M. K. Chakrabarti

Specific Enhancement by Fentanyl of the Effects of Intrathecal Bupivacaine on Nociceptive Afferent But Not on Sympathetic Efferent Pathways in Dogs

BackgroundBupivacaine alone, or in combination with opioids, has been shown to provide adequate pain relief without motor paralysis. This study examined the effects of bupivacaine administered intrathecally on sympathetic efferent and Aδ- and C-fiber-mediated afferent pathways in dogs and the interactions with intrathecal fentanyl. MethodsSpontaneous activity in renal sympathetic nerves was observed, as were reflex somatosympathetic responses mediated by Aδ and C fibers evoked by supramaximal electrical stimulation of the tibial and radial nerve. Bupivacaine was administered intrathecally in doses of 0.5, 1, 2, and 3.5 mg, each in 0.5 ml, and 7 mg in 1 ml with or without pretreatment with 5.4 mg intrathecal fentanyl (ED25 for depression of C tibial reflexes) in each of five preparations. ResultsBupivacaine caused a dose-dependent inhibition of both Aδ- and C-fiber-mediated somatosympathetic responses evoked by tibial nerve stimulation. The depression of radial and tibial nerve reflexes and spontaneous renal sympathetic activity was similar. Pretreatment with fentanyl (5.4 μg, intrathecally) depressed tibial C-fiber reflexes by only 23.8% without any significant effect on either tibial Aδ or radial Aδ and C fiber responses. Fentanyl markedly enhanced the effect of subsequent doses of bupivacaine on tibial Aδ and C reflexes without any additional effect on either spontaneous sympathetic activity or radial responses. ConclusionsIntrathecal bupivacaine has no selectivity for the afferent and efferent pathways, and intrathecal fentanyl acts synergistically to enhance the effect of bupivacaine on the afferent pathway without a measurable effect on sympathetic outflow.

Clonidine Has Comparable Effects on Spontaneous Sympathetic Activity and Afferent Aδ– and C-fiber-mediated Somatosympathetic Reflexes in Dogs

Background:Clonidine, an α2-adrenergic agonist, has been studied as an adjunct or alternative to spinal opioids in the management of moderate to severe pain. This study examined the relative effects of clonidine on efferent spontaneous sympathetic activity and afferent Aδ and C fiber-mediated somatosympathetic responses. Methods:Spontaneous and evoked sympathetic activity in renal sympathetic nerves, mediated by Aδ and C fibers by means of supramaximal electrical stimulation of the radial and tibial nerves, were observed in anesthetized dogs. Incremental doses of clonidine were administered intrathecally or intravenously in each of five preparations followed by intravenous naloxone 2 mg and yohimbine 5 mg. Results:Both spontaneous sympathetic outflow and afferent Aδ– and C fiber-mediated somatosympathetic responses evoked by tibial nerve stimulation were depressed in a similar dose dependent manner by clonidine administered intrathecally or intravenously in a dose ratio of approximately 1:4. Intrathecal clonidine inhibited and eliminated both local spontaneous sympathetic outflow and tibial nerve evoked sympathetic responses but had no significant depressant effect on the radial nerve evoked sympathetic reflexes. When administered intravenously clonidine had a similar depressant effect on both radial and tibial nerve elicited reflexes and spontaneous sympathetic activity. Conclusions:Clonidine, administered intrathecally or intravenously, has a similar depressant effect on both spontaneous sympathetic outflow and afferent Aδ– and C fiber–mediated somatosympathetic reflexes. When administered intrathecally it has little effect on reflexes evoked via the descending pathway by radial nerve stimulation.

Relative effects of intrathecal administration of fentanyl and midazolam on aδ and c fibre reflexes

The effects of fentanyl and midazolam, administered intrathecally, on somatosympathetic reflexes evoked by tibial nerve stimulation were investigated in 12 anaesthetized and paralysed dogs. Fentanyl depressed both the C and A delta fibre evoked reflexes in a dose ratio of approx 1:2. In contrast, midazolam had a greater effect on A delta compared with C fibre reflexes; while A delta reflexes were abolished by a total dose of 3 mg midazolam, C fibre reflexes were depressed by only 50%. The effect of fentanyl was reversed by naloxone (2 mg, i.v.) and that of midazolam by flumazenil (1 mg, i.v.). The results suggest that fentanyl and midazolam have different relative effects on A delta and C fibre pathways.

An assessment of the Humphrey ADE anaesthetic system in the Mapleson A mode during spontaneous ventilation

The Humphrey ADE anaesthetic breathing system in the Mapleson A mode has been compared with the Magill system in spontaneously breathing conscious volunteers and anaesthetised patients. In the latter, rebreathing occurred at a significantly lower fresh gas flow with the ADE system than when the Magill system was used (mean 45.6 ml/kg/minute und 56.5 ml/kg/minute respectively). There was no significant difference between the fresh gas flow at which rebreathing orcurred in conscious volunteers.

