Topical anesthetic



Local anesthesia

Local anesthesia is any technique to induce the absence of sensation in part of the body generally for the aim of inducing local analgesia, that is, local insensitivity to pain, although other local senses may be affected as well. It allows patients to undergo surgical and dental procedures with reduced pain and distress. In many situations, such as cesarean section, it is safer and therefore superior to general anesthesia. It is also used for relief of non-surgical pain and to enable diagnosis of the cause of some chronic pain conditions. Anesthetists sometimes combine both general and local anesthesia techniques.

The following terms are often used interchangeably:

• Local anesthesia, in a strict sense, is anesthesia of a small part of the body such as a tooth or an area of skin.

• Regional anesthesia is aimed at anesthetizing a larger part of the body such as a leg or arm.

• Conduction anesthesia is a comprehensive term, which encompasses a great variety of local and regional anesthetic techniques.

Medical

Local anesthetic

A local anesthetic is a drug that causes reversible local anesthesia and a loss of nociception. When it is used on specific nerve pathways (nerve block), effects such as analgesia (loss of pain sensation) and paralysis (loss of muscle power) can be achieved.

Clinical local anesthetics belong to one of two classes: aminoamide and aminoester local anesthetics. Synthetic local anesthetics are structurally related to cocaine. They differ from cocaine mainly in that they have no abuse potential and do not act on the sympathoadrenergic system, i.e. they do not produce hypertension or local vasoconstriction, with the exception of Ropivacaine and Mepivacaine that do produce weak vasoconstriction.

Local anesthetics vary in their pharmacological properties and they are used in various techniques of local anesthesia such as:

• Topical anesthesia (surface)

• Infiltration

• Plexus block

• Epidural (extradural) block

• Spinal anesthesia (subarachnoid block)

Adverse effects depend on the local anesthetic agent, method, and site of administration and is discussed in depth in the local anesthetic sub-article, but overall, adverse effects can be:

1. localized prolonged anesthesia or paresthesia due to infection, hematoma, excessive fluid pressure in a confined cavity, and severing of nerves & support tissue during injection,

2. systemic reactions such as depressed CNS syndrome, allergic reaction, vasovagal episode, and cyanosis due to local anesthetic toxicity.

3. lack of anesthetic effect due to infectious pus such as an abscess.

Topical anesthetic,

A topical anesthetic is a local anesthetic that is used to numb the surface of a body part. They can be used to numb any area of the skin as well as the front of the eyeball, the inside of the nose, ear or throat, the anus and the genital area[1]. Topical anesthetics are available in creams, ointments, aerosols, sprays, lotions, and jellies[2]. Examples include benzocaine, butamben, dibucaine, lidocaine, oxybuprocaine, pramoxine, proparacaine, proxymetacaine, and tetracaine (also named amethocaine).

Usage

Topical anesthetics are used to relieve pain and itching caused by conditions such as sunburn or other minor burns, insect bites or stings, poison ivy, poison oak, poison sumac, and minor cuts and scratches[3].

Topical anesthetics are used in ophthalmology and optometry to numb the surface of the eye (the outermost layers of the cornea and conjunctiva) to:

• Perform a contact/applanation tonometry.

• Perform a Schirmer's test (The Schirmer's test is sometimes used with a topical eye anesthetic, sometimes without. The use of a topical anesthetic might impede the reliability of the Schirmer's test and should be avoided if possible.).

• Remove small foreign objects from the uppermost layer of the cornea or conjunctiva. The deeper and the larger a foreign object which should be removed lies within the cornea and the more complicated it is to remove it, the more drops of topical anesthetic are necessary prior to the removal of the foreign object to numb the surface of the eye with enough intensity and duration.

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Infiltration (medical)

Infiltration is the diffusion or accumulation (in a tissue or cells) of substances not normal to it or in amounts in excess of the normal. The material collected in those tissues or cells is called infiltrate.

Classification

As part of a disease process, infiltration is sometimes used to define the invasion of cancer cells into the underlying matrix or the blood vessels. Similarly the term may describe the deposition of amyloid protein. During leukocyte extravasation white blood cells move in response to cytokine chemicals (chemotaxis), from within the blood out to infiltrate into the diseased or infected tissues. The presence of lymphocytes in tissue in greater than normal numbers is likewise called infiltration.

