Regional Anesthesia
Epidural/Spinal Anesthesia
PHARMACOLOGY OF EPIDURAL/SPINAL ANESTHESIA
1. How are hyperbaric solutions of local anesthetics prepared? Where does the solution go upon injection or upon attaining the supine position? Which procedures most commonly use hyperbaric solutions?
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1. Hyperbaric solutions of local anesthetics (LAs) are prepared by adding glucose to increase the density to >1.0008. The hyperbaric solutions gravitate to dependent areas; in the supine position, they gravitate to the thoracic kyphosis at T6. Hyperbaric solutions are often used for the following:
Obstetrics: cesarean section and spontaneous vaginal delivery (SVD);
Herniorrhaphy;
Any intra-abdominal surgery;
Groin incision (radical orchiectomy);
Gynecologic surgery requiring T10 dermatomal level of anesthesia: cerclage, dilation, and curettage, or cone biopsy.
Solutions include 0.75% bupivacaine with 8.25% glucose, 5% lidocaine (Xylocaine) with 7.5% glucose, and 0.5% tetracaine (Pontocaine) with 5% glucose.
2. How are hypobaric solutions of local anesthetics prepared? Where does the solution go upon spinal injection?
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2. Hypobaric solutions are prepared by diluting the LA with distilled water to a density <0.9998. These solutions “float” to the least dependent areas.
3. For what procedures is an isobaric solution used? Name two isobaric solutions.
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3. Isobaric solutions are used for the following:
Lower-limb amputations or orthopedic or vascular procedures,
Genitourinary surgery,
Perineal surgery,
Rectal surgery.
Examples of isobaric solutions include 2% lidocaine and 0.5% bupivacaine, both of which are epidural solutions.
4. What are the two theories regarding the mechanism for prolonged duration of action by adding vasoconstrictors to spinal anesthesia? Name two vasoconstrictors often added to local anesthetics to prolong their duration of action. Which has been shown to be more effective?
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4. Vasoconstrictors prolong spinal anesthesia by the following:
Vasoconstriction,
Direct antinociceptive effects in the spinal cord.
Vasoconstriction of blood vessels supplying the dura and spinal cord decreases blood flow, leading to decreased vascular absorption of LA. More LA contacts the neural tissue for a longer period, causing more intense, prolonged blockade. Epinephrine is a more effective vasoconstrictor than phenylephrine.
5. Will vasoconstrictors prolong the duration of spinal anesthesia for bupivacaine (Marcaine), lidocaine (Xylocaine), and tetracaine (Pontocaine) in abdominal surgery?
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5. Vasoconstrictors are more efficacious in prolonging tetracaine blockade than they are with lidocaine or bupivacaine. In epidural anesthesia, epinephrine produces a more profound motor blockade of bupivacaine and etidocaine (Duranest).
6. What is the main side effect of prilocaine? At what dose do you see this effect?
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6. Prilocaine doses >600 mg are associated with methemoglobinemia. Etidocaine provides profound skeletal muscle relaxation with 3 to 4 hours of sensory analgesia.
7. What is the beneficial effect of etidocaine (Duranest) in surgery?
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7. Etidocaine provides profound skeletal muscle relaxation with 3 to 4 hours of sensory analgesia.
8. Why is ropivacaine (Naropin) preferred by some over bupivacaine? What is the initial epidural dosage of ropivacaine and that for continuous labor epidural infusion? What is the dose for a surgical procedure?
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8. Ropivacaine is an amide LA, metabolized by the liver. The onset, depth, and duration of sensory block are similar to those of bupivacaine, but it has fewer motor blockade properties. Ropivicaine is less potent than bupivicaine. Ropivacaine has been shown to cause significantly less depression of cardiac conductivity (less QRS widening) than bupivacaine. For labor and delivery, the initial epidural dosage is 0.2% ropivacaine, 10 to 20 mL, given in incremental doses of 3 to 5 mL. Continuous infusion is 6 to 14 mL/h of 0.2%. For surgery, 0.5% ropivacaine, 20 to 30 mL, is recommended.
9. Which factors determine the quality of an epidural blockade?
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9. Factors influencing the quality of epidural blockade are as follows:
LA employed;
Dose, volume, and concentration of LA;
Addition of vasoconstrictor;
Site and speed of injection;
Patient position;
Patient age, height, and clinical status.
10. How do age and pregnancy affect the spread of epidural anesthesia?
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10. Age and pregnancy enhance epidural spread of LA:
The effect of age results from the compliance of the epidural space, and That of pregnancy results from the following:
Distention of epidural venous plexus (inferior vena cava compression),
Hormonal factors that increase sensitivity to LAs.
COMPLICATIONS OF SPINAL/EPIDURAL ANESTHESIA
1. Name some of the minor complications and some of the major complications associated with spinal/epidural anesthesia.
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1. Minor complications of regional anesthesia include the following:
Hypotension,
High spinal,
Postdural puncture headache (PDPH),
Back pain.
Major complications involve neurologic injuries such as the following:
Isolated nerve injury,
Meningitis,
Cauda equina syndrome.
2. What should be the accepted limit for hypotension before treatment in pregnancy? Why? How can you prevent or minimize hypotension? Which are the drugs of choice for treating hypotension and why?
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2. With a labor epidural, it is probably best not to allow mean arterial pressure to drop by >20%. Maintaining maternal blood pressure (systolic blood pressure [SBP]) >100 mm Hg ensures placental perfusion. Hypotension can be minimized by administering a fluid bolus and replacing fluid deficits, as well as by optimizing patient position to ensure adequate venous return. Patients can be placed in a slight Trendelenburg (5 to 10 degrees) with a left lateral tilt (10 to 20 degrees) to improve venous return without greatly exaggerating cephalad spread of the spinal anesthesia.
Vasopressors that constrict veins more than arteries are better for correcting hypotension from spinal anesthesia. Examples are ephedrine, phenylephrine, and mephentermine sulfate.
3. What is believed to be the cause of postdural puncture headache (PDPH)? Which factors influence the frequency of PDPH? What characterizes a PDPH? What is the treatment?
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3. PDPH is believed to be caused by decreased cerebrospinal fluid (CSF) pressure resulting from leakage of CSF fluid through the opening in the dural sheath created by the lumbar puncture (LP) needle. The incidence is related to age, sex, pregnancy, and size, angle, and direction of the needle. The headache must have a postural component and is usually frontal or occipital with associated photophobia and tinnitus. Treatment is bed rest, analgesics, and hydration. Caffeine, theophylline, sumatriptan, and adrenocorticotrophic hormone have been suggested but the results have been disappointing. Blood patch is the most effective option. Blood patch is more successful after 4 days of conservative treatment but should be offered earlier if headache is significant.
4. What are the symptoms of a high spinal? What is the treatment?
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4. Symptoms of a high spinal are as follows:
Agitation,
Nausea,
Hypotension.
Treatment is oxygen (O2), restoring blood pressure (BP), reassurance, and intubation for inadequate ventilation.
5. What is the treatment for backache postspinal?
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5. Epidural hematoma and cauda equina syndrome should be excluded based on the absence of clinical neurologic signs. Backache should be treated with reassurance, rest, heat, and analgesics.
6. What is the treatment for nausea associated with a spinal?
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