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:
1. Ketamine, which is a NMDA receptor antagonist. It can block "wind up" and somatic pain, which is pain originating
from muscle, bone or skin, rather than visceral pain. Ketamine is a beneficial adjunct to other analgesic drugs such as opioids
but it is not an effective analgesic when used alone. Higher doses can cause dissociative effects (Ketamine reaction). An
initial loading dose of 0.25-0.5 mg/kg IV bolus should be given prior to starting the CRI, and induction with Ketamine/Diazepam
will provide an effective loading dose. The CRI rate is 2-5mcg/kg/min
2. Lidocaine is a local anesthetic and anti-arrhythmic for treating ventricular premature contractions (VPCs) and
ventricular tachycardia (V-tach). It can provide analgesia when given IV, but is not recommended for use in cats because cats
are very sensitive to Lidocaine's side effects. It is usually used in conjunction with Morphine, Morphine and Ketamine, or
Fentanyl and Ketamine. The CRI rate for dogs is 25-50 mcg/kg/min.
3. Morphine is a pure opioid agonist commonly used in conjunction with Ketamine alone or in a combination with Ketamine
and/or Lidocaine (MK or MLK). Cats can be sensitive to Morphine's dysphoric effects, so use cautiously or avoid use entirely.
The CRI rate is 2-6 mcg/kg/min.
4. Hydromorphone is also a pure opioid agonist that can be used in conjunction with Ketamine alone or in combination
with Ketamine and Lidocaine. The CRI rate 0.1-0.8 mcg/kg/min.
5. Fentanyl is a pure opioid agonist that has to be used as a CRI when administered intravenously due to its short-lived
effect of a maximum of 30 minutes. It can also be administered with a transdermal patch. Fentanyl can be used alone or combined
with Ketamine and/or Lidocaine. The CRI rate is 1-5 mcg/kg/hr
6. (Dex)medetomidine is an Alpha-2 agonist that can treat pain when used with opioids as it has a synergistic effect
with them. It can also be used alone to keep patients calm and/or sedated (i.e. tracheostomy patients). Avoid use in compromised
animals as it can cause respiratory depression, hypotension, bradycardia, and increases the chance of atrioventricular (AV)
block. The CRI rate is 1-3 mcg/kg/hr.
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:
1. Add 36mg (2.4ml of 15mg/ml) Morphine and 340mcg (0.34ml of 1000mcg/ml) Medetomidine per one liter of crystalloid
fluids. Administer at the fluid maintenance rate (weight in pounds multiplied by 1.25 hourly) in dogs. This is useful in
2. Add 1200 mcg (24ml of 50mcg/ml) Fentanyl to one liter crystalloid fluids, and administer at 1ml/pound/hr.
3. MLK: add 60mg (4ml of 15mg/ml) Morphine, 60mg (0.6ml of 100mg/ml) Ketamine, and 35ml of 20mg/ml Lidocaine to one liter of crystalloid fluids and administer at 1ml/pound/hr.
4. Fentanyl can be substituted for Morphine in this by adding 1200 mcg (24ml of 50mcg/ml) to the liter of crystalloid
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.