The effects of intravenous clonidine on ventilation

Comparison of Oral Clonidine and Gabapentin Premedication for Attenuation of Pressor Response to Laryngoscopy and Endotracheal Intubation

Background:

During the administration of general anesthesia, direct laryngoscopy and endotracheal intubation cause an increase in heart rate, arterial pressure, and dysrhythmias in upto 90% of patients. These changes can be particularly hazardous for patients with cerebral or coronary diseases. Both clonidine and gabapentin have been used for anesthetic effects, but a better drug for controlling hemodynamic parameters is being investigated.

Aims:

The study was done to evaluate and compare the efficacy of oral clonidine 0.3 mg and oral gabapentin 900 mg as a premedication for attenuation of pressor response to laryngoscopy and endotracheal intubation.

Materials and Methods:

After obtaining approval from the ethics committee, 75 patients, American Society of Anesthesiologists physical status classes I and II between the ages of 18 and 60 years scheduled to undergo elective noncardiac surgical procedure were enrolled in the study. Patients were randomized into three groups of 25 each who received 0.3 mg clonidine, 900 mg gabapentin, and placebo. The hemodynamic parameters were recorded at various time intervals along with any adverse effects.

Statistical Analysis:

Quantitative variables were compared using unpaired t-test between the two groups and ANOVA for three groups. Qualitative variables were compared using the Chi-square test/Fisher's exact test. P < 0.05 was considered statistically significant.

Results:

In our study, we found that both clonidine and gabapentin are effective premedicants by oral route 2 h before induction of anesthesia to blunt the hemodynamic response to laryngoscopy and intubation as compared to placebo. Between clonidine and gabapentin, clonidine was found to be more effective with respect to blunting of systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial pressure (MAP), although found to be statistically significant only at 15 min with respect to SBP and DBP.

Conclusion:

Using clonidine or gabapentin, one can effectively provide stable hemodynamic conditions during laryngoscopy and endotracheal intubation, but more so with clonidine.

Safety of clonidine used for long-term sedation in paediatric intensive care: A systematic review

AIM

Although not approved, the α-adrenoceptor agonist clonidine is considered an option for long-term sedation protocols in paediatric intensive care. We reviewed adverse effects of clonidine occurring in this indication.

METHODS

Relevant literature was systematically identified from PubMed and Embase. We included interventional and observational studies on paediatric patients admitted to intensive care units and systemically long-term sedated with clonidine-containing regimes. In duplicates, we conducted standardised and independent full-text assessment and extraction of safety data.

RESULTS

Data from 11 studies with 909 patients were analysed. The studies were heterogeneous regarding patient characteristics (age groups, comorbidity, or comedication) and sedation regimes (dosage, route, duration, or concomitant sedatives). Just four randomised controlled trials (RCTs) and one observational study had comparison groups, using placebo or midazolam. For safety outcomes, our validity evaluation showed low risk of bias only in three studies. All studies focused on haemodynamic problems, particularly bradycardia and hypotension. Observed incidences or subsequent interventions never caused concerns. However, only two RCTs allowed meaningful comparisons with control groups. Odds ratios showed no significant difference between the groups, but small sample sizes (50 and 125 patients) must be considered; pooled analyses were not reasonable.

CONCLUSION

All evaluated studies concluded that the use of clonidine in paediatric intensive care units is safe. However, a valid characterisation of the safety profile remains challenging due to limited, biased and heterogeneous data and missing investigation of long-term effects. This evaluation demonstrates the lack of data, which prevents reliable conclusions on the safety of clonidine for long-term sedation in critically ill children. For an evidence-based use, further studies are needed.

Pharmacology of Common Analgesic and Sedative Drugs Used in the Neonatal Intensive Care Unit

In this review of analgesic and sedation medication in neonates, important classes of old and newer medications commonly used in the neonatal intensive care unit setting are discussed. In addition to drug metabolism, efficacy, and safety for individual drugs, new insights into multimodal analgesic approaches suggest ways in which multiple analgesic drug classes can be combined to maximize efficacy and minimize toxicity. Opiate pharmacogenetics and the potential for a precision therapeutics approach is explored, with a final description of gaps in knowledge and a call for future research of pain and sedation control in the neonatal population.

