Anesthetic Potency of Intravenous Infusion of 20% Emulsified Sevoflurane and Effect on the Blood-Gas Partition Coefficient in Dogs

BACKGROUND

Intravenous (IV) infusions of volatile anesthetics in lipid emulsion may increase blood lipid concentration, potentially altering the anesthetic agent's blood solubility and blood-gas partition coefficient (BGPC). We examined the influence of a low-lipid concentration 20% sevoflurane emulsion on BGPC, and the anesthetic potency of this emulsion using dogs.

METHODS

We compared BGPC and anesthetic characteristics in 6 dogs between the IV anesthesia of emulsion and the sevoflurane inhalation anesthesia in a randomized crossover substudy. Minimum alveolar concentrations (MACs) were determined by tail-clamp stimulation by using the up-and-down method. Blood sevoflurane concentration and partial pressure were measured by gas chromatography; end-tidal sevoflurane concentration was measured using a gas monitor. The primary outcome was BGPC at the end of IV anesthesia and inhalation anesthesia. Secondary outcomes were time to loss/recovery of palpebral reflex, finish intubation and awakening, MAC, blood concentration/partial pressure at MAC and awakening, correlation between blood partial pressure and gas monitor, and the safety of emulsions.

RESULTS

BGPC showed no difference between IV and inhaled anesthesia (0.859 [0.850-0.887] vs 0.813 [0.791-0.901]; P = .313). Induction and emergence from anesthesia were more rapid in IV anesthesia of emulsion than inhalation anesthesia. MAC of emulsion (1.33% [1.11-1.45]) was lower than that of inhalation (2.40% [2.33-2.48]; P = .031), although there was no significant difference in blood concentration. End-tidal sevoflurane concentration could be estimated using gas monitor during IV anesthesia of emulsion. No major complications were observed.

CONCLUSIONS

IV anesthesia with emulsion did not increase the BGCP significantly compared to inhalation anesthesia. It was suggested that the anesthetic potency of this emulsion may be equal to or more than that of inhalation.

Dexmedetomidine combined with etomidate or emulsified isoflurane for induction reduced cardiopulmonary response in dogs

To investigate the effects of etomidate, emulsified isoflurane, and their combination with dexmedetomidine on physiological parameters, electrocardiogram (ECG) results, and the quality of induction and recovery during isoflurane maintenance anaesthesia. 5 mixed-breed dogs received each of four treatments: etomidate (E group); emulsified isoflurane (EI group); both dexmedetomidine and etomidate (DE group); or both dexmedetomidine and emulsified isoflurane (DEI group). All drugs were IV injection administered for induction, followed by 1.5 MAC (minimal alveolar concentration) of isoflurane to maintain anaesthesia. Rectal temperature (RT), respiratory rate (RR), heart rate (HR), mean arterial pressure (MAP), and ECG were measured at baseline, 0, 5, 10, 20, 40, and 60 minutes after intubation. The quality of induction and recovery was evaluated for all dogs. All the anaesthetic procedures provided good conditions for induction of anaesthesia. The quality of induction and recovery in the E group was worse than other groups. The decrease of RR in the E and DE groups was stronger than that in the EI and DEI groups. The dogs in the E group had the most significant prolongation of the Q-T interval and changes in the S-T segment. Deviation and extension of the S-T segment were noted in the El group. The dogs in the DE and DEI groups had fewer changes in the ECG results than those in the E and EI groups. The addition of dexmedetomidine caused less effect on cardiopulmonary parameters and the ECG results than either etomidate or emulsified isoflurane alone. Thus, etomidate or emulsified isoflurane in combination with dexmedetomidine may be useful clinically for the induction of anaesthesia.

Stable perfluorocarbon emulsions for the delivery of halogenated ether anesthetics

BACKGROUND

Research into injectable volatile anesthetics has been ongoing for approximately 40 years, with limited success, in an attempt to address the deficiencies of inhalational anesthesia. The purpose of this work was to formulate and optimize volatile anesthetic carrier emulsions based on our prior work in perfluorocarbon emulsions.

METHODS

Perfluorocarbons were screened for their volatilty and emulsion stability. Optimal anesthetic emulsions were manufactured by high pressure homogenization of a select, clinically relevant perfluorocarbon, isoflurane and a surfactant-containing aqueous phase. Longitudinal particle size, polydispersity and isoflurane content analysis was performed. Observational studies of in vivo efficacy and safety were performed in 225-300 g Lewis Rats (n = 34) with blood chemistry and post study tissue pathology analysis.

