Sympathetic Nervous System and Spinal Anesthesia

Sympathetic Outflow

What is the sympathetic outflow from the spinal cord?

The sympathetic nervous system is also known as the thoracolumbar outflow. It extends from the T1 segment to the L2 segment of the spinal cord.
Sympathetic fibers emerge from the anterolateral part of the spinal cord.
They move up to form ganglia like the stellate ganglion,
middle cervical sympathetic ganglion, and superior cervical sympathetic ganglion, supplying areas like the face.

What is the difference between sympathetic and parasympathetic outflow?

Sympathetic outflow is thoracolumbar (T1 to L2).
Parasympathetic outflow is craniosacral: cranial (via the vagus nerve) and sacral (S2, S3).

Horner's Syndrome and Sympathetic Block

What is Horner's syndrome?

Horner's syndrome is caused by a sympathetic block.
Symptoms include anhydrosis (loss of sweating), flushing, and warmth on the affected side of the face.

Why don't we typically see Horner's syndrome after a spinal block for a Cesarean section, even though a T4 block causes a total sympathectomy?

In a spinal block, the sympathetic block can extend up to T1, causing a total sympathetic block.
Horner's syndrome is usually not observed because the block is bilateral.
Since it occurs on both sides simultaneously, there is no contralateral side to compare it to, making the signs difficult to detect.

Lidocaine: Preparations and Concentrations

Why are there different concentrations of lidocaine for different routes of administration?

The concentration varies based on the route of administration and the required tissue penetration.
Higher concentrations are needed for routes with unreliable absorption, such as on mucosa, to ensure an effective dose reaches the nerve endings.

What are the concentrations of various lidocaine preparations and their uses?

10% Spray: Used for topical anesthesia of mucosa. One puff delivers 0.1 ml, containing 10 mg of lidocaine.
4% Topical: For application on mucosa.
2% Jelly: Used by urologists for urethral anesthesia. The lower concentration (2%) ensures safety even if a large volume is absorbed from the inflamed urethra.
5% Ointment: Used for anal fissures. A higher concentration is needed for penetration through mucosa and possible fecal matter.
5% Hyperbaric (Intrathecal): For spinal anesthesia. A high concentration allows for a small volume (1.5-2 ml) to deliver the required mass of drug (75-100 mg) without increasing intrathecal volume.
2% (with adrenaline): Used as a test dose for epidurals to rule out intrathecal placement.
1% or 2%: For infiltrative anesthesia and nerve blocks like brachial plexus blocks.
Viscous 2%: Used for oral cavity anesthesia, swished and spit out to anesthetize the anterior tongue and oral cavity.
4%: For airway blocks like transtracheal block.
1% (with adrenaline): For individual nerve blocks like superior laryngeal or glossopharyngeal nerve blocks.
5% Patch: For local anesthesia, requiring a high concentration to penetrate the skin.
0.5%: For Intravenous Regional Anesthesia (IVRA or Bier's block).

How do you calculate the drug amount from a percentage concentration?

1% means 1 gram in 100 ml.
10% means 10 grams in 100 ml.
10 grams = 10,000 mg in 100 ml, so 1 ml of a 10% solution contains 100 mg (10,000 mg / 100 ml).
Therefore, 5% lidocaine means 50 mg in 1 ml. To give 100 mg, you would administer 2 ml.
2% lidocaine means 20 mg in 1 ml. To give a 60 mg test dose, you administer 3 ml.

Why is a 5% concentration used for spinal anesthesia instead of a lower percentage like 1%?

The goal in spinal anesthesia is the mass of the drug (e.g., 75-100 mg of lidocaine), not the volume.
Using a 5% solution allows this mass to be delivered in a small volume (1.5-2 ml).
If a 1% solution were used, 10 ml would be needed to deliver the same 100 mg, which would unnecessarily increase the intrathecal volume.

Lidocaine: Clinical Applications and Dosing

What is a test dose in epidural anesthesia and why is it used?

A test dose is used to rule out accidental intrathecal or intravascular placement of an epidural catheter.
For non-pregnant patients, 3 ml of 2% lidocaine with adrenaline (60 mg) is used.
If injected intrathecally, this dose is sufficient to produce a motor block, indicating malposition.

How does the test dose for an epidural differ in pregnant patients?

In pregnant patients, the dose required to produce a motor block is lower (45 mg) due to factors like increased progesterone levels, higher neuronal sensitivity, and reduced CSF volume.
Therefore, a test dose of 3 ml of 1.5% lidocaine with adrenaline is used.

What are the non-local anesthetic uses of lidocaine?