Clonidine or remifentanil for adequate surgical conditions in patients undergoing endoscopic sinus surgery: a randomized study

Background

Deliberate hypotension is one way to achieve a bloodless surgical field in endoscopic sinus surgery (ESS). We compared two anaesthesia regimens to induce deliberate hypotension and attempted to determine the most efficient one.

Methods

Fifty-nine patients undergoing ESS were minimized into two groups. In the CLO group, patients received I.V. sufentanil 0.15 µg/kg together with I.V. clonidine 2–3 µg/kg. In the REMI group, patients received remifentanil at a rate of up to 1 µg/kg/min. Fromme scores were collected 15 min after the incision and at the end of the procedure. Mean arterial pressure readings (MAP), heart rate readings, time to eyes opening, time to extubation, pain scores, analgesic requirements, and oxygen needs were collected and compared.

Results

There were no significant differences in Fromme scores between the two groups. The averaged MAP from 15 min to the end of the procedure was significantly lower in the REMI group; these patients also received more ephedrine. Significantly fewer patients in the CLO group needed oxygen therapy to keep their Pulse Oximeter Oxygen Saturation within 3% of their preoperative values. Patients in this group also needed less piritramide in the recovery room, and their pain scores were lower at discharge from the recovery room.

Discussion

Although both anaesthesia regimens offered a similar quality of surgical field, this study suggests that clonidine had a better average safety profile. Furthermore, patients who received this regimen required fewer painkillers immediately after surgery.

Sedation with Dexmedetomidine or Propofol Impairs Hypoxic Control of Breathing in Healthy Male Volunteers

Background

In contrast to general anesthetics such as propofol, dexmedetomidine when used for sedation has been put forward as a drug with minimal effects on respiration. To obtain a more comprehensive understanding of the regulation of breathing during sedation with dexmedetomidine, the authors compared ventilatory responses to hypoxia and hypercapnia during sedation with dexmedetomidine and propofol.

Methods

Eleven healthy male volunteers entered this randomized crossover study. Sedation was administered as an intravenous bolus followed by an infusion and monitored by Observer’s Assessment of Alertness/Sedation (OAA/S) scale, Richmond Agitation Sedation Scale, and Bispectral Index Score. Hypoxic and hypercapnic ventilatory responses were measured at rest, during sedation (OAA/S 2 to 4), and after recovery. Drug exposure was verified with concentration analysis in plasma.

Results

Ten subjects completed the study. The OAA/S at the sedation goal was 3 (3 to 4) (median [minimum to maximum]) for both drugs. Bispectral Index Score was 82 ± 8 and 75 ± 3, and the drug concentrations in plasma at the sedation target were 0.66 ± 0.14 ng/ml and 1.26 ± 0.36 μg/ml for dexmedetomidine and propofol, respectively. Compared with baseline, sedation reduced hypoxic ventilation to 59 and 53% and the hypercapnic ventilation to 82 and 86% for dexmedetomidine and propofol, respectively. In addition, some volunteers displayed upper airway obstruction and episodes of apnea during sedation.

Conclusions

Dexmedetomidine-induced sedation reduces ventilatory responses to hypoxia and hypercapnia to a similar extent as sedation with propofol. This finding implies that sedation with dexmedetomidine interacts with both peripheral and central control of breathing.

Comparison of lumbar epidural bupivacaine with fentanyl or clonidine for postoperative analgesia in children with cerebral palsy after single-event multilevel surgery

AIM

To compare diazepam use, muscle spasm, analgesia, and side effects when clonidine or fentanyl are added to epidural bupivacaine in children with cerebral palsy after multilevel orthopaedic surgery.

METHOD

Fifty children were prospectively randomized to receive clonidine (n=24, mean age 10y 10mo [SD 2y 11mo]) or fentanyl (n=26, mean age 10y 11mo [SD 2y 10mo]).

RESULTS

There was no difference in primary outcome measures: median diazepam use (fentanyl 0, interquartile range [IQR] 0-0; clonidine 0, IQR 0-0; p=0.46), any muscle spasm (no muscle spasms in: fentanyl, 36%; clonidine, 62%; p=0.11), painful muscle spasm (fentanyl 40%; clonidine 25%; p=0.46), or pain score ≥6 (none: fentanyl 44%; clonidine 42%; p=0.29). There were differences in secondary outcome measures: no vomiting (clonidine 63%; fentanyl 20%); vomiting occurred more frequently with fentanyl (32% vomited more than three times; clonidine none; p=0.001). Fentanyl resulted in more oxygen desaturation (at least two episodes: fentanyl 20%; clonidine 0; p<0.001). Clonidine resulted in lower mean (SD) area under the curve for systolic blood pressure (fentanyl 106.5 [11.0]; clonidine 95.7mmHg [7.9]) and heart rate (fentanyl 104.9 beats per minute [13.6]; clonidine 85.3 [11.5]; p<0.001).

