Open Access
Open Peer Review

This article has Open Peer Review reports available.

How does Open Peer Review work?

Effects of low-dose clonidine on cardiovascular and autonomic variables in adolescents with chronic fatigue: a randomized controlled trial

  • Even Fagermoen1, 2Email author,
  • Dag Sulheim3, 4,
  • Anette Winger5,
  • Anders M. Andersen6,
  • Johannes Gjerstad7, 8,
  • Kristin Godang9,
  • Peter C. Rowe10,
  • J. Philip Saul11,
  • Eva Skovlund12, 13 and
  • Vegard Bruun Wyller1, 14
BMC Pediatrics201515:117

https://doi.org/10.1186/s12887-015-0428-2

Received: 7 September 2014

Accepted: 20 August 2015

Published: 10 September 2015

Abstract

Background

Chronic Fatigue Syndrome (CFS) is a common and disabling condition in adolescence with few treatment options. A central feature of CFS is orthostatic intolerance and abnormal autonomic cardiovascular control characterized by sympathetic predominance. We hypothesized that symptoms as well as the underlying pathophysiology might improve by treatment with the alpha2A–adrenoceptor agonist clonidine.

Methods

A total of 176 adolescent CFS patients (12–18 years) were assessed for eligibility at a single referral center recruiting nation-wide. Patients were randomized 1:1 by a computer system and started treatment with clonidine capsules (25 μg or 50 μg twice daily, respectively, for body weight below/above 35 kg) or placebo capsules for 9 weeks. Double-blinding was provided. Data were collected from March 2010 until October 2012 as part of The Norwegian Study of Chronic Fatigue Syndrome in Adolescents: Pathophysiology and Intervention Trial (NorCAPITAL). Effect of clonidine intervention was assessed by general linear models in intention-to-treat analyses, including baseline values as covariates in the model.

Results

A total of 120 patients (clonidine group n = 60, placebo group n = 60) were enrolled and started treatment. There were 14 drop-outs (5 in the clonidine group, 9 in the placebo group) during the intervention period. At 8 weeks, the clonidine group had lower plasma norepinephrine (difference = 205 pmol/L, p = 0.05) and urine norepinephrine/creatinine ratio (difference = 3.9 nmol/mmol, p = 0.002). During supine rest, the clonidine group had higher heart rate variability in the low-frequency range (LF-HRV, absolute units) (ratio = 1.4, p = 0.007) as well as higher standard deviation of all RR-intervals (SDNN) (difference = 12.0 ms, p = 0.05); during 20° head-up tilt there were no statistical differences in any cardiovascular variable. Symptoms of orthostatic intolerance did not change during the intervention period.

Conclusions

Low-dose clonidine reduces catecholamine levels in adolescent CFS, but the effects on autonomic cardiovascular control are sparse. Clonidine does not improve symptoms of orthostatic intolerance.

Trial registration

Clinical Trials ID: NCT01040429, date of registration 12/28/2009.

Background

Chronic Fatigue Syndrome (CFS) is a disabling condition with unknown pathophysiology. In adolescents, prevalence has been estimated from 0.1 to 2.4 % depending on definition of CFS and method of estimation [1, 2]. Apart from a single trial of intravenous immunoglobulin in adolescents with CFS [3], no pharmacotherapy has proven beneficial in this patient population.

Orthostatic intolerance is common with a prevalence of more than 25 % in adults with CFS [4], and more than 90 % in children with CFS [5, 6]. Previously, dysregulation of autonomic cardiovascular control has been demonstrated in adults as well as adolescents, characterized by increased sympathetic and decreased parasympathetic nervous activity [710]. This autonomic imbalance might reflect alteration of central control mechanism [11, 12], and provide a target for pharmacotherapy [7, 13].

Clonidine is a centrally acting agonist to the presynaptic alpha2A receptor, thereby attenuating sympathetic nervous activity and enhancing parasympathetic activity, even in low doses [1416]. Thus, clonidine has well-known antihypertensive properties. A pilot study suggested normalization of cardiovascular variables in adolescent CFS patients receiving low-dose clonidine [17]. However, a single nucleotide polymorphism (SNP) of the alpha2A receptor gene might possible modify the effect of clonidine treatment [18].

The aim of this study was to investigate the effects of low-dose clonidine on autonomic cardiovascular control in adolescent CFS. We hypothesized that clonidine would improve symptoms of orthostatic intolerance and normalize cardiovascular variables and indices of autonomic nervous activity at rest as well as during orthostatic challenges. The study is part of the NorCAPITAL-project (The Norwegian Study of Chronic Fatigue Syndrome in Adolescents: Pathophysiology and Intervention Trial; ClinicalTrials ID: NCT01040429, date of registration 12/28/2009).

Methods

Patients

All hospital pediatric departments in Norway (n = 20) as well as primary care pediatricians and general practitioners were invited to refer patients aged 12 – 18 years to the national referral center for young CFS patients at Oslo University Hospital. The referring units were equipped with written information for distribution to potential study participants and their parents/next-of-kin. If consent was given, a standard form required the referral unit to confirm the result of clinical investigations considered compulsory to diagnose pediatric CFS according to national Norwegian recommendations (pediatric specialist assessment, comprehensive hematology and biochemistry analyses, chest x-ray, abdominal ultrasound, and brain magnetic resonance imaging). Also, the referring units were required to confirm that the patient a) was unable to follow normal school routines due to fatigue; b) was not permanently bedridden; c) did not have any concurrent medical or psychiatric disorder that might explain the fatigue; d) did not experience any concurrent demanding life event (such as parents’ divorce) that might explain the fatigue; e) did not use prescribed pharmaceuticals (including hormone contraceptives) regularly. A previous demanding life event was not an exclusion criterion. Completed forms were consecutively conveyed to the study center and carefully evaluated by either of two authors (DS or EF). Patients considered eligible to this study were invited to a clinical encounter at our study center after which a final decision on inclusion was made.

In agreement with clinical guidelines [19, 20], this study applied a “broad” case definition of CFS, requiring three months of unexplained, disabling chronic/relapsing fatigue of new onset. We did not require that patients meet any other accompanying symptom criteria. Details of inclusion and exclusion criteria are provided in Table 1.
Table 1

Criteria for inclusion and exclusion

 

Inclusion criteria

Exclusion criteria

CFS patients

Persisting or constantly relapsing fatigue lasting 3 months or more.

