Clonidine: understanding its disposition, sites and mechanism of action

Stability of Morphine Sulfate-Clonidine and Sufentanil-Clonidine Mixtures

BACKGROUND

Spinal analgesia is recommended for intractable cancer pain. Morphine-clonidine and sufentanil-clonidine are often used in association in intrathecal drug delivery systems, injected by intraabdominal pumps. To refill these pumps and to limit patient transport, it may be necessary to ship the mixtures in polypropylene syringes to peripheral establishments located near patient homes. The purpose of this study is to determine the stability of morphine-clonidine and sufentanil-clonidine mixtures in polypropylene syringes to ensure the best and safest transport conditions and in implantable pumps for intrathecal use.

MATERIALS AND METHODS

The stability study method was conceived according to the International Council for Harmonization guidelines. For polypropylene syringes, four different mixtures of morphine-clonidine and sufentanil-clonidine were assessed over seven days. Two storage temperatures were tested (5 ± 3 °C and 25 ± 2 °C). For implantable pumps, two different mixtures of morphine-clonidine and sufentanil-clonidine were assessed over 28 days and stored at 37 °C.

RESULTS

For the morphine-clonidine mixtures in polypropylene syringes, all mixtures remained stable for five days in both storage conditions (5 ± 3 °C and 25 ± 2 °C) because of relative concentrations systematically positioned between 90% and 110% (95% CIs of the mean of three samples). The two mixtures in implantable pumps remained stable for 28 days. For the sufentanil-clonidine mixtures in polypropylene syringes, cold conservation kept all the preparations stable for seven days, whereas a quick degradation was observed after only two days for ambient storage conditions. This result is similar to that with an implantable pump, in which the concentration is <90% on day 7 for low concentration mixtures. No visual modification, no significant pH modification, and no changes in turbidity assays were observed in either study.

CONCLUSION

This study shows the stability of the morphine-clonidine mixtures in syringes stored at 5 °C for five days and in implantable pumps stored at 37 °C for 28 days. For the sufentanil-clonidine mixtures, the results show stability in syringes for seven days at 5 °C. Pump results show stability of seven days for low concentrations and 28 days for high concentrations.

The Options for Neuraxial Drug Administration

Neuraxial drug administration, i.e., the injection of drugs into the epidural or intrathecal space to produce anesthesia or analgesia, is a technique developed more than 120 years ago. Today, it still is widely used in daily practice in anesthesiology and in acute and chronic pain therapy. A multitude of different drugs have been introduced for neuraxial injection, only a part of which have obtained official approval for that indication. A broad understanding of the pharmacology of those agents is essential to the clinician to utilize them in a safe and efficient manner. In the present narrative review, we summarize current knowledge on neuraxial anatomy relevant to clinical practice, including pediatric anatomy. Then, we delineate the general pharmacology of neuraxial drug administration, with particular attention to specific aspects of epidural and intrathecal pharmacokinetics and pharmacodynamics. Furthermore, we describe the most common clinical indications for neuraxial drug administration, including the perioperative setting, obstetrics, and chronic pain. Then, we discuss possible neurotoxic effects of neuraxial drugs, and moreover, we detail the specific properties of the most commonly used neuraxial drugs that are relevant to clinicians who employ epidural or intrathecal drug administration, in order to ensure adequate treatment and patient safety in these techniques. Finally, we give a brief overview on new developments in neuraxial drug therapy.

Thoracic Epidural Clonidine Attenuates Haemodynamic Responses Induced by Endobronchial Intubation

Laryngoscopy and endobronchial intubation usually cause transient hypertension and tachycardia. We investigated whether thoracic epidurally injected 3 μg/kg clonidine attenuates cardiovascular responses to intubation compared with 2 μg/kg fentanyl and 1 mg/kg lidocaine. Epidural catheterization was performed at the T6–T7 or T7–T8 intervertebral space, and saline or clonidine in saline was injected 20 min before anaesthetic induction. Anaesthesia was induced using 5 mg/kg thiopental sodium and 0.1 mg/kg vecuronium. Laryngoscopy and endobronchial intubation were performed 2 min later. Mean blood pressure and heart rate were measured throughout anaesthetic induction. In the control group and the fentanyl group, mean blood pressure and heart rate 3 min after endobronchial intubation were elevated significantly compared with baseline. In the clonidine group, however, mean blood pressure and heart rate did not increase compared with baseline. The control group had higher mean blood pressure and heart rate than the clonidine group 3 min after endobronchial intubation. Thoracic epidural clonidine may attenuate the haemodynamic response to endobronchial intubation.

Supplementation of retrobulbar block with clonidine in vitreoretinal surgery: effect on postoperative pain

STUDY OBJECTIVE

To evaluate the effect of clonidine when added to local anesthetics on duration of postoperative analgesia during retrobulbar block.

DESIGN

Prospective, randomized controlled trial.

SETTING

Operating room and Postanesthesia Care Unit of a university-affiliated hospital.

SUBJECTS

80 ASA physical status 1, 2, and 3 patients undergoing vitreoretinal surgery with or without scleral buckling.

INTERVENTIONS

Patients in the control group (n = 40) received a retrobulbar block with 4.5 mL of lidocaine-bupivacaine and 0.5 mL of saline. Clonidine group patients (n = 40) received 4.5 mL of lidocaine-bupivacaine and 0.5 μg/kg of clonidine in a 0.5 mL volume.

MEASUREMENTS

The time to first analgesic request, frequency of postoperative pain, and number of postoperative analgesic requests per patient were assessed.

MAIN RESULTS

37 patients in the control group (92.5%) versus 24 patients (60%) in the clonidine group reported pain postoperatively (P = 0.001), with a shorter time to first analgesic request noted in the control group (4.9 ± 3 vs 11.9 ± 5.3 hrs; P < 0.001). The median number of postoperative analgesic requests per patient during the first 24 hours was higher in the control group than the clonidine group [2 (0-3) vs. 1 (0-3); P < 0.001].

