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:

o 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)

o Isobaric: Baricity = O, remains in the level of the injection; positioning does not affect spread of LA

o Hyperbaric: Baricity > 1, med in dextrose, ↑ density than CSF → “fall”, positioning is important

Density, Specific Gravity, and Baricity of Different Substances and Local AnestheticsDensity Specific Gravity Baricity

Water 0.9933 1.0000 0.9930CSF 1.0003 1.0069 1.0000HypobaricTetracaineLidocaine

0.33% in water0.5% in water

0.9980N/A

1.00461.0038

0.99770.9985

IsobaricTetracaineLidocaineBupivacaine

0.5% in 50% CSF2% in water0.5% in water

0.99981.00030.9993

1.00641.00661.0059

0.99951.00030.9990

HyperbaricTetracaineLidocaineBupivacaineBupivacaine

0.5% in 5% dextrose5% in 7.5% dextrose0.5% in 8% dextrose0.75% in 8% dextrose

1.01361.02651.02101.0247

1.02031.03331.02781.0300

1.01331.02651.02071.0227

Levels for major procedures:

Body Landmark Dermatome Level ProcedureLittle finger C8 (All cardio

accelerator fibers blocked)

Nipples T4 Upper abdominal surgeryXiphoid T6 Intestinal, gynecologic, and urologic

surgeryInferior edge of Scapula

T7

Umbilicus T10 Vaginal delivery of a fetus, and hip surgeryL1 Thigh surgery and lower leg amputationsL2 Foot and ankle surgery

Spinal cord ends at L1-2, place spinals below this level.Superior iliac crest L4

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Inferior iliac crest S2S2 to S5 (saddle block)

Perineal and anal surgery

Spinal EpiduralSmaller doses of LA (mg) → mg block → Larger doses of LA (ml) → Volume block → →Lower potential for toxicity →Higher potential for toxicityFaster onset Slower onsetLimited Duration Flexible durationLower failure rate (CSF is definitive for location) Higher failure rateBaricity 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 AnesthesiaDose (mg) Duration (min)

PlainWith 0.2mgEpinephrine Onset (min)to T10 to T4

Commonly Used

Lidocaine 5% Bupivacaine 0.75%

50–758–12

75–10014–20

60–7090–110

75–100100–150

3–55–8

Less Commonly Used

Tetracaine 0.5%Mepivacaine 2%Ropivacaine 0.75%Levobupivacaine 0.5%Chloroprocaine 3%

6–10N/A15–1710–1530

12–1660–8018–20N/A45

70–90140–160140–200135–17080–120

120–180N/AN/AN/A130–170

3–52–43–54–82–4

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)

30 min (90 min)

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800mg (1000mg)

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 mlChest T12-L2 8-12mlUpper Abdomen (Cholecyctectomy, gastric resection)

L2 12-16ml

Lower AbdomenColon resection, aortic aneurysm, retro pubic prostatectomy

L2 12-16ml

Herniorrhaphy L3 8-12mlhysterectomy L3 10-14mlLower ExtremitiesAnesthesia L4 10-14mlSympathetic block L2 8-12mlPerineum (TURP, vaginal hysterectomy)

L4 8-12ml

Back and flank (nephrectomy) L2 10-14mlVaginal delivery1st stage labor L3 5-7ml2nd and 3rd stage labor L3 10-12mlNagelhout 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 DurationMorphine 2-5mg 20-30min 30-60min 12-24hrHydromorphone 1 mg 10-15min 20-30min 8-15hrsNagelhout 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.

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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 Symptomstypically 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.

o Appear within first 12-24 hours after surgeryo Exclusively a pain syndrome, no bowel or bladder dysfunctiono All neuro, MRI and Electrophysiologic examinations are normalo Most often resolve within 3 days, rarely last a week.

Pain is often more severe than the surgical procedure. TreatmentNSAIDs, possibly opioids if severe pain, and trigger point injections, time to allow

symptoms to pass

Caudal equina syndromeresult 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 damageRisk factors

Poor subarachnoid dispersion of LAo Block failure, followed by a repeat injectiono Fine-gauge or pencil point needleo Spinal micro cathetero Continuous infusiono Hyperbaric anesthetic solutiono 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

Treatmentsteroids, anti-inflammatory medications, decompression if related to hematoma

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

Symptoms includecan initially be hard to see in pts with an epidural or spinal block

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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 onepidural/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 patchregarded 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 complicationso Infectiono Neurologiclow back pain with neuro impairment of the lower extremities, subdural hematoma,

arachnoiditis, radicular back pain, pneumocephalus, seizures, and acute meningeal irritation.

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Spinal

Anatomy

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

o Paramedian approach Skin Subcutaneous fat Ligamentum flavum Dura mater Subdural space Arachnoid mater Subarachnoid space

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Dermatomeo The fourth thoracic (T4) dermatome

corresponds to the nipples.o The sixth thoracic (T6) dermatome

corresponds to the xiphoid.o 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)

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Medications

Table 4. Dose, Duration, and Onset of Local Anesthetics Used in Spinal Anesthesia

Dose (mg) Duration (min)

Plain

With 0.2 mg

EpinephrineOnset (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

o 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. neostigmineo Factors determining spinal level blockade and spread

Baricity of the local anesthetic solution Position of the patient during and just after injection

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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 effectso Contraindications

Absolute Patient refusal

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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

o 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

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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

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When a lumbar approach to the epidural space is used in adults, the depth from the skin to the ligamentum flavum is commonly near 4 cm; in 80% of patients the epidural space is cannulated at a distance of 3.5 to 6 cm from the skin. In a small number of patients the lumbar epidural space is as near as 2 cm from the skin.

