Castration of piglets: the analgesic effects of intratesticular and intrafunicular lidocaine injection

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1 Veterinary Anaesthesia and Analgesia, 2005, 32, 1 9 RESEARCH PAPER Castration of piglets: the analgesic effects of intratesticular and intrafunicular lidocaine injection Henning A Haga* DVM, PhD & Birgit Ranheim DVM, PhD *Department of Companion Animal Clinical Sciences, The Norwegian School of Veterinary Science, Oslo, Norway Department of Food Safety and Infection Biology, The Norwegian School of Veterinary Science, Oslo, Norway Correspondence: Henning A Haga, Department of Companion Animal Clinical Sciences, The Norwegian School of Veterinary Science, PO Box 8146 Dep, N-0033 Oslo, Norway. andreas.haga@veths.no Abstract Objective The aim of this study was to evaluate the analgesic effect of intratesticular and intrafunicular lidocaine for the surgical castration of piglets and to investigate the degree of nociception induced by lidocaine injection. Study design Prospective controlled experimental study. Animals Forty-seven male Norwegian landrace piglets with normal testicular anatomy, aged 22 (±2.6 SD) days and weighing 7.4 ± 1.4 kg. Materials and methods Anaesthesia was induced and maintained using halothane delivered in oxygen. End-tidal halothane was stabilized at 1.3% for 20 minutes before mean arterial blood pressure (MAP) pulse rate and electroencephalography (EEG) monitoring began. After 5 minutes of data collection, scrotal skin was desensitized with lidocaine before either an intrafunicular (IF) (n ¼ 15) or an intratesticular (IT) (n ¼ 16) lidocaine injection was made. Pigs in the control group (n ¼ 16) did not receive lidocaine. Ten minutes later, a scalpel and an emasculator were used to cut the funiculus spermaticus. The MAP, pulse rate and EEG were monitored continuously for 5 minutes after castration. Results During castration, MAP increased significantly, while pulse rate and EEG theta power fell significantly more in control, compared with the IT or IF groups. EEG alpha power fell more in the control group than in the IF group. No significant differences were found between the IF and IT groups. EEG, MAP and pulse rate responses to castration in the control group were significantly larger than the response to lidocaine injection. Conclusion/clinical relevance Injecting lidocaine into the funiculus spermaticus or into the testes is effective in reducing signs of nociception caused by castration. Lidocaine injection is less noxious than castration without local anaesthetic. Keywords castration, electroencephalography, local anaesthesia, nociception, pain, piglets. Introduction Piglet castration is routinely performed in several countries in an attempt to prevent boar-tainting of meat. It is traditionally carried out at a young age without anaesthesia. The procedure was previously regarded as relatively innocuous because of its brevity and the popularly held misconception that the underdeveloped nervous systems of neonatal mammals limits their nociceptive abilities. Recent reports have challenged this; neonatal mammals may have increased pain perception and be predisposed to developing hypersensitivity (Fitzgerald & Beggs 2001). The castration of piglets induces painrelated behaviour during (Weary et al. 1998; Horn et al. 1999; Taylor & Weary 2000; Taylor et al. 1

2 2001) and after (McGlone & Hellman 1988; McGlone et al. 1993; Hay et al. 2003) surgery. Under current Norwegian law, castration may only be performed by a veterinarian using an anaesthetic on piglets less than 28 days old. The benefits of anaesthetizing piglets for surgical castration are also receiving attention in other European countries, and both local anaesthesia and general anaesthesia induced by injection or inhalation have been evaluated (McGlone & Hellman 1988; White et al. 1995; Kohler et al. 1998; Nyborg et al. 2000; Baumann & Bilkei 2002). An anaesthetic technique suitable for routine piglet castration should be quickly performed, cost-effective, induce minimal stress and provide post-operative analgesia. Rapid complete recovery is also desirable: piglets may be crushed by the sow and are prone to hypothermia. The use of local anaesthetic fulfils most of these criteria. Several studies have shown that local anaesthetic administered into the substance of the testicle (IT) or into the funicular (IF) or the cavum vaginalis reduces nociception induced by castration (McGlone & Hellman 1988; White et al. 1995; Nyborg et al. 2000). To our knowledge, a comparison of the nociceptive