Another current disease process or demanding life event that might explain the fatigue

 

Functional disability resulting from fatigue to a degree that prevent normal school attendance

Another chronic disease

 

Age ≥ 12 years and < 18 years

Permanent use of drugs (including hormones) possibly interfering with measurements

  

Permanently bed-ridden

  

Positive pregnancy test

  

Pheocromocytoma

  

Evidence of reduced cerebral and/or peripheral circulation due to vessel disease

  

Polyneuropathy

  

Renal insufficiency

  

Known hypersensitivity towards clonidine or inert substances (lactose, saccarose) in capsule

  

Abnormal ECG (apart from ectopic beats)

  

Supine heart rate < 50 beats/min

  

Supine systolic blood pressure < 85 mmHg

  

Upright systolic blood pressure fall > 30 mmHg

Healthy control subjects

Age ≥ 12 years and < 18 years

Another chronic disease

  

Permanent use of drugs (including hormones)

Study design

All included patients underwent a baseline investigational program at our research unit. Thereafter, they were randomized to 9 weeks of treatment with oral clonidine capsules or placebo capsules in a 1:1 ratio, using a computer-based routine for stratified randomization (block size: 4); 18 months disease duration (the median disease duration in a previous follow-up study [21]) served as the stratification criterion. Because of practical issues, randomization was performed prior to final decision on enrolment; the procedure was carried out by a research nurse not otherwise affiliated with the study. Outcome was assessed by an investigational program identical to the baseline program at week 8 and week 30; in this article, only results from week 8 are reported. Patients and researchers were blinded to treatment allocation at all stages.

Clonidine dosages were 50 μg B.I.D for body weight >35 kg, and 25 μg B.I.D for body weight < 35 kg. Catapresan® 25 μg clonidine hydrochloride tablets (Boehringer Ingelheim, Germany) were enclosed in orange opaque, demolition-restraint lactose capsules (Apoteket Produktion & Laboratorier, Kungens Kurva, Sweden). Identical capsules without Catapresan® were used as placebo comparator. Half the dose was given for the first 3 days of the intervention period in order to minimize adverse introductory effects. Blood samples for clonidine concentration analyses were taken approximately two weeks after start of the intervention, and at the second visit.

NorCAPITAL was approved by the Norwegian National Committee for Ethics in Medical Research and the Norwegian Medicines Agency. Data were collected in the period March 2010 until October 2012. Written informed consent was obtained from all participants, and from parents/next-of-kin if required.

Investigational program

A one-day in-hospital assessment included clinical examination, blood sampling (antecubital venous puncture), and 20° head-up tilt test (HUT), and always commenced between 7.30 and 9.30 a.m. Patients were instructed to fast overnight and abstain from tobacco products and caffeine for at least 48 h, to bring a morning spot urine sample in a sterile container, and to apply the local anesthetic lidocaine (Emla®) on the skin in the antecubital area one hour in advance. At week 8, CFS patients were told to postpone their prescribed morning study drug dose (clonidine/placebo) until after blood sampling and HUT. All procedures were undertaken in a quiet, warm room in a fixed sequence and by three researchers only (DS, EF and AW). Blood samples were obtained in a fixed sequence from antecubital venous puncture after at least five minutes supine rest in calm surroundings. Samples of oral mucosa were collected for genetic analyses. Following the in-hospital assessment, a self-administered questionnaire was completed.

Laboratory analyses

The blood samples for plasma norepinephrine (NA) and epinephrine (A) analyses were obtained in vacutainer tubes treated with ethylene glycol tetraacetic acid (EGTA)–Glutathione. The samples were placed on ice for approximately 30 min; thereafter, plasma was separated by centrifugation (3000 rpm, 15 min, 4 °C) and frozen at – 80 °C until assayed. Samples were analyzed for plasma NA and A by high-performance liquid chromatography (HPLC) with a reversed-phase column and glassy carbon electrochemical detector (Antec, Leyden Deacade II SCC, Zoeterwoude, The Netherlands) using a commercial kit (Chromsystems, München, Germany) [2224]. All samples were measured in singlet, with serial samples from a given individual run at the same time to minimize run-to-run variability. The intra- and interassay coefficient of variation (CV) were 3.9 and 10.8 %, respectively. The detection limit was 5.46 pM.

Urine samples for NA and A analyses were collected in 10 ml universal containers. Immediately after collection the urine was acidified to pH ≈ 2.5, thereafter, stored at 2–8 °C until assayed. Urine treated this way is stable at least 5 days. The analyses were performed consecutively. The same HPLC protocol as for plasma measurement was used for the measurement of urin NA/A. The intra- and interassay coefficient of variation (CV) for urine were 3.9 and 5.2 %, respectively.

The blood samples for clonidine determinations were collected in 4 mL heparin tubes. After centrifugation for 12 min at 1000 g at room temperature, the plasma fraction was frozen at −20 °C until analysis. A slight modification of the method described by Müller et al. [25] was used for plasma clonidine assaying. The assay was validated based on FDA guidelines [26]. The samples were separated on an Alliance HT 2795 HPLC system and detected by a Micromass Quattro micro API MS/MS-instrument. System control, data acquisition and integration were performed by Masslynx software Ver 4.1.2008 (all from Waters, Milford, MA, USA). The MS/MS conditions were optimized by manual tuning during pump-infusion of neat solutions. The assay was set up to quantify from 0.10 μg/L to 5.00 μg/L clonidine in plasma. Quality control samples were included in all sample series, and placed both before and after the patient samples in each analytical run. The median intra assay CV was 1 % at 5 μg/L, 5 % at 0.75 μg/L and 10 % at 0.10 μg/L. The inter assay CV was 6 % at 5 μg/L, 5 % at 0.75 μg/L and 12 % at 0.10 μg/L. Limit of detection, defined as a peak-to-peak signal to noise ratio of 5:1, verified by the Masslynx software, was 0.025 μg/L. Accuracy was 97 % (median) at 5 μg/L, 97 % at 0.75 μg/L, and 107 % at 0.10 μg/L.

The genotyping of the alpha2A receptor single nucleotide polymorphism (SNP) rs1800544 was carried out by predesigned TaqMan SNP genotyping assay (Applied Biosystems, Foster City, CA, USA), using the SDS 2.2 software (Applied Biosystems). As previously described, approximately 10 % of the samples were re-genotyped, and the concordance rate was 100 % [27]. Genotyping was also performed in 68 healthy individuals having the same distribution of gender and age as the CFS patients.

Head-up tilt-test

Head-up tilt-test (HUT) was performed using an electronically operated tilt table with foot-board support (Model 900–00, CNSystems Medizintechnik, Graz, Austria). Patients were connected to the Task Force Monitor (TFM) (Model 3040i, CNSystems Medizintechnik, Graz, Austria), a combined hardware and software device for noninvasive recording of cardiovascular variables. 5 min was used for supine recordings, after which the participants were head-up tilted to 20° for 15 min. Details of the HUT protocol have been described elsewhere [9]. The feasibility of this protocol for studying adolescent CFS patients has been demonstrated in several previous studies [9, 28]. In particular, the low tilt angle (20°) does not normally precipitate syncope, which is otherwise a common problem among adolescents being subjected to stronger orthostatic challenges [29]. Still, 20° head-up tilt is sufficient to demonstrate hemodynamic alterations and compensatory autonomic responses.

