OB Regional Anesthesia Review

OB Regional Anesthesia Review

Baricity:

equal to the density of the local anesthetic divided by the density of the CSF
main determinant of how the local anesthetic is distributed when injected into the CSF
Local anesthetics can be…..when compared to CSF:
Hypobaric: Baricity<1.0, Med in H2O, ↓ density than CSF → rise; positioning of pt after injection is important (for perineal/anal sx, in prone/jackknife)
Isobaric: Baricity = O, remains in the level of the injection; positioning does not affect spread of LA
Hyperbaric: Baricity > 1, med in dextrose, ↑ density than CSF → “fall”, positioning is important

Density, Specific Gravity, and Baricity of Different Substances and Local Anesthetics
Density / Specific Gravity / Baricity
Water / 0.9933 / 1.0000 / 0.9930
CSF / 1.0003 / 1.0069 / 1.0000
Hypobaric
Tetracaine
Lidocaine / 0.33% in water
0.5% in water / 0.9980
N/A / 1.0046
1.0038 / 0.9977
0.9985
Isobaric
Tetracaine
Lidocaine
Bupivacaine / 0.5% in 50% CSF
2% in water
0.5% in water / 0.9998
1.0003
0.9993 / 1.0064
1.0066
1.0059 / 0.9995
1.0003
0.9990
Hyperbaric
Tetracaine
Lidocaine
Bupivacaine
Bupivacaine / 0.5% in 5% dextrose
5% in 7.5% dextrose
0.5% in 8% dextrose
0.75% in 8% dextrose / 1.0136
1.0265
1.0210
1.0247 / 1.0203
1.0333
1.0278
1.0300 / 1.0133
1.0265
1.0207
1.0227

Levels for major procedures:

Body Landmark / Dermatome Level / Procedure
Little finger / C8 (All cardio accelerator fibers blocked)
Nipples / T4 / Upper abdominal surgery
Xiphoid / T6 / Intestinal, gynecologic, and urologic surgery
Inferior edge of Scapula / T7
Umbilicus / T10 / Vaginal delivery of a fetus, and hip surgery
L1 / Thigh surgery and lower leg amputations
L2 / Foot and ankle surgery
Spinal cord ends at L1-2, place spinals below this level.
Superior iliac crest / L4
Inferior iliac crest / S2
S2 to S5 (saddle block) / Perineal and anal surgery
Spinal / Epidural
Smaller doses of LA (mg) → mg block → / Larger doses of LA (ml) → Volume block →
→Lower potential for toxicity / →Higher potential for toxicity
Faster onset / Slower onset
Limited Duration / Flexible duration
Lower failure rate (CSF is definitive for location) / Higher failure rate
Baricity has an effect / Baricity has no effect (no CSF to create Baricity)

Doses for Spinal Blocks:

Dose, Duration, and Onset of Local Anesthetics Used in Spinal Anesthesia
Dose (mg) / Duration (min)
Plain / With 0.2mg
Epinephrine / Onset (min)
to T10 / to T4
Commonly Used
Lidocaine 5%
Bupivacaine 0.75% / 50–75
8–12 / 75–100
14–20 / 60–70
90–110 / 75–100
100–150 / 3–5
5–8
Less Commonly Used
Tetracaine 0.5%
Mepivacaine 2%
Ropivacaine 0.75%
Levobupivacaine 0.5%
Chloroprocaine 3% / 6–10
N/A
15–17
10–15
30 / 12–16
60–80
18–20
N/A
45 / 70–90
140–160
140–200
135–170
80–120 / 120–180
N/A
N/A
N/A
130–170 / 3–5
2–4
3–5
4–8
2–4
Drug / Onset / Maximum Dose
(with epinephrine);
Max mg dose / Duration
(with Epinephrine)
Lidocaine / Rapid / 4.5 mg/kg (7 mg/kg)
300mg (500 mg) / 120 min (240 min)
Bupivacaine / Slow / 2.5 mg/kg (3 mg/kg)
175mg (225mg) / 4 hours (8 h)
Chloroprocaine / Rapid (short duration) / 10 mg/kg (15 mg/kg)
800mg (1000mg) / 30 min (90 min)

Doses for spinal blocks depend on:

Height of pt → determines the volume of the subarachnoid space
Segmental level of anesthesia desired
Duration of anesthesia desired

Doses for Epidural blocks (Volume blocks): the volume depends on the location the catheter tip in the epidural space as the size of the segmental epidural spaces increases down the spinal cord as the spinal cord occupied less and less space.

