Spinal Anesthesia (pt 1) Flashcards
What is Spinal Anesthesia?
Reversible chemical blockade of neuronal transmission by a local anesthetic injected into the Cerebral Spinal Fluid (CSF).
Where do the actions of spinal anesthesia occur?
On anterior and posterior nerve roots as they pass through the CSF to peripheral nerves
-To a lesser extent, on the spinal cord itself
-Temporary interruption of sensory, autonomic, and motor nerve fibers renders the patient insensitive to surgical stimulation.
Immediately after spinal nerves exit via the intervertebral foramen, they divide into _____ ?
Ventral and Dorsal Primary Rami.
-DPR: supply skin and muscles of the back
-VPR: supply anterior lateral muscles, skin of neck and trunk and limbs.
Both also contribute a small nerve to sympathetic ganglia.
Joint together eventually to form plexi in a specific region
What are Dermatomes?
Areas of the skin that are innervated by a specific nerve root.
-Help to determine where a block is working
-Help to understand where injury/pain is coming from
What nerve forms the dermatomes of the face?
Ophthalmic, Maxillary, and Mandibular divisions of the Trigeminal Nerve
What dermatome level is C4?
Clavicle
What dermatome level is C8?
Little finger
What dermatome level is T4?
Nipple
What dermatome level is T6 - T8?
Xiphoid, lower rib cage
What dermatome level is T10?
Umbilicus
Above ___ is a high spinal, which causes what symptoms?
T2
-Severe hypotension, dyspnea, respiratory arrest
What dermatome level is T12?
Last rib
Explain how/why Differential Nerve Blockade works?
The anatomy of the nerve root accounts for differential sensitivity of nerve types to local anesthetic.
-Small diameter nerve fibers are found close to the nerve root surface (Shorter diffusion path of LA)
-Large diameter nerve fibers are found deep to the nerve bundle (longer diffusion path of LA)
Which fibers are more sensitive to local anesthetic blockade?
Large myelinated fibers are more sensitive to local anesthetic blockade than smaller myelinated and unmyelinated fibers.
What order are fibers blocked in (A, B, C, etc)?
1) B Fibers
2) C Fibers
3) A Delta Fibers
4) Larger A Gamma (muscle spindles), A Beta (Touch, pressure)
5) A Alpha (Proprioception, motor) fibers are last.
What fibers are found at the outside of the root?
B Fibers
-Preganglionic sympathetic efferents
-Autonomics. This is why the first thing you’ll see with a block is a drop in BP.
What fibers are found immediately inferior to B Fibers?
C and A-Delta fibers
-C: Pain
-A Delta: Pain, temperature
-Pain, temperature, and touch afferents
-Post ganglionic sympathetics
What is the order of sensitivity with large myelinated to unmyelinated?
Large Myelinated > Smaller myelinated > unmyelinated
-Once it gets to them, the large myelinated fibers are very sensitive
What are other factors that contribute to a differential nerve block?
-Myelination
-Nodes of Ranvier
-Size of fibers
-Location depth
-Na and K Channels on each nerve
Sympathetic blockade (BP changes is 1st sign) is usually ___ segments higher than ____ blockade (temp, light touch, pain), which is usually ____ or greater segments before ____ blockade.
Sympathetic blockade (BP changes is 1st sign) is usually 2 segments higher than Sensory blockade (temp, light touch, pain), which is usually 2 or greater segments before Motor blockade.
-Sometimes, sympathetics can go up to 6 segments higher (bad).
-Could end up having a bigger block than what you thought.
How do you assess your patient for sensory block?
-Loss of temperature sensation from cool alcohol swab is most sensitive indicator of initial onset of sensory block
-Sharp/broken tongue depressor is most accurate for overall sensory block
-Decrease in BP may be first sign it’s working (B fibers)
-Loss of sensation to cold (alcohol swab) occurs before sharp and at a higher level (C, A-Delta)
-Initial motor block is myelinated A Beta & A Gamma fibers
-Profound block is A Alpha
What are the 2 most important factors that impact the Spinal (Intrathecal) Level?
-Baricity
-Patient position
How do you determine baricity?
In relation to the specific gravity of CSF (1.004-1.008).
-Hypobaric: Less baricity than CSF (floats up)
-Isobaric: stays where it is injected
-Hyperbaric: most frequently used. Sinks
What are the highest points of the spinal curve?
-Cervical & Lumbar Lordosis
-C3 and L3 high points
What are the lowest points of the spinal curve?
-Thoracic and Sacral Kyphosis
-T6 and S2 low points
What other factors affect the height of a spinal block?
-Age
-Height (if very tall, can increase dose as more distance to travel)
-Weight: Obese/Pregnant have decreased intrathecal space (less dose)
-Technique of injection: site, direction of needle/bevel, rate of injection (barbotage) - controversial
-Spinal Fluid (rate of circulation, volume, pressure - coughing and straining)
-LA: Amount, concentration, volume, vasoconstrictors (slow down how fast it is absorbed)
MOST IMPORTANT is baricity and patient position!!!!
How does age impact your height of spinal block?
