Spinal Anesthesia: What is it? Vertebral Anatomy, Patient Position, Procedure, Administration and Side Effects

It involves the administration of local anesthetic in the subarachnoid space.

The spinal canal extends from the foramen magnum to the sacral hiatus.

The limits of the bony canal are the anterior vertebral body, the lateral pedicles, the spinous processes and the laminae posteriorly.

Vertebral anatomy

Three interlaminar ligaments unite the vertebral processes:

  1. The supraspinatus ligament superficially connects to the apices of the spinous processes.
  2. The interspinous ligament connects the spinous processes on its horizontal surface.
  3. The flavum ligament connects the caudal border of the vertebrae above to the cephalic border of the lamina below. This ligament is composed of elastic fibers and is generally recognized by its greater resistance to the passage of a needle.

The spinal cord extends along the vertebral canal during fetal life, ends around L3 at birth, and progressively moves cephalad to reach the adult position near L1 by 2 years of age.

The nerve roots of the medullary, lumbar, sacral, and coccygeal cone branch distally to form the cauda equina.

Spinal needles are placed in this area of ​​the canal (below L2) because the mobility of the nerves reduces the risk of trauma from the needle.

The spinal cord is made up of three meninges:

  1. The pia mater.
  2. The arachnoid, which is located between the pia and the dura.
  3. The dura mater, which is a tough fibrous sheath that runs longitudinally along the spinal cord and is caudally attached to S2.

The subarachnoid space lies between the pia mater and the arachnoid mater and extends from the junction of the dura mater at S2 to the superior cerebral ventricles .

The space contains the spinal cord, nerves, cerebrospinal fluid, and blood vessels that supply the spinal cord. Cerebrospinal fluid is a clear colorless fluid that fills the subarachnoid space.

The total volume of cerebrospinal fluid is 100 to 150 ml, while the volume in the spinal subarachnoid space is 25 to 35 ml.

Cerebrospinal fluid is continuously formed at a rate of 450 ml per day by secretion or ultrafiltration of plasma from the choroidal arterial plexuses located in the lateral, third, and fourth ventricles.

Cerebrospinal fluid is reabsorbed into the bloodstream through the arachnoid villi and granulations that protrude through the dura until it is in contact with the endothelium of the cerebral venous sinuses .

Patient position

The lateral decubitus, prone, and sitting positions can be used for the administration of spinal anesthesia.

  • In the lateral position, the patient is positioned with the affected side up if a hypobaric or isobaric technique is to be used and with the affected side down if a hyperbaric technique is to be used. The spine is horizontal and parallel to the edge of the table.
  • The sitting position is useful for the lower spinal blocks required in certain gynecological and urological procedures, and is commonly used in obese patients to aid in midline identification. It is often used in conjunction with hyperbaric anesthetics.
  • The prone or ventral ulna position is used in conjunction with hypobaric or isobaric anesthetics for rectal, perineal, and anus procedures. A prone position can be used for the administration of spinal anesthesia and subsequent surgery.

Process

Newer needles have a pencil point design with a side opening. These needles can reduce the incidence of postdural headache, compared to traditional ‘sharp point’ needles when dividing, rather than cutting dural fibers during insertion.

Twenty-four and twenty-five gauge needles bend easily and are often inserted through a 19 gauge introducer needle. The 22 gauge needle is stiffer and steers easily when inserted.

It may be useful in older patients in whom access may be more difficult and the incidence of postdural headache is low.

The following should be taken into account:

  • The patient should be monitored with standard monitors including electrocardiogram, blood pressure, and oxygen saturation.
  • Intermediate spaces L2-3, L3-4, or L4-5 are commonly used for spinal anesthesia.
  • A large area of ​​the skin is disinfected with an appropriate antiseptic solution. Care must be taken to avoid contamination of the spinal kit with an antiseptic solution as the solution is potentially neurotoxic.
  • The stylet is checked for proper fit inside the needle.

The stylet should always be kept in place when advancing the needle so that the lumen of the needle does not block the tissue. If paresthesia occurs during placement, the needle should be withdrawn immediately.

As the needle advances past this ligament, there will be a sudden loss of resistance as the needle “jumps” through the dura.

The stylet is removed and correct placement is confirmed by observing the free flow of cerebrospinal fluid in the center of the needle.

The needle is rotated 90 ° if necessary to confirm or restore good flow of cerebrospinal fluid.

Administration of anesthesia

The syringe containing the predetermined dose of local anesthetic is attached to the needle, the cerebrospinal fluid is gently aspirated into the syringe, producing birefringence within the dextrose-containing solutions and confirming free flow. The medicine is injected slowly.

Repeating the aspiration of cerebrospinal fluid at the end of the injection confirms that the tip of the needle is still within the subarachnoid space. The needle is withdrawn and the patient is gently positioned in the desired position.

Monitor blood pressure, pulse, and respiratory function closely (every 60 to 90 seconds) for 10 to 15 minutes and determine rising anesthetic level by observing response to a light prick or alcohol swab.

Stabilization of the local anesthetic level takes approximately 20 minutes.