INTERPRETATION

Clonidine and fentanyl provide adequate analgesia with low rates of muscle spasm, resulting in low diazepam use. The choice of epidural additive should be based upon the most tolerable side-effect profile.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.

[Dexmedetomidine and clonidine: a review of their pharmacodynamy to define their role for sedation in intensive care patients]

Alpha-2 adrenergic agonists ("alpha-2 agonists") present multiple pharmacodynamic effects: rousable sedation, decreased incidence of delirium in the setting of critical care, preservation of respiratory drive, decreased whole body oxygen consumption, decreased systemic and pulmonary arterial impedance, improved left ventricular systolic and diastolic function, preserved vascular reactivity to exogenous catecholamines, preserved vasomotor baroreflex with lowered set point, preserved kidney function, decreased protein catabolism. These pharmacodynamic effects explain the interest for these drugs in the critical care setting. However, their exact role for sedation in critically ill-patients remains open for further studies. Given the few double-blind randomized multicentric trials available, the present non exhaustive analysis of the literature aims at presenting the utilization of alpha-2 agonists as potential first-line sedative agents, in the critical care setting. Suggestions regarding the use of alpha-2 agonists as sedatives are detailed.

[Effects of intravenous clonidine on recovery and postanaesthetic analgesic requirements.]

Pain and pain-related sympathoadrenergic reactions (hypertension, tachycardia) accompanied by nausea, vomiting and shivering are the most common side effects of recovery from anaesthesia. The alpha(2)agonist clonidine acts as a sedative, anxiolytic, antihypertensive, antiemetic, antisialogogue and decreases the incidence of shivering. Thus, we studied the effects of intraoperatively administered clonidine on the recovery period and the postoperative analgesic requirements in patients undergoing maxillofacial surgery.

METHODS

After approval by the local Ethics Commitee and after informed consent had been given, 40 patients scheduled for elective maxillofacial surgery were included in this double-blind, randomized study. As a supplement to standardized general anaesthesia (isoflurane, N(2)O), the patients received either clonidine 5 mug/kg or placebo during the last hour of the operation. Blood pressure, heart rate, time of recovery, and sedation and pain scores were measured postoperatively. The occurrence of nausea, vomiting or shivering was noted, as were the requirements of piritramide for analgesia, which was administered on demand in titrating dosages, and of nifedipine for systolic blood pressure exceeding 180 mm Hg.

RESULTS

The two groups were comparable regarding biometric parameters, ASA-classification and duration of anaesthesia. Clonidinetreated patients were later in opening their eyes (22.5+/-11.9 min vs 17.9+/-10.9; n.s.) and the ability to state their dates of birth returned later (32.2+/-11.6 min vs. 25.7+/-12.8;P<0.05). Pain was more frequent in the placebo group (P<0.05 after 30 min), and there-fore, these patients required much more piritramid (P<0.01). The sedation scores showed no significant differences. No vomiting occurred in the clonidine group, and shivering was less frequent (P<0.01). The placebo group received more nifedipine (P<0.05) because the rate-pressure product was higher (P<0.01).

DISCUSSION

Opiates are frequently used as analgesics after maxillofacial surgery, even though their most common side effect-respiratory depression, nausea and vomiting-are particularly dangerous in these patients because of the obstruction of the upper respiratory tract. Self-titration of the opiate dosage on demand can decrease the incidence of serious side effects. Clonidine administered intraoperatively caused a profound reduction in analgesic requirements in this study. Additional opiate administration in the postoperative period was unnecessary in nearly all clonidine-treated patients. The attenuating effect on sympathoadrenergic reactions leads to lowering of the rate-pressure product and may be of advantage for patients suffering from arterial hypertension, angina pectoris or bronchial asthma. The slower emergence from anaesthesia following clonidine administration is probably caused by double-blind study properties preventing full consideration of the decreased isoflurane requirements after clonidine.

