Soaker-type catheter placement
A soaker-type catheter, also referred to as a diffusion or wound catheter, is simply fenestrated tubing that is sterilely placed at a painful site for the continuous or intermittent administration
of local anesthetics. Soaker-type catheters are easily placed during a surgical procedure, such as a limb amputation or large
tumor resection, just before closure. In addition, they can be positioned nonsurgically at the site of a lesion that is painful
but cannot be resected or when a course of palliative care has been chosen.
Efficacy and adverse effects. The veterinary literature regarding the use of soaker-type catheters is limited, but human medical experience is extensive
and, for the most part, positive. A 2007 study demonstrated the viability of soaker-type catheters in postoperative pain management
in dogs, cats, and goats.36 In another veterinary study, continuous infusion of bupivacaine into normal dogs' stifles for eight hours demonstrated plasma
concentrations well below the toxic dose.37 Unfortunately, no studies have evaluated the efficacy of continuous infusion of local anesthetics in veterinary patients
with surgical or traumatic wounds. Results from human trials are largely encouraging, with most showing an improvement in
patient pain scores or a decrease in opioid requirements,38-46 whereas others have not found a benefit.47-50 Of great interest is the reduction in chronic pain in people a minimum of four years after bone graft harvesting with the
use of continuous bupivacaine infusion at the graft site for 48 hours.51
Divergent conclusions about the utility of soaker-type catheters may be due to different catheter placement depths (fascial
layers vs. subcutaneous tissues), study end points (opioid consumption vs. length of hospital stay or patient satisfaction),
innervation at various surgical sites (inguinal region vs. sternum), and other study design disparities. Few serious adverse
effects attributable to local anesthetic infiltration are reported in veterinary patients. One case of lidocaine toxicosis
with continuous infusion of 3 mg/kg/hr for three days has been described in a dog36 ; additional complications could include infection, hematoma formation, intravascular injection and subsequent cardiovascular
collapse, or intraneural administration of local anesthetics and resulting nerve damage.
Figure 4. A commercially available soaker-type catheter and drug reservoir; the catheter has been inserted into the soft tissue
over the humerus of a dog with osteosarcoma to provide palliative analgesia.
Obtaining a catheter. Soaker-type catheters can be purchased commercially (Diffusion/Wound Catheter—MILA International; On-Q PainBuster Post-Op
Relief System—I-Flow Corp) (Figure 4) or can be fashioned in a sterile manner from materials commonly found in the veterinary clinic. Commercial catheters are
made of plastic tubing, often polyurethane, and typically range in length from 2.5 to 10 in. The fenestrated areas vary in
length and are designed to spread local anesthetic over a predetermined area (5 to 22 cm).
Homemade catheters are readily constructed intraoperatively.52,53 For instructions on how to make a soaker-type catheter, see the boxed text "How to make a soaker-type catheter." The catheter
can be made ahead of time and gas-sterilized with either hydrogen peroxide or ethylene oxide if facilities are available.
How to make a soaker-type catheter
Placing the catheter. Intraoperative placement of either a commercially manufactured or homemade soaker-type catheter is quick and easy, being
similar in nature to placing a drain during closure. Place the catheter in the deepest layer of the wound, and make sure all
fenestrations are within the muscle layers or subcutaneous tissues. Loose tacking absorbable sutures can be used in the deep
layer, but the catheter should slide out easily for removal. If a drain is also to be placed, position the soaker-type catheter
dorsally and the drain ventrally. The catheter can be tacked to the skin with suture to keep it in place and then bandaged
as necessary (Figure 5).
Figure 5. A red rubber soaker-type catheter in place, along with a wound drain, in a dog that underwent radical excision of
a soft tissue sarcoma. Note that the catheter is placed dorsally in the wound. A tacking suture holds the catheter to the
skin, and the catheter will be wrapped to further
secure it to the dog. Bupivacaine (1 mg/kg) will be administered through the catheter every six hours.
Soaker-type catheter placement can be accomplished outside the operating room as well when a painful lesion is not to be surgically
addressed. Commercially available catheters are advantageous in such situations because they have tear-away applicators that
allow for minimal tissue trauma during placement. Homemade versions could also be placed in such a situation but would require
some tissue dissection to appropriately localize the catheter. Tunneling the catheter some distance in the subcutaneous tissues
before exiting the skin will help provide stability to the placement.
Elastomeric and electronic reservoirs that allow for continuous infusion of local anesthetic can be purchased from the same
companies that supply commercial soaker-type catheters. Syringe pumps can also be used to provide continuous infusion. Lidocaine
or mepivacaine are likely the safest choices for continuous infusion, and care should be taken to avoid toxic doses, especially
in cats. Infusions of lidocaine or mepivacaine of 1 to 2 mg/kg/hr should be safe and effective in most cases. For intermittent
administration, bupivacaine at 1 to 2 mg/kg is the best option because of its long duration of action (four to six hours).
Catheters are typically left in place for one to three days, but can be left in longer if strict aseptic technique is followed.
The techniques described above are easy to perform and require simple materials and drugs available in most small-animal veterinary
practices. By providing preemptive and multimodal analgesia, reducing the need for systemic analgesics and anesthetics and
the attendant side effects, and decreasing the perioperative stress response through the use of local anesthetic techniques,
you can improve the overall quality of the analgesic care in your patients. Look for subsequent articles in this series throughout
the year covering additional helpful and easy-to-perform anesthetic techniques.