Providing adequate pain relief to veterinary patients is challenging; and not just because they tend to mask the degree to which they are suffering.
The most common way we treat pain that results from surgery, accident, or illness is to give a bolus of medication (or medications) on a relatively set schedule. For example, a dog who has been hit by a car might receive a nonsteroidal anti-inflammatory every 12 hours and tramadol every 8 hours, or a cat recovering from surgery could be given buprenorphine every 6 hours. The problem with these types of dosing regimens is that they inevitably lead to peak and trough drug concentrations in the body.
Riding the roller coaster of pain relief is obviously not ideal. Patients will not only go through periods of suffering during the troughs, but they may also be at higher risk for undesirable side effects during peak drug concentrations. The situation can be improved somewhat by combining two different medications given at different intervals (like the canine example mentioned above), but that does not fully resolve the problem and can produce a dosing schedule that is hard to follow.
When a pet is hospitalized, a technique called a constant rate infusion (CRI) is a useful alternative. With a CRI, medications are added to a bag of intravenous fluids, and the whole cocktail is then “infused” into the patient’s bloodstream at a “constant rate.” Drugs that are typically used in this way include morphine, hydromorphone, fentanyl, lidocaine, ketamine, midazolam, and dexmedetomidine. Combination therapy using two or more drugs at the same time is the norm.
These are all powerful drugs. Obviously, we don’t want to make a mistake in their dosing. For this reason, most veterinary clinics use fluid pumps for their patients who are on CRIs. These machines allow doctors and technicians to set an ideal rate of administration, and alarms will sound if it rises or falls too far away from what is programmed. It is possible to use a CRI without a fluid pump by calculating how many drips per minute need to flow through the chamber on the IV line, but rates can change with a pet’s position, when the line kinks, etc., so patients need to be watched closely.
Don’t let the phrase “constant rate” fool you. The level of pain relief that is provided by a CRI can easily be adjusted by turning the rate of the infusion up or down. Things can get a little complicated when we’re trying to provide for a pet’s fluid and pain relief needs using a single bag, however, since there is no way to turn down the fluids without also providing less pain relief and vice versa. Ideally, we only provide part of the patient’s fluids in combination with the pain relievers and use a separate, additional bag of IV fluids to cover the balance. In this way, we can make adjustments to one without affecting the other. If this isn’t feasible, oftentimes the simplest remedy is to make a new cocktail based on the pet’s current needs and dispose of the old.
Now if your veterinarian recommends a constant rate infusion for your pet, you can shock him or her by nodding knowingly and saying, “Sounds like a good idea doc, when can you get it started?”
Dr. Jennifer Coates