The uptake of isoflurane during anaesthesia.

The uptake of isoflurane at a constant end‐expired concentration of 1.5% in oxygen was studied in 15 women, ASA 1 or 2, undergoing elective total abdominal hysterectomy. The anaesthetic was administered by a simple computer‐controlled to‐and‐fro closed system. After an initial period of wash‐in to the system, the rate of uptake of isoflurane decreased bi‐exponentially with a rapid reduction during the first 15 min. Perturbations from this bi‐exponential decline reflect changes in cardiac output. The mean (SD) cumulative use of isoflurane was 4.5 (0.43) ml after 30 min and 7.3 (0.79) ml after 60 min.

A computer-controlled closed anaesthetic breathing system.

We describe the design and working of a computer‐controlled, closed anaesthetic breathing system which rapidly achieves and maintains a prescribed end‐tidal concentration ofisoflurane in oxygen. The system is simple to set up and not expensive; the only nonstandard component is a modified glass syringe. We have demonstrated that gas analysers may contribute as much as the patient to the accumulation of nitrogen within the breathing system. Details of our clinical experience with the system are presented in an accompanying article.

specific Actions of Halothane, Isoflurane, and Desflurane on Sympathetic Activity and A [greek small letter delta] and C Somatosympathetic Reflexes Recorded in Renal Nerves in Dogs

BackgroundThis was a study of the relative effects on directly recorded sympathetic activity of desflurane, isoflurane, and halothane.MethodsRenal sympathetic nerve activity (RSNA) was recorded with bipolar electrodes in renal nerves exposed retroperitoneally in anesthetized ([Greek small letter alp

Uptake of desflurane during anaesthesia

The amount of desflurane required to maintain an end‐expired concentration of 8% was measured in 30 ASA 1 and 2 patients undergoing elective spinal surgery. The anaesthetic was administered using a computer‐controlled closed circle system. After an initial period during which the expired concentration of desflurane was stabilised (4 min) the rate of uptake showed a bi‐exponential decline. Mean cumulative usage of desflurane was 10.1 ml of liquid at 30min, 14.8ml at 60min, 25.4ml at 120min, 35.8 at 180min.

The effect of sevoflurane on spontaneous sympathetic activity, A delta and C somatosympathetic reflexes, and associated hemodynamic changes in dogs

This study examined the effect of sevoflurane on spontaneous renal sympathetic nerve activity (RSNA), A delta-and C-fiber-mediated somatosympathetic reflexes, and hemodynamic changes in anesthetized dogs. RSNA, and A delta and C reflexes evoked by electrical stimulation of the radial nerve were observed in multifiber recordings of efferent activity in renal sympathetic nerves. Sevoflurane was administered at 1%, 2%, 3%, and 4% end-tidal concentrations for periods of 20 min. The mean A delta reflexes decreased by 20%, 39%, and 54% (P < 0.05 to < 0.01), and the C reflexes decreased by 38%, 62%, and 74% (P < 0.05 to < 0.01) at concentrations of 2%, 3%, and 4%, respectively. The relatively greater effect on C reflexes was significant (P < 0.05) and comparable with the effect of [micro sign]-opioids. There was no change in mean RSNA, heart rate (HR), and cardiac output (CO) up to 3% sevoflurane, but these decreased by 36%, 24%, and 13% (P < 0.05), respectively, at 4% sevoflurane. Sevoflurane 1%-4% caused a virtually linear reduction in systemic vascular resistance (SVR) from 7% (P < 0.05) to 44% (P < 0.05), together with a reduction in mean arterial pressure (MAP) that was significant for concentrations greater than 2%. The results indicate that sevoflurane causes a greater depression of C compared with A delta reflexes, and that the reduction in MAP was entirely due to a decrease in SVR up to 3%, whereas at 4% sevoflurane, reductions in sympathetic activity, HR, and CO also contributed its depressor effect. Implications: The relatively greater depressant effect of sevoflurane on C compared with A delta nociceptive somatosympathetic reflexes is similar to [micro sign]-opioids. The hypotensive effect of sevoflurane was significant at 2% concentration, whereas heart rate, cardiac output and sympathetic activity were reduced only at concentrations greater than 3%. (Anesth Analg 1998;86:1079-83)

The Ohmeda Rascal II

The Ohmeda Rascal II is a multigas analyser and pulse oximeter for dedicated anaesthetic use. It uses the Raman scattering of laser light to identify and quantify oxygen, nitrogen, carbon dioxide, nitrous oxide and three volatile anaesthetic agents. Its response times equal or better the published response times of infrared or photo‐acoustic devices. It is linear within the clinical ranges of all gases and vapours, simple to use, requires no maintenance, holds its calibration well, and is a suitable monitor for clinical and research use.