Site of procedure / Dose in ml
Chest / T12-L2 / 8-12ml
Upper Abdomen (Cholecyctectomy, gastric resection) / L2 / 12-16ml
Lower Abdomen
Colon resection, aortic aneurysm, retro pubic prostatectomy / L2 / 12-16ml
Herniorrhaphy / L3 / 8-12ml
hysterectomy / L3 / 10-14ml
Lower Extremities
Anesthesia / L4 / 10-14ml
Sympathetic block / L2 / 8-12ml
Perineum (TURP, vaginal hysterectomy) / L4 / 8-12ml
Back and flank (nephrectomy) / L2 / 10-14ml
Vaginal delivery
1st stage labor / L3 / 5-7ml
2nd and 3rd stage labor / L3 / 10-12ml

Nagelhout p. 1067

Opioids in Regional Anesthesia:

Epidural anesthesia:

Hydrophilic opioids (Morphine) spread easily within CSF and can enhance surgical anesthesia and provide postoperative pain control.

Lipophilic opioids (Fentanyl) rapidly absorbed into systemic circulation → no strong effect.

Epidural Opioids / Bolus Dose / Onset / Peak / Duration
Morphine / 2-5mg / 20-30min / 30-60min / 12-24hr
Hydromorphone / 1 mg / 10-15min / 20-30min / 8-15hrs

Nagelhout p.1069

Spinal anesthesia: Spinal doses are approximately 10 times less than the epidural doses.

Morphine: 0.1-0.5mg can provide pain control for ~24hrs, but necessitates in-hospital monitoring for respiratory depression → is the traditional opioid for prolonged pain management.

Fentanyl: 25mcg for short surgical procedures, pts can be discharged home the same day.

Transient neurological symptoms (TNS) syndrome of pain and dysesthesias that may occur in up to 1/3 of pts receiving intrathecal doses of lidocaine.

Pathogenesis of TNS is believed to represent concentration dependent neurotoxicity of local anesthetics
Rarely occurs with other local anesthetics
Mechanism responsible is unknown
Increased incidence in pts in lithotomy position, positioning for knee arthroscopy, and outpatient procedures
Symptomstypically include back pain, weakness, and numbness radiating to one or both buttocks and down the legs, or both. Does not consistently manifest with sensory/motor deficits or reflex abnormalities.
Appear within first 12-24 hours after surgery
Exclusively a pain syndrome, no bowel or bladder dysfunction
All neuro, MRI and Electrophysiologic examinations are normal
Most often resolve within 3 days, rarely last a week.
Pain is often more severe than the surgical procedure.
TreatmentNSAIDs, possibly opioids if severe pain, and trigger point injections, time to allow symptoms to pass

Caudal equina syndromeresult of an injury below the level of the conus, or caudal end of the cord, typically below L2. Persistent paresthesia and limited motor weakness are the most common injuries

Nerve roots within the subarachnoid space are highly vulnerable to chemical damage, particularly the sacral roots which are poorly myelinated.
Typically results from compression within the lumbar spinal canal or from chemical damage

Risk factors

Poor subarachnoid dispersion of LA
Block failure, followed by a repeat injection
Fine-gauge or pencil point needle
Spinal micro catheter
Continuous infusion
Hyperbaric anesthetic solution
Lithotomy position
Unintentional intrathecal injection of a large volume intended for the epidural space
Incorrect formulation, with unsuitable preservative or antioxidant
Intrathecal LIDOCAINE, particularly 5%
Can be caused by an epidural hematoma

Treatmentsteroids, anti-inflammatory medications, decompression if related to hematoma

Epidural hematomadamage to vessel or anticoagulation can cause a hematoma to form in epidural space. Incidence <1 in 150,000

Symptoms includecan initially be hard to see in pts with an epidural or spinal block

Severe, localized back pain with delayed radicular radiation that may mimic disk herniation
Weakness
Numbness
Urinary incontinence
Fecal incontinence

Risk factors

Difficult or traumatic epidural needle/catheter placement or spinal placement
Coagulopathy or therapeutic anticoagulation
Spinal deformity
Spinal tumor

Early recognition is critical as delay of more than 8 hours in decompressing the spine reduces the odds of good recovery. Treatment is surgical removal of the pressure on the spine.