Instantaneous RR intervals (RRI) and heart rate (HR) were obtained from the electrocardiogram (ECG). Continuous arterial blood pressure was obtained noninvasively using photoplethysmography on the right middle finger. Mean arterial blood pressure (BP) was calculated by numerical integration of the recorded instantaneous BP. The recorded value was calibrated against conventional oscillometric measurements of arterial BP on the left arm every five minutes according to the TFM manufacturer’s recommendation. Impedance cardiography with electrodes placed on the neck and upper abdomen was used to obtain a continuous recording of the temporal derivative of the transthoracic impedance (dZ/dt). Beat-to-beat stroke volume was calculated from the impedance signal [30].

Power spectral analysis (frequency-domain method) of HR variability and systolic blood pressure (SBP) variability was automatically provided by the TFM, using an adaptive autoregressive model [31]. Power was calculated in the Low Frequency (LF) range (0.05 to 0.17 Hz), and High Frequency (HF) range (0.17 to 0.4 Hz). In addition, time-domain indices of variability were computed from the RRIs: The standard deviation of all RR-intervals (SDNN), the proportion of successive RRIs with a difference greater than 50 ms (pNN50), and the square root of the mean square differences of successive RRIs (r-MSSD).

Heart rate variability (HRV) is considered an index of autonomic cardiac modulation. In the frequency-domain, vagal (parasympathetic) activity is the main contributor to HF variability, whereas both vagal and sympathetic activity contributes to LF variability [32]. The LF/HF ratio is considered an index of sympathovagal balance. SBP variability is regarded an index of sympathetic modulation of peripheral resistance vessels [33]. For time-domain indices, vagal (parasympathetic) activity is the main contributor to pNN50 and r-MSSD, whereas SDNN is a measure of total variability, analogous to the Total Power index in the frequency domain.

Data from each HUT procedure was exported to Microsoft Excel for further calculations. Beat-to-beat stroke index (SI) was calculated dividing stroke volume by body surface area, and beat-to-beat total peripheral resistance index (TPRI) was calculated as mean BP divided by the product of SI and HR. For each participant, the following epochs of the recordings were chosen: Baseline (270 to 30 s before tilt up) and Early tilt (30 to 270 s after tilt). In each epoch we computed the median value for the conventional cardiovascular variables as well as the indices of HR and SBP variability; this procedure reduces the influence of erroneous outliers, such as ectopic heart beats. Thereafter, the delta values (Early Tilt – Baseline) which are considered indices of the cardiovascular response to orthostatic challenge were computed for each participant. This analytic approach has been proven feasible in several previous report from our group [911].

Questionnaire

The participants received a comprehensive questionnaire consisting of several validated inventories, as has been described in detail elsewhere [28].

The Autonomic Symptom Profile (ASP) [34], which has been used in previous Norwegian CFS studies but which is not validated for the Norwegian language, was slightly modified in order to fit our age group. A composite score reflecting orthostatic symptoms was constructed from 8 single items from the ASP, addressing experiences of dizziness in specific situations (such as rising suddenly from supine position, taking a shower, etc.). The total sum score is from 0 to 8; higher values reflect more pronounced orthostatic problems. In addition, other symptoms related to autonomic cardiovascular control, such as palpitations and pale and cold hands, were charted on a 1–5 Likert scale.

The questionnaire also included the CFS symptom inventory for adolescents [28, 35]. This inventory was used to subgroup the CFS patients according to the 1994 CFS case definition [36].

Statistics

Determination of sample size is described elsewhere [28]. Outcome of clonidine intervention was assessed by general linear models (ANCOVA) in intention-to-treat analyses, including baseline values as covariates in the model [37]. The net intervention effect was calculated from the parameters of the fitted general linear model. Differential effects in subgroups adhering to the 1994 CFS case definition, genotype of the alpha2A receptor single nucleotide polymorphism (SNP) rs1800544, and sex, were explored by including these variables as interaction terms. Dose–response relationships for patients allocated to clonidine were explored by linear regression analyses. Missing values were imputed as last observation carried forward from the pre-medication test. In order to obtain near-normally distributed variables, ln-transformation was carried out for supine values of LF-HRV, HF-HRV, Total Power-HRV, LF/HF ratio and LF-SBP. Square root transformation was carried out for 20° head-up tilt values of LF-HRV, HF-HRV and Total Power-HRV. Genotype frequency among patients and healthy controls were explored with chi-square analyses.

SPSS statistical software (SPSS Inc., Chicago, IL, USA) was applied for all statistical analyses, and all tests were carried out two-sided. A p-value ≤ 0.05 was considered statistically significant. Corrections for multiple comparisons were not applied.

Results

A total of 176 CFS patients were referred to the study, of which 151 were eligible for randomization (Fig. 1). A total of 120 patients were enrolled and started treatment; 60 patients in the clonidine group and 60 patients in the placebo group. At week 8, there were 5 dropouts in the clonidine group and 9 dropouts in the placebo group (Fig. 1). Further baseline demographic and clinical characteristics are given in Table 2.
Fig. 1

Study flowchart. Study flowchart. A total of 176 adolescents with CFS were assessed for eligibility. Of these, 151 fulfilled randomization criteria, whereas 120 started treatment. At week 8, 106 participants were still participating in the intervention program, 55 in the clonidine group and 51 in the placebo group

Table 2

Background characteristics

 

Clonidine (n = 60)

Placebo (n = 60)

Gender - no. (%)

  

 Male

13 (22)

21 (35)

 Female

47 (78)

39 (65)

Age - years, mean ± SD

15.3 ± 1.5

15.5 ± 1.6

BMI - kg/m2, mean ± SD

21.6 ± 4.4

21.5 ± 4.0

Adheres to 1994 CFS case definition - no. (%)

  

 No

14 (24)

15 (26)

 Yes

45 (76)

43 (74)

Genotype a – no. (%)

  

 C/C

32 (53)

35 (58)

 C/G

25 (42)

19 (32)

 G/G

3 (5)

6 (10)

Disease duration - months, median (range)

18 (4 to 72)

18 (5 to 104)

Disease duration – months, mean ± SD

19.4 ± 13.0

23.5 ± 17.0

School absenteism - %, mean ± SD

66 ± 29

64 ± 31

Smokers – more than once a week – no.