Effects of clonidine on breathing during sleep and susceptibility to central apnoea

We hypothesized that administration of clonidine would decrease the hypocapnic apnoeic threshold (HAT) and widen the CO(2) reserve during non-REM sleep.

METHODS

Ten healthy subjects (4 females) (age 22.3 ± 3.0 years; BMI 25.5 ± 3.4 kg/m(2)) were randomized to receive placebo or 0.1 mg/45 kg of clonidine on 2 separate nights. Ventilation and upper airway resistance were monitored during wakefulness and sleep. Two separate experiments were performed: Protocol 1 (n=8), CO(2) reserve, HAT and HcVR were determined using non-invasive hyperventilation (NIV) to induce hypocapnia for at least 3 min; Protocol 2 (n=6), peripheral hypocapnic ventilatory response (HcVR) was determined by NIV using short (3 breaths) hyperventilation.

RESULTS

Clonidine decreased the systolic blood pressure by 12 ± 10 mmHg but did not affect baseline ventilation or upper airway resistance during wakefulness or sleep. Protocol (1), clonidine was associated with decreased HAT relative to placebo (37.3 ± 3.3 mmHg vs. 39.7 ± 3.4 mmHg, P<0.05), increased CO(2) reserve (-3.8 ± 1.3 mmHg vs. -2.8 ± 1.2 mmHg, P<0.05), and decreased HcVR (1.6 ± 0.6 L/min/mmHg vs. 2.5 ± 1.3 L/min/mmHg, P<0.05). Protocol (2), administration of clonidine did not decrease peripheral HcVR compared to placebo (0.5 ± 0.3 L/min/mmHg vs. 0.7 ± 0.3 L/min/mmHg, P=NS).

CONCLUSION

Clonidine is associated with diminished susceptibility to hypocapnic central apnoea without significant effect on ventilation or upper airway mechanics. Reduced susceptibility to hypocapnic central apnoea is not explained by the peripheral chemoreceptor pathway. This suggests a central rather than a peripheral effect of clonidine on the susceptibility to hypocapnic central apnoea.

Dexmedetomidine and clonidine induce long-lasting activation of the respiratory rhythm generator of neonatal mice: possible implication for critical care

Dexmedetomidine and clonidine are alpha-2 adrenoceptor agonists increasingly used in the critical care unit as sedative agents for their benzodiazepine-sparing effects and their limited depressing effect on breathing. However adverse effects on breathing have been also reported with alpha-2 adrenoceptor agonists and their central effects on the respiratory rhythm generator are poorly known. We therefore examined the effects of dexmedetomidine, clonidine, the alpha-2 adrenoceptor antagonist yohimbine and the benzodiazepine midazolam on the activity of the isolated respiratory rhythm generator of neonatal mice using medullary preparations where the respiratory rhythm generator continued to function in vitro. For the first time, we showed that 5min bath applications of dexmedetomidine or clonidine activated the respiratory rhythm generator for periods over than 30min. Second, we showed that the long-lasting effect of dexmedetomidine implicated receptors other than alpha-2 adrenoceptors as it persisted after their blockade with yohimbine. Third, we reported that 5min bath applications of the benzodiazepine midazolam significantly depressed the respiratory rhythm generator, and that this depression was prevented by pre-treatment with either dexmedetomidine or clonidine. Although further experiments are still required to identify the mechanisms through which dexmedetomidine and clonidine activate the respiratory rhythm generator, our current in vitro results in neonatal mice support the use of dexmedetomidine and clonidine in the critical care unit.

[Perioperative management of patients with obstructive sleep apnoea]

There is lack of studies investigating procedures aiming at a decrease in perioperative mortality in patients with obstructive sleep apnoea (OSA). During anesthetic evaluation, identification of patients with OSA as well as using a risk score has been recommended by the American Society of Anesthesiology in order to identify the best perioperative strategy. Perioperative attention should be focused on a secure airway and the duration of monitoring that is necessary regarding severity of OSA, surgical stress and respiratory function. Postoperatively, residual neuromuscular blockade and a supine position have to be avoided. Continuous pulse oximetry should be used as long as patients remain at increased risk and should be applied until oxygen saturation remains above 90% with room air during sleep. Opioids should be excluded for pain management whenever possible, and CPAP or NIPPV should be administered as soon as feasible after surgery to patients who have been receiving it preoperatively.

