Continuous rate infusions in intraoperative pain management (Proceedings)
CRI stands for continuous rate infusion, and its use is becoming more prevalent in the veterinary field as a method to control intraoperative and postoperative pain. It was not long ago that the best options for surgical pain management were intramuscular or bolus injections of opioids, which remain acceptable options, but CRIs can be a better option for patients undergoing prolonged, invasive or painful procedures. The purpose of this article is to outline the benefits of CRIs and list some of the more commonly drugs used. Please note that there are many different drug combinations, and the anesthetist should consider the signalment, medical history, behavior of the patient, surgical procedure, and the length of time they are expected to be hospitalized for each combination used.
The goal in pain management is to maximize benefits while minimizing negative side effects. Intramuscular (IM) and subcutaneous (SQ) injections cause pain on varying levels, and even intravenous drug boluses can be absorbed unpredictably and inconsistently causing the patient erratic highs and lows. A CRI provides steady and adequate analgesia while the amount being administered can constantly be adjusted. Any patient on a CRI should be monitored closely to ensure they are getting adequate analgesia and not experiencing dramatic side effects.
Though contemporary anesthesia inhalants such as Isoflurane and Sevoflurane are much safer than their predecessors, they are still potent vasodilators that can lower blood pressure. Their administration should be minimized during a surgical procedure, which is why the anesthetist should first decrease the inhalant anesthesia when a patient becomes hypotensive. Since inhalants do not provide analgesia, the concurrent administration of a CRI greatly decreases the amount of inhalant anesthesia used, reducing the potential of anesthetic hypotension and subsequent complications, and makes pain management much easier. To add perspective, Isoflurane may have to be administered at 3.5-4% (up to 5%) when the patient lacks adequate perioperative analgesia, but with an effective CRI, Isoflurane often can be kept at or below 1%. Also, recovery can be traumatic for the patient without proper pre- and intraoperative analgesia for their pain can be difficult to manage due to "wind up". Wind-up is a term used for when the patient is in so much pain that it is difficult to get the pain and anxiety under control. It usually requires higher doses of drugs (potentially causing more side effects) to "catch up" with the pain, and maintain adequate analgesia. Far lower drug doses are needed when preemptive analgesia is used, and the lower doses often maintain pain control post-operatively.One of the newest approaches to anesthesia is total intravenous anesthesia (TIVA) in which the patient is intubated and delivered 100% oxygen while anesthesia is delivered via a CRI. This can be a safer choice for critically ill patients who are already hypotensive or patients undergoing a procedure that is expected to cause hypotension.
Some common drugs used in CRIs include:
The patient should still be premedicated with proper analgesia prior to staring any CRI.
Calculating CRIs can be challenging, and accuracy is imperative. There are computer programs that calculate CRIs automatically.
There are also combination "shortcuts" such as:
It is highly recommended to administer CRIs through a syringe pump or diluted in an IV fluid bag that is hooked up to a pump for some of these drugs have a narrow margin of safety and must be administered carefully.
Hansen, B. Management of Pain in the Critically Ill. Proceedings International Veterinary Emergency and Critical Care Symposium 2003.
Palmer, Darci. Analgesic Constant Rate Infusions, Veterinary Support Personnel Network. 11/20/2008.
Plumb, Donald. Veterinary Drug Handbook. Fifth Edition. Wiley-Blackwell. 2005.