Minimum effective local anaesthetic dose for spinal anaesthesia

Editor—As a comparison of the dose of local anaestheticrequired for spinal anaesthesia with either levobupivacaineor ropivacaine the study by Sell and colleagues¹ is both interestingand informative. However, I have a number of concerns aboutthe paper that has been based on that study. In the title, and subsequently, the authors describe plain solutionsof both drugs as being ‘isobaric’ yet the densityfigures they quote in the paper are well below the normal rangefor CSF. These solutions are not isobaric with cerebral spinalfluid and they spread in a manner that is different from thosethat are truly isobaric, the primary difference being much widervariability     

Minimum effective local anaesthetic dose for spinal anaesthesia

Editor—In their response¹ to my criticisms² of their paper³Sell and colleagues accept some of     

Density determination of local anaesthetic opioid mixtures for spinal anaesthesia

Purpose

To measure the density of hyperbaric and isobaric local anaesthetics before and after addition of neuroaxial opioids to define a method for calculating any local anaesthetic/opioid mixture density based on individual component densities.

Methods

Density was determined using a volumetric pycnometer (25.0281 ± 0.0013 ml). The density of local anaesthetics (bupivacaine, lidocaine), opioids (fentanyl, morphine) and multiple anaesthetic/opioid mixtures were measured in quadruplicate and expressed in g·ml^?1, at 37°C (mean ± SD). Regression analysis was used to derive a formula for calculating the density of any anaesthetic/opioid mixture.

Results

Individual components had the following densities (g·ml^?1): bupivacaine 0.75%; 1.0252 ± 0.0001, lidocaine 5%; 1.0249 ± 0.0001, bupivacaine 0.5%; 0.9994 ± 0.0001, lidocaine 2%; 1.0000 ± 0.0001, 50μg·ml^?1 fentanyl; 0.9936 ± 0.0001, and 0.5 mg·ml^?1 morphine; 1.0001 ± 0.0001. Using regression analysis, linear relationships were demonstrated between density (D) of anaesthetic/opioid mixture and the proportion of anaesthetic in the mixture (fractional volume of anaesthetic) (r = 0.9999,P < 0.001). The following formula was derived; Density_Mixture = (D_{Local anaesthetic} ? D_Opioid) × Fractional Volume Anaesthetic + D_Opioid Comparison of calculated and measured densities for multiple clinically relevant anaesthetic/opioid mixtures showed a significant degree of correlation (r = 0.9996,P < 0.001).

Conclusion

Density of spinal anaesthetic/opioid mixtures can be calculated from the component densities and the proportion of anaesthetic in the mixture.     

Influence of the baricity of a local anaesthetic agent on sedation with propofol during spinal anaesthesia

Background: This study examined the effect of different levels of spinalanaesthesia, induced by solutions of different baricity butcontaining the same amount of local anaesthetic agent, on therequirement for sedation with propofol. Methods: Thirty-six patients undergoing varicose vein surgery under spinalanaesthesia were randomly allocated to receive tetracaine 15mg in 3 ml of either glucose 5% (hyperbaric) or CSF (isobaric).I.V. propofol was started 5 min after the intrathecal injectionand was titrated to maintain a bispectral index (BIS) scoreof 65–75. The propofol requirements to maintain this rangein the two groups were compared every 5 min. Results: The propofol requirement was always lower in the hyperbaricgroup, with the differences becoming statistically significant20 min after the intrathecal injection. Total consumption ofpropofol over the 55 min of the study was also less in the hyperbaricgroup. Conclusion: The known difference in level of spinal anaesthetic block inducedby solutions of different baricity, but the same dose of localanaesthetic, was associated with different requirements forpropofol sedation as determined by BIS assessment.     

Total spinal anaesthesia as a complication of local anaesthetic test-dose administration through an epidural catheter

We describe a case of total spinal anaesthesia, which occurred after a 3-ml lignocaine (20 mg ml(-1)) test dose was administered through an epidural catheter in a 79-year-old patient scheduled for gastrectomy under combined general and epidural anaesthesia. The surgery was postponed, and the patient required admission to the intensive therapy unit. Spinal MRI from the total spinal cord did not reveal any pathology. During the next 24 h the patient recovered and after 11 days was successfully operated on under general anaesthesia. No late complications followed. We presume that during placement, the epidural catheter had migrated to the spinal canal as a result of technical difficulties. Although controversial, we consider that administering a standard test dose of local anaesthetic via an epidural catheter is recommended, especially in high-risk patients and when epidural space identification or catheter placement poses technical difficulties. A test dose of local anaesthetic does not fully prevent complications.     