1

0

a The alpha2A receptor single nucleotide polymorphism (SNP) rs1800544. C = Cytosine, G = Guanine

At week 8, the clonidine group had statistically significantly lower plasma norepinephrine (p = 0.05) and urine norepinephrine/creatinine ratio (p = 0.002) as compared to the placebo group (Table 3). At supine rest, the clonidine group had higher heart rate variability in the low-frequency band (LF-HRV, absolute unites) (p = 0.007) and as well as higher SDNN (p = 0.05) (Table 4). No other significant differences were observed. In particular, symptoms of orthostatic intolerance did not change during the intervention period.
Table 3

Outcome of clonidine intervention – symptom scores and catecholamines

 

Baseline

Week 8 (during treatment)

Symptoms scores

  

Orthostatic symptoms – total score

  

 Clonidine group, mean

3.8

3.5

 Placebo group, mean

3.5

3.5

 Difference (95 % CI)

 

−0.05 (−0.5 to 0.4)

 p-value (clonidine vs. placebo)

 

0.84

Palpitations - score

  

 Clonidine group, mean

2.4

2.2

 Placebo group, mean

2.2

2.2

 Difference (95 % CI)

 

0.06 (−0.3 to 0.4)

 p-value (clonidine vs. placebo)

 

0.73

Pale and cold hands - score

  

 Clonidine group, mean

3.0

2.7

 Placebo group, mean

3.0

2.8

 Difference (95 % CI)

 

−0.1 (−0.5 to 0.3)

 p-value (clonidine vs. placebo)

 

0.62

Catecholamines

  

Plasma norepinephrine - pmol/L

  

 Clonidine group, mean

2040

1557

 Placebo group, mean

1942

1761

 Difference (95 % CI)

 

−205 (−406 to −4)

 p-value (clonidine vs. placebo)

 

0.05

Plasma epinephrine - pmol/L

  

 Clonidine group, mean

327

291

 Placebo group, mean

415

299

 Difference (95 % CI)

 

−8 (−44 to 29)

 p-value (clonidine vs. placebo)

 

0.68

Urine norepinephrine/creatinine ratio - nmol/mmol

  

 Clonidine group, mean

13.3

9.6

 Placebo group, mean

13.7

13.6

 Difference (95 % CI)

 

−3.9 (−6.4 to −1.5)

 p-value (clonidine vs. placebo)

 

0.002

Urine epinephrine/creatinine ratio - nmol/mmol

  

 Clonidine group, mean

1.7

1.2

 Placebo group, mean

1.6

1.6

 Difference (95 % CI)

 

−0.4 (−0.8 to 0.1)

 p-value (clonidine vs. placebo)

 

0.11

Missing values were imputed based on the principle of last observation carried forwards. Thus, all calculations are based on 120 individuals (60 in each intervention group except one to two in each group with missing values at baseline). Means and differences at week 8 are estimated from the parameters of the general linear model

Table 4

Outcome of clonidine intervention – cardiovascular variables

 

Baseline

Week 8 (during treatment)

Supine

  

Heart rate - beats/min

  

 Clonidine group, mean

70

67

 Placebo group, mean

72

69

 Difference (95 % CI)

 

−2.0 (−4.1 to 0.1)

 p-value (clonidine vs. placebo)

 

0.06

SBP – mmHg

  

 Clonidine group, mean

103

104

 Placebo group, mean

107

103

 Difference (95 % CI)

 

1.4 (−1.0 to 3.9)

 p-value (clonidine vs. placebo)

 

0.25

MBP – mmHg

  

 Clonidine group, mean

77

78

 Placebo group, mean

80

77

 Difference (95 % CI)

 

1.3 (−0.7 to 3.4)

 p-value (clonidine vs. placebo)

 

0.19

DBP – mmHg

  

 Clonidine group, mean

65

64

 Placebo group, mean

66

63

 Difference (95 % CI)

 

0.8 (−1.0 to 2.7)

 p-value (clonidine vs. placebo)

 

0.37

SI - ml/m2

  

 Clonidine group, mean

47

46

 Placebo group, mean

46

46

 Difference (95 % CI)

 

0.2 (−2.1 to 2.4)

 p-value (clonidine vs. placebo)

 

0.86

TPRI - mmHg/L/min/m2

  

 Clonidine group, mean

9.1

9.4

 Placebo group, mean

8.9

8.9

 Difference (95 % CI)

 

0.5 (−0.1 to 1.1)

 p-value (clonidine vs. placebo)

 

0.11

SDNN – ms

  

 Clonidine group, mean

74

78

 Placebo group, mean

66

66

 Difference (95 % CI)

 

12.0 (−0.2 to 23.7)

 p-value (clonidine vs. placebo)

 

0.05

r-MSSD – ms

  

 Clonidine group, mean

79

83

 Placebo group, mean

65

70

 Difference (95 % CI)

 

13.1 (−3.2 to 29.5)

 p-value (clonidine vs. placebo)

 

0.11

pNN50 - %

  

 Clonidine group, mean

40

40

 Placebo group, mean

31

38

 Difference (95 % CI)

 

2.2 (−3.0 to 7.3)

 p-value (clonidine vs. placebo)

 

0.40

LF-HRV – nu

  

 Clonidine group, mean

40

42

 Placebo group, mean

43

38

 Difference (95 % CI)

 

3.7 (−0.5 to 8.0)

 p-value (clonidine vs. placebo)

 

0.08

HF-HRV – nu

  

 Clonidine group, mean

60

58

 Placebo group, mean

57

62

 Difference (95 % CI)

 

−3.7 (−8.0 to 0.5)

 p-value (clonidine vs. placebo)

 

0.08

LF-HRV* - ms2

  

 Clonidine group, mean

628

679

 Placebo group, mean

451

487

 Ratio (95 % CI)

 

1.4 (1.1 to 1.8)

 p-value (clonidine vs. placebo)

 

0.007

HF-HRV* - ms2

  

 Clonidine group, mean

962

961

 Placebo group, mean

600

825

 Ratio (95 % CI)

 

1.2 (0.9 to 1.5)

 p-value (clonidine vs. placebo)

 

0.28

Total Power-HRV* - ms2

  

 Clonidine group, mean

1991

2053

 Placebo group, mean

1352

1638

 Ratio (95 % CI)

 

1.3 (1.0 to 1.6)

 p-value (clonidine vs. placebo)

 

0.06

LF/HF-ratio*

  

 Clonidine group, mean

0.65

0.70

 Placebo group, mean

0.75

0.59

 Ratio (95 % CI)

 

1.2 (1.0 to 1.4)

 p-value (clonidine vs. placebo)

 

0.09

LF-SBP – nu

  