Important to review what meds the pt is onepidural/spinals should not be done on pts taking antiplatelet drugs unless they have been held for a specific time frame. Also pts on LMWH at high risk. Timing of catheter removal is crucial as removal can cause hematoma formation as well.

Blood patchregarded by many as the gold standard treatment for PDPH. Blood is drawn using sterile technique. All air is removed from the syringe. It is then injected into the epidural space near the area of the where the dura was inadvertently punctured.

Idea is that the blood will help “patch” the hole in the dural space preventing continued leak of CSF
Possibly blood in epidural space helps more to tamponade the leak of CSF vs actually seal the puncture.
Exact volume used is debatable, average 12-15 mL is used, but can inject up to 20 mL. With larger volume. The injection is terminated when volume is instilled or pt complains of severe back, leg, or neck pain or pressure.
Headache is often relieved immediately
After procedure pt should lie flat and rest for 1-2 hours. Pt may resume ambulation after 2 hours but should avoid vigorous physical activity, heavy lifting, or any activity involving a valsalva maneuver.

Procedure may be repeated if first fails. Often second procedure provides relief. If second fails to relieve PDPH, get neuro consult.
Two main complications
Infection
Neurologiclow back pain with neuro impairment of the lower extremities, subdural hematoma, arachnoiditis, radicular back pain, pneumocephalus, seizures, and acute meningeal irritation.

Spinal

Anatomy

Midline approach
Skin
Subcutaneous fat
Supraspinous ligament
Interspinous ligament
Ligamentum flavum
Dura mater
Subdural space
Arachnoid mater
Subarachnoid space
Paramedian approach
Skin
Subcutaneous fat
Ligamentum flavum
Dura mater
Subdural space
Arachnoid mater
Subarachnoid space

Dermatome
The fourth thoracic (T4) dermatome corresponds to the nipples.
The sixth thoracic (T6) dermatome corresponds to the xiphoid.
The tenth thoracic (T10) dermatome corresponds to the umbilicus.

Table 1. Dermatomal Levels of Spinal Anesthesia for Common Surgical Procedures
Procedure / Dermatomal Level
Upper abdominal surgery / T4
Intestinal, gynecologic, and urologic surgery / T6
Transurethral resection of the prostate
Vaginal delivery of a fetus, and hip surgery / T10
Thigh surgery and lower leg amputations / L1
Foot and ankle surgery / L2
Perineal and anal surgery / S2 to S5 (saddle block)

Medications

Table 4. Dose, Duration, and Onset of Local Anesthetics Used in Spinal Anesthesia
Dose (mg) / Duration (min)
Plain / With 0.2 mg
Epinephrine / Onset (min)
to T10 / to T4
Commonly Used
Lidocaine 5%
Bupivacaine 0.75% / 50–75
8–12 / 75–100
14–20 / 60–70
90–110 / 75–100
100–150 / 3–5
5–8
Less Commonly Used
Tetracaine 0.5%
Mepivacaine 2%
Ropivacaine 0.75%
Levobupivacaine 0.5%
Chloroprocaine 3% / 6–10
N/A
15–17
10–15
30 / 12–16
60–80
18–20
N/A
45 / 70–90
140–160
140–200
135–170
80–120 / 120–180
N/A
N/A
N/A
130–170 / 3–5
2–4
3–5
4–8
2–4

