Proceedings of the Anaesthetic Research Society Meeting

British Journal of Anaesthesia 96 (2): 275–87P (2006)

doi:10.1093/bja/aei288

ABSTRACTS Proceedings of the Anaesthetic Research Society Meeting Burleigh Court, Loughborough November 24–25, 2005 (The name of the author presenting the paper is shown in bold type. *Indicates non-member. All authors have certified that, where appropriate, studies have been conducted with the approval of the relevant Human Ethics Committee or Animal Experimental Review Committee.)

VERBAL PRESENTATIONS Sex and site differences in human pain: the role of visual cues and stress hormone response K. McGinn1*, E. Keogh3*, D. Stephens2* and A. Holdcroft1 1 Department of Anaesthesia and 2Department of Mathematics, Imperial College London, London, UK. 3Department of Psychology, University of Bath, Bath, UK

Our previous research demonstrated that in response to a heat stimulus there were significant sex differences across all sites tested and differences between sites irrespective of sex, in visual analogue scale and verbal rating scale responses given by normal subjects.1 The sites studied were the arm, the lower abdomen and lower back. In this study we aimed to determine possible mechanisms. In addition to reproductive cycle variations, we postulated that visual cues may contribute to the pain responses and that the site of stimulation may affect the output of stress hormones. Stress hormones may be released in response to noxious stimuli and may modulate pain sensations.2 Using a thermal stimulus our objective was to determine the dose–response curves to a standardized sequence of stimuli on the abdomen (with and without sight of the stimulus) and lower back of males and females using a fixed choice method and then to compare the results with scores of pain intensity and blood concentrations of stress hormones. Twenty each of male and female subjects were recruited after checking their inclusion criteria (aged 18–40 yr, fit and healthy, not on medication). Informed written consent was obtained. Subjects refrained from alcohol, caffeine and nicotine before the study. For all subjects a baseline anxiety, depression and stress questionnaire was completed and for females a detailed reproductive history was obtained. Testing was conducted in a standardized environment. Using a computer-controlled thermode (Somedic), temperatures for each test site ranged from 40 to 49
C in 1
C intervals. Subjects were asked to make a forced choice (i.e., yes or no) in answer to the question ‘Was it painful?’ The sequence of stimuli and sites (AV, abdomen; ANV, abdomen non-visual; BNV, back non-visual) were selected using computer generated random numbers. Visual cues were removed by drawing a curtain across the lower thorax. Blood samples were obtained before and after the test at the first site. The samples collected in lithium–heparin tubes were analysed for norepinephrine and epinephrine using a reverse phase-ion pair high performance liquid chromatography assay with electrochemical detection.

A significant difference between sexes was observed at 48–49
C for all sites (P<0.001) and for epinephrine concentrations between sites [changes from baseline as mean (SEM) abdomen (AV+ANV) 34.32 (3.74) back 55.17 (7.01) pg ml1; P=0.039] but not between sexes [females baseline 48.9 (4.9), after first site 90.9 (8.9); males baseline 49.1 (4.5), after first site 88.3 (8.1) pg ml1]. The AV and ANV site responses were significantly different from BNV stimulation (P<0.001). When the data were combined with that from the previous study, significant differences were observed between males and females taking hormonal contraceptives (P=0.002). These results demonstrate no effect of visual cues on pain response, but endogenous and exogenous hormones appear to be contributory factors to subject differences in responses to thermal pain sensation. Acknowledgements: The authors thank the ARS and the Westminster Medical School Research Trust for funds. Keywords: pain; gender difference; stress response

References 1 Patel A, Keogh E, Stephens D, Holdcroft A. Br J Anaesth 2005; 94: 402P 2 Zimmer C, Basler HD, Vedder H, Lautenbacher S. Clin J Pain 2003; 19: 233–9

Effect of pulsed magnetic field therapy on pain reported by human volunteers in a laboratory model of acute pain M. I. Fernandez*, P. J. Watson* and D. J. Rowbotham University Division of Anaesthesia, Critical Care and Pain Management, University of Leicester and University Hospitals of Leicester, Leicester Royal Infirmary, Leicester LE1 5WW, UK

It has been claimed that pulsed magnetic field therapy (PMFT) is a non-invasive, safe and easy-to-use technique for the treatment of pain.1 2 Magnetic field is applied over the site of injury or inflammation using a probe-like device. However, there are few data from well-designed clinical or experimental studies confirming the efficacy of PMFT. Therefore, we utilized an acute pain model to perform a randomized, double-blind, cross-over study on volunteers. Local Ethics Committee approval and written informed consent was obtained. To elicit pain, we modified a technique described previously;3 saline 5% was infused into the brachioradialis muscle of the non-dominant arm via a 27-G needle for 10 min (18 ml h1 for 5 min, then 36 ml h1). Ten healthy male volunteers (18–40 yr) underwent this procedure on two separate occasions at least 1 week


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Proceedings of the Anaesthetic Research Society Meeting

100 90

Control

80

Active

Mean VAS

70 60 50 40 30 20 10 0 Time (min) Fig 1 Mean VAS pain scores after sham (control) and active interventions. There were no significant differences in mean VAS pain scores. There were no significant differences with respect to mean (SEM) maximum pain score [sham 60 (8), active 63 (9) mm; P=0.66, 95% CI 18 to 12 mm] or AUCp [sham 463 (50), active 499 (90) mm min1; P=0.64, 95% CI 201 to 129].

apart. They were randomized to receive either active or sham PMFT (commencing 5 min before infusion) for 30 min. Two identical commercially available machines were used (PulsePack 6000, Quantum Techniks); the active machine delivered a M-wave magnetic pulse (1.25 Hz, 3 ms width). The sham machine delivered no magnetic energy. Pain was assessed at 15-s intervals using a validated electronic visual analogue scale (VAS) device. Using data from previous work, we calculated that nine volunteers were required to detect a difference of 20 mm on the VAS (a=0.05, b=0.20). Area under the VAS pain curve was calculated using the trapezoid method. Data were checked for normality using the Kolmogorov–Smirnov test and compared with the paired t-test and ANOVA for repeated measures (Fig. 1). We have been unable to demonstrate an analgesic effect of PMFT using this device in this experimental model. Keywords: pain, experimental model; technique, pulsed magnetic field therapy

to analgesic drugs are rarely reported despite histamine release by some of the drugs. We aimed to measure the patient characteristics of reported suspected adverse drug reactions to analgesic drugs used mainly in the perioperative period. These were the opioids alfentanil, fentanyl, morphine, nalbuphine, omnopon, pethidine, and remifentanil and the non-steroidal ketorolac. We report a retrospective analysis of the patient characteristics of Yellow Card reports of suspected ADRs from 1965 to 2004 as classified in the Adverse Data Reporting On-line Tracking system of the Medicines and Healthcare products Regulatory Agency. In total, 1312 reactions were retrieved. A single drug was reported in 908 cases, 39 (4%) of which were fatal and 219 categorized as ‘allergic’. Allergic phenomenon reporting varied from 2/33 (6%) for remifentanil to 11/53 (22%) for alfentanil. More females than expected reported ‘allergic’ reactions to alfentanil (males:females=1:11; P=0.002). Age was a factor in ‘allergic’ reactions to morphine or omnopon (P=0.003 and 0.04 respectively). ‘Cardiac’ reactions were common with remifentanil (19/53, 36%) and alfentanil (19/53, 36%). Prospective studies are needed to investigate these findings further. The majority of suspected reactions to analgesic drugs were not reported to be of an allergic nature. In the course of this study we identified opportunities for improvement of adverse reaction reporting in this area, for example, in aspects of the information provided by healthcare professionals. Acknowledgements: The authors thank the Anaesthetic Research Society Heath Family Grant and the Medicines and Healthcare products Regulatory Agency. Keywords: analgesia, perioperative; analgesics, opioids; complications, drug reactions

References 1 Mertes PM, LaxenaireM-C. Eur J Anaesthesiol 2002; 19: 240–62 2 Fisher MM, Baldo BA. Ann Fr Anesth Reanim 1993; 12: 97–104

The effect of continuous sciatic nerve blockade via a neural sheath catheter on stump and phantom limb pain in lower limb amputees V. Jaggernauth1*, M. McCarthy2*, M. Lapworth2*, L. Toogood1*, K. Latif1* and J. P. Thompson1

References 1 Trock DH. Rheum Dis Clin North Am 2000; 26: 51–62 2 Ludwig W. Biol Ther 1996; 14: 230–1 3 Graven-Nielsen T, Arendt-Nielsen L, Svensson P, et al. J Musculoskel Pain 1997; 5: 49–69

