Chapter 23
Loco-Regional Anesthesia
Block it out!
Carrie Schroeder
Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, USA
- Q. What is the difference between local and regional anesthesia?
- A. Although these two terms are often used interchangeably, there are fundamental differences between providing local and regional anesthesia. In the strictest sense, local anesthesia is simply providing anesthesia to a small area via subcutaneous infiltration of a local anesthetic. An example of this would be blocking a small area around a laceration by infiltration of lidocaine near the peripheral margins of the wound. Regional anesthesia refers to anesthesia of a large region via delivery of local anesthetic to more proximal nerves. An example of this would be a lumbosacral epidural with bupivacaine. A common term that is used to encompass both local and regional anesthesia is loco-regional anesthesia.
- Q. What are the components of the pain pathway? What parts of the pain pathway are targeted with loco-regional anesthesia?
- A. There are five steps of the nociceptive process [1]:
- Transduction: peripheral sensory nerve endings known as nociceptors are activated by a painful stimulus, which is converted into an electrical impulse.
- Transmission: the electrical impulse travels up the peripheral nerve towards the dorsal horn of the spinal cord.
- Modulation: in the dorsal horn of the spinal cord, endogenous systems either upregulate or downregulate the electrical impulse.
- Projection: the electrical stimulus travels through the spinal cord to the brain.
- Perception: the painful stimulus is perceived by the brain.
Loco-regional anesthesia targets transduction (e.g., infiltration of lidocaine near the margins of a wound will desensitize nociceptors in the tissue bed), transmission (e.g., injection of a local anesthetic near/on peripheral sensory nerves, as in a sciatic nerve block), and modulation (e.g., epidural administration of a local anesthetic that blocks dorsal root ganglion and dorsal horn sensory neurons that are involved in processing pain from the periphery).
- Q. Why is loco-regional anesthesia important if the patient is under anesthesia?
- A. When a patient is under general anesthesia, it is generally accepted that they do not “feel” pain as the perception of pain is absent due to the effect of general anesthetics on the cerebral cortex and other brain centers. However, the physiologic responses to painful stimuli are very much present even in the anesthetized state. This is easily observed in the anesthetized patient when painful surgical stimuli result in an increased heart rate, respiratory rate, and blood pressure. While these are the changes easily observed in monitored parameters, there are a whole number of physiologic changes that occur under the surface. These include sympathetic nervous system stimulation resulting in decreased renal blood flow, increased central sensitization to pain (“wind up pain”), sodium and water retention, decreased wound healing, and immune compromise [1]. Furthermore, the painful patient generally requires greater amounts of anesthetic in order to remain adequately anesthetized, often resulting in adverse cardiovascular and respiratory effects.
Loco-regional anesthesia is the most successful element of analgesia in that the pain process is blocked entirely, thus avoiding negative consequences of the physiologic responses to pain. When possible, and applicable to the procedure being performed, incorporation of loco-regional anesthesia is a highly effective multimodal approach to treating pain and will reduce patient requirements for other analgesic drugs such as opioids and NSAIDs. That said, the expected duration of blockade with a loco-regional technique should be considered and other analgesic drugs should be available as soon as it is anticipated that the loco-regional effects are waning.
- Q. What are local anesthetics?
- A. Local anesthetics are pharmacologic agents that function primarily to block neuronal transmission. This results in loss of sensory, motor, and autonomic functions of the nerve that has been blocked. The degree of sensory blockade relative to motor and autonomic blockade can depend on the specific local anesthetic used and the dose/route of administration. All of the local anesthetics exert their effects by blocking sodium channels and differ in their onset time and duration of effects.
- Q. What are the common local anesthetics used in veterinary medicine? Do they have different durations of effect?
- A. The properties of commonly used local anesthetics are listed in Table 23.1. Fast onset refers to nearly immediate effect while moderate onset refers to roughly 15–30 min for effect. Onset time is typically slightly more rapid and duration shorter with epidural or spinal administration versus peripheral nerve blockade.
Table 23.1 Local anesthetic drugs, onset time, duration, and toxic doses.
Drug
Onset
Duration (hours)
Toxic dose (dog)
Lidocaine
Fast
1–3
10 mg/kg (dog)
6 mg/kg (cat)
Mepivacaine
Fast
2–4
>5 mg/kg (dog)
>2.5 mg/kg (cat)
Bupivacaine
Moderate
4–12
3 mg/kg (dog)
2 mg/kg (cat)
Ropivacaine
Moderate
5–8
5 mg/kg
- Q. What are signs of local anesthetic toxicity? What are the toxic doses in dogs and cats?
- A. Local anesthetic toxicity may occur in loco-regional anesthesia if inadvertent intravascular injections occur or if dose calculations are incorrect, especially in very small patients. It is important to carefully calculate drug dosages. Dilute local anesthetic agents as necessary and aspirate prior to injection to ensure absence of blood to avoid intravascular injection.
Should local anesthetic toxicity occur, clinical signs include [2]:
- nystagmus
- muscle twitching
- seizures
- CNS depression
- hypotension
- death.
See Table 23.1 for toxic doses in dogs and cats. Individual patient sensitivity can vary and ancillary local anesthetic administration (e.g., lidocaine constant rate infusion) should be factored in when estimating total dose administered to a patient.
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- A. There are five steps of the nociceptive process [1]:
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