As part of medical intervention, local anaesthetics may be injected at more than one point so as to infiltrate an area prior to a surgical procedure. However the term may also apply to unintended iatrogenic leakage of fluids from phlebotomy or intravenous drug delivery procedures, a process also known as extravasation or "tissuing"..

Etiology

Infiltration may be caused by:

• Puncture of distal vein wall during venipuncture

• Puncture of any portion of the vein wall by mechanical friction from the catheter/needle cannula

• Dislodgement of the catheter/needle cannula from the intima of the vein which may be a result of a poorly secured IV device or the selection of which venous site is used.

• Improper cannula size or excessive delivery rate of the fluid .

Epidural

An epidural catheter after insertion. The site has been prepared with tincture of iodine. Depth markings may be seen along the shaft of the catheter.

The term epidural is often short for epidural analgesia, a form of regional analgesia involving injection of drugs through a catheter placed into the epidural space. The injection can cause both a loss of sensation (anaesthesia) and a loss of pain (analgesia), by blocking the transmission of signals through nerves in or near the spinal cord.

The epidural space is the space inside the bony spinal canal but outside the membrane called the dura mater (sometimes called the "dura"). In contact with the inner surface of the dura is another membrane called the arachnoid mater ("arachnoid"). The arachnoid encompasses the cerebrospinal fluid that surrounds the spinal cord.

Difference from spinal anesthesia

Spinal anaesthesia is a technique whereby a local anaesthetic drug is injected into the cerebrospinal fluid. This technique has some similarity to epidural anaesthesia, and the two techniques may be easily confused with each other. Differences include:

• The involved space is larger for an epidural, and subsequently the injected dose is larger, being about 10-20 ml in epidural anesthesia compared to 1,5-3,5 ml in a spinal.

• In an epidural, an indwelling catheter may be placed that avails for additional injections later, while a spinal is usually one-shot only; though a continuous spinal can also be administered. Especially, in pain management with morphine pumps.

• The onset of analgesia is approximately 15–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.

• An epidural usually doesn't cause significant neuromuscular block at the lower effective analgesic dosages, while a spinal more often does.

• An epidural may be given at a thoracic or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.

• With epidural, it is possible to create segmental blocks as opposed to spinal where the block involves all segments below the highest level of anesthesia.

• The extension of the block with epidural anesthesia is highly dependent on the volume and rate of injection. The position of the patient has little to no impact on the level of the block.

• With spinal anesthesia, the density of the solution combined with the position of the patient influences the level of the block significantly.

Consequently, epidural is safer if a higher level of block is required.

Indications

Injecting medication into the epidural space is primarily performed for analgesia. This may be performed using a number of different techniques and for a variety of reasons. Additionally, some of the side-effects of epidural analgesia may be beneficial in some circumstances (e.g., vasodilation may be beneficial if the patient has peripheral vascular disease). When a catheter is placed into the epidural space (see below) a continuous infusion can be maintained for several days, if needed. Epidural analgesia may be used:

• For analgesia alone, where surgery is not contemplated. An epidural for pain relief (e.g. in childbirth) is unlikely to cause loss of muscle power, but is not usually sufficient for surgery.

• As an adjunct to general anaesthesia. The anaesthetist may use epidural analgesia in addition to general anaesthesia. This may reduce the patient's requirement for opioid analgesics. This is suitable for a wide variety of surgery, for example gynaecological surgery (e.g. hysterectomy), orthopaedic surgery (e.g. hip replacement), general surgery (e.g. laparotomy) and vascular surgery (e.g. open aortic aneurysm repair). See also caudal epidural, below.

• As a sole technique for surgical anaesthesia. Some operations, most frequently Caesarean section, may be performed using an epidural anaesthetic as the sole technique. Typically the patient would remain awake during the operation. The dose required for anaesthesia is much higher than that required for analgesia.