RESULTS

Emulsion particle size and isolflurane content in select emulsions were stable at room temperature greater than 300 days. This stability was depedent on perfluorocarbon molecular weight and boiling point. in vivo, emulsions demonstrated a rapid onset and offset. Variability in onset metrics (loss of righting reflex, pain reflexes and time to recovery) was less than 40% amongst individual emulsion preparations (n = 9) utilized in induction trials. No adverse effects due to the intravenous administration of emulsions were observed in blood chemistry results or post-study pathological examination.

CONCLUSIONS

These formulations showed stability, safety and efficacy. In addition to induction and general anesthesia, these emulsions could have utility in global health or in military applications where equipment and resources are limited.

Long-Term Evaluation of Continuous Epidural Anesthesia in an Improved Canine Model

Background

Continuous epidural analgesia with catheterization is a useful technique, because it has a wide adaptation range and provides prolonged analgesia. Problems that may arise from long-term epidural analgesia are changes in the analgesic area, duration of analgesia and catheter-related problems. Few articles have evaluated gradual changes of long-term epidural analgesia. In the animal models used in those studies, the catheter was inserted by an invasive surgical procedure. In the present study, we evaluated changes in a canine model in which the catheter was inserted by a minimally invasive procedure.

Objectives

To evaluate long-term changes in the efficacy of epidural analgesia in an improved canine model in which the epidural catheter was inserted and fixed for 5 weeks using a minimally invasive procedure.

Materials and Methods

Six beagles underwent epidural catheterization under general anaesthesia. The catheter tip was located in the sixth lumbar region; the catheter peripheral end was passed subcutaneously through the neck. Physiological saline was continuously infused (1.0 mL/h) via the catheter throughout the study. The efficacy of epidural analgesia was assessed weekly with 2% lidocaine once a week in 5 weeks. Peripheral blood analysis including interleukin-6 (IL-6) level in the cerebrospinal fluid (CSF), histological evaluations and epidurography were performed to evaluate the mechanisms underlying the changes.

Results

No dog died and the catheters were kept in place. The efficacy of analgesia was well maintained until 4 weeks; at 5th week, the efficacy decreased by half. The spread of injected medium was not observed in the cranial direction at 5th week and the tip of catheter was capped with granulation tissue. Throughout the study period, white blood cell counts and C-reactive protein levels were slightly high for catheterization and the IL-6 level in CSF was below detectable limits.

Conclusions

This was the longest study period with continuous epidural analgesia administered in canines. The effective period of epidural anesthesia was 4 weeks in this study. We speculate that time-dependent decrease in the anaesthetic efficacy was attributed to formation of granulation tissue surrounding the catheter tip.

Emulsified isoflurane enhances thermal transient receptor potential vanilloid-1 channel activation-mediated sensory/nociceptive blockade by QX-314

BACKGROUND

QX-314 produces nociceptive blockade, facilitated by permeation through transient receptor potential vanilloid-1 (TRPV1) channels. TRPV1 channel can be activated by noxious heat and sensitized by volatile anesthetics. The authors hypothesized that emulsified isoflurane (EI) could enhance thermal TRPV1 channel activation-mediated sensory/nociceptive blockade by QX-314.

METHODS

Rats were perineurally injected with QX-314 (Sigma-Aldrich Co. Ltd. Shanghai, China) alone or QX-314 combined with EI, followed by heat exposure on the injection site. The tail-flick and tail-clamping tests were used to assess sensory and nociceptive blockade, respectively; a sciatic nerve block model was used to assess motor and sensory blockade. Effects of EI on thermal activation of TRPV1 channels were evaluated on rat dorsal root ganglia neurons by whole-cell patch-clamp recordings.

RESULTS

Heat exposure enhanced sensory/nociceptive blockade by QX-314 in rat tails, but not motor blockade in sciatic nerve block model. QX-314 alone or QX-314 + 42°C produced no nociceptive blockade. QX-314 + 48°C produced 100% nociceptive blockade with duration of 12.5 ± 2.0 h (mean ± SEM). By adding 2% EI, QX-314 + 42°C produced 80% nociceptive blockade with duration of 8.1 ± 1.9 h, which was similar to the effect of QX-314 + 46°C (7.7 ± 1.1 h; P = 0.781). The enhancement of heat on sensory/nociceptive blockade of QX-314 was prevented by TRPV1 channel antagonist. The temperature thresholds of TRPV1 channel activation on dorsal root ganglia neurons were significantly reduced by EI.

CONCLUSIONS

Thermal activation of TRPV1 channels enhanced long-lasting sensory/nociceptive blockade by QX-314 without affecting motor blockade. The addition of EI reduced temperature thresholds for inducing long-lasting sensory/nociceptive blockade due to QX-314.

