Subarachnoid Block - Indications, Contraindications, Anatomy, Technique, and Complications.

Introduction:

The subarachnoid block (SAB) is an excellent procedure. Anesthetic drugs are administered to the CSF (cerebrospinal fluid) in the lower section of the spinal column to create numbness. During the process, the patient remains conscious. It is a simple block to learn for the beginner and a satisfactory procedure for the patient, surgeon, and anesthesiologist. As expertise is achieved, the beginner anesthetist quickly acquires confidence. Expect success rates of 95 % or higher. SAB offers ideal perioperative working practices for treatments performed below the umbilicus. SAB has two unique features. Firstly, it may be administered immediately and has a quick onset. Second, a complete blockage of the lower birth canal and perineum is guaranteed. The third benefit of SAB over other localized procedures is the lack of circulatory absorption and hence the absence of the possibility of developing systemic toxicity. SAB is beneficial when immediate instrumental delivery is necessary. It appears to be a feasible alternative to other regularly used procedures, such as pudendal block, epidural and caudal block, or general anesthetic. The block's simplicity masks the risks of the method. Ensure no mistakes while performing the subarachnoid block since it may be fatal in the hands of an untrained or incompetent anesthesiologist.

What Are the Indications for the Subarachnoid Block?

• SAB is appropriate for most surgical operations below the umbilicus (T10).

• Perineal surgeries, abdominal gynecological treatments, prostatectomy, cesarean deliveries, and hernia repairs are all standard procedures. In addition, SAB is used for the majority of hip and knee joint replacements.

• Patients prefer to remain awake, and minimal sedation is typically less stressful.

• SAB benefits older people, particularly those suffering from cardiovascular and pulmonary illness. These people are less likely to be confused after surgery.

• SAB is recommended for individuals with renal, hepatic, or metabolic disorders (for example- diabetes).

• SAB is much better for both mother and baby during delivery.

• In remote practices, SAB is a safer and less expensive method of anesthesia. Therefore, when a relatively inexperienced anesthesiologist is required to deliver anesthesia in an emergency, SAB is a better alternative than general anesthesia.

• In remote practices, SAB is a safer and less expensive method of anesthesia. Therefore, when a relatively inexperienced anesthesiologist is required to deliver anesthesia in an emergency, SAB is a better alternative than general anesthesia.

• SAB is an attractive alternative for providing emergency administration of anesthesia for lower limb surgery in an intoxicated patient with a full stomach.

What Are the Contraindications for the Subarachnoid Block?

1. Absolute Contraindications:

• Refusal of the patient.

• Infection at the lumbar puncture site.

• Any derangement in the clotting time and bleeding time.

• Patients under anticoagulants.

2. Relative Contraindications:

• Hypovolemia.

• Aortic valve stenosis.

• Any infection due to toxic bacteria.

• An increase in intracranial pressure.

3. It is important to note that no surgical or anesthetic technique is risk-free. Therefore, rational thinking and clinical judgment are essential when choosing a proper method for anesthesia.

4. Despite being more or less contraindicated, a SAB may be the optimal approach in case of emergency lower limb surgery in a hypovolemic patient with a chest injury.

What Is the Anatomy of the Subarachnoid Block?

• A long tubular space known as the vertebral canal, often referred to as the spinal canal, is created when vertebral foramina are positioned next to one another. The spinal cord is located in it.

• It originates where the medulla oblongata and foramen magnum meet at the level of the foramen magnum. It extends downward and ends at the first lumbar vertebra (L1) level in adults or the second lumbar vertebra (L2) in infants.

• At this point, it tapers to form the conus medullaris. Around 45 centimeters is how long the cord is. The dura, arachnoid, and pia mater are three of the meninges layers that enclose the spinal cord. The dura, arachnoid, and pia mater protect the spinal cord.

• The extradural or epidural space, which comprises areolar tissue, fat, lymphatics, arteries, and the internal vertebral venous plexus, exists as a possible, low-pressure gap between the dura and bony spinal canal. The subdural space is a potential space between the dura and the arachnoid mater.

• Below the cord, the dura produces a sac that terminates at the level of the second sacral segment (S2), then continues down each nerve root before joining the epineurium of the spinal nerves.

• The subarachnoid space is between the arachnoid and the pia mater and accommodates the cerebrospinal fluid. The spinal fluid serves as the site of injection of various anesthetic medications.

