Interaction between dexmedetomidine and α-adrenergic receptors: emphasis on vascular actions

Comparison of TWA and PEP as indices of α2- and ß-adrenergic activation

RATIONALE

Pre-ejection period (PEP) and T-wave amplitude (TWA) have been used to assess sympathetic nervous system (SNS) activity. Here we report two single-blinded, placebo-controlled intravenous (IV) drug application studies in which we pharmacologically modified SNS activity with epinephrine (study 1) as well as dexmedetomidine (alpha2-agonist) and yohimbine (alpha2-antagonist) (study 2). Restricted heart rate (HR) intervals were analyzed to avoid confounding effects of HR changes.

OBJECTIVE

Study 1 served to replicate previous findings and to validate our approach, whereas study 2 aimed to investigate how modulation of central SNS activity affects PEP and TWA.

METHODS

Forty healthy volunteers (58% females) participated in study 1 (between-subject design). Twelve healthy men participated in study 2 (within-subject design). TWA and PEP were derived from ECG and impedance cardiography, respectively.

RESULTS

Epinephrine shortened PEP and induced statistically significant biphasic TWA changes. However, although the two alpha2-drugs significantly affected PEP as expected, no effects on TWA could be detected.

CONCLUSION

PEP is better suited to reflect SNS activity changes than TWA.

Perioperative coronary artery spasms in patients undergoing catheter ablation of atrial fibrillation

PURPOSE

Catheter ablation (CA) is an established treatment for atrial fibrillation (AF). Although coronary artery spasms (CAS) during or after ablation procedures have been described as a rare complication in some case reports, the incidence and characteristics of this complication have not been fully elucidated. The present observational study aimed to clarify the CAS in a large number of patients experiencing AF ablation.

METHODS

A total of 2913 consecutive patients (male: 78%, mean 66 ± 10 years) who underwent catheter ablation of AF were enrolled.

RESULTS

Nine patients (0.31%, mean 66 ± 10 years, 7 males) had transient ST-T elevation (STE). Eight out of the 9 patients had STE in the inferior leads. STE occurred after the transseptal puncture in 7 patients, after the sheath was pulled out of the left atrium in 1, and 2 h after the ablation procedure in 1. Six patients had definite angiographic CAS without any sign of an air embolization on the emergent coronary angiography. In the3 other patients, the STE improved either directly after an infusion of nitroglycerin or spontaneously before the CAG. The patients with CAS had a higher frequency of a smoking habit (89% vs. 53%; P = .04), smaller left atrial diameter (36 ± 6 vs. 40 ± 7; P = .07), and lower CHADS2 score (0.6 ± 0.5 vs. 1.3 ± 1.1; P = .004) than those without.

CONCLUSIONS

Although the incidence was rare (0.31%), CAS should be kept in mind as a potentially life-threatening complication throughout an AF ablation procedure especially performed under conscious sedation.

Dexmedetomidine-induced hemodynamic instability in patients undergoing orthopedic upper limb surgery under brachial plexus block: a retrospective study

Background

Hemodynamic instability is a frequent adverse effect following administration of dexmedetomidine (DMED). In this study, we evaluated the incidence of DMED-induced hemodynamic instability and its predictive factors in clinical regional anesthesia practice.

Methods

One hundred sixteen patients who underwent orthopedic upper limb surgery under brachial plexus block with intravenous DMED administration were retrospectively identified. The primary outcome was the incidence of DMED-induced hemodynamic instability. The participants were allocated to a stable or unstable group by their hemodynamic instability status. Patients’ characteristics were compared between the groups. The relationship between the potential risk factors and development of DMED-induced hemodynamic instability was analyzed with a logistic regression model.

Results

DMED-induced hemodynamic instability was observed in 14.7% of patients (17/116). The unstable group had more women than the stable group (76.5% vs. 39.4%, P = 0.010). When patients were classified into four subgroup according to body mass index (underweight, normal weight, overweight, and obesity), there was significant difference in the composition of the subgroups in the two groups (P = 0.008). In univariate analysis, female sex, obesity, and pre-existing hypertension were significant predictors of DMED-induced hemodynamic instability. Multivariate analysis demonstrated that female sex (adjusted OR 3.86, CI 1.09; 13.59, P = 0.036) and obesity (adjusted OR 6.41, CI 1.22; 33.57, P = 0.028) were independent predictors of DMED-induced hemodynamic instability.