responses to different routes of local anaesthetic administration or the subsequent antinociceptive effects has not been made. Nociceptive stimuli during general anaesthesia may induce changes in blood pressure, pulse rate or in the electroencephalogram (EEG) which may be used as indicators of nociception. The aim of this study was to use these indicators under halothane anaesthesia to evaluate the analgesic effect of IT or IF lidocaine injection during piglet castration. A second aim was to compare the nociception induced by IT lidocaine injection with injection into the funiculus spermaticus. Materials and methods Animals Ten pregnant Norwegian landrace sows were housed at the research facility and fed a commercial diet (Format edel norm; Felleskjøpet, Kløfta, Norway). Each piglet was given iron dextran by subcutaneous injection, (200 mg Fe) (Idofer, Boehringer Ingelheim, København, Denmark) within 3 days of farrowing. From 3 days of age, the piglets were offered a commercial grain diet (Format Kvikk; Felleskjøpet, Norway) and in litters from sows with agalactia, a commercial milk-replacer (Pluss melkeerstatning; Felleskjøpet, Norway) was also offered. Forty-seven male piglets, from 10 different litters, with a mean age of 22 ± 2.6 (SD) days and a mean body mass of 7.4 ± 1.4 kg were included in the study. All animals had normal testicular anatomy and were judged to be healthy after clinical examination. All piglets were destroyed under anaesthesia at the end of the study. The protocol was approved by the National Animal Research Authority. Anaesthesia Anaesthesia was induced with halothane (Halothane; Halocarbon Laboratories, River Edge, NJ, USA) delivered in oxygen and administered by mask. When anaesthesia was judged to be adequate for endotracheal intubation, lidocaine (Xylocain 100 mg ml )1 ; AstraZeneca, Oslo, Norway) was sprayed onto the larynx and the trachea was intubated. A paediatric circle anaesthesia system was immediately connected to the endotracheal tube and the pig was placed in left lateral recumbency on a hot water blanket before intermittent positive pressure ventilation was imposed. Instrumentation The pigs were connected to an anaesthetic monitor (AS/3, Datex Engström, Helsinki, Finland) for monitoring oxygen saturation, end-tidal CO 2 concentration, inspiratory and expiratory halothane concentration, rectal temperature and ECG. Gas was drawn to the monitor from the proximal end of the endotracheal tube. Lidocaine (0.3 ml SC) (Lidokain 10 mg ml )1 ; Nycomed Pharma AS, Asker, Norway) was administered in proximity to the saphenous artery, which was then exposed through a skin incision. A mm catheter (Venflon; Beckton Dickinson, Helsingborg, Sweden) was then introduced. Tubing filled with physiological saline was connected to the catheter and attached to an electronic transducer (PX-241; Edwards Lifesciences, Irvine, CA, USA) zeroed at the level of the thoracic inlet. To avoid biasing the results, the measured physiological variables were maintained within specified intervals. Rectal temperature was kept above 37.5 C while mean arterial pressure (MAP) was maintained above 50 mmhg by infusing Ringer s acetate and dextran (Macrodex; Pharmalink, Upplands Väsby, Sweden) solution. Dobutamine (Dobutamine 2.0 mg ml )1 ; Abbott Laboratories, Chicago, IL, USA) was infused using a syringe driver 2 Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9

3 (P6000/TIVA, IVAC, San Diego, CA, USA) at a rate that maintained MAP >50 mmhg. The skin at the EEG electrode location was shaved and defatted with diethyl ether before electrodes (Zipprep Aspect Medical Systems, Natick, MA, USA) were applied. To record a two-channel reference EEG, an electrode was placed 1 cm caudal to the lateral angle of the eye and 1 cm medial to the temporal line on each side of the head. A reference electrode was placed in the midline 2 cm caudal to the recording electrodes. A ground electrode was placed on the neck behind the left ear. Before recording began, the electrode impedance was checked and maintained below 2000 ohms at 16 Hz. The electrodes were connected to an EEG monitor (A-1000; Aspect Medical Systems, Newton, MA, USA) that was capable of calculating some of the Fourier-transformed variables that have been used to characterize anaesthesia (Rampil 1997). The monitor filters were set as follows high frequency filter: 50 Hz, 50/60 Hz filter: 50 Hz and low frequency filter: 0.25 Hz. All measurements of power are reported in db with respect to lv 2. The EEG data used were calculated as the mean of the recordings from both