Anesthesia

Neurology - Overview

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  • Presented by H. Schaefer, MN.

Anesthesia Basics

  • Definition: Sedation of a patient for the purposes of a medical procedure/intervention.

Types of Anesthesia

  1. Local Anesthesia

    • Localized Effect: Loss of sensation in a focused body area or region (known as regional anesthesia).

    • Mechanism: Nerve block via local anesthetics.

    • Use Cases:

      • Minor procedures such as sutures for lacerations.

      • Some major procedures (e.g., oral surgery, labor & delivery).

  2. General Anesthesia

    • Systemic Effect: Results in loss of consciousness.

    • Composition: Combines multiple drugs for optimal effect.

    • Application: Used for major procedures (e.g., abdominal surgery).

  3. Monitored Anesthesia Care (MAC)

    • Effect: Systemic effect with lower levels of sedation.

    • Features: Maintains vital signs (VS) without the need for intubation (endotracheal tube).

    • Levels of Sedation: Includes conscious sedation—patients are sleepy yet able to awaken and respond when prompted, maintaining VS without assistance.

Local Anesthesia Details

  • Drug Class: Sodium channel blockers.

    • Mechanism of Action:

    • Prevents influx of sodium into neurons.

    • No action potential occurs, leading to no cellular depolarization and inability to communicate sensory information to the cerebral cortex.

    • Affects both efferent and afferent pathways, impacting sensory and motor functions.

  • Key Drugs:

    • Lidocaine, Prilocaine, Bupivacaine, Ropivacaine.

    • Onset Time: Less than 2 minutes.

    • Duration: Drug and dose-dependent.

    • Example durations:

      • Lidocaine & Prilocaine: 1-2 hours.

      • Bupivacaine: 2-4 hours (suitable for longer procedures, allows for postoperative analgesia).

Cocaine as a Local Anesthetic

  • Classification: Same drug class as other local anesthetics.

  • Usage: Rarely used due to high addiction risk.

  • Pharmacokinetics & Pharmacodynamics (PKPD):

    • Local action (e.g., intranasal route induces numbness of nasal mucosa).

    • Distribution in CNS and acts as a dopamine & reward system agonist.

Local Anesthesia Routes

  • Application Methods:

    1. Topical:

    • Application on the surface (e.g., cream, spray).

    1. Subcutaneous (SC) Infiltration:

    • Injection into tissue near nerve endings.

    1. Nerve Block:

    • Injection near a large nerve.

Synergistic Agents

  • Epinephrine (Adrenalin):

    • Provides localized vasoconstriction, controls bleeding, and increases the duration of action of the anesthetic.

  • Sodium Bicarbonate:

    • Alkalizes tissue, particularly useful in cases of bacterial infection (bacterial acid).

  • Adjunct Analgesia:

    • Use of opioids, NSAIDs, or Tylenol as necessary once anesthesia wears off.

Topical Anesthetics for Dermal Analgesia

  • Product: EMLA cream (lidocaine 2.5% and prilocaine 2.5%).

  • Packaging: 30 g, with specific DIN (Drug Identification Number).

Local Anesthesia: Epidural Method

  • Administration: Infusion of anesthetic into the epidural space.

    • Ensured by the absence of cerebrospinal fluid (CSF) return in the needle.

    • Anesthetic acts on the spinal nerve, disrupting impulse transmission to/from CNS.

  • Key Locations for Catheter Placement: Cervical, Thoracic, Lumbar.

  • Onset Time: Approximately 20-30 minutes.

  • Administration Type: Continuous infusion via an indwelling catheter.

    • Dosage is higher for the epidural route as compared to the spinal route.

Spinal Nerves Anatomy

  • Dura, Spinal Cord, and Ligamentum Flavum are key components of the anatomy involved in epidural anesthesia.

  • Epidural Catheter Placement Checks:

    • Markings on the catheter tip specify placement (e.g., at 10 cm, 15 cm).

Surgical Applications of Anesthesia

Thoracic Surgery

  • Procedures such as thoracotomy, pectus repair, and thoracic aortic aneurysm repair.

Upper Abdominal Surgery

  • Involves esophagectomy, gastrectomy, pancreatectomy, hepatic resection.

Lower Abdominal Surgery

  • Examples include colectomy, bowel resection, and abdominal perineal resection.

Dermatomes Mapping

  • Understanding dermatomes is crucial for localizing sensation and anesthesia effects on the skin.

Local Anesthesia: Intrathecal Route (Spinal Injection)

  • Method: Delivery of the drug directly into cerebrospinal fluid (CSF).

    • Single dosage injected into the intrathecal space, always placed below L2 to avoid spinal cord damage.

  • Verification of Needle Position: Confirmation when CSF returns in the needle puncture.

  • Onset: Rapid effect due to direct delivery into CSF.

  • Considerations:

    • Tonicity of solution and patient positioning influence action location.

    • Common for abdominal and pelvic procedures.

