Local anaesthetics

Local anaesthetics are sodium channel blockers which block pain sensation in a smaller area (inducing regional anaesthesia) by preventing transmission of pain signals from pain receptors to the CNS. They are administered intramuscularly, subcutaneously, intravenously, intraarticularly, topically, but not orally. Local anaesthetics are used to prevent or eliminate pain during surgical procedures.

Indications

Surface anaesthesia – by applying directly to mucous membrane or skin
- As a spray, gel, ointment, etc.
Infiltration anaesthesia – by giving multiple injections targeting small nerve branches, as done at the dentist
Nerve block anaesthesia – giving injections close to major nerve trunks, which anaesthetizes the entire are innervated by this trunk
Spinal anaesthesia – by administering directly into the subarachnoid space, into the CSF
Epidural anaesthesia – by administering directly into the epidural space
IV regional anaesthesia – giving IV and applying a tourniquet to an extremity

Lidocaine is the most used local anaesthetic for the skin as it has the highest penetration to the skin. It can also be given IV as an antiarrhythmic, although this is rare. As a gel lidocaine can be used prior to catherization of the bladder or in the form of a spray prior to performing laryngoscopy.

Mechanism of action

Local anaesthetics work by reversibly blocking voltage-gated Na+ channels. These channels are responsible for the upstroke phase of the action potential in nerve fibres. By blocking them will there be no depolarization, so nociceptive nerve signals won’t be carried to the brain. These drugs physically “plug” and therefore block the channels. At high concentrations they may block other ion channels as well.

Side effects

Voltage-gated Na+ channels are found everywhere, so when the local anaesthetics are absorbed into the plasma channels all over the body can be affected.

CNS effects:
- Tremor
- Confusion, agitation
- Paralysis
- Seizures
Cardiovascular effects:
- Bradycardia (negative chronotropic effect)
- AV block (negative dromotropic effect)
- Decreased contractility (negative inotropic effect)
- Arrhythmias
- Vasodilation
Allergic reactions
- Mostly to ester local anaesthetics

Spinal anaesthesia carries some extra risk. Not only sensory nerves travel in the spinal cord, but autonomic and motor nerves as well. The following effects may occur:

Sympathetic block -> Bradycardia, hypotension
Parasympathetic block -> urinary retention
Paralysis of phrenic nerve

Regional anaesthesia

Regional anaesthesia refers to using local anaesthetics to render a specific area of the body anesthetised. It is used for operative anaesthesia but also for post-operative analgesia. They don’t cause loss of consciousness, but they may be combined with general anaesthetics to achieve this.

Many adult surgeries can be performed with regional anaesthesia alone, without general anaesthesia. Regional anaesthesia is becoming more and more popular, as they have various advantages compared to general anaesthesia:

It’s better for elderly
Cheaper
There’s no need for airway management, avoiding potential complications
Less opioids are needed
Less post-operative nausea and vomiting (PONV)
Reduced postoperative pain
Few cardiovascular and pulmonary side effects

Types

There are various types:

Central regional anaesthesia (neuraxial block) – anaesthesia of the spinal cord
- Epidural anaesthesia
- Subarachnoid (spinal) anaesthesia
- Combined spinal-epidural anaesthesia
Peripheral regional anaesthesia (nerve block) – anaesthesia of peripheral nerves
- Major (multiple nerves or a plexus)
  - Cervical plexus block
  - Brachial plexus block
  - Interscalene block
  - Supraclavicular block
  - Infraclavicular block
  - Axillary block
- Minor (single nerve)
  - Sciatic nerve block
  - Femoral nerve block
  - Digital nerve block
- Intravenous regional anaesthesia
- Intraarticular anaesthesia
Local anaesthesia
- Infiltration anaesthesia
- Topical anaesthesia

Central regional anaesthesia

Central regional anaesthesia, also called neuraxial block, refers to either epidural or subarachnoid anaesthesia, forms of regional anaesthesia. Both types may cause haemodynamic changes, especially hypotension, due to blockade of the sympathetic fibres.

Indications

Regional anaesthesia is preferable (compared to general anaesthesia) in:

Surgical oncology
Orthopaedic/traumatological surgery
C-section
Lower limb revascularisation
Obese patients
Elderly patients (prevents post-operative cognitive dysfunction (POCD)
Paediatric surgery (in addition to general anaesthesia)

Epidural anaesthesia

Epidural anaesthesia involves injection of local anaesthetic with or without opioids into the epidural space to act on spinal nerve roots. Nerve roots around the site of injection are affected, while leaving the nerve roots above and below mostly unaffected. Epidural anaesthesia may be administered at any vertebral level. The drugs are administered via a catheter for continuous infusion.

Epidural anaesthesia is often used for surgeries in the lower body, like C-section, hernia repair, knee surgery, or during labour. It may also be used for postoperative or chronic pain management with an epidural catheter.

Spinal anaesthesia

Spinal anaesthesia, also called subarachnoid anaesthesia, involves injection of local anaesthetic with or without opioid into the subarachnoid space to act on the spinal cord. Unlike epidural anaesthesia, all motor and sensory function below the level of injection is blocked. Spinal anaesthesia is usually administered as a single injection.

Spinal anaesthesia may only be placed in the lumbar region. It has more rapid onset than epidural, but also causes more rapid haemodynamic changes than epidural. Like epidural anaesthesia, it is often used for surgeries of the lower body, below the level of the umbilicus.

Epidural and spinal anaesthesia may be combined, called combined spinal-epidural anaesthesia (CSE).

Peripheral regional anaesthesia

Peripheral regional anaesthesia is also known as a nerve block or conduction anaesthesia, a form of regional anaesthesia. With this modality local anaesthetic is administered around the nerve supplying the area to be anaesthetised. This may take place some distance from the surgical site, like for example a brachial plexus block for hand surgery. The result is anaesthesia of the area supplied by the nerve distal to the area of injection.

Indications

Peripheral regional anaesthesia are often used for smaller surgical procedures, like:

Excisional biopsy of naevi or other skin lesions
Wedge resection of ingrown toenail
Suturing of wounds

Nerve block

Nerve blocks are often used for anaesthesia of fingers (example) and toes as well (digital nerve block), in which case local anaesthetic is injected to each side of the digit, as close to the nerves passing there as possible. There is one dorsal and one palmar/plantar digital nerve on each side of the digit, and so local anaesthetic should be deposited at four sites across two injections.

Local anaesthesia

Local anaesthesia involves either infiltration anaesthesia or topical anaesthesia.

Infiltration anaesthesia is similar to nerve blocks but differ in that the aim of infiltration anaesthesia is not to anaesthetise the nerve itself, but the nerve endings in a certain area of tissue. Local anaesthetic is injected directly into and around the area to be anaesthetised, from which it will disperse in the tissue. This is simpler than nerve blocks and preferred for most cases, except for toes and fingers. Infiltration anaesthesia is often used for wounds which are to be examined or sutured, or for skin biopsy.

Topical anaesthesia refers to applying local anaesthetic topically to the area to be anesthetised, often as a spray or gel. This is useful for the upper airway before laryngoscopy, endoscopy, or intubation, and before bladder catheterisation. Topical anaesthesia is especially useful in children.

Intravenous regional anaesthesia

Intravenous regional anaesthesia (IVRA) or Bier block involves isolating an exsanguinated limb from the circulation with a tourniquet and injecting the local anaesthetic intravenously. It’s simple, safe, and effective, and especially suitable for short procedures and outpatient surgery.