Local anesthesia is produced by local anesthetic agents reversibly blocking sodium transport across neural membranes. The anesthetic may be absorbed through a mucous membrane, infiltrated into an operative area, injected in close proximity to a peripheral nerve, or placed in the extradural (epidural and caudal anesthesia) or subarachnoid (spinal anesthesia) space. Subcutaneous infiltration is useful for minor surgical procedures, such as excising lumps and suturing wounds. Wound edges may also be infiltrated with a long acting anesthetic to aid postoperative pain relief. Infiltration should not be undertaken through infected tissues and it is difficult to obtain adequate anesthesia of inflamed areas.
This section considers common sites for neural blockade. The choice of anesthetic depends on the length of anesthesia required (short acting, e.g. lidocaine, acts within a few minutes and lasts 30 – 60 minutes, whereas long acting, e.g. bupivacaine takes 15 – 30 minutes to act and lasts for a few hours). The dosage varies with the preparation used, the effect required, the vascularity of the region, and the age, weight, physique and clinical condition of the patient: 1 and 2% solutions are usually available, but for larger areas and epidural use, solutions of 0.25 and 0.5% are appropriate. A large number of proprietary preparations exist, and you must read the manufacturer’s instructions and seek expert advice for the agent you intend to use; particularly note recommended doses for children. The maximum dose is usually 200 mg for an adult for short acting and 400 mg in 24hours for long acting preparations. This maximum dosage may be doubled when preparations include adrenalin; this produces local vasoconstriction and reduces the usual dilatory effect of the local anesthetic. Adrenalin preparations also prolong the anesthetic effect. The usual concentration of adrenalin is 1:200,000 (up to 1:80,000 in dental practice). NB Adrenalin preparations must NEVER be used in digital anesthesia, as they may produce severe vasoconstriction of the digital vessels with the potential of ischemic damage.
An aseptic technique is essential when injecting a local anesthetic. It often precedes an operative procedure, and full skin preparation and toweling of the region may be required. Adequate time must be allowed for the anesthetic to work before commencing any operative procedure. You must be aware of the potential complications of local anesthetic. These are dose related, and more likely to occur with inadvertent intravascular injection. You must therefore always withdraw the plunger of the syringe before injecting the anesthetic, and the injection must be given slowly, to detect any immediate untoward effects before the full dose is administered.
Read the manufacturer’s instructions concerning drug interactions, these include beta-blockers, vasoconstrictor and some antiviral preparations. The effects of toxicity, and the occasionally encountered hypersensitivity, include myocardial depression and arrhythmias, and cerebral excitation; you should therefore monitor the effects of an injection for at least 30 minutes. Management may include CPR and the maintenance of fluid and electrolyte balance. Other complications relate to the damage of adjacent structures and anatomical knowledge is essential when undertaking regional blockade.
The scalp commonly requires suturing after head injuries. The first priority is to establish the severity of the injury and to ensure that you are not dealing with a skull fracture or an underlying intracerebral hemorrhage. During this time, digital pressure can be applied to the wound edge and then firm bandaging to reduce the heavy blood loss that may occur from this vascular structure. Anesthesia is usually by local infiltration, allowing debridement, removing just enough hair to determine the extent of the laceration. The mobility of the scalp over the periosteum and the rapid healing, due to its vascularity, allow you to close most defects.
The whole scalp can be anesthetized by injecting anesthetic around the nerves supplying it. The supraorbital nerve emerges from the orbit through the supraorbital notch or foramen (figure 27a), and the supratrochlear lies medial to it. The contribution from the maxillary nerve is through the lacrimal, emerging from the lateral aspect of the orbit, and the zygomatic branches emerging from the zygomatic bone (figure 27b). The auriculotemporal nerve passes a centimeter anterior to the upper part of the ear (figure 27c). The lesser occipital (figure 27d) passes cranially near the posterior margin of the ear and the greater occipital (figure 27e) lies 3–4 cm from the midline.
The infraorbital and mental nerves are in line with the supraorbital, 1.5 cm from the midline. The infraorbital (figure 28a) leaves the maxilla a centimeter below the infraorbital margin, and supplies the lower eyelid, cheek, upper lip and anterior nares. The mental (figure 28b) emerges from the mental foramen, directed laterally, about 1.5 cm above the lower mandibular margin, and supplies the lower lip and chin.
Ocular surgery may be undertaken under local anethesia. As well as infiltrating the nerves already described, supplying the skin around the orbit, peribulbar injection (figure 29a–c) is through the lid or, by retracting it, through the underlying conjunctival sac. The needle passes backwards below the eyeball along the orbital floor for 3–4 cm.
The mandibular nerve (figure 30) emerges through the foramen ovale. This site, in the pterygoid fossa, is about 4 cm deep to the mandibular notch. The surface markings of the notch can be found by asking the subject to open their mouth. Place your finger on the condyle of the mandible and, on closing the mouth, your finger rests over the mandibular notch. Pass the needle horizontally and medially for 4 cm in the coronal plane, from this point.