Anatommy and physiology of cnb


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Anatommy and physiology of cnb

  1. 1. Co-ordinator:- Dr. Chavi sethi (M.D) Speaker:- Dr. Sushil kr. patel
  2. 2. Anatomy • The key to safe and effective administration of an CNB blockade begins with a thorough understanding of the anatomy of the vertebral column, ligaments, and blood supply, the epidural space, spinal canal, and associated structure. Vertebral Column-The vertebral column consists of 7 cervical, 12 thoracic, and 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 . The primary functions of the vertebral column are to maintain erect posture, to encase and protect the spinal cord, and to provide attachment sites for the muscles responsible for movements of the head and trunk. The normal spinal column is straight when viewed dorsally or ventrally.When viewed from the side, there are two ventrally convex curvatures in the cervical and lumbar regions, giving the spinal column the appearance of a double C. Structure of Vertebrae-Each vertebra is composed of a vertebral body and a bony arch. The arch consists of two anterior pedicles and two posterior laminae. The transverse processes are located at the junction of the pedicles and lamina, and the spinous process is located at the junction of the laminae. The spinous processes 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.
  3. 3. Primary Curve
  4. 4. Secondary Curves in Lateral
  5. 5. Typical Vertebrae
  6. 6. Typical Lumbar
  7. 7. Joints of the Vertebral Column • vertebrae articulate at the intervertebral and facet joints. • intervertebral joints are located between adjacent vertebral bodies. They maintain the strength of attachment between vertebrae. • facet joints form between articular processes. The facet joints are heavily innervated by the medial branch of the dorsal ramus of the spinal nerves. This innervation serves to direct contraction of muscle that moves the vertebral column
  8. 8. Ligaments • Vertebrae are joined together by a series of ligaments and disks. 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. • posterior surface of the vertebral bodies is connected by the posterior longitudinal ligament, which also forms the anterior wall of the vertebral canal. The other ligaments of importance : • 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
  9. 9. ligament
  10. 10. A and P Longitudinal Ligament
  11. 11. Spinal Cord • Runs through the vertebral canal • Extends from foramen magnum to second lumbar vertebra or lower end of 1st vertebrae • Regions – Cervical – Thoracic – Lumbar – Sacral – Coccygeal • Gives rise to 31 pairs of spinal nerves – All are mixed nerves • Not uniform in diameter – Cervical enlargement: supplies upper limbs – Lumbar enlargement: supplies lower limbs • Conus medullaris- tapered inferior end – Ends between L1 and L2 • Cauda equina - origin of spinal nerves extending inferiorly from conus medullaris.
  12. 12. Vertebral Relationships
  13. 13. spinal cord, which is continuous cephalad with the brainstem through the foramen magnum and terminates distally in the conus medullaris. This distal termination, because of differential growth rates between the bony vertebral canal and the central nervous system (CNS), varies from L3 in infants to the lower border of L1 in adults. • Most of us develop the impression that the spinal nerve roots are uniform in size and structure, but that there is large interindividual variability in nerve root size. These differences may help explain the interpatient differences in neuraxial block quality when equivalent techniques are used on seemingly similar patients. • Another anatomic relationship may affect neuraxial blocks; although the dorsal (sensory) roots are generally larger than the anterior (motor) roots, the dorsal roots are often blocked more easily. • This seeming paradox is explained by organization of the dorsal roots into component bundles, which creates a much larger surface area on which the local anesthetics act, thus explaining why larger sensory nerves are blocked more easily than smaller motor nerves.
  14. 14. Meninges • Connective tissue membranes – Dura mater: outermost layer; continuous with epineurium of the spinal nerves – Arachnoid mater: thin and wispy – Pia mater: bound tightly to surface • Forms the filum terminale – anchors spinal cord to coccyx • Forms the denticulate ligaments that attach the spinal cord to the dura • Spaces – Epidural: external to the dura • Anesthestics injected here • Fat-fill – Subdural space: serous fluid – Subarachnoid: between pia and arachnoid • Filled with CSF
  15. 15. Spinal Cord and its membrane . • 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 eight cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pairs of spinal nerves. • The roots inferior to the conus medullaris become the cauda equina before exiting through the lumbar and sacral foramina. After the spinal nerves leave the spinal canal through the intervertebral foramina, they divide into the anterior and posterior primary rami. • The posterior primary rami innervate the skin and muscles of the back. The anterior rami supply the rest of the trunk and the limbs. Each spinal nerve supplies a specific region of skin referred to as a dermatome . • 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. For instance, destruction of at least three consecutive spinal nerves is required to produce a total sensory loss of the dermatome supplied by the middle nerve of the three.
