Volatile anesthetic sensitivity of T-type calcium currents in various cell types

[Intraoperative Neuromonitoring: Electroencephalography]

Intraoperative neuromonitoring using electroencephalography (EEG) enables anaesthesiologists to monitor the depth of anaesthesia. It is intended to reduce the occurrence of intraoperative wakefulness, postoperative delirium and postoperative cognitive deficits and to shorten process times in the operating room. This article shows how to interpret the raw EEG, spectrograms and processed indices for different age groups and anaesthetics and summarizes the resulting clinical benefits. While propofol and volatile anesthetics produce characteristic frontal EEG signatures with a high activity of coherent α- and δ-waves, ketamine triggers an increase in rapid γ-waves, which leads to incorrectly high indices (BIS, PSI, NI) despite deep anaesthetic levels.In children, frontal α-waves do not appear until the age of approx. 6 months and valid indices (BIS, PSI, NI) can only be derived starting at an age of approx. 12 months. Furthermore, children of preschool and elementary school age often show epileptiform discharges in the EEG during induction of anaesthesia, what is linked to emergence delirium. In adults, the intraoperative frontal α-power decreases significantly with increasing age and older patients tend to have an increased occurrence of burst suppression patterns during anaesthesia. Clinical benefits of EEG-based neuromonitoring comprise reduced doses of anaesthesia, shorter wake-up times after surgery and a lower incidence of intraoperative awareness during total intravenous anaesthesia. Moreover, anaesthesia guided by processed EEG indices can reduce the incidence of postoperative delirium and postoperative cognitive deficits in older patients. In-depth knowledge about intraoperative EEG changes that go beyond the interpretation of processed indices could lead to a further reduction in intra- and postoperative complications in the future.

Topical Sevoflurane: A Novel Treatment for Chronic Pain Caused by Venous Stasis Ulcers

In the US, an estimated 1 - 2% of chronic venous insufficiency (CVI) patients (of 6 - 7 million nationwide) develop at least one venous stasis ulcer (VSU) during their illness. Of these, approximately 40% develop subsequent ulcers, making VSU prognostically poor. Current management of VSU is costly, with poor prognosis, high recurrence rate, inadequate pain management, and significantly reduced quality of life (QoL). Topical volatile anesthetic agents, such as sevoflurane, offer improved pain relief and symptom control in patients suffering from chronic VSU. The immediate impact of topical sevoflurane in reducing pain associated with ulcer bed debridement has several implications in improving the quality of life in patients with CVI induced ulcers and in the prognosis and healing of the ulcers. This review summarizes a topical formulation of a volatile anesthetic and its implications for the management of VSUs.

The Effects of General Anesthetics on Synaptic Transmission

General anesthetics are a class of drugs that target the central nervous system and are widely used for various medical procedures. General anesthetics produce many behavioral changes required for clinical intervention, including amnesia, hypnosis, analgesia, and immobility; while they may also induce side effects like respiration and cardiovascular depressions. Understanding the mechanism of general anesthesia is essential for the development of selective general anesthetics which can preserve wanted pharmacological actions and exclude the side effects and underlying neural toxicities. However, the exact mechanism of how general anesthetics work is still elusive. Various molecular targets have been identified as specific targets for general anesthetics. Among these molecular targets, ion channels are the most principal category, including ligand-gated ionotropic receptors like γ-aminobutyric acid, glutamate and acetylcholine receptors, voltage-gated ion channels like voltage-gated sodium channel, calcium channel and potassium channels, and some second massager coupled channels. For neural functions of the central nervous system, synaptic transmission is the main procedure for which information is transmitted between neurons through brain regions, and intact synaptic function is fundamentally important for almost all the nervous functions, including consciousness, memory, and cognition. Therefore, it is important to understand the effects of general anesthetics on synaptic transmission via modulations of specific ion channels and relevant molecular targets, which can lead to the development of safer general anesthetics with selective actions. The present review will summarize the effects of various general anesthetics on synaptic transmissions and plasticity.

