Local anaesthesia works by blocking the conduction of sensory stimuli in afferent nerve fibres. Because thin nerve fibres are blocked before thick nerve fibres, impairment of stimulus conduction occurs in the following sequence: pain – cold – heat – contact- pressure- motor.
Methods
- Freezing the skin with ethyl chloride spray. The technique is described in the section “Skills.” Used for incisions of superficial abscesses.
- Surface anaesthesia of mucous membranes using drops or sprays (0.5% lidocaine). Used in ophthalmology (tonometry, removal of foreign bodies) and in otorhinolaryngology (paracentesis and indirect laryngoscopy).
- Infiltration anaesthesia involves infiltration of the skin and subcutis, surrounding the tumour or edges of a wound with a local-acting anaesthetic. The technique is described in the section “Skills.” Used in debridement and minor surgical interventions.
- Guided anaesthesia involves distally guided anaesthesia using the Oberst technique. A nerve sheath is selectively infiltrated with a local-acting anaesthetic so that the area for which this sensory nerve is responsible becomes numb. Feeling is lost from the base to the tip. The technique is described in the section “Skills.” This type of local anaesthesia is used for operations involving the fingers, toes, and penis.