Results of 40 yr Yellow Card reporting for commonly used perioperative analgesic drugs J. Richardson* and A. Holdcroft Department of Anaesthesia, Pain Medicine and Intensive Care, Imperial College London, Chelsea and Westminster Hospital, London SW10 9NH, UK

Adverse drug reactions (ADRs) may cause morbidity or mortality. Analgesics used perioperatively are heterogeneous but associated ADRs may complicate anaesthesia and recovery. Research into ADRs in anaesthesia has tended to focus on reactions to neuromuscular blocking agents, and a significantly increased proportion of reports in females have been observed.1 2 ADRs and allergies

1 University Department of Cardiovascular Sciences, Clinical Division of Anaesthesia, Critical Care and Pain Management and 2Department of Vascular Surgery, Leicester Royal Infirmary, Leicester LE1 5WW, UK

Up to 80% of patients undergoing major lower limb amputation subsequently develop phantom limb pain (PLP).1 Perioperative nerve blockade with local anaesthetic infusions may improve postoperative analgesia but the incidence of long-term PLP remains high.2 With LREC approval and informed written consent, 26 patients underwent major lower limb amputation under standardized general anaesthesia with postoperative morphine via a PCA system. Patients were randomly assigned to receive a 4 ml bolus followed by a perineural infusion of either bupivacaine 0.75% or saline 0.9% at 2 ml h1 into the sciatic nerve sheath, started at the end of surgery and continued for 96 h. PCA morphine consumption, VAS (scale 0– 100 mm) for stump pain at rest or on movement (SP-Rest and SP-Move) and PLP were measured before surgery, in the early postoperative period, and at 6 and 12 months. Quality of life questionnaires (SF-36 and McGill pain inventory) were also administered at these times. Data were compared using Kruskal–Wallis and Fisher exact tests via SPSS for Windows (release 11.01).

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Table 1 VAS for stump and phantom pain. Data presented as median (IQR). *P<0.08 Group

6 months

Placebo (n=6) Bupivacaine (n=5)

12 months

SP-Rest

SP-Move

PLP

SP-Rest

SP-Move

PLP

5 (1–19) 0*

10 (3–30) 0*

12 (2–30) 0 (0–30)

8 (3–61) 0 (0–12)

21 (8–60) 0* (0–8)

12 (1–34) 0 (0–21)

Six patients were withdrawn because of early protocol violations and data on 20 patients were analysed. However, complete followup data were available on 11 patients. Preoperative VAS pain scores were high in both groups (median = 50 mm in the placebo group and 41 mm in the bupivacaine group). There was a trend towards higher early postoperative VAS pain scores and PCA morphine consumption in the placebo group but differences were not statistically significant. However, the bupivacaine group reported less stump pain at 6 and 12 months (P<0.05). At 12 months 1 out of 5 patients in the bupivacaine group reported PLP compared with 5 out of 6 in the placebo group (P<0.05). In this study (Table 1), perineural infusion of bupivacaine reduced the incidence of late persistent stump and phantom pain after major lower limb amputation. Further follow-up studies are required to confirm these findings.

25

Parecoxib n=63

Mean VAS

20

Diclofenac n=63

19.4 17.4 14.9

16.2

15

15.2 10.9

10 5 0 30

60

90

Time after awakening (min) Fig 2 Mean VAS scores at 30, 60 and 90 min post recovery.

Keywords: pain, phantom; technique, nerve block

References

Keywords: analgesics, parecoxib, diclofenac; surgery, arthroscopy

1 Nikolajsen L, Jensen TS. Br J Anaesth 2001; 87: 107–16 2 Lambert AW, Dashfield A, Cosgrove C, et al. Reg Anesth Pain Med 2001; 26: 316–21

Acknowledgement: The authors acknowledge an educational grant provided by Pharmacia towards the cost of the study.

Comparison of parecoxib with diclofenac in knee arthroscopy 1

2

2

1

P. Dalrymple *, S. Boumphrey , J. Langton and R. Griffiths 1 Peterborough Hospital NHS Trust and 2Plymouth Hospitals NHS Trust, UK

Parecoxib is a cyclo-oxygenase 2 selective, non-steroidal antiinflammatory drug (NSAID). This twin centred, double blind, randomized, controlled study aimed to compare the analgesic effects of parecoxib with diclofenac, a non-selective NSAID in common usage, in patients undergoing day case knee arthroscopy. One hundred and thirty-eight (ASA I–II) patients were randomized into two groups receiving either preoperative oral diclofenac (100 mg) with i.v. placebo on induction or preoperative oral placebo with i.v. parecoxib (40 mg) on induction. Only the anaesthetist was unblinded to the patients’ analgesia. A standard general anaesthetic was administered. Induction was with alfentanil 1 mg and propofol. Maintenance was with a laryngeal mask, spontaneous ventilation with oxygen/nitrous oxide and isoflurane. Twenty millilitres of bupivacaine 0.5% was instilled into the joint cavity, by the surgeon, at the end of the procedure. One hundred millimetres visual analogue scales (VASs) were recorded at 30, 60 and 90 min intervals postawakening. A record of rescue opiate analgesic prescribing and fitness for day case discharge were also recorded. Sixty-three patients in each group completed the study. There were no significant differences in the VAS scores (Fig. 2) between the two groups at 30, 60 or 90 min (P>0.05). Fourteen patients in the parecoxib group required rescue opiate analgesia, compared with 15 in the diclofenac group. Eighty-four per cent of the parecoxib group were fit for day case discharge, compared with 86% in the diclofenac group. We found no significant difference in either analgesic efficacy, need for rescue analgesia or fitness for day case discharge.

Preliminary investigation of the effect of indomethacin and FR-139317 on LPS-induced changes in responsiveness of the porcine isolated coronary artery W. Qi1*, J. X. Wei1, R. P. Mahajan1 and V. G. Wilson2 1

University Department of Anaesthesia and Intensive Care and School of Biomedical Sciences, Queen’s Medical Centre, Nottingham NG7 2UH, UK 2

We have reported that L-NAME, an inhibitor of nitric oxide synthase, prevents lipopolysaccharide (LPS)-induced changes in the responsiveness of the porcine-isolated coronary artery (PCA).1 This has been interpreted as evidence for the induction of nitric oxide synthase (iNOS), and generation of nitric oxide, after exposure to LPS. Pro-inflammatory cytokines have been reported to induce cyclooxygenase and endothelin converting enzyme in human vascular smooth muscle,2 3 suggesting that locally produced prostanoids and endothelin also modulate vascular responses. We have examined the effect of indomethacin, an inhibitor of cyclooxygenase, and FR-139317, a selective ET-A receptor antagonist,4 on the action of LPS in the PCA. Paired 5 mm segments of the PCA were dissected from pig hearts and incubated in Krebs Henseleit solution at 37
C (containing antibiotics) in the presence or absence of LPS 100 mg ml1. After 16–18 h the segments were prepared for isometric tension recording1 and exposed to cumulatively increasing concentrations of either U46619 or KCl, in the presence or absence of a combination of indomethacin 3 mM and FR-139317 1 mM. Responses have been expressed as percentage of the maximum contraction, and the negative logarithm of the concentration causing 50% of the maximum response calculated. The values are shown as the mean–SEM.

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Table 2 The effect of indomethacin and FR-139317 on the magnitude (g wt) and potency (pD2) of KCl and U46619 contractions of the PCA after exposure to 100 mg ml1 LPS. Statistically different from the vehicle segment (*P<0.01)

Control LPS Control LPS

KCl (n=6) KCl (n=11) U46619 (n=6) U46619 (n=11)