• For post-operative analgesia, after an operation where the epidural was used as either the sole anesthetic, or was used in combination with general anesthesia. Analgesics are given into the epidural space for a few days after surgery, provided a catheter has been inserted. Through the use of a patient-controlled epidural analgesia (PCEA) infusion pump, a patient has the ability to give an occasional extra dose of post-surgical pain medications administered through the epidural.

• For the treatment of back pain. Injection of analgesics and steroids into the epidural space may improve some forms of back pain. See below.

• For the treatment of chronic pain or palliation of symptoms in terminal care, usually in the short- or medium-term.

Technique of insertion

Epidural anaesthesia requires a high level of technical proficiency to avoid serious complications, and should always be performed by a trained anaesthetist, using a strict aseptic technique to reduce the risk of infection.

Insertion site

The anaesthetist palpates the patient's back and identifies a suitable anatomical gap between the bony spinous processes prior to the procedure. The level of the spine at which the catheter is best placed depends mainly on the site and type of an intended operation or the anatomical origin of pain. The iliac crests are commonly used for reference in order to locate the L4 vertebra, which is well below the termination of the spinal cord. Since innervation of the chest and abdomen travels under the ribs, the anaesthetist can palpate along the corresponding rib to determine placement of the catheter tip.

Most commonly, the anaesthetist conducting an epidural places the catheter in the mid-lumbar, or lower back region of the spine, although occasionally a catheter is placed in the thoracic (chest) or cervical (neck) region. In adults, the spinal cord terminates around the level of the disc between L1 and L2(in neonates it extends to L3 but can reach as low as L4), below which lies a bundle of nerves known as the cauda equina ("horse's tail"). Hence, lumbar epidurals carry a very low risk of injuring the spinal cord.

Locating the epidural space

The skin is infiltrated with local anaesthetic such as lidocaine over the identified space. The insertion point is usually in the midline, although other approaches, such as the paramedian approach, may occasionally be employed. In the paramedian approach, the needle tip passes along a shelf of vertebral bone called the lamina until just before reaching the ligamentun flavum and the epidural space. 'Walking' the needle tip off this lamina allows the clinician to be confident that they are close to the epidural space. This is particularly important in the thoracic spine, where the spinal cord is larger (than in the lumbar spine) and nearly fills the spinal canal increasing the risk of dural puncture and cord damage.

A particular type of needle known as a Tuohy needle is almost invariably used. This needle was specially designed for locating the epidural space safely, and has several specific features for this purpose.

The Tuohy needle is inserted to the ligamentum flavum, is attached to a syringe in the peripheral end, and slowly advanced between two spinous processes. The loss of resistance to injection technique is used to identify the epidural space. This technique is to apply constant pressure on the piston of the syringe towards the barrel as if unfusing, and the loss of resistance is where it is be possible to inject through the syringe, so the piston can easily move into the barrel. This technique works because the ligamentum flavum is extremely dense, and injection into it is almost impossible. In the epidural space, on the other hand, there is negative or neutral pressure.

The syringe may contain air or saline. The principles are the same, but the specifics of the technique are different due to the greater compressibility of air with respect to saline.

Loss of resistance indicates a high likelihood that the tip of the needle has entered the epidural space. A sensation of "pop" or "click" may be felt as the needle breaches the ligamentum flavum just before entering the epidural space. A technique involving constant application of pressure to identify the epidural space whilst advancing the Tuohy needle was described as Dogliotti's principle in 1933. An innovative technique for teaching this sensation of 'loss of resistance' using a banana was described by Leighton in Anesthesiology 70:368-9; 1989 - "A greengrocer's model of the epidural spaceTraditionally anesthesiologist have used either air or saline for identifying the epidural space, depending on their personal preference. However, evidence is accumulating that saline may result in more rapid and satisfactory quality of analgesia In addition to the loss of resistance technique, realtime observation of the advancing needle is becoming more common. This may be done using a portable ultrasound scanner, or with fluoroscopy (moving X-ray pictures)

Anaesthetic drugs

A patient receiving an epidural for pain relief typically receives a combination of local anesthetics and opioids. This combination works better than either type of drug used alone. Common local anesthetics include lidocaine, bupivacaine, ropivacaine, and chloroprocaine. Common opioids include morphine, fentanyl, sufentanil, and pethidine (known as meperidine in the U.S.). These are injected in relatively small doses.