Comparative Evaluation of Volatile Anaesthetic Agents for Attenuation of Venous Cannulation Pain: A Prospective, Randomized Controlled Study

INTRODUCTION

Topical application of volatile anaesthetic agents has been found to attenuate the response to a mechanical stimulus; however, this effect of volatile anaesthetic on perception of pain during venous cannulation is not known.

AIM

To compare the efficacy of topically administered volatile anaesthetic agents for attenuating venous cannulation pain.

MATERIALS AND METHODS

This prospective, randomized, placebo controlled and double blind study was conducted on 120 patients, aged 20-60years. They were of American Society of Anaesthesiologists (ASA) I or II physical status, of either sex, planned for elective surgeries. These patients were randomized into 4 groups, of 30 each. Equipotent doses of halothane (1ml), isoflurane (1.5ml), sevoflurane (2.7ml) and sterile water (2.5ml; Control) were topically administered on the volar surface of forearm wrapped with cotton and aluminium foil; venous cannulation was performed with 18G intravenous cannula after 30 min. These patients were assessed for the incidence and severity of pain upon venous cannulation {visual analog scale (VAS), 0-100mm; 0 = no pain and 100 = worst imaginable pain}. Data were analysed by one-way ANOVA, Chi-square test and Kruskal-Wallis test. The p<0.05 was considered as significant.

RESULTS

A significant reduction in the incidence of venous cannulation pain was observed in the halothane (79%) group as compared to control (100%; p<0.05), isoflurane (100%; p<0.05) and sevoflurane (100%; p<0.05) groups. The severity of venous cannulation pain as assessed by median (interquartile range, Q1-Q3). VAS scores was reduced in the halothane {10 (10-20); p<0.001}, isoflurane {20 (10-30); p<0.001} and sevoflurane {20 (20-30); p<0.001} groups as compared to the control group {40 (30-40)}; VAS score in the halothane group was significantly less as compared to isoflurane (p<0.05) and sevoflurane (p<0.05) groups.

CONCLUSION

Topical application of halothane is most effective in reducing incidence and severity of venous cannulation pain; however, topical application of isoflurane and sevoflurane decreases only the severity of venous cannulation pain.

Effects of sevoflurane and propofol on frontal electroencephalogram power and coherence

BACKGROUND

The neural mechanisms of anesthetic vapors have not been studied in depth. However, modeling and experimental studies on the intravenous anesthetic propofol indicate that potentiation of γ-aminobutyric acid receptors leads to a state of thalamocortical synchrony, observed as coherent frontal alpha oscillations, associated with unconsciousness. Sevoflurane, an ether derivative, also potentiates γ-aminobutyric acid receptors. However, in humans, sevoflurane-induced coherent frontal alpha oscillations have not been well detailed.

METHODS

To study the electroencephalogram dynamics induced by sevoflurane, the authors identified age- and sex-matched patients in which sevoflurane (n = 30) or propofol (n = 30) was used as the sole agent for maintenance of general anesthesia during routine surgery. The authors compared the electroencephalogram signatures of sevoflurane with that of propofol using time-varying spectral and coherence methods.

RESULTS

Sevoflurane general anesthesia is characterized by alpha oscillations with maximum power and coherence at approximately 10 Hz, (mean ± SD; peak power, 4.3 ± 3.5 dB; peak coherence, 0.73 ± 0.1). These alpha oscillations are similar to those observed during propofol general anesthesia, which also has maximum power and coherence at approximately 10 Hz (peak power, 2.1 ± 4.3 dB; peak coherence, 0.71 ± 0.1). However, sevoflurane also exhibited a distinct theta coherence signature (peak frequency, 4.9 ± 0.6 Hz; peak coherence, 0.58 ± 0.1). Slow oscillations were observed in both cases, with no significant difference in power or coherence.

CONCLUSIONS

The study results indicate that sevoflurane, like propofol, induces coherent frontal alpha oscillations and slow oscillations in humans to sustain the anesthesia-induced unconscious state. These results suggest a shared molecular and systems-level mechanism for the unconscious state induced by these drugs.