Vehicle

Indomethacin/ FR-139317

Emax (g wt) pD2

Emax (g wt) pD2

15.40–1.89 12.81–1.21 13.18–1.12 13.27–1.35

14.80–1.84 7.70–0.95* 14.41–1.49 12.57–1.09

1.61–0.02 1.55–0.02 7.96–0.08 7.70–0.13

1.56–0.02 1.40–0.02* 7.84–0.10 7.35–0.05*

The differences between mean values were assessed by a Student’s paired t-test. As shown in Table 2, the presence of indomethacin 3 mM and FR-139317 1 mM did not affect either the maximum response or the potency of KCl and U46619. After exposure to 100 mg ml1 LPS, however, the maximum response to KCl was reduced by 39.9–4.73% (n=11), while the potency of U46619 and KCl were significantly reduced. The inhibitory action of indomethacin and FR-139317 on KCl and U46619, after exposure to LPS, suggests an up-regulation of COX, endothelin-converting enzyme, or both to produce factors that support contraction and offset the effect of iNOS. Keywords: technique, myography; cytokine, LPS; sepsis, vascular response

acid or dehydroascorbic acid for 24 h in the presence of the iron chelator deferioxamine, then washed and the culture media replaced with medium containing 2 mg ml1 lipopolysaccharide (LPS, endotoxin) only for 18 h. The rate of monocyte ROS production, expression of manganese superoxide dismutase (MnSOD), catalase and glutathione peroxidase were determined and then related to mitochondrial membrane potential and cytochrome c oxidase release. Results showed no difference in the rate of ROS formation in monocytes when pre-exposed to ascorbic acid vs control (88–13 and 99–18 fluorescent units min1 respectively). However, a marked increase in the rate of ROS formation was observed when cells were pre-exposed to dehydroascorbic acid (137–2 units min1). After the exposure to either ascorbic acid or dehydroascorbic acid, MnSOD expression increased, but catalase expression only increased in the presence of dehydroascorbic acid. No change in glutathione peroxidase expression or mitochondrial membrane potential was seen with any pre-treatment. However, both ascorbic acid and dehydroascorbic acid pre-exposure resulted in reduced release of cytochrome c oxidase from mitochondria. These results show that dehydroascorbic acid acts as a prooxidant under physiological conditions, but ascorbic acid is just as capable of inducing mitochondrial protection upon LPS exposure. We have shown previously that ascorbic acid is handled abnormally during sepsis3 and it is interesting to speculate on the in vivo effects of vitamin C supplementation during oxidative stress.2 Keywords: cytokines, LPS; dehydroascorbic acid; mitochondrial damage

References References 1 Wei JX, Wilson VG, Mahajan RP. Br J Anaesth 2005; 94: 410P 2 Wood M, Bishop-Bailey D, Pepper JR, et al. J Cardiovasc Pharmacol 1998; 31: S348–50 3 Wood M, Mitchell J, Wood E, et al. Mol Pharmacol 1999; 55: 902–9 4 Dashwood MR., Allen SP, Luu TN, et al. Br J Pharmacol 1994; 112: 386–9

1 Galley HF, Howdle PD, Walker BE, Webster NR. Free Radic Biol Med 1997; 23: 768–74 2 Long CL, Maull KI, Krishnan RS, et al. J Surg Res 2003; 109: 144–8 3 Galley HF, Davies MJ, Webster NR. Free Radic Biol Med 1996; 20: 139–43

Surface expression and secretion of TREM-1 by neutrophils Dehydroascorbic acid as a pro-oxidant: can it protect against endotoxin induced mitochondrial damage in vitro?

A. M. Mahdy, D. A. Lowes, J. E. Bruce, H. F. Galley and N. R. Webster Academic Unit of Anaesthesia and Intensive Care, University of Aberdeen, Scotland

D. A. Lowes, H. F. Galley and N. R. Webster Academic Unit of Anaesthesia and Intensive Care, University of Aberdeen, Scotland

Uncontrolled production of reactive oxygen species (ROS) is believed to be involved in the pathophysiology of sepsis and the development of multiple organ failure. In septic patients, circulating vitamin C is markedly depleted and thus may potentiate the negative effects of oxidative stress.1 2 Vitamin C is regarded as an umbrella term for both ascorbic acid (reduced) and dehydroascorbic acid (oxidized) which are accumulated by cells using different transport systems. Ascorbic acid is taken up by the sodium exchange pumps, SVCT-1 and -2 and is regarded as the minor route of intracellular vitamin C accumulation. However, dehydroascorbic acid is taken up by the glucose transport proteins, GLUT-1, -3 and -4. Once inside the cell, dehydroascorbic acid is rapidly reduced to ascorbic acid by both enzymatic and non-enzymatic mechanisms with the production of ROS. We hypothesized that cellular uptake of dehydroascorbic acid may act as a ‘physiological pro-oxidant’ that may prime cells to better resist the damaging effects of ROS produced during endotoxin exposure. Following an approval and an informed consent from the ethical committee, monocytes were isolated from EDTA blood samples from four healthy volunteers using density gradient sedimentation and adherence. Monocytes were exposed to either 100 mM ascorbic

Triggering receptor expressed on myeloid cells (TREM-1) is expressed on neutrophils and monocytes. TREM-1 has an alternatively spliced variant that lacks the transmembrane region, resulting in the receptor being secreted in a soluble form (sTREM-1). Soluble TREM-1 is found in plasma in experimental and clinical sepsis and has been advocated as a prognostic marker in septic shock. The exact cellular source of sTREM-1 has not yet been identified and there are no data relating sTREM-1 secretion to TREM-1 cell surface expression in lipopolysaccharide (LPS) activated neutrophils. Based on available evidence, we hypothesized that LPS stimulation of neutrophils would lead to an upregulation of TREM-1 cell surface expression and an increase in sTREM-1 release. After the Local Research Ethics Committee’s approval and informed consent, 30 ml venous blood was collected from 18 volunteers. Isolated neutrophils were adjusted to 1·107 cells ml1 and viability was assessed using trypan blue exclusion. Cells were incubated for 16 h with either no LPS, 1 mg ml1 LPS or LPS plus cycloheximide 10 mg ml1. Culture supernatants were collected and stored at 20
C for sTREM-1 assay and harvested cells were resuspended for determination of TREM-1 receptor cell surface expression by flow cytometry. Normal human serum (10%) was used to block non-specific Fc binding and then cells were incubated with monoclonal anti-human TREM-1, followed by an isotypespecific conjugated secondary antibody and then a conjugated

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Proceedings of the Anaesthetic Research Society Meeting

∗∗∗

300 sTREM-1 (pg ml−1)

Depression of function (%)

350

250 200 150

∗∗

100 Memory 80 60

GABA-induced depolarizations

40 20 0 0

100

1

2 MAC

50

Fig 4 Effect of MAC on memory and GABA induced depolarizations.

0 No LPS

LPS

LPS/CHX

Fig 3 Soluble TREM-1 concentrations in culture supernatants from neutrophils isolated from 18 subjects. No LPS=unstimulated cells; LPS=cells stimulated with LPS 1 mg ml1; LPS/CHX=cells stimulated with LPS 1 mg ml1 in the presence of cycloheximide 10 mg ml1. Box and whisker plots show median, 25th and 75th percentile and full range. **P<0.001 and ***P<0.001 compared with unstimulated cells.

anti-human CD16b antibody, which binds specifically to neutrophils. To exclude non-specific staining, isotype-matched control antibodies were used. In our study, TREM-1 surface expression was constitutive, was not upregulated upon LPS-stimulation and was not affected by the presence of cycloheximide. On the other hand, sTREM-1 release from neutrophils was significantly increased by LPS stimulation (P<0.0001), an effect that was abrogated by the protein inhibitor cycloheximide (Fig. 3). We conclude that sTREM-1 is secreted by human neutrophils in response to LPS challenge in a process involving de novo protein synthesis and that is not accompanied by an upregulation of the TREM-1 receptor on the cell surface. This could be seen as a protective mechanism through which neutrophils produce their own TREM-1 ‘decoy’ receptor as a downregulatory response to limit the effects of persistent activation of the TREM-1 pathway. Manipulation of alternative mRNA splicing to influence the ratio of expression of sTREM-1/TREM-1 in a favourable direction may represent a future novel therapeutic approach for sepsis. Acknowledgements: The authors thank TENOVUS Scotland for funding this project. J.E.B. was an ARS vacation scholar. Keywords: cytokine, LPS; neutrophils, TREM-1

Why is the brain so sensitive to anaesthetics? (1) R. S. Cormack1*, G. G. Lockwood2 and D. C. White1 1

Northwick Park, UK. 2Hammersmith Hospitals, UK

It is generally agreed that anaesthetics act by depressing conduction through synapses. Figure 4 shows that memory (Lockwood GG, unpublished data), doubtless a function mediated by many synapses, is much more sensitive to anaesthesia than GABA channels.1 At first sight it might be concluded that these two phenomena are not related. However, studies on GABA channels and other neurotransmitters performed in vitro must relate to individual synapses. In contrast, synapses in the CNS are connected together in a complex manner. From Figure 4 it can be seen that although one-third of one MAC abolishes memory1 it has only a small effect on GABA-induced depolarization.1 In fact one MAC only depresses GABA depolarization by 8%.