 Clonidine group, mean

39.3

38.0

 Placebo group, mean

38.1

36.9

 Difference (95 % CI)

 

1.1 (−3.0 to 5.2)

 p-value (clonidine vs. placebo)

 

0.60

LF-SBP* - mmHgs2

  

 Clonidine group, mean

3.8

3.7

 Placebo group, mean

3.0

3.2

 Ratio (95 % CI)

 

1.1 (0.9 to 1.5)

 p-value (clonidine vs. placebo)

 

0.34

Response to 20° head-up tilt

  

Heart rate - beats/min

  

 Clonidine group, mean

5.2

4.9

 Placebo group, mean

4.8

4.9

 Difference (95 % CI)

 

0.0 (−1.1 to 1.2)

 p-value (clonidine vs. placebo)

 

0.97

SBP – mmHg

  

 Clonidine group, mean

0.74

−0.59

 Placebo group, mean

0.15

−0.01

 Difference (95 % CI)

 

−0.58 (−2.2 to 1.0)

 p-value (clonidine vs. placebo)

 

0.48

MBP - mmHg

  

 Clonidine group, mean

1.19

0.61

 Placebo group, mean

0.94

1.23

 Difference (95 % CI)

 

−0.63 (−2.1 to 0.8)

 p-value (clonidine vs. placebo)

 

0.39

DBP - mmHg

  

 Clonidine group, mean

1.13

1.2

 Placebo group, mean

1.58

1.8

 Difference (95 % CI)

 

−0.59 (−2.0 to 0.8)

 p-value (clonidine vs. placebo)

 

0.40

SI - ml/m2

  

 Clonidine group, mean

−5.9

−4.5

 Placebo group, mean

−5.1

−5.3

 Difference (95 % CI)

 

0.9 (−0.4 to 2.1)

 p-value (clonidine vs. placebo)

 

0.17

TPRI - mmHg/L/min/m2

  

 Clonidine group, mean

0.66

0.44

 Placebo group, mean

0.60

0.62

 Difference (95 % CI)

 

−0.18 (−0.47 to 0.11)

 p-value (clonidine vs. placebo)

 

0.22

SDNN - ms

  

 Clonidine group, mean

−5.1

−7.9

 Placebo group, mean

−4.4

−0.7

 Difference (95 % CI)

 

−7.2 (−16.0 to 1.6)

 p-value (clonidine vs. placebo)

 

0.11

r-MSSD - ms

  

 Clonidine group, mean

−18

−24

 Placebo group, mean

−16

−17

 Difference (95 % CI)

 

−7.6 (−19.6 to 4.4)

 p-value (clonidine vs. placebo)

 

0.11

pNN50 - %

  

 Clonidine group, mean

−14

−11

 Placebo group, mean

−9

−13

 Difference (95 % CI)

 

1.2 (−3.1 to 5.4)

 p-value (clonidine vs. placebo)

 

0.59

LF-HRV - nu

  

 Clonidine group, mean

8.3

6.1

 Placebo group, mean

6.7

9.2

 Difference (95 % CI)

 

−3.1 (−7.4 to 1.1)

 p-value (clonidine vs. placebo)

 

0.15

HF-HRV - nu

  

 Clonidine group, mean

−8.3

−6.1

 Placebo group, mean

−6.7

−9.2

 Difference (95 % CI)

 

3.1 (−1.1 to 7.4)

 p-value (clonidine vs. placebo)

 

0.15

LF-HRV# - ms2

  

 Clonidine group, mean

−320

−161

 Placebo group, mean

−176

−171

 n.a.

 

n.a.

 p-value (clonidine vs. placebo)

 

0.87

HF-HRV# - ms2

  

 Clonidine group, mean

−828

−640

 Placebo group, mean

−523

−629

 n.a.

 

n.a.

 p-value (clonidine vs. placebo)

 

0.99

Total Power-HRV# - ms2

  

 Clonidine group, mean

−1107

−790

 Placebo group, mean

−668

−736

 n.a.

 

n.a.

 p-value (clonidine vs. placebo)

 

0.78

LF/HF-ratio

  

 Clonidine group, mean

0.35

0.34

 Placebo group, mean

0.44

0.55

 Difference (95 % CI)

 

−0.21 (−0.46 to 0.04)

 p-value (clonidine vs. placebo)

 

0.09

LF-SBP - nu

  

 Clonidine group, mean

2.5

4.4

 Placebo group, mean

3.2

3.7

 Difference (95 % CI)

 

0.7 (−2.4 to 3.8)

 p-value (clonidine vs. placebo)

 

0.66

LF-SBP - mmHgs2

  

 Clonidine group, mean

−2.6

−1.0

 Placebo group, mean

−0.6

−0.2

 Difference (95 % CI)

 

−0.7 (−1.7 to 0.3)

 p-value (clonidine vs. placebo)

 

0.17

Missing values were imputed based on the principle of last observation carried forwards. Thus, all calculations are based on 120 individuals (60 in each intervention group). Means and differences at week 8 are estimated from the parameters of the general linear model

For variables annotated with a *, modeling was performed on ln-transformed variables; all means are based on back-transformation of the variables, and ratios instead of differences are reported. For variables annotated with a #, modeling was performed on square root-transformed variables; all means are based on back-transformation of the variables, but neither differences nor ratios can be computed, as indicated with the label n.a. (not applicable). CI = Confidence Interval; SBP = Systolic Blood Pressure; MBP = Mean arterial Blood Pressure; DBP = Diastolic Blood Pressure; SI = Stroke Index; TPRI = Total Periferal Resistance Index; RRI = R-R Interval; HRV = heart rate variability; HF = High Frequency; LF = Low Frequency; SDNN = standard deviation of all RR-intervals; pNN50 = the proportion of successive RRIs with adifference greater than 50 ms; r-MSSD = the square root of the mean square differences of successive RRIs; nu = normalized units; n.a. = not applicable because of square root transformation of variables; n = number of patients, for most variables equal to 60 because of imputation

Urine norepinephrine/creatinine ratio was negatively related to plasma clonidine concentration (B = −14.5, p = 0.004). TPRI supine (B = 4.1, p = 0.01), heart rate variability in the low-frequency band supine (LF-HRV, absolute unites) (B = 1423, p = 0.02) and HRV-Total Power supine (B = 4353, p = 0.04) were positively related to plasma clonidine concentration. No other dose response-relationships were found.

Subgrouping according to the 1994 CFS case definition, genotype frequency of the alpha2A receptor SNP rs1800544 and sex did not reveal any differential response to the intervention. Also, the genotype frequency was equal among CFS patients and healthy controls (p = 0.75).