Potential network mechanisms mediating electroencephalographic beta rhythm changes during propofol-induced paradoxical excitation

Propofol, like most general anesthetic drugs, can induce both behavioral and electroencephalographic (EEG) manifestations of excitation, rather than sedation, at low doses. Neuronal excitation is unexpected in the presence of this GABA(A)-potentiating drug. We construct a series of network models to understand this paradox. Individual neurons have ion channel conductances with Hodgkin-Huxley-type formulations. Propofol increases the maximal conductance and time constant of decay of the synaptic GABA(A) current. Networks range in size from 2 to 230 neurons. Population output is measured as a function of pyramidal cell activity, with the electroencephalogram approximated by the sum of population AMPA activity between pyramidal cells. These model networks suggest propofol-induced paradoxical excitation may result from a membrane level interaction between the GABA(A) current and an intrinsic membrane slow potassium current (M-current). This membrane level interaction has consequences at the level of multicellular networks enabling a switch from baseline interneuron synchrony to propofol-induced interneuron antisynchrony. Large network models reproduce the clinical EEG changes characteristic of propofol-induced paradoxical excitation. The EEG changes coincide with the emergence of antisynchronous interneuron clusters in the model networks. Our findings suggest interneuron antisynchrony as a potential network mechanism underlying the generation of propofol-induced paradoxical excitation. As correlates of behavioral phenomenology, these networks may refine our understanding of the specific behavioral states associated with general anesthesia.

Ethanol influences on native T-type calcium current in thalamic sleep circuitry

Ethanol is known to disrupt normal sleep rhythms; however, the cellular basis for this influence is unknown. This study uses an in vitro slice preparation coupled with electrophysiological recordings to probe neuronal responses to acute ethanol exposure. Recordings were conducted in ferret and rat thalamic slices, since thalamic circuitry is an integral component of sleep/wake cycles and sleep spindles. A key mediator of spindle wave activity is the low-threshold calcium current (T-type current). The T-type current underlies burst responses in the lateral geniculate and thalamic reticular nuclei that are important in spindle propagation. Whole cell patch recordings in thalamic brain slices revealed that ethanol has a differential, dose-dependent effect on the native T-type current in thalamic relay cells. Low concentrations of ethanol (2.5, 5, and 10 mM) enhance T-type current (n = 35), whereas higher concentrations of ethanol (20 and 50 mM) decrease T-type current (n = 27). To address whether this dose-dependent effect was due to variation between cells, in a subset we verified the differential effect within the same cell (n = 7). In an effort to examine whether the biphasic effects on the current were due to the order of ethanol exposures, we varied the order of high and low ethanol concentrations within the same cell. The ability of ethanol to perturb calcium currents in thalamic relay cells may provide a mechanistic framework for the well documented disruptions in sleep/wake behavior in subjects with ethanol exposure.

Effects of halothane and propofol on excitatory and inhibitory synaptic transmission in rat cortical neurons

General anesthetics are thought to act on both excitatory and inhibitory neuronal pathways at both post- and presynaptic sites. However, the literature in these regards is somewhat controversial. The aim of the present study was to reassess the relative importance of the various anesthetic actions using a common preparation. Rat cortical neurons in primary culture were used to record spontaneous miniature postsynaptic currents by the whole-cell patch-clamp technique. Halothane at clinically relevant concentrations prolonged the decay phase of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of tetrodotoxin and at higher concentrations decreased the frequency of mIPSCs. The mIPSC amplitudes underwent little change. Spontaneous action potential-dependent IPSCs recorded in the absence of tetrodotoxin were similarly affected by halothane. Halothane also decreased the frequency of spontaneous miniature non-N-methyl-D-aspartate (NMDA) excitatory postsynaptic currents (mEPSCs) as well as spontaneous action potential-dependent NMDA EPSCs and non-NMDA EPSCs without affecting their decay phase. The halothane effect on mIPSC and mEPSC frequency was dependent on the external calcium concentration. In contrast to halothane, the only effect of propofol was the prolongation of the decay phase of mIPSCs and IPSCs. The prolongation of mIPSCs and IPSCs by halothane and propofol coupled with the ineffectiveness on mEPSCs and EPSCs suggests a selective postsynaptic modulation of GABA(A) receptors. The additional calcium-dependent inhibition of mIPSC and mEPSC frequency by halothane (but not propofol) suggests a more general mechanism by this anesthetic on presynaptic transmitter release.