Summary

In study session VI you learnt

• The vertebral column is a bony structure that extends from the foramen magnum to the sacral hiatus and it consists of 7 cervical, 12 thoracic, and 5 lumbar vertebrae as well as the sacrum, and coccyx. It provides a structural support, protection of the spinal cord and nerves, and mobility
• The vertebral column consists of the vertebral body anterior to vertebral foramen and the vertebral (or neural) arch composed of a pedicle, transverse process, superior and inferior articular processes, and spinous process which surrounds and protects the spinal cord posteriorly lying in the vertebral foremen.
• Ligaments maintain the shape of the vertebral column and provide support. Vertebral bodies and disks are connected and supported on the ventral side by anterior and posterior longitudinal ligaments. On the dorsal side of the vertebral column (structures that the anesthesia provider will pass through when placing a needle for neuraxial blockade) the ligamentum flavum, interspinous ligament, and supraspinous ligament provide support.
• The spinal cord is the continuation of the brain and occupies the vertebral canal. It extends from the upper border of the first cervical vertebra (the atlas) to the upper border of the second lumbar vertebra. Spinal puncture above L2 vertebra may result in cord damage
• The cord is divided into segments by the spinal nerves that leave it. There are 31 pairs of spinal nerves each arising by the fusion of the anterior and posterior nerve root. The nerve roots are blocked by the analgesic solution during spinal anesthesia. The analgesic solutions block the finest fibres first. The sensory nerves, being of smaller diameter than the motor, are blocked first.
• The area of skin supplied by the branches of each pairs of nerves (Ventral [motor] and a Dorsal [sensory]) is known as dermatome.
• The cerebrospinal fluid occupies the space between the pia mater and the arachnoid mater called sub arachnoid space. The local anesthetic is deposited into it during spinal anesthesia. The baricity of a spinal anesthetic solution is a ratio: the density of the anesthetic solution divided by that of CSF. Baricity is the most useful index of determining how spinal anesthetic solutions distribute themselves when added to CSF.
• Local anesthetics are drugs which produce a reversible loss of sensation in a portion of the body by preventing nerve impulse conduction or transmission.
• Local anesthetic agents can be separated chemically into basically two groups: amino ester (cocaine, tetracaine) & amino amide (lidocaine, bupivacaine) agents. The main differences between amonoamide and amino ester types of local anesthetics are their route of metabolism & allergenic potential. Esters are hydrolyzed in plasma by the cholinesterase enzymes, where as the amide compounds undergo enzymatic degradation in the liver.
• For spinal anesthesia we commonly use bupivacaine, lidocaine and tetracaine
• The primary determinant of intrinsic anesthetic potency is lipid solubility which determines the agent's ability to penetrate lipid cell membrane. Additional factors include fiber size, type, and myelination, pH and vasodilator/vasoconstrictor properties
• The most important factors that influence the frequency of success full conduction blockade is the anesthetist's experience. Other factors include dosage of local anesthetics, site of injection, addition of vasoconstrictors and pregnancy.
• Epinephrine containing local anesthetics should never be injected into end organs such as ears, nose, penis, fingers, or toes. Epinephrine may cause vasoconstriction and subsequent necrosis of tissue.