Local anesthetics Flashcards
3 parts of LA
- lipophilic group: usually benzene ring
- hydrophilic group: tertiary amine
- intermediate bond: hydrocarbon chain
intermediate bond
- Ester: -CO-
2. Amide: -HNC-
Ester LA
No ‘i’ prior to -caine
- Procaine
- Cocaine
- Chloroprocaine
- tetracaine
pseudocholinesterase
Amide
(contain an ‘i’ before -caine
- lidocaine
- bupivacaine
- ropivacaine
- mepivacaine
- prilocaine
Uptake: from fastest to slowest
Faster uptake–>higher plasma concentrations–>increasing chance of local anesthetic toxicity
Intravenous > tracheal > intercostal > caudal > paracervical > epidural > brachial plexus > sciatic > subcutaneous
factors influencing absorption and onset of action
- lipid solubility–>penetrate nerve membrane–>potency
2. pKa: lower–>more un-ionized–>speed of action
biotransformation & excretion
Ester: pseudocholinesterase —>water-soluble–>urine
Amide: liver P-450–>kidney
enhance LA effects
- epinephrine: 5mcg/ml (1:200,000) to 20mcg/ml (1:50,000)–>vasoconstricting
- opioids
- alpha-adrenergic agonists: clonidine
- steroids
informed consent: brachial plexus
bleeding, infection, nerve damage, persistent paresthesia or weakness, shortness of breath, and local anesthetic toxicity
paresthesia
a sensation of pricking, tingling, or creeping on the skin that has no objective cause
Intralipid (20% lipid emulsion)
bolus of 1.5 mL/kg over 1 min–> 0.25 mL/kg/min x 30-60 min–>repeated boluses as needed for persistent asystole.
post pain control vs surgical anesthesia
- Bupivacaine & ropivacaine: long duration–>postop pain
2. Lidocaine & mepivacaine: surgical anesthesia (not post op pain d/t short duration)
dyspnea
- phrenic nerve: close to brachial plexus–>anesthetized–>unilateral diaphragm paralysis
- pneumothorax: MC in supraclavicular
- LA toxicity
Increase risk of cardiovascular toxicity of bupivacaine
- pregnancy: progesterone
- hypoxemia
- respiratory acidosis (not alkalosis)
- hyperkalemia (rather than hypo)
What percentage of amide LA are excreted unchanged in urine
<5%
Local anesthetic systemic toxicity (LAST)<—unintended intravascular injection or absorption
- intercostal > caudal epidural > lumbar epidural > brachial plexus > subcutaneous
- Risk factors: age, cardiac disease, hepatic dysfunction, hypoxemia, and acidosis
- CNS is more sensitive than cardiovascular system, except bupivacaine & etidocaine
LA CNS toxicity
- benign: perioral numbness, tinnitus
- On cnetral inhibitory pathways: shivering, muscle tremors, tonic-clonic seizure
- depressant: coma, hypoventilation, respiratory arrest
Cauda equina syndrome (CES)
- bowel & bladder dysfunction
- perineal sensory loss
- LE motor weakness
Note: d/t diffuse injury to CE
Risk factors: high dose & consentration, restricted distribution, vasoconstrictors, LA (lidociane highest)
Transient neurologic symptoms (TNS)
Pain
1. uni- or bi-lateral
2. buttocks, LB, LEs
3. onset: usually 12-24 hrs after SA or EA
4. duration: 6hrs to 4 days
5. risk factors: lidocaine, lithotomy, obesity, knee arthroscopy, outpt surgery
Note: - neurologic PE
Cardiac Toxicity
- CNS excitatory–>increase: HR, BP
- Direct cardiac–>arrhythmia, prolongation of PR & QRS, depression of SA & AV nodes, QT prolongation, VT, torsade de pintes, VF
Regional anesthesia
- performed after Std ASA monitoring
2. Available: 20% intralipid emulsion, AMBU bag, O2
ASRA
www.asra.com
Allergy to LA
- Amide: extremely rare
- Ester: cross-reactivity to p-aminobenzoic acid (PABA)–>allergy (Ester LA metabolized to PABA)
- Preservatives: parabens
preservatives/additives
- Methylparaben: in some multi-dose formulation of LA–>paraben–>allergy
- Metabisulfate: –>urticarial, flusing, pruritus, airway obstruction
- EDTA: i.t. –>nerve injury; Epid–>back muscle spasm
- Epi: 1) prolongation; 2) test dose
- Na2CO3: faster onset
Epinephrine test dose
- epinephrine: 1:200,000
- Intravascuar injuection
1) increased T vave
2) increased/decreased HR
3) increased BP
Methemoglobinemia
- Prilocaine–>O=toluidine in liver–>oxidize HgB
- usu. seen >600mg
- cyanosis refractory to O2
- metHgb >15%—>anxiety, dyspnea, HA, weakness, n/V–>50%–>confusion, seizures, arrhythmias, hemodynamic instability, death
- metHgb: co-oximetry
- treated: methylene blue
ECG w/ LAST
Initial ECG changes indicating cardiac toxicity from local anesthetics include peaked T waves, elevated ST segments, and a prolongation of PR and QRS intervals. Next, depression of the SA and AV nodes is observed. Finally, the QT interval is prolonged, leading to ventricular tachycardia, torsade de pointes and ventricular fibrillation.
_____________ of amide local anesthetics is the most important factor determining clearance from plasma.
Protein Binding
