Neuraxial Anesthesia Lecture 7 PDF

Neuraxial Anesthesia Dr. Inas Shatnawi Anesthesia specialist Neuraxial Anesthesia Studies suggest that postoperative morbidity and mortality may be reduced when neuraxial blockade is used either alone or in combination with general anesthesia. Neuraxial blocks may red...

Neuraxial Anesthesia Dr. Inas Shatnawi Anesthesia specialist Neuraxial Anesthesia Studies suggest that postoperative morbidity and mortality may be reduced when neuraxial blockade is used either alone or in combination with general anesthesia. Neuraxial blocks may reduce the incidence of venous thrombosis and pulmonary embolism, cardiac complications in high-risk patients, bleeding and transfusion requirements, vascular graft occlusion, and pneumonia and respiratory depression following upper abdominal or thoracic surgery in patients with chronic lung disease.  The spinal column forms a double C, being convex anteriorly in the cervical and lumbar regions. The meninges are composed of three layers: the pia, arachnoid and the dura mater. Cerebrospinal fluid (CSF) is contained between the pia and arachnoid maters in the subarachnoid space. The epidural space is a better-defined potential space within the spinal canal that is bounded by the dura and the ligamentum flavum.  The spinal cord normally extends from the foramen magnum to the level of L1 in adults. In children, the spinal cord ends at L3 and moves up with age. Neuraxial Anesthesia The principal site of action for neuraxial blockade is believed to be the nerve root. Local anesthetic is injected into CSF (spinal anesthesia) or the epidural space (epidural and caudal anesthesia) and blocks the nerve root in the subarachnoid space or epidural space, respectively. Direct injection of local anesthetic into CSF for spinal anesthesia allows a relatively small dose and volume of local anesthetic to achieve dense sensory and motor blockade. The same local anesthetic concentration is achieved within nerve roots only with much larger volumes and quantities of local anesthetic molecules during epidural and caudal anesthesia. Neuraxial Anesthesia Indications: Neuraxial blocks may be used alone or in conjunction with general anesthesia for most procedures below the neck. As a primary anesthetic, neuraxial blocks are most useful in lower abdominal, inguinal, urogenital, rectal, and lower extremity surgery. Lumbar spinal surgery may also be performed under spinal anesthesia. Upper abdominal procedures (eg, gastrectomy) have been performed with spinal or epidural anesthesia, but because it can be difficult to safely achieve a sensory level adequate for patient comfort, these techniques are not commonly used. 1. Somatic block Sensory blockade interrupts both somatic and visceral painful stimuli. The size and character of the fiber types, and the fact that the concentration of local anesthetic decreases with increasing distance from the level of injection, explains the phenomenon of differential blockade during neuraxial anesthesia. Differential blockade typically results in sympathetic blockade (judged by temperature sensitivity) that may be two segments or more cephalad than the sensory block (pain, light touch), which, in turn, is usually several segments more cephalad than the motor blockade. 2. Autonomic block Interruption of efferent autonomic transmission at the spinal nerve roots during neuraxial blocks produces sympathetic blockade. Sympathetic nerve fibers exit the spinal cord with the spinal nerves from T1–L2. Neuraxial anesthesia does not block the Vagus nerve (main parasympathetic supply). The physiological responses of neuraxial blockade therefore result from decreased sympathetic tone and/or unopposed parasympathetic tone. 2. Autonomic block Neuraxial blocks produce variable decreases in blood pressure that may be accompanied by a decrease in heart rate. A high sympathetic block not only prevents compensatory vasoconstriction but may also block the sympathetic cardiac accelerator fibers that arise at T1–T4. Profound hypotension may result from arterial dilation and venous pooling combined with bradycardia. Unopposed vagal tone may explain the sudden cardiac arrest sometimes seen with spinal anesthesia. 2. Autonomic block Many considerations should be undertaken to minimize the degree of hypotension. Make sure the patient is not dehydrated or volume depleted. Although, volume loading with 10–20 mL/kg of intravenous fluid in a healthy patient before initiation of the block has been shown to fail to prevent hypotension. Left uterine displacement in the third trimester of pregnancy helps to minimize physical obstruction to venous return. Despite these efforts, hypotension may still occur and should be treated promptly. Excessive or symptomatic bradycardia should be treated with atropine, and hypotension should be treated with vasopressors. Ephedrine has direct and indirect β-adrenergic effects that increase heart rate and contractility and also produce vasoconstriction. 2. Autonomic block Surgical trauma produces a systemic neuroendocrine response. This systemic response includes increased concentrations of ACTH, cortisol, epinephrine, norepinephrine, and vasopressin levels, as well as activation of the renin– angiotensin–aldosterone system. Clinical manifestations include intraoperative and postoperative hypertension, tachycardia, hyperglycemia and altered renal function. Neuraxial blockade can partially or totally suppress the neuroendocrine stress response. 