-Ligamentum Flavum gets tougher
-Intrathecal space is more compressed
-Higher spread of Local Anesthetic
What is the baricity of CSF?
1
What is the baricity of hypobaric solutions?
-Tetracaine 0.33% / water = 0.9977
-Lidocaine 0.5% / water = 0.9985
What is the baricity of isobaric solutions?
-Tetracaine 0.5% / 50% CSF = 0.9995
-Lidocaine 2% / water = 1.003
-Bupivicaine 0.5% / water = 0.999
What is the baricity of hyperbaric solutions?
-Tetracaine 0.5% / 5% dextrose = 1.0133
-Lidocaine 5% / 7.5% dextrose = 1.0265
-Bupivicaine 0.5% / 8% dextrose = 1.0207
-Bupivicaine 0.75% / 8% dextrose = 1.0227
What is specific gravity?
The ratio of a density of a substance to the density of water
-Affected by proteins, glucose, uremia
-temperature can affect it
-Spec grav of CSF = 1.0069
What is the definition of baricity?
The resting position of 2 fluids with differing specific gravities.
What is the primary objective of neuraxial anesthesia? **
-To block afferent fibers located in the Dorsal Roots
-Motor and sympathetic fibers are close and they get blocked as they pass through the ventral root.
Why is Lidocaine 5% not used anymore for Spinal Anesthesia (SAB)?
-Potential to cause transient neurological syndrome (TNS)
-Can cause Cauda Equina syndrome (permanent)
Which is the most cardiotoxic local anesthetic?
Bupivicaine
-May need lipids to reverse
List some common preservatives added to local anesthetics.
-Parabens (high allergy/anaphylaxis potential)
-Sulfites (neurotoxic)
-EDTA (muscle pain, can cause tetany in paraspinous muscles)
T/F: it’s ok to use preservative containing, multi-use vials for spinals.
FALSE
-Want preservative free, single dose vials for anything going in the spine.
-Multi-use can be used for PNB, but not for spinals.
What is the MOA of local anesthetics?
-Limit the sodium channels and stop the propagation of a nerve impulse.
-Small amounts act directly on the spinal cord itself by way of diffusion through the pia mater (slow process)
-LA can find its way to the cord via the Virchow-Robin Spaces
What are Virchow-Robin Spaces?
Insertion sites of tiny blood vessels on the spinal cord where local anesthetics can get around the sides of them and act directly on the cord.
The more _____ soluble the drug, the more it can penetrate the membrane and get to the roots (site of action).
Lipid soluble.
Why is Epinephrine added to local anesthetics?
-Alpha 1 agonist
-Produces direct analgesia when placed on the cord
-Increases duration of LA by causing constriction of blood vessels (slows absorption of local)
-Usual dose is 0.1-0.2 mL of Epi 1:1000 for spinal (Epi Wash)
Epinephrine has the greatest effect on which local anesthetic?
Greatest effect on Tetracaine
Then Lidocaine, then Bupivicaine.
What is the dose of Phenylephrine added to Spinals?
-Pure Alpha 1 Agonist
-0.5 - 2 mg
What is the purpose of adding Clonidine to Local Anesthetics?
-Alpha 2 Agonist Activity
-High lipid solubility
-Local anesthetic effects
-Prolongs DOA
Why is NaHCO3 added to local anesthetics?
To increase the pH to decrease the onset (quicker)
Describe Opioids added to a Spinal
-Stimulate opioid receptors in the brainstem (Substantia Gelatinosa) and Spinal Cord
-Mu receptors are responsive to Morphine, Meperidine, Sufentanil, Fentanyl, and Alfentanil
-Mu 2 receptors are responsible for dec HR, RR, Euphoria
-SEs: N/V, itching, urinary retention
What is the opioid reversal?
Naloxone - competitive antagonist
What is the number 1 complaint of opioid additives to Local Anesthetics?
Itching (especially in OB)
Why is Fentanyl widely used as an additive to local anesthetics?
-It adheres to lipoproteins in the spinal cord due to its high lipid solubility
-Less drug available to diffuse to respiratory centers
-Dose: 10-25 mcg
-Onset: 5-10 min
-DOA: 2-4 hours
-Can be used in combo with morphine
What is important to know regarding Morphine added to local anesthetics?
-Use preservative free for spinals!
-Hydrophilic and not highly bound to spinal cord, so it moves around (can hit respiratory centers)
-Be very careful - administer only if patient is being monitored
-Onset: 60-90 min
-DOA: 6-8 hours
-Dose: 0.1 - 0.5 mg (spinal)
-Won’t be able to give more opioids for breakthrough pain
-Caution with risk of delayed respiratory depression**
For Upper abdominal surgery, what Dermatome level do you want to block?
T4
For intestinal, gyn, and urologic surgeries, what Dermatome level do you want to block?
T6
For a TURP, what Dermatome level do you want to block?
T10
For a vaginal delivery or hip surgery, what Dermatome level do you want to block?
T10
For thigh surgery and lower leg amputations, what Dermatome level do you want to block?
L1
For Foot and Ankle surgery, what Dermatome level do you want to block?
L2
For perineal and anal surgery, what Dermatome level do you want to block?
S2-S5 (saddle block)