Hyaluronidase reduces local anaesthetic volumes for sub-Tenon's anaesthesia

BACKGROUND: Volumes of local anaesthetics for sub-Tenon's anaesthesia vary. Lower volumes produce less akinesia, whereas higher volumes increase chemosis and intra-ocular pressures. Hyaluronidase is often added to local anaesthetics to improve akinesia without increasing the volume of the injection, but this is controversial. This randomized, sequential allocation study examines the addition of hyaluronidase on the minimum local anaesthetic volume (MLAV) required for a sub-Tenon's block. METHODS: Sixty-two patients having sub-Tenon's blocks for cataract surgery were randomized into two groups. The control group (n=31) received 2% w/v lidocaine and the study group (n=31) received 2% w/v lidocaine with hyaluronidase 15 IU ml(-1). Using parallel up-down sequential allocation from a 4 ml starting volume, the volumes in both groups were changed using a testing interval of 1 ml according to the quality of globe akinesia. The median effective local anaesthetic volume (MLAV) was calculated for both groups using probit regression. RESULTS: The groups were similar for age, sex, and ocular axial length. The MLAV in the hyaluronidase group was 2.6 ml [95% confidence interval (CI), 2.1-3.l] and 6.4 ml (95% CI, 5.1-8.1) in the control group (P<0.002). CONCLUSIONS: Hyaluronidase permits a significant 2.4-fold (95% CI, 1.8-3.4) reduction in MLAV for sub-Tenon's anaesthesia.     

Peribulbar anaesthesia using a mixture of local anaesthetic and vecuronium

The aim of this double-blind, randomised study was to assess the effects of the addition of 0.5 mg of vecuronium bromide to a standard local anaesthetic mixture used for peribulbar anaesthesia. We studied 60 patients undergoing regional anaesthesia for intra-ocular surgery and were primarily interested in the quality of globe and lid akinesia. All received a mixture of 5 ml 2% lignocaine with 1:200 000 adrenaline, 5 ml 0.75% bupivacaine and 150 IU hyaluronidase with either 0.9% saline 0.25 ml (group A, n  = 30) or vecuronium bromide 0.25 ml (0.5 mg) (group B, n  = 30). Eye movements assessed at both 5 and 10 min were significantly reduced in the vecuronium group (group B) (p < 0.05). We conclude that the addition of vecuronium at a dose of 0.5 mg to the standard local anaesthetic mixture improves the quality of globe and lid akinesia.     

Minimum effective anaesthetic concentration of hyperbaric lidocaine for spinal anaesthesia

Purpose

Minimum effective anaesthetic concentration (MEAC) of lidocaine for spinal anaesthesia, defined as the concentration at which a spinal anaesthetic agent produces surgical anaesthesia within 20 min of administration in 50% of patients, was determined in a randomised, double-blind study in young patients undergoing knee and ankle surgery.

Methods

Using the combined spinal-epidural technique, 48 or 72 mg hyperbaric lidocaine containing dextrose 7.5% was administered intrathecally to 43 patients at concentrations ranging from 0.2–0.9%. The choice of lidocaine concentration was determined by Dixon’s up-and-down method, Complete anaesthesia was defined as: ( I ) pinprick anaesthesia at or higher than T₁₂, (2) anaesthesia to transcutaneous tetanic electric stimulation (50 Hz at 60 mA for five seconds) in the knees and (3) complete leg paralysis; all occurring in both lower extremities within 20 min. Epidural anaesthesia was initiated if anaesthesia was incomplete.

Results

In the 48 mg group, MEAC was 0.54% (95% Cl-0.21–0.87). Anaesthetic effect was variable with mean duration of anaesthesia of 29 min (range: 20–50 min) and maximum pinprick sensory level ranging from T₂-T₁₀. In the 72 mg group, successful anaesthesia was achieved consistently at a concentration of 0,3%, i.e., MEAC was < 0.3%. Mean duration of complete anaesthesia was 46 min (range: 30–60 min) with maximum sensory level from T₃–T₈.

Discussion

Spinal anaesthesia can be accomplished with very dilute lidocaine solutions (< 0.9%). The value of MEAC is dose-dependent, i.e., complete anaesthesia can be accomplished with lower concentrations by increasing the dose of spinal anaesthetic administered.     