2. Autonomic block Neuraxial block-induced sympathectomy allows vagal tone dominance and results in a small, contracted gut with active peristalsis. Postoperative epidural analgesia with local anesthetics and minimal systemic opioids hastens the return of gastrointestinal function after open abdominal procedures. 2. Autonomic block Loss of autonomic bladder control results in urinary retention until the block wears off. If no urinary catheter is placed perioperatively, it is prudent to use the regional anesthetic of shortest duration sufficient for the surgical procedure and to administer the minimal safe volume of intravenous fluid.  Contraindication Major contraindications to neuraxial anesthesia include: 1. Patient refusal 2. Bleeding tendency 3. Severe hypovolemia 4. Elevated intracranial pressure 5. Infection at the site of injection. Other relative contraindications include 1. Severe valvular heart diseases 2. Sepsis or bacteremia could theoretically predispose patients to spread of the infectious agents into the epidural or subarachnoid space. 3. Patients with preexisting neurological deficits may report worsening symptoms following a block. 4. Patients with dementia, psychosis, or emotional instability Neuraxial Blockade in the Setting of Anticoagulants & Antiplatelet Agents Oral Anticoagulants (Warfarin)  a normal prothrombin time (PT) and INR should be documented prior to the block. Antiplatelet Drugs: 1. aspirin and other (NSAIDs) drugs do not increase the risk of spinal hematoma. 2. More potent agents should be stopped, and neuraxial blockade should be administered only after their effects have worn off. Ticlopidine (Ticlid)  14 days clopidogrel (Plavix) 7 days Neuraxial Blockade in the Setting of Anticoagulants & Antiplatelet Agents Standard (Unfractionated) Heparin Prophylactic “minidose” subcutaneous heparin is not a contraindication to neuraxial anesthesia. Neuraxial anesthesia should be avoided in patients on therapeutic doses of heparin and with increased partial thromboplastin time. Low-Molecular-Weight Heparin (LMWH): neuraxial anesthesia is preferred 12 hours after the last dose. Technical Considerations Neuraxial blocks should be performed only in a facility in which all the equipment and drugs needed for intubation, resuscitation, and general anesthesia are immediately available. Standard monitoring should be applied. Nonpharmacologic patient preparation is also very helpful. The patient should be told what to expect so as to minimize anxiety. Supplemental oxygen via a face mask or nasal cannula may be required to avoid hypoxemia when sedation is used. Technical Considerations Standard sterility should be applied. Surface Anatomy Spinous processes are generally palpable and help to define the midline. Cervical  The most prominent one is that of C7. Thoracic  the spinous process of T7 is usually at the same level as the inferior angle of the scapulae Lumbar  A line drawn between the highest points of both iliac crests usually crosses either the body of L4 or the L4–L5 interspace.  Patient Positioning 1- Sitting position: The anatomic midline is often easier to appreciate when the patient is sitting than when the patient is in the lateral position; particularly with very obese patients. Patients sit with their elbows resting on their thighs or a bed side table, or they can hug a pillow. Flexion of the spine maximizes the “target” area between adjacent spinous processes and brings the spine closer to the skin surface. Patient Positioning 2- Lateral Decubitus Many clinicians prefer the lateral position for neuraxial blocks. Patients lie on their side with their knees flexed and pulled high against the abdomen or chest, assuming a “fetal position.” An assistant can help the patient assume and hold this position. Patient Positioning 3- Buie’s (Jackknife) Position Th is position may be used for anorectal procedures. The advantage is that the block is done in the same position as the operative procedure, so that the patient does not have to be moved following the block. The disadvantage is that CSF will not freely flow through the needle, so that correct subarachnoid needle tip placement will need to be confirmed by CSF aspiration. Spinal anesthesia Spinal anesthetic solutiinjection inhibit conduction in nerve roots as they course through the subarachnoid space. The needle is advanced from skin through the deeper structures until two “pops” are felt. The first is penetration of the ligamentum flavum, and the second is penetration of the dura–arachnoid membrane. Successful dural puncture is confirmed by withdrawing the stylet to verify free flow of CSF. Spinal needles Spinal needles are available in many sizes, lengths, and bevel and tip designs. All should have a tightly fitting removable stylet that completely occludes the lumen to avoid tracking epithelial cells into the subarachnoid space. They can be divided into : 1. Sharp tip (ex. Quincke needle: a cutting needle with end injection.) 