  16. 16. Spinal Cord and its membrane • pia mater is a highly vascular membrane that closely invests the spinal cord . • arachnoid mater is a delicate, nonvascular membrane closely attached to the outermost layer, the dura. • Of these two membranes, it is thought that the arachnoid functions as the principal barrier to drugs crossing in and out of the CSF and is estimated to account for 90% of the resistance to drug migration. • In the subarachnoid space -- CSF, spinal nerves, a trabecular network between the two membranes, and blood vessels that supply the spinal cord and lateral extensions of the pia mater and dentate ligaments, which provide lateral support from the spinal cord to the dura mater . • Although the spinal cord ends at the lower border of L1 in adults, the subarachnoid space continues to S2.
  17. 17. Membrane...... • Third and outermost membrane in the spinal canal is a randomly organized fibroelastic membrane, the dura mater (or theca). • This layer is a direct extension of the cranial dura mater and extends as the spinal dura mater from the foramen magnum to S2, where the filum terminale (an extension of the pia mater beginning at the conus medullaris) blends with the periosteum on the coccyx . • There is a potential space between the dura mater and the arachnoid, the subdural space, that contains only small amounts of serous fluid and thus allows the dura and arachnoid to move over each other. • This space is not intentionally used by anesthesiologists, although injection into it during spinal anesthesia may explain the occasional failed spinal anesthetic and the rare “total spinal” after epidural anesthesia when there is no indication of errant injection of local anesthetic into CSF.
  18. 18. •Surrounding the dura mater is another space that is often used by anesthesiologists, the epidural space. The spinal epidural space extends from the foramen magnum to the sacral hiatus and surrounds the dura mater anteriorly, laterally, and more usefully, posteriorly. •The epidural space is bounded anteriorly by the posterior longitudinal ligaments, laterally by the pedicles and intervertebral foramina, and posteriorly by the ligamentum flavum. •Contents of the epidural space include the nerve roots that traverse it from foramina to peripheral locations, as well as fat, areolar tissue, lymphatics, and blood vessels, including the well-organized Batson venous plexus. • •This lack of epidural space uniformity also extends to age-related differences. There is evidence that adipose tissue in the epidural space diminishes with age. • Another anatomic change in epidural space anatomy that has long been promoted is that the intervertebral foramina decrease in size with increasing age. •This decrease has been linked conceptually to higher block levels for similar epidural doses of local anesthetic. considered together, it may be that the decrease in epidural space adipose tissue with age may dominate the age-related changes in epidural dose reqirement
  19. 19. 13-20 Meninges of Vertebra and Spinal Cord
  20. 20. conti....... • Posterior to the epidural space is the ligamentum flavum (the so-called yellow ligament), which also extends from the foramen magnum to the sacral hiatus. Although classically portrayed as a single ligament, it is really composed of two ligamenta flava, the right and the left, which join in the middle and form an acute angle with a ventral opening . • The ligamentum flavum is not uniform from skull to sacrum, nor even within an intervertebral space. Ligament thickness, distance to the dura, and skin-to- dura distance vary with the area of the vertebral canal . The two ligamenta flava are variably joined (fused) in the midline, and this fusion or lack of fusion of the ligamenta flava even occurs at different vertebral levels in individual patients. • Immediately posterior to the ligamentum flavum are the lamina and spinous processes of vertebral bodies or the interspinous ligaments. Extending from the external occipital protuberance to the coccyx posterior to these structures is the supraspinous ligament, which joins the vertebral spines .
  21. 21. Surface Anatomy • Surface structures assist in determining the level of entry into the epidural and subarch space. The spinous processes help define the location of the midline as they are usually midline structures. The cervical and lumbar spinous processes are horizontally directed whereas the T4 through T9 thoracic spinous processes have a sharp caudal angulation . • Needle entry into the cervical and lumbar regions should be directed horizontally with a slight upward angulation, whereas in the upper thoracic region, a midline approach to the epidural space is more difficult because of the angulation of the spinous processes. A paramedian approach is usually more successful • safest point of entry into the epidural space is below the level of the spinal cord. In adults, this corresponds to the lower border of the L1 vertebrae, and in children, at the lower border of the L3 vertebrae. Epidural insertion in adults is commonly introduced at either the L3-4 interspinous space or one higher, L2-3. A line drawn between the superior aspect of the iliac crests crosses either the spinous process of L4 or the L4-5 interspace. • The interspinous space above this point (L3-4 interspinous space) or one higher (L2-3) can safely be chosen for needle entry into the epidural space of adults. Some research has challenged the accuracy of the iliac crest in assessing the level, but this is still a generally accepted surface landmark. By approximately the age of 8 years, the same interspaces can be chosen for children; however, under the age of 7, to avoid accidental cord injury the caudal approach to the epidural space is safer.