Emulsified isoflurane increases convulsive thresholds of lidocaine and produces neural protection after convulsion in rats

BACKGROUND

Local anesthetic-induced convulsions remain a concern of anesthesiologists when performing regional anesthesia. Our previous study found that the lidocaine requirement for IV regional anesthesia was reduced with coadministration of emulsified isoflurane. We designed this study to examine whether emulsified isoflurane could increase the convulsive threshold of lidocaine and produce protection after a lidocaine-induced convulsion.

METHODS

In experiment 1, the median convulsive dose of lidocaine with or without the addition of emulsified isoflurane was determined using the up-and-down method. In experiment 2, emulsified isoflurane (0.032 mL/kg for isoflurane), midazolam (1.6 mg/kg), 30% Intralipid (solvent of emulsified isoflurane) or saline was infused to treat lidocaine-induced convulsions, respectively. Convulsive behavior was scored by the modified Racine scale. Cognitive function and the pathology of hippocampus cornu ammonis 3 pyramid neurons of rats were evaluated on days 1, 3, 5, and 7 after convulsions.

RESULTS

In experiment 1, the median convulsive dose of lidocaine alone producing convulsions was 18.7 ± 2.6 mg/kg, and it was increased to 22.7 ± 2.6 (P = 0.010) and 26.7 ± 2.6 mg/kg (P < 0.001) with coadministration of emulsified isoflurane at doses of 0.016 and 0.032 mL/kg isoflurane, respectively. In experiment 2, both emulsified isoflurane and midazolam significantly suppressed lidocaine-induced tonic-clonic seizures. Rats treated with emulsified isoflurane regained full consciousness (convulsive score = 0) significantly earlier than rats treated with midazolam (8.7 ± 2.4 vs 19.5 ± 3.9 minutes, P < 0.001). Cognitive impairment and hippocampus cornu ammonis 3 pyramid neuron abnormalities were found after convulsions and improved with the administration of both emulsified isoflurane and midazolam.

CONCLUSION

Emulsified isoflurane increased the convulsive threshold of lidocaine and preserved neurological function in rats experiencing lidocaine-induced convulsions.

A Phase I, Dose-escalation Trial Evaluating the Safety and Efficacy of Emulsified Isoflurane in Healthy Human Volunteers

Background

This first-in-human volunteer phase I clinical trial aimed to evaluate the safety, tolerability, and anesthesia efficacy of emulsified isoflurane (EI), an intravenously injectable formulation of isoflurane.

Methods

Seventy-eight healthy volunteers were recruited in this open-label, single-bolus, dose-escalation, phase I trial and were allocated into 16 cohorts. Each volunteer received a single bolus injection of EI. The dose started with 0.3 mg/kg (for isoflurane) and was planned to end with 64.6 mg/kg. Postdose vital signs, physical examination, laboratory tests, chest radiograph, 12-lead electrocardiogram, and development of any adverse event were closely monitored as safety measurements. Effectiveness in producing sedation/anesthesia was assessed by Modified Observer’s Assessment of Alertness/Sedation and Bispectral Index.

Results

The dose escalation ended as planned. The most common adverse events associated with EI were injection pain (77 of 78, 98.7%) and transient tachycardia (22 of 78, 25.6%). Only at high doses (≥38.3 mg/kg) did EI cause transient hypotension (5 of 78, 6.4%) or apnea (11 of 78, 14.1%), but all the affected volunteers recovered uneventfully. Fast onset of unconsciousness (typically 40 s after injection) was developed in all volunteers receiving doses of 22.6 mg/kg or greater. Waking-up time and depression in Modified Observer’s Assessment of Alertness/Sedation correlated well with EI dose. None of the postdose tests revealed any abnormal result.

Conclusions

EI is safe for intravenous injection in human volunteers in the dose range of 0.3 to 64.6 mg/kg. At doses of 22.6 mg/kg or higher, EI produced rapid onset of unconsciousness in all volunteers followed by fast, predictable, and complete recovery.

Inhibition of voltage-gated sodium channels by emulsified isoflurane may contribute to its subarachnoid anesthetic effect in beagle dogs

BACKGROUND

Volatile anesthetics, in addition to their general anesthesia action, have been proven to produce regional anesthetic effect in various animal models. The major aim of this study was to examine whether emulsified isoflurane (EI) could also produce subarachnoid anesthesia and to investigate its possible mechanism.

METHODS

Beagle dogs were randomly assigned into 5 groups (n = 6/group): intrathecally receiving 1% lidocaine 0.1 mL/kg, 30% intralipid 0.1 mL/kg (control), or 8% EI at doses of 0.05, 0.075, or 0.1 mL/kg, respectively. Consciousness state, motor function of limbs, and response to nociceptive stimulus were observed after drug administration. The effect of EI on voltage-gated Na channel was recorded from isolated spinal neurons of rats, using the whole-cell patch-clamp technique. Inhibition of peak sodium currents and effect of EI on Na channel gating were analyzed.