Occasionally other agents may be used, such as clonidine or ketamine.

Epidural infusion pump with opioid (sufentanil) in a locked box

For a short procedure, the anaesthetist may introduce a single dose of medication (the "bolus" technique). This will eventually wear off. Thereafter, the anaesthetist may repeat the bolus provided the catheter remains undisturbed.

For a prolonged effect, a continuous infusion of drugs may be employed. A common solution for epidural infusion in childbirth or for post-operative analgesia is 0.2% ropivacaine or 0.125% bupivacaine, with 2 μg/mL of fentanyl added. This solution is infused at a rate between 4 and 14 mL/hour, following a loading dose to initiate the nerve block.

There is some evidence that an automated intermittent bolus technique provides better analgesia than a continuous infusion technique, though the total doses are identical.

Extensions or alternative techniques

Combined spinal-epidurals

For some procedures, the anaesthetist may choose to combine the rapid onset and reliable, dense block of a spinal anaesthetic with the post-operative analgesic effects of an epidural. This is called combined spinal and epidural anaesthesia (CSE).

The anaesthetist may insert the spinal anaesthetic at one level, and the epidural at an adjacent level. Alternatively, after locating the epidural space with the Tuohy needle, a spinal needle may be inserted through the Tuohy needle into the subarachnoid space. The spinal dose is then given, the spinal needle withdrawn, and the epidural catheter inserted as normal. This method, known as the "needle-through-needle" technique, may be associated with a slightly higher risk of placing the catheter into the subarachnoid space.

Caudal epidurals

The epidural space may be entered through the sacrococcygeal membrane, using a 22g catheter-over-needle or regular 21G needle. Injecting a volume of 1 cc/kg of local anaesthetic here provides good analgesia of the perineum and groin areas. This is typically a single-injection technique and a catheter is not normally placed. This is known as a caudal epidural or "caudal".

The caudal epidural is an effective and safe analgesic technique in children undergoing groin, pelvic or lower extremity surgery. It is usually combined with general anaesthesia since children cannot tolerate the injection awake.

Potential problems

Side effects

In addition to blocking the nerves which carry pain, local anaesthetic drugs in the epidural space will block other types of nerves as well, in a dose-dependent manner. Depending on the drug and dose used, the effects may last only a few minutes or up to several hours. Epidural typically involves using the opiates fentanyl or sufentanil, with bupivacaine, Fentanyl is a powerful opiate with a potency and side effects 80X that of morphine. Sufentanil is another opiate, 5 to 10Xs more potent than Fentanyl. Bupivacaine is markedly toxic, causing excitation: nervousness, tingling around the mouth, tinnitus, tremor, dizziness, blurred vision, or seizures, followed by depression: drowsiness, loss of consciousness, respiratory depression and apnea. Bupivacaine has caused several deaths by cardiac arrest when epidural anesthetic has been accidentally inserted into vein instead of epidural space in the spine. Epidural correctly administered results in three main effects:

• Loss of other modalities of sensation (including touch, and proprioception)

• Loss of muscle power (hence, a risk of falling)

• Loss of function of the sympathetic nervous system, which controls blood pressure

Pain nerves are most sensitive to the effects of the epidural. This means that a good epidural can provide analgesia without affecting muscle power or other types of sensation. The larger the dose used, the more likely it is that the side-effects will be problematic.

For example, a laboring woman may have a continuous epidural during labor that in 85% of cases provides good analgesia without impairing her ability to move around in bed. If she requires a Caesarean section, she is given a larger dose of epidural bupivacaine. After a few minutes, she can no longer move her legs, or feel her abdomen. If her blood pressure drops below 80/50 she is given an intravenous bolus of ephedrine or phenylephrine infusion to compensate. During the operation, she feels no pain.