channels. Local anaesthesia and castration The pigs were given either no local anaesthetic, or lidocaine, 4 mg kg )1 (10 mg ml )1 lidocaine and 5 lg ml )1 adrenaline) (Xylocain-Adrenalin; Astra- Zeneca). Two-thirds of the injection volume of lidocaine was administered into the testis or funiculus spermaticus, and one-third was injected subcutaneously into the scrotum. These three different groups were termed: control; testis (IT) and funiculus (IF) respectively. A mm needle (Terumo, Leuven, Belgium) was used for the injections. Castration was performed 10 minutes after the lidocaine injection. Using a No. 22 scalpel blade, two sagittal incisions were made through the skin and subcutaneous structures down to the visceral layer of the vaginal tunic. The testicle was grasped and exposed without excessive force, and the funiculus spermaticus was clamped with an emasculator (Hauptner, Solingen, Germany) applied for 15 seconds. Recording of data The end-tidal halothane percentage was stabilized at 1.3% for 20 minutes prior to recording EEG, MAP and pulse rate. To avoid affecting the cardiovascular data, ventilation volume and frequency and dobutamine and fluid infusion rates were held stable for the 5 minutes before data recording, and throughout the procedure. Mean arterial pressure and pulse rate were both monitored through the pressure line and were manually noted every 30 seconds. EEG data were automatically stored every 5 seconds in a computer connected to the EEG monitor. Data recording started 5 minutes before lidocaine injection and continued for 5 minutes after castration. A schematic illustration of the study design is given in Fig. 1. Statistical analysis and randomization Treatment was block randomized and stratified with respect to litter. EEG variables subjected for analysis were: absolute delta power (delta), absolute theta power (theta), absolute alpha power (alpha), absolute beta power (beta), total power (totpow), the ratio of theta to delta power (theta/ delta), the ratio of alpha to delta power (alpha/ delta), the ratio of beta to delta power (beta/delta), spectral edge frequency 95% (SEF 95%) and median frequency (MED). A longer time period of cardiovascular data was analysed because the cardiovascular responses developed more slowly and lasted longer than the EEG response. The individual mean values of MAP and pulse rate were calculated for the following periods: 5 minutes prior to lidocaine injection; 5 minutes from the start of lidocaine injection; 5 minutes before castration; and 5 minutes from the start of castration. For the EEG variables the individual mean values for the following periods were calculated: during injection; during castration; and for time periods equivalent in length before these two events. For evaluation of the response to injection, the differences between the mean values calculated before injection and the mean values calculated from the beginning of injection were calculated. For evaluation of the response to castration the differences between the mean values immediately before castration and the mean values from the beginning of castration were calculated. All differences were tested for normal distribution using a Shapiro Wilks test and tested for being significantly different from zero using a Student s t-test or a Wilcoxon signed rank test where appropriate. The variables where significant responses were found were included in further analysis. The responses in these variables to castration in the three groups were Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9 3

4 End-tidal halothane% Variable 1.3% end tidal halothane Fluid and dobutamine infusion rates Recording of EEG, MAP and pulse rate Variable Constant infusion rate Recording Time (minutes) Events: Induction Injection Castration Euthanasia Figure 1 A diagram showing study design. Time is given at the x-axis and a bar illustrating end-tidal halothane percentage, infusion rates and data recording is plotted at the y-axis. compared using ANOVA or a Kruskal Wallis where appropriate. If a significant difference was found between groups, the groups were further compared using a Tukey Kramer honest significance difference test. The responses to castration in the control group were compared with the responses to lidocaine injection in the two other groups using an ANOVA or Kruskal Wallis test when appropriate. If a significant difference was found between groups, they were further compared using a Tukey Kramer honest significance difference test or a Bonferronicorrected Wilcoxon test when appropriate. The significance