BOX 4-3Advanced Analgesic Techniques

• Low-dose epidural administration of opioids or opioid-local anesthetic combinations can result in analgesia whose quality is similar to if not better than that achieved with systemic administration. This method depends on technical expertise and may be challenging to implement in very small animals. Epidural administration of drugs has not been studied in nonmammalian vertebrates.
• Local anesthetics can be injected into joints, wounds, and body cavities (abdominal or pleural) by continuous or intermittent injection through intra-wound catheters, greatly reducing the need for systemic administration of other analgesics (Liu et al. 2006). The relatively short duration of the action of local anesthetics may limit their utility in situations where redosing is difficult. Lidocaine is used intravenously to provide analgesia after tissue injury (Omote 2007).
• Oral administration of some analgesics is feasible (e.g., NSAIDs, opioids, gabapentin), but for some drugs (opioids) first-pass (species-dependent) metabolism limits bioavailability, necessitating dose adjustment, use of a different route of administration, or selection of another drug. Compounding of drugs into palatable forms that animals are willing to consume is possible, but without data to support a particular method, one must be concerned about absorption, shelf life, and efficacy.
• Dilution of injectable analgesics to make them easier to use or to improve provision in very small animals must be done with the understanding that formulations may not work as well and that shelf life is not predictable.
• Continuous infusion of certain types of analgesics (e.g., opioids, ketamine, α₂-adrenoreceptor agonists) avoids “peaks and valleys” in drug concentration and may provide better coverage for moderate to severe pain. Transdermal preparations are available in formulations suitable for larger animals and may be useful in producing uninterrupted analgesia. Sustained-release formulations make it possible to avoid periods of inadequate drug administration. For further consultation please see Carroll 2008; Flecknell 2009; Gaynor and Muir 2002; Hellyer et al. 2007; Krugner-Higby et al. 2008; Lamont and Mathews 2007; Robertson 2005; Tranquilli et al. 2007; Valverde and Gunkel 2005.