  22. 22. Anatomic Landmarks to Identify Vertebral Levels Before Epidural Injection Anatomic Landmark Features 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
  23. 23. Dermatomal Levels of Spinal Anesthesia for Common Surgical Procedures Procedure Dermatomal Level Upper abdominal surgery T4 Intestinal, gynecologic, and urologic surgery T6 Transurethral resection of the prostate T8 Vaginal delivery of a fetus, and hip surgery T10 Thigh surgery and lower leg amputations L1 Foot and ankle surgery L2 Perineal and anal surgery S2 to S5 (saddle block)
  24. 24. Anatomical and physiological differences in children • There are certain features of paediatric anatomy and physiology which are different from the adult and thus make the central neuraxial blockade a good alter-native anaesthetic technique. • spinal cord ends at L3 level at birth and reaches L-1 by 6-12 months. • The dural sac is at the S4 level at birth and reaches S2 by the end of the first year. The line joining the two supe-rior iliac crests (inter-cristal line) crosses at L5-S1 in-terspace at birth, L5 vertebra in young children and L3/4 interspace in adults. • It is for this reason that the lumbar puncture be done at a level below which the cord ends, safest being at or below the inter cristal line. • bones of the sacrum are not fused posteriorly in children enabling an access to the subarachnoid space even at this level. • Another feature which is unique in infants is that there is only one anterior concave curvature of the vertebral column at birth. The cervical lordosis begins in the first 3 months of life with the child's ability to hold the head upright. The lumbar lordosis starts as the child begins to walk at the age of 6-9 months. Therefore, the spread of isobaric local anaesthetic is different in infants particularly as compared to adults.
  25. 25. conti..... • The subarachnoid space is incompletely divided by the denticulate ligament laterally, and the subarachnoid septum medially. • The volume of cerebrospinal fluid CSF is 4 -1 which is double the adult volume. Moreover, in infants half of this volume is in the spinal space whereas adults have only one-fourth. This significantly affects the pharmocokinetics of intrathecal drugs. • The spinal fluid hydrostatic pressure of 30-40mm H 2 O in horizontal position is also much less than that in adults. • The neck can be in extension for lateral positioning while performing a lumbar puncture as cervical flexion is of no benefit in children and in fact, may obstruct the airway during the procedure. It can also be performed in sitting position with the head extended. • The physiological impact of sympathectomy is minimal or none in smaller age groups. • fall in blood pressure and a drop in the heart rate are practically not seen in children less than five years. Therefore there is no role of preloading with fluids before a subarachnoid block.
  26. 26. conti..... • This may be due to the immature sympathetic nervous system in children younger than five-eight years or a result of the relatively small intravascular volume in the lower extremities and splanchnic system limiting venous pooling and relatively vasodilated peripheral blood vessels. • Infants respond to high thoracic spinal anaesthesia by reflex withdrawal of vagal parasympathetic tone to the heart. It is one of the reasons why spinal anaesthesia has been the technique of choice in critically ill and moribund neonates who present for surgery in grave haemodynamic instability. • most important concern with the use of intrathecal local anaesthetics in infants and young children is the risk of toxicity. This age group is particularly prone to direct toxicity to the spinal cord when administered in large doses. • Neonates with immature hepatic metabolism and decreased plasma proteins like albumin and alpha 1 acid glycoprotein have higher serum levels of unbound amide local anaesthetics, which are normally highly protein bound (90%). • A relatively higher cardiac output and regional blood flow in infants also increases the drug uptake from neuraxial spaces and can predispose them to local anaesthetic toxicity besides decreasing the duration of action. Infants may have decreased levels of plasma pseudocholinesterase which may augment local anaesthetic toxicity especially with the ester group.