level was set to 5% and two-sided tests were used. Commercially available software (Microsoft Excel for Windows 2000, Redmond, WA, USA and Jump version 5.01, SAS Institute Inc., Cary, NC, USA) was used for data handling and statistical analysis. Results All results are reported as mean ± SD. The duration of lidocaine injection was 30 ± 9.1 seconds and the duration of surgery was 62.7 ± 6.3 seconds. For the control, testis and funiculus groups, the dobutamine infusion rates were 20.7 ± 19.9, 23.0 ± 22.5 and 15.0 ± 15.1 lg kg )1 minute )1 respectively. The Ringer s acetate infusion rates were 5.9 ± 3.5, 5.7 ± 3.6 and 5.4 ± 2.6 ml kg )1 hour )1 and the dextran infusion rates were 5.2 ± 4.0, 4.5 ± 2.7 and 4.3 ± 1.9 ml kg )1 hour )1. Values of mean, standard deviations, group sizes and results of the statistical comparison between groups are given in Tables 1 and 2. Where a significant difference was found between groups during castration, the mean value within each group is graphically depicted in Figs 2 5. The responses to castration and injection in the three groups are given in Tables 1 and 2. A significant decrease in pulse rate, delta, theta, alpha and totpow was encountered when castrating the control group, while MAP and MED increased. In the IT group, significant increases in MAP and decreases in delta and totpow were found during castration. In the IF group, a significant increase in MAP and a fall in theta were observed. During testicular injection, the pulse rate and MAP increased significantly. When injecting lidocaine into the funiculus spermaticus, the pulse rate fell significantly. When comparing responses to castration in the three groups, MAP (Fig. 2) increased significantly more in the control group than in the IT or IF groups (Table 1), pulse rate (Fig. 3) and theta (Fig. 4) fell significantly more in the control group than in the IT or IF group (Table 1) and alpha (Fig. 5) fell significantly more in the control group than in the IF group (Table 1). No significant differences were found between the IF or IT group. There were no significant differences found in delta, totpow or MED between the groups during castration. MAP, pulse rate, delta, theta, alpha and totpow changed significantly more in the control group during castration than in the IT or IF group during injection (Table 2). No significant differences in response to injection were found between the IT or IF groups and no significant differences between groups were found in MED. Discussion Castration without local anaesthesia induced a more pronounced nociceptive response than castration with lidocaine injection either into the testis or into the funiculus spermaticus, indicating that injecting lidocaine into these structures reduces pain. No significant difference could be found between the two groups in which lidocaine was used, so it was impossible to demonstrate differences in the analgesic effect of intratesticular versus 4 Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9

5 Table 1 Response to castration in control (no lidocaine administered) funiculus (IF) (lidocaine injected into the funiculus spermaticus) and testis (IT) (intratesticular lidocaine) groups Control castration Testis castration Funiculus castration p-values Pulse rate (beats minute )1 ) )5.5 (5.4) n ¼ 16 )0.71 (3.24) n ¼ 16* )0.14 (3.40) n ¼ 15* MAP (mmhg) 8.0 (4.8) n ¼ (3.13) n ¼ 16* 4.31 (4.36) n ¼ 15* Delta (db) )0.59 (1.10) n ¼ 15 )0.36 (0.57) n ¼ 14 )0.24 (0.66) n ¼ à Theta (db) )0.69 (0.51) n ¼ 15 )0.11 (0.41) n ¼ 14* )0.19 (0.32) n ¼ 15* Alpha (db) )0.94 (0.99) n ¼ 15 )0.47 (1.23) n ¼ 14 )0.02 (0.69) n ¼ 15* Totpow (db) )0.54 (0.95) n ¼ 15 )0.31 (0.45) n ¼ 14 )0.20 (0.48) n ¼ à MED (Hz) 0.17 (0.33) n ¼ 15 )0.009 (0.253) n ¼ 14 )0.003 (0.131) n ¼ à Mean ± SD are given for cardiovascular and EEG variables. The number of observations within each time period is given. *Denotes a significant difference (p < 0.05) from the control group. p-values for comparisons between all groups are given. Statistical tests used: ANOVA, àkruskal Wallis. MAP, mean arterial pressure. Table 2 Response to injection in the funiculus (lidocaine injected into the funiculus spermaticus) and testis (intratesticular lidocaine) group Testis injection Funiculus injection p-values Pulse rate 1.36 (2.12) n ¼ 16* )1.44 (2.56) n ¼ 15* (beats minute )1 ) MAP (mmhg) 1.56 (1.96) n ¼ 16* 0.45 (2.47) n ¼ 15* Delta (db) 0.15 (0.62) n ¼ 14* 0.28 (0.91) n ¼ 15* 0.008à Theta (db) )0.17 (0.43) n ¼ 14* (0.421) n ¼ 15* Alpha (db) )0.12 (0.52) n ¼ 14* )0.20 (0.68) n ¼ 15* Totpow (db) (0406) n ¼ 14* 0.20 (0.65) n ¼ 15* à MED (Hz) ) (0.313) n ¼ 14 )0.036 (0.17) n ¼ à The data for response to castration in the control group are given in Table 1. Mean ± SD are given for cardiovascular and EEG variables. The number of observations within each time period is given. *Denotes a significant difference from the control group. The p values for the comparison between all groups are given. Statistical tests used: ANOVA, àkruskal Wallis. MAP, mean arterial pressure. 