Constant rate infusion (CRI) analgesia is a way of providing pain control by ensuring that the blood levels of the drugs are
held constant. In practice, it entails maintaining a venous access. This technique can be used during anesthesia as part of
balancing the anesthetic technique and continued to the postoperative period. It can also be started right after the end of
anesthesia for patients that are known to have painful surgeries. For patients with primary disease, e.g., acute pancreatitis,
with pain as one of the signs, CRI analgesia can also be used.
The focus of this presentation will be on the use and techniques of CRI for pain control in small animal patients.
When using CRI, a stable plasma or tissue concentration of the drug in the body should be achieved. It is important to remember
that once a drug is administered, part of it is also eliminated. The technique involves maintaining a stable drug concentration
in the central component. If this drug concentration is maintained, a steady state is considered to be achieved. This indicates
that the rate of infusion matches the rate of elimination. The concentration of the drug at steady state depends on the infusion
rate and the body clearance of the drug (see formula below).
Concentration at steady state (Cp
ss) = Infusion rate/Body clearance of the drug (β•Vd')
In practice, this steady state can be achieved in an efficient manner by giving an IV bolus dose and then the drug is administered
as a CRI. A delay between the start of an infusion and the establishment of a steady state is expected. The longer the half-life
of the drug, the longer it takes to reach a steady state or plateau. By 5 half-lives, it is noted that the amount of drug
in the body provided by the infusion will reach 97 percent of the steady state value.
There are advantages of administering analgesic drug(s) as CRI compared with intermittent IV or IM injections. CRI prevents
the sudden peaks and valleys associated with intermittent IV boluses. With IV boluses, there is a fast rise in plasma concentration
that may result in adverse effects. It is also possible that drugs with rapid elimination given as IV boluses rapidly fall
below the therapeutic concentration. Even with IM and subcutaneous injections, the blood level can go below the therapeutic
range. In painful patients, this will lead to breakthrough pain making the pain control more difficult. Because of the constant
plasma drug concentration in CRI, the control of pain will be uniform and consistent. Based on our clinical experience, CRI
appears to be a very effective technique in patients that are in severe pain. Depending upon the condition and response of
the patient, the rate of infusion can also be changed to produce the desired effect. It was shown in a study that CRI of
analgesic agent will result in less total amount of drug used to provide pain control compared with intermittent boluses.
This results in savings with the cost of the drugs. CRI has also been shown to result in faster recovery from the drug effect.
This finding can be useful in patients that have worsening hemodynamic condition. Terminating the CRI will result in quicker
lowering of the plasma drug concentration. Overall, it is easier to adjust the amount of drug(s) being administered in relation
to the severity of pain.
Despite the many positive features of CRI technique, it is not as widely used as the other methods of administering analgesic
drugs. In a busy practice, it may be less efficient because the solution has to be prepared especially if more than one drug
will be used. There is also a need for constant supervision by a staff member since the IV lines can be disconnected or kinked.
To administer drugs accurately with CRI, the use of syringe pump is helpful. Unfortunately, syringe pumps are still relatively
expensive. Some drugs used in CRIs are not extensively studied. The best infusion rates for different conditions need to be
determined. In general, we administer these drugs to effect. These drugs can accumulate in the body if given for a long time
resulting in adverse effects. Further studies are needed to determine the best protocol for prolonged administration (greater
than 24 hours). Different drugs may be used simultaneously as a multimodal approach to providing analgesia. This presents
additional question as to the stability and compatibility of these drugs when mixed together. When adverse effect occurs,
it may be more difficult to pinpoint the drug causing the problem because of the multiple drugs used.