Neuraxial Anesthesia - Epidurals
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Absolute and Relative?
- Patient refusal
- Untreated hypovolaemia
- Increased ICP
- Infection at site
- Allergy to LA
- Coagulopathy - (epidural block can be placed 4h after the last dose of sc heparin, 12h after the last dose of low-molecular-weight heparin, 7 days after clopidogrel, INR <1.5; platelets <100)
- Fixed CO states (inability to increase cardiac output in response to sympathectomy)
- Anatomical abnormalities of spine
- 7 cervical
- 12 thoracic
- 5 lumbar vertebrae.
- At the caudal end, the 5 sacral vertebrae are fused to form the sacrum;
- And the 4 coccygeal vertebrae are fused to form the coccyx
Primary functions x3?
The primary functions of the vertebral column are to:
- 1. Maintain erect posture,
- 2. Encase and protect the spinal cord,
- 3. Provide attachment sites for the muscles responsible for movements of the head and trunk
Structure of Vertebrae?
- Vertebra is composed of a vertebral body and a bony arch.
- Arch consists of two anterior pedicles and two posterior laminae.
- Transverse processes are located at the junction of the pedicles and lamina
- Spinous process is located at the junction of the laminae and vary in their angulation in the cervical, thoracic, and lumbar regions.
- The spinous processes are almost horizontal in the cervical, lower thoracic, and lumbar regions, but become significantly more sharply angled in the midthoracic region.
- The greatest degree of angulation is found between the T3 and T7 vertebrae, making insertion of an epidural needle in the midline more difficult.
- The shape and size of the vertebrae differ from the cervical to the lumbar region secondary to function.
- Vertebral bodies are smaller in the cervical region an become progressively larger in the lumbar area where they support the greatest amount of weight.
Ligaments - x6 and attachments?
Anteriorly, the vertebral bodies are separated by the intervertebral disks. The ligament connecting them runs from the base of the skull to the sacrum and is called the anterior longitudinal ligament.
The posterior surface of the vertebral bodies is connected by the posterior longitudinal ligament, which also forms the anterior wall of the vertebral canal.
Intertransverse ligaments: connects transverse processes
Supraspinous ligaments: attaches to the apices of the spinous processes, extends from sacrum to skull where it becomes the ligamentum nuchae
Interspinous ligaments: connects spinous processes
Ligamentum flavum: thick, elastic ligament, connects the laminae, composed of a right and left ligament that joins in the middle forming an acute angle; narrows toward the articular processes
Describe Spinal Cord and Nerves?
The spinal cord extends from the foramen magnumto the L1-2 vertebral level in adults, and L3 vertebral level in children before becoming the conus medullaris.
- From the spinal cord extends a series of dorsal and ventral roots that converge to form mixed spinal nerves.
- The mixed nerves contains motor, sensory, and in many cases, autonomic fibers.
- There are:
- - 8 cervical,
- - 12 thoracic,
- - 5 lumbar,
- - 5 sacral,
- - 1 coccygeal pairs of spinal nerves.
There is overlap between adjacent segmental nerves. Loss of a single spinal nerve will produce an area of altered sensation, but won’t result in total sensory loss.
Surface Landmarks for Dermatomal Blockade x7?
- C8 - Little finger -> Cardioaccelerator fibers blocked (T1 to T4)
- T1, T2 - inner aspect of the arm -> Above fibers blocked but to lesser degree
- T4 - Nipple line, root of scapula -> Cesarean section, Appendectomy, upper abdominal surgery
- T7 - Inferior border of scapula; Tip of xiphoid -> Splanchnic (T5 to L1) blockage; lower abdominal surgery; T5 to T7 for thoracotomy or fractured ribs (at relevant interspace)
- T10 - Umbilicus -> Usual level for LE procedures, hip surgery, TURP, vaginal delivery
- L2 to L3 - Anterior thigh -> Appropriate for knee, foot surgery
- S1 - Heel of foot -> Part of sacral plexus, difficult to block
Meninges x3, spaces and relationship to other structures?
- 3 layers of membranes.
- The innermost layer = pia mater, attaches intimately to the surface of the spinal cord and roots of the spinal nerves. As the roots of the spinal nerves extend distally, the pia mater transforms into the second layer = arachnoid.
- The aranchoid detaches from the roots and reflects back across the pia, enclosing the spinal cord within a cavity called the subarachnoid space. The space is filled with cerebrospinal fluid and transmits blood vessels to and from the spinal cord. Superficial to the arachnoid is the thick dura mater. The space between the arachnoid and dura is called the subdural space. Because the arachnoid is pushed against the dura mater by the pressure of the CSF, the subdural space is negligible. It contains a small amount of serous fluid which allows the dura and arachnoid to move over each other. Because of its exceedingly small volume, it is referred to as a potential space.
- Unintentional injection into the subdural space during epidural anesthesia explains the occasional patchy epidural, high epidural/spinal level or total spinal after epidural placement.
- Key features: diffuse, spotty anesthesia with a delayed onset for 15 to 30 min, clinical presentation of a high or total spinal.
- The subdural space extends into the skull, so agents injected into this space can affect higher levels in the brain than epidural medications - watch for neurologic changes.
Anatomic Landmarks x6?
- C7 Vertebral prominence, the most prominent process in the neck
- T3 Root of the spine of the scapula
- T7 Inferior angle of the scapula
- L4 Line connecting iliac crests
- S2 Line connecting the posterior inferior iliac spines
- Sacral hiatus Groove or depression just above or between the gluteal clefts above the coccyx
Epidural Space - borders & internal structures?
- The epidural space is smaller than the subarachnoid space, extends from the base of the skull to the sacral hiatus, and surrounds the dura mater anteriorly, laterally, and posteriorly.
- The epidural space is bound posteriorly by the ligamentum flavum and laterally by the pedicles and the intervertebral foramina.
- It is a space filled with the fat, areolar tissue, lymphatics, veins, and nerve roots that traverse it, but no free fluid.
- The volume of fat is greater in obese individuals and less in the elderly. It is postulated that the decrease in epidural fat explains the age-related changes in epidural dose requirements.The epidural space is rich in blood vessels, including Batson’s venous plexus. Batson’s plexus is continuous with the iliac vessels in the pelvis and the azygos system in the abdominal and thoracic body walls.
- Because this plexus has no valves, blood from any of the connected systems can flow into the epidural vessels. This is especially important in obstetrics when compressed caval vessels can lead to engorgement of the epidural veins, increasing the risk of catheter entry into a vein.
- The engorgement is even greater at the intervertebral foramina where the vessels egress from the vertebral canal. Therefore, the incidence of penetrating a blood vessel with an “off-midline” needle insertion is more likely.
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