Very large doses of epidural anaesthetic can cause paralysis of the intercostal muscles and diaphragm (which are responsible for breathing), and loss of sympathetic function to the heart itself, causing a profound drop in heart rate and blood pressure. This requires emergency treatment, and in severe cases may require airway support. This happens because the epidural is blocking the heart's sympathetic nerves, as well as the phrenic nerves, which supply the diaphragm.

It is considered safe practice for all patients with epidurals to be confined to bed to prevent the risk of falls.

The sensation of needing to urinate is diminished, which often requires the placement of a urinary catheter for the duration of the epidural

Opioid drugs in the epidural space are relatively safe (as well as effective). However, very large doses may cause troublesome itch, and rarely, delayed respiratory depression.[25][26][27][28]

Difference from epidural anaesthesia

Epidural anaesthesia is a technique whereby a local anaesthetic drug is injected through a catheter placed into the epidural space. This technique has some similarity to spinal anaesthesia, and the two techniques may be easily confused with each other. Differences include:

• The involved space is larger for an epidural, and subsequently the injected dose is larger, being about 10–20 mL in epidural anaesthesia compared to 1.5–3.5 mL in a spinal.

• In an epidural, an indwelling catheter may be placed that avails for additional injections later, while a spinal is almost always a one-shot only.

• The onset of analgesia is approximately 15–30 minutes in an epidural, while it is approximately 5 minutes in a spinal.

• An epidural often does not cause as significant neuromuscular block unless specific local anaesthetics are used which block motor fibres as readily as sensory nerve fibres, while a spinal more often does.

• An epidural may be given at a cervical, thoracic, or lumbar site, while a spinal must be injected below L2 to avoid piercing the spinal cord.

Injected substances

Bupivacaine (Marcaine) is the local anaesthetic most commonly used, although lignocaine (lidocaine), tetracaine, procaine, ropivacaine, levobupivicaine and cinchocaine may also be used. Sometimes a vasoconstrictor such as epinephrine is added to the local anaesthetic to prolong its duration. Of late, many anaesthesiologists are preferring to add opioids like morphine, fentanyl, or buprenorphine, or non-opioids like clonidine, to the local anaesthetic used in a spinal injection, to give a smoother effect and to provide prolonged pain relief once the action of the spinal local anaesthetic has worn off.

Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space. Usually, the hyperbaric, (for example, hyperbaric bupivacaine) is chosen, as its spread can be effectively and predictably controlled by the anaesthesiologist, by tilting the patient. Hyperbaric solutions are made more dense by adding dextrose to the mixture.

Mechanism

Regardless of the anaesthetic agent (drug) used, the desired effect is to block the transmission of afferent nerve signals from peripheral nociceptors. Sensory signals from the site are blocked, thereby eliminating pain. The degree of neuronal blockade depends on the amount and concentration of local anaesthetic used and the properties of the axon. Thin unmylenated C-fibres associated with pain are blocked first, while thick, heavily mylenated A-alpha motor neurons are blocked last. The desired result is total numbness of the area. A pressure sensation is permissible and often occurs due to incomplete blockade of the thicker A-beta mechanoreceptors. This allows surgical procedures to be performed with no painful sensation to the person undergoing the procedure.

Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake.

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Indications

This technique is very useful in patients having an irritable airway (bronchial asthma or allergic bronchitis), anatomical abnormalities which make endotracheal intubation very difficult (micrognathia), borderline hypertensives where administration of general anaesthesia or endotracheal intubation can further elevate the blood pressure, procedures in geriatric patients.

Contraindications

Non-availability of patient's consent, local infection or sepsis at the site of lumbar puncture, bleeding disorders, space occupying lesions of the brain, disorders of the spine and maternal hypotension.

Complications

Can be broadly classified as immediate (on the operating table) or late (in the ward or in the P.A.C.U. post-anaesthesia care unit):

• Spinal shock.

• Cauda equina injury.

• Cardiac arrest.

• Hypothermia.

• Broken needle.

• Bleeding resulting in haematoma, with or without subsequent neurological sequelae due to compression of the spinal nerves

• Infection: immediate within six hours of the spinal anaesthetic manifesting as meningism or meningitis or late, at the site of injection, in the form of pus discharge, due to improper sterilization of the LP set.

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