Local anesthetics are being increasingly recognized for their important role in surgical pain management. A thorough discussion
of it's mechanism of action, toxicity and applications is found in the previous session.
Wound Diffusion Catheters
A new strategy to extend the duration of local anesthesia may markedly help patients with moderate to severe surgical injury.
Implantation of a catheter into the surgical wound site prior to closure allows repeated or continuous infusion of local anesthetics
into the affected area. In humans, relatively costly FDA approved catheters are used. For veterinary use, two moderately priced
types are commercially available. The basic form is a soft pliable catheter with tiny holes along the end that is implanted;
functioning somewhat like a garden "soaker hose". Delivery of local anesthetic by continuous infusion of lidocaine, or intermittent
bolus dose of bupivacaine can be used. An elastomeric balloon pump is a modestly priced option for ambulatory patients. A
recent review of wound incision catheters for surgery in humans, concluded that the overall: "Continuous wound catheters consistently
demonstrated analgesic efficacy in terms of reduced pain scores or opioid use for all surgical subgroups, despite heterogeneity
in type of surgical procedure, location of wound catheter, mode of delivery of local anesthetic, dose of local anesthetic,
and analgesic mixture" (Liu et al, 2006). In most cases, duration was approximately 2 days. Beneficial outcomes included reduction
of pain scores at rest and with activity, reduction of daily consumption of opioids, and trends towards better patient satisfaction
and length of hospital stay. Veterinary clinical studies have been reported involving the use of continuous wound infusion
of local anesthetics. The most studied surgical indication was total ear canal ablation, but use for extensive soft tissue
resection in cats (fibrosarcoma resection) is also reported. Studies reported to date have used either bupivacaine or lidocaine
infusion, and, as with human studies, pain was generally perceived to be adequately managed with the infusion, and complication
rates low and not perceived to be problematic. Currently, anecdotal reports are that practitioners are using such wound infusion
catheters for limb amputation, ear canal ablation, intercostal and sternal thoracotomy, celiotomy, and major soft tissue tumor
excision, with excellent results and few complications.
Evidence is mounting for the beneficial effects of intravenous lidocaine (IVL) on pain after soft tissue surgery in humans
and dogs. IVL markedly reduces the inhalant anesthetic requirements of patients and thus is it imperative that vaporizer settings
be adjusted accordingly when IVL is used during surgery. Many types of surgical pain, as well as that of major trauma, burn
and pancreatitis pain may benefit from this use. IVL can be recommended as a safe and sparing adjunct to opioid and other
analgesics for surgery, trauma, and pancreatitis at a dose of 50 mcg/kg/min, in dogs; and this has been used for 24 – 48 hours.
In humans it has also been shown to speed the return of bowel function, decrease postoperative pain, minimize opioid consumption,
and shorten the hospital stay after abdominal surgery; Systemic, intravenous infusion of lidocaine has also been shown to
elicit a sustained effect on neuropathic pain in humans, and may have a specific point of action in the brain.
Several reports of concentrations and effects of IVL in cats strongly warn that it should not be used intraoperatively in
cats, because of their inability to metabolize the drug, and practitioners are advised to wait until further evidence supporting
its use in non-anesthetized cats is reported.
Note: Formulas for a combination morphine, lidocaine, and ketamine constant rate IV infusion has been described in dogs. The
combination is profoundly analgesic, fairly sedating, and is superior for the most painful post-operative states. The drug
concentrations and fluid rates may be adjusted to fit the needs of the individual patients. Rate calculators are available
http://www.vin.com/ (Library/Calculators) and