Respiratory, metabolic and hemodynamic effects of clonidine in ventilated patients presenting with withdrawal syndrome

OBJECTIVE

To investigate the respiratory, metabolic and hemodynamic effects of clonidine in ventilated patients presenting with withdrawal syndrome after sedation interruption.

DESIGN

Prospective, interventional, single-center study in 30 ventilated ICU patients.

INTERVENTIONS

Metabolic [oxygen consumption (VO(2)), CO(2) production (VCO(2)), resting energy expenditure (REE)], respiratory [minute ventilation (V (E)), tidal volume (V (T)), respiratory rate (RR)] and hemodynamic (HR, SAP, MAP) parameters were measured in 30 ventilated ICU patients. Measurements were performed first under sedation with remifentanil-propofol, then after sedation interruption, and finally after clonidine administration (0.9-1.8 mg of clonidine in two doses of 10 min interval).

RESULTS

Sedation interruption produced significant increases in the hemodynamic parameters (SAP and MAP by 33%, HR by 37%), and metabolic rate (increase in VO(2) by 70%, VCO(2) by 88% and REE by 74%), leading to high respiratory demands (increase in V (E) from 9 to 15 l/min). The V (E) was increased due to a twofold increase in the RR; V (T) remained constant. In 25 out of 30 patients, clonidine administration decreased the hemodynamic (SAP, MAP and HR), metabolic (VO(2), VCO(2), REE) and respiratory parameters to values close to those observed with sedation. Clonidine induced mild sedation and patients became more cooperative with the ventilator. All patients responding to clonidine were weaned from the ventilator in 2 days (median, range 1-18 days).

CONCLUSION

Patients with withdrawal syndrome had significantly elevated hemodynamic, metabolic and respiratory demands. Clonidine significantly decreased these demands, induced mild sedation and facilitated patient cooperation with the ventilator, enabling ventilator weaning.

Clonidine-evoked respiratory effects in anaesthetized rats

The respiratory effects of stimulation of alpha2-adrenergic receptors were studied in spontaneously breathing anaesthetized rats that were neurally intact, or bilaterally vagotomized, or subjected to bilateral combined midcervical vagotomy and section of the carotid sinus nerves. An intravenous clonidine bolus (15 microg kg(-1)) evoked a prolonged slowing of the respiratory rate in all the neural states explored. Vagotomy reduced the early clonidine-evoked decline, but not the augmentation of tidal volume that followed the decline. After section of the carotid sinus nerves, clonidine challenge continued to decrease the respiratory rate, but not the tidal volume. Blockade of alpha2-adrenergic receptors with intravenous doses of SKF 86466 (200 microg kg(-1)) abolished all respiratory effects of the clonidine challenge. In all the neural states studied, clonidine evoked a significant short-lived rise in mean arterial blood pressure followed by a decrease below the respective prechallenge value. The SKF 86466 pretreatment lowered mean arterial blood pressure control values and reduced the magnitude of postclonidine changes. These results indicate that: (i) clonidine-evoked activation of alpha2-adrenergic receptors affects the two components of the breathing pattern differently, and this occurs beyond the lung vagi; and (ii) changes in tidal volume result from excitation of the carotid bodies and are coupled with centrally mediated slowing of the respiratory rhythm.

The role of α2-agonists in neurosurgery

Alpha2-agonists have been extensively used and studied in anaesthesia and intensive care medicine. A list of benefits includes anxiolysis, blood pressure stabilization, analgesia, anaesthetic sparing effects and sedation without respiratory depression or significant cognitive impairment. Fear of inadvertent hypotension, bradycardia or post-operative sedation, and the variability of the haemodynamic response to different doses or rates of administration, have meant that universal acceptance in clinical practice has not yet been seen. Recently, the actions of these agents on the alpha2-adrenoreceptor and the imidazoline receptor have been more accurately identified, helping to explain the differences between the actions of various agents. The two readily available agents, clonidine and dexmedetomidine have already been used in many different anaesthetic situations, for a wide variety of surgical procedures. We believe that both of these alpha2-agonists are useful adjuncts for the management of the neurosurgical patient during surgery, and in the intensive care unit.

Oral clonidine does not change ventilatory response to carbon dioxide in sevoflurane-anesthetized children

BACKGROUND

Clonidine is a useful premedicant for pediatric anesthesia. The drug has potential for ventilatory depression. The aim of the current study was to determine the effects of clonidine premedication on the ventilatory response to hypercapnia during sevoflurane anesthesia using the carbon dioxide (CO(2)) steady state method.

