Development of regional anesthesia started with the isolation of local anesthetics, the first being cocaine (the only naturally occurring local anesthetic). The first regional anesthetic technique performed was spinal anesthesia, and the first operation under spinal anesthesia was in 1898 in Germany by August Bier. Before this, the only local anesthetic techniques were topical anesthesia of the eye and infiltration anesthesia.
The central nervous system (CNS) comprises the brain and spinal cord. The term neuraxial anesthesia refers to the placement of local anesthetic in or around the CNS. Spinal anesthesia is a neuraxial anesthesia technique in which local anesthetic is placed directly in the intrathecal space (subarachnoid space). The subarachnoid space houses sterile cerebrospinal fluid (CSF), the clear fluid that bathes the brain and spinal cord. There are roughly about 130 to 140 mL of CSF in an adult human which continually cycles throughout the day. Approximately 500 mL of CSF gets produced daily.
Other neuraxial techniques include epidural and caudal anesthesia, each having its particular indications. Spinal anesthesia is only performed in the lumbar spine for reasons which will be discussed later in this article, used for surgical procedures involving the lower abdomen, pelvis and lower extremities.
Administration of spinal anesthesia requires appropriate positioning and understanding of neuraxial anatomy. The goal is to deliver appropriately dosed anesthetic into the intrathecal (subarachnoid) space.
The spine comprises seven cervical, 12 thoracics, five lumbar, and five fused sacral vertebral bones. The different vertebral bones earn their names based on their relative positions and structural differences. The vertebrae are stacked end to end with articulating joints and ligaments, and a hollow space running through them called the spinal canal. This canal houses the spinal cord. The spinal nerves exit the spinal canal via lateral spaces formed between pedicles from adjacent vertebrae.
As mentioned earlier, spinal anesthesia is only performed in the lumbar area, specifically the mid to low lumbar levels to avoid damage to the spinal cord and also to prevent intrathecally-injected medications from having any activity in the upper thoracic and cervical regions. The caudal end of the spinal cord is the conus medullaris and usually is at the lower border of the first or sometimes the second lumbar vertebral body. In pediatric patients, it is a little more inferior, generally ending around L3. In the adult population, the mean conus position is the lower third of L1 (range: the middle third of T12 down to the upper third of L3). The variation in conus positions follows a normal distribution. No significant difference in conus position is seen between male and female patients or with increasing age. The dural sac usually extends to S2/3. For these reasons, insertion of the spinal needle for spinal anesthesia is usually at the L3/4 or L4/5 interspace. Spinal cord trauma is more likely when choosing higher interspaces, especially in obese patients.
On entry, and starting at the skin, the needle traverses a number of structures. The structures traversed depend on the approach (see technique).
Understanding dermatomal anatomy is imperative for an understanding of the level of blockade of target structures. For example, for lower abdominal cesarean sections, the incision is usually made below the T10 dermatome. However, coverage of up to T4 dermatome is required to prevent discomfort or pain from peritoneal tugging; this is especially evident with uterine manipulation. Patients complain of "pulling on their inside." some corresponding dermatomal landmarks are:
Neuraxial anesthesia is used as a sole anesthetic or in combination with general anesthesia for most procedures below the neck. As mentioned in the introduction, spinal anesthesia is in common use for surgical procedures involving the lower abdomen, pelvis, perineal and lower extremities; it is beneficial for procedures below the umbilicus.
There needs to be patient counseling regarding the procedure, and signed informed consent is necessary. Since the procedure is usually performed on awake or slightly sedated patients, the indication for spinal anesthesia and what to expect during placement of neuraxial, risks, benefits, and alternative procedures are some of the discussions that can help allay anxiety. It is crucial to let patient understand that they will have little or no ability to move their lower extremities until the resolution of the block.
Spinal anesthesia is best for short procedures. For more extended procedures or procedures that would compromise respiration, general anesthesia is usually preferable.
There are major known contraindications to neuraxial anesthesia (spinal and epidural). The absolute contraindications are lack of consent from the patient, elevated intracranial pressure (ICP), primarily due to intracranial mass and infection at the site of procedure (risk of meningitis).
Other relative contraindications are severe mitral and aortic stenosis and left ventricular outflow obstruction as seen with hypertrophic obstructive cardiomyopathy.
In the setting of coagulopathy, placement of neuraxial block requires re-evaluation. The American Society of Regional Anesthesia (ASRA) publishes updated guidelines that detail timing for neuraxial anesthesia for patients on oral anticoagulants, antiplatelets, thrombolytic therapy, unfractionated and low molecular weight heparin. Review the latest guidelines before proceeding with the procedure.
Overall because these are elective procedures, it is imperative to undergo a risk/benefit analysis before proceeding.
Since the performance of neuraxial procedures is under aseptic technique, the clinician is expected to maintain a sterile environment. Cap, masks, hand wash, sterile gloves are required. For a successful procedure, adequate preparation is requisite.
There should be adequate equipment count and ample space to accommodate patient and personnel. Monitors to assess the patient's circulation (blood pressure, continuous EKG), oxygenation (continuous pulse oximetry), and temperature should be set up and ready. The clinician performing process must be proficient in using and interpreting monitors. If planning sedation, means to assist patient ventilation, oxygenation, and circulatory support should be in place. Intravenous access should be established before starting. A certified anesthesiologist should be present in the event that the patient will require general anesthesia.
There are commercially available spinal anesthesia kits. Contents of kits usually include chlorhexidine with alcohol, drape and local infiltrating anesthetic (usually 1% lidocaine). Other contents include the spinal needle (Quincke, Whitacre, Sprotte, or Greene), 3 ml and 5 ml syringes, and preservative-free spinal anesthetic solution. Solutions may range from lidocaine, ropivacaine, bupivacaine, procaine or tetracaine.
