physiol Regul Integr Comp Physiol 282, R1044–R1045.
Thrasher TN (2004). Baroreceptors and the long-term control of blood pressure. Exp Physiol 89, 331–341.
Tochikubo O, Kawano Y, Miyajima E, Toshihiro N & Ishii M (1997). Circadian variation of haemodynamics and baroreflex functions in patients with essential hypertension. Hypertens Res 20, 157–166.[Medline]
Weber MA (2002). The 24-hour blood pressure pattern: does it have implications for morbidity and mortality? Am J Cardiol 89, 27A–33A.[Medline]
Weber MA, Drayer JI, Nakamura DK & Wyle FA (1984). The circadian blood pressure pattern in ambulatory normal subjects. Am J Cardiol 54, 115–119.[CrossRef][Medline]
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