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1.  The use of a stimulating catheter for total knee replacement surgery - preliminary results 
British journal of anaesthesia  2005;95(2):250-254.
There is continuing debate as to whether the use of electrical stimulation that aids in localizing nerves is also beneficial for optimizing placement of nerve catheters and will lead to improved clinical outcomes such as reductions in pain scores and opioid consumption.
We undertook a retrospective, non-randomized comparison of stimulating and non-stimulating nerve catheters in 419 patients undergoing total knee replacement between December 2002 and July 2004. Pre-operatively, patients received sciatic and femoral nerve blocks, with a catheter for the femoral nerve. In 159 patients, a stimulating (Stimucath, Arrow International, Reading, PA) and, in 260 patients, a non-stimulating (Contiplex, BBraun, Melsungen, Germany) catheter system was used. Postoperatively, pain scores and morphine consumption were recorded at 4-hour intervals until the first postoperative morning. In a subset of 85 patients, the postoperative evaluation period was lengthened to three days.
Post-operative visual analogue scores (VAS) for pain were similar in the two groups during the first 24 hours (P = 0.305). In patients followed for three days, VAS scores did not differ on any of the days (P = 0.427). Total morphine consumption did not differ on the first post-operative day (Stimulating: 12.4 [10.1-14.7] vs. non-stimulating: 10.4 [8.9-11.8]; mean [95% CI]; P=0.140) or on subsequent days.
The practical advantages of the stimulating catheter, as by reported by previous investigators, were not obvious in this clinical situation. In terms of outcome measures such as pain scores and morphine consumption, we found no significant differences between stimulating and non-stimulating catheters.
PMCID: PMC1770892  PMID: 15923268
Continuous femoral nerve block; stimulating catheters; total knee replacement
2.  Clonidine Produces a Dose-dependent Impairment of Baroreflex-mediated Thermoregulatory Responses to Positive End-expiratory Pressure in Anaesthetised Humans 
British journal of anaesthesia  2005;94(4):536-541.
Background. Perioperative hypothermia is common and results from anaesthetic-induced inhibition of thermoregulatory control. Hypothermia is blunted by baroreceptor unloading caused by positive end-expiratory pressure (PEEP) and is mediated by an increase in the vasoconstriction threshold. Premedication with clonidine impairs normal thermoregulatory control. We therefore determined the effect of clonidine on PEEP-induced hypothermia protection.
Methods. Core temperature was evaluated in patients undergoing combined general and epidural anaesthesia for lower abdominal surgery. They were assigned to an end-expiratory pressure of zero (ZEEP) or 10 cmH2O PEEP. The PEEP group was divided into three blinded subgroups: placebo (Clonidine-0), clonidine 150 μg (Clonidine-150), and clonidine 300 μg (Clonidine-300). Placebo or clonidine was given orally 30 minutes before surgery. We evaluated core temperature and thermoregulatory vasoconstriction. We also determined epinephrine, norepinephrine, and angiotensin II concentrations and plasma renin activity.
Results. Core temperature after 180 minutes of anaesthesia was 35.1 ± 0.1°C in the ZEEP group. PEEP significantly increased final core temperature to 35.8 ± 0.2°C (Clonidine-0 group). Clonidine produced a linear, dose-dependent impairment of PEEP-induced hypothermia protection: final core temperatures of 35.4 ± 0.1°C in the clonidine-150 group and 35.1 ± 0.2°C in the Clonidine-300 group. Similarly, clonidine produced a linear and dose-dependent reduction in vasoconstriction threshold: Clonidine-0=36.4 ± 0.1°C, Clonidine-150=35.8 ± 0.1°C, and Clonidine-300=35.4 ± 0.2°C. Plasma norepinephrine and angiotensin II concentrations and renin activity were consistent with the thermoregulatory responses.
Conclusion. Baroreceptor unloading by PEEP normally moderates perioperative hypothermia. However, clonidine premedication produces a linear, dose-dependent impairment of this benefit.
PMCID: PMC1362957  PMID: 15708868
baroreceptor reflex; clonidine; hypothermia; positive end-expiratory pressure (PEEP); thermoregulation
3.  Magnesium Sulfate Only Slightly Reduces the Shivering Threshold in Humans 
British journal of anaesthesia  2005;94(6):756-762.
