Subarachnoid Spinal Block

Overview

Background

Subarachnoid (spinal) block is a safe and effective alternative to general anesthesia when the surgical site is located on the lower extremities, perineum (eg, surgery on the genitalia or anus), or lower body wall (eg, inguinal herniorrhaphy). Because of the technical challenges of readily identifying the epidural space and the toxicity associated with the large doses of local anesthetics needed for epidural anesthesia, spinal anesthesia was the dominant form of neuraxial anesthesia well into the 20th century.

Subarachnoid block can be used as the sole source of anesthesia. Alternatively, spinal and epidural anesthesia can be used jointly, taking advantage of the qualities of both techniques: the rapid, dense sensorimotor blockade of a spinal anesthetic and the opportunity to redose the patient with an epidural catheter anesthetic.

Spinal anesthesia produces intense sensory and motor blockade as well as sympathetic blockade. As opposed to epidural anesthesia, in which medications are instilled outside the dura mater, the goal of spinal anesthesia is to instill the desired medications into the cerebrospinal fluid (CSF). The sensorimotor block produced requires smaller doses of local anesthetics (hence, local anesthetic toxicity is rarely a concern) and is often more dense in character.

Although the focus of this topic is subarachnoid block, comparison with epidural anesthesia may be informative. For instance, brief periods (less than 24 hours) of postoperative analgesia can be facilitated by adding an opioid to the local anesthetic injected into the cerebrospinal fluid. Prolonged postoperative analgesia is best ensured by insertion of an epidural catheter, using an opioid and local anesthetic combination infused continuously over the first few postoperative days. See the table below for a comparison of subarachnoid and epidural anesthesia.

Table 1. Comparative Benefits of Subarachnoid and Epidural Anesthesia (Open Table in a new window)

Indications

Spinal anesthesia is a safe and effective alternative to general anesthesia when the surgical site is located on the lower extremities, perineum (eg, surgery on the genitalia or anus), or lower body wall (eg, inguinal herniorrhaphy). Cesarean deliveries are routinely performed under spinal anesthesia, as are total hip arthroplasty and total knee arthroplasty.

Advantages include avoidance of general anesthesia and the airway management concerns that accompany general anesthesia. However, that is not to suggest that spinal anesthesia is always the best course in a patient likely to have difficulties with endotracheal intubation. All patients with difficult airways, no matter what anesthetic plan is chosen, should have a well thoughtout plan for airway management, should it be needed.

Additional benefits may include reducing the metabolic stress response to surgery, reduction in blood loss, decrease in the incidence of venous thromboembolism, reduction in pulmonary compromise (particularly in patients with advanced pulmonary disease), and the ability to monitor the patient’s mental status.

Contraindications

Strong contraindications include patient refusal, lack of patient cooperation, difficulties with positioning, and increased intracranial pressure. Other contraindications include situations that require some risk-benefit analysis include hypovolemia, coagulation disturbances, stenotic valvular disease, bacteremia, and infection at the site of needle insertion.

Spinal anesthesia has also been noted to result in symptomatic deterioration in patients with multiple sclerosis.
Patients with chronic low back pain may decline spinal anesthesia out of concerns for increased low back pain. Performing spinal anesthesia in patients with degenerative lumbar spine disease or a prior history of lumbar surgery may prove technically difficult, but these are not necessarily contraindications.

Allergy to local anesthetics may also be a contraindication, but true allergies are usually found with ester-based local anesthetics (eg, tetracaine), not the amide-based local anesthetics (eg, bupivacaine), so finding a suitable local anesthetic is not challenging.

Technical Considerations

Best Practices

Although one-shot injection techniques are the norm, continuous spinal anesthesia has enjoyed periods of popularity while also being demonized. In the early 1990s, spinal microcatheters (27-G) were introduced but were followed by an increased incidence of postoperative cauda equina syndrome.
In cases in which cauda equina syndrome developed postoperatively, microcatheters were used; in response to an unsuitable rise in anesthetized dermatomal levels, unusually large of doses of local anesthetics (usually lidocaine) were administered to effect a sufficient spinal anesthetic.

What may have happened was that insufficient turbulence was created through injection through the microcatheter, the local anesthetic pooled distally in the lumbar intrathecal space (below the natural lumbar lordosis), and with repeated local anesthetic doses, administered in hopes of advancing the dermatomal level of local anesthetic effect, toxic local anesthetic levels were created in the region of the cauda equina.

Continuous spinal techniques may be regaining a slow resurgence in popularity, but patients should be carefully chosen. Instead of microcatheters, larger conventional epidural catheters should be used. Because of the larger rent in the dura, postdual puncture headache is an increased risk; therefore, patients who are less likely to have postdual puncture headache, such as older patients, are better candidates. The wisdom that excessive doses of local anesthetics are best not injected into the intrathecal space has been hard earned.

Procedure Planning

With any sympathectomy, blood pressure is expected to decrease secondary to increased venous capacitance and decreased peripheral vascular resistance. Incidence of hypotension is estimated at 35%. Bradycardia secondary to blockade of sympathetic-mediated cardioaccelerator nerves (T1-T4) may contribute to decreased cardiac output. The incidence is around 13%, and bradycardia is more likely to be found in children or adults with baseline heart rates less than 60 per minute and may be reversed with the anticholinergic medications atropine or