Mechanism of action and indications
Spinal anesthesia is used for surgeries on organs located below the diaphragm: the stomach, intestines, liver, biliary tract, spleen, pelvic organs, and lower extremities.
The clinical effect is achieved through the sequential blockade of three types of neural structures:
- Posterior roots (sensory fibers): leads to the complete loss of pain, temperature, and tactile sensitivity.
- Anterior roots (motor fibers): causes motor paralysis and deep myorelaxation.
- Preganglionic sympathetic fibers (running within the anterior roots): their blockade alters vascular innervation, specifically dilating arterioles in the innervation zone of the splanchnic nerves.
Preparation and puncture technique
The procedure requires syringes graduated in tenths of a milliliter with tightly fitted plungers, as well as special needles with stylets. The puncture site is treated with diethyl ether and alcohol, and the skin is infiltrated with a 0.25% procaine solution.
The main goal of correct patient positioning is maximum flexion of the spine, which widens the intervertebral spaces. The patient either sits across the table with knees raised and shoulders lowered, or lies on their side, drawing the knees up to the abdomen and tucking the chin to the chest.
Steps of the procedure:
- The needle is inserted strictly along the midline between the spinous processes of $L_{III}$ and $L_{IV}$ (or $L_{II}$ and $L_{III}$) with a slight downward tilt (5–10°).
- The physician overcomes the resistance of the supraspinous, interspinous, and ligamenta flava. A "give" or "pop" is felt upon exiting the ligaments.
- The next slight resistance is the puncture of the dura mater.
- Needle advancement is stopped, and the stylet is removed. If clear cerebrospinal fluid appears, the procedure is performed correctly. If no fluid is present, the needle can be slightly rotated or advanced by 1–2 mm.
Anesthetic baricity and positioning
After a successful puncture, 2–3 ml of cerebrospinal fluid is drawn into the syringe, mixed with the anesthetic, and injected into the spinal canal. The subsequent patient position depends on the density (baricity) of the solution to prevent its spread toward the medulla oblongata.
- Hypobaric solutions (have lower density): 2% lidocaine solution, 0.5% bupivacaine solution. They "float" upward in the cerebrospinal fluid. The head of the table must be lowered by 15°.
- Hyperbaric solutions (have higher density): 5% procaine solution, 0.75% bupivacaine solution in dextrose. They spread downward ("sink"). In this case, the head of the table is elevated.
Complications and their prevention
The most common complication is arterial hypotension. Due to the blockade of sympathetic fibers, the blood vessels of the abdominal cavity, pelvis, and legs dilate, blood pools in these areas, and blood pressure drops. For prevention, vasoconstrictors are administered before surgery, and treatment involves vasopressors with plasma expanders and leg wrapping (centralization of blood circulation).
If the anesthetic spreads upward in the subarachnoid space, paralysis of the intercostal muscles occurs. This causes respiratory disturbances up to respiratory arrest, requiring oxygen therapy or mechanical ventilation.
Overdose (intoxication) occurs if a large amount of the drug accidentally enters the bloodstream. Symptoms: skin flushing, anxiety, tachycardia, convulsions, and in severe cases, coma and collapse. Mild forms are managed with barbiturates and oxygen; severe cases require mechanical ventilation, fluid infusion, and cardiac massage.
Late complications include headache, motor paresis, and purulent meningitis (in case of aseptic technique breach).
Contraindications
Spinal anesthesia is absolutely contraindicated in:
- Traumatic shock.
- Severe intoxication associated with peritonitis and arterial hypotension.
- Inflammatory skin diseases at the intended puncture site.
- Spinal deformities.
Briefly: answers to topical questions
Which meningeal layers does the needle pierce during a lumbar puncture?
During a lumbar puncture, the needle pierces the dura mater (dura mater). During the procedure, after passing through the ligamentous apparatus (interspinous, supraspinous, and yellow ligaments), tissue resistance disappears. Then, the dura mater is punctured, which is accompanied by the sensation of another slight resistance or a characteristic "pop". After passing this layer, the needle enters directly into the subarachnoid space, which is reliably indicated by the appearance of clear cerebrospinal fluid.
What types of needles are used for spinal anesthesia?
Special needles with a closely fitted stylet are used to perform spinal anesthesia.
At what spinal level does the spinal cord end in adults?
In adults, the spinal cord ends at the level of the first or second lumbar vertebra. The lower boundary is variable and depends on body build and spinal anatomy; it roughly corresponds to the intervertebral disc between L1 and L2. Below this level, the substance of the spinal cord is absent; the filum terminale continues from the conus medullaris, and the roots of the lumbar, sacral, and coccygeal nerves form the cauda equina.
Remembering solution baricity is simple: HYPObaric solutions are light, like balloons, and float UP (the head of the table must be lowered). HYPERbaric solutions are heavy, like stones, and fall DOWN (the head of the table must be raised).
Class and exam questions
What to do if blood appears when removing the stylet from the needle?
In such a situation, the needle must be removed. The procedure should be repeated in a different intervertebral space.
Why does motor paralysis occur during spinal anesthesia?
Myorelaxation occurs because the local anesthetic blocks the anterior (motor) roots of the spinal cord.
How does an allergic reaction to an anesthetic manifest and how is it treated?
Manifestations depend on individual sensitivity: skin rash, itching, angioedema, as well as laryngospasm and bronchospasm are possible. Antihistamines, glucocorticoids, and antispasmodics are used for treatment.
What else to study on this topic
- Pathogenesis of blood circulation decentralization during vasodilation
- Specifics of using hyperbaric and hypobaric solutions
- Mechanism of respiratory failure development during intercostal muscle blockade
- Clinical presentation of local anesthetic overdose
- Preoperative history taking for complication prevention
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