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Analgesia – Pain Management

Indications for Local Analgesia
Vicki L. Campbell, DVM, DACVECC, DACVAA (UT)

Local analgesia is an underused technique that can greatly enhance the overall analgesia in a patient; lead to use of less systemic drugs; and may be used in situations in which sedation, heavy systemic analgesia or anesthesia is contraindicated. Many animals that present to emergency are in shock, and those that are not in decompensated shock may be in compensated shock. Those in compensated shock are able to compensate because their sympathetic nervous system is maintaining their blood pressure and helping to maintain their oxygen delivery. In the intensive care unit, critically ill animals are dynamic with potentially minute-to-minute changes in cardiac output, blood pressure and oxygen delivery.

Almost all sedatives, analgesics and anesthetics blunt the sympathetic nervous system to some extent. This effect puts animals in the emergency department and critical care unit at risk for decompensation when receiving systemic drugs for analgesia, sedation or anesthesia. Local analgesic techniques may aid in decreasing systemic drug requirements in these patients.

Although shock patients in general are at risk for systemic decompensation with systemic sedatives and anesthetics, there are specific conditions in which these drugs should be particularly avoided or delayed if at all possible. Specific examples include head trauma, pulmonary contusions, pneumothorax, myocardial contusions (and subsequent arrhythmias), diaphragmatic hernia, liver fractures, splenic fractures, urinary tract rupture, severe anemia/hypoproteinemia and neurologic abnormalities. Avoidance of anesthesia and sedation in these cases is beneficial because anesthetic drugs can worsen/induce arrhythmias; positive pressure ventilation may contribute to or worsen pneumothorax; lungs that have pulmonary contusions are more prone to atelectasis and subsequent hypoxemia; anesthetic drugs can alter blood flow to the brain and worsen head trauma/neurologic status; urinary tract injuries may cause severe life-threatening electrolyte disturbances (especially hyperkalemia); and liver/spleen fractures may lead to intraoperative hemorrhage and hypotension.

In addition, many anesthetic drugs are protein-bound and become more bioavailable in animals that are acidotic, a common consequence of shock and trauma. Avoidance of anesthesia until full assessment and proper resuscitation is attained is critical. However, sometimes sedation or anesthesia in these cases cannot be avoided. Use of local analgesic techniques in these life-threatening situations frequently decreases the need for systemic drug use and makes for a safer overall procedure.

Because local anesthetics directly block nerve impulses, they decrease pain in an alternative way compared with systemic analgesics. When used preemptively, this decreases the likelihood of wind-up of the pain pathways, ultimately helps prevent hyperalgesia, and aids in the multimodal approach to patient analgesia.

Drug selection is an important part of the local anesthetic protocol. Local anesthetics work by blocking nerve impulses. At a cellular level this occurs by blocking sodium channels in the nerve membrane.  When sodium is blocked, the nerve cannot conduct an impulse, and therefore no sensation can be transmitted. Local anesthetics cause analgesia but can also cause complete loss of motor function depending on the properties of the drug, location, myelination of the nerve, dose and size of the nerve fibers. Generally, local anesthetics cause nerve blockade in a particular order by first numbing pain, then warmth, touch, deep pressure and finally motor function. However, large peripheral nerves are an exception to this and tend to have motor blockade before sensory blockade, as well as cause proximal extremity analgesia prior to distal extremity blockade. Local anesthetic drugs differ in their side effects, onset of action and duration of action. Consideration of these drug factors should influence drug selection.

Drug Dosage Use
Lidocaine 1-2 mg/kg Short-acting analgesia for local infiltration, intrapleural and intraperitoneal blocks, and occasionally epidural use
Bupivicaine 1-2 mg/kg Longer-acting analgesia for local infiltration, intrapleural, intraperitoneal and epidural use.
Dexmedetomidine 0.001-0.005 mg/kg Epidural, intraarticular, or perineurally

 

Preservative-free morphine 0.1 mg/kg Epidural

 

Fentanyl 0.01 mg/kg Epidural

 

Buprenorphine 0.003-0.006 mg/kg Epidural