Continuous spinal anesthesia allows repeated injection of small aliquots of medication to produce the desired level of sensory block.

With this technique, a high or rapid sympathetic block can be avoided (of particular concern in the case of a compromised patient). A 20-gauge catheter is inserted through a 17-gauge epidural needle.

The catheter is advanced 2 to 4 cm into the subarachnoid space. Stimulation of the nerve roots during insertion of the catheter requires repositioning of the catheter.

The spinal layering technique is frequently used in orthopedics to prolong the duration of anesthesia by administering a dose of the drug through the spinal needle, waiting several minutes, and then adding an additional drug.

Physiological changes caused by spinal anesthesia

Bloqueo neuronal

The smaller C fibers that carry autonomous impulses are more easily blocked than the larger sensory and motor fibers.

As a result, the level of autonomic block extends above the level of sensory block in two to six segments. This phenomenon is called differential block .

Similarly, sensation-transmitting fibers are more easily blocked than larger motor fibers, so sensory block will extend above the level of motor block.

Cardiovascular

The hypotension is directly proportional to the degree of sympathetic blockade produced. Sympathetic blockage results in dilation of the arteries and venous capacitance vessels, leading to decreased systemic vascular resistance and decreased venous return.

If the block is below T4, increased activity of the baroreceptors leads to increased activity of cardiac sympathetic fibers and vasoconstriction of the upper extremities.

Blockage above T4 disrupts cardiac sympathetic fibers, leading to bradycardia, decreased cardiac output, and a further decrease in blood pressure.

These changes are more marked in patients who are hypovolemic, elderly, or have venous return obstruction.

Risk factors for bradycardia after spinal anesthesia include baseline bradycardia, fitness level 1, use of beta-blockers, age less than 50, prolonged PR interval, and sensory level above T6.

Respiratory

Low spinal anesthesia has no effect on ventilation. With the ascending height of the block in the thoracic area, there is a progressive paralysis of the ascending intercostal muscle.

This has little effect on ventilation in the supine surgical patient with intact phrenic nerve mediated diaphragmatic function.

However, ventilation in patients with poor respiratory reserve, such as morbid obesity , can be profoundly affected.

Paralysis of the intercostal and abdominal muscles decreases the effectiveness of coughing, which can be important in patients with chronic obstructive pulmonary disease.

Generally, a spinal level of T4 does not result in impaired ventilation, but respiratory compromise can occur in patients with limited respiratory reserve or higher spinal levels.

Visceral effects

  • Bladder: Sacral blockage (S2 to S4) produces an atonic bladder that can hold large volumes of urine. Blockage of afferent and efferent sympathetic innervation to the sphincter and detrussor muscle produces urinary retention.
  • Intestine: the sympathetic block (T5 to L1) produced by spinal anesthesia leads to the contraction of the small and large intestine due to a predominance of the parasympathetic tone.
  • Neuroendocrine: Epidural T5 blockade inhibits part of the neural component of the stress response by blocking sympathetic afferents to the adrenal medulla and blocking the sympathetic and somatic pathways that mediate pain.

Other components of the stress response and central release of humoral factors are not affected.

The afferent vagal fibers of the upper abdominal viscera are not blocked and can stimulate the release of hypothalamic and pituitary hormones, such as antidiuretic hormone and adrenocorticotropic hormone.

Glucose tolerance and insulin release are normal.

Thermoregulation

The hypothermia can occur due to several mechanisms.

The main cause is the redistribution of central heat to the periphery secondary to vasodilation, which makes forced heating of the air particularly effective in raising the patient’s temperature.

The core temperature may decrease even though the surface temperature is maintained and patients may feel warm despite a decrease in temperature.

Thermoregulation is impaired due to loss of vasoconstriction to preserve heat below the level of sympathectomy. The chill is common.

Central nervous system effects

Spinal anesthesia may have direct effects to suppress consciousness, probably secondary to decreased afferent stimulation of the reticular activation system.

During spinal or epidural anesthesia, the requirements for sedative agents may decrease.

Determinants of the level of spinal block

Baricity of the local anesthetic solution: Local anesthetic solutions can be described as hyperbaric, hypobaric, or isobaric in relation to the specific gravity of the cerebrospinal fluid (1004-1007 g / ml).

Drug dose: the anesthetic level varies directly with the dose of the agent used.

Drug volume: The greater the volume of the drug injected, the more the drug will spread within the cerebrospinal fluid. This is especially applicable to hyperbaric solutions.

Cerebrospinal fluid turbulence : Turbulence created within the cerebrospinal fluid during or after injection will increase the spread of the drug and the level obtained.

Turbulence is created by rapid injection, slippage (the repeated aspiration and reinjection of small amounts of cerebrospinal fluid mixed with drugs), coughing, and excessive movement of the patient.

Cerebrospinal fluid volume : The volume of the lumbosacral cerebrospinal fluid is inversely correlated with the degree of spread of the local anesthetic. There are no good predictors of lumbosacral cerebrospinal fluid volume, but weight has some correlation.

Increased intra-abdominal pressure: Pregnancy, obesity, ascites, and abdominal tumors increase the pressure within the inferior vena cava.