2. The blunt tip (pencil-point) needles  decreased the incidence of postdural puncture headache. Ex. The Whitacre needles have rounded points and side injection. Ex. The Sprotte is a side-injection needle with a long opening. In general, the smaller the gauge needle, the lower the incidence of headache. Factors Influencing Level of Spinal Block 1. Baricity of the local anesthetic solution: migration of the local anesthetic cephalad in CSF depends on its density relative to CSF (baricity). 2. Position of the patient during and immediately after injection With the patient in a head-down position, a hyperbaric solution spreads cephalad, and a hypobaric anesthetic solution moves caudad and vice versa. When a patient remains in a lateral position, a hyperbaric spinal solution will have a greater effect on the dependent (down) side, whereas a hypobaric solution will achieve a higher level on the nondependent (up) side 3. Drug dosage; the larger the dosage, the more cephalad the level of anesthesia. Factors Influencing Level of Spinal Block Hyperbaric solutions tend to move to the most dependent area of the spine (normally T4–T8 in the supine position). In the supine position, this should limit a hyperbaric solution to produce a level of anesthesia at or below T4. In the sitting position, “saddle block” can be achieved by keeping the patient sit ting for 3–5 min following injection, so that only the lower lumbar nerves and sacral nerves are blocked. Other factors: weight, height, age, pregnancy, the site of the injection. Spinal Anesthetic Agents Only preservative-free local anesthetic solutions are used. Addition of vasoconstrictors (epinephrine (0.1–0.2 mg)) and opioids enhance the quality and/or prolong the duration of spinal anesthesia. Hyperbaric bupivacaine and tetracaine are two of the most commonly used agents for spinal anesthesia. Both are relatively slow in onset (5–10 min) and have a prolonged duration (90–120 min). Lidocaine and procaine have a relatively rapid onset (3–5 min) and short duration of action (60–90 min).  Epidural Anesthesia Continuous epidural anesthesia is a neuraxial technique offering a range of applications wider than the typical all-or-nothing, single dose spinal anesthetic. An epidural block can be performed at the lumbar, thoracic, or cervical level. Sacral epidural anesthesia is referred to as a caudal block Epidural techniques are used for: surgical anesthesia, obstetric analgesia, postoperative pain control, and chronic pain management. Epidurals can be used as a single shot technique or with a catheter that allows intermittent boluses and/ or continuous infusion. Epidural anesthesia is slower in onset (10–20 min) and may not be as dense as spinal anesthesia. For example, by using relatively dilute concentrations of a local anesthetic combined with an opioid, an epidural provides analgesia without motor block. Epidural needle and catheter The standard epidural needle is typically 17–18 gauge, 3 or 3.5 inches long, and has a blunt bevel with a gentle curve of 15–30° at the tip. The Tuohy needle is most commonly used. The blunt, curved tip helps to push away the dura after passing through the ligamentum flavum instead of penetrating it. Placing a catheter into the epidural space allows for continuous infusion. Epidural catheters are useful for intraoperative epidural anesthesia and/or postoperative analgesia. Typically, a 19- or 20-gauge catheter is introduced through a 17- or 18-gauge epidural needle. Epidural Anesthesia Toxic side effects are likely if a “full epidural dose” is injected intrathecally or intravascularly. Safeguards against toxic epidural side effects include test and incremental dosing. A test dose is designed to detect both sub-arachnoid and intravascular injection: The classic test dose combines local anesthetic and epinephrine. Aspirating prior to injection is insufficient to avoid intravenous injection. Incremental dosing is a very effective method of avoiding serious complications Factors Affecting Level of Block Factors affecting the level of epidural anesthesia may not be as predictable as with spinal anesthesia. In adults, 1–2 mL of local anesthetic per segment to be blocked is a generally accepted guideline. For example, to achieve a T4 sensory level from an L4-L5 injection would require about 12–24 ml. Additives to the local anesthetic, particularly opioids, tend to have a greater effect on the quality of epidural anesthesia than on the duration of the block. Epidural Anesthetic Agents Commonly used short- to intermediate-acting agents for surgical anesthesia include chloroprocaine, lidocaine, and mepivacaine. Longer acting agents include bupivacaine, levobupivacaine, and ropivacaine. Only preservative-free local anesthetic solutions or those specifically labeled for epidural or caudal use are employed.  Caudal anesthesia The caudal space is the sacral portion of the epidural space. Caudal epidural anesthesia is a common regional technique in pediatric patients. It may also be used for anorectal surgery in adults. In children, caudal anesthesia is typically combined with general anesthesia for intraoperative supplementation and postoperative analgesia. It is commonly used for procedures below the diaphragm, including urogenital, rectal, inguinal, and lower extremity surgery. The hiatus may be felt as a groove or notch above the coccyx and between two bony prominences; the sacral cornua.