Neuraxial Anesthesia Practice Flashcards

Anatomy of the Vertebral Column and Spine

• Vertebral Segments and Curves:

  • Cervical Spine: Consists of $7$ vertebrae. The curve is concave, referred to as Lordosis.

  • Thoracic Spine: Consists of $12$ vertebrae. The curve is convex, referred to as Kyphosis.

  • Lumbar Spine: Consists of $5$ vertebrae. The curve is concave, referred to as Lordosis.

  • Sacrum: Consists of $5$ fused vertebrae. The Sacral Hiatus is formed by the lamina of $S4$ and $S5$.

  • Coccygeal: Fused vertebrae making up the tailbone.

• Specific Vertebrae and Landmarks:

  • Atlas: The first cervical vertebra ($C1$).

  • Axis: The second cervical vertebra ($C2$). It is the first vertebra that can be palpated and counted.

  • C7: The vertebral prominence; the most prominent process at the base of the neck.

  • T7: Located at the inferior tip of the scapula.

  • Tuffier’s Line: An anatomic landmark drawn across the top of the iliac crests, crossing at either the $L4$ vertebra or the $L4\text{--}L5$ interspace.

  • S2: Located at the level of the posterior superior iliac spines.

  • L5–S1: This is the largest interspace in the vertebral column.

• Supine Positioning and Distribution:

  • The highest points of the spine when a patient is in the supine position are $C5$ and $L3$.

  • The lowest points of the spine when a patient is in the supine position are $T5$ and $S2$.

  • These curves are critical for the distribution of local anesthetics (LA) within the subarachnoid space.

Ligaments and Meninges

• Vertebral Ligaments:

  • Anterior and Posterior Longitudinal Ligaments: Bind the vertebral bodies together.

  • Supraspinous Ligament: A thick ligament connecting the tips of the spinous processes.

  • Interspinous Ligament: Connects the spinous processes on their horizontal surfaces.

  • Ligamentum Flavum (Yellow Ligament): The strongest and thickest ligament, especially in the lumbar area. It connects the laminae of the vertebrae, is composed of elastic fibers, and helps maintain upright posture. It contains small blood vessels from the vertebral plexuses.

• Meningeal Layers (PAD the Cord):

  • Dura Mater (Tough Mother): The outermost, thick protective layer.

  • Arachnoid Mater (Cobweb Mother): The middle layer; it is non-vascular and continues down to the level of $S2$.

  • Pia Mater (Soft Mother): The innermost layer, in direct contact with the spinal cord. It is highly vascular and ends where the spinal cord ends at $L1$.

Anatomy of the Spinal Cord and Spaces

• Spinal Cord Dimensions:

  • Extends from the foramen magnum to the level of $L1$ in adults.

  • At birth, the spinal cord ends at $L3$. By $1$ year of age, it moves to the child/adult level of $L1$.

  • Conus Medullaris: The tapered, lower end of the spinal cord.

  • Cauda Equina (Horse's Tail): The bundle of nerve roots located below $L1$ that travel caudad before exiting.

  • Filum Terminale (Coccygeal Ligament): Fibrous tissue that secures the lower end of the spinal cord to the coccyx.

• Spinal Nerves:

  • There are $31$ pairs of spinal nerves exiting from $C1$ to $S5$.

  • At the cervical level, nerves exit above the corresponding vertebrae ($C1\text{--}C7$). Because there is a $C8$ nerve root but only $7$ cervical vertebrae, exiting shifts below the vertebrae starting at $T1$.

  • Anterior (Ventral) Roots: Responsible for motor (Efferent) function. Mnemonic: AVM (Anterior Ventral Motor).

  • Posterior (Dorsal) Roots: Responsible for sensory (Afferent) function. Mnemonic: PDS (Posterior Dorsal Sensory).

• Anatomical Spaces:

  • Epidural Space: A potential space outside the dural sac, located between the dura mater and the ligamentum flavum.

  • Subdural Space: A potential space between the dura mater and the arachnoid mater.

  • Subarachnoid Space: The space below the arachnoid mater containing cerebrospinal fluid (CSF). Also known as the Intradural or Intrathecal space.

Cerebrospinal Fluid (CSF) Physiology

• Characteristics:

  • An ultrafiltrate of blood plasma that is clear, colorless, and odorless.

  • Maintained in hydrostatic and osmotic equilibrium.

• Production and Turnover:

  • Generated primarily in the choroid plexus of the lateral ventricles ($95\%$).

  • $5\%$ is produced in the third and fourth ventricles (Aqueduct of Sylvius, Foramen of Magendie, Foramina of Monro).

  • Total production is approximately $500\,ml/day$. CSF turns over about $3.0$ times a day.

• Volume and Pressure:

  • Total volume: $100\text{--}150\,cc$.

  • Volume in subarachnoid space: approximately $30\text{--}80\,cc$.

  • Normal fluid pressure: $10\text{--}20\,cmH_2O$.

  • Spinal Cord Perfusion Pressure Equation:     $SCPP = MAP - CSFP$

• CSF Composition:

  • pH: $7.3\text{--}7.6$

  • Specific Gravity at $37^\circ C$: $1.005$ (range $1.003\text{--}1.009$)

  • Protein: $15\text{--}45\,mg\%$

  • Glucose: $60\%$ of serum glucose

  • Chloride: $120\text{--}130\,meq/L$

  • Sodium: $140\text{--}150\,meq/L$

  • Bicarbonate: $25\text{--}30\,meq/L$

Vascular Supply to the Spinal Cord

• Anterior Spinal Artery (1):

  • Originates from vertebral arteries.

  • Supplies the ventral (anterior) $2/3$ of the cord (motor function).

  • Provides $75\%$ of the total blood supply. High risk for ischemia due to limited collaterals.

• Posterior Spinal Arteries (2):

  • Originate from the posterior inferior cerebellar arteries at the base of the brain.

  • Supply the dorsal (posterior) $1/3$ of the cord (sensory function).

  • Provides $25\%$ of the blood supply, with a rich collateral supply offering relative protection from ischemia.

• Artery of Adamkiewicz:

  • A critical blood supply arising from the aorta, usually between $T9\text{--}T12$, typically on the left side.

  • Supplies the anterior lower $2/3$ of the cord.

Mechanism of Action and Nerve Blockade

• Principle Site of Action: The nerve root.

  • Spinal Anesthesia: LA is injected into the CSF and bathes nerve roots.

  • Epidural/Caudal Anesthesia: LA is injected into the epidural space and bathes nerve roots (a slower process for onset).

• Order of Blockade by Fiber Type:

  1. B fibers: Preganglionic autonomic (sympathetic) fibers. These are blocked first.

  2. C & A-delta fibers: Pain (dull and sharp) and temperature. Temperature is commonly the first sensory modality lost.

  3. A-gamma fibers: Proprioception.

  4. A-beta fibers: Touch and pressure.

  5. A-alpha fibers: Motor movement. These are the last to be blocked.

• Differential Blockade Levels:

  • Sympathetic/Autonomic level: Typically $2\text{--}6$ segments higher than the sensory level.

  • Motor block level: Typically $2$ segments below the sensory level.

  • Sensory level: The midpoint of the block.

• Regression of Block: Occurs in reverse order. Sensation returns to the toes last. Mnemonic: "All Students Must Pass" (ASMP).

Sensory Dermatomes

• C3–C4: Top of the shoulder / Neck.

• T4: Nipple line.

• T6: Xiphoid process / bottom of the sternum.

• T10: Umbilicus.

• T12: Pubic bone / Inguinal area.

• L1: Inguinal ligament.

• L2–L3: Knee / anterior thigh.

• L4–L5: Anterior ankle and foot.

• S1: Heel.

• S3–S5: Perineal (genital) area.

Physiological Manifestations of Neuraxial Blockade

• Cardiovascular Manifestations:

  • Sympathetic blockade leads to venous and arterial vasodilation, causing decreased Blood Pressure (BP), Heart Rate (HR), and contractility.

  • Cardiac Accelerators ($T1\text{--}T4$): High sympathectomy blocking these fibers results in profound hypotension and bradycardia due to unopposed vagal (parasympathetic) tone. This can lead to sudden cardiac arrest.

  • Compensatory Mechanisms: Vasoconstriction above the block level occurs if the block is below $T5$.

  • Interventions: Volume loading ($10\text{--}20\,ml/kg$), co-loading, and vasopressors (Ephedrine for HR/BP, Phenylephrine/Neo for BP). Left Uterine Displacement (LUD) is critical for patients in the 3rd trimester.

• Pulmonary Manifestations:

  • Phrenic Nerve ($C3\text{--}C5$): Innervates the diaphragm. Generally unaffected unless the block is very high.

  • High Spinal: Causes dyspnea due to proprioceptive blockade of accessory muscles.

  • Total Spinal: Leads to apnea caused by medullary hypoperfusion and brainstem ischemia.

• GI and Renal Manifestations:

  • GI: Parasympathetic dominance leads to a contracted, small gut with increased peristalsis. Nausea/Vomiting (N/V) is common due to hypotension and vagal stimulation.

  • Urinary: Autonomic bladder control is lost (S2–S4 sacral blockade). This can cause an atonic bladder and urinary retention, which might outlast the block.

• Thermoregulation:

  • Vasodilation causes redistribution of central heat, leading to low core temperatures though the patient may feel warm.

  • Shivering is common and treated with Demerol or Clonidine.

Indications and Contraindications

• Indications: Lower abdomen, perineum, and lower extremity procedures. Post-op pain management, C-sections, and labor pain.

• Absolute Contraindications:

  • Patient refusal.

  • Allergy to local anesthetics.

  • Infection at the injection site.

  • Coagulopathy or bleeding diathesis.

  • Increased intracranial pressure (risk of brain herniation).

  • Septicemia/bacteremia.

  • Severe hypovolemia/shock.

  • Critical aortic or mitral stenosis.

• Relative Contraindications:

  • CNS disease (e.g., Multiple Sclerosis).

  • Chronic back pain.

  • Peripheral neuropathies (e.g., Diabetes Mellitus).

  • Uncooperative patient.

  • Severe spinal deformity.

Spinal Anesthesia Technique (Subarachnoid Block)

• Layers Passed Through:

  1. Skin

  2. Subcutaneous tissue

  3. Supraspinous ligament

  4. Interspinous ligament

  5. Ligamentum flavum

  6. Epidural space

  7. Dura mater

  8. Arachnoid mater

  9. Subarachnoid space

• Needle Types:

  • Quincke: Cutting-tip needle with end injection.

  • Whitacre / Sprotte: Pencil-point (blunt) needles with side injection. Pencil-point needles reduce the risk of Post-Dural Puncture Headache (PDPH).

• Baricity (Density relative to CSF):

  • Hyperbaric: Heavier than CSF (Local + Glucose). Injected LA "sinks" to dependent areas (typically $T4\text{--}T8$ in supine position).

  • Hypobaric: Lighter than CSF (Local + Sterile Water). LA "floats" or rises.

  • Isobaric: Equal to CSF density (Local mixed $1:1$ with CSF). LA stays at the level of injection.

• Dosing Adjustments for Height:

  • Standard doses are based on a $66\text{-inch}$ ($5'6"$) patient.

  • Add or subtract the following for every $6$ inches in height deviation:

    • Tetracaine: $2\,mg$

    • Bupivacaine: $1.5\,mg$

    • Lidocaine: $10\,mg$

Epidural Anesthesia Technique

• Characteristics:

  • The epidural space is wider in the midline: $5\text{--}6\,mm$ in the lumbar region and $3\text{--}5\,mm$ in the mid-thoracic region.

  • The distance from skin to the epidural space is $4\,cm$ in $50\%$ of the population and $4\text{--}6\,cm$ in $80\%$.

• Needles and Catheters:

  • Tuohy Needle: $17\text{--}18\,g$, $3\text{--}3.5$ inches long. It has a curved ($15\text{--}30^\circ$) tip to guide the catheter and avoid dural puncture.

  • Catheter Placement: Usually advanced $3\text{--}5\,cm$ (ideal is $4\,cm$) into the epidural space.

• Identification Methods:

  • Loss of Resistance (LOR): Using air or saline in a glass syringe.

  • Hanging Drop: Using saline at the needle hub.

• The Test Dose:

  • Standard test dose: $3\,ml$ of Lidocaine $1.5\%$ with Epinephrine $1:200,000$.

  • Epinephrine content: $0.005\,mg/ml$ ($15\,\mu g$ total in $3\,ml$).

  • IV Injection signs: Heart rate increase $> 20\%\,BP/HR$ within $30$ seconds.

  • Spinal (Intrathecal) Injection signs: Motor blockade within $3$ minutes.

• Dosing Principle: Volume is the key to height. Use $1\text{--}2\,ml$ of local anesthetic per segment to be blocked. Dose is decreased by $50\%$ in elderly/neonates and by $30\%$ in pregnancy.

Complications of Neuraxial Anesthesia

• Post-Dural Puncture Headache (PDPH):

  • Cause: CSF leakage from dural puncture exceeding production, leading to low intracranial pressure and traction on meningeal structures.

  • Hallmark: Postural headache (better when flat, worse when upright).

  • Risk Factors: Young females, pregnancy, use of large-gauge cutting needles.

  • Treatment: Bed rest (lying flat), fluids, Caffeine (stimulates CSF production and vasoconstricts), and Epidural Blood Patch ($15\text{--}20\,ml$ of autologous blood).

• Systemic Toxicity (LAST):

  • Caused by inadvertent intravascular injection. Signs include tinnitus and metallic taste.

  • Absorption rate: Intercostal $>$ Caudal $>$ Epidural $>$ Brachial Plexus $>$ SQ.

• Neurological Injury:

  • Transient Neurologic Syndrome (TNS): Pain following block resolution lasting $2\text{--}7$ days, commonly associated with $5\%$ Lidocaine use.

Anticoagulation Guidelines (ASRA)

• Warfarin: Stop $3\text{--}5$ days prior; INR must be $< 1.3$.

• NSAIDs/Aspirin: No contraindication for block or catheter removal.

• Clopidogrel (Plavix): Stop $7$ days prior.

• Ticlopidine (Ticlid): Stop $14$ days prior.

• Heparin (Standard/Unfractionated): Delay block until $1$ hour before next dose. Catheter removal $1$ hour prior to or $4$ hours after dosing.

• LMWH (Lovenox): Wait $24$ hours after dose before performing block. Remove catheter at least $10\text{--}12$ hours after the last dose, and wait $2$ hours after removal to restart the medication.

• Herbal Meds: Garlic, ginger, ginseng, and ginkgo affect coagulation; should be stopped though guidelines are less definitive.