Conclusions

Female and obese patients are more likely to have hemodynamic instability following intravenous DMED administration in clinical regional anesthesia practice. This study suggests that DMED dose may be diminished to prevent hypotensive risk in these populations.

Trial registration

This article was retrospectively registered at WHO clinical trial registry platform (Trial number: KCT0005977).

The Current Role of Dexmedetomidine as Neuroprotective Agent: An Updated Review

Dexmedetomidine, selective α2-adrenergic agonist dexmedetomidine, has been widely used clinically for sedation and anesthesia. The role of dexmedetomidine has been an interesting topic of neonatological and anesthetic research since a series of advantages of dexmedetomidine, such as enhancing recovery from surgery, reducing opioid prescription, decreasing sympathetic tone, inhibiting inflammatory reactions, and protecting organs, were reported. Particularly, an increasing number of animal studies have demonstrated that dexmedetomidine ameliorates the neurological outcomes associated with various brain and spinal cord injuries. In addition, a growing number of clinical trials have reported the efficacy of dexmedetomidine for decreasing the rates of postoperative neurological dysfunction, such as delirium and stroke, which strongly highlights the possibility of dexmedetomidine functioning as a neuroprotective agent for future clinical use. Mechanism studies have linked dexmedetomidine’s neuroprotective properties with its modulation of neuroinflammation, apoptosis, oxidative stress, and synaptic plasticity via the α2-adrenergic receptor, dependently or independently. By reviewing recent advances and preclinical and clinical evidence on the neuroprotective effects of dexmedetomidine, we hope to provide a complete understanding of the above mechanism and provide insights into the potential efficacy of this agent in clinical use for patients.

Mechanisms Contributing to the Generation of Mayer Waves

Mayer waves may synchronize overlapping propriobulbar interneuronal microcircuits constituting the respiratory rhythm and pattern generator, sympathetic oscillators, and cardiac vagal preganglionic neurons. Initially described by Sir Sigmund Mayer in the year 1876 in the arterial pressure waveform of anesthetized rabbits, authors have since extensively observed these oscillations in recordings of hemodynamic variables, including arterial pressure waveform, peripheral resistance, and blood flow. Authors would later reveal the presence of these oscillations in sympathetic neural efferent discharge and brainstem and spinal zones corresponding with sympathetic oscillators. Mayer wave central tendency proves highly consistent within, though the specific frequency band varies extensively across, species. Striking resemblance of the Mayer wave central tendency to the species-specific baroreflex resonant frequency has led the majority of investigators to comfortably presume, and generate computational models premised upon, a baroreflex origin of these oscillations. Empirical interrogation of this conjecture has generated variable results and derivative interpretations. Sinoaortic denervation and effector sympathectomy variably reduces or abolishes spectral power contained within the Mayer wave frequency band. Refractorines of Mayer wave generation to barodeafferentation lends credence to the hypothesis these waves are chiefly generated by brainstem propriobulbar and spinal cord propriospinal interneuronal microcircuit oscillators and likely modulated by the baroreflex. The presence of these waves in unitary discharge of medullary lateral tegmental field and rostral ventrolateral medullary neurons (contemporaneously exhibiting fast sympathetic rhythms [2-6 and 10 Hz bands]) in spectral variability in vagotomized pentobarbital-anesthetized and unanesthetized midcollicular (i.e., intercollicular) decerebrate cats supports genesis of Mayer waves by supraspinal sympathetic microcircuit oscillators. Persistence of these waves following high cervical transection in vagotomized unanesthetized midcollicular decerebrate cats would seem to suggest spinal sympathetic microcircuit oscillators generate these waves. The widespread presence of Mayer waves in brainstem sympathetic-related and non-sympathetic-related cells would seem to betray a general tendency of neurons to oscillate at this frequency. We have thus presented an extensive and, hopefully cohesive, discourse evaluating, and evolving the interpretive consideration of, evidence seeking to illumine our understanding of origins of, and insight into mechanisms contributing to, the genesis of Mayer waves. We have predicated our arguments and conjectures in the substance and matter of empirical data, though we have occasionally waxed philosophical beyond these traditional confines in suggesting interpretations exceeding these limits. We believe our synthesis and interpretation of the relevant literature will fruitfully inspire future studies from the perspective of a more intimate appreciation and conceptualization of network mechanisms generating oscillatory variability in neuronal and neural outputs. Our evaluation of Mayer waves informs a novel set of disciplines we term quantum neurophysics extendable to describing subatomic reality. Beyond informing our appreciation of mechanisms generating sympathetic oscillations, Mayer waves may constitute an intrinsic property of neurons extant throughout the cerebrum, brainstem, and spinal cord or reflect an emergent property of interactions between arteriogenic and neuronal oscillations.