To attempt an explanation on the difference in sensitivity shown, it is necessary to consider the effect of anaesthetic inhibition on transmission through synapses in series. If conduction through a synapse is depressed by 50% then it can be calculated that passage through six synapses virtually abolishes conduction through the chain. This contrasts with the situation in which the depressed synapses are regarded as being in parallel. In this case the degree of depression through the network is the same as that in each individual synapse. These findings are supported by observations2 that show MAC is mediated by motor tracts within the spinal cord, and that unconsciousness is produced by fractions of MAC. Keywords: anaesthesia, depth; brain, memory, GABA

References 1 Puil E, El-Beheiry H. Br J Pharmacol 1990; 101: 61–6 2 King BS, Rampil IJ. Anesthesiology 1994; 84: 1484–92

Why is the brain so sensitive to anaesthetics? (2) R. S. Cormack1*, G. G. Lockwood2 and D. C. White1 1

Northwick Park, UK. 2Hammersmith Hospitals, UK

In order to progress from series and parallel connections to multiple interconnections we require a more general analysis of network function. We have adopted a particular system of artificial neural networks to investigate the effects of artificial anaesthesia. A GRAM network1 is an interconnected set of artificial nodes, each of which may be thought of as representing a functional neuronal unit (e.g., a synapse, a neurone, a cortical column, etc.) according to the context. Each node has a set of input connections from the ‘outside world’ and from its peers within the network; we refer to these as external and internal connections, respectively. The activity at all the input connections forms an input pattern. The nodes are taught (programmed) which output to make (fire or not fire) in response to particular input patterns. When it is presented with a novel input pattern, it responds in the manner it has been taught for the learned input pattern that is ‘closest’ to the novel input. The network can be characterized by the ratio of internal to external input connections at each of its nodes. Just as the nodes learn to respond to input patterns, the whole network may be thought of as responding to the totality of external input connections, but in this case it forms an output pattern rather than a simple on/off response. The learning at each node imbues the entire network with the ability to form a particular output pattern in response to a particular external input pattern. We have anaesthetized nodes by limiting their ability to accommodate departures from their set of learned input patterns. We investigated the effect of anaesthesia on the network by measuring its ability to form the correct output pattern in response to a

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R=Ratio of external:internal input connections

A

Network performance (%)

100 R=2

RT-PCR NOP peptide primers cDNA from 5 of 10 healthy volunteers

R=4

Lanes 1+7: Ladder Lanes 2–6: cDNA from PBMCs

80 60 40

1

R=0.25

3

4

5

6

7

B

R=1

20

2

RT-PCR NOP receptor primers cDNA from 5 of 10 healthy volunteers

R=0.5 0 0

10

20

30

40

50

Lanes 1+7: Ladder Lanes 2–6: cDNA from PBMCs

Increasing anaesthesia (arbitrary units) Fig 5 Network performance changes with increasing anaesthesia.

1

novel input pattern. We found that networks with fewer internal inputs than external inputs function less effectively than those with a large proportion of internal inputs, but are more resistant to anaesthesia. Networks with greater input complexity fail suddenly when anaesthesia exceeds a critical point. This very general result implies that networks with the greatest degree of internal processing have the finest function but are catastrophically sensitive to anaesthesia (Fig. 5).

4

5 6

7

Table 3 Plasma UII concentrations in smoking and non-smoking male volunteers. Data are presented in pg ml1 as median (IQR). *P=0.034 (control vs smokers); **P=0.01 (control vs cigarette<10 min)

Reference RIA ILMA

Human peripheral blood mononuclear cells express the mRNA encoding for the nociceptin peptide and its receptor

3

Fig 6 (A) RT–PCR NOP peptide primers. cDNA from 5 of 10 healthy volunteers. (B) RT–PCR NOP receptor primers. cDNA from 5 of 10 healthy volunteers.

Keywords: anaesthesia, depth; neurones, network

1 Aleksander I. Comput Contr J 1990; 1: 259–65

2

Control

Smoker (n=20)

Cigarette <10 min (n=11)

Cigarette >10 min (n=9)

(n=20) 1.67 (1.0–2.27) 0.42 (0.42–0.97)

2.62 (1.87–3.46)* 0.42 (0.42–0.65)

3.10 (1.87–4.60)** 0.42 (0.42–0.62)

2.2 (1.50–2.80) 0.42 (0.42–1.11)

We conclude that PBMCs express the RNA encoding for both the nociceptin receptor and its endogenous ligand nociceptin (N/OFQ). If these immunocytes deliver N/OFQ to neural or vascular tissue then a role for nociceptin in a neuroimmune/neurovascular axis can be proposed.

J. P. Williams*, J. McDonald, J. P. Thompson, D. J. Rowbotham and D. G. Lambert University Department of Cardiovascular Sciences, Division of Anaesthesia, Critical Care and Pain Management, LRI, Leicester LE1 5WW, UK

Classically there are considered to be three subtypes of opioid receptor; MOP (m), DOP, (d), KOP (k).1 In 1994–5 a fourth opioid receptor and its ligand (nociceptin receptor and N/OFQ respectively) were found. The nociceptin receptor–N/OFQ system is involved in a range of physiological functions including nociception and modulation of the cardiovascular system.2 In a previous study we failed to demonstrate MOP receptors on peripheral blood mononuclear cells (PBMCs).3 In this study we have attempted to demonstrate the expression of both the mRNA encoding for the nociceptin peptide and its receptor in PBMCs using a reverse transcription polymerase chain reaction (RT–PCR) technique. After LREC’s approval, venous blood was drawn from 10 healthy male volunteers (aged 23–40 yr) and PBMCs were isolated by density gradient sedimentation. The PBMCs were then washed in phosphate buffered saline and total RNA extracted using a standard phenol/chloroform technique. RT–PCR was then performed using sense and anti-sense primers directed at the nociceptin receptor and nociceptin peptide. Primer pairs were chosen to sit on different exons of the genomic DNA message so complementary DNA derived from RNA could be differentiated from genomic DNA. Figure 6A and B shows the results of RT–PCR of mRNA with primers for the NOP peptide and NOP receptor.

Keywords: blood cells, peripheral mononuclear cells; nociceptors

References 1 McDonald J, Lambert DG. CEACCP 2005; 5: 22–5 2 Mogil JS, Pasternak GW. Pharmacol Rev 2001; 53: 381–415 3 Williams JP, Cote TE, Thompson JP, et al. Br J Anaesth 2004; 93: 165–6P

Does smoking increase plasma Urotensin II concentrations? S. J. Gold, J. P. Williams, E. E. F. Helm*, J. Sadler*, W. Song*, J. P. Thompson, L. L. Ng* and D. G. Lambert University Department of Cardiovascular Sciences, Clinical Division of Anaesthesia, Critical Care and Pain Management, Leicester Royal Infirmary, Leicester LE1 5WW, UK

Urotensin II (UII) is a peptide hormone originally isolated from the Goby fish. It is similar in structure to the mammalian hormone somatostatin and has been isolated in humans.1–3 Human UII acts on the UT receptor and is the most potent arterial vasoconstrictor identified to date.1–3 There are few data relating its effects in humans, and the results of plasma measurements have been conflicting,2 perhaps because different measurement techniques have been

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used. The effects of smoking on plasma UII concentrations are unknown. The primary aim of our study was to demonstrate whether smoking had any effect on plasma UII concentrations in otherwise healthy volunteers. The secondary aim was to compare results obtained from assaying simultaneously using both Radioimmunoassay (RIA) and an Immunoluminometric Assay (ILMA). After LREC’s approval and with informed consent, blood was obtained from 20 healthy males and 20 healthy male smokers. A power calculation from previous data suggested that this would detect a difference of 20% between groups (a=0.05, b=0.1). Five millilitres of whole blood were collected into EDTA monovette containers containing 3 TIU of aprotinin and maintained on ice. These samples were centrifuged at 3000g for 20 min at 4
C, plasma was then removed and stored at 70
C. UII was extracted and analysed using a commercial RIA (Phoenix)4 and an ‘in house’ ILMA5 with minimum sensitivities of 1 pg ml1 and 0.42 pg ml1 respectively. Data were analysed by Kruskall-Wallis test. Controls and smokers were well matched for height, weight and age. The results from the RIA suggest that smokers have higher plasma UII concentrations; this difference is greater if a cigarette has been smoked in the preceding 10 min. Results from the ILMA do not support this finding (Table 3). The RIA results suggest that cigarette smoking may increase plasma UII concentrations. The RIA and ILMA assays for UII yielded conflicting results, the reasons for which are unclear but most likely result from the differing antibodies that form the basis of the assays.

suggests that cilial distribution may be uneven along the trachea2 and this represents a possible source of error in laboratory investigations. Low power Scanning Electron Microscope (SEM) imaging (·500) allows inclusion of high and low density areas, however, higher powered study (·550+) risks bias from this irregular cilial coverage. We have systematically evaluated cilial distribution to better understand its implications for this type of research. Twenty rat tracheae (adult female Wistars) were imaged using SEM and image analysis software applied to determine zones with cilial density 50–100% of the maximum density on the specimen (Fig. 7). The bands are larger at the superior region and are not associated with cartilage, when carrying out investigations into the effect of agents it might be best to discard this initial portion comprising the first two cartilages. Regular bands of high and low cilial density, mean 288 (SD 19) and 162 (7) mm respectively, alternate along the trachea with low density bands adjacent to underlying cartilage.

