Spinal anesthesia.docx

1、Spinal anesthesiaSpinal anesthesiaSpinal anesthesia involves the administration of local anesthetic into the subarachnoid space. A.Anatomy 1.The spinal canal extends from the foramen magnum to the sacral hiatus. The boundaries of the bony canal are the vertebral body anteriorly, the pedicles lateral

2、ly, and the spinous processes and laminae posteriorly Vertebral anatomy.2.Three interlaminar ligaments bind the vertebral processes together: a.Superficially, the supraspinous ligament connects the apices of the spinous processes. b.The interspinous ligament connects the spinous processes on their h

3、orizontal surface. c.The ligamentum flavum connects the caudal edge of the vertebrae above to the cephalad edge of the lamina below. This ligament is composed of elastic fibers and is usually recognized by its increased resistance to passage of a needle. 3.The spinal cord extends the length of the v

4、ertebral canal during fetal life, ends at about L-3 at birth, and moves progressively cephalad to reach the adult position near L-1 by 2 years of age. The conus medullaris, lumbar, sacral, and coccygeal nerve roots branch out distally to form the cauda equina. It is in this area of the canal (below

5、L-2) that spinal needles are placed, because the mobility of the nerves reduces the danger of trauma from the needle. 4.The spinal cord is invested in three meninges: a.The pia mater. b.The dura mater, which is a tough fibrous sheath running longitudinally the entire length of the spinal cord and is

6、 tethered caudally at S-2. c.The arachnoid, which lies between the pia and dura mater. 5.The subarachnoid space lies between the pia mater and the arachnoid and extends from the attachment of the dura at S-2 to the cerebral ventricles above. The space contains the spinal cord, nerves, cerebrospinal

7、fluid (CSF), and blood vessels that supply the cord. 6.CSF is a clear colorless fluid that fills the subarachnoid space. The total volume of CSF is 100 to 150 mL, whereas the volume in the spinal subarachnoid space is 25 to 35 mL. CSF is continuously formed at a rate of 450 mL/day by secretion or ul

8、trafiltration of plasma from the choroid arterial plexuses located in the lateral, third, and fourth ventricles. CSF is reabsorbed into the bloodstream through the arachnoid villi and granulations that protrude through dura to lie in contact with the endothelium of the cerebral venous sinuses. B.Phy

9、siology 1.Neural blockade. Smaller C fibers conveying autonomic impulses are more easily blocked than the larger sensory and motor fibers. As a result, the level of autonomic blockade extends above the level of the sensory blockade by two to three segments. This is termed differential blockade. Simi

10、larly, fibers conveying sensation are more easily blocked than the larger motor fibers so that sensory blockade will extend above the level of motor blockade. 2.Cardiovascular. Hypotension is directly proportional to the degree of sympathetic blockade produced. Sympathetic blockade results in dilata

11、tion of arteries and venous capacitance vessels, leading to decreased systemic vascular resistance and decreased venous return. If the block is below T-4, increased baroreceptor activity produces an increase in activity to the cardiac sympathetic fibers and vasoconstriction of the upper extremities.

12、 Blockade above T-4 interrupts cardiac sympathetic fibers, leading to bradycardia, decreased cardiac output, and a further decrease in blood pressure. These changes are more marked in patients who are hypovolemic, elderly, or have obstruction to venous return (e.g., pregnancy). These effects can be

13、minimized with prehydration, vasopressors, and anticholinergics. 3.Respiratory. Low spinal anesthesia has no effect on ventilation. With ascending height of the block into the thoracic area, there is a progressive ascending intercostal muscle paralysis. This has little effect on ventilation in the s

14、upine surgical patient who still has diaphragmatic function mediated by the phrenic nerve. Ventilation in patients with poor respiratory reserve, such as the morbidly obese, however, may be profoundly impaired. Both intercostal and abdominal muscle paralysis decrease the efficiency of coughing, whic

15、h may be important in patients with chronic obstructive pulmonary disease. Epidural analgesia with opioids and low dose local anesthetics, which produces minimal motor blockade, is helpful in the postoperative care of thoracic surgical patients. 4.Visceral effects a.Bladder. Sacral blockade (S2-S4)

16、results in an atonic bladder that is able to retain large volumes of urine. Blockade of sympathetic efferents (T5-L1) results in an increase in sphincter tone, producing retention. A urinary catheter should be placed if anesthesia or analgesia is maintained for a prolonged period. b.Intestine. Sympa

