beginning neuraxial anesthesia (an overview) local anesthetics (an introduction)

Beginning Neuraxial Anesthesia(an overview)

Local Anesthetics(an introduction)

Neuraxial Anesthesia Indications

• Any operation in the lower abdomen and below

Contraindications

• Absolute– Patient doesn’t want it– Infection at site of

puncture– Increased ICP– Uncorrected

hypovolemia– Uncorrected

coagulopathy

• Relative– Systemic infection– Neuruologic diseases

like MS

Spinal vs. Epidural

• Spinal– More definite endpoint– Easier to do– Faster onset– More intense sensory

and motor block– Limited duration– Fewer failures

• Epidural– Less definite endpoint– More difficult to do– Slower onset– Less intense sensory

and motor block possible (labor)

– Unlimited duration– Postop analgesia

possible– More failures

Sedation/Analgesia

Position

ABSOLUTELY NO RITUALS!

Where?

Spinal - L2-3 and below

Epidural - anywhere

Skin anesthesia

• Do a good intradermal skin wheal• Other, deeper soft tissues are not painful• The periostium is painful but impossible to

anesthetize easily, so don’t try

Please memorize this image. When you are performing an epidural or spinal puncture use the image of the ligaments as a guide to imagine where the needle tip is at all times.

If at any time you think the plunger is stuck, STOP. Remove the syringe and check that the plunger moves freely.

Pay attention to what you are FEELING as the needle advances. If you feel as though the ligamentum flavum has been penetrated but there has been no LOR to injection, STOP. Reassess plunger action and resistance to injection.

Pay attention to DEPTH as the needle advances. If you feel as though you should have penetrated the ligamentum flavum by now but there has been no LOR to injection, STOP. Reassess plunger action and resistance to injection.

Spinal Anesthesia

We do it the same as we do an epidural except we use flimsy

needles and we don’t stop in the epidural space

Because the needles are so flimsy, we use an introducer needle

Interspinous lig

Epidural spaceDura

CSF

Ligamentum flavum

FAILURE!

So, we’re in!

What do we inject?

Ask your attending

Epidural injections:ChloroprocaineLidocaineBupivacaine

Epidural injections:What concentration?How much?

Ask your attending

Epidural injections:What concentration?

Chloroprocaine – 3%Lidocaine – 1-2%Bupivacaine – 0.625-0.5%

(low conc. for labor)How much?

Roughly 10-20 ml

Spinal injections:What solution?How much?

Ask your attending

Spinal injections:What solution?

Chloroprocaine – 2-3% (no dextrose)

Lidocaine – 5%/0.75% dextrose

Bupivacaine – 0.75%/0.825 dextrose

Spinal injections:How much?

Chloroprocaine – 2 ml (40-60 mg)

Lidocaine – 1-2 ml (50-100 mg)

Bupivacaine – 1-2 ml (7.5-15 mg)

Conversion of % concentration to mg/ml:

1% solution = 1gm per 100 ml (1000 mg per 100 ml) = 10 mg/ml

% solution X 10 = mg/ml

e.g., 0.5% bupivacaine X 10 = 5 mg/ml Dose is volume X concentration:

10 ml of 0.5% bupivacaine = 50 mg dose

Dose is important in determining toxicity

Manufacturer Maximum Recommended Doses

• Chloroprocaine– 800 mg no epinephrine– 1000 mg with epinephrine

• Lidocaine– 300 mg no epinephrine– 500 mg with epinephrine

• Bupivacaine– 175 mg no epinephrine– 225 mg with epinephrine

Concept of baricity

• Baricity is the relationship of the density of the local anesthetic solution to the density of the cerebrospinal fluid.

If the LA solution is:• Less dense than CSF it is hypobaric (floats)• Equal in density to CSF it is isobaric (stationary)• More dense than CSF it is hyperbaric (sinks)

• As a concept, baricity refers only to spinal anesthesia and not to epidural anesthesia

Spinal solutions

• Hyperbaric solutions (with dextrose)– Intra-abdominal operations (including inguinal

hernia and vaginal procedures)– All operations can be done with this solution

• Isobaric solutions (epidural solutions without dextrose)– Lower extremity operations (hip and below)

• Hypobaric solutions (diluted with DW)– Not really useful

1 ml 5% lido with dextrose during injection

1 ml 5% lido with dextrose immediately after injection

• The effect of baricity on the distribution of bupivacaine in spinal model

Hyperbaric

IsobaricH

ypobaric

In spite of the crudeness of this model, the levels of anesthesia predicted by the model are remarkably similar to the levels of anesthesia observed in patients

Immediately after injection

20 min. after injection

Hyperbaric

IsobaricH

ypobaric

What could go wrong?