In the lumbar region, the ligamentum flavum is 5 to 6 mm thick in the midline, whereas in the thoracic region it is 3 to 5 mm 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 6 cm.

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 2 mL of saline solution, and a small (0.25 mL) 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.

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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 10 mL of solution before threading the catheter. If a catheter is inserted, it should be inserted only 2 to 3 cm 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 5 cm 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).

Needle Puncture: Cervical Epidural

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In the cervical epidural technique, the patient is typically in a sitting position with the head bent forward and supported on a table.

The spinous processes of the cervical vertebrae are nearly perpendicular to the long axis of the vertebral column; thus, a midline technique is applicable for the cervical epidural block.

The most prominent vertebral spinous processes, those of C7 and T1, are identified with the neck flexed. The second (index) and third fingers of the palpating hand straddle the space between C7 and T1, and the epidural needle is slowly inserted in a plane approximately parallel to the floor (or parallel to the long axis of the cervical vertebral spinous processes). Abutment of the needle onto the ligamentum flavum will be appreciated at a depth similar to that seen in the lumbar epidural block (i.e., 3.5 to 5.5 cm), and needle placement is then performed using the loss-of-resistance technique as in the other epidural methods. The hanging-drop method is also an option for identification of the cervical epidural space.

Potential Problems

systemic toxicity resulting from intravenous injection of the intended epidural anesthetic o One way to minimize intravenous injection of the pharmacologic doses of local anesthetic

needed for epidural anesthesia is to verify needle or catheter placement by administering a test dose before the definitive epidural anesthetic injection. The current recommendation for the test dose is 3 mL of local anesthetic solution containing 1:200,000 epinephrine (15 µg of epinephrine).

o Even if the test dose is negative, the anesthesiologist should inject the epidural solution incrementally, be vigilant for unintentional intravascular injection, and have all necessary

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equipment and drugs available to treat local anesthetic–induced systemic toxicity.

o

Unintentional administration of an epidural dose into the spinal fluid. o Blood pressure and heart rate should be supported pharmacologically and ventilation should be

assisted as indicated. o Usually atropine and ephedrine will suffice to manage this situation, or at least will provide time

to administer more potent catecholamines. If the entire dose (20 to 25 mL) of local anesthetic is administered into the cerebrospinal fluid, tracheal intubation and mechanical ventilation are indicated because it will be approximately 1 to 2 hours before the patient can consistently maintain adequate spontaneous ventilation.

o When epidural anesthesia is performed and a higher-than-expected block develops after a delay of only 15 to 30 minutes, subdural placement of the local anesthetic must be considered. Treatment is symptomatic, with the most difficult part involving recognition that a subdural injection is possible.

If neurologic injury occurs after epidural anesthesia, a systematic approach to the problem is necessary. The performance of cervical or thoracic epidural techniques demands special care with hand and needle

control because the spinal cord is immediately deep to the site of both these epidural blocks. Epidural hematoma created with the needles or catheters.

o Concern about epidural hematoma formation is greater in patients who have been taking antiplatelet drugs such as aspirin or who have been receiving preoperative anticoagulants.

o Perioperative anticoagulant regimens that demand special consideration are the use of low–molecular-weight heparin (LMWH) or potent antiplatelet drugs concurrently with epidural block.

o It is currently recommended that no procedure, including withdrawal or manipulation of an epidural catheter, should occur within 12 hours after a dose of LMWH, and the next dose of LMWH should be delayed for at least 2 hours after atraumatic epidural needle or catheter insertion or manipulation. The antiplatelet drugs (e.g., ticlopidine, clopidogrel, and platelet glycoprotein IIb/IIIa receptor antagonists) are sometimes combined with aspirin and other anticoagulants.

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Post–dural puncture headache can result from epidural anesthesia when unintentional subarachnoid puncture accompanies the technique.

o When using the larger-diameter epidural needles (18 and 19 gauge), it can be expected that at least 50% of patients experiencing unintentional dural puncture will have a postoperative headache

Pearls

Epidural catheters can be malpositioned in a number of ways. If a catheter is inserted too far into the epidural space, it can be routed out of foramina, resulting in patchy epidural block.

The catheter can also be inserted into the subdural or subarachnoid space or into an epidural vein. Similarly, the use of epidural catheters may be complicated by a prominent dorsomedian connective tissue band (epidural septum or fat pad), which is found in some patients.

Another means of facilitating the success of epidural anesthesia is to allow the block enough “soak time” before beginning the surgical procedure.

There appears to be a plateau effect in the doses of epidural local anesthetics; that is, once a certain quantity of local anesthetic has been injected, more of the same agent does not significantly increase the block height but rather may make the block denser, perhaps improving quality.

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