CONCLUSIONS

The addition of clonidine 0.5 μg/kg to the local anesthetics of a retrobulbar block for vitreoretinal surgery decreases the frequency of postoperative pain and prolongs the time of analgesia.

Obesity and hypertension: mechanisms, cardio-renal consequences, and therapeutic approaches

The increasing prevalence of obesity in the industrialized world is causing an alarming epidemic. Almost 70% of American adults are overweight or obese. The link between increasing body weight and hypertension is well established. Obesity hypertension through metabolic, endocrinic, and systemic hemodynamic alteration causes structural vascular and cardiac adaptations that trigger concentric, eccentric left ventricular hypertrophy and electrophysiological changes, which may increase the risk for congestive heart failure and sudden cardiac death as a result of arrhythmias. The increased renal blood flow in conjunction with a decreased renal vascular resistance causes renal hyperperfusion and hyperfiltration. Such changes lead to glomerulomegaly, focal segmental glomerulosclerosis, tubulointerstitial inflammation, and fibrosis that characterize the renal damage in obese hypertensive subjects. We propose that weight reduction, with the addition of other nonpharmacological approaches that included exercise and reduction in alcohol intake, should be the first choice to treat obesity hypertension. Salt restriction may be helpful only in salt-sensitive patients. The benefits of diet in obese patients include improvement of insulin sensitivity, reduction in sympathetic nervous and renin angiotensin system activities, and restoration of leptin sensitivity. As a consequence of these and other metabolic changes, the previously described systemic and renal hemodynamic alterations improved and the cardiovascular and renal morphological changes induced by obesity were lessened. After reviewing the medications available, we believe that owing to the cardiovascular and renal morbidity and mortality that characterized obesity hypertension, the ACEI or ARBs offer the best cardio-renal protection and should be the pharmacologic treatment of choice. If these alone do not control BP adequately, then a low-dose diuretic should be added as a second approach. Although we strongly believe in our proposal, more multicenter long-term clinical pharmacological trials are needed to evaluate the efficacy and safety of the antihypertensive approaches in the treatment of obesity hypertension.

Intrathecal clonidine reduces the incidence of ischemia-provoked ventricular arrhythmias in a canine postinfarction heart failure model

BACKGROUND

Intrathecal clonidine (ITC) is used clinically to manage neuropathic pain but frequently causes hypotension and bradycardia due to centrally mediated sympatholytic effects.

OBJECTIVES

The purpose of this study was to evaluate the cardiac electrophysiologic effects of thoracic ITC and its effects on ischemia-provoked ventricular arrhythmias.

METHODS

Twelve mongrel dogs with healed myocardial infarctions and heart failure were evaluated. ITC was delivered locally via catheter to the T2-T4 spinal segments and was dosed to reduce heart rate (HR) by >20% to 25%. Electrophysiologic testing was performed before and after ITC. Transient (4-minute) myocardial ischemia was induced via left circumflex coronary artery occlusion on two separate occasions to provoke ventricular arrhythmias (ventricular tachycardia [VT]/ventricular fibrillation [VF]). Ischemic episodes were separated by 1 to 2 days, and dogs were randomly assigned to receive ITC or intrathecal saline flush (control) prior to the first or the second ischemic episode.

RESULTS

ITC produced significant decrease in HR (31%) and increases in PR interval (22%), Wenckebach cycle length (122%), and atrial and ventricular effective refractory periods (19% and 9%, respectively) but had no significant effect on systemic blood pressure. The occurrence of VT/VF was reduced from 9 of 12 to 3 of 12 dogs when ITC was administered prior to transient myocardial ischemia (P = .04). ITC also blunted ischemia-induced HR increase by 74%.

CONCLUSION

ITC reduced ischemia-induced VT/VF in a canine model of healed myocardial infarction with superimposed heart failure and acute ischemia. Results from electrophysiologic testing were consistent with a clonidine-induced reduction in cardiac sympathetic activity from the spinal cord. These data suggest that ITC administration may be a novel approach to treating ventricular arrhythmias.

Withdrawal following sufentanil/propofol and sufentanil/midazolam

PURPOSE

Patients in the ICU after long-term administration of an opioid/hypnotic often develop delirium. To assess the nature of this phenomenon, patients in a surgical ICU following ventilatory support and sedation with an opioid/hypnotic/sedative were studied.

METHODOLOGY

Following sufentanil/midazolam (group 1; n =14) or sufentanil/propofol (group 2; n =15) sedation, patients were evaluated for changes in mean arterial blood pressure and heart rate, the activity of the central nervous system (sensory evoked potentials, spectral edge frequency of EEG), and the endogenous opioids plasma concentrations (beta-endorphin, met-enkephalin). Data obtained were correlated with the individual intensities of withdrawal symptoms 6-, 12-, and 24 h following sedation.

RESULTS

Following a mean duration of ventilation of 7.7 days (+/-3.6 SD) in groups 1 and 3.5 (+/-1.7 SD) in group 2, withdrawal intensities peaked within the 6th hour after cessation. Plasma beta-endorphin and met-enkephalin levels were low during sedation, and only the sufentanil/midazolam group demonstrated a postinhibitory overshoot. Withdrawal symptom intensities demonstrated an inverse correlation with beta-endorphin and met-enkephalin levels, a direct linear correlation with amplitude height of the evoked potential, and blood pressure and heart rate changes. Withdrawal intensities did not correlate with EEG power spectral edge frequency.

CONCLUSION

The endorphinergic system is suppressed when a potent exogenous opioid like sufentanil is given over a long period of time. Following sedation, abstinence symptoms seem to be related to postinhibitory increased endorphin synthesis. This is mostly seen in the combination of sufentanil/midazolam. In addition, an increase in the amplitude of the sensory-evoked potential suggests a postinhibitory excitatory state within the nociceptive system.