Dexmedetomidine as an Alternative Anesthetic Agent for Flap Surgery: An Intravital Evaluation in the Cremaster Muscle Flap

INTRODUCTION

Flap surgery is one of the most commonly used techniques of reconstructive surgery for effective repair of damaged tissue. Optimal anesthetic technique and anesthetic agent plays an important role in flap perfusion. This study aimed to evaluate the effects of dexmedetomidine infusion on the microcirculation in the cremaster muscle flap by direct in vivo monitoring.

MATERIALS AND METHODS

We randomly divided 9 Wistar albino rats into 3 groups. The rats in the control group underwent the surgical procedure (isolation of the cremaster muscle) alone; the rats in the experimental groups 1 and 2 received an infusion of dexmedetomidine (10 and 30 min) after the surgical procedure.

RESULTS

The means of vessel diameters, number of functional capillaries, and movements of leukocytes in all groups were evaluated using intravital microscopic examination. The diameters of the arterioles and venules of the cremaster muscle significantly increased in the dexmedetomidine groups. The number of functional capillaries was higher in the dexmedetomidine groups than in the control group. No difference was observed in the movements of leukocytes between the control and experimental groups. Dexmedetomidine significantly increased the diameters of the arterioles and venules of the cremaster flap and the number of functional capillaries.

CONCLUSION

On the basis of the effects of dexmedetomidine on microcirculation, we suggest that dexmedetomidine continue to be used as an anesthetic agent, and may be considered also for reconstructive procedures, particularly flap surgery.

Sedation During Surgery for Movement Disorders and Perioperative Neurologic Complications: An Observational Study Comparing Local Anesthesia, Remifentanil, and Dexmedetomidine

BACKGROUND

The anesthetic management of patients requiring surgery for movement disorders needs to balance microrecording quality and patient cooperation with safety and comfort. Anesthetics can alter microrecording, although the effect on outcome is debatable. They also provide a rested and cooperative patient and minimize complications such as intracranial hemorrhage by providing better hemodynamic control. Most teams use local anesthesia with monitored anesthesia care or conscious sedation with propofol. Recently, dexmedetomidine has emerged as an alternative that, at low doses, does not affect microrecording, and that does not impair respiratory drive.

METHODS

In the past 15 years, we have used in our institution local anesthesia, remifentanil, or dexmedetomidine sedation. We compared functional outcome and rate of complications in a group of 145 patients with similar characteristics.

RESULTS

We found 5 (3.4%) intracranial hemorrhages. Two (1.4%) were symptomatic. The remifentanil group had the highest risk of having systolic blood pressure >160 mm Hg during surgery (odds ratio [OR], 2.8; 95% confidence interval [CI], 0.9-9.9), whereas the dexmedetomidine group had the lowest (OR, 0.7; 95% CI, 0.2-1.8), compared with the local anesthesia group. Surgical time was shortest with dexmedetomidine (mean, 283 minutes) and longest with local anesthesia only (mean, 328 minutes). Functional outcome (Unified Parkinson's Disease Rating Scale, Part III motor component scale) was similar among groups. The dexmedetomidine group had a statistically significant lower risk of perioperative neurologic events compared with the local anesthesia group (OR, 0.09; 95% CI, 0.002-0.68).

CONCLUSIONS

Sedation can be used safely without affecting outcome, and dexmedetomidine provides better hemodynamic management. Clinical significance remains unclear and larger studies need to be undertaken.