This pressure increases the volume of blood within the epidural venous plexus, concomitantly reducing the volume of cerebrospinal fluid within the spinal column, allowing greater dissemination of the injected local anesthetic. In obese patients, this effect is enhanced by the increase in fat in the epidural space.

Spinal curvatures: Lumbar lordosis and thoracic kyphosis influence the spread of hyperbaric solutions. Drug injected above the L3 level while the patient is in the lateral position will extend cephalad and be limited by the thoracic curvature at T4.

Side effects

A nerve injury is rare, but it can be a serious problem.

Several types of nerve injuries can occur:

Direct nerve injury related to needle or catheter placement: Pain during catheter insertion or drug injection is a warning sign of potential nerve injury as a result of needle or catheter placement and requires repositioning the needle or catheter.

Transient paresthesias, which can occur during neuraxial block placement, generally resolve immediately and do not have long-term sequelae.

Transient neurological syndrome: it is a spontaneous severe radicular pain that is evident after the resolution of the spinal anesthetic and can last from 2 to 7 days. Symptoms include buttock and thigh pain.

Transient neurological syndrome generally responds to conservative measures, such as non-steroidal anti-inflammatory drugs and warm compresses.

The incidence is higher with lidocaine administration, but it has also been observed with tetracaine, bupivacaine, and mepivacaine. Obesity, outpatient surgery, arthroscopic knee surgery, and lithotomy position are additional risk factors.

Back pain : There is an incidence of back pain after spinal anesthesia that may be related to the relaxation of the ligaments that occurs with anesthesia.

There is a similar incidence of back pain after general anesthesia, also related to the effects of anesthetic agents and muscle relaxants on back structures.

Bleeding: Puncture of an epidural vein during needle insertion can result in blood, or a mixture of blood and cerebrospinal fluid, emerging from the spinal needle. If the fluid is not removed quickly, the needle must be withdrawn and reinserted.

Spinal hematoma: In a surgical emergency the overall incidence is around 1 in 150,000. Signs and symptoms of severe back pain and persistent neurological deficit usually present within 48 hours. The risk is higher among patients who present with coagulopathies or are anticoagulated.

In these cases, close postoperative follow-up to detect signs compatible with hematoma is justified. Diagnosis is usually made with MRI imaging, and treatment is through evacuation of emergent hematoma.

Because catheter removal and needle placement can cause spinal hematoma, anesthesiologists must monitor the patient’s coagulation status and use of anticoagulants not only at the time of needle placement but also at the time of catheter removal.

Dural puncture headache: usually develops in 3 days; 70% of headaches resolve in 7 days and 90% in 6 months. The classic “spinal headache” is frontal and occipital in distribution; less often, the temporal area is affected.

The headache is aggravated by upright posture and is relieved by lying down. Other manifestations include visual disturbances or hearing impairment. Young patients and female gender are risk factors. The incidence can be reduced by using smaller needles and non-cutting needles (eg, pencil point needles).

Initial treatments for symptoms include rehydration, maintenance of the supine position, pain relievers including opioids, and caffeine. Caffeine exerts its effect by vasoconstriction of the cerebral vessels.

Hypotension: The incidence of hypotension can be reduced by administering lactate Ringer’s solution before blocking.

Patients with decreased cardiac function require care in the administration of large volumes of fluid, because the translocation of fluid from the peripheral to the central circulation during the recession of the block and the return of systemic vascular tone could lead to volume overload and pulmonary edema.

Treatment of hypotension includes increasing venous return and treating severe bradycardia. The Trendelenburg position, administration of fluids, elevation of the lower extremities for autotransfusion of blood, or the use of vasopressors may be necessary.

Bradycardia: Bradycardia can be treated with atropine or glycopyrrolate. If bradycardia is severe and is accompanied by hypotension , ephedrine or epinephrine can be used.

Dyspnea: is a common complaint with high spinal levels. It is caused by proprioceptive blockage of the afferent fibers of the abdominal muscles and the chest wall. Reassuring the patient may be all that is needed, although adequate ventilation must be ensured.

Apnea: can be caused by reduced spinal blood flow that accompanies severe hypotension or by direct blockade of C3 to C5, which inhibits the production of the phrenic nerve. Immediate ventilatory support is required.

Urinary retention: Urinary retention can last longer than sensory and motor block. A urinary catheter should be placed if anesthesia or analgesia is maintained for a long period of time.

Nausea and vomiting: These are usually caused by hypotension or unopposed vagal stimulation. Treatment consists of restoring blood pressure, administering oxygen and atropine.

Infection: After spinal anesthesia, an infection is extremely rare. However, meningitis, arachnoiditis, and epidural abscess can occur. Possible etiologies include chemical contamination and viral or bacterial infection. Consultation and prompt diagnosis and treatment are essential.

Pruritis: occurs commonly with the use of neuraxial opioids and is more common with intrathecal compared to epidural administration. The exact mechanism is unclear. Drug treatments include nalbuphine, naloxone, naltrexone, diphenhydramine, ondansetron, and propofol.

Chills: have a high incidence and can be treated with meperidine. Clonidine has been shown to have similar efficacy.