• The subarachnoid space spreads laterally to the dorsal root ganglia with the nerve roots. It terminates at the level of S2 in adults and is still lower in children.

• The Tuffier line, sometimes called an intercristal line and an imaginary plane connecting the iliac crests on both sides, is an important landmark when determining the correct vertebral space. It roughly designates the fourth lumbar (L4) spinous process.

How Is the Patient Prepared for SAB?

• Intravenous access is gained using a large-bore cannula (such as an 18G), and the patient is administered with the proper intravenous fluid. Standard monitors are connected, and the person relaxes into a seated position.

• The medical team should record the patient's electrocardiogram (ECG), heart rate (HR), blood pressure (BP), mean arterial pressure (MAP), and oxygen saturation before administering the drug.

• Throughout the intraoperative phase, it is crucial to continuously monitor the above mentioned parameters at regular intervals, frequently every five minutes.

• During the procedure, caution is required to maintain the emergency medications accessible. In addition, the patient must be given a vasopressor if the MAP goes beyond 65 mmHg or falls below 20 % of its baseline value.

What Is the Technique of SAB?

• The subarachnoid block causes anesthesia at or beneath the level of T4 because the thoracolumbar curve peaks and the local hyperbaric anesthetic disperses at the T4 level when a patient is lying horizontally with the face and torso facing up. As a type of low spinal anesthetic, saddle spinal block focuses on the sacral portions, preserving the sympathetic outflow and creating the least amount of hemodynamic disturbance.

• Determining the proper location to inject the drug is essential to achieving the optimum anesthetic dose and preventing damage to the cord's tail end.

• To prevent this, locate the spinous process at the level of the L4 lumbar vertebra somewhere at the intercristine line, which connects the iliac crests on either side.

• Next, locate the gap between the lumbar vertebrae L3 and L4. All spinal blocks should be performed at or caudal to this area to ensure that spinal injury is kept to a minimum. During the process, extreme caution is advised to maintain aseptic conditions.

• To achieve analgesia between the third and fourth lumbar vertebral vertebrae, a hypodermic needle is introduced into the skin in the midline. During the procedure, a tiny cutaneous wheal appears on the skin's surface.

• With the patient in a sitting posture, a spinal needle of 22 to 25 gauge is placed in the midline at the level of the abovementioned interspace with a 15-degree cephalad angulation.

• Advance the syringe penetrates through the subcutaneous tissue, usually presenting minimal resistance.

• The anesthetist will observe an increase in tissue resistance as it advances deeper into the supraspinous and interspinous ligaments.

• Continue to advance the needle until two "pops" are experienced. The first is due to ligament flavum penetration, which has its most significant thickness in the lumbar area, ranging from two to five centimeters. A tear in the dura-subarachnoid membrane causes the second pop.

• Remove the stylet and see spontaneous clear cerebrospinal fluid (CSF) flow via the needle to establish an accurate dural puncture. It may be essential to aspirate the CSF using a tiny gauge (25G) needle to determine its location in the subarachnoid area.

• Aspirate the CSF to confirm the needle has not displaced from its original position after attaching the syringe with the anesthetic medicine to the needle.

• Difficulty aspirating after securing the syringe to the needle, despite early free flow, indicates that the syringe has shifted and needs to be repositioned.

• Following the administration of the medication, the patient is directed to remain in the same posture for three to ten minutes. This minor change in the procedure explains how to obtain actual low spinal anesthesia. Next, both sides utilize an alcohol swab or a pinprick test to determine the dermatomal extent of sensory block by temperature perception (cold).

What Are the Complications of SAB?

• Injury to the nerve, damage to the nerve roots or spinal cord, and cauda equina syndrome (numbness around the anus, loss of control of the bowel and bladder, and pain radiating below the legs) are all potential risks of lack of anesthesiologists' experience.

• Accidental intravascular injection and hemorrhage (epidural hematoma) are potentially possible complications. In addition, back pain and post-dural puncture headache (PDPH) are both rather prevalent.

• There is always the possibility of infection (meningitis, epidural abscess), a complete or partial spinal block, and a failed block.

• Postoperative urinary retention.

• Hypotension is a result of chemical sympathectomy.

Conclusion:

The subarachnoid block is a valuable surgical anesthetic technique for various obstetric, urological, and perianal operations. Spinal approaches are usually preferable over general anesthesia because they avoid the necessity for tracheal intubation, which causes airway discomfort and a variety of respiratory issues.