Discussion

This study shows that clonidine reduces catecholamine levels in adolescent CFS. However, the effects on cardiovascular autonomic control are sparse, and clonidine does not improve symptoms of orthostatic intolerance.

Previous studies have documented that adult as well as adolescent CFS patients are characterized by enhanced sympathetic and attenuated parasympathetic nervous activity [7, 9, 38, 39]. In particular, CFS patients have increased levels of catecholamines [40, 41] and a sympathetic predominance of cardiovascular autonomic control possibly due to central alterations [9, 11, 42]. In this study, clonidine lowered catecholamine levels as expected. Of note, urine norepinephrine, which is considered an index of sympathetic nervous activity over time [43], decreased dose-dependently.

Clonidine had limited impact on standard cardiovascular variables, both at rest and during orthostatic challenge. This finding was surprising. In previous studies of healthy individuals as well as hypertensive patients, clonidine dosages similar to those applied in this study have been shown to decrease both blood pressures and heart rate, and these alterations of hemodynamics were paralleled by a decrement of catecholamines [15, 4447]. Furthermore, in healthy subjects, clonidine also attenuates indices of cardiovascular sympathetic nervous modulation (such as LF-HRV), both in supine and sitting positions [44]. In this study, there was a clonidine-mediated increase in LF-HRV at supine rest, as well as a positive relationship between LF-HRV and clonidine plasma concentration. The interpretation of LF-HRV-indices is not straight forward; these results, however, might suggest an enhancement of sympathetic heart rate modulation, resembling the effects of clonidine in essential hypertension [48]. This is in contrast to effects of clonidine in healthy subjects [44]. A previous study suggests early sympathetic baroreceptor activation and diminished baroreceptor reserve in CFS [11]. We speculate that clonidine, by way of reducing sympathetic tone (as evident from the catecholamine-lowering effect), might in fact increase the sympathetic nervous system modulatory effects [49].

Taken together, the findings presented in this study suggest an alteration of clonidine pharmacodynamics in CFS. One possible explanation is genetically determined differences of the alpha2A receptor protein, which is the ligand for clonidine. A single nucleotide polymorphism (SNP) (rs1800544) in the alpha2A receptor gene implies substitution of guanine (G) for cytosine (C) at position 1291, and has functional consequences [18]. However, the genotype frequencies among CFS patients and a comparable group of healthy controls were almost identical, and subgroup analysis based on genotype revealed no differences in response to treatment. Another possible explanation is altered expression of adrenoceptors, as has previously been demonstrated in CFS [50] as well as in other conditions with high levels of catecholamines [51].

The possibility of increased long-term cardiovascular risk in CFS patients remains a concern [52]. In addition to increased sympathetic nervous activity, CFS patients are also characterized by slight inflammatory activation [28] and elevated nocturnal blood pressure and heart rate [53], which in turn are associated with development of atherosclerosis. Further research is warranted to clarify the eventual need of prophylactic measures.

A possible limitation of this study is the wide inclusion criteria and no a priori-definition of the degree of school absenteeism necessary to fulfil the diagnostic criteria, which might have obscured results applying to a subgroup only. However, the study population corresponds closely to the population who is diagnosed as CFS by pediatricians; thus, we assume the external validity to be strong. Furthermore, subgrouping based upon the 1994 CFS case definition did not change the results. We have not done subgrouping based on caffeine use. Another limitation of this study is the 4 min epochs used for time-domain analyses of heart rate variability, as opposed to the 5 min epochs recommended [32]. It is considered inappropriate to compare time-domain indices (especially SDNN) obtained from recordings of different durations; while the present study does not violate this principle, caution should be shown when comparing our results to other studies. Strengths of this study include high compliance and low drop-out-rates, and the successful blinding of all (staff and patients) clinically involved in the study.

Conclusions

Low-dose clonidine reduces catecholamine levels in adolescent CFS. However, the effects on cardiovascular autonomic control are sparse, and clonidine does not improve symptoms of orthostatic intolerance.

Abbreviations

BP: 

Blood pressure

CFS: 

Chronic fatigue syndrome

HF: 

High frequency

HR: 

Heart rate

HRV: 

Heart rate variability

HUT: 

Head-up tilt test

LF: 

Low frequency

RRI: 

Instantaneous RR intervals

SBP: 

Systolic blood pressure

SNP: 

Single nucleotide polymorphism

Declarations

Acknowledgements

We thank Kari Gjersum for secretary assistance; Hamsana Chandrakumar, Esther Gangsø, Anne Marie Halstensen, Adelheid Holm, Berit Widerøe Njølstad, Pelle Rohdin, and Anna Marie Thorendal Ryenbakken for practical assistance; Berit Bjelkåsen for development of the computerized randomization procedure; Liv Thrane Bjerke for pharmacy services; Gaute Døhlen, Bjørn Bendz, Knut Engedal, and Ola Didrik Saugstad for study monitoring; all referring units; and finally all participants and their parents/next-of-kin.

The study was funded by: Health South–East Hospital Trust; The University of Oslo; Oslo and Akershus University College of Applied Sciences; The Norwegian Competence Network of Paediatric Pharmacotherapy; Simon Fougner Hartmann’s Family Foundation; Eckbo’s Family Foundation.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
Institute of Clinical Medicine, Medical Faculty, University of Oslo
(2)
Department of Anaesthesiology and Critical Care, Oslo University Hospital
(3)
Department of Paediatrics, Oslo University Hospital
(4)
Department of Paediatrics, Lillehammer County Hospital
(5)
Institute of Nursing Sciences, Oslo and Akershus University College of Applied Sciences
(6)
Department of Pharmacology, Oslo University Hospital
(7)
National Institute of Occupational Health
(8)
Department of Biosciences, University of Oslo
(9)
Section of Specialized Endocrinology, Department of Endocrinology, Oslo University Hospital Rikshospitalet
(10)
Department of Paediatrics, Johns Hopkins University School of Medicine
(11)
Department of Paediatrics, Medical University of South Carolina
(12)
Department of Pharmaceutical Science, University of Oslo
(13)
Norwegian Institute of Public Health
(14)
Department of Paediatrics, Akershus University Hospital