Keywords: complications, smoking; hormones, urotensin II

Organ culture of human bronchial explants

Acknowledgement: The authors acknowledge Peninsula Medical School & Plymouth Hospitals NHS Trust. Keywords: cilia distribution; rat, trachea

References 1 Kay RJ, Moate RM, Bray I, et al. Anesthesiology 2002; 97: 275–7 2 Oliveira MJ, Pereira AS, Guimaraes L, et al. Lung 2003; 181: 275–82

H. V. Rankin*, R. M. Moate*, A. J. Moody*, P. MacNaughton*, J. Rahamim*, M. E. F. Smith* and J. R. Sneyd

References 1 2 3 4

Douglas SA, Ohlstein HA. Trends Cardiovasc Med 2000; 10: 229–37 Douglas SA. Curr Opin Pharmacol 2003; 3: 159–67 Douglas SA, Dhanak D, Johns DG. Trends Pharmacol Sci 2004; 25: 76–85 Thompson JP, Watt P, Sanghavi S, Strupish JW, Lambert DG. Anesth Analg 2003; 97: 1501–3 5 Ng LL, Loke I, O’Brien RJ, Squire IB, Davies JE. Circulation 2002; 106: 2877–80

Electron Microscopy Centre, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

We have previously described the effect of altered oxygen fractions on cultured rat tracheal rings.1 Although rat trachea affords a reliable tissue source for microscopic examination the relevance of subsequent findings to man is always uncertain. We have explored tissue culture of explanted human bronchial tissue as an alternative material for experimental use. Lobectomy specimens (n=3 adults) resected during surgery for pulmonary malignancy were dissected and a portion of lobar bronchus distal to the resection margin (retained for histology) was removed, subdivided into segments and cultured (21% oxygen, 5% carbon dioxide, balance nitrogen at 37
C, DMEM) using individual wells and maintaining an air interface to the ciliated surface of each segment. Segments were removed for scanning electron

Uneven cilial distribution in rat trachea H. V. Rankin*, R. M. Moate*, A. J. Moody*, P. MacNaughton* and J. R. Sneyd Electron Microscopy Centre, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

Increased oxygen fraction reduces cilial abundance in incubated segments of rat trachea.1 Microscopic examination of rat trachea

1000 Denuded area Ciliary bands

Width (µm)

800 600 400 200 0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

Cartilage number from larynx (0) to bifurcation (17) Fig 7 Width of cilia bands mapped out down one representative trachea.

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14

15

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microscopy on alternate days and cilial coverage quantified using image analysis (Fig. 8). Culture of segmented human bronchus from surgical resection specimens is feasible and may offer an alternative model to rat derived tissue. Acknowledgements: The authors acknowledge Peninsula Medical School & Plymouth Hospitals NHS Trust. Keywords: bronchus, organ culture

Reference 1 Kay RJ, Moate RM, Bray I, et al. Anesthesiology 2002; 97: 275–7

Evaluation of flow direction within a conventional circle breathing system

Keywords: equipment, breathing systems, circle

M. P. Crowley and T. C. Smith Department of Anaesthesia, Alexandra Hospital, Redditch B97 5QJ, UK

References

The increasing popularity of the circle system within the UK, driven by economic and environmental factors, is generating interest in computer modelling to find the route to greater efficiency.1 2 A number of computer simulations exist for volatile agents but they fail to accurately mimic clinical changes in circle gas and vapour concentrations, particularly in the early phases of anaesthesia.3 The widely held understanding of the circle (which is used in these models) depends on the inclusion of two unidirectional valves placed one each in the inspiratory and expiratory limbs to ensure that flow only occurs in one direction around the circuit. This abstract describes the testing of our hypothesis that flow can occur in both directions in the circle according to respiratory phase. A conventional circle system was set up as in the diagram (Fig. 9). Two Wright’s respirometers were incorporated; one in the inspiratory limb to measure the volume in the forward direction, the second in the distal expiratory limb to measure volume in the reverse 100 80 60 %

Patient 3 Patient 2 Patient 1

40 20 ∗

0 0

1

2

3

4

6

8

direction. A fresh gas flow (FGF) of 5 litre min1 of 100% oxygen was delivered to the circle. A 2 litre bag attached to the patient connector was ventilated using a Penlon bag-in-bottle driven by a Penlon Nuffield 200 ventilator using the following settings: flow rate 0.25 litre s1, inspiratory time 2 s, expiratory time 4 s. Following a period of stabilization, minute gas volumes for each respirometer were recorded at 1 min intervals for a total of 30 min. The average gas flows were 5.5 litre min1 inspiratory and 3.4 litre min1 distal expiratory. The reverse flow occurred entirely during the expiratory phase of the ventilator cycle after a short pause whilst the bag-in-bottle refilled. The presence of reverse flow within the circle is confirmed for this configuration. This may explain the discrepancy of the computer simulations. Further study is planned to quantify the effects of different FGFs, ventilator settings and spontaneous breathing with this and other configurations.

10 12 14 16

Number of days in culture Fig 8 Cilial coverage (%) in incubated explanted human bronchial segments. *Segment incorrectly mounted during electron microscopy processing.

1 Lockward GG., White DC. Br J Anaesth 2001; 87: 559–63 2 Smith I. Abstract, ALFA London, 2003 3 Philip JP. Gasman Simulation User Manual. Med man Simulations Inc., 2002; A-5

Measurement of change in illumination provided by laryngoscope blades under tension V. Varadarajan*, M. A. Hampson*, A. R. Wilkes and J. E. Hall Department of Anaesthetics and Intensive Care Medicine, Wales College of Medicine, Heath Park, Cardiff CF14 4XN, UK

The concern about the risk of cross-infection has led to the increasing use of disposable laryngoscope blades in anaesthesia. The performance of disposable blades, in particular the illumination provided by the blades, varies widely.1 The International Organisation for Standardization (ISO) proposed a draft to measure the movement of the field of illumination provided by laryngoscope blades when under tension.2 This has been measured and it was found that most blades did not comply with the requirement in the draft standard.3 The aim of this study was to measure if the application of tension to laryngoscope blades caused a change in the level of illumination. Twelve adult (Mac 3) and 11 paediatric (Miller 1) blades were assessed. A standard ‘green spec’ handle with rechargeable batteries (Heine XP, Albert Waeschle, Creekmoor, UK) was used for the fibreoptic blades and a second handle was used for the standard bulbs (Flexicare Medical, Mountain Ash, UK); the Vital View blades required their own handle (Vital Signs, Littlehampton, UK). A force of 58 N was applied 10 mm from the tip of each blade. This force is well within the range anaesthetists use during intubation and is also the value in the ISO draft for measuring the shift in illumination.2 Illumination was measured using a lux meter 10 mm from the tip of the blade (Testo 545, Testo Ltd, Alton, UK).

Diagram of the test circle layout FGF Wright's respirometer indicating direction of flow measurement

2 litre ‘lung’ Soda lime

One-way valve APL valve

Fig 9 Test circle layout.

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Table 4 Laryngoscope blades and illumination. *Fibreoptic, ybulb in blade Adult blades

Illumination (lux)

Truphatek Lite blade slimy Lite blade IIy Vital signs Straight* Curved* Proact Metal max 90y Metal max 100y Metal max GS90* Penlon Crystal* Ru¨sch Bioblad* Intersurgical* Heine XP* Timesco Europay

Paediatric blades

No force

With force

2150 3659

1912 3218

263 507

170 470

510 498 3673 4743 2159 504 2240 514

483 490 3856 4977 2213 579 2368 580

Illumination (lux)

Proact Metal max 90y Metal max 100y Metal max GS90* Timesco Europay Heine XP* Penlon Crystal* Intersurgical* Timesco Freeway* Callisto* Truphatek Green lite* Vital Signs*

Effective width (°)

25

No force

With force

670 1010 2906 1240 3253 1994 2949

649 1105 2652 1088 2963 2332 3575

339 987

452 1028

2394 357

2485 438

Mac 2 Mac 4

20 15 10 5 0 5

6

7

8

9

10

11

12

13

14

15

Depth of insertion (cm) Fig 10 The continuous line represents the EIT+MIT for Mac2 and 4 when XT/IT=66%. The vertical bars are for XT/IT at 55% (lower) and 75% at each depth of insertion.