Lidocaine has several other important uses:

Anti-arrhythmic (Class 1B): It is a sodium channel blocker used to treat ventricular ectopics (VPCs) to prevent ventricular tachycardia/fibrillation.
Bronchodilator: For resistant bronchospasm, given intravenously for its membrane-stabilizing effect on bronchial smooth muscle.
Pre-induction agent: To suppress the sympathetic response to laryngoscopy and intubation, and to reduce the pain of propofol injection.
Component of Opioid-Free Anesthesia (OFA): Used as an IV infusion for its analgesic properties, both intraoperatively and postoperatively.
Chronic Pain Management: As IV infusions or 5% patches for conditions like post-herpetic neuralgia.
Test for Hepatic Blood Flow: By measuring the metabolite MEGX after lidocaine injection.

What is the role of lidocaine in Opioid-Free Anesthesia (OFA)?

In opioid-free anesthesia, lidocaine is used to provide analgesia.
It is given as a loading dose of 1-1.5 mg/kg before intubation, followed by a continuous infusion (e.g., 15-20 mcg/kg/min or 1 mg/kg/hr), often continued into the postoperative period for pain management.

What is the dosage regimen for lidocaine as an anti-arrhythmic agent?

Loading dose: 1-1.5 mg/kg IV bolus (given slowly).
Infusion: To maintain a therapeutic plasma concentration of 2-3 mcg/ml, a specific tapering infusion is used:
- First 20 minutes: 60 mcg/kg/min
- Next 2 hours: 30 mcg/kg/min
- Subsequently: 15 mcg/kg/min
This is often continued for 12-18 hours while the underlying cause is addressed.

What is the maximum safe dose of lidocaine?

Adults:
- Plain: 3 mg/kg
- With Adrenaline: 7 mg/kg
Children:
- Plain: 5 mg/kg
- With Adrenaline: 10 mg/kg

Higher doses can be used in specific techniques like tumescent anesthesia due to the low concentration and vasoconstriction.

What is tumescent anesthesia and why can such a high dose of lidocaine be used?

Tumescent anesthesia is used in procedures like liposuction.
Large volumes of a very low concentration of lidocaine (0.05%) with epinephrine are injected subcutaneously.
The large volume compresses blood vessels, and the epinephrine causes vasoconstriction, which together dramatically slow systemic absorption, preventing toxic plasma levels even with total doses of 35-50 mg/kg.

Lidocaine: Metabolism and EMLA Cream

What is the primary metabolite of lidocaine and why is it important?

The primary metabolite is Monoethylglycinexylidide (MEGX).
It is important because:

80% of lidocaine's antiarrhythmic effect is attributed to MEGX.
Measuring MEGX levels after a lidocaine bolus can be used as a test to assess hepatic blood flow, as lidocaine is rapidly metabolized in the liver (first-order kinetics).

What is EMLA cream and how does it work?

EMLA stands for Eutectic Mixture of Local Anesthetics.
It is a 1:1 mixture of 2.5% lidocaine and 2.5% prilocaine.
When mixed, they form a eutectic mixture with a melting point of 18°C, which is below room temperature.
This allows both drugs to be in a liquid state and easily mixed to form a cream.
It is applied under an occlusive dressing for 45-60 minutes to anesthetize the skin to a depth of about 5 mm.

Does EMLA cream cause vasoconstriction or vasodilation?

EMLA cream initially causes local vasoconstriction.
However, after about one hour of application, it produces vasodilation.
Therefore, it is recommended to apply it for a full hour and then remove it before cannulation to benefit from the vasodilation.

Long-Acting Local Anesthetics: Bupivacaine, Ropivacaine, and Levobupivacaine

What are the common preparations of Bupivacaine, Ropivacaine, and Levobupivacaine?

Bupivacaine: Available as 0.25%, 0.5%, and heavy 0.5% with 8% dextrose for intrathecal use.
Ropivacaine: Available as 0.2%, 0.5%, 0.75%, and 1%. For intrathecal use, it is available as 0.5% and 0.75%. It is 1.5 times less potent than bupivacaine.
Levobupivacaine: Available as 0.25%, 0.5% for infiltration, and 0.5% isobaric or 0.5% heavy with 8% dextrose for intrathecal use. It is equipotent to bupivacaine.

What are the advantages of Ropivacaine over Bupivacaine?

Less Motor Block: Due to its lower lipophilicity, it provides better sensory blockade with less motor blockade, making it ideal for labor analgesia.
Reduced Cardiotoxicity: It is the S-enantiomer, which is associated with significantly less cardiotoxicity than the racemic mixture (bupivacaine).

When is isobaric Levobupivacaine particularly useful?