Additives
Vasoconstrictors – epinephrine and phenylephrine – limit systemic reabsorption and prolong duration of action by keeping the local anesthetic in contact with the nerve fibers
Alpha-2-adrenergic agonists enhance pain relief and prolong sensory block and motor block
i.e. clonidine
Acetylcholinesterase inhibitors prevent the breakdown of acetylcholine and produce analgesia when injected intrathecally
i.e. neostigmine
Factors determining spinal level blockade and spread
Baricity of the local anesthetic solution
Position of the patient during and just after injection
Dose of the anesthetic injected

Table 2. Determinants of Local Anesthetic Spread in the Subarachnoid Space
Properties of local anesthetic solution
Baricity
Dose
Volume
Specific gravity
Patient characteristics
Position during and after injection
Height (extremely short or tall)
Spinal column anatomy
Decreased CSF volume (increased Intraabdominal pressure due to increased weight, pregnancy, etc.)
Technique
Site of injection
Needle bevel direction

Risks/side effects
Contraindications
Absolute
Patient refusal
Sepsis at the site of injection
Hypovolemia
Coagulopathy
Indeterminate neurological disease
Increased intracranial pressure
Relative
Infection distinct from the site of injection
Unknown duration of surgery

Complications
Permanent neurologic injury - can occur after needle introduction into the spinal cord or nerves, spinal cord ischemia, bacterial contamination of the subarachnoid space, or hematoma formation
After low-molecular-weight heparin (LMWH) administration, delay spinal anesthesia for 10 to 12 hours
If blood is noted during needle placement, delay LMWH therapy for 24 hours
In cases of continuous spin anesthesia and accidental LMWH therapy, remove the catheter 10-12 hours after the last dose of LMWH
Avoid spinal anesthesia for 14 days after the last dose of ticlopidine (Ticlid) and 7 days after clopidogrel (Plavix)
Patients should not receive glycoprotein IIb/IIIa inhibitors for 4 weeks after surgery, and do not attempt spinal anesthesia until platelet function returns to normal
Caudal equina syndrome – associated with the use of continuous spinal micro catheters. Use of hyperbaric 5% lidocaine for spinal anesthesia is also associated with an increased incidence of cauda equina syndrome
Arachnoiditis – known to occur after intrathecal steroid injection. Causes include infection, myelograms from oil-based dyes, blood in the intrathecal space, neuroirritant, neurotoxic/neurolytic substances, surgical interventions in the spine, intrathecal corticosteroids, and trauma
Meningitis – bacterial or aseptic
Post dural puncture headache – result of loss of CSF with incidence up to 25% after spinal anesthesia. Headache is characteristically worse when the head is elevated and becomes milder or completely relieved when the patient is supine.

High spinal – can result in profound respiratory impairment, most likely due to brainstem ischemia. If the blood pressure and cardiac out become too low due to vasodilation, cerebral blood flow can be impaired
Cardiovascular collapse – rare event. Bradycardia usually precedes cardiac arrest, and early aggressive treatment of bradycardia is warranted. Treatment with atropine, ephedrine, and epinephrine with ACLS protocol initiated

Epidural Block

Patient Selection

-cooperative

-can also be used in the cervical and thoracic areas as well—levels at which spinal anesthesia is not advised.

-a continuous technique has been found to be helpful in providing epidural local anesthesia or opioid analgesia after major surgical procedures.

Pharmacologic Choice

Chloroprocaine, an amino ester local anesthetic, is a short-acting agent that allows efficient matching of the length of the surgical procedure and the duration of epidural analgesia, even in outpatients. 2-Chloroprocaine is available in 2% and 3% concentrations; the latter is preferable for surgical anesthesia and the former for techniques not requiring muscle relaxation.

Lidocaine is the prototypical amino amide local anesthetic and is used in 1.5% and 2% concentrations epidurally. Concentrations of mepivacaine necessary for epidural anesthesia are similar to those of lidocaine; however, mepivacaine lasts from 15 to 30 minutes longer at equivalent dosages. Epinephrine significantly prolongs (i.e., by approximately 50%) the duration of surgical anesthesia with 2-chloroprocaine and either lidocaine and mepivacaine. Plain lidocaine produces surgical anesthesia that lasts from 60 to 100 minutes.

