Local Anaesthetics

last authored: March 2010, David LaPierre
last reviewed:

 

 

Introduction

Local anesthetics are used to block nerve conduction, preventing pain sensation. They are used for minor procedures. Physicians often underestimate the amount of discomfort inflicted by medical procedures. It is important to have a clear understanding of the methods available to reduce patient discomfort and to have a liberal attitude in their use.

Local anesthetics can be used in place of general anesthetic to reduce nausea and vomiting, decrease cardiovascular risk, encourage earlier hospital discharge.

 

Local anaesthetics are a key element in ensuring patient comfort during many medical procedures, however, it is important to recognize that the simple acts of explaining a procedure, offering reassurance and placing the patient in a comfortable position, go a long way toward reducing patient anxiety and discomfort.

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Indications

Local anesthetics are used for minor procedures, such as wound exploration and cleaning, suturing, fracture reduction, and excisions. They can also be used intraoperatively to reduce pain during recovery.

 

Local anaesthetic agents are most commonly used in one of three ways:

• topical application
• direct local infiltration
• regional anaesthesia (nerve block)

Some procedures are so extensive, painful or time consuming that they are best performed in the operating room under a general anaesthetic. In other circumstances it may be reasonable to take a balanced approach and combine light intravenous sedation and pain relief with a local agent. This is particularly useful in circumstances where a local anaesthetic alone may not provide adequate comfort, such as drainage of a large cutaneous abscess.

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Clinical Usage and Tips

Local anaesthetics are lipid soluble and so can cross the BBB. As such, it is a good idea to keep local anaesthetics local! Anesthetics with epinephrine are very useful for reducing bleeding at the site of injection via vasoconstriction. This can be especially helpful when attempting to visualize the field of view.

 

Epinephrine also keeps the anesthetic local, allowing greater doses to be given by reducing the risk of systemic adverse events.

 

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Contraindications

Always ask about a history of allergies prior to administration of local anaesthetics. True anaesthetic allergies are extremely uncommon. Most episodes attributed to allergy are, in fact, vasovagal reactions, hyperventilation from pain or the effects of drug over dosage. A careful history will help discern these patients from those with a true allergic reaction i.e. wheezing, hives, low BP.

Patients who are allergic to the ester group will usually be fine with an amide - and vise versa. Unfortunately, the preservative in multiuse vials of amides is an ester (methylparaben); use a single dose vial instead.

 

You may use an unrelated agent such as Diphenhydramine (Benadryl) - this is an antihistamine which is also a decent local anaesthetic for infiltration - but not for regional blocks.

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Subcutaneous Injections

© 2006-2007 PocketSnips, Northern Ontario School of Medicine. (www.pocketsnips.org)   Video: Injection of Local Anesthetic. Used with permission.   The video is not a substitute for medical advice.

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Regional Blocks

Digital blocks

SIMPLE - single injection median palmar lidocaine with epinephrine (shown in image).

• inject into the prximal crease of the palmar surface of the finger.
• Will cover entire volar finger and the dorsal side up to the PIP joint.
• better as there is only one injection

Dorsal block

• place your finger under theirs
• inject once you can see or feel tenting on their palmar side

 

Metacarpal block

• does the ray
• good for boxer's fracture reduction
• ensure you are not too proximal
• inject dorsally straight down until you see/feel tenting
• aspirate slightly before injecting

 

Hematoma block

Used for fractures; inject directly into the fracture hematoma. Often used by emergency room physicians and orthopedics, often under conscious sedation.

 

Median nerve blocks

Anesthesia can do a brachial plexus block, under local anesthesia and with ultrasound guidance.

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Overdose and Adverse Events

Sigificant side effects can occur with lidocaine overdose, such as:

• peri-oral numbness (often first symptom)
• convulsions
• blurry vision
• allergic reaction
• metallic taste

Lidocaine overdose can be lethal following autonomic instability.

• tachycardia
• arrhythmias with lidocaine (even though that is what it is sometimes used for!)

There is no antidote for lidocaine, but there is for epinephrine.

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Mechanism of Action

Lidocaine blocks fast-acting sodium channels and is also used for arrhythmias.

Esters are rapidly metabolized by blood esterases, so have a short half-life.

Amides are hydrolyzed in the liver, so longer/more varied half-life.

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To sort this

Agent	Typical Application	Dose and Route
Lidocaine Viscous	Oral mucous membranes i.e. To allow laryngoscopy	2 - 4 % spray or gargle (< 4 mg/kg)
Cocaine	Nasal mucous membranes i.e. For cautery of a nose bleed.	4 % solution usually applied to cotton pledget. (< 3 mg/kg)
TAC Tetracaine 0.5%, Adrenaline 0.05% Cocaine 11.8 %	Applied to intact skin ~ 30 min before a procedure i.e. IV start. Debatable use in wounds. Most often used in children. Toxicity if applied to mucous membranes.	3 - 5 ml applied topically.
EMLA	Eutectic mixture of local anaesthetics	Cream which is applied topically

Local	Regional (Nerve Block)
Easier, No specific landmarks/technique	Requires specific knowledge of anatomy etc.
Local Anaesthetic: injected into the open wound through the wound edge	Less painful than going through the skin
Faster onset (1 - 2 minutes)	Slower onset (5 - 20 minutes)
Affects specific wound area only	Affects region supplied by nerve(s)
Causes distortion of local tissues/landmarks	No local distortion, preserves appearance
May increase local tissue damage/infection	None
Not suitable for some areas i.e. digits, ears etc.	Good option for these cases
Multiple wounds - multiple injections	Good for multiple or complex wounds in one nerve distribution.
Little risk of neuro vascular injury	Possible injury to nerve or blood vessel

 

 

 

Additional Resources

 

 

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Topic Development

created: DLP, Aug 09

authors: DLP, Aug 09

editors:

reviewers:

 

 

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