METHODS

Sixty children (3-13 yr) were assigned to receive clonidine 4 microg x kg(-1) or placebo. Anesthesia was maintained with spontaneous breathing and 2% sevoflurane. Minute ventilation (VE), respiratory rate (RR), endtidal CO(2) pressure (P(ECO(2)), and arterial hemoglobin oxygen saturation (SpO2) were measured with a facemask tightly fitted before and during 7% CO(2) inhalation.

RESULTS

Compared with placebo, oral clonidine failed to reduce VE volume before CO(2) loading under general anesthesia with 2% sevoflurane. Inhalation of CO(2) increased VE. Oral clonidine did not attenuate the increase in VE induced by hypercapnic challenge under sevoflurane anesthesia. There were no differences in RR, P(ECO(2), or SpO2 between the placebo and clonidine groups before and during CO(2) loading.

CONCLUSION

These data suggest that oral clonidine is a suitable premedication for sevoflurane anesthesia under spontaneous breathing conditions in children.

Novel concepts for analgesia in pediatric surgical patients. Cyclo-oxygenase-2 inhibitors, alpha 2-agonists, and opioids

Pain has been an understated concern in infants and children. Failure to recognize pain in the past has resulted in undue suffering by infants and children of all ages, but with the introduction of instruments to measure pain and a widespread appreciation of the severity of this problem in this age group, pain is rapidly coming under control. Novel concepts in both old and new analgesics have created safe and effective pain management strategies for infants and children. This review examines three analgesics and their potential roles in infants and children: cyclo-oxygenase (COX)-2 inhibitors, alpha 2-agonists and opioids.

Ventilatory effects of alpha2-adrenoceptor blockade in awake goats

We sought to determine the extent to which alpha2-adrenoceptor (alpha2-AR) pathways exert a tonic influence on respiratory rhythm under normal physiological conditions. The ventilatory effects of alpha2-AR blockade with SKF-86466 were examined in awake adult goats. The antagonist effect of SKF-86466 at alpha(2)-ARs was evident both as a reversal of the ventilatory disturbances produced by the alpha2-AR agonist clonidine (5 microg/kg IV) and as a rightward shift in the potency of the agonist-evoked response after SKF-86466 pretreatment. Systemic administration of SKF-86466 in cumulative doses (25-250 microg/kg) or as a single bolus injection (500 microg/kg) produced dose-dependent increases in breathing in all animals. The excitatory effect was primarily mediated by increases in respiratory frequency that persisted for at least 120 min after injection (500 microg/kg). The time course and magnitude of the ventilatory response to SKF-86466 was similar in carotid body denervated animals. The ventilatory response to isocapnic hypoxia but not normoxic hypercapnia was significantly elevated after SKF-86466 administration (500 microg/kg). SKF-86466 (25-1000 microg/kg) produced dose-dependent increases in heart rate in all animals but did not significantly change mean arterial blood pressure at any dose. The results demonstrate that alpha2-AR pathways exert a tonic inhibitory influence on respiratory rhythm in the awake goat.

Alpha2A-adrenoceptor mediated tachypnea in awake goats

To further elucidate the role of alpha2-adrenoceptors (alpha2-ARs) in the control of respiratory rhythm we examined the ventilatory effects of guanfacine (a preferentially selective alpha2A-AR agonist) and clonidine (a non-selective alpha2-AR agonist) in awake adult goats. Systemic administration of guanfacine in cumulative doses (20 microg/kg; 140-180 microg/kg total cumulative dose) increased breathing in all animals in a dose-dependent manner. The excitatory effect was entirely mediated by increases in respiratory frequency. The magnitude of the guanfacine-induced tachypnea was similar to that produced by systemic administration of cumulative doses of clonidine (1-2 microg/kg; 4-10 microg/kg total cumulative dose) in the same animals studied on a separate day. Both guanfacine- and clonidine-induced tachypnea was reversed by the preferentially selective alpha2A-AR antagonist RX821002 (2-6 microg/kg IV). Unlike clonidine however, guanfacine administration did not produce slow arrhythmic breathing episodes (irregular TE intervals and central apneas) that are characteristic of alpha2-AR stimulation with alpha2-AR agonists in the awake goat. The results suggest that alpha2-AR agonist-induced ventilatory excitation (tachypnea) requires the activation of alpha2A-ARs whereas clonidine-induced ventilatory depression (arrhythmic breathing) requires the activation of an alternate alpha2-AR subtype (presumably alpha2C-ARs). The results further demonstrate that alpha2-AR pathways exert an important influence on respiratory rhythm in the awake goat.