RESULTS

Emulsified isoflurane produced subarachnoid anesthesia in a dose-dependent manner, and at the dose of 0.1 mL/kg, the effect of 8% EI was similar to 1% lidocaine. Sodium channel currents were inhibited by EI at clinically relevant concentrations, with the IC50 (median inhibitory concentration) at 0.69 ± 0.08 mM. Voltage activation of Na channels was positive, shifted by isoflurane at the concentration of 0.77 mM, and V½ of activation (voltage for half-maximal activation) shifted from -12.4 ± 2.7 mV to -7.3 ± 2.3 mV (P < 0.01).

CONCLUSIONS

Emulsified isoflurane produced dose-dependent subarachnoid anesthesia, and this effect might be mediated by inhibition of EI on voltage-gated Na channels in the spinal cord.

A model for the preferential delivery of isoflurane to the spinal cord of the goat

To identify the blood supply of the caprine central nervous system, six anaesthetised goats were perfused with coloured suspension into the brachiocephalic artery, the aorta, the iliac artery and the femoral artery. The subsequent distribution indicated that the brain and the main segments of the spinal cord were supplied by the brachiocephalic artery and aorta, respectively. Ten similarly anaesthetised goats then received emulsified isoflurane randomly via either the proximal part of the descending aorta (arterial group) or an ear vein (venous group). In the arterial group, the isoflurane partial pressure (P(iso)) in femoral arterial blood was almost double the P(iso) in jugular venous blood. The model showed that preferential delivery of isoflurane to the goat spinal cord in situ was possible and could be used for further research into the mechanisms of anaesthetic action, particularly factors affecting immobility.

Reversible conduction block in isolated toad sciatic nerve by emulsified isoflurane

BACKGROUND

Studies have shown that the local use of volatile anesthetics can produce local anesthetic effects. We designed this study to evaluate the characteristics of nerve conduction block of emulsified isoflurane (EI) and compare its nerve blockade with 1%lidocaine, by measuring compound nerve action potential (CNAP) parameters in isolated toad sciatic nerve.

METHODS

One hundred isolated toad sciatic nerves were selected and randomly assigned to 10 groups of 10 each, administered 2% to 8% EI (v/v) (EI(8) group, etc.), 1% lidocaine, 30% Intralipid(R) (Huarui Pharmacy, Wuxi, Jiangsu, China), and Ringer solution (RS) for 10 minutes, respectively. All nerves were then washed and soaked with RS for 10 minutes and 30 minutes. The nerve conduction block effect was represented by CNAP parameters that were recorded by an extracellular recording technique per minute.

RESULTS

The results showed that the negative amplitudes of CNAP were decreased by EI and lidocaine (P < 0.05), and the conduction velocities of CNAP were also decreased at some time points (D7-W3) (P < 0.05). After RS washing, the 2 parameters recovered gradually. The changes in the 2 parameters induced by EI had slower onset rates and faster recoveries than those produced by lidocaine (7 minutes vs 1 minute and 9 minutes vs 30 minutes). The nerve blockade induced by EI was dose dependent (P < 0.05), and the half maximal inhibition concentration of EI was 5.46%.

CONCLUSIONS

EI produced completely reversible and dose-dependent nerve conduction inhibition, which had slower onset and faster recovery compared with those produced by lidocaine.

Update on pharmacology of hypnotic drugs

PURPOSE OF REVIEW

This review is intended to provide an update on pharmacology of hypnotic drugs and current state of published research for new or improved agents.

RECENT FINDINGS

Albeit no completely new drugs have been launched in the last few years, research on pharmacology of existing drugs is still ongoing, and new formulations of existing drugs are proposed (propofol, isoflurane). Xenon, an old but so far unavailable drug, has elicited new interests and this review will examine the recent publications on this fascinating agent.

SUMMARY

These results will improve our handling of existing drugs and open new perspectives on drug monitoring through measurement of propofol concentrations in expired air.

Is a new paradigm needed to explain how inhaled anesthetics produce immobility?

A paradox arises from present information concerning the mechanism(s) by which inhaled anesthetics produce immobility in the face of noxious stimulation. Several findings, such as additivity, suggest a common site at which inhaled anesthetics act to produce immobility. However, two decades of focused investigation have not identified a ligand- or voltage-gated channel that alone is sufficient to mediate immobility. Indeed, most putative targets provide minimal or no mediation. For example, opioid, 5-HT3, gamma-aminobutyric acid type A and glutamate receptors, and potassium and calcium channels appear to be irrelevant or play only minor roles. Furthermore, no combination of actions on ligand- or voltage-gated channels seems sufficient. A few plausible targets (e.g., sodium channels) merit further study, but there remains the possibility that immobilization results from a nonspecific mechanism.