Constant rate infusions (CRI) of analgesic drugs are an excellent way to manage pain in both dogs and cats. A CRI of analgesic agents has several advantages over multiple repeated injections for pain relief, including:
1. A more stable plane of analgesia with less incidence of break-through pain (which can be difficult to treat);
2. A lower drug dosage delivered at any given time, resulting in a lower incidence of dose-related side effects;
3. Greater control over drug administration (easy to change the dose);
4. Decreased need for stimulation of resting patients to administer drugs; and
5. Decreased cost (when compared to technician time, needles and syringes required for repeat injections).
Drugs that are useful for CRIs include fentanyl, hydromorphone, morphine, butorphanol, ketamine, lidocaine and a myriad of combinations of these drugs. Dosages are available in the tables below.
The opioid class of drugs includes some of the most potent analgesic drugs available and opioids should be considered for any patient experiencing moderate to severe pain. Although opioids are generally sedating in dogs, they can cause excitement in cats. Fortunately, the low dose of opioids delivered in a CRI rarely results in sedation or excitement. However, if excitement does occur, a light dose of a sedative (eg, acepromazine or dexmedetomidine) can be administered to the cat and the CRI rate maintained (if excitement is mild) or reduced (if excitement is moderate). If sedation occurs, the dose of the CRI can be decreased. Fentanyl, hydromorphone and morphine are potent full agonist opioids that provide profound dose-related analgesia. These full agonists are the most commonly used opioids but butorphanol, an agonist-antagonist, has an advantage in that this drug is more likely to provide sedation than excitement in cats. However, butorphanol provides only moderate analgesia and has a ceiling effect for pain relief (ie, a point is reached where higher dosages result in more sedation but not more analgesia). Thus, butorphanol is only appropriate for short-term mild to moderate pain and should be used as part of a multi-modal protocol rather than as a sole agent.
Table 1: Dosages for constant rate infusions (CRIs) used in CATS.
Lidocaine can be administered systemically to provide analgesia but its mechanism of action when used systemically is not entirely clear. Proposed mechanisms include blockade of sodium channels or potassium currents in the dorsal horn of the spinal cord and direct inhibition of abnormal electrical charges from injured or inflamed peripheral nerves. Lidocaine CRIs are extremely useful in dogs but are somewhat controversial in cats because: 1) cats appear to be more sensitive to the lidocaine-induced side effects than other species are, and 2) there is evidence that lidocaine may cause excessive cardiovascular depression in cats. Point 1 is potentially (although not unequivocally) true and a lower dosage of lidocaine is recommended for cats than is recommended for dogs. Point 2 is most commonly reported in anesthetized cats and the cardiovascular depression could result from a physiologic interaction between lidocaine and anesthetic agents. Also, some argue that lidocaine CRI has been used successfully for anti-arrhythmic therapy in cats without undue cardiovascular depression and should be appropriate for analgesia, especially since the dose for analgesic therapy is actually on the low end of the dose used for anti-arrhythmic therapy. Because of the uncertainty of lidocaine effects in cats, some veterinarians feel that lidocaine CRI is not warranted in the cat at all while others feel that it is an appropriate means to treat pain, especially in patients where other options may be limited. If lidocaine CRI is chosen, using low dosages in conscious cats (ie, not under anesthesia) is recommended. Lidocaine CRIs are commonly used in dogs, especially in dogs with gastro-intestinal pain (eg, pain from exploratory laparotomy, gastric dilatation-volvulus [GDV], pancreatitis, parvovirus, etc…).
Table 2: Dosages for constant rate infusions (CRIs) used in DOGS.