Bupivacaine, an amino amide, is a widely used long-acting local anesthetic for epidural anesthesia. It is used in 0.5% and 0.75% concentrations, but analgesic techniques can be performed with concentrations ranging from 0.125% to 0.25%. Its duration of action is not prolonged as consistently by the addition of epinephrine, although up to 240 minutes of surgical anesthesia can be obtained when epinephrine is added.

Ropivacaine, another long-acting amino amide, is also used for regional and epidural anesthesia. For surgical anesthesia it is used in 0.5%, 0.75%, and 1% concentrations. Analgesia can be obtained with concentrations of 0.2%. Its duration of action is slightly less than that of bupivacaine in the epidural technique, and it appears to produce slightly less motor blockade than a comparable concentration of bupivacaine.

In addition to the use of epinephrine as an epidural additive, some anesthesiologists recommend modifying epidural local anesthetic solutions to increase both the speed of onset and the quality of the block produced. One recommendation is to alkalinize the local anesthetic solution by adding bicarbonate to it to achieve both these purposes. Nevertheless, the clinical advisability of routinely adding bicarbonate to local anesthetic solutions should be determined by local practice protocols.

Placement

Anatomy

When a lumbar approach to the epidural space is used in adults, the depth from the skin to the ligamentum flavum is commonly near 4cm; in 80% of patients the epidural space is cannulated at a distance of 3.5 to 6cm from the skin. In a small number of patients the lumbar epidural space is as near as 2cm from the skin.
In the lumbar region, the ligamentum flavum is 5 to 6mm thick in the midline, whereas in the thoracic region it is 3 to 5mm thick.
In the thoracic region, the depth from the skin to the epidural space depends on the degree of cephalad angulation used for the paramedian approach as well as the body habitus of the patient.
In the cervical region the depth to the ligamentum flavum is approximately the same as that in the lumbar region, 4 to 6cm.

Needle Puncture: Lumbar Epidural

identify the midline structures, and the bony landmarks
If a single-shot epidural technique is chosen, a Crawford needle is appropriate; if a continuous catheter technique is indicated, a Tuohy or other needle with a lateral-facing opening is chosen.
The midline approach is most often indicated for a lumbar epidural procedure.
The needle is slowly advanced until the change in tissue resistance is noted as the needle abuts the ligamentum flavum.
At this point, a 3- to 5-mL glass syringe is filled with 2mL of saline solution, and a small (0.25mL) air bubble is attached to the needle, and if the needle tip is in the substance of the ligamentum flavum, the air bubble will be compressible. If the ligamentum flavum has not yet been reached, pressure on the syringe plunger will not compress the air bubble.
Once compression of the air bubble has been achieved, the needle is grasped with the nondominant hand and pulled toward the epidural space, while the dominant hand (thumb) applies constant steady pressure on the syringe plunger, thus compressing the air bubble. When the epidural space is entered, the pressure applied to the syringe plunger will allow the solution to flow without resistance into the epidural space.
An alternative technique is the hanging-drop technique for identifying entry into the epidural space. In this technique, when the needle is placed in the ligamentum flavum a drop of solution is introduced into the hub of the needle. No syringe is attached, and when the needle is advanced into the epidural space, the solution should be “sucked into” the space.
The incidence of unintentional intravenous cannulation with an epidural catheter may be decreased by injecting 5 to 10mL of solution before threading the catheter. If a catheter is inserted, it should be inserted only 2 to 3cm into the epidural space because threading it farther may increase the likelihood of catheter malposition. Obstetric patients require catheters to be inserted to 3 to 5cm into the epidural space to minimize dislodgement during labor analgesia.

Needle Puncture: Thoracic Epidural

In this technique, the paramedian approach is preferred because it allows easier access to the epidural space because the spinous processes in the mid-thoracic region overlap each other from cephalad to caudad.
The loss-of-resistance technique and insertion of the catheter are carried out in a manner identical to that used for lumbar epidural block. Again, the hanging-drop technique is an alternative method of identifying the thoracic epidural space, although the classic Bromage needle–syringe grip is my first choice for the thoracic epidural block (Fig. 41-10).