Spinal anesthesia (SA), also known as a subarachnoid block, temporarily stops sensation and movement in the lower half of the body. This technique involves injecting medication directly into the cerebrospinal fluid (CSF), which bathes the spinal cord. The medication, typically a local anesthetic, acts directly on the nerves, causing a rapid onset of numbness and muscle paralysis below the waist. Spinal anesthesia is distinct from an epidural block because it is a single injection into the CSF, while an epidural involves placing a catheter just outside the dura mater for continuous delivery.
Preparing the Patient and Monitoring
The process begins with preparation to ensure patient safety and comfort before the needle insertion. An intravenous (IV) line is established to administer necessary fluids and potentially a mild sedative. Standard monitoring equipment is attached immediately, including a blood pressure cuff, a pulse oximeter to track blood oxygen saturation, and electrocardiogram (ECG) leads to monitor heart rhythm.
These monitors allow the healthcare team to watch for physiological changes, such as a drop in blood pressure, that can occur after the anesthetic takes effect. Once the patient is positioned, the area of the lower back where the injection will occur is cleansed with an antiseptic solution like chlorhexidine or iodine. This sterilization process reduces the risk of infection before the procedure begins.
Locating the Injection Site
Patient positioning is required to open the spaces between the vertebrae for needle insertion. Patients are asked to either sit up and lean forward, curling their shoulders and neck inward, or lie on their side in a fetal position with knees drawn toward the chest. Both positions maximize the separation of the spinal bones.
The provider palpates and identifies the bony landmarks along the spine, aiming for an interspace below the L2 vertebra to avoid injury to the spinal cord. The most common insertion sites are between the L3 and L4 or the L4 and L5 vertebrae. After the target space is identified, a local anesthetic is injected to numb the skin and underlying soft tissue. This minimizes the discomfort of the subsequent spinal needle insertion.
Step-by-Step Drug Administration
After the site is numbed, the actual injection begins with the placement of an introducer needle. This short, wider needle is inserted through the skin and into the ligamentous tissue. It creates a stable, straight path for the finer spinal needle, which is then passed through the center of the introducer.
As the needle advances, the provider notes distinct changes in resistance as it passes through several layers of tissue. These layers include the supraspinous ligament, the interspinous ligament, and the ligamentum flavum. The final resistance is felt as the needle penetrates the dura mater, the tough outer membrane surrounding the spinal cord. A sudden loss of resistance signals entry into the subarachnoid space, the target area containing the CSF.
Confirmation of correct placement is achieved when cerebrospinal fluid (CSF) drips or “flashes back” from the hub of the spinal needle. This confirms the needle tip is correctly positioned. The syringe containing the local anesthetic, such as bupivacaine or lidocaine, is then attached to the needle.
The anesthetic solution is injected slowly into the CSF, allowing the medication to mix and spread to the nerve roots that control sensation and movement. Once the full dose has been administered, the spinal needle and the introducer are removed together. The entire process of insertion, confirmation, and injection typically takes only a few minutes.
Onset of Effect and Immediate Post-Procedure Care
The effects of spinal anesthesia begin quickly, often within two to five minutes of medication delivery. Patients first experience a warm, tingling sensation in their legs and feet as the anesthetic blocks the sensory nerves. This is rapidly followed by a feeling of heaviness, then complete numbness and the inability to move the legs.
Once the needle is removed, the patient is repositioned for the surgical procedure. Close monitoring is necessary during this time, as the medication can cause blood vessels to relax, leading to a drop in blood pressure. The anesthesia team watches the blood pressure and heart rate, administering IV fluids or medications to maintain stable circulation if needed.
Sensation and movement typically begin to return within one to four hours. The patient is kept under observation until the effects have resolved and they regain full movement of their lower extremities.