SAB
Meaning of SAB: One injection, no catheter, and a needle placed deliberately below the point where the spinal cord itself stops.
Definition of SAB
SAB stands for subarachnoid block, better known as spinal anesthesia or a spinal block. A local anesthetic is injected through the low back into the cerebrospinal fluid that bathes the nerve roots, below the level at which the spinal cord ends. It numbs the lower body for a set period while the person remains awake.
How does it differ from an epidural?
| Spinal block | Epidural | |
|---|---|---|
| Where the drug goes | Into the cerebrospinal fluid itself | Into the fat filled space just outside the covering |
| Delivery | A single injection, needle removed | A fine catheter left in place |
| Dose | Small | Considerably larger volume |
| Onset | Within minutes | Gradual, over a longer stretch |
| Duration | Fixed by the drug chosen | Extended as needed through the catheter |
Both are regional techniques, meaning they numb a region rather than producing unconsciousness as general anesthesia does. The two are sometimes combined, using the spinal component for a fast, dense block and the catheter for pain relief afterward.
Where exactly is the needle placed?
The spinal cord itself ends high in the lower back, around the first or second lumbar vertebra in an adult, while the sac of fluid continues further down. The injection is made below that ending point, so the needle passes among a loose bundle of nerve roots that drift aside rather than through the cord. The technique is the same approach used for a spinal tap, the difference being that one removes fluid for testing and the other adds an anesthetic to it. Because the drug enters the fluid directly, a very small dose produces a dense block.
What follows the injection?
Sensation fades from the feet upward, usually with warmth first, then loss of pain, then loss of movement. The same nerves also control blood vessel tone, so vessels in the lower body widen and blood pressure commonly falls soon after the block sets in, which is why pressure is watched closely and fluids are often running. The bladder is numbed too, so urination may need help until the block wears off. A headache that is worse sitting or standing can follow a puncture of the covering membrane. The block is used for procedures on the lower abdomen, pelvis, hips and legs, including cesarean birth and joint replacement.
Also known as
Used in a sentence
The procedure was performed under SAB, allowing the patient to remain awake throughout.
Good to know
A spinal block delivers a single injection into cerebrospinal fluid, distinct from an epidural, which places a catheter just outside that fluid filled space and allows anesthetic to be topped up over time.