Clinical Considerations Common to Spinal & Epidural Blocks
Neuraxial blocks may be used alone or in conjunction with general anesthesia for most procedures below the neck. Indeed, in some centers outside of North America, minimally invasive coronary artery surgery has been performed with thoracic epidural anesthe-sia alone. As a primary anesthetic, neuraxial blocks have proved most useful in lower abdominal, ingui-nal, 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.
If a neuraxial anesthetic is being considered, the risks and benefits must be discussed with the patient, and informed consent should be obtained. The patient must be mentally prepared for neuraxial anesthesia, and neuraxial anesthesia must be appro-priate for the type of surgery. Patients should under-stand that they will have little or no lower extremity motor function until the block resolves. Proce-dures that require maneuvers that might compro-mise respiratory function (eg, pneumoperitoneum or pneumothorax) or are unusually prolonged are typically performed with general anesthesia, with or without neuraxial blockade.
Major contraindications to neuraxial anesthe-sia include patient refusal, bleeding diathesis,severe hypovolemia, elevated intracranial pressure (particularly with an intracranial mass), and infec-tion at the site of injection. Other relative contrain-dications include severe aortic or mitral stenosis and severe left ventricular outflow obstruction (hyper-trophic obstructive cardiomyopathy); however, with close monitoring and control of the anesthetic level, neuraxial anesthesia can be performed safely in patients with valvular heart disease, particularly
if extensive dermatomal spread of anesthesia is not required (eg, “saddle” block spinal anesthetics).
Relative and controversial contraindications are also shown in Table 45–1. Inspection and pal-pation of the back can reveal surgical scars, scolio-sis, skin lesions, and whether the spinous processes can be identified. Although preoperative screening tests are not required in healthy patients undergo-ing neuraxial blockade, appropriate testing should be performed if the clinical history suggests a bleed-ing diathesis. Neuraxial anesthesia in the presence of sepsis or bacteremia could theoretically predispose patients to hematogenous spread of the infectious agents into the epidural or subarachnoid space, as has been shown for lumbar puncture in the presence of septicemia.
Patients with preexisting neurological deficits or demyelinating diseases may report worsening symptoms following a block. It may be impossible to discern effects or complications of the block from preexisting deficits or unrelated exacerbation of pre-existing disease. For these reasons, some risk-averse practitioners argue against neuraxial anesthesia in such patients. A preoperative neurological exami-nation should thoroughly document any deficits. In a retrospective study examining the records of 567 patients with preexisting neuropathies, 2 of the patients developed new or worsening neuropathy following neuraxial anesthesia. Although this find-ing indicates a relatively low risk of further injury, study investigators suggest that an injured nerve is vulnerable to additional injury, increasing the likeli-hood of poor neurological outcomes.
Regional anesthesia requires at least some degree of patient cooperation. This may be difficult or impossible for patients with dementia, psycho-sis, or emotional instability. The decision must be individualized. Unsedated young children may not be suitable for pure regional techniques; however, regional anesthesia is frequently used with general anesthesia in children.
Whether a block should be performed in the set-ting of anticoagulants and antiplatelet agents can be problematic. The American Society of Regional Anesthesia and Pain Medicine (ASRA) has issued several guidelines on this subject. Because guidelines are frequently revised and updated, practitioners are advised to seek the most recent edition. Although the incidence of epidural hematoma is reported to be quite low (1 in 150,000 epidurals), ASRA is concerned that the actual incidence may be somewhat higher. Moreover, the use of anticoagulant and antiplatelet medications continues to increase, placing an ever larger number of patients at potential risk of epidural hematomas. Because of the rarity of epidural hemato-mas, most guidelines are based on expert opinion and case series reviews, as clinical trials are not feasible.
If neuraxial anesthesia is to be used in patients receiving warfarin therapy, a normal prothrombin time and international normalized ratio should be documented prior to the block. Anesthesia staff should always consult with the patient’s primary physicians whenever considering the discontinua-tion of antiplatelet or antithrombotic therapy.
By themselves, aspirin and other nonsteroidal antiin-flammatory drugs (NSAIDs) drugs do not increase the risk of spinal hematoma from neuraxial anesthesia procedures or epidural catheter removal. This assumes a normal patient with a normal coagulation profile who is not receiving other medications that might affect clotting mechanisms. In contrast, more potent agents should be stopped, and neuraxial blockade should generally be administered only after their effects have worn off. The waiting period depends on the specific agent: for ticlopidine (Ticlid), it is 14 days; clopido-grel (Plavix), 7 days; abciximab (Rheopro), 48 hr; and eptifibatide (Integrilin), 8 hr. In patients with a recently placed cardiac stent, discontinuation of antiplatelet therapy can result in stent thrombosis and acute ST-segment elevation myocardial infarction. Risks versus benefits of a neuraxial technique should be discussed with the patient and the patient’s primary doctors.