Factors influencing preoperative stress response in coronary artery bypass graft patients

Background

In many studies investigating measures to attenuate the hemodynamic and humoral stress response during induction of anaesthesia, primary attention was paid to the period of endotracheal intubation since it has been shown that even short-lasting sympathetic cardiovascular stimulation may have detrimental effects on patients with coronary artery disease. The aim of this analysis was, however, to identify the influencing factors on high catecholamine levels before induction of anaesthesia.

Methods

Various potential risk factors that could impact the humoral stress response before induction of anaesthesia were recorded in 84 males undergoing coronary aortic bypass surgery, and were entered into a stepwise linear regression analysis. The plasma level of norepinephrine measured immediately after radial artery canulation was chosen as a surrogate marker for the humoral stress response, and it was used as the dependent variable in the regression model. Accordingly, the mean arterial blood pressure, heart rate and the calculated pressure-rate product were taken as parameters of the hemodynamic situation.

Results

Stepwise regression analysis revealed that the oral administration of low-dose clonidine (mean dose 1.75 μg·kg^-1) on the morning of surgery was the only significant predictor (p = 0.004) of the high variation in preoperative norepinephrine plasma levels. This intervention decreased norepinephrine levels by more than 40% compared to no clonidine administration, from 1.26 to 0.75 nmol·l^-1. There was no evidence for dose-responsiveness of clonidine. All other potential predictors were removed from the model as insignificant (p > 0.05). The use of beta-blocker, ace-inhibitors, ejection fraction, and body mass index were significant determinants for the hemodynamic situation (heart rate, mean arterial pressure, pressure rate product) of the patient during the pre-induction period.

Conclusion

The oral administration of clonidine is the only significant predictor for the observed variation of norepinephrine levels during the preoperative period. Lack of significant dose responsiveness suggests that even a low dose of the drug can attenuate the preoperative stress response and thus is recommended in cardiovascular high risk patients.

Epidural clonidine suppresses the baroreceptor-sympathetic response depending on isoflurane concentrations in cats

Epidural administration of clonidine induces hypotension and bradycardia secondary to decreased sympathetic nerve activity. In this study, we sought to elucidate the change in baroreflex response caused by epidural clonidine. Thirty-six cats were allocated to six groups (n = 6 each) and were given either thoracic epidural clonidine 4 micro g/kg or lidocaine 2 mg/kg during 0.5, 1.0, or 1.5 minimum alveolar anesthetic concentration (MAC) isoflurane anesthesia. Heart rate (HR), mean arterial blood pressure (MAP), and cardiac sympathetic nerve activity (CSNA) were measured. Depressor and pressor responses were induced by IV nitroprusside 10 micro g/kg and phenylephrine 10 micro g/kg, respectively. Baroreflex was evaluated by the change in both CSNA and HR relative to the peak change in MAP (deltaCSNA/deltaMAP and deltaHR/deltaMAP, respectively). These measurements were performed before and 30 min after epidural drug administration. Epidural clonidine and lidocaine decreased HR, MAP, and CSNA by similar extents. deltaCSNA/deltaMAP and deltaHR/deltaMAP for depressor response were suppressed with epidural lidocaine and clonidine in all groups but the clonidine 0.5 MAC isoflurane group (0.197 +/- 0.053 to 0.063 +/- 0.014 and 0.717 +/- 0.156 to 0.177 +/- 0.038, respectively, by epidural lidocaine [P < 0.05] but 0.221 +/- 0.028 to 0.164 +/- 0.041 and 0.721 +/- 0.177 to 0.945 +/- 0.239, respectively, by epidural clonidine during 0.5 MAC isoflurane). Those for pressor response were suppressed in all groups. We conclude that thoracic epidural clonidine suppresses baroreflex gain during isoflurane anesthesia >1.0 MAC but may offer certain advantages compared with epidural lidocaine during 0.5 MAC isoflurane by virtue of preserving baroreflex sensitivity when inadvertent hypotension occurs.

Determination and quantification of clonidine in human blood serum

Clonidine ((2-[2,6-dichlorophenyl]amino)-2-imidazoline) preferentially stimulates central alpha(2)-adrenoceptors, which leads to inhibition of sympathetic tone, resulting in a lowering of arterial pressure and of heart rate. Additionally, many other desirable and undesirable effects are described, including analgesia, sedation and withdrawal reactions, which consist of a sudden rise in arterial pressure, nervousness, agitation and increased heart rate. The present study has the goal to develop a simple and effective method for the analysis of trace amounts of clonidine in human blood serum. Special emphasis is necessary to make application of electron impact ionization and separation of the analyte fragments in a quadruple mass analyzer suitable. The procedure comprises solid phase extraction followed by formation of the pentafluorobenzyl derivative. Further purification is achieved by phase transfer extraction into an acidic aqueous solution succeeded by re-extraction into dichloromethane. After solvent exchange, an aliquot is injected into the gas chromatograph equipped with a DB5 MS capillary column and a mass spectrometric detector. Chromatograms are recorded in single ion monitoring mode. Quantification is accomplished by internal standardization with moxonidine [4-chloro-5-(2-imidazolin-2-yl-amino)-6-methoxy-2-methylpyridine].

Continuous intraventricular clonidine infusion in controlled morphine withdrawal--case report

A patient with atypical bilateral facial pain reported the loss of analgesic effect of intracerebroventricular morphine delivered continuously via an implanted pump, accompanied by intolerable adverse side effects associated with the administered high dose of morphine. Clonidine was substituted for morphine over a period of 3 weeks to achieve a drug holiday. The patient did not have significant withdrawal symptoms or major discomfort from pain, leading to a reduced quality of life during this period. Six months after the treatment, the patient continues to require a significantly lower daily dose of morphine. Morphine withdrawal with clonidine substitution produced a significant improvement in the analgesic efficacy of morphine and in the quality of life in the absence of undesirable side effects.