Isobaric levobupivacaine is useful for lower limb and orthopedic surgeries.
Because it is isobaric, it does not move significantly within the CSF, remaining near the injection site.
This results in a lower level of block (e.g., up to T10) and a prolonged duration of anesthesia, which is beneficial for longer procedures.

What are the maximum safe doses for long-acting local anesthetics?

Bupivacaine: 2 mg/kg
Levobupivacaine: 3 mg/kg
Ropivacaine: 3 mg/kg

Adrenaline as an Adjuvant

What are the advantages of adding adrenaline to a local anesthetic?

There are five key advantages:

Prolongs Duration of Block: By causing local vasoconstriction, it reduces the systemic absorption of the local anesthetic, keeping it at the nerve site longer.
Reduces Surgical Bleeding: Local vasoconstriction decreases blood loss in the surgical field.
Decreases Systemic Toxicity: Slower absorption means lower peak plasma concentrations, allowing for the use of higher, safer total doses.
Enhances Blockade: Adrenaline's alpha-2 agonistic property can produce direct analgesia and enhance the nerve block by acting on C-fibers.
Serves as a Marker for Intravascular Injection: If accidentally injected into a blood vessel, the adrenaline causes a rapid increase in heart rate and blood pressure, alerting the clinician.

Adjuvants in Neuraxial and Regional Anesthesia

What are common adjuvants used with local anesthetics besides adrenaline?

Opioids: Morphine is the gold standard for neuraxial use due to its hydrophilicity, providing prolonged analgesia. Fentanyl is a shorter-acting option.
Alpha-2 Agonists (Clonidine, Dexmedetomidine): Enhance analgesia and prolong block duration.
Dexamethasone: Used particularly in peripheral nerve blocks (e.g., brachial plexus) to significantly prolong the duration of both motor and sensory blockade, sometimes up to 24 hours.

What are the typical neuraxial doses for common adjuvants?

Intrathecal Morphine: 100 mcg (for the Indian population, due to higher sensitivity to respiratory depression).
Epidural Morphine: 2-4 mg bolus.
Intrathecal Fentanyl: 15-25 mcg.
Intrathecal Clonidine: 30 mcg.
Epidural Clonidine (e.g., caudal): 3 mcg/kg.
Intrathecal Dexmedetomidine: 5-10 mcg.
Epidural Dexamethasone: 4-8 mg.

Local Anesthetic Systemic Toxicity (LAST)

What is the CC/CNS ratio and why is it important?

The CC/CNS ratio is the ratio of the plasma concentration of a local anesthetic that causes cardiovascular collapse to the concentration that causes CNS toxicity.
A higher ratio indicates a greater margin of safety, as CNS symptoms (which serve as an early warning) will appear well before life-threatening cardiac toxicity.

Lidocaine: Ratio of ~7 (high safety margin).
Bupivacaine: Ratio of 2-3 (narrow safety margin, highly cardiotoxic).
Ropivacaine/Levobupivacaine: Ratio of ~5 (intermediate safety).

Why is bupivacaine more cardiotoxic than lidocaine?

This is explained by the "fast-in, fast-out" vs. "fast-in, slow-out" theory on cardiac sodium channels.
Lidocaine blocks the channel but rapidly dissociates during diastole ("fast-out").
Bupivacaine blocks the channel but dissociates very slowly ("slow-out"), leading to a prolonged and profound block of cardiac conduction.
This effect is exaggerated during tachycardia because the shorter diastole allows even less time for the drug to dissociate.

What patient factors increase the risk of LAST?

Extremes of age (neonates and elderly):
- Neonates: Low levels of alpha-1 acid glycoprotein, immature liver enzymes, low muscle mass for sequestration, and high cardiac output.
- Elderly: Reduced liver function (homeostenosis) and sarcopenia (decreased muscle mass).
Pregnancy: Low alpha-1 acid glycoprotein, increased cardiac output, and increased progesterone (which sensitizes sodium channels).
Liver disease: Impaired metabolism of amide local anesthetics.
Acidosis:
Burn patients:

Besides patient factors and drug type, what other factors influence the risk of LAST?

Route of administration: The BIER (Biceps-Intercostal-Epidural-Rectus sheath) mnemonic highlights highly vascular areas with rapid absorption:
- Blood (Intercostal/Interpleural)
- Intercostal
- Epidural (Caudal)
- Rectus sheath/Paravertebral/Sciatic/Femoral
Rate of administration: Rapid injection can lead to a sudden, high peak plasma concentration.
Site of injection: Highly vascular areas lead to rapid absorption.
Method of administration: Continuous infusions can lead to cumulative toxicity over time.
Accidental intravascular injection: Even a small dose can be immediately toxic.