Effects of a height and weight adjusted dose of local anaesthetic for spinal anaesthesia for elective Caesarean section

In this prospective, randomised, double-blind study, we compared the effects of two dosage regimens. Pregnant patients at term were randomly assigned to two groups to be given diamorphine 0.4 mg in hyperbaric bupivacaine 0.5% 2.4 ml or diamorphine 0.4 mg in a volume of hyperbaric bupivacaine 0.5% adjusted according to the patient's height and weight. Adequate anaesthesia was provided in all patients in both groups. The onset of the sensory block for cold and pinprick was faster with the fixed dose regimen (p = 0.01). There were more spinal blocks to above the first thoracic dermatome in the fixed dose group (17.1% vs. 2.2%, p = 0.022). Hypotension occurred in 71.7% vs. 50.0% of patients in the fixed dose and adjusted dose groups respectively (p = 0.035). In the fixed dose group, more patients required ephedrine to treat hypotension (79.5% vs. 56.8%, p = 0.022) and a larger median dose was administered (9 mg vs. 6 mg, p = 0.042). The decrease in mean (SD) arterial pressure was less in the adjusted group (35.0 (16.4) mmHg vs. 28.0 (13.5) mmHg, p = 0.036).     

Peribulbar anaesthesia: a double-blind comparison of three local anaesthetic solutions

A prospective, randomised, double-blinded study comparing three agents for peribulbar anaesthesia is reported. Sixty patients undergoing extracapsular cataract extraction under local anaesthesia were randomly allocated to receive peribulbar anaesthesia with lignocaine 2% with adrenaline; prilocaine 3% with felypressin 0.03 IU.ml-1 or 2% lignocaine and 0.5% bupivacaine in a ratio of 1:1, using a standardised two-injection technique. The pain of injection, time of onset of the block and the operating conditions at the start and finish of surgery were assessed. Peribulbar anaesthesia using lignocaine 2% was significantly more painful than the other solutions. The onset of anaesthesia adequate for surgery was similar in all three groups. Prilocaine 3% with felypressin was associated with the greatest number of blocks providing total akinesia of the eye. Inadequate duration of anaesthesia was seen in only one case; the solution used for this block was 2% lignocaine.     

Low spinal anaesthesia combined with local anaesthesia for caesarean section--an evaluation

A combination of low spinal anaesthesia using hyperbaric cinchocaine (Nupercaine; Ciba) 0,25% and local anaesthetic abdominal wall infiltration for caesarean section was evaluated for use in peripheral hospitals in the developing world. The technique described is not suitable for this purpose. Further research into this technique is desirable, and recommendations regarding an improved protocol are made.     

Study of unilateral postherniorraphy analgesia with local anaesthetic and monitored anaesthesia care

Local anaesthetic with monitored anaesthetic care (MAC) is a very good technique for unilateral-inguinal herniorraphy. We looked at the analgesia produced by the infiltration with local anaesthetic, the intensity of pain in the immediate postsurgery period; the efficiency of oral analgesics and the satisfaction of the patients. Between January and July 1997, 63 patients underwent unilateral-inguinal herniorraphy (Shouldice type) using local anaesthetic (300 mg of mepivacain 1% and 50 mg of bupivacain 0.25%) and MAC (fentanyl, mydazolan and propofol). The intensity of pain was measured using two evaluation scales: visual analogue scale (EV) and verbal scale (Eve). When the patients asked for an analgesic they were given magnesic metamizol (Nolotil), every 6 h. Five patients (8%) felt no pain and 58 felt pain 4 h 36 min after local anaesthetic infiltration (EV=2.5; Eve=1.45) of these 58 patients, 49 took a first dose of ‘Nolotil’ 6 h 40 min after local anaesthetic induction (EV=4; Eve=1.97), 43 received a second dose of ‘Nolotil’ at 13 h 40 min (EV=3; Eve=1.49) and 22 a third dose at 17 h 40 min (EV=3.2; Eve=1.7). Every patient that was very satisfied with the anaesthetic technique, said that the postsurgery pain was bearable and they would be happy to be operated on again with the same anaesthetic-surgery technique. The efficacy of the anaesthetic technique (local anaesthetic with conscious sedation) was very good, 8% of the patients never felt pain and 21% never received any analgesic. The time passed until the first analgesic dose was 6 h 40 min, and the tolerance of the pain was excellent.