What could go wrong with spinal anesthesia?

• It doesn’t work• It goes too high (total spinal)• It doesn’t go high enough• It causes hypotension• It doesn’t last long enough• It causes a spinal headache

The Two Components of

Spinal Headache• There must have

been a lumbar puncture

• The headache is related to posture– Worst when

standing or sitting– Gone or improved

with recumbency

Effect of Age on the Incidence of Spinal

Headache

Vandam and Dripps, JAMA 1956;161:586-591

0

2

4

6

8

10

12

14

16

Perc

ent H

eada

che

10-19 20-29 30-39 40-49 50-59 60-69 70-79 80-89

Age

This and AARP discounts are two of the few advantages to aging!

Needle tip design is important

• 25 gauge Quincke or cutting needle has 5% incidence of spinal headache in OB patients.

• 25 gauge Whitacre or pencil tipped needle has <1% incidence of spinal headache in OB patients

What could go wrong with epidural anesthesia?

• It doesn’t work• It goes too high (total spinal)• It doesn’t go high enough• It causes hypotension• It doesn’t last long enough• It causes a spinal headache (but it’s not

supposed to)• It produces spinal anesthesia• It goes intravascular causing systemic toxicity

Or the catheter could have penetrated the dura and be located intra-thecally

Epidural Test Dose

• 3 ml of 1.5% lidocaine with 1:200,000 epi

– 1:200,000 = 5 ug/ml X 5 ml = 25 ug epi• will cause tachycardia and is used to detect and

intravascular injection

– 3 ml X 15 mg/ml = 45 mg lido• will cause spinal anesthesia and is used to detect

an intrathecal injection

The Local Anesthetic Molecule• Local

anesthetics consist of an aromatic ring and an amine, separated by a hydrocarbon chain

Two types of local anesthetics based on the hydrocarbon chain linkage:

Esters have [-CO-O-] linkageAmides have [-N-CO-] linkage

N C

O

O C C NCH3

CH3

H9C4

NH2 C

O

O C C N

C2H5

C2H5

NH2 C

O

O C C N

Cl

C2H5

C2H5

Procaine

Chloroprocaine

Tetracaine

ESTERS

N

O

NH

CH3

CH3 CH3

H

N

O

NH

CH3

CH3 CH2

CH2

CH2

CH3

H

M epivacaine

Ropivacaine

Bupivacaine

N

O

NH

CH3

CH3 CH2

CH2

CH3

HHAmide

Bupivacaine Analogues

AmideLidocaineAnalogues

Toxicity

• Directly related to lipid solubility (potency)– Bupivacaine > Lidocaine > Chloroprocaine

• The more potent the LA, the more toxic the LA is– It takes a lower dose to produce the toxicity

• Two types of toxicity– Central nervous system– Cardiovascular

• Central Nervous System– Earliest signs and symptoms are those of

excitation owing to depression of inhibitory cells allowing excitatory cell preponderance

• Tinnitus• Light headedness• Confusion• Circum-oral numbness• Tonic-clonic convulsions

Toxicity of Local Anesthetics

• Central Nervous System– Excitation is followed by depression

Drowsiness Unconsciousness Respiratory Arrest

SZ

SZ

Treatment of CNS Toxicity• STOP INJECTING• If seizure, depress the CNS with

benzodiazepines (midazolam, ativan, diazepam), or propofol, or thiopental

• Support airway and breathing• Intubation if necessary• Wait until consciousness returns

– It is unlikely there will be a good block as the local obviously went intravascular

• Or convert to general and continue with the operation

Cardiovascular ToxicityHYPERTENSION - TACHYCARDIA OWING TO CNS EXCITATION

NEGATIVE INOTROPY

DECREASED CARDIAC OUTPUT

MILD - MODERATE HYPOTENSION

PERIPHERAL VASODILATATION

PROFOUND HYPOTENSION

SINUS BRADYCARDIA

CONDUCTION DEFECTS VENTRICULAR ARRYTHMIAS

CARDIOVASCULAR COLLAPSE

Treatment of Bupivacaine CardiotoxicityWeinberg, GL. RAPM 27:568, 2002

• Early Response• ABC• ACLS Protocol

An Intralipid Protocol• Intralipid bag available

• LA arrest unresponsive to ACLS:

1 ml/kg intralipid IV over one minute

Repeat X2 at 3-5 min. intervals

0.25 ml/kg/min intralipid IV until stable

Picard J, Meek T: Lipid emulsion to treat overdose of local anaesthetic: the gift of the glob. Anaesthesia 2006; 61: 107-9

Prevention Is Better Than Treatment

Good Luck!

Have Fun!!

Be Careful!!!