“Minidose” subcutaneous heparin prophylaxis is not a contraindication to neuraxial anesthesia or epidural catheter removal. In patients who are to receive systemic heparin intraoperatively, blocks may be performed 1 hr or more before heparin administration. A bloody epidural or spinal does not necessarily require cancellation of surgery, but discussion of the risks with the surgeon and careful postoperative monitoring is needed. Removal of an epidural catheter should occur 1 hr prior to, or 4 hr following, subsequent heparin dosing.
Neuraxial anesthesia should be avoided in patients on therapeutic doses of heparin and with increased partial thromboplastin time. If the patient is started on heparin after the placement of an epi-dural catheter, the catheter should be removed only after discontinuation or interruption of heparin infu-sion and evaluation of the coagulation status. The risk of spinal hematoma (with or without neuraxial punc-ture) is unclear in the setting of full anticoagulation for cardiac surgery. Prompt diagnosis and evacuation of symptomatic epidural hematomas increase the likelihood that neuronal function will be preserved.
Many cases of spinal hematoma associated with neuraxial anesthesia followed the introduction of the
“low-molecular weight heparin” (LMWH) enoxapa-rin (Lovenox) in the United States in 1993. Many of these cases involved intraoperative or early post-operative LMWH use, and several patients were receiving concomitant antiplatelet medication. If an unusually bloody needle or catheter placement occurs, LMWH should be delayed until 24 hr post-operatively, because this trauma may increase the risk of spinal hematoma. If postoperative LMWH thromboprophylaxis will be utilized, epidural catheters should be removed 2 hr prior to the first LMWH dose. If already present, the catheter should be removed at least 10 hr after a dose of LMWH, and subsequent dosing should not occur for another 2 hr.
Neuraxial anesthesia should not be performed if a patient has received fibrinolytic or thrombolytic therapy.
Should lumbar neuraxial anesthesia, when used in conjunction with general anesthesia, be performed before or after induction of general anesthesia? This is controversial. The major arguments for having the patient asleep are that (1) most patients, if given a choice, would prefer to be asleep, and (2) the pos-sibility of sudden patient movement causing injury is markedly diminished. The major argument for neuraxial blockade while the patient is still awake is that the patient can alert the clinician to paresthe-sias and pain on injection, both of which have been associated with postoperative neurological deficits. Although many clinicians are comfortable perform-ing lumbar epidural or spinal puncture in anes-thetized or deeply sedated adults, there is greater consensus that thoracic and cervical punctures should, except under unusual circumstances, only be performed in awake patients. Pediatric neuraxial blocks, particularly caudal and epidural blocks, are usually performed under general anesthesia.
Neuraxial blocks should be performed only in a facil-ity in which all the equipment and drugs needed for intubation, resuscitation, and general anesthesia are immediately available. Regional anesthesia is greatly facilitated by adequate patient premedication.
Nonpharmacologic patient preparation is also very helpful. The patient should be told what to expect so as to minimize anxiety. This is particularly important in situations in which premedication is not used, as is typically the case in obstetric anesthesia. Supple-mental oxygen via a face mask or nasal cannula may be required to avoid hypoxemia when sedation is used. Minimum monitoring requirements include blood pressure and pulse oximetry for labor anal-gesia. Monitoring for blocks rendered in surgical anesthesia is the same as that in general anesthesia. Epidural steroid injections for management of pain (when little or no local anesthetic is injected) do not require continuous monitoring.
Spinous processes are generally palpable and help to define the midline. Ultrasound can be used when landmarks are not palpable (Figure 45–9). The spi-nous processes of the cervical and lumbar spine are nearly horizontal, whereas those in the thoracic spine slant in a caudal direction and can overlap significantly (Figure 45–2). Therefore, when per-forming a lumbar or cervical epidural block (with maximum spinal flexion), the needle is directed with only a slight cephalad angle, whereas for a thoracic block, the needle must be angled significantly more cephalad to enter the thoracic epidural space. In the cervical area, the first palpable spinous process is that of C2, but the most prominent one is that of C7 (vertebra prominens). With the arms at the side, the spinous process of T7 is usually at the same level as the inferior angle of the scapulae ( Figure 45–10). A line drawn between the highest points of both iliac crests (Tuffier’s line) usually crosses either the body of L4 or the L4–L5 interspace. Counting spinous processes up or down from these reference points identifies other spinal levels. A line connecting the posterior superior iliac spine crosses the S2 posterior foramina. In slender persons, the sacrum is easily palpable, and the sacral hiatus is felt as a depression just above or between the gluteal clefts and above the coccyx, defining the point of entry for caudal blocks.