75 Figure 2 Mean MAP (mmhg) before, during and after lidocaine injection and castration. The three curves illustrate the control group where no lidocaine was administered, the funiculus group where lidocaine was injected into the funiculus spermaticus and the testis group where intratesticular lidocaine was used. MAP (mmhg) Injection Castration Time (minutes) Control Funiculus Testis intrafunicular lidocaine administration. The EEG and cardiovascular responses to lidocaine injection were less than the response to castration without local anaesthesia, indicating that piglet castration without local anaesthesia is more painful than injecting lidocaine into the testis or the funiculus spermaticus. Both lidocaine injection and castration after lidocaine resulted in some response, indicating that castration is noxious, even after lidocaine injection. The MAC of halothane in pigs has been reported to be 0.91% (Tranquilli et al. 1983). The current study was performed at 1.3% end tidal Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9 5

6 Pulse rate (beats minute 1 ) Injection Castration Time (minutes) Testis Funiculus Control Figure 3 Mean pulse rate (beats minute )1 ) before, during and after lidocaine injection and castration. The three curves illustrate the control group where no lidocaine was administered, the funiculus group where lidocaine was injected into the funiculus spermaticus and the testis group where intratesticular lidocaine was used. Theta power (db) Injection Castration Time (minutes) Funiculus Testis Control Figure 4 Mean theta power (db) before, during and after lidocaine injection and castration. The three curves illustrate the control group where no lidocaine was administered, the funiculus group where lidocaine was injected into the funiculus spermaticus and the testis group where intratesticular lidocaine was used. Alpha power (db) Injection Castration Time (minutes) Testis Funiculus Control Figure 5 Mean alpha power (db) before, during and after lidocaine injection and castration. The three curves illustrate the control group where no lidocaine was administered, the funiculus group where lidocaine was injected into the funiculus spermaticus and the testis group where intratesticular lidocaine was used. halothane, which is equivalent to 1.4 MAC. Even at this depth of anaesthesia castration without local anaesthesia induced a marked nociceptive response, indicating that castrating unanaesthetized piglets probably causes severe pain. Manipulating and cutting the funiculus spermaticus has been identified as the most painful step during piglet castration (White et al. 1995; Taylor & Weary 2000) and so any technique advocated for this operation must effectively diminish the nociception induced by the handling and cutting of this structure. It may seem illogical to administer lidocaine distal to the area where an analgesic effect is desired, because lidocaine must distribute proximal to spermatic cord nerves to have an effect. Several studies have identified a beneficial effect of pre-castration intratesticular lidocaine (McGlone & Hellman 1988; Von Waldmann et al. 1994; Horn et al. 1999). Recently, an autoradiographic study using radioactive lidocaine demonstrated proximal drug distribution into the spermatic cord after 10 minutes (Ranheim et al. 2003). Intrafunicular local anaesthetic administration also has an effect upon castration-induced nociception (Nyborg et al. 6 Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9