A dose response study of clonidine with local anesthetic mixture for peribulbar block: a comparison of three doses

Clonidine prolongs anesthesia and analgesia of local anesthetics in various neural blocks as well as the duration of retrobulbar block. We assessed the dose-response relationship of clonidine added to lidocaine in peribulbar block. Sixty patients undergoing cataract surgery were given peribulbar block with 7 mL of 2% lidocaine and hyaluronidase with either saline (Control) or clonidine in 0.5-microg/kg (0.5 Clon), 1.0-microg/kg (1.0 Clon), or 1.5-microg/kg (1.5 Clon) doses. The onset and duration of lid and globe akinesia, globe anesthesia and analgesia, postoperative analgesic requirement, and adverse effects (hypotension, bradycardia, hypoxia, sedation, and dizziness) were recorded. The success rate and onset of block were comparable in all groups. The duration of lid and globe akinesia, globe anesthesia and analgesia was significantly (P < 0.01) prolonged in patients receiving 1.0 and 1.5 microg/kg clonidine as compared with the Control group. Perioperative pain scores and analgesic requirement were significantly less in these groups. 0.5 microg/kg clonidine did not increase the duration of anesthesia and analgesia significantly. Hypotension and dizziness were observed more in patients receiving 1.5 microg/kg clonidine as compared with other groups. We conclude that 1.0 microg/kg clonidine with a mixture of lidocaine (2%) significantly prolonged the duration of anesthesia and analgesia after peribulbar block with limited side effects.

IMPLICATIONS

We studied the effect of the addition of 0.5, 1.0 and 1.5 microg/kg clonidine to a lidocaine-hyaluronidase mixture on the onset and duration of peribulbar block and perioperative analgesia. A dose of 1.0 microg/kg produced a significant increase in duration of anesthesia and analgesia with minimal side effects.

Obesity and hypertension

Substantial evidence from epidemiological data supports a link between obesity and hypertension. However, the relationship between the two disorders is not straightforward and most likely represents an interaction of demographic, genetic, hormonal, renal, and hemodynamic factors. Age, race, and sex also modulate the strength of the association between obesity and hypertension. Hyperinsulinemia, which is characteristic of obesity, can contribute to the probability of developing hypertension by activating the sympathetic nervous system (SNS) and by causing sodium retention. The pressor effect of insulin in obesity may be further enhanced by the observation that its vasodilator action can be blunted in obese subjects. Preliminary data have shown that leptin, whose levels are increased in most obese individuals, can contribute to hypertension in obesity through its effects on insulin, SNS, and sodium excretion. The kidney may also have a role in the pathophysiology of hypertension in obesity. Abnormal renal sodium handling coupled with structural changes in the kidney of an obese patient can raise blood pressure. In addition, obesity is associated with distinct cardiovascular hemodynamic alterations and development of eccentric myocardial hypertrophy. Most of these obesity-associated abnormalities, as well as hypertension itself, can be reversed by weight loss. Furthermore, weight loss can prevent, or at least delay, the development of hypertension in patients with high-normal blood pressure. Weight reduction should be the first-line treatment in every obese hypertensive patient. However, the majority of patients will need pharmacologic intervention in conjunction with weight loss. Selection of antihypertensive agents in the overweight patient should take into account the mechanisms leading to hypertension and the metabolic abnormalities that characterize the obese patient.

Clonidine does not impair redistribution hypothermia after the induction of anesthesia

Clonidine is commonly given for premedication, and it impairs normal thermoregulatory responses to warm and cold stimuli while depressing sympathetic tone. We studied the effect of premedication by clonidine on redistribution hypothermia induced by the induction of anesthesia. Sixteen ASA physical status I or II patients were randomly assigned to receive either clonidine 150 micrograms or a placebo. Anesthesia was induced 45 min later by thiopental, fentanyl, and vecuronium i.v. and was maintained by the administration of 0.6% isoflurane. We monitored central core (tympanic) temperature and skin surface temperatures at the forearm and the fingertip during the 2 h after the induction of anesthesia before surgery. We estimated skin blood flow at the level of the forearm by using laser Doppler during the same period. The core temperature decreased comparably in the two groups of patients, from 37.1 +/- 0.2 degrees C to 35.3 +/- 0.4 degrees C and from 37.1 +/- 0.2 degrees C to 35.5 +/- 0.3 degrees C in the clonidine and placebo groups, respectively. The forearm-fingertip surface temperature gradient decreased similarly in the two groups. There was no evidence of cutaneous vasoconstriction. The laser Doppler index at the fingertip increased similarly in the two groups, as did the forearm-fingertip temperature gradient. We conclude that premedication with clonidine does not significantly impair the profile of central hypothermia induced by heat redistribution after the induction of anesthesia.

IMPLICATIONS

The induction of general anesthesia is associated with redistribution hypothermia. This study shows that premedication with oral clonidine does not worsen the decrease in core temperature resulting from general anesthesia.

Clonidine reduces hyperpolarization-activated inward current (Ih) in rat hypoglossal motoneurons

We used intracellular recording techniques to investigate the actions of clonidine on hypoglossal motoneurons (HMs) in rat brainstem slices. Clonidine (10-100 microM) produced a small (2-6 mV), dose-dependent hyperpolarization in HMs, accompanied by an increase in peak input resistance (RN). It also slowed the time course of the depolarizing 'sag' of the voltage response to constant hyperpolarizing current steps. These effects were mimicked by the alpha2-adrenoceptor (alpha2-AR) agonist guanabenz, but not by the Ih-imidazoline receptor agonists moxonidine or rilmenidine. Recorded in single-electrode voltage clamp mode, clonidine decreased input conductance of HMs and reduced the amplitude of a hyperpolarization-activated inward current (Ih). Clonidine's effect on Ih was three-fold: it shifted the half-activation voltage (V1/2) in the hyperpolarizing direction (by 4.4 +/- 0.7 mV at a dose of 10 microM), decreased the maximal current (by approximately 20%), and slowed the time course of Ih activation at all voltage steps. At the most hyperpolarized potential steps, clonidine slowed activation of Ih dramatically, yielding a striking increase in the activation time constant. The alpha2-AR antagonists yohimbine and idazoxan reduced clonidine's effect on V1/2 and on the Ih activation time course, but neither blocked clonidine's reduction of the maximal current, nor its strong slowing of Ih activation at the most hyperpolarized steps. We were unable to mimic or occlude clonidine's actions with the adenylate cyclase inhibitor SQ 22536 nor with the non-specific protein kinase inhibitor H-7. We conclude that clonidine hyperpolarizes HMs via a reduction of the amount of Ih that is active at rest, and that the response is mediated in part by alpha2-ARs. Some effects of clonidine on these neurons do not appear to be receptor-mediated, and may be due to physical block by clonidine of Ih channels.

