Local anesthesia in piglets undergoing castration—A comparative study to investigate the analgesic effects of four local anesthetics on the basis of acute physiological responses and limb movements

Abstract

Surgical castration of male piglets without analgesia is a painful procedure. This prospective, randomized and double-blinded study aimed to evaluate the analgesic effects of four different local anesthetics for piglet castration during the first week of life. In total, 54 piglets aged 3 to 7 days were distributed into 6 treatment groups: handling (H); castration without pain relief (sodium chloride, NaCl); and castration with a local anesthetic: 4% procaine (P), 2% lidocaine (L), 0.5% bupivacaine (B) or 20 mg/ml mepivacaine (M). By excluding stress and fear as disruptive factors via a minimum anesthesia model, all piglets received individual minimum alveolar concentration (MAC) isoflurane anesthesia. Twenty minutes before castration, all treatment groups except group H received one injection per testis. Then, 0.5 ml of a local anesthetic or NaCl was injected intratesticularly (i.t.), and 0.5 ml was administered subscrotally. Acute physiological responses to noxious stimuli at injection and castration were evaluated by measuring blood pressure (BP), heart rate (HR), cortisol, epinephrine, norepinephrine and chromogranin A (CgA); limb movements were quantified. The results confirm that castration without analgesia is highly painful. Surgical castration without pain relief revealed significant changes in mean arterial blood pressure (MAP) and HR. Local anesthetic administration significantly reduced changes in BP and HR associated with castration. Piglets receiving a preoperative local anesthetic exhibited the fewest limb movements during castration, while the NaCl group exhibited the most. Injection itself was not associated with significant changes in MAP or HR. However, many piglets exhibited limb movements during injection, indicating that the injection itself causes nociceptive pain. No significant differences were found between groups regarding parameters of plasma cortisol, catecholamines and CgA. In conclusion, all four local anesthetics administered are highly effective at reducing signs of nociception during castration under light isoflurane anesthesia. However, injection of a local anesthetic seems to be painful.

Efficacy of Intra-Operative Topical Wound Anaesthesia to Mitigate Piglet Castration Pain—A Large, Multi-Centred Field Trial

“…Studies of piglet nociceptive motor and vocal response identify traction on, and severing the spermatic cords [7,14,15] as inducing the greatest pain [7,[14][15][16]52] of the procedure. Previous studies identified pre-emptive lidocaine or bupivacaine injection as effective to mitigate the pain-induced (nociceptive) motor response [22,28,36,37]. Most recently, Saller et al [28] reported that, used in combination with light general anaesthesia to minimise the motor response to restraint and handling stress, Lidocaine 2%, Bupivacaine 0.5%, Mepivacaine 2% and Procaine administered via injection 20 min prior to castration were all effective to block the nociceptive motor response to castration as compared with those injected with normal saline (placebo).…”

“…Although reports vary [18][19][20][21], analgesics such as Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) may be of use to address inflammatory-related pain that develops in the postoperative period [8,22], but generally lack efficacy to ameliorate pain during the procedure and in the early post-operative period when pain is most acute [4,23,24]. Standard methods for addressing surgical procedural pain, such as the use of general anaesthesia [18,[25][26][27][28][29] or injected local anaesthetics, although effective to mitigate acute intra-operative pain, used alone, they provide little post-operative pain mitigation [22,24,25,28,[30][31][32][33][34][35][36][37][38][39][40]. Furthermore, their use may be impractical and/or impeded on-farm by cost, logistics, occupational health and safety, food safety or welfare concerns.…”

Optimal Methods of Documenting Analgesic Efficacy in Neonatal Piglets Undergoing Castration

“…Pre-emptive use of local anaesthesia via intra-testicular (i.t.) or infundibular injection, or via topical wound instillation, has been associated with reduced cortisol levels as compared with untreated animals in some trials [24,39,44], while not in others [24,36,46,74], or only where local anaesthetics and NSAIDs have been used in combination [20]. As detailed above, the lack of efficacy of local or general anaesthesia to reduce cortisol or ACTH does not, however, represent lack of efficacy to mitigate pain.…”

“…Neural markers of pain mitigation, such as the expression of the c-fos gene and its protein product, Fos, in neurons of the spinal cord [75], are significantly reduced when piglets are castrated under effective local or general anaesthesia, as compared with piglets castrated without anaesthesia [43]. Furthermore, this is associated with a dramatic reduction in the nociceptive motor and vocal response to castration [31,32,74]. Additionally, reduced postoperative hyperalgesia has been documented in local anaesthetic-treated piglets [31,32].…”

“…On this basis, a range of outcome variables have been used to assess piglet pain during and following castration, and to assess amelioration of pain due to use of local anaesthetics or analgesia. These include; (a) physiological responses during the procedure [22,24,27,28,[33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][64][65][66][67][68][74][75][76][77][78], (b) nociceptive motor responses during the procedure [27,28,31,47,49,50,64,74]; (c) vocal responses during the procedure [22,[24][25][26][27][28]31,46,[50][51]…”