7 2000). The present study supported these results even though identified nociceptive responses persisted after lidocaine had been used. This is probably caused by uneven distribution of lidocaine in the affected tissues resulting in incomplete sensory block. Ten minutes after intratesticular lidocaine injection, the concentration in the cremaster muscle is low (Ranheim et al. 2003) which may represent an area of incomplete sensory block. The scrotal ligament and the intra-abdominal part of the spermatic cord are other areas where sensory block may be incomplete. In a previous study, intratesticular lidocaine injection caused nocifensive responses (Von Waldmann et al. 1994) raising the possibility that the administration of anaesthesia may be more painful than the surgery itself. However, the findings of the current study do not support this: the response to lidocaine injection was less than the response to castration without local anaesthetic. Studies evaluating the analgesic effect of lidocaine administration for piglet castration have to date, used motor responses (Von Waldmann et al. 1994; Horn et al. 1999) highest energy frequency calls (HEF) (White et al. 1995) or post-operative behaviour (McGlone & Hellman 1988) to quantify effect, while another study used dorsal horn c-protein expression (Nyborg et al. 2000). In the current study, all pigs were stabilized at the same level of general anaesthesia before castration began and so the cerebrocortical and cardiovascular responses could be collected and analysed without inflicting pain. Blood pressure and pulse rate have traditionally been used to indicate nociception during general anaesthesia and have been compared with EEG variables in isoflurane-anaesthetized pigs. However, blood pressure is the most sensitive indicator of nociception (Haga et al. 2001). It is commonly assumed that increasing blood pressure and pulse rate is proportional to increasing nociception, but the use of cardiovascular variables as nociceptive indicators in the conscious animal is limited by the fact that many stimuli, emotional states and homeostatic mechanisms also influence the cardiovascular system (Le Bars et al. 2001) i.e. anaesthetic depth may influence the cardiovascular response. For example, in rats, increasing halothane concentrations cause nociception-induced pressor responses to become depressor response (Gibbs et al. 1989). To reduce these problems, a predetermined end-tidal halothane concentration was used in the current study and the cardiovascular variables were stabilized before castration in order to isolate the effect of nociceptive stimulation upon cardiovascular and EEG variables. Pulse rate and blood pressure may fall during intense surgical stimulation of genitalia, which some have attributed to reflex vagal activity (Smith 1992). In the control group, a decrease in pulse rate and an increase in MAP occurred in response to castration. Nociception-induced increases in blood pressure in horses undergoing castration is caused by an increase in peripheral vascular resistance (Taylor et al. 1998). A combination of reflex vagal responses and vasoconstriction may explain a paradoxical increase in blood pressure despite a fall in pulse rate; this has also been seen in horses during castration (Haga & Dolvik in press). On this basis, we believe it is appropriate to assume that a smaller decrease in pulse rate or increase in blood pressure during castration indicates less intense nociception. Nociception may induce both synchronization and desynchronization of the EEG, and so it is not possible to predict the effect of nociception on the EEG (Otto 2000). However, desynchronization has been used to indicate nociception during general anaesthesia in several species (Greene et al. 1992; Ekström et al. 1993; Von Waldmann et al. 1994; Murrell et al. 2003), and in several studies, has been inhibited by analgesics (Greene et al. 1992; Von Waldmann et al. 1994; Murrell et al. 2000). Desynchronization may be regarded as a change in the direction of the conscious EEG, but at 1.4 MAC of halothane, it is unlikely that consciousness is present. Halothane was used in the current work because EEG studies under volatile agent anaesthesia indicate that desynchronization is more easily observed with halothane compared with other agents (Johnson & Taylor 1998). During castration in the current study, a significant fall in totpow and an increase in MED were found, indicating EEG desynchronization in response to surgery. The mean age of the piglets in this study was 22 days. Piglets may be castrated earlier, and it is commonly held that castration of very young piglets is less painful. If this were true, then the finding that castration induces greater nociceptive responses than lidocaine injection in smaller piglets would be open to challenge, i.e., pain inflicted by injecting lidocaine may be greater than pain inflicted by castration in younger animals. The influence of age upon castration-induced pain has been investigated. Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9 7