17、thetic blockade (T5-L1) produced by spinal anesthesia has a promotility effect on the gut because of predominance of parasympathetic tone. 5.Renal blood flow is maintained, because of autoregulation by local tissue factors, except with severe hypotension. Urine production is usually unaffected. 6.Ne

18、uroendocrine. Peridural block to T-5 inhibits part of the neural component of the stress response, through its blockade of sympathetic afferents to the adrenal medulla and blockade of sympathetic and somatic pathways mediating pain. Other components of the stress response and central release of humo

19、ral factors are unaffected. Vagal afferent fibers from upper abdominal viscera are not blocked and can stimulate release of hypothalamic and pituitary hormones, such as antidiuretic hormone and adrenocorticotropic hormone. Glucose tolerance and insulin release are normal. 7.Thermoregulation. Vasodil

20、ation of the lower limbs can produce hypothermia. C.Technique 1.Spinal needle. Newer needles such as the Sprotte and Whitacre feature a pencil-point design with a lateral opening. These needles may reduce the incidence of postdural puncture headache (to 1%) compared with traditional “cutting tip” ne

21、edles by splitting rather than cutting dural fibers during insertion. Needles that are 24 and 25 gauge are easily bent and are often inserted through a 19-gauge introducer needle. The 22-gauge Quincke needle is more rigid and is more easily directed and inserted. It can be useful in older patients i

22、n whom access may be more difficult and the incidence of postdural puncture headache is low. 2.Patient position. The lateral decubitus, prone, and sitting positions can be used for administration of spinal anesthesia. a.In the lateral position, the patient is placed with the affected side up if a hy

23、pobaric or isobaric technique is to be used and with the affected side down if a hyperbaric technique is to be used. The spine is horizontal and parallel to the edge of the table. The knees are drawn up toward the chest and the chin flexed downward onto the chest to obtain maximal flexion of the spi

24、ne. b.The sitting position is useful for low spinal blocks required in certain gynecologic and urologic procedures and is commonly used in obese patients to assist in identification of the midline. It is used in conjunction with hyperbaric anesthetics. The head and shoulders are flexed downward onto

25、 the trunk with the arms resting on a Mayo stand. An assistant should be available to stabilize the patient, and the patient should not be oversedated. c.The prone position is used in conjunction with hypobaric or isobaric anesthetics for procedures on the rectum, perineum, and anus. A prone jackkni

26、fe position can be used for both administration of spinal anesthesia and the subsequent surgery. 3.Procedure a.The L2-3, L3-4, or L4-5 interspaces are commonly used for spinal anesthesia. The L3-4 interspace or the spinous process of L-4 are aligned with upper borders of the superior iliac crests. b

27、.Disinfect a large area of skin with an appropriate antiseptic solution. Care must be taken to avoid contamination of the spinal kit with antiseptic solution, which is potentially neurotoxic. c.Check the stylet for correct fit within the needle. d.Raise a skin wheal with 1% lidocaine and a 25-gauge

28、needle at the spinal puncture site. e.Approaches 1.Midline. Place the spinal needle (or introducer) through the skin wheal and into the interspinous ligament. The needle should be in the same plane as the spinous processes and angulated slightly cephalad toward the interlaminar space . Spinal needle

29、 insertion, lateral view. For the classic midline approach, the needle is introduced in the middle of the interspace and advanced with a slight cephalad angulation. If correctly angled (A), it will enter the interspinous ligament, ligamentum flavum, and epidural space. If bone is contacted, it may b

30、e the inferior spinous process (B), and cephalad redirection will identify the correct path. If angling cephalad causes contact with bone again at a shallower depth (C), it is probably the superior spinous process. If bone is encountered at the same depth after several attempts at redirection (not s

31、hown), the needle is most likely on the lamina lateral to the interspace, and the position of the true midline should be reassessed. (From Mulroy MF. Regional anesthesia: an illustrated procedural guide, 2nd ed. Boston: Little, Brown and Company, 1996:79, with permission.)2.Paramedian. This approach

32、 is useful in patients who cannot adequately flex their back because of pain or whose interspinous ligaments may be ossified. Place the spinal needle 1.5 cm lateral and slightly caudad (approximately 1 cm) to the center of the selected interspace. Aim the needle medially and slightly cephalad, passing lateral to the supraspinous ligament. If the lamina is contacted, redirect the needle and walk the tip off the lamina in a medial and cephalad direction. 3.Needle placement. Always keep the stylet in place when advancing the needle so that the needles lumen does not become plugged with tissue