Plasma concentrations and disposition of clonidine following a constant 14-day epidural infusion in cancer patients

Epidural administration of clonidine, a partial alpha 2-adrenergic agonist, provides effective relief for patients with intractable cancer pain. However, clonidine's long plasma elimination half-life suggests a potential for plasma accumulation following repeated doses or constant infusion. Adult male and female cancer patients experiencing severe, intractable pain were administered a continuous epidural infusion (30 micrograms/h) of clonidine hydrochloride for 14 days. Plasma clonidine concentrations were determined for 31 patients using a radioimmunoassay with a limit of quantitation of 0.062 ng/mL. Mean (+/- SD) plasma clonidine concentration and calculated total body clearance after 7 days of infusion (respectively, 2.19 +/- 1.17 ng/mL, n = 24; 279 +/- 184 mL/min, n = 27) were comparable to those following 14 days of infusion (2.50 +/- 1.51 ng/mL, n = 19; 272 +/- 163 mL/min, n = 21). Clonidine does not appear to accumulate in the plasma compartment during prolonged (14-day) continuous epidural infusion in cancer patients.

Effects of intravenous and oral clonidine on hemodynamic and plasma-catecholamine response due to endotracheal intubation

STUDY OBJECTIVE

To investigate the effects of intravenous (IV) versus oral clonidine on alterations of heart rate (HR), mean arterial pressure (MAP), cardiac output (CO), and plasma-catecholamines due to endotracheal intubation.

DESIGN

Randomized, double-blind, placebo-controlled study.

SETTING

University hospital surgery operating room.

PATIENTS

33 ASA physical status I patients were randomly assigned to either receive clonidine 3 micrograms/kg IV immediately prior to anesthesia induction, clonidine 4 micrograms/kg orally 90 minutes prior to anesthesia induction, or placebo.

INTERVENTIONS

Insertion of a 14 G cannula in a large cubital vein for the determination of plasma-catecholamines using local anesthesia. Insertion of a radial artery catheter for measuring blood pressure (BP) using local anesthesia. Transthoracic echocardiography determined CO.

MEASUREMENTS AND MAIN RESULTS

Heart rate, MAP, CO, and plasma-catecholamine concentrations were measured. Measurements were performed prior to induction, during intubation, and 10 minutes after intubation. During endotracheal intubation, MAP was significantly lower in the IV clonidine group compared with the placebo and the oral clonidine groups. Cardiac output was significantly lower in the IV clonidine group only. In contrast to the placebo group, norepinephrine plasma concentrations did not increase in either clonidine group. Significant alterations of epinephrine plasma concentrations due to intubation were not observed in either group. Hemodynamics after intubation were not impaired by clonidine treatment.

CONCLUSIONS

In conclusion, IV clonidine reduced stress response to endotracheal intubation compared with placebo. Oral clonidine at the dose used was less effective in blunting hemodynamic stress response than IV clonidine.

Comparative assessment of the anaesthetic and analgesic effects of intramuscular and epidural clonidine in humans

PURPOSE

The aim of the study was to assess and compare in analogous controlled experimental conditions, the anaesthetic sparing and analgesic effects of the same dose of clonidine administered by the intramuscular (im) and epidural (ep) routes.

METHODS

We used a randomized, double blind and placebo controlled protocol. Sixty patients undergoing abdominal hysterectomy were distributed into three groups who, 30 min before surgical incision, received: 300 micrograms ep clonidine plus im saline; ep saline plus 300 micrograms im clonidine; or ep and im saline (ss). General anaesthesia was maintained with 60% N2O in O2, and isoflurane administered at concentrations to maintain mean arterial pressure (MAP) and heart rate (HR) within 20% of basal values. Isoflurane requirements (mass spectrometry), cardiovascular variables (MAP, HR), and plasma concentrations of glucose, cortisol and prolactin were evaluated at critical time points. In the recovery room (RR), sedation (Ramsay) and pain intensity (VAS) were estimated at the time of analgesia request (TAR).

RESULTS

Intramuscular and ep clonidine decreased isoflurane requirements similarly by about 85% (P < 0.001). Patients in the ep group had lower MAP (P < 0.03) and HR (P < 0.001) than in the im group, but im and ep clonidine similarly blunted the plasma prolactin increase induced by intubation. In RR, ep but not im clonidine (P < 0.01) induced postoperative analgesia demonstrated by a prolonged TAR 80.8 +/- 7.3 (ep) 35.9 +/- 3.2 (im) and 44.5 +/- 5.1 (ss) min and a lower VAS (P < 0.05).

CONCLUSIONS

Epidural and intramuscular clonidine decreased isoflurane requirements similarly, but only the epidural route provided postoperative analgesia, suggesting a spinal site for the analgesic action.