    • Risks: Potential for respiratory depression if the diaphragm is impacted.

Baricity of Local Anesthetics

  1. Isobaric:

    • Stays in place, density/specific gravity similar to CSF (approximately 1.003-1.008, similar to normal saline).

  2. Hypobaric:

    • Floats upwards, having a lower density than CSF (<1.003, akin to sterile water).

  3. Hyperbaric:

    • Settles to dependent aspects of the subarachnoid space, greater density than CSF (>1.008, like dextrose).

Side Effects & Nursing Assessment

  • Goal of Anesthesia: Induced loss of sensation.

  • Monitoring Needs:

    • Vital signs (with special focus on respiratory rate, especially with spinal anesthesia).

    • Watch for hypotension and respiratory depression.

    • Test sensation and motor functions during and post-procedure.

    • Be aware of site hematomas and infections, catheter migration, backache, urinary retention, and potential spinal cord injury.

  • Critically Risky Event: Wrongful CSF infiltration due to catheter migration from epidural to spinal, risking overdose.

General Anesthesia (GA)

  • Definition: Drug-induced loss of consciousness; patients are not rousable even by painful stimulation.

  • Treatment Goals:

    1. Analgesia

    2. Unconsciousness and amnesia

    3. Loss of reflexes (this is dependent on the procedure).

Drugs Utilized in General Anesthesia

  1. Analgesics:

    • Opioids administered intravenously (IV) such as fentanyl and morphine.

  2. Induction and Maintenance:

    • Induction causes the beginning of loss of consciousness, while maintenance is crucial for sustaining deep sedation (surgical anesthesia).

    • Both inhaled and IV anesthetics can be used, often coupled with benzodiazepines or ketamine.

    • Mechanism of Action (MOA):

      • Decrease action potentials, increase GABA activation, and other CNS effects.

  3. Loss of Reflexes:

    • Administer neuromuscular blocking agents (often referred to as paralytics).

Inhaled & IV General Anesthetics

  • Inhaled General Anesthetics:

    • Examples include nitrous oxide, halothane, and isoflurane.

    • They work by decreasing action potentials and increasing GABA among other CNS effects.

  • Intravenous Anesthetics:

    • Barbiturate-like drug: Propofol (Diprivan).

    • Mechanism includes increasing GABA, with rapid onset and a short half-life, requiring continuous infusion.

    • The dose must be critically managed due to potential hypotension requiring vital supports.

Neuromuscular Blocking Agents

  • Type: Anticholinergics, blocking acetylcholine binding at nicotinic receptors affecting SNS, PNS, and somatic nervous systems.

  • Characteristics:

    • No penetration of the blood-brain barrier (BBB), resulting in muscle paralysis including diaphragm involvement—ventilation becomes necessary.

    • Useful for complex procedures necessitating muscle relaxation.

    • Quick onset; generally administered by continuous IV infusion.

  • Drugs: Vecuronium, Rocuronium, Pancuronium, and Succinylcholine (noted for its short half-life).

Monitored Anesthesia Care (MAC): Conscious Sedation

  • Definition: A combination of sedative and analgesic medications inducing a depression of consciousness while preserving the ability to maintain an airway and awaken the patient.

  • Drugs Used:

    • Ketamine (note that higher doses are needed compared to depression treatment).

    • Benzodiazepines (e.g., Midazolam).

    • Adjunct medications may include opioids.

CNS Drugs - Cautions

  • Avoid combining with other CNS drugs (for instance, ethanol).

  • Be aware of drug-drug interactions that may yield additive effects or pose a risk of toxicity.

  • Patients must be assessed for effects and dosages titrated as necessary.

  • Evaluate the impact of CNS effects/side effects on their activities of daily living (ADLs).

  • Consider risks associated with addiction and overdose.

Physical Dependence on CNS Drugs

  • Definition: Body adapts to the presence of the drug, leading to tolerance necessitating higher doses for the same effect and withdrawal symptoms upon abrupt discontinuation lasting 2-4 weeks.

  • Symptomatology:

    • Sympathetic nervous system effects such as vital sign changes, blurred vision, and loss of appetite.

    • Other symptoms include fever, psychosis, agitation, anxiety, panic, seizures, and disorientation with impaired memory and focus.

  • Withdrawal Protocols: Must be carefully observed to mitigate the adverse effects during cessation.

Abuse & Psychological Dependence

  • Diagnostic Criteria: As per DSM-5, a diagnosis of substance use disorder is made if three or more criteria are met within the same 12-month period. These include:

    1. Significant time spent acquiring, using, and recovering from the substance.

    2. Disruption of important activities due to substance use.

    3. Usage exceeding intended amounts.

    4. Compulsive substance use in spite of harm.

    5. Unsuccessful efforts to curtail use.

    6. Development of tolerance, requiring higher amounts.

    7. Experience of withdrawal symptoms upon discontinuation (indicating the presence of physical dependence).