The anatomic midline is often easier to appreciate when the patient is sitting than when the patient is in the lateral decubitus position ( Figure 45–11). This is particularly true 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 (arching the back “like a mad cat” maximizes the “target” area between adjacent spinous pro-cesses and brings the spine closer to the skin surface (Figure 45–12).
Many clinicians prefer the lateral position for neuraxial blocks (Figure 45–13). Patients lie on their side with their knees flexed and pulled high against the abdo-men or chest, assuming a “fetal position.” An assistant can help the patient assume and hold this position.
This position may be used for anorectal procedures utilizing an isobaric or hypobaric anesthetic solution . 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 subarach-noid needle tip placement will need to be confirmed by CSF aspiration. A prone position is typically used when fluoroscopic guidance is required.
The spine is palpated, and the patient’s body position is examined to ensure that the plane of the back is perpendicular to that of the floor. This ensures that a needle passed parallel to the floor will stay midline as it courses deeper (Figure 45–4). The depression
between the spinous processes of the vertebra above and below the level to be used is palpated; this will be the needle entry site. A sterile field is established with chlorhexidine or a similar solution. A fenes-trated sterile drape is applied. After the preparation solution has dried, a skin wheal is raised at the level of the chosen interspace with local anesthetic using a small (25-gauge) needle. A longer needle can be used for deeper local anesthetic infiltration.
Next, the procedure needle is introduced in the midline. Remembering that the spinous processes course caudad from their origin at the spine, the needle will be directed slightly cephalad. The subcu-taneous tissues offer little resistance to the needle. As the needle courses deeper, it will enter the supraspi-nous and interspinous ligaments, felt as an increase in tissue resistance. The needle also feels more firmly implanted in the back. If bone is contacted superfi-cially, a midline needle is likely hitting the lower spi-nous process. Contact with bone at a deeper level usually indicates that the needle is in the midline andhitting the upper spinous process, or that it is lateral to the midline and hitting a lamina. In either case, the needle must be redirected. As the needle pene-trates the ligamentum flavum, an obvious increase in resistance is encountered. At this point, the proce-dures for spinal and epidural anesthesia differ. For epidural anesthesia, a sudden loss of resis-tance (to injection of air or saline) is encountered as the needle passes through the ligamentum flavum and enters the epidural space. For spinal anesthesia, the needle is advanced through the epi-dural space and penetrates the dura–subarachnoid membranes, as signaled by freely flowing CSF.
The paramedian technique may be selected if epi-dural or subarachnoid block is difficult, particu-larly in patients who cannot be positioned easily (eg, severe arthritis, kyphoscoliosis, or prior spine surgery) (Figure 45–14). Many clinicians routinely use the paramedian approach for thoracic epidural
puncture. After skin preparation and sterile drap-ing (as previously described), the skin wheal for a paramedian approach is raised 2 cm lateral to the inferior aspect of the superior spinous process of the desired level. Because this approach is lateral to most of the interspinous ligaments and pen-etrates the paraspinous muscles, the needle may encounter little resistance initially and may not seem to be in firm tissue. The needle is directed and advanced at a 10–25° angle toward the midline. If bone is encountered at a shallow depth with the paramedian approach, the needle is likely in con-tact with the medial part of the lower lamina and should be redirected mostly upward and perhaps slightly more laterally. On the other hand, if bone is encountered deeply, the needle is usually in contact with the lateral part of the lower lamina and should be redirected only slightly craniad, more toward the midline (Figure 45–15).
With knowledge of the sensory dermatomes (see appendix), the extent of sensory block can be assessed by a blunted needle.
Although it has not, as of yet, transformed the prac-tice of neuraxial blockade in the same manner as it has for other procedures, ultrasound guidance can facilitate neuraxial blockade in patients with poorly palpable landmarks. As with other uses of ultra-sound, specific training is required for practitioners to identify correctly the landmarks and interspaces necessary for neuraxial blockade.
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