Background: Hypothermia may be an effective treatment for stroke or acute myocardial infarction; however, it provokes vigorous shivering, which causes potentially dangerous hemodynamic responses and prevents further hypothermia. Magnesium is an attractive antishivering agent because it is used for treatment of postoperative shivering and provides protection against ischemic injury in animal models. We tested the hypothesis that magnesium reduces the threshold (triggering core temperature) and gain of shivering without substantial sedation or muscle weakness.
Methods: We studied nine healthy male volunteers (18-40 yr) on two randomly assigned treatment days: 1) Control and 2) Magnesium (80 mg·kg-1 followed by infusion at 2 g·h-1). Lactated Ringer's solution (4°C) was infused via a central venous catheter over a period of approximately 2 hours to decrease tympanic membrane temperature ≈1.5°C·h-1. A significant and persistent increase in oxygen consumption identified the threshold. The gain of shivering was determined by the slope of oxygen consumption vs. core temperature regression. Sedation was evaluated using verbal rating score (VRS, 0-10) and bispectral index of the EEG (BIS). Peripheral muscle strength was evaluated using dynamometry and spirometry. Data were analyzed using repeated-measures ANOVA; P<0.05 was statistically significant.
Results: Magnesium reduced the shivering threshold (36.3±0.4 [mean±SD] vs. 36.6±0.3°C, P=0.040). It did not affect the gain of shivering (Control: 437±289, Magnesium: 573±370 ml·min-1·°C-1, P=0.344). The magnesium bolus did not produce significant sedation or appreciably reduce muscle strength.
Conclusions: Magnesium significantly reduced the shivering threshold; however, due to the modest absolute reduction, this finding is considered to be clinically unimportant for induction of therapeutic hypothermia.
PMCID: PMC1361806  PMID: 15749735
Magnesium; Temperature; Thermoregulation; Therapeutic hypothermia; Brain protection; Cardiac protection; Shivering
4.  Impact of age on both BIS values and EEG bispectrum during anaesthesia with sevoflurane in children 
British Journal of Anaesthesia  2005;94(6):810-820.
The aim of this study was to evaluate the potential relationship between age, BIS (Aspect™) and the EEG bispectrum during anesthesia with sevoflurane. BIS and raw EEG sampled at 400 Hz were recorded at a steady state of 1 MAC sevoflurane in 100 children, and during a decrease from 2 MAC to 0.5 MAC in a sub-group of 29 children. The bispectrum of the EEG was estimated on successive epochs of 20 seconds using MATLAB© software, independently of the Aspect™ device. For analysis, the bispectrum was divided into 36 frequencies of coupling (Pi) - the MatBis. A multiple correspondence analysis (MCA) was used to establish an underlying structure of the pattern of each individual’s MatBis at the steady state of 1 MAC. Clustering of children into homogeneous groups was conducted by a hierarchical ascending classification (HAC). The level of statistical significance was set at 0.05. At the steady state of 1 MAC sevoflurane, the BIS values for all 100 children ranged from 20 to 74 (median 40). Projection of both age and BIS value recorded at 1 MAC (T10) onto the structured model of the MCA showed them to be distributed along axis F1 of this model. In contrast, projection of children’s position during the decrease in sevoflurane concentration was linked to axis F2. At 1 MAC sevoflurane, six homogeneous groups of children were obtained through the HAC. Groups 5 (30 months; range 23–49) and 6 (18 months; range 6–180) were the younger children and group 1 (97 months; range 46–162) the older. Groups 5 and 6 had the highest median values of BIS (54; range 50–59)(55; range 26–74) and the group 1 the lowest values (29; range 22–37). The EEG bispectrum, as well as the BIS (Aspect XP™) measured at 1 MAC sevoflurane appeared to be strongly related to the age of children.
PMCID: PMC2043092  PMID: 15833781
Adolescent; Aging; physiology; Anesthetics, Inhalation; pharmacology; Body Weight; physiology; Child; Child, Preschool; Dose-Response Relationship, Drug; Electroencephalography; drug effects; Humans; Infant; Methyl Ethers; pharmacology; Monitoring, Intraoperative; methods; Signal Processing, Computer-Assisted; Depth of Anesthesia; EEG; Bispectrum; PCA; Classification; Factorial Analysis; BIS; Monitoring

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