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.

The way the needle is held is unimportant. What is important is that you have control of the needle and that the needle is INSERTED AND MOVED SLOWLY AND DELIBERATELY. Three different ways to hold the needle for the loss of resistance with saline

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.

Please note the following: 1. The syringe is filled with saline and there is NO AIR in the syringe. 2. The syringes shown here are not filled enough to start with. Put 4 – 5 ml of saline in the syringe. 3. Do not inject all of the saline before you have engaged the needle in the interspinous ligament or the ligamentum flavum. 4. Once engaged in a ligament (any ligament) it is impossible to inject the saline 5. Once there is resistance to injection, keep CONSTANT UNREMITTING THUMB PRESSURE ON THE PLUNGER WHILE SLOWLY ADVANCING THE NEEDLE until there is no 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 space Dura CSF Ligamentum flavum

FAILURE!

So, we’re in! What do we inject?

Ask your attending

Epidural injections: Chloroprocaine Lidocaine Bupivacaine

Epidural injections: What concentration? How much?

Ask your attending

Epidural injections: What concentration? Chloroprocaine – 3% Lidocaine – 1-2% Bupivacaine – % (low conc. for labor) How much? Roughly 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 ( mg) Bupivacaine – 1-2 ml ( 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 Isobaric Hypobaric l 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 Isobaric Hypobaric

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: 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-] linkage Amides have [-N-CO-] linkage

ESTERS

Amide Bupivacaine Analogues

Amide Lidocaine Analogues

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 l Drowsiness l Unconsciousness l Respiratory Arrest

SZ SZ

Recording from a Single Cell Seizure like activity is due to direct excitation, and not depression of inhibitory cells allowing excitatory cell preponderance

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 Toxicity HYPERTENSION - 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 Cardiotoxicity Weinberg, GL. RAPM 27:568, 2002 Early Response ABC ACLS Protocol

Treatment of Bupivacaine Cardiotoxicity ii Antiarrhytmic therapy –Benzodiazepines or propofol for seizures and CNS induced arrhythmias –Epinephrine may promote arrhythmias –Amiodarone [now first line ACLS antiarrhythmic] may be useful –Lidocaine may compete with bupivacaine or could be additive (data are mixed on using lidocaine to treat bupivacaine induced VT)

Treatment of Bupivacaine Cardiotoxicity iii Vasopressor support –Epinephrine –Vasopressin 40 U now in ACLS protocol May be less arrhythmogenic than epinephrine Produces greater coronary artery perfusion pressures that last longer than epinephrine Causes less acidosis than repeated doses of epinephrine (Krismer, et al Anesth Analg 2001;93:734–42) –Epinephrine + Vasopressin A combination of epinephrine and vasopressin produces better survival than either epinephrine or vasopressin alone (Mayr, et al Anesth Analg 2004;98:1426 –31)

Treatment of Bupivacaine Cardiotoxicity iiia Contraindications –Calcium channel blockers –Phenytoin –Bretyllium no longer supported

Treatment of Bupivacaine Cardiotoxicity iv Novel therapies –Lipid infusion (soybean oil… ?propofol) Bupivacaine inhibits transport of fatty acids into mitochondria Provides substrate Improves survival and increase dose required for toxicity in rats Sequesters lipid soluble bupivacaine in blood –Insulin/Glucose/Potassium May restore K+ gradients for repolarization May promote ATP source

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!!!