Department of Clinical Sciences, College of Veterinary Medicine, Gainesville, FL, USA
Pain diffusion catheters are an excellent way to provide on mode of analgesia in your multi-modal pain control regimen. Pain diffusion catheters are a way of providing continuous local analgesia to the surgical site for up to 72 hours postoperatively.
Although there are descriptions in the veterinary literature of how to make your own pain catheter using a red rubber feeding tube and your MacGyver veterinary skills, I would recommend using a commercially made catheter as they are purpose made and at $15–20 a catheter, very cost effective. These catheters are 5-Fr, fenestrated, closed-ended catheters that accept an injection port. The catheter has a dark line on it that indicates where the fenestrated portion of the catheter ends, and there are several lengths available, depending on the size of your surgical site.
Pain diffusion catheters can be used in a variety of postoperative wounds. Common indications in my practice include: amputations, mastectomy, large mass removals, thoracotomies, ear canal ablations and laparotomy incisions. These are not recommended for the ventral approach to the neck as the local anesthetic will block the recurrent laryngeal nerves. Similarly, it is important to be aware of nerves at sites where you are placing a pain diffusion catheter. For example, a pain diffusion catheter placed at the site of a total ear canal ablation will affect the facial nerve and this will result in the need to apply eye lubrication post operatively and the inability of the surgeon to assess facial nerve function.
The pain diffusion catheter should be selected based on the length of the wound. It is important that none of the fenestrated portion of the catheter exits the entry site, as this will result in local anesthetic leaking during administration and will increase the potential for contamination of the surgical site. It is also important that the entry site is as small as possible, as a large entry site will also increase leakage and the potential for contamination. To achieve this, a fine-tipped mosquito forceps is used to penetrate or almost penetrate the skin from inside to out. If necessary, a scalpel blade is used to create the smallest incision possible that allows for the tips of the mosquito to penetrate the skin. The mosquito forceps are then opened slightly and the tip of the pain diffusion catheter is grasped and pulled into the incision site. The pain diffusion catheter should not be placed within the incision site. The pain diffusion catheter is sutured in place using the tabs provided by the manufacturer and an injection cap is placed on the end of the catheter.
Once the incision is completely closed, bupivacaine is then administered via the injection port. Dose recommendations vary, but it is important to keep in mind the potential for toxicity with the administration of a local anesthetic. I generally use 1.5 mg/kg every 6 hours. Because bupivacaine is acidic and this can cause discomfort on injection and because it is beneficial for the patient to recover with bupivacaine on board, it is recommended that the first dose of bupivacaine is administered while the patient is still under general anesthetic. After this, the bupivacaine dosing should be repeated every 6 hours. In my experience, delays in administration beyond 6 hours can result in patient discomfort on administration because the local anesthetic block may have completely or partially worn off. One technique that has been reported in veterinary medicine is to add bicarbonate to local anesthetics to minimize discomfort on administration. Although this may prevent discomfort, it will also result in precipitation of the bicarbonate within the catheter which will block the fenestrations and render the catheter unusable. If a closed suction drain is used at the same surgical site, a pain diffusion catheter can still be used. However, the charge on the closed suction drain should be released for an hour after each bupivacaine injection to prevent the bupivacaine from being removed by the closed suction drain.
The pain diffusion catheter should be used as part of a multimodal analgesic plan. This may include a nonsteroidal anti-inflammatory, if indicated, and an opioid. Patients should be assessed regularly and their pain medication adjusted as needed. However, it does appear that the opioid requirement for dogs that have a local anesthetic block of their incision with a pain catheter have a decreased opioid requirement. This will have a lot of benefits to the patient, including resuming eating, drinking, walking, urinating and defecating faster after surgery.