Local Anaesthetics for Perioperative Pain Management: Underestimated Potential?
CLASSIC USE OF LOCAL ANAESTHETICS IN A BALANCED ANAESTHESIA PROTOCOL AND PERIOPERATIVE PAIN MANAGEMENT
A combination of local or locoregional anaesthesia with general anaesthesia has several very important advantages: 1) A satisfactory surgical plane of anaesthesia can be obtained with much lesser doses of general anaesthetics (volatile or intravenous agents). This means less cardiovascular and respiratory depression, a more stable anaesthesia, less metabolisation efforts, quicker recoveries. 2) The phenomenon know as "winding-up" of the central nervous system (CNS) will be at least less pronounced. Winding up results in a prolonged sensitization of the CNS and hyperalgesia. With the combination of the local techniques and a general (light) anaesthesia the postoperative analgesia will be better/longer or at least easier to control with additional medication. Complications are few if the recommended maximum dosis of the local anaesthetics are respected.
The advantages of this philosophy cannot be overestimated and are particularly attractive and easy to apply in practice. The classic agents are the more short acting (1-2 hrs) lidocaine (1% to 2%) with a fast (5 min) onset and the longer (4-6 hrs) acting bupivacaine (0.25%-0.7%) with a slow (20 min) onset. Both agents can be mixed together.
The possibilities include: a) topical application; b) tissue infiltration; c) ring block; d) nerve blockade; e) intravenous regional anaesthesia; f) spinal anaesthesia (epidural, subdural).
Off all these techniques topical application, tissue infiltration, ring blocks are very easy to do but their potential benefit is often unrecognised for reasons difficult to explain. Intravenous regional anaesthesia is also an easy procedure. Epidural anaesthesia is in the reach of each practitioner.
1. Topical application
Even if a general anaesthesia procedure is done, noxious output from the operated area can be reduced by application of local anaesthetic ointments, liquid solutions or sprays (lidocaine). Remember that the intact skin is not penetrated except following application of a special formula of prilocaine (EMLA). Application on nasal and oral mucous membranes, esophagus, trachea and urogenital mucosae is possible. Straightforward simple applications during surgical procedures are e.g., splash of lidocaine on the spermatic cord during male castration and on the ovarian pedicle during ovariectomy. Local anaesthetics do not retard wound healing and an increase in the tendency to diffuse bleeding has not been documented as a problem.
2. Infiltrations/ring block
These remain a very simple but often neglected possibility. Aspiration before injection will limit the risk for intravascular injection. Examples of a possible use are e.g., removal of small skin tumors, suturing skin wounds, surgery on the toes (distal phalanx amputation, interdigital wounds or cysts, wounds etc), incisional infiltration of subcutis and fascia before a caesarean or a laparotomy in critical cases.
3. Intravenous regional anesthesia (Bier's block)
Although a simple and effective technique for distal extremity surgery not exceeding 1.5 hrs this anaesthetic technique is not always favoured by some surgeons because of the lack of bleeding in the operated area. It can be very useful for relatively minor surgery on the extremities of critical or debilitated patients. Using a tourniquet and/or cuff combined with intravenous lidocaine an effective analgesia after 5-10 minutes in all tissues distal to the tourniquet/cuff can be obtained, sensibility returns within minutes after release of the tourniquet or cuff.
4. Plexus Brachialis block
This is an excellent technique to be used, e.g., for radius/ulna surgery. It is a block of the n. axillaris, n. medianus, n. ulnaris, n. musculocutaneus, n. radialis following successful infiltration of the area of the plexus brachialis situated at the level of the shoulder joint at the medial side of the scapula. The exact quantities of local anaesthetic to use are not described and depend on the size of the patient and the toxic dosis (e.g., 2-20 ml lidocaine 1 or 2 % mixture). Electrolocation of the plexus with a nerve stimulator is a useful way to obtain maximum block with a small amount of local anaesthetic. If successful there will be analgesia and motoric paralysis (not appreciated when this block is performed under anaesthesia). Analgesia extends from the elbow to distal. Successful block will be apparent when conducting general anaesthesia: low concentration of inhalation agent necessary, stable vital parameters. Complications are rare but pneumothorax, nerve trauma, bleeding and intravascular injection are possible.
5. Intercostal blocks
These are used before the start of a thoracotomy (most logic approach for "preventive") or when starting to close the thorax; the technique can also be used for analgesic support when managing rib fractures. The technique consists of: injection in the intercostal muscles at the level of the caudal aspect of the rib close to its origin 0.2 to 1 ml 2% lidocaine (or bupivacaine, or a mix of bupivacaine and lidocaine) and this in two intercostal spaces before and two spaces after the thoracotomy incision; this can be done from outside or from inside the thorax.
6. Spinal anesthesia
Mostly used is epidural anesthesia. Accidental puncture of the dura and collection of cerebrospinal fluid however is possible when attempting an epidural injection. This will happen more often in the cat were the dural sac is longer than in the dog. The combination of an epidural anaesthesia and general anaesthesia is a very good technique for surgery caudal to the ribs (orthopedics, soft tissues, caesareans...). It allows a light level of general anaesthesia, a stable plane of anaesthesia, and post operative pain relief. In general local anaesthetics are used but they can be combined with opioids.
The local anaesthetic can be combined with an opioid for epidural injection. They will have an interesting and practical synergistic analgesic effect. This way a good and prolonged postoperative analgesic effect can be obtained. Addition of 0.1 mg/kg methadone or morphine can give up to 10 hours of postoperative analgesia (motoric effects having disappeared).
Local anaesthetics offer the opportunity to obtain prolonged postoperative pain relief by using a long acting preparation like bupivacaine, by repeating the described applications (e.g., re-doing an epidural), by combining with other drugs (e.g., epidural with local anaesthetic and an opioid), or by continuous infusion in the epidural space via an epidural catheter. Incisional and intraabdominal application of lidocaine preceding closure of laparotomy wounds has been described in human and in dogs as a mean of postoperative analgesia.
INTRAVENOUS LIDOCAINE IN BALANCED ANAESTHESIA PROTOCOL AND POSTOPERATIVE PAIN MANAGEMENT
Intravenous lidocaine has been shown to reduce the minimum alveolar concentration (MAC) of isoflurane (and halothane) in several species including dogs. For this reason it can be used in a balanced anaesthesia protocol for critical patients. Furthermore it has distinct analgesic properties and can be used as and adjunct in iv infusion to manage difficult cases with postoperative pain; it is claimed particularly useful in protocols for visceral pain and for more chronic pain. Dose iv: 1.5-3 mg/kg followed by a loading dose of 5-200 mcg/kg/min.
SUPPLEMENTARY USEFUL ASPECTS OF LIDOCAINE THERAPY
Lidocaine as an antiarrhythmic agent is classically used to treat ventricular extrasystoles. Dogs presented with gastric volvulus often show these rhythm disturbances. Lidocaine therapy can both treat/prevent ventricular extrasystoles and offer at the same additional analgesia during and after surgery. There is also a large body of evidence from animal experiments that supports the potential of lidocaine as an anti-oxidant and inflammatory modulator useful in preventing reperfusion injury. Therefore it seems attractive to consider it for the treatment/ prevention of systemic inflammatory response (SIRS) and multiple organ dysfunction syndrome (MODS). Although conclusive clinical evidence is lacking lidocaine infusions are also used for their prokinetic properties to prevent/treat postoperative ileus (human, horses). Propulsive activity at the level of the proximal duodenum is enhanced.
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