Effects of a newly developed transdermal clonidine delivery system (M-5041T) on EEG sleep-wake cycle in relation to plasma concentration in rabbits

1. The effects of a transdermal clonidine delivery system (M-5041T) on EEG sleep pattern with relation to plasma concentrations in unrestrained rabbits were investigated and compared with those of intravenous (i.v.) administration of clonidine. 2. Although M-5041T did not affect the EEG recorded from cortex and hippocampus at doses up to 2.5 mg/kg, slow theta waves in hippocampal EEG accompanied by low-voltage slow waves in cortex were induced at a higher dose of 12.5 mg/kg. On i.v. injection (0.25 mg/kg), EEG tracings with bursts of high-voltage slow waves in cortical EEG and slow theta waves in hippocampus were observed. 3. At doses of 0.5 and 2.5 mg/kg, M-5041T did not cause any alterations of the sleep-wake cycle, and plasma concentrations of 1-2 ng/ml were maintained for an 8-hr observation period. However, this delivery system significantly suppressed the incidence of rapid-eye movement sleep (REMS) from 11.9 to 4.7% and enhanced drowsiness (DW) from 9.0 to 21.0% during the 8-hr recording period at 12.5 mg/kg with a plasma concentration of up to 10 ng/ml. Contrary to transdermal administration, i.v. clonidine (0.25 mg/kg) completely blocked light and deep slow wave sleep as well as REMS with a plasma concentration indicated more than 10 ng/ml at 2 hr post administration. Recovery to a normal sleep-wake cycle was eventually established thereafter. The incidence of REMS and DW were significantly decreased from 11.9 to 6.3% and increased from 9.0 to 25.5%, respectively. 4. Concurrent monitoring of clonidine concentrations in cerebrospinal fluid (CSF) indicated that CSF concentrations after patching M-5041T, as well as i.v. clonidine, were almost equal to plasma levels. 5. These results suggest that alteration of the sleep-wake cycle with clonidine occurs depending upon brain concentrations, which increase to a level similar to that in plasma after administration, and that M-5041T at doses of less than 2.5 mg/kg could establish effective hypertensive therapy without obvious effects on the cycle.

Epidural clonidine analgesia for intractable cancer pain. The Epidural Clonidine Study Group

Although the vast majority of patients with cancer pain receive effective analgesia from standard therapy, a few patients, particularly those with neuropathic pain, continue to experience severe pain despite large doses of systemic or intraspinal opioids. Animal studies suggest intraspinal alpha 2-adrenergic agonists may be effective in such cases. Eighty-five patients with severe cancer pain despite large doses of opioids or with therapy-limiting side effects from opioids were randomized to receive, in a double-blind manner, 30 micrograms/h epidural clonidine or placebo for 14 days, together with rescue epidural morphine. Pain was assessed by visual analog score (VAS), McGill Pain Questionnaire, and daily epidural morphine use. Success was defined as a decrease in either morphine use of VAS pain, with the alternative variable either decreasing or remaining constant. Blood pressure, heart rate, and degree of nausea and sedation were monitored. Successful analgesia was more common with epidural clonidine (45%) than with placebo (21%). This was particularly prominent in those with neuropathic pain (56% vs. 5%). Pain scores were lower at the end of the treatment period in patients with neuropathic pain treated with clonidine rather than placebo, whereas morphine use was unaffected. Clonidine, but not placebo, decreased blood pressure and heart rate. Hypotension was considered a serious complication in 2 patients receiving clonidine and in 1 patient receiving placebo. This study confirms the findings from previous animal studies which showed the effective, potent analgesic properties of intraspinal alpha 2-adrenergic agonists and suggests that epidural clonidine may provide effective relief for intractable cancer pain, particular of the neuropathic type.

Involvement of imidazoline-preferring receptors in regulation of sympathetic tone

We examined the contribution of imidazoline-preferring receptors (IPR) and alpha 2-adrenoceptors at different levels of the central nervous system in the antihypertensive and sympathoinhibitory actions of rilmenidine in 2 conscious animal models, the spontaneously hypertensive rat (SHR) and the normotensive rabbit. In conscious SHRs, we compared the potency of rilmenidine and clonidine administered intravenously into the lateral cerebral ventricle, the cisterna magna, and into the subarachnoidal space of the thoracolumbar spinal cord. In SHRs, we found that rilmenidine was more potent and more effective by the intrathecal than the intracisternal route. By contrast, clonidine was most effective after administration into the cisterna magna. Intravenous administration of rilmenidine or clonidine induced dose-dependent and prolonged decreases in blood pressure and heart rate. Neither rilmenidine nor clonidine altered mean arterial pressure or heart rate when given into the lateral cerebral ventricle. These data suggest that in SHRs the spinal cord may be an important site for the antihypertensive action of rilmenidine. We therefore characterized the receptor type involved. We observed in conscious SHRs that intrathecal post-treatment with idazoxan, a mixed alpha 2-adrenoceptor and IPR antagonist, abolished the antihypertensive effect of rilmenidine, whereas 2-methoxyidazoxan, a selective alpha 2-adrenoceptor antagonist, caused only a partial reversal of the blood pressure effects of rilmenidine. These results suggest that rilmenidine acts mainly through IPR rather than alpha 2-adrenoceptors in the spinal cord. In view of these findings, we compared the hypotensive actions of rilmenidine and clonidine, administered into the lateral cerebral ventricle, the cisterna magna, and the subarachnoid space of the thoracolumbar spinal cord in conscious normotensive rabbits. Both drugs were less potent and effective when administered intrathecally than intracisternally. These experiments suggest that the hypotensive action of rilmenidine and clonidine in the rabbit is mediated through receptors mainly located in the brainstem. Further, we found that idazoxan reversed the hypotensive action of rilmenidine more readily than 2-methoxyidazoxan. Surprisingly, both idazoxan and 2-methoxyidazoxan completely reversed the depressor effects of clonidine. Therefore, in the rabbit, rilmenidine acts through IPR located in the brainstem and clonidine acts predominantly through alpha 2-adrenoceptors. In conclusion, our studies demonstrate that IPR are involved in the vasodepressor action of rilmenidine in both conscious SHRs and rabbits. However, although the main site of action of rilmenidine in SHRs may be located in the thoracolumbar spinal cord, in the rabbit it appears to be in the brainstem.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of clonidine in human plasma by gas chromatography-electron-impact mass spectrometry

A new and sensitive gas chromatographic-mass spectrometric (GC-MS) assay for routine analysis of clonidine in human plasma is described. Quantification is carried out with simple electron-impact ionization mass spectrometry and solid-phase cartridges for preliminary extraction from plasma. The pentafluorobenzyl derivative of clonidine yields an intense ion fragment at m/z 354, and the lower limit of detection is 0.025 ng/ml for a 1-ml plasma sample. The practical applicability of this method is demonstrated by determining plasma concentrations of clonidine in a clinical study of a new transdermal delivery system for clonidine.