The illumination provided by the blades varied widely (Table 4). Application of force did not cause a major decrease in illumination and in fact some of the blades actually showed an increase in the level of illumination when under tension. Keywords: equipment, laryngoscope; laryngoscope, illumination

References 1 Wilkes AR, Hampson MA, Mecklenburgh JS, Hall JE. Anaesthesia 2005; 60: 303–4 2 ISO TC 121/SC 2/N778. Available from www.iso.org 3 Varadarajan V, Wilkes AR, Hall JE, Hampson MA. Eur J Anaesthesiol 2005; 22: 67

Effective angular blade width for Macintosh blades sizes 2 and 4 at various XT/IT ratios D. Sethuraman* and P. Charters University Hospital Aintree, Liverpool, UK

Since the original theoretical analysis of laryngoscope blade shape was published,1 others have used the method more or less unaltered for further blade comparisons.2 3 Limitations of this method included the small numbers on which the expected XT/IT ratio (66%) was based and no allowance for expected variation. Recent data from a study of lateral X-rays of patients known to be normal intubations but not undergoing direct laryngoscopy4 in

fact gave XT/IT ratio of 57% (–5.5%). For this study, we compared XT/IT ratios of 55, 66 and 75% for Mac 2 and Mac 4 blades. Effective angular blade width is the sum of angles MIT, the deflecting angle, and EIT, the angle reflecting forward space. Only MIT changes with the XT/IT ratio. Blade shapes were accurately reproduced from scaled digital scans. A CAD program [Total CAD, IMSI (UK) Ltd, Middlesex] was used to measure the angles. The combined (MIT+EIT) score of Mac 4 and Mac 2 blades give better results at an XT/IT ratio of 55%. The Mac 4 has the best possible effective blade width with a score of 0 for depth of insertions between 7 and 15 cm. Even with an XT/IT ratio of 75%, it still performs well with a combined score of <5
for depths of insertion between 7 and 13 cm. As expected, the Mac 2 is less predictable (Fig. 10). When the laryngoscope blade is positioned in the valleculum, we have previously suggested that space for tongue displacement would be greatest when the mouth is relatively closed and this equates to higher XT/IT ratios. Careful clinical studies are needed to determine the trade-off between space for tongue displacement vs small effective blade widths (here associated with small XT/IT ratios). These results suggest that while it may be possible to design blades with an effective angular blade width of zero, the Mac 4 is very close to this ideal anyway. Keywords: equipment, laryngoscopes

References 1 Marks RRD, Hancock R, Charters P. Can J Anaesth 1993; 40: 262–70 2 Friesen JHP. Can J Anaesth 2002; 49: 95A. Available from http://members. shaw.ca/friesenscope

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3 Yardeni IZ, Gefen A, Smolyarenko V, Zeidel A, Beilin B. Acta Anaesthesiol Scand 2002; 46: 1003–9 4 Cherian A, Groom P, Charters P. Br J Anaesth 2005; 95: 570P

Emergency Care Simulator for training medical students to identify the critically ill A. K. Rajasekaran*, M. Pugh*, M. Pimblett* and A. S. Laurence Department of Anaesthetics, Royal Preston Hospital, Preston, UK

Assessment of different methods of ventilation via needle cricothyroidotomy using a laryngo-tracheal lung model N. J. Flint1*, W. C. Russell1* and J. P. Thompson2 1 Department of Anaesthesia and 2University Department of Cardiovascular Sciences, Clinical Division of Anaesthesia, Critical Care and Pain Management, Leicester Royal Infirmary, Leicester LE1 5WW, UK

Needle or cannula cricothyroidotomy is an accepted method for enabling ventilation in emergency life-threatening airway difficulties. Several different methods have been described to achieve ventilation in this setting. The Difficult Airway Society recommends using a high-pressure source1 (e.g., the Manujet, VBM, Germany); life support teaching (e.g., ATLS) suggests using standard oxygen tubing with a Y-connector for control of ventilation. A specific flow modulator device is also commercially available (ENK, Cook, Denmark). Alternatively, some texts suggest that an anaesthetic circuit, a self-inflating bag or the oxygen flush of an anaesthetic machine may be used.2 The aim of this study was to assess the minute ventilation achieved with these methods via a needle cricothyroidotomy, using a bench-top laryngo-tracheal lung model3 with delivery via cannulae of different sizes. Delivered minute ventilation was measured for each method using a rate of 20 min1 and an I:E ratio of 1:2 via 13, 14, 16 and 20 G cannulae. Each measurement was repeated six times. There was a large variation in delivered minute volumes between different methods (Table 5). The Manujet reliably achieved >10 litre min1 ventilation with all but the smallest cannula. The oxygen flush was only able to deliver minute ventilation of >3 litre min1 using an unobstructed cannula of 14 G or larger. The Ambu bag failed to deliver any measurable ventilation other than with the 13 G specialist cannula. If extrapolated to the clinical setting, these results suggest that low-pressure systems are unlikely to provide adequate ventilation via a cannula cricothyroidotomy and that a high-pressure system should be used. Keywords: breathing, positive pressure ventilation; technique, cricothyroidotomy

Sim-Man attempts to simulate an actual patient to assist the training of health professionals. However, its use for medical student training is less common. We undertook a cross-over trial with third year medical students in Lancashire Teaching Hospitals NHS Trust. Participation was voluntary with signed consent, and had to fit in with other components of the curriculum. After an initial introduction and recruitment, all participants were given a 1 h (including discussion) lecture on early warning score (EWS) assessment, its uses and application. A week later, participants were randomly divided into two groups; Group LS were given a reinforcement lecture while group SL were given a group teaching session using the Laerdel Sim-Man. A further week later, all subjects had individual assessments on Sim-Man of a critical patient scenario. Efforts were made to avoid contact between those who had completed the assessment with those yet to be assessed, and was completed in one long session. One further week later, the groups crossed over. Those originally given a lecture (LS) had a group Sim-Man session, while group SL had the 1 h reinforcement lecture. Finally, after another week, all were given another individual assessment on Sim-Man, using a different critical patient scenario. Both assessment scenarios involved assessing knowledge (by identical eight-question MCQ), skill assessed on Sim-Man based on time to assess the correct EWS. Finally, a self-assessment questionnaire was completed before and after the reinforcing lecture and Sim-Man training session (four times for each subject) asking a 5 point score (0–4) for confidence, clinical skill and knowledge. The year comprised 26 medical students, and 17 were willing and available to complete our assessment. MCQ scores were similar at both assessment times for both groups. The time to arrive at a correct EWS on the Sim-Man improved at the 2nd session for both groups. The self-assessment questionnaire scores also improved after each session and were better after the 2nd session but this is not clear from the median score (Table 6). There was a significant improvement in time to EWS at the 2nd session in both groups. Because of the small number of subjects, statistics is questionable between groups. We cannot show which order of teaching is better, but it is clear that repetition did improve times to correct EWS diagnosis. A larger group of subjects would give a better idea, but the assessments can be very tiring on the authors and logistics of a larger group make assessments on the same day extremely difficult. Keywords: equipment, simulators; training, medical students

References 1 Henderson JJ, Popat MT, Latto IP, et al. Anaesthesia 2004; 59: 675–94 2 Benumof JL. Airway Management. Principles and Practice. St. Louis: Mosby, 1996; 458–73 3 Ng A, Harvey N, Russell W, Thompson JP. Anesth Analg 2002; 95: 764–9

Table 6 Time to correct EWS and questionnaire scores. **P<0.01; *P<0.02 (Wilcoxon paired rank, compared with 1st session) Table 5 Minute ventilation for each method in litre min1. Values are mean (95% CI) Method

20 G

16 G

14 G

13 G

Manujet Three way tap ENK O2 Flush Ambu bag

2.1 2.9 1.1 0.0 0.0

10.3 3.7 2.0 0.7 0.0

12.6 5.3 3.1 3.1 0.0

15.7 6.0 6.1 3.9 2.1

(0.1) (0.2) (0.3) (0.0) (0.0)

(0.2) (0.4) (0.3) (0.0) (0.0)

(0.2) (0.3) (0.4) (0.1) (0.0)

(0.2) (0.8) (0.7) (0.1) (0.3)