References

  1. Nijhof SL, Maijer K, Bleijenberg G, Uiterwaal CS, Kimpen JL, van der Putte EM. Adolscent chronic fatigue syndrome: prevalence, incidence, and morbidity. Pediatrics. 2011;127:e1169–75.View ArticlePubMedGoogle Scholar
  2. Crawley E. The epidemiology of chronic fatigue syndrome/myalgic encephalitis in children. Arch Dis Child. 2014;99:171–4.View ArticlePubMedGoogle Scholar
  3. Rowe KS. Double-blind randomized controlled trial to assess the efficacy of intravenous gammaglobulin for the management of chronic fatigue syndrome in adolescents. J Psychiatr Res. 1997;31:133–47.View ArticlePubMedGoogle Scholar
  4. Hoad A, Spickett G, Elliott J, Newton J. Postural orthostatic tachycardia syndrome is an under-recognized condition in chronic fatigue syndrome. QJM. 2008;101:961–5.View ArticlePubMedGoogle Scholar
  5. Stewart JM, Gewitz MH, Weldon A, Arlievsky N, Li K, Munoz J. Orthostatic intolerance in adolescent chronic fatigue syndrome. Pediatrics. 1999;103:116–21.View ArticlePubMedGoogle Scholar
  6. Stewart JM, Gewitz MH, Weldon A, Munoz J. Patterns of orthostatic intolerance: The orthostatic tachycardia syndrome and adolescent chronic fatigue. J Pediatrics. 1999;135:218–25.View ArticleGoogle Scholar
  7. Okamoto LE, Raj SR, Peltier A, Gamboa A, Shibao C, Diedrich A, et al. Neurohumoral and haemodynamic profile in postural tachycardia and chronic fatigue syndromes. Clin Sci. 2012;122:183–92.View ArticlePubMedGoogle Scholar
  8. Bou-Holaigah I, Rowe PC, Kan J, Calkins H. The relationship between neurally mediated hypotension and the chronic fatigue syndrome. JAMA. 1995;274:961–7.View ArticlePubMedGoogle Scholar
  9. Wyller VB, Due R, Saul JP, Amlie JP, Thaulow E. Usefulness of an abnormal cardiovascular response during low-grade head-up tilt-test for discriminating adolescents with chronic fatigue from healthy controls. Am J Cardiol. 2007;99:997–1001.View ArticlePubMedGoogle Scholar
  10. Wyller VB, Barbieri R, Thaulow E, Saul JP. Enhanced vagal withdrawal during mild orthostatic stress in adolescents with chronic fatigue. Ann Noninvasive Electrocardiol. 2008;13:67–73.View ArticlePubMedGoogle Scholar
  11. Wyller VB, Barbieri R, Saul P. Blood pressure variability and closed-loop baroreflex assessment in adolescent chronic fatigue syndrome during supine rest and orthostatic stress. Eur J Appl Physiol. 2011;111:497–502.View ArticlePubMedGoogle Scholar
  12. Boneva RS, Decker MJ, Maloney EM, Lin JM, Jones JF, Helgason HG, et al. Higher heart rate and reduced heart rate variability persist during sleep in chronic fatigue syndrome: a population-based study. Auton Neurosci. 2007;137:94–101.View ArticlePubMedGoogle Scholar
  13. Lewis I, Pairman J, Spickett G, Newton JL. Clinical characteristics of a novel subgroup of chronic fatigue syndrome patients with postural orthostatic tachycardia syndrome. J Intern Med. 2013;273:501–10.View ArticlePubMedGoogle Scholar
  14. Szabo B. Imidazoline antihypertensive drugs: a critical review on their mechanism of action. Pharmacol Ther. 2002;93:1–35.View ArticlePubMedGoogle Scholar
  15. Anavekar SN, Jarrott B, Toscano M, Louis WJ. Pharmacokinetic and pharmacodynamic studies of oral clonidine in normotensive subjects. Eur J Clin Pharmacol. 1982;23:1–5.View ArticlePubMedGoogle Scholar
  16. Cividjian A, Toader E, Wesseling KH, Karemaker JM, McAllen R, Quintin L. Effect of clonidine on cardiac baroreflex delay in humans and rats. Am J Physiol Regul Integr Comp Physiol. 2011;300:949–57.View ArticleGoogle Scholar
  17. Fagermoen E, Sulheim D, Winger A, Andersen AM, Vethe NT, Saul JP, et al. Clonidine in the treatment of adolescent chronic fatigue syndrome: a pilot study for the NorCAPITAL trial. BMC Res Notes. 2012;5:418.View ArticlePubMedPubMed CentralGoogle Scholar
  18. Small KM, Liggett SB. Identification and functional characterixation of alpha2-adrenoceptor polymorphisms. Trend Pharm Sci. 2001;22:471–7.View ArticleGoogle Scholar
  19. National Institute for Health and Clinical Excellence. Chronic fatigue syndrome/myalgic encephalomyelitis (or encephalopathy). Diagnosis and management of CFS/ME in adults and children. NICE clinical guideline 2007, no. 53. London, England: Royal College of Pediatrics and Child Health.Google Scholar
  20. Royal College of Paediatrics and Child Health. Evidence Based Guideline for the Management of CFS/ME in Children and Young People. London England: National Institute for Health and Clinical Excellence; 2004.Google Scholar
  21. Sulheim D, Hurum H, Helland IB, Thaulow E, Wyller VB. Concurrent improvement of circulatory abnormalities and clinical symptoms in adolescent chronic fatigue syndrome. Biopsychosoc Med. 2012;6:10.View ArticlePubMedPubMed CentralGoogle Scholar
  22. Tsunoda M. Recent advances in methods for the analysis of catecholamines and their metabolites. Anal Bioanal Chem. 2006;386:506–14.View ArticlePubMedGoogle Scholar
  23. Kågedal B, Goldstein DS. Catecholamines and their metabolites. J Chromatogr. 1988;29:177–233.View ArticleGoogle Scholar
  24. Hjemdahl P. Catecholamine measurements by high-performance liquid chromatography. Am J Physiol. 1984;247:E13–20.PubMedGoogle Scholar
  25. Müller C, Ramic M, Harlfinger S, Hünseler C, Theisohn M, Roth B. Sensitive and convenient method for the quantification of clonidine in serum of pediatric patients using liquid chromatography/tandem mass spectrometry. J Chromatogr A. 2007;1139:221–7.View ArticlePubMedGoogle Scholar
  26. US Department of Health and Human Services, Food and Drug Administration. Guidance for Industry. Bioanalytic method validation. MD, USA, 2001.http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/UCM070107.pdf (2015.02.18).
  27. Olsen MB, Jacobsen LM, Schistad EI, Pedersen LM, Rygh LJ, Røe C, et al. Pain intensity the first year after lumbar disc herniation is associated with the A118G polymorphism in the opioid receptor mu 1 gene: evidence of a sex and genotype interaction. J Neurosci. 2012;32:9831–4.View ArticlePubMedGoogle Scholar