  27. 27. Physiologic Effects of Neuraxial Blockade
  28. 28. Somatic Blockade • Neuraxial anesthesia effectively stops the transmission of painful sensation and abolishes the tone of skeletal muscle, enhancing operating conditions for the surgeon. • Sensory blockade involves somatic and visceral painful stimulation. • Motor blockade involves skeletal muscles. Neuraxial anesthesia results in a phenomenon known as differential blockade. • This effect is due to the activity of local anesthetics and anatomical factors. Local anesthetic factors include the concentration and duration of contact with the spinal nerve root. • As the local anesthetic spreads out from the site of injection the concentration becomes less, which may in turn effect which nerve fibers are susceptible to blockade. Anatomical factors are related to various fiber types found within each nerve root. • Small myelinated fibers are easier to block than large unmyelinated fibers. In general, the differential blockade found after neuraxial blockade is as follows: sympathetic blockade is 2-6 dermatome segments higher than sensory and sensory blockade is generally 2 dermatome levels higher than motor.
  29. 29. Autonomic Blockade • Neuraxial blockade effectively blocks efferent autonomic transmission of the spinal nerve roots, producing a sympathetic block and a partial parasympathetic block. • Sympathetic fibers are small, myelinated, and easily blocked. During neuraxial blockade, sympathetic block occour prior to sensory, followed by motor. • sympathetic nervous system (SNS) is described as thoracolumbar since sympathetic fibers exit the spinal cord from T1 to L2. • The parasympathetic nervous system (PNS) has been described as craniosacral since parasympathetic fibers exit in the cranial and sacral regions of the CNS. It should be noted that neuraxial blockade does not affect the vagus nerve (10th cranial nerve). • End result of neuraxial blockade is a decreased sympathetic tone with an unopposed parasympathetic tone. This imbalance will result in many of the expected alterations of normal homeostasis noted with the administration of epidural and spinal anesthesia.
  30. 30. Cardiovascular Effects • Neuraxial blockade can impact the cardiovascular system by causing the following changes: 1. Decrease in blood pressure (33% incidence of hypotension in non-obstetric populations) 2. Decrease in heart rate (13% incidence of bradycardia in non-obstetric populations) 3. Decrease in cardiac contractility • Sympathectomy is the term used to describe blockade of sympathetic outflow. Nerve fibers that affect vasomotor tone of the arterial and venous vessels arise from provider wants to block with neuraxial blockade. • The sympathetic dermatome ranges from 2-6 levels higher than the sensory dermatome level. Sympathectomy is directly related to the height of the block and results in venous and arterial vasodilatation. • The venous system contains about 75% of the total blood volume while the arterial system contains about 25%. Dilation of the venous system is predominantly responsible for decreases in blood pressure since the arterial system is able to maintain much of its vascular tone. T5-L1, which is generally within the area that the anesthesia
  31. 31. •Total peripheral vascular resistance in the normal patient (normal cardiac output and normovolemic) will decrease 15-18%. •In the elderly the systemic vascular resistance may decrease as much as 25% with a 10% decrease in cardiac output. •Heart rate may decrease during a high block due to blockade of the cardioaccelerator fibers (T1-T4). Heart rate may also decline as a result of a decrease in SVR, decreased right atrial filling, and decreases in the intrinsic chronotropic stretch receptor response. • that epidural and spinal anesthesia differ in their effect on arterial blood pressure? A common concept is that the decrease in arterial blood pressure is more gradual and of less magnitude with epidural than with spinal anesthesia of comparable levels. During the T10 block, there was no significant change in organ blood flow; during the T1 block, with a 22% decrease in mean arterial pressure, cerebral and myocardial blood flow was insignificantly altered
  32. 32. Respiratory Effects • Neuraxial blockade plays a very minor role in altering pulmonary function. Even with high thoracic levels of blockade, tidal volume is unchanged. There is a slight decrease in vital capacity. • This is the result of relaxation of the abdominal muscles during exhalation. • phrenic nerve is innervated by C3-C5 and is responsible for the diaphragm. phrenic nerve is extremely hard to block, even with a high spinal. • In fact, apnea associated with a high spinal is thought to be related to brainstem hypoperfusion and not blockade of the phrenic nerve. This is based on the fact that spontaneous respiration resumes after hemodynamic resuscitation has occurre. • risk and benefits of neuraxial anesthesia should be carefully weighed for the patient with severe lung disease. • Patients with chronic lung disease depend on intercostal and abdominal muscles to aid their inspiration and exhalation. • Neuraxial blockade may reduce the function of these muscles, having a detrimental impact on the patient’s ability to breathe, as well as affect the ability to clear secretions and cough. • For procedures above the umbilicus, a pure regional anesthetic may not be beneficial for the patient with chronic lung disease. • However, postoperative analgesia with thoracic epidurals has been found to be helpful to the patient with severe lung disease undergoing a thoracic or abdominal procedure. • .