Number of subjects Time to EWS 1st session (min) Pre questionnaire; median (range) Post questionnaire; median (range) Time to EWS 2nd session (min) Pre questionnaire; median (range) Post questionnaire; median (range)

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Lecture. . . Sim-Man

Sim-Man. . . Lecture

8 6.2 (4.3–8.1)** 2 (1–2) 2 (2–3) 3.7 (2.5–5.2)** 2 (2) 2.5 (2–3)

9 5.5 (3.3–8)* 1 (1–2) 3 (2–3) 4.1 (2.5–5.1)* 2 (1–3) 3 (2–4)

Proceedings of the Anaesthetic Research Society Meeting

Fisher’s exact test—a clinician’s perspective

Effect of carotid endarterectomy on early changes in cerebrovascular reactivity in patients with unilateral significant carotid stenosis

R. S. Cormack Northwick Park Hospital, Harrow HA1 3UJ, UK

Decisions based on comparing two proportions are often needed in Medicine; a viable reference standard is essential. Fisher never explained how to find the second tail in his test and text-book suggestions are incoherent.1 2 A method is proposed which uses the strategy of science, not pure mathematics.3 How likely is it that this difference, 4/35 vs 0/36, could arise by chance? To answer this a pack of 71 cards (4 red, the rest black) is shuffled and dealt into packs of 35 and 36—then the distribution of red cards is to be noted. Repeating this many times measures the probabilities of the five possible outcomes (Fig. 11). In computer simulations these ‘experimental p’ values verify predictions from Fisher’s equation, but problems arise when finding the total P. Irwin4 proposed that p for the observed case should be added to any p that is ‘as small, or smaller’. But consider these three cases, in which the 2nd sample is 34, 35 and 36, all other data as above; total P for the three cases is then 0.11, 0.11 and 0.05. This absurdity is avoided by using experimental p values, not from a vast number of trials, but from averaged results of a fairly small number; total P is then 0.123, 0.116, and 0.110, avoiding the discontinuity. This modified Irwin method is completely exact in the way that Fisher intended, that is, unlike chi-squared, it uses no approximations, consequently it predicts exactly results with the model defined, easily verified by computer simulations. Some distinguished statisticians support the modified Irwin method and the software is available free to ARS members. Keywords: statistics, Fisher’s exact test

References 1 2 3 4

Cormack RS. J R Stat Soc Ser A 1984; 147: 455 Cormack RS. JAMA 1986; 262: 3129 Cormack RS. Metron 1986; 44: 44–30 Irwin JO. Metron 1935; 12: 83–94

0.4

4/35 v. 0/36

Probability

0.3

1 Academic Unit of Anaesthesia and 2Vascular Surgical Unit, The General Infirmary at Leeds, Great George Street, Leeds LS1 3EX, UK

Impaired cerebrovascular reactivity (CVR) in patients with significant carotid disease is attributed to chronic post-stenotic hypoperfusion producing maximal dilatation of cerebral resistance vessels.1 CVR to carbon dioxide has been shown to be improved between 3 days and 6 months following carotid endarterectomy (CEA) in patients with a significant unilateral internal carotid stenosis.2 3 However, it is not known if this improvement takes place immediately after CEA. The time course of recovery of CVR is of practical interest to the anaesthetist as blood pressure lability in the presence of impaired CVR may be associated with cerebral hypo- or hyperperfusion. CVR to carbon dioxide was studied before surgery and 1 day after operation in patients undergoing CEA for symptomatic unilateral disease. Middle cerebral artery flow velocity (MCAv) was measured on the side of the surgery using transcranial Doppler (MultiDop X4, TCD-8.01, DWL Elektronische System GmbH, Singen, Germany). End-tidal carbon dioxide (E0 CO2) was determined using an infrared analyser (model Binos 1, Leybold-Haraeus Limited, Koln, Germany) and measured at baseline (normoventilation) and then with an elevated E0 CO2, achieved with the patient breathing through a system that included a length of dead-space tubing. CVR was expressed as the percentage change in MCAv divided by the change in E0 CO2 (kPa). Statistical analysis was performed using Excel 2002 (Microsoft Corp.) and Stata SE8 (Stata Corp., College Station, Texas). Twelve patients (10 male), average age 73.4 yr (SD 6.5 yr), were studied. There were no significant postoperative complications. The average preoperative CVR was 17.8% kPa1 (SD 8.8% kPa1) and the average postoperative CVR 19.1% kPa1 (SD 10.1% kPa1). The average change in CVR between the pre- and postoperative tests was 1.3% kPa1 (SD 9.8% kPa1). This change was not statistically significant (P=0.39). We did not show a significant improvement in CVR 1 day after surgery in patients with unilateral disease. This may reflect persistent impairment immediately following surgery or preserved reactivity in this group of patients. CVR is a complex issue that warrants further study. Acknowledgement: G.A.C. was funded by a Research Fellowship from the Association of Anaesthetists of Great Britain and Ireland.

0.2

0.1

G. A. Chapman1, D. Dellagrammaticas2, M. J. Gough2 and S. J. Howell1

Observed case

Keywords: brain, circulation, regulation; surgery, carotid end artectomy

0 4

3

2

1

0

Number of red cards in 1st sample Fig 11 Probability of five different outcomes.

References 1 Russell DA, Gough MJ. Eur J Vasc Endovasc Surg 2004; 28: 115–23 2 Reinhard M, Roth M, Mu¨ller T, et al. Stroke 2004; 35: 1381–7 3 Silvestrini M, Troisi E, Matteis M, et al. Stroke; 1996; 27: 1970–3

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Factors affecting indices of cerebral autoregulation: a computational modelling study

Table 7 Mean (range) colony forming units

I. K. Moppett and J. G. Hardman

Test period

No mask no gum

Mask no gum

No mask gum

Mask gum

Mean (range)

5 (0–20)

5 (0–31)

11 (0–100)

4 (0–33)

University Department of Anaesthesia, Queen’s Medical Centre, Nottingham, UK

Impairment of cerebral autoregulation may be detrimental after traumatic brain injury, yet formal testing using induced hypertension and hypotension is difficult practically and potentially detrimental. Several groups have proposed indices of autoregulatory ability based on the relationship between slow, spontaneous fluctuations in blood pressure and various surrogate measures of cerebral perfusion: (i) mean reactivity (Mx) [mean arterial pressure (MAP) and middle cerebral artery flow velocity (MCAFV)]; (ii) pressure reactivity (PRx) [MAP and intracranial pressure (ICP)]; (iii) oxygen reactivity (ORx) [cerebral perfusion pressure and brain tissue oxygenation (PbrO2)].1–3 Because of the invasive nature of the measurements required, there are few data describing how these tests should perform in the presence of physiological changes in normal and pathological states. The development of an integrated computational model of cerebral physiology allows us to study these factors in detail. In a perfectly autoregulating system an inverse relationship between MAP and ICP would be expected (PRx: 1) and no correlation between perfusion pressure and MCAFV or PbrO2 (Mx, ORx: 0). The brain tissue oxygen index (bPtiO2) is the gradient of the ORx relationship and is expected to become larger when autoregulation is disturbed. The Nottingham Cerebral Simulator (NCS) is a validated, polycompartmental, iterative model of cerebral blood flow and gas exchange. Two cerebral hemispheres are contained within a relatively non-compliant cranial space surrounded by pressurized cerebrospinal fluid. Arteriolar resistance is variable, responding to changes in MAP, Paco2 and Sao2 . The effects on the indices of changes in Paco2 (4.3–7.3 kPa), Sao2 (0.8–0.98), MAP (50– 150 mm Hg), CSF dynamics and autoregulatory strength were assessed. Each of the indices was calculated as the Pearson correlation coefficient between the two relevant variables: each of these variables was represented by 40 5 s averages. MAP was given random periodic fluctuations around its baseline, with amplitude 3 mm Hg, lasting 45 s. All of the indices were found to be dependent upon MAP with larger values when MAP is beyond the autoregulatory plateau. Hypercapnia caused a graded increase in each of the indices, which was greater than the effect seen with vasodilation caused by hypoxaemia. Changes in intracranial compliance and CSF production had little effect on the indices. Reduction in autoregulatory strength caused each of the indices to increase, and this was partially reversible through hypocapnia. The results from the NCS modelling suggest that there are no marked differences in the behaviour of each of these indices in response to clinically relevant changes in physiological variables. Given that most of the clinical data for these indices have come from patients with high ICPs it is useful to know that CSF dynamics per se do not seem to affect the behaviour of the indices. Keywords: brain, circulation, regulation; technique, computer simulation

