Spinal analgesia, (or commonly called spinal anesthesia or sub-arachnoid block (S.A.B.)) is a form of regional anaesthesia involving injection of a local anaesthetic into the cerebrospinal fluid (CSF), generally through a fine needle, usually 3.5 inches long. For extremely obese patients, some anaesthesiologists are known to prefer spinal needles which are seven inches long. The tip of the spinal needle should, theoretically, have a short or small bevel. Recently pencil point needles have been made available (Whitakre's).
There are hyperbaric, isobaric and hypobaric solutions of anesthetics to choose for the spinal anesthesia. Usually, the hyperbaric is chosen, as its spread can be effectively and predictably controlled by the anaesthesiologist.
Bupivacaine is the local anaesthetic most commonly used, although lignocaine (lidocaine), tetracaine, procaine, ropivacaine, levobupivicaine and cinchocaine are also available. Sometimes a vasoconstrictor such as epinephrine is added to the local anesthetic to prolong its duration. Of late, many anaesthesiologists are preferring to add opioids like fentanyl or buprenorphine, or non-opioids like clonidine, to the local anaesthetic used in spinal, to give a smoother 'effect' and to provide prolonged pain relief once the action of the 'spinal' has worn off.
Regardless of the anaesthetic agent (drug) used, the desired effect is to block the transmission of nerve signals to and from the affected area. Sensory signals from the site are blocked, thereby eliminating pain, and motor signals to the area eliminate movement. In effect, the result is total numbness of the area and paralysis. This allows surgical procedures to be performed with little or no sensation whatsoever to the person undergoing the procedure, and provides a still patient or area for the surgeon to work on.
Some sedation is sometimes provided to help the patient relax and pass the time during the procedure, but with a successful spinal anaesthetic the surgery can be performed with the patient wide awake. Spinal anaesthetics are limited to procedures involving most structures below the upper abdomen. To administer a spinal anaesthetic to higher levels may affect the ability to breathe by paralyzing the intercostal respiratory muscles, or even the diaphragm in extreme cases (called a "high spinal", or a "total spinal", with which consciousness is lost), as well as the body's ability to control the heart rate via the cardiac accelerator fibers.
Baricity refers to the density of a substance compared to the density of human cerebral spinal fluid. Baricity is used in anaesthesia to determine the manner in which a particular drug will spread in the intrathecal space.
The first planned spinal anesthesia for surgery in man was administered by August Bier (1861-1949) on 16th August 1898, in Kiel, when he injected 3 ml of 0.5% cocaine solution into a 34 year old laborer. After using it on 6 patients, he and his assistant each injected cocaine into the other's spine. They recommended it for surgeries of legs, but gave it up due to the toxicity of cocaine.
Presently, spinal analgesia (or S.A.B.) is the mainstay of anaesthesia in countries like India and parts of Africa, excluding the major centres. Thousands of spinal anaesthetics are administered daily in hospitals & nursing homes.At a low cost, a surgery of up to two hours duration can be performed.
Indications: This technique is very useful in patients having an irritable airway (bronchial asthma or allergic bronchitis), anatomical abnormalities which make endotracheal intubation very difficult (micrognathia), borderline hypertensives where administration of general anaesthesia or endotracheal intubation can further elevate the blood pressure, procedures in geriatric patients. It is the technique of choice for diabetic patients.
Contraindications: Non-availability of patient's consent, local infection or sepsis at the site of lumbar puncture, bleeding disorders, space occupying lesions of the brain, disorders of the spine.