Dexmedetomidine inhibits vasoconstriction via activation of endothelial nitric oxide synthase

Despite the complex vascular effects of dexmedetomidine (DEX), its actions on human pulmonary resistance arteries remain unknown. The present study tested the hypothesis that DEX inhibits vascular tension in human pulmonary arteries through the endothelial nitric oxide synthase (eNOS) mediated production of nitric oxide (NO). Pulmonary artery segments were obtained from 62 patients who underwent lung resection. The direct effects of DEX on human pulmonary artery tension and changes in vascular tension were determined by isometric force measurements recorded on a myograph. Arterial contractions caused by increasing concentrations of serotonin with DEX in the presence or absence of L-NAME (endothelial nitric oxide synthase inhibitor), yohimbine (α₂-adrenoceptor antagonist) and indomethacin (cyclooxygenase inhibitor) as antagonists were also measured. DEX had no effect on endothelium-intact pulmonary arteries, whereas at concentrations of 10^–8~10^–6 mol/L, it elicited contractions in endothelium-denuded pulmonary arteries. DEX (0.3, 1, or 3×10^–9 mmol/L) inhibited serotonin-induced contraction in arteries with intact endothelium in a dose-dependent manner. L-NAME and yohimbine abolished DEX-induced inhibition, whereas indomethacin had no effect. No inhibitory effect was observed in endothelium-denuded pulmonary arteries. DEX-induced inhibition of vasoconstriction in human pulmonary arteries is mediated by NO production induced by the activation of endothelial α₂-adrenoceptor and nitric oxide synthase.

The effects of dexmedetomidine administration on the pulmonary artery pressure and the transpulmonary pressure gradient after the bidirectional superior cavopulmonary shunt

The hemodynamic effects of dexmedetomidine (DEX) on pulmonary artery pressure (PAP) are not fully understood in patients with a single-ventricle physiology. The objective of this retrospective study was to characterize the effect of perioperative DEX infusion on PAP and the transpulmonary pressure gradient after a bidirectional superior cavopulmonary shunt (BCPS) operation. We retrieved physiologic data including the heart rate, incidence of cardiac pacing, systolic and diastolic arterial blood pressure (ABP), and superior vena cava (SVC) and inferior vena cava (IVC) pressures from the medical records of all patients <12 months of age who underwent a BCPS operation. Patients with an additional Norwood or Damus-Kaye-Stansel procedure, those with additional pulmonary blood flow, and those without both a SVC and an IVC catheter were excluded from the present study. Following the BCPS operation, the SVC pressure is equivalent to the PAP. Similarly, the IVC pressure is equivalent to the common atrial pressure (CAP). Accordingly, we can directly assess the transpulmonary pressure gradient, defined as the difference between the PAP and the CAP, using simultaneous SVC and IVC pressure measurements. Twenty-nine patients were included in the present study. We did not find any increase in the PAP, CAP, PAP/systolic ABP ratio, or the transpulmonary pressure gradient as of 6 h after admission to the intensive care unit when the patients were treated with DEX infusion at a median (interquartile ranges) dose of 0.6 mcg/kg/h (0.4, 0.64 mcg/kg/h). We concluded that the administration of DEX to children with a single-ventricle physiology is acceptable.

Optimal doses of sevoflurane and propofol in rabbits

BACKGROUND

Although sevoflurane and propofol are commonly used anesthetics in rabbits, optimal doses of remain unclear. We thus assessed the optimal hypnotic doses of sevoflurane and propofol, and evaluated the influence of dexmedetomidine on sevoflurane and propofol requirements.

METHODS

Twenty-eight Japanese white rabbits were randomly assigned to one of four groups (n=7 each). Rabbits were given either sevoflurane, propofol, sevoflurane+dexmedetomidine, or propofol+dexmedetomidine (injected 30 μg∙kg(-1)∙hr(-1) for 10 min followed by an infusion of 3.5 μg∙kg(-1)∙hr(-1)). Hypnotic level was evaluated with Bispectral Index (BIS), a well-validated electroenchalographic measure, with values between 40 and 60 representing optimal hypnosis. BIS measurements were made 10 minutes after the adjustment of target end-tidal sevoflurane concentration in the sevoflurane group and sevoflurane+dexmedetomidine group, and at 10 min after the change of infusion rate in the propofol group and propofol+dexmedetomidine group.

RESULTS

BIS values were linearly related to sevoflurane concentration and propofol infusion rate, with or without dexmedetomidine. Sevoflurane concentration at BIS=50 was 3.9±0.2% in the sevoflurane group and 2.6±0.3% in the sevoflurane+dexmedetomidine group. The propofol infusion rate to make BIS=50 was 102±5 mg∙kg(-1)∙hr(-1) in the propofol group, and 90±10 mg∙kg(-1)∙hr(-1) in the propofol+dexmedetomidine group.