Spinal anesthesia should be only be performed by highly trained and certified medical personnel; this is typically by a board-certified anesthesiologist or anesthesiologist in training under the supervision of an anesthesiologist. Other fellowship-trained staff that performs spinal anesthesia are pain management physicians with training in physical medicine and rehabilitation (PM&R), neurology, emergency medicine.
Like in any other procedure, an assistant is always helpful. Since the patient's back will be facing the physician performing the procedure, a supporting staff member, usually a trained nurse, is typically present to assist with equipment. The supporting staff also helps in supporting the patient from the ventral side to help them maintain their posture and keep them safe in their position, especially if the patient has undergone sedation.
Before induction of neuraxial anesthesia, a thorough history and physical examination should take place. Pertinent in history is an understanding of previous exposure to anesthetic medication, review of allergies, family history of any anesthetic problems.
The physical exam generally focuses on the site of spinal anesthesia placement. The back should receive a full examination. A check for systemic or local skin infections, spine abnormalities (e.g., scoliosis, spinal stenosis, previous back surgery, spina bifida, history of tethered cord), the pre-procedural neurological exam for strength and sensation are also crucial for assessment and documentation.
A procedural time-out should be performed, confirming the patient's identity, planned procedure, allergy, check for consent and verbal statement of coagulation status.
Once the patient has has undergone approriate selection, the optimal patient position for the procedure must be estblished.
The procedure is usually carried out with the patient in the sitting or lateral decubitus position. Patient's comfort is tantamount. The goal of positioning is to help establish a straight path for needle insertion between the spinal vertebrae. The most commonly used position is the sitting position. This is because, in the lateral decubitus position, the spinal anatomy is usually not laterally symmetrical as it is in the sitting position.
With the patient positioned in the sitting position and leg hanging from the side of the bed, he/she should be encouraged to maintain a flexed spine position to help open up the interspace. The sitting position is appropriate for spinal anesthesia with a hyperbaric solution.
Either left or right lateral decubitus positions are viable options as well.
After the patient is in proper position, the access site is identified by palpation. This is usually very difficult to achieve with obese patients because of the amount of subcutaneous fat between the skin and spinous process. The space between 2 palpable spinous processes is usually the site of entry. The patient should wear a hat or cover for his/her hair to maintain asepsis.
Strict aseptic technique is always necessary, achievble with chlorhexidine antiseptics with alcohol content, adequate hand-washing, mask, and cap. Cleaning always starts from the chosen site of approach in circles and then away from the site. Allow time for the cleaning solution to dry. In the spinal kit, the drape placement is on the patient's back to isolate the area of access. Local anesthetic (usually about 1 ml 1% lidocaine) is used for skin infiltration, and a wheal is created at the site of access chosen, either midline or paramedian.
In the midline approach, the spinal approach to the intrathecal space is midline with a straight line shot. After infiltration with lidocaine, the spinal needle is introduced into the skin, angled slightly cephalad. The needle traverses the skin, followed by the subcutaneous fat. As the needle courses deeper, it will engage the supraspinous ligament and then the interspinous ligament; the practitioner will note this as an increase in tissue resistance. Next later will be the ligamentum flavum, and this would present like a "pop." On popping through this ligament, is the approach to the epidural space, which is the point of placement for epidurally-administered medications and catheters. This also presents the point where loss of resistance is felt to injection of saline or air. For spinal anesthesia, the clinician proceeds with needle insertion until penetration of the dura-subarachnoid membranes, which is signaled by free-flowing CSF. It is at this point that the administration of spinal medication takes place.
For the paramedian approach, the skin wheal from the local anesthetic is placed about 2 cm from midline, and the spinal needle advances at an angle toward the midline. In this approach, the supraspinous and interspinous ligaments are usually not encountered. Hence, there is little resistance encountered until reaching the ligamentum flavum
Appropriate patient selection and care should be established to help obviate common complications associated with neuraxial anesthesia. While many of the complications are of very low incidence, it’s worth being aware of them. Severe complications are believed to be extremely rare, but the frequency is probably underestimated. Some common ones are:
Neuraxial anesthesia offers many benefits that not available with general anesthesia. Neuraxial anesthesia has made it possible to perform many major procedures on an awake patient. For example, cesarian sections can be better and safely performed via neuraxial anesthesia than with general anesthesia, which allows the establishment of bonding between a mother and her neonate to take place immediately.
Neuraxial anesthesia has demonstrated itself a useful adjunct to general anesthesia. The use of thoracic epidurals as post-op pain modality in post-thoracotomy patients has helped to improve patient's respiratory status. Other beneficial effects are better pain control than intravenous narcotics, less need for systemic opioids, earlier recovery of bowel functions, and easier participation in physical therapy.
Neuraxial anesthesia is a beneficial anesthesia modality that has helped for a wide range of surgical procedures. It does come with risks of which healthcare providers need to be aware. Patient selection is critical and should be driven by careful history and physical examination. The indication for neuraxial anesthesia needs to match the surgical needs of the patient. After surgery, the post-op team needs to be aware of the procedure, and the patient needs to monitored by well-trained personnel.
Patient's hemodynamics require monitoring in the immediate post-op period until resolution of the anesthetic. Nurses and physicians from other field managing the patient need to be aware of the nature of anesthesia that patient underwent.
Upon discharge home, the patient should be able to be contacted by personnel who performed the procedure, and the patient should receive a list of possible complications. The patient should be asked about headaches, backache, and checked for any neurological deficits post-procedure and return of bowel and bladder function. The patient should be reassured and evaluated regarding any complication that may have resulted. A visit to the anesthesia clinic and/or appropriate specialist is the recommendation in the event of any complications.
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