8 In one study, it appeared that castration affected behaviour to a greater extent in 7-week-old pigs compared with 2-week-old pigs, although the results were statistically insignificant (McGlone & Hellman 1988). In a later study, no behavioural difference was found between castration at 1, 5, 10, 15 or 20 days of age (McGlone et al. 1993). Heart rate and the rate of HEF have also been used to evaluate pain induced by castration in the age interval 1 24 days (White et al. 1995). Younger piglets gave fewer HEF when castrated, indicating less pain in the younger piglets. In a later study using HEF, it was found that pigs at 3 days of age gave fewer HEF than pigs at 10 or 17 days when castrated (Taylor et al. 2001). The youngest pigs also gave fewer HEF when sham-castrated compared with castration at 10 and 17 days of age, indicating that the reduced number of HEF at 3 days of age was a direct effect of age rather than an effect of less pain. Experimental studies in other neonatal mammals indicate an enhanced ability to perceive pain and an ability to develop hypersensitivity to it (Fitzgerald & Beggs 2001). Taken together, these results indicate that neonatal pigs do not experience less pain when being castrated, and that the validity of our results may extend to younger animals. Inhalant anaesthesia has adverse cardiovascular effects which may affect drug disposition. Despite our efforts to maintain cardiovascular function and physiological variables within normal limits, the local distribution of lidocaine may differ in anaesthetized and conscious animals, with local distribution being slower in the former because of impaired circulation. Consequently, the current study may underestimate the effect of lidocaine injection. It may be argued that the technique used for cutting the funiculus spermaticus in this study was more painful than other techniques. The Funiculus spermaticus may be severed using scalpels, scissors or emasculators or it may be torn by testicular traction. No difference was found in HEF when comparing traction with scalpel incision (Taylor & Weary 2000). When castrating pigs, limiting haemorrhage becomes increasingly important with greater size. Scalpels provide clean incisions with more haemorrhage. As the injection techniques evaluated are supposed to be effective up to the age of 4 weeks, we chose one that would minimize bleeding, while aware that this may have affected the comparison of nociception induced by injection and castration. In conclusion, lidocaine injection into either the testis or the funiculus spermaticus reduces nociception induced by castration. Our data show that lidocaine injection is probably less painful for piglets than being castrated without local anaesthetic. Acknowledgements This study was financially supported by the Norwegian Ministry of Agriculture and the Norwegian Animal Health Authority. We acknowledge Technician Jens Røhnebæk for assistance during the study. References Baumann B, Bilkei G (2002) Castration of piglets under general anaesthesia. Agrarforschung 9, Ekström PM, Short CE, Geimer TR (1993) Electroencephalography of detomidine-ketamine-halothane and detomidine- ketamine-isoflurane anesthetized horses during orthopedic surgery. A comparison. Vet Surg 22, Fitzgerald M, Beggs S (2001) The neurobiology of pain: developmental aspects. Neuroscientist 7, Gibbs NM, Larach DR, Skeehan TM et al. (1989) Halothane induces depressor responses to noxious stimuli in the rat. Anesthesiology 70, Greene SA, Moore MP, Keegan RD et al. (1992) Quantitative electroencephalography for monitoring responses to noxious electrical stimulation in dogs anaesthetised with halothane or with halothane and morphine. In: Animal Pain (1st edn). Short CE, Poznak AV (eds). Churchill Livingstone, New York, pp Haga HA, Dolvik NI (in press) Electroencephalographic and cardiovascular parameters as nociceptive indicators in the isoflurane anaesthetised horse. Vet Anaesth Analg (in press). Haga HA, Tevik A, Moerch H (2001) Electroencephalographic and cardiovascular indicators of nociception during isoflurane anaesthesia in pigs. Vet Anaesth Analg 28, Hay M, Vulin A, Genin S et al. (2003) Assessment of pain induced by castration in piglets: behavioral and physiological responses over the subsequent 5 days. Appl Anim Behav Sci 82, Horn T, Marx G, Borell E (1999) Behaviour of piglets during castration with and without a local anaesthesia. Deutsche Tierarztliche Wochenschrift 106, Johnson CB, Taylor PM (1998) Comparison of the effects of halothane, isoflurane and methoxyflurane on the electroencephalogram of the horse. Br J Anaesth 81, Ó Association of Veterinary Anaesthetists, 2005, 32, 1 9

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