References 1 Czosnyka M, Smielewski P, Kirkpatrick P, et al. Neurosurgery 1997; 41: 11–17 2 Lang EW, Lagopoulos J, Griffith J, et al. J Neurol Neurosurg Psychiatr 2003; 74: 1053–9 3 Soehle M, Jaeger M, Meixensberger J. Neurol Res 2003; 25: 411–17

Facemasks reduce bacterial dispersal while chewing gum K. A. Demaine*, M. R. J. Parker* and S. D. Monaghan* Department of Anaesthesia, Manchester Royal Infirmary, Oxford Road, Manchester M13 9WL, UK

It is common to see operating theatre personnel chewing gum during an operating list. The effects of this practice on bacterial dispersion and wound infection rates are currently unknown. Potential sources of bacterial origin could include oral and nasal cavities or skin shedding. The objective of this study was to determine whether chewing gum increases bacterial shedding and to ascertain the effect of a surgical face mask on bacterial dispersion. After gaining approval and informed consent from the ethics committee, 25 healthy volunteers were seated with a blood agar plate 30 cm below and 10 cm in front of the mouth. None had a history of current oral pathology or were taking steroids or antibiotics. Bacterial dispersion was assessed at four consecutive discrete time points: (1) period of no face mask and no gum, (2) period with a face mask and no gum, (3) period of chewing gum in the absence of a face mask, and (4) period of chewing gum with a face mask. Each test period lasted 20 min. For environmental control, a plate was placed 50 cm from the test plate and remained in place throughout the four test periods. Plates were incubated at 37
C for 24 h. Establishment of bacterial colonies on the agar plate and the respective colony counts were used as end points of the study. The Mann-Whitney U-test was used to compare groups. Chewing gum in the absence of a mask was associated with a significant increase in bacterial colony counts in some subjects (12/21). This bacterial dispersion was significantly reduced in subjects who chewed gum while wearing a face mask (P=0.037) (Table 7). We conclude that chewing gum without wearing a face mask was associated with an increased risk of bacterial dispersion and potential wound contamination. Acknowledgement: The authors acknowledge a grant from the Manchester Royal Infirmary Anaesthetic Endowment Fund. Keywords: complication, infection; intervention, chewing gum

References 1 2 3 4

McLure HA, Talboys CA, Yentis SM, et al. Anaesthesia 1998; 53: 624–6 Schweizer R. Lancet 1976; 2: 1129–30 Dawes C, Tsang RW, SuelzleT. Arch Oral Biol 2001; 46: 625–32 Macpherson LM, Dawes C. J Dent Res 1993; 72: 1391–7

An observational study assessing the efficacy of HME (heat and moisture exchanger) filters at preventing contamination with biological material on their internal surface on the machine side T. E. Sheraton*, C. Modestini*, J. E. Hall and A. R. Wilkes Department of Anaesthetics and Intensive Care Medicine, Wales College of Medicine, Heath Park, Cardiff CF14 4XN, UK

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Proceedings of the Anaesthetic Research Society Meeting

Table 8 Filter type and effectiveness in preventing bacterial contamination Filter type

BB25M BB22/15M 335/55427 (Hygroboy) 355/5427 (Hygrobaby)

Number tested

82 12 9 5

Median [min–max] bioluminescence level (RLU)

Number with RLU>30 (machine side)

Patient side

Machine side

21 (4–14 748) 18 (4–1234) 153 (15–5578)

7 (2–5870) 18 (4–71) 8 (2–377)

14 4 1

6 (5–65)

1

221 (13–517)

Breathing system filters are intended to be used to prevent crossinfection in anaesthesia. However, there is a lack of information on whether filters prevent contamination of the breathing system by the patient. The contamination on 108 used filters of four different types obtained from operating theatres was measured. Two types were pleated hydrophobic (BB22/15M and BB25M, Pall Medical, Portsmouth, UK) used for adult patients and two were electrostatic (Hygroboy and Hygrobaby, Tyco Healthcare, Gosport, UK) used for paediatric patients. The filters were swabbed over their internal surfaces on the side of both patient and machine. The swabs were assessed for contaminating soil with the use of adenosine triphosphate bioluminescence. The assays were performed using the Biotrace Clean-Trace system (Biotrace Plc, Bridgend, UK), according to the manufacturer’s instructions and swabbing guidelines.1 Results were expressed in Relative Light Units (RLUs), which is proportional to the amount of organic soiling present on the swab. The filters were deemed to have failed if the RLU value on the machine side was >50. This value was based on data from unused filters and extrapolation of cut-off levels taken from the food industry. The ability to re-use breathing systems is based on the assumption that breathing filters prevent infectious material crossing from the patient to the breathing system. Current standard testing of filters2 has no set ‘pass’ level and is performed in the laboratory setting. Contamination appeared to be present on the machine side of the filters (Table 8). Bioluminescence may be used in the clinical setting to elucidate factors that might increase the chance of crosscontamination between patients. Acknowledgement: The authors thank Biotrace for supplying the Biotrace Clean-Trace System. Keywords: complication, bacterial contamination; equipment, heat and moisture exchange filters

References 1 Davidson CA, Griffith CJ, Peters AC, Fielding LM. Luminescence 1999; 14: 33–8 2 Wilkes T. Business Briefings: Medical Device Manufacturing and Technology, 2004; 70–1

general anaesthesia (GA) in this obstetric unit has decreased from 87.8% of cases in 1983 to 17.6% in 2004. However, some patients still need a GA for fetal distress, contraindication to or failure of regional anaesthesia (RA), and failure to consent to RA. Obstetric patients can be difficult to intubate; the most recent Confidential Enquiry into Maternal and Child Health (2000–2) has again emphasized that difficulties with intubation continue to cause anaesthetic deaths.1 Maternity anaesthetic records, where failed intubation had occurred, have been kept since 1982 by J.S., as the incidents occurred. We reviewed the traceable records of all these cases. Information was recorded on a purpose-designed form, and results were subjected to non-parametric analysis (x2-test). P<0.05 was taken as statistically significant. For more than 22 yr, there have been 28 cases of failure to intubate in 8132 GAs (0.34%). The annual Caesarean section (CS) rate increased from 8% in 1983 to 21.5% in 2004. For comparison, the 22 yr have been divided into four periods of 5, 6, 5 and 6 yr2 3 (some of the data for 1992 are missing). The overall incidence of failed intubation was 1 in 290 GAs. The incidence of failed intubation is decreasing: 1:197 in 1983–7; 1:350 in 1988–93; 1:336 in 1994–8; and 1: 545 in 1999–2004. Out of the 28 cases of failure to intubate, we were able to identify the case notes of 21 patients. The mean (range) age of the patients was 32.1 (24–42) yr and BMI was 33 (23.9–56) kg m2. Seventeen patients underwent non-elective procedures: 12 CS, 4 manual removal of placenta, and 1 forceps application. Six of the patients who had a GA had had an epidural: one epidural had failed, one patient requested a GA, and the reason for GA is not recorded in the other four. Predisposing factors to difficult intubation were recorded in five cases. Three of the records noted Grade 2 laryngoscopy, eleven Grade 3, one Grade 4, and six had made no such record. The failed intubation protocol was followed in all but one case. Failure to intubate was twice as common during out of hours work. It was four times more likely to occur during an emergency. In 16 of the cases, the first anaesthetist was a registrar; in three cases a senior registrar; and an associate specialist or consultant in one case each. The second anaesthetist who came to help was a consultant in 50% of cases. All patients were woken up after the failed intubation and no maternal mortality was recorded. Seven patients were re-anaesthetized within 48 h by rapid sequence induction and the airway was intubated. Maternal morbidity (? aspiration) was reported in two patients: both had a good outcome. Sixteen patients have since had another GA and one of them had a repeat failed intubation 2 yr later. The annual incidence of failed intubation has decreased as the usage of RA has risen. The reason for this is not clear. The trainees of 2004 have less exposure to GA for CS than the trainees of 1982. Keywords: anaesthesia, obstetric; complications, intubation failure

References 1 Confidential Enquiry into Maternal and Child Health. London: RCOG Press, 2005 2 Rahman K, Jenkins JG. Anaesthesia 2005; 60:168–71 3 Saravanakumar K, Cooper GM. Br J Anaesth 2005; 94: 690

Twenty-eight cases of failure to intubate in 8132 obstetric general anaesthetics for more than 22 yr S. S. Tripathi* and J. Sprigge* Arrowe Park District General Hospital, Wirral CH49 5PE, UK

Arrowe Park Hospital Maternity Unit opened in 1982 and serves a population of 350 000. For more than 22 yr, the use of

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