  33. 33. • Thoracic and abdominal surgical procedures are associated with decreased phrenic nerve activity resulting in decreased diaphragmatic function and FRC (functional reserve capacity). This can lead to atelectasis and hypoxia due to ventilation/perfusion mismatching. • Thoracic epidural analgesia has been found to decrease the incidence of pneumonia, respiratory failure, improve oxygenation, and decrease the amount of time that the patient may require for postoperative ventilation •
  34. 34. Gastrointestinal Effects •Another organ system affected during neuraxial blockade is the gastrointestinal tract. •Nausea and vomiting may be associated with neuraxial block in up to 20% of patients and are primarily related to gastrointestinal hyperperistalsis caused by unopposed parasympathetic (vagal) activity. •Atropine is effective in treating nausea associated with high (T5) subarachnoid anesthesia. • This gastrointestinal hyperperistalsis has the advantage of providing excellent surgical conditions because of a contracted gut. • An often-cited advantage of regional anesthesia in patients with compromised gastrointestinal function (e.g., hepatic dysfunction) is that less physiologic impairment is possible than with general anesthesia. • decrease in hepatic blood flow during spinal anesthesia parallels the decrease in mean arterial blood pressure.When epidural analgesia is continued into the postoperative period, there may be a protective effect on the gastric mucosa because intramucosal pH is higher during postoperative epidural analgesia than during systemic analgesia.
  35. 35. Renal Effects • Neuraxial blockade has little effect on the blood flow to the renal system. • Autoregulation maintains adequate blood flow to the kidneys as long as perfusion pressure is maintained. • Neuraxial blockade effectively blocks sympathetic and parasympathetic control of the bladder at the lumbar and sacral levels. • Urinary retention can occur due to the loss of autonomic bladder control. Detrusor function of the bladder is blocked by local anesthetics. Normal function does not return until sensory function returns to S3. • Risk factors for prolonged blockade of the detrusor muscle include the use of long acting local anesthetics, age > 50 years, volume of fluids administered, and surgical procedure. Prolonged blockade of the detrusor muscle may lead to bladder over distention and urinary retention. This should be taken into consideration if no urinary catheter will be placed. If possible, short acting medications should be used. • It should be monitor the amount of intravenous fluids administered to prevent over distention of the bladder. • patient with a history of an enlarged prostrate is at risk for urinary retention. Patients should be monitored for urinary retention.
  36. 36. Metabolic and Endocrine Effects • Surgery produces a host of neuroendocrine responses related to inflammatory response and activation of somatic and visceral afferent nerve fibers. • This response results in the release of adrenocorticotropic hormone, cortisol, epinephrine, norepinephrine, vasopressin, and activation of the renin- angiotension-aldosterone system. • release of these substances has the following clinical manifestations: hypertension, tachycardia, hyperglycemia, protein catabolism, depressed immune response, and alteration in renal function. As noted earlier, neuraxial blockade can effectively block this response. • In intra-abdominal surgery, it may only partially suppress its effects. For lower extremity surgery, it can totally suppress these effects. The effect of neuraxial blockade is beneficial by reducing catecholamine release, decreasing stress related arrhythmias, and decreasing the incidence of ischemia.
  37. 37. caudal Anatomy • The sacrum results from fusion of the five sacral vertebrae. • The sacral hiatus, which is failure of the laminae of S5 and usually part of S4 to fuse in the midline, is the detail of interest. • It results in a variably shaped and sized inverted V–shaped bony defect covered by the posterior sacrococcygeal ligament, a functional counterpart to the ligamenta flava. • The sacral hiatus is identified by locating the sacral cornua, remnants of the S5 articular processes . This bony defect allows access to the sacral canal, although needle insertion through this defect may be difficult because of the frequency of anatomic variation. • The sacral canal contains the terminal portion of the dural sac, which typically ends cephalad to a line joining the posterior superior iliac spines, or S2.
  38. 38. caudal Anatomy • Variation is found in this feature as well, with the termination of the dural sac being lower in children, ease of palpating the sacral hiatus in children may make the pediatric caudal technique easier overall. • In addition to the dural sac, the sacral canal contains a venous plexus, which is part of the valveless internal vertebral venous plexus. • It is estimated from magnetic resonance imaging (MRI) studies that the volume of the caudal canal in adults, excluding the foramina and dural sac, is about 10 to 27 mL. Perhaps this wide variability in volume accounts for some of the variation in block height with caudal anesthesia
  39. 39. Thank you for your attention