  28. Sulheim D, Fagermoen E, Winger A, Andersen AM, Godang K, Müller F, et al. Disease mechanisms and clonidine treatment in adolescent chronic fatigue syndrome: a combined cross-sectional and randomized clinical trial. JAMA Pediatr. 2014;168:351–60.View ArticlePubMedGoogle Scholar
  29. de Jong-de Vos van Steenwijk CC, Wieling W, Johannes JM, Harms MP, Kuis W, Wesseling KH. Incidence and hemodynamic characteristics of near-fainting in healthy6- to 16-year old subjects. J Am Coll Cardiol. 1995;25:1615–21.View ArticleGoogle Scholar
  30. Fortin J, Habenbacher W, Heller A, Hacker A, Grüllenberger R, Innerhover J, et al. Non-invasive beat-to-beat cardiac output monitoring by an improved method of transthoracic bioimpedance measurement. Comput Biol Med. 2006;36:1185–203.View ArticlePubMedGoogle Scholar
  31. Bianchi AM, Mainardi LT, Meloni C, Chierchia S, Cerutti S. Continuous monitoring of the sympatho-vagal balance through spectral analysis. Eng Med Biol Mag. 1997;16:64–73.View ArticleGoogle Scholar
  32. Task force of the European society of cardiology and the North American society of pacing electrophysiology. Heart rate variability. Standards of measurement, physiological interpretation, and clinical use. Circulation. 1996;93:1043–65.View ArticleGoogle Scholar
  33. Malpas S. Neural influences on cardiovascular variability: possibilities and pitfalls. Am J Physiol Heart Circ Physiol. 2002;282:H6–20.PubMedGoogle Scholar
  34. Suarez GA, Opfer-Gehrking TL, Offord KP, Atkinson EK, O’Brien PC, Low PA. The autonomic symptom profile: a new instrument to assess autonomic symptoms. Neurology. 1999;52:523–8.View ArticlePubMedGoogle Scholar
  35. Wagner D, Nisenbaum R, Heim C, Jones JF, Unger ER, Reeves WC. Psychometric properties of the CDC symptom inventory for assessment for Chronic Fatigue Syndrome. Popul Health Metr. 2005;3:8.View ArticlePubMedPubMed CentralGoogle Scholar
  36. Fukuda K, Straus SE, Hickie I, Sharpe MC, Dobbins JG, Komaroff A. The chronic fatigue syndrome: a comprehensive approach to its definition and study. Ann Int Med. 1994;121:953–9.View ArticlePubMedGoogle Scholar
  37. Vickers AJ, Altman DG. Statistics notes: Analysing controlled trials with baseline and follow up measurements. BMJ. 2001;323:1123–4.View ArticlePubMedPubMed CentralGoogle Scholar
  38. Pagani M, Lucini D, Mela GS, Langewitz W, Malliani A. Sympathetic overactivity in subjects complaining of unexplained fatigue. Clin Sci. 1994;87:655–61.View ArticlePubMedGoogle Scholar
  39. Stewart J, Weldon A, Arlievsky N, Li K, Munoz J. Neurally mediated hypotension and autonomic dysfunction measured by heart rate variability during head-up tilt testing in children with chronic fatigue syndrome. Clin Auton Res. 1998;8:221–30.View ArticlePubMedGoogle Scholar
  40. Timmers HJ, Wieling W, Soetekouw PM, Bleijenberg G, Van Der Meer JW, Lenders JW. Hemodynamic and neurohumoral responses to head-up tilt in patients with chronic fatigue syndrome. Clin Auton Res. 2002;12:273–80.View ArticlePubMedGoogle Scholar
  41. Wyller VB, Saul JP, Walløe L, Thaulow E. Sympathetic cardiovascular control during orthostatic stress and isometric exercise in adolescent chronic fatigue syndrome. Eur J Appl Physiol. 2008;102:623–32.View ArticlePubMedGoogle Scholar
  42. De Becker P, Dendale P, De Meirleir K, Campine I, Vandenborne K, Hagers Y. Autonomic testing in patients with chronic fatigue syndrome. Am J Med. 1998;105:22S–6.View ArticlePubMedGoogle Scholar
  43. Grouzmann E, Lamine F. Determination of catecholamines in plasma and urine. Best Pract Res Clin Endocrinol Metab. 2013;5:713–23.View ArticleGoogle Scholar
  44. Lazzeri C, La Villa G, Mannelli M, Janni L, Franchi F. Effects of acute clonidine administration on power spectral analysis of heart rate variability in healthy humans. J Auton Pharmacol. 1998;18:307–12.View ArticlePubMedGoogle Scholar
  45. Anavekar SN, Howes LG, Jarrott B, Syrjanen M, Conway EL, Louis WJ. Pharmacokinetics and antihypertensive effects of low dose clonidine during chronic therapy. J Clin Pharmacol. 1989;29:32.View ArticleGoogle Scholar
  46. Arndts D, Doevendans J, Kiersten R, Heintz B. New aspects of the pharmacokinetics and pharmacodynamics of clonidine in man. Eur J Clin Pharmacol. 1983;24:21–30.View ArticlePubMedGoogle Scholar
  47. Veith RC, Beset JD, Halter JB. Dose-dependent supression of norepineprhine appearance rate in plasma by clonidine in man. J Clin Endocrinol Metab. 1984;59:151.View ArticlePubMedGoogle Scholar
  48. Lazzeri C, La Villa G, Mannelli M, Janni L, Barletta G, Montano N, et al. Effects of clonidine on power spectral analysis of heart rate variability in mild essential hypertension. J Auton Nerv Syst. 1998;74:152–9.View ArticlePubMedGoogle Scholar
  49. Saul JP. Beat-to-beat variations of heart rate reflect modulation of cardiac autonomic outflow. News Physiol Sci. 1990;5:32–7.Google Scholar
  50. Light AR, Bateman L, Jo D, Hughen RW, Vanhaitsma TA, White AT, et al. Gene expression alterations at baseline and following moderate exercise in patients with Chronic Fatigue Syndrome and Fibromyalgia Syndrome. J Int Med. 2012;271:64–81.View ArticleGoogle Scholar
  51. Streeten DH, Anderson Jr GH. Mechanisms of orthostatic hypotension and tachycardia in patients with pheochromocytoma. Am J Hypertens. 1996;9:760–9.View ArticlePubMedGoogle Scholar
  52. Zhou Y, Xie G, Wang J, Yang S. Cardiovascular risk factors significantly correlate with autonomic nervous system activity in children. Can J Cardiol. 2012;28:477–82.View ArticlePubMedGoogle Scholar
  53. Hurum H, Sulheim D, Thaulow E, Wyller VB. Elevated nocturnal blood pressure and heart rate in adolescent chronic fatigue syndrome. Acta Paediatr. 2011;100:289–92.View ArticlePubMedGoogle Scholar

Copyright

© Fagermoen et al. 2015

Advertisement