65: Spinal Anesthesia

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CHAPTER 65 Spinal Anesthesia

James Duke, MD, MBA

1 What are the advantages of spinal anesthesia over general anesthesia?

image The metabolic stress response to surgery and anesthesia is reduced by subarachnoid block (SAB).
image Particularly in elective hip surgery, there may be up to a 20% to 30% reduction in blood loss.
image SAB decreases the incidence of venous thromboembolic complications by as much as 50%.
image Pulmonary compromise appears to be less.
image Endotracheal intubation is avoided.
image Mental status can be followed.

2 What are the usual doses of common local anesthetics used in spinal anesthesia and the duration of effect?

See Table 65-1.

TABLE 65-1 Local Anesthetic Dosing for Spinal Anesthesia

image

3 Where are the principal sites of effect of spinal local anesthetics?

The principal sites are the spinal nerve roots and spinal cord. Interestingly nerve roots may have different anatomic configurations (rootlets vs. roots); and there may be variability in fascial compartmentalization of the roots, especially between the dorsal and ventral nerve roots, accounting in part for differences between motor and sensory blockade.

4 What factors determine the termination of effect?

Resorption of the agent from the cerebrospinal fluid (CSF) into the systemic circulation limits duration. Addition of a vasoconstrictor that slows resorption prolongs duration of effect. Vasoconstrictor efficacy decreases with local anesthetics with intrinsic longer durations of effect.

5 Describe the factors involved in distribution (and extent) of conduction blockade

image Patient characteristics include height, position, intra-abdominal pressure, anatomic configuration of the spinal canal, and pregnancy. There is great interindividual variation in lumbosacral CSF volumes; magnetic resonance imaging has shown volumes ranging from 28 to 81 ml. Lumbar CSF volumes correlate well with the height and regression of the block. With the exception of an inverse relation with weight, no external physical measurement reliably estimates lumbar CSF volumes. CSF volumes are also reduced in pregnancy.
image Needle direction and site when anesthetic is injected are important.
image The total injected dose of local anesthetic is important, whereas the volume or concentration of injectant is unimportant.
image The baricity of the local anesthetic solution is important. Baricity is defined by the ratio of the density of the local anesthetic solution to the density of CSF. A solution with a ratio >1 is hyperbaric and tends to sink with gravity within the CSF. An isobaric solution has a baricity of 1 and tends to remain in the immediate area of injection. A ratio <1 is a hypobaric solution, which rises in the CSF.

6 At what lumbar levels should a spinal anesthetic be administered? What structures are crossed when performing a spinal block?

The selected level should be below L1 in an adult and L3 in a child to avoid needle trauma to the spinal cord. As an anatomic landmark, the L3-L4 interspace is located at the line intersecting the top of the iliac crests. Either a midline or paramedian approach can be used. The anatomic layers passed through include skin, subcutaneous structures, supraspinous ligament, interspinous ligament, ligamentum flavum, dura mater, and arachnoid membrane.

7 What are the most common complications of spinal anesthesia?

Common complications include hypotension, bradycardia, increased sensitivity to sedative medications, nausea and vomiting (possibly secondary to hypotension), postdural puncture headache (PDPH), and residual back pain and paresthesias (usually associated with the use of lidocaine. Less frequent but more ominous complications include nerve injury, cauda equina syndrome, meningitis, total spinal, and hematoma/abscess formation. Particular issues associated with these complications are discussed subsequently.

8 What are the physiologic changes and risk factors found with subarachnoid block–associated hypotension?

Hypotension occurs as a result of a loss of sympathetically mediated peripheral vascular resistance. Arterial and central venous pressure decrease with only mild decreases in heart rate, stroke volume, and cardiac output. Hypovolemia, age greater than 40 years, sensory level greater than T5, baseline systolic blood pressure below 120 mm Hg, and performance of the block at or above L3-L4 increase the incidence of hypotension. Hypotension (and possibly decreased cerebral blood flow) may be responsible for nausea and vomiting observed with SAB. Patients should receive a bolus of crystalloid or colloid (250 to 1000 ml) before SAB. Because of the pattern of distribution, colloid is more effective, although more expensive. Volume expansion and intravenous sympathomimetics usually reverse hypotension should it occur. Trendelenburg position may raise the level of blockade and should be used with caution and vigilance. Volume loading should also be used cautiously in patients with limited cardiac reserve. In these patients, as the block recedes, vascular tone increases, raising the central blood volume, which may precipitate heart failure. Strategies to create a unilateral block may also decrease the hypotension associated with SAB.

9 What are the etiology and risk factors for subarachnoid block–associated bradycardia?

Bradycardia may occur secondary to unopposed vagal tone from a high sympathectomy, blockade of the cardioaccelerator fibers (T1-T4), and the Bezold-Jarisch reflex (slowing of the heart rate secondary to a decrease in venous return). Patients with underlying increased vagal tone (children and adults with resting heart rates <60) are at increased risk. Bradycardia may be treated with anticholinergic agents (atropine) or β-adrenergic agonists such as ephedrine.

10 Why are patients who have received spinal anesthetics especially sensitive to sedative medications? What is deafferentation?

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