CONCLUSIONS

The optimal end-tidal concentration of sevoflurane alone was thus 3.9%, and optimal infusion rate for propofol alone was 102 mg∙kg(-1)∙hr(-1). Dexmedetomidine reduced sevoflurane requirement by 33% and propofol requirement by 11%.

Safe Sedation and Hypnosis using Dexmedetomidine for Minimally Invasive Spine Surgery in a Prone Position

Dexmedetomidine, an imidazoline compound, is a highly selective α2-adrenoceptor agonist with sympatholytic, sedative, amnestic, and analgesic properties. In order to minimize the patients' pain and anxiety during minimally invasive spine surgery (MISS) when compared to conventional surgery under general anesthesia, an adequate conscious sedation (CS) or monitored anesthetic care (MAC) should be provided. Commonly used intravenous sedatives and hypnotics, such as midazolam and propofol, are not suitable for operations in a prone position due to undesired respiratory depression. Dexmedetomidine converges on an endogenous non-rapid eye movement (NREM) sleep-promoting pathway to exert its sedative effects. The great merit of dexmedetomidine for CS or MAC is the ability of the operator to recognize nerve damage during percutaneous endoscopic lumbar discectomy, a representative MISS. However, there are 2 shortcomings for dexmedetomidine in MISS: hypotension/bradycardia and delayed emergence. Its hypotension/bradycardiac effects can be prevented by ketamine intraoperatively. Using atipamezole (an α2-adrenoceptor antagonist) might allow doctors to control the rate of recovery from procedural sedation in the future. MAC, with other analgesics such as ketorolac and opioids, creates ideal conditions for MISS. In conclusion, dexmedetomidine provides a favorable surgical condition in patients receiving MISS in a prone position due to its unique properties of conscious sedation followed by unconscious hypnosis with analgesia. However, no respiratory depression occurs based on the dexmedetomidine-related endogenous sleep pathways involves the inhibition of the locus coeruleus in the pons, which facilitates VLPO firing in the anterior hypothalamus.

Sedation for critically ill or injured adults in the intensive care unit: a shifting paradigm

As most critically ill or injured patients will require some degree of sedation, the goal of this paper was to comprehensively review the literature associated with use of sedative agents in the intensive care unit (ICU). The first and selected latter portions of this article present a narrative overview of the shifting paradigm in ICU sedation practices, indications for uninterrupted or prolonged ICU sedation, and the pharmacology of sedative agents. In the second portion, we conducted a structured, although not entirely systematic, review of the available evidence associated with use of alternative sedative agents in critically ill or injured adults. Data sources for this review were derived by searching OVID MEDLINE and PubMed from their first available date until May 2012 for relevant randomized controlled trials (RCTs), systematic reviews and/or meta-analyses and economic evaluations. Advances in the technology of mechanical ventilation have permitted clinicians to limit the use of sedation among the critically ill through daily sedative interruptions or other means. These practices have been reported to result in improved mortality, a decreased length of ICU and hospital stay and a lower risk of drug-associated delirium. However, in some cases, prolonged or uninterrupted sedation may still be indicated, such as when patients develop intracranial hypertension following traumatic brain injury. The pharmacokinetics of sedative agents have clinical importance and may be altered by critical illness or injury, co-morbid conditions and/or drug-drug interactions. Although use of validated sedation scales to monitor depth of sedation is likely to reduce adverse events, they have no utility for patients receiving neuromuscular receptor blocking agents. Depth of sedation monitoring devices such as the Bispectral Index (BIS©) also have limitations. Among existing RCTs, no sedative agent has been reported to improve the risk of mortality among the critically ill or injured. Moreover, although propofol may be associated with a shorter time to tracheal extubation and recovery from sedation than midazolam, the risk of hypertriglyceridaemia and hypotension is higher with propofol. Despite dexmedetomidine being linked with a lower risk of drug-associated delirium than alternative sedative agents, this drug increases risk of bradycardia and hypotension. Among adults with severe traumatic brain injury, there are insufficient data to suggest that any single sedative agent decreases the risk of subsequent poor neurological outcomes or mortality. The lack of examination of confounders, including the type of healthcare system in which the investigation was conducted, is a major limitation of existing pharmacoeconomic analyses, which likely limits generalizability of their results.