Current Concepts

Wound Soaker Catheters for Control of Post-Operative Pain in Dogs and Cats

Alicia Karas, Emily McCobb, Lois Wetmore, Cheryl Blaze

History and Rationale

One of this decade’s remarkable advances in managing post-surgical pain involves delivering local anesthetic directly to the wound, and is easily adopted for use in veterinary patients. Traditional nerve block techniques eliminate the pain of surgery but mastery can require significant expertise.  In addition, commonly used local anesthetics have relatively short durations (lidocaine, 1 – 2 hours; bupivacaine, 4 – 6 hours). The availability of implantable infusion catheters makes it possible to use repeated dosing or continuous infusion of local anesthetics into surgical wounds, and improves pain control. FDA approved human use catheters are available, but costly. Two modestly priced catheters are commercially available for veterinary use (see below). Basically, they consist of a pliable catheter with tiny holes along the implanted end, functioning somewhat like a garden “soaker hose” (See figure 1a-c). The catheter is buried in the wound bed during surgical closure.


Constant infusion of the local anesthetic can be used for patients staying in hospital. If electronic pumps are unavailable, or early discharge is planned, an elastomeric balloon pump can be used to achieve continuous infusion. Alternatively,  intermittent injection of bupivacaine (every 4 – 6 hours) can be performed.

In humans, “wound soaker” catheters are used for cardiothoracic, abdominal, amputation and mastectomy surgeries, as well as for other major surgery types. Placement of the catheter is generally done at the end of surgery, and infusion duration is approximately 2 days. Benefits include reduction of mean pain scores at rest and with activity, reduction of daily consumption of opioids, and trends towards better patient satisfaction and reduced length of hospital stay.

To date, a small number of veterinary clinical studies of wound soaker catheters have been reported in the literature. The most studied surgical indication has been  total ear canal ablation in dogs, but use for  resection of fibrosarcomas in cats  is also reported. We have published a review of clinical use of wound soaker catheters in our hospitals in 56 canine, feline and caprine patients. Collectively, studies reported to date have used either bupivacaine or lidocaine infusion. As with human studies, pain control was optimized, there was a reduced need for strong opioids, and the complication rate was low. Ancillary benefits include reduction in the level of sedation (or opioid side effects) and reduced hospital stay.

Concerns are sometimes raised about the effects of local anesthetics on wound healing, tissue integrity and  infection.  However, over the past decade it has become apparent that benefits of local anesthetics are not limited to simple inhibition of nerve conduction. They have been shown to have broad anti-inflammatory effects, including reduced production of eicosanoids, thromboxanes, leukotrienes, histamine, and inflammatory cytokines. They can also have antimicrobial effects. Although vital to the overall immune response, inflammation can be responsible for severe morbidity in patients, in addition to its impact on the generation and maintenance of pain. Reported clinical complication rates for various methods of wound infusion of local anesthetics in humans are extremely low, with the exception of direct constant infusion into joints, which is not currently recommended. Therefore, it is time to re-examine the usefulness of peripheral local anesthetics in veterinary pain medicine.

Instructions for use of wound soaker catheters for surgeries with high potential for pain

  • Pre-, intra-, and post – operative analgesics are administered, generally starting with a strong opioid (e.g. hydromorphone, oxymorphone) +/- other adjunctive pain medications, and tapering to oral “go home” medications. Pain is assessed at regular intervals. Medications should be dosed according to patient evaluation, as weaker opioids (e.g. buprenorphine, tramadol) may be sufficient in conjunction with local anesthetic infusions.
  • Prior to surgical closure, a soaker catheter is chosen so that the infusion holes will span the long axis of the wound, but be completely contained within it. The catheter is placed in the deepest layer of the wound, ideally with the injection port exiting  dorsally.  Deep placement helps assure that the catheter is close to major nerve trunks.
  • The wound is closed over the catheter, which is fastened to the skin where it exits by means of a waterproof tape butterfly or other knotted suture technique. Care is taken not to occlude or kink the soft catheter. (See Innovation section for views of wound soaker catheters in place)
  • A priming dose of bupivacaine (1.0 mg/kg) is injected slowly with the patient still under general anesthesia
  • Local anesthetic presence in the wound is continued by either:
    • Continuous infusion of lidocaine (dog only)
    • Intermittent bolus injection of bupivacaine (dog or cat)
  • Administration of local anesthetic is continued for at least 24 hours, and up to 3 days.  The catheter should remain in place for at least 6 hours after the last dose of local anesthetetic to ensure that pain does not recur. If the patient is comfortable without the use of the local anesthetic drug the catheter may be removed. Note: the use of elastomeric “disposable” pumps requires that a flow controller of the appropriate rate be chosen.

Dosing

Dogs

The lidocaine continuous infusion rateis 1.0 -2.0 mg/kg/hour.  The dilution of lidocaine (from 1 – 2%) is adjusted so that the following approximate volumes are used:

30 – 40 kg, limb amputation wound 3 – 4 mls/hour
5   – 25 kg, limb amputation wound 1 – 3 mls/hour
30 – 40 kg, thoracotomy wound 2 – 3 mls/hour
5   – 25 kg, thoracotomy wound 0.75 – 2 mls/hour

The intermittent bupivacaine dose is 0.5 mg/kg every 4 – 6 hours. The 0.5% solution can be diluted to 0.25% with saline to increase coverage:

30 – 40 kg, limb amputation wound 4 – 8 mls/dose
5   – 25 kg, limb amputation wound 1.5 – 6 mls/dose
30 – 40 kg, thoracotomy wound 2 – 3.5 mls/dose
5   – 25 kg, thoracotomy wound 0.75 – 2 mls/dose
Cats

Bupivacaine is diluted to 0.25%, with injection of 0.5 mg/kg every 4 – 6 hours.

The volume of drug used may be adjusted somewhat according to the size of the wound bed and the adequacy of pain control.  Palpation of the entire wound, initially with the amount of pressure that you can comfortably apply over a closed eyelid and then slightly more, should elicit little reaction from the patient. Technical staff should be trained, and the catheter hub, syringes, pumps and lines should be clearly marked, to prevent accidental intravenous injection of local anesthetic – a significant hazard.

In our study, the most common complication was disconnection of the catheter from the infusion set.  This can be avoided by the use of an intermittent injection technique or by securely attaching the infusion set to the catheter and using a tension loop.

Sources of catheters: