8.8:
Local Anesthetics: Clinical Application as Spinal Anesthesia
Spinal anesthetics are given during surgeries of the lower abdomen and limbs to block sensory and motor neurons.
They are injected into the subarachnoid space of the lower lumbar region, where they come in contact with the surrounding cerebrospinal fluid or CSF.
Once in the CSF, local anesthetics or LAs are rapidly absorbed by the nerves causing a nerve blockade.
The level of blockade depends on the LA concentration. Usually, low concentrations of LAs are sufficient to block sensory neurons, while at high LA concentrations, motor neurons are also blocked.
The CSF also dilutes LAs while they diffuse away from the injection site.
The direction of diffusion depends on the density or baricity of the LA solution compared to the CSF. Due to gravity, hyperbaric solutions flow down in the CSF, while hypobaric solutions tend to rise. Isobaric solutions stay near the injection site, slowly diffusing in all directions.
8.8:
Local Anesthetics: Clinical Application as Spinal Anesthesia
Spinal anesthetics are given during lower abdomen and limb surgeries to block sensory and motor neurons. They are administered in the mid to low lumbar regions, primarily acting on the cauda equina's nerve roots. The blockade level depends on the local anesthetic (LA) concentration. Usually, low LA concentrations are sufficient to block sensory fibers, while only high LA concentrations block motor fibers. Other factors like injection volume and speed, the patient's posture, and the drug solution's baricity relative to the cerebrospinal fluid also affect the extent of anesthesia. For example, the baricity of the drug solution determines the direction of migration of the anesthetics. Isobaric solutions remain at the injection site and diffuse in all directions.
The drugs most commonly used to cause spinal anesthesia are lidocaine, tetracaine, and bupivacaine. Generally, lidocaine is used for short procedures, bupivacaine for intermediate procedures, and tetracaine for more lengthy procedures. LAs are often administered with epinephrine to increase the duration and intensity of blockade.
Spinal anesthesia is safer and preferred over general anesthesia as it has minimum systemic toxicity and causes muscle relaxation without losing consciousness.