Sedative, analgesic and cognitive effects of clonidine infusions in humans

This placebo-controlled, randomized study evaluated, on separate days, the dose-response relationship for 1 h infusions of clonidine 1, 2 and 4 microg kg(-1) h(-1), in eight healthy volunteers aged 22-30 yr. Response end-points included sedation (bispectral index, visual analogue scale and observer assessment of sedation), analgesia to a cold pressor test, memory (recall of word lists), cognitive function (digit symbol substitution test (DSST)), respiratory function (respiratory rate, end-tidal carbon dioxide, oxygen saturation) and haemodynamic stability (heart rate and mean arterial pressure). Clonidine infusions resulted in significant and progressive sedation, but all subjects were easily awoken to perform tests and evaluations. Statistically significant analgesia, memory impairment and reduced performance on the DSST occurred during 4 microg kg(-1) h(-1) infusions (resulting in a plasma concentration of 2 ng ml(-1). There were no statistically significant changes in cardiorespiratory variables throughout the study.

Sedative, amnestic, and analgesic properties of small-dose dexmedetomidine infusions

This research determined the safety and efficacy of two small-dose infusions of dexmedetomidine by evaluating sedation, analgesia, cognition, and cardiorespiratory function. Seven healthy young volunteers provided informed consent and participated on three occasions with random assignment to drug or placebo. Heart rate, blood pressure, respiratory rate, ETCO(2), O(2) saturation, and processed electroencephalogram (bispectral analysis) were monitored. Baseline hemodynamic measurements were acquired, and psychometric tests were performed (visual analog scale for sedation; observer's assessment of alertness/sedation scale; digit symbol substitution test; and memory). The pain from a 1-min cold pressor test was quantified with a visual analog scale. After a 10-min initial dose of saline or 6 microg. kg(-1). h(-1) dexmedetomidine, volunteers received 50-min IV infusions of saline, or 0.2 or 0.6 microg. kg(-1). h(-1) dexmedetomidine. Measurements were repeated at the end of infusion and during recovery. The two dexmedetomidine infusions resulted in similar and significant sedation (30%-60%), impairment of memory (approximately 50%), and psychomotor performance (28%-41%). Hemodynamics, oxygen saturation, ETCO(2), and respiratory rate were well preserved throughout the infusion and recovery periods. Pain to the cold pressor test was reduced by 30% during dexmedetomidine infusion. Small-dose dexmedetomidine provided sedation, analgesia, and memory and cognitive impairment. These properties might prove useful in a postoperative or intensive care unit setting. IMPLICATIPNS: The alpha(2) agonist, dexmedetomidine, has sedation and analgesic properties. This study quantified these effects, as well as cardiorespiratory, memory and psychomotor effects, in healthy volunteers. Dexmedetomidine infusions resulted in reversible sedation, mild analgesia, and memory impairment without cardiorespiratory compromise.

Respiratory effects of dexmedetomidine in the surgical patient requiring intensive care

STATEMENT OF FINDINGS: The respiratory effects of dexmedetomidine were retrospectively examined in 33 postsurgical patients involved in a randomised, placebo-controlled trial after extubation in the intensive care unit (ICU). Morphine requirements were reduced by over 50% in patients receiving dexmedetomidine. There were no differences in respiratory rates, oxygen saturations, arterial pH and arterial partial carbon dioxide tension (PaCO2) between the groups. Interestingly the arterial partial oxygen tension (PaO2) : fractional inspired oxygen (FIO2) ratios were statistically significantly higher in the dexmedetomidine group. Dexmedetomidine provides important postsurgical analgesia and appears to have no clinically important adverse effects on respiration in the surgical patient who requires intensive care.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

[Benefit-risk and monitoring modalities of different techniques and methods of postoperative analgesia]

This review aimed to determine the benefits-risks ratio of postoperative analgesia. The various agents usually used for intravenous postoperative analgesia (paracetamol, NSAID's, opioids), and the techniques for postoperative analgesia (PCA, epidural, perinervous block) are analysed. The rules proposed for the monitoring of postoperative analgesia are considered.

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.