Influence of anaesthesia on the cardiovascular effects of rilmenidine and clonidine in spontaneously hypertensive rats

1. The acute cardiovascular effects of two alpha 2-adrenoceptor agonists, rilmenidine and clonidine, were studied in 15-week-old male spontaneously hypertensive rats (SHRs). The effects of these drugs were compared with intravenous (i.v.) and intracerebroventricular (i.c.v.) administration in conscious and pentobarbitone-anaesthetized SHRs, in which aortic blood pressure (BP) was continuously recorded. 2. In conscious SHRs, i.v. doses of either rilmenidine (30, 100, 300 micrograms kg-1) or clonidine (3, 10, 30 micrograms kg-1) induced dose-dependent short-lasting increases in BP followed by moderate decreases associated with bradycardia, while the same three doses of both drugs given i.c.v. were devoid of BP and heart rate (HR) effects. 3. Pentobarbitone-anaesthesia increased the sympathetic control of BP and suppressed the cardiac baroreflex sensitivity. 4. In anaesthetized SHRs, i.v. injections of the same 3 doses of rilmenidine and clonidine induced a slight increase in BP, rapidly followed by profound and long-lasting BP and HR decreases. Surprisingly, when given i.c.v., these 3 doses lowered BP and HR to the same extent but in a more progressive manner. 5. The lack of efficacy of both drugs in conscious SHRs after the i.c.v. administration of i.v. active doses and the lack of more marked and rapid effects in anaesthetized SHRs, after i.c.v. than after i.v. injections, question the involvement of a major central site of action for these antihypertensive alpha 2-adrenoceptor agonists. Moreover, these results show that the cardiovascular effects of these drugs are profoundly influenced by baseline sympathetic nervous system activity which is enhanced by pentobarbitone-anaesthesia.

Sympathoinhibitory effects of rilmenidine may be mediated by sites located below the brainstem

1. To determine the site of action of rilmenidine, we examined its effets on arterial blood pressure (BP), heart rate (HR) and postganglionic renal sympathetic nerve activity (RSNA) after intracerebroventricular (i.c.v.) administration (300 micrograms kg-1), in groups (all n = 6) of conscious and freely moving, pentobarbitone-anaesthetized and pentobarbitone-anaesthetized and spinally transected, fifteen week-old male spontaneously hypertensive rats (SHRs). 2. In conscious SHRs, which exhibited a low sympathetic nerve activity (RSNA: 3.4 +/- 0.9 muV), rilmenidine was inactive on systolic BP (SBP), diastolic BP (DBP), HR and RSNA. 3. In intact pentobarbitone-anaesthetized SHRs, which exhibited an elevated sympathetic nerve activity (RSNA: 10.6 +/- 0.9 muV), rilmenidine exerted potent antihypertensive (delta SBP: -37 +/- 4%; delta DBP: -43 +/- 6%), bradycardic (delta HR: -32 +/- 3%) and sympathoinhibitory (delta RSNA: -68 +/- 9%) activities. 4. In pentobarbitone-anaesthetized SHRs with cervical spinal cord transection, BP was markedly decreased and sympathetic nerve activity (RSNA: 10.3 +/- 3.1 muV) returned to the level observed in conscious SHRs (RSNA: 3.6 +/- 0.5 muV). In these conditions, rilmenidine remained sympathoinhibitory (delta RSNA: -74 +/- 5%). 5. In conclusion, we have shown that pentobarbitone-anaesthesia enhances the peripheral sympathetic tone by a central action, as the spinal cord transection allows RSNA to return to normal levels and that, spinal or ganglionic structures could be a major site of the sympathoinhibitory action of rilmenidine.

Pharmacokinetics of clonidine after epidural administration in surgical patients. Lack of correlation between plasma concentration and analgesia and blood pressure changes

The pharmacokinetics of epidural clonidine 150 micrograms was studied in 13 patients who had undergone abdominal hysterectomy. Plasma clonidine concentrations were measured up to 19 h in eight patients. In another five patients frequent blood sampling was performed only during the first 20 min to define early vascular uptake better. Peak plasma clonidine concentrations of 1.08 +/- 0.35 ng ml-1 (mean +/- s.d.) were reached between 5 and 10 min after injection. Plasma elimination half-life was 829 +/- 157 min and plasma clearance was 177 +/- 28 ml min-1. There was a significant decrease in arterial blood pressure within 10 min of the injection of clonidine. The maximum decrease in systolic blood pressure, from a pre-injection value of 135 +/- 24.7 to 99 +/- 14.4 mmHg (18.0 +/- 3.3 to 13.2 +/- 1.9 kPa), occurred at 60 min. Blood pressure remained significantly lower than the pre-injection value for 4 h. There was no change in heart rate. Verbal analogue pain scores, on a scale 0-10, decreased from a median of 7.6 before clonidine to 5.0 after 30 min (P less than 0.05). The median score at 60 min was 4.3. Thereafter, pain scores were not significantly different from the control score. We conclude that epidural clonidine 150 micrograms produces only moderate and short-lived postoperative analgesia. Absorption of clonidine from the epidural space into the blood is very rapid and may contribute to the hypotension that occurs.

