CRI use for anesthesia and analgesia (Proceedings)

Pain management in veterinary patients has come a long way.  There are a myriad of analgesic drugs available and numerous techniques exist to further provide top-notch analgesia and pain management to animals undergoing even the most painful surgeries. Constant rate infusions (CRIs) are increasing in their usage, thanks in large part to their wide versatility. There are pharmacologic agents used as CRIs that can provide analgesia, maintain anesthesia, and provide blood pressure support if needed.

Analgesics

Delivery of analgesic drugs as a CRI is typically far superior to bolus doses for a number of reasons. By administering a constant, steady infusion of a drug, stable levels of tissue concentration are reached. This prevents the peaks and valleys of comfort versus discomfort that occurs when bolus doses of analgesics are give as a patient begins to seem painful. When giving bolus doses, drugs peak, providing analgesia, then fall to below therapeutic levels allowing for breakthrough pain. CRI administration eliminates that rise and fall. CRIs can be easily adjusted to meet the needs of each individual patient, allowing for lower amounts of the drug to be given. Less money is spent on the actual drug as well as on supplies (syringes, needles, etc). Lower doses also decrease the incidence and severity of side effects.

Analgesic choices

Many classes of analgesic drugs can be given as a CRI including opioids, local anesthetics (specifically lidocaine), NMDA receptor antagonists (ketamine), and alpha-2 agonists (dexmedetomidine).  Drug dosages are shown in Table 1.

Opioids work by binding with specific receptors in the central nervous system (CNS) and can, depending on the opioid, provide relief for mild to severe pain. Morphine, fentanyl, hydromorphone, and butorphanol are commonly used in opioid CRIs. Morphine, fentanyl, and hydromorphone can be used for moderate to severe pain, while butorphanol is only appropriate for use in mild to moderately painful cases. CRIs tend to reduce the severity of opioid side effects such as vomiting, dysphoria, and respiratory depression, but patients should still be monitored closely for any sign of distress.

Lidocaine works by blocking sodium ion channels and causing membrane stabilization. It can reduce the amount of opioid analgesic and inhalant gases required to maintain anesthesia. Lidocaine is relatively inexpensive and has anti-arrythmic and anti-inflammatory properties. It is may be useful in cases where gastrointestinal pain is involved(GDV, laparotomies, etc). Cats have an increased sensitivity to local anesthetics and it is currently not recommended to use lidocaine infusion on feline cases.

Ketamine works by antagonizing the NMDA receptors which are responsible for central sensitization, hypersensitization, and “wind-up” pain. Ketamine is not capable of providing adequate analgesia in its own right, however, when administered in combination with opioid analgesics, it can lower anesthetic requirements of the patient.

Dexmedetomidine has mild analgesic properties, as well as anxiolytic and sedative properties. It works by simulating alpha-2 receptors in the CNS. Dexmedetomidine CRIs are most commonly used in the postoperative phase, for patients that are anxious and/or vocal despite an appropriate anesthetic regimen.

Anesthetic maintenance

Total intravenous anesthesia (TIVA) with propofol may be indicated for short, painless procedures. It may also been needed in situations where a patient cannot be intubated (tracheal procedures), where patients need to remain intubated and unconscious for extended periods (cases requiring long term ventilation assistance), or patients with autosomal disorders where inhalants cannot be used (malignant hyperthermia).  In these cases propofol is administered with a bolus dose of 2-5 mg/kg followed by 0.05-0.2mg/kg/min. Adequate oxygenation is always a concern, so if possible it is recommended to intubate patients and maintain them on 100% oxygen. Standard anesthetic monitoring is still needed in TIVA patients. It may be harder to control depth when using TIVA, so good monitoring can act as an early alert system to patients that become too light or too deep, much the same as when anesthetizing a patient with volatile anesthetics.

Pressor support

Dopamine and dobutamine are the choices most commonly used to provide blood pressure support in hypotensive patients. Both drugs increase cardiac contractility, and assuming the patient has appropriate vascular volume, these drugs can most boost low blood pressure. The range for both dobutamine and dopamine is 1-10 mcg/kg/min. There can be a wide variation in patient reactions, so it is recommended to begin with low end dosing and adjust as needed.

Equipment

One downfall of CRI administration is that it requires specific equipment for the best and safety route of delivery. It is possible to deliver CRIs using either a fluid pump, syringe pump, or standard drip set. It is ideal to use a syringe pump, which can be programmed with all the variables involved in administering the infusion, but pumps can range from hundreds to thousands of dollars. If CRIs are being used on a regular basis, infusion pumps could be worth the investment-less waste, less man-power needed, fewer errors. Fluid pumps are a nice piece of equipment to have if you are simply injecting your drugs into a bag; they are programmable and can be set to deliver specific volumes over time. It is still on the technician to figure out the best concentration of the CRI for the length of time being it will be delivered. If neither a fluid pump or syringe pump is available, a standard drip set can be used. However, extreme attention needs to be paid to ensure the patient receives the appropriate fluid rate.

Table. 1: Analgesic CRI drug dosages

Drug

IV Loading Dose

CRI Dose

Notes

Butorphanol

0.1-0.2mg/kg 0.1-0.2mg/kg/hr

Can be expensive

Dexmedetomidine

1-2 mcg/kg 0.5-2 mcg/kg/hr Fewer negative side effects noted

Fentanyl

2-5 mcg/kg 2-25 mcg/kg/hr Lower end dose for post-operative management

Hydromorphone

0.02-0.05 mg/kg 0.01-0.04 mg/kg/hr Lower end of dose for feline patients

Ketamine

0.5 mg/kg 2-10 mcg/kg/min Often used with an opioid

Lidocaine

1-2 mg/kg 25-50 mcg/kg/min NOT IN CATS

Morphine

0.1-0.3 mg/kg SLOWLY 0.1-0.3 mg/kg/hr Lower end dose for cats

Morphine-Lidocaine-Ketamine

M: 0.1 mg/kg

L: 0.5 mg/kg

K: 0.25 mg/kg

M: 0.12 mg/kg/hr

L: 1.5 mg/kg/hr

K: 0.12 mg/kg/hr

Can use fentanyl/hydro in place of morphine

Omit lidocaine for cats

References

Skarda, Roman T. and Tranquilli, William J. 2007. Veterinary Anesthesia and Analgesia, 4th edition. Ames (IA): Blackwell Publishing