Cardiovascular and endocrine effects of clonidine premedication in neurosurgical patients

The present study was conducted to examine the haemodynamic and endocrine effects of clonidine, given as sole preanaesthetic medication, in neurosurgical patients. Nineteen patients of ASA physical status I and II, subjected to craniotomy, randomly received po premedication of either clonidine (300 micrograms, n = 9) or placebo (n = 10). Blood pressure and heart rate were monitored continuously, while arterial blood samples were collected at specific times, from induction of anaesthesia to recovery, for the measurement of plasma concentrations of epinephrine, norepinephrine, cortisol, aldosterone, and glucose. Clonidine treatment led to a decrease in mean arterial blood pressure (MABP), heart rate (HR), and plasma cortisol and aldosterone concentrations throughout the study, compared with placebo (P less than 0.05). Clonidine, however, did not prevent increases in MABP (16 +/- 5 mmHg, mean +/- SE, P less than 0.05) and HR (18 +/- 4 bpm, P less than 0.05) during induction of anaesthesia, which was comparable to the placebo group. Plasma catecholamine concentrations did not differ between the two groups. Plasma glucose concentrations increased in both groups at the end of the study (P less than 0.05), but were lower in clonidine-treated patients (P less than 0.05). Though statistically significant, the observed inhibitory haemodynamic and endocrine effects of clonidine seem to be of minor clinical importance. As the action of clonidine on cerebral blood flow regulation is not well known, we see no advantage in the preanaesthetic administration of clonidine to neurosurgical patients with normal cardiovascular status.

Thermographic observations on rats with experimental neuropathic pain

Infrared thermographic images were obtained from the plantar hind paws of rats with an experimental nerve injury that produces signs of neuropathic pain. Thermograms confirmed that the experimental neuropathy produces signs resembling those of patients with neuropathic pain. The hind paws on the nerve-damaged side were abnormally hot, abnormally cold, or apparently normal 8-16 days post injury, a variability that is seen clinically in neuropathic pain patients. Abnormally cold hind paws became warm as soon as the injured sciatic nerve was transected, indicating that the underlying vasoconstriction was mediated by neural impulse activity. Xylazine (Rompun), a sympatho-inhibitory alpha 2-adrenoceptor agonist that normally increases cutaneous temperature, caused the hind paw on the control side to warm, as anticipated, while causing paradoxical cooling of abnormally hot hind paws, and even of 'normal temperature' paws on the nerve-injured side. These findings shed light on possible mechanisms underlying abnormal deviations of skin temperature as a symptom of nerve injury. The findings also attest to the usefulness of the experimental animal model of neuropathic pain and of the thermographic method.

A differential effect of yohimbine on adrenal and neuronal catecholamine release during bilateral carotid occlusion in the dog

This study reports on the effects of yohimbine and clonidine on the release of adrenal and renal catecholamines (epinephrine, E; norepinephrine, NE; and dopamine, DA) in response to bilateral carotid occlusion (BCO, 3 min) in vagotomized dogs anesthetized with sodium pentobarbital. The model used allowed us to simultaneously compare adrenal catecholamine secretion with neuronal NE release in the kidney. In control dogs, the net output (ng/min/g tissue) of adrenal E (70.5 +/- 19.7), NE (22.2 +/- 5.9) and DA (2.6 +/- 0.8) increased markedly (P less than 0.01) during BCO to a maximum level of 265.1 +/- 87.9, 97.4 +/- 30.6 and 10.5 +/- 3.2, respectively. Similarly, the net output (ng/min/g tissue) of renal NE (0.66 +/- 0.06) and DA (0.09 +/- 0.02) increased significantly (P less than 0.01) to 1.00 +/- 0.11 and 0.15 +/- 0.04, respectively. Aortic systolic pressure (mm Hg) (140.8 +/- 8.0) and heart rate (beats/min) (162.7 +/- 5.1) also increased (P less than 0.01) to 212.5 +/- 19.3 and 179.5 +/- 5.4, respectively. In dogs treated with yohimbine (0.3 mg/kg, i.v.), the net increase in adrenal catecholamine output was diminished by approximately 47% (P less than 0.05). In contrast, the net increase in renal NE output was potentiated by 41% (P less than 0.05). The net increase in heart rate was also enhanced significantly (P less than 0.01) in the presence of yohimbine. In dogs receiving clonidine (15 micrograms/kg, i.v.) the increases in net output of both adrenal and renal catecholamine were abolished. Similarly, pressor and heart rate responses were abolished in the presence of clonidine. The results indicate that yohimbine exerted a differential effect on renal sympathetic nerves (increase) and adrenal medullae (decrease) in modulating catecholamine release in response to BCO, while clonidine abolished both neural NE release and adrenal catecholamine secretion. This study suggests that a presynaptic alpha 2-adrenoceptor-mediated mechanism, the blockade of which enhances neural NE release at peripheral sympathetic nerve terminals in many tissues, may not be involved in the modulation of adrenal catecholamine secretion during BCO.

Inhibition of serotonin-induced ACTH release in man by clonidine

1. Clonidine, an alpha 2-adrenergic agonist, is thought to inhibit noradrenergic neuronal activity (NNA) in the central nervous system (CNS) by a presynaptic alpha 2-receptor mechanism. Central NNA is thought to be the primary monoaminergic stimulus to pituitary ACTH release. Fenfluramine, a serotonin releasing agent and uptake inhibitor, causes ACTH release in normal man. 2. The present study investigated the effect of clonidine on fenfluramine-induced ACTH release in six normal volunteers. Four protocols were used: 1.5 mg/kg body weight oral fenfluramine; 4.3 micrograms/kg body weight oral clonidine; oral clonidine + oral fenfluramine 1 h later; placebo clonidine. Plasma ACTH and cortisol were measured at intervals for 5 h after clonidine and for 4 h after fenfluramine. 3. The mean plasma ACTH and cortisol levels and the mean maximal changes in these levels were significantly lower during the clonidine + fenfluramine test than during fenfluramine alone. Plasma ACTH and cortisol levels were not lowered significantly more by clonidine than by placebo. 4. In conclusion, clonidine blocked the ACTH-releasing activity of fenfluramine in normal humans. This inhibition of active ACTH release may result from clonidine blockade of fenfluramine-induced activation of central NNA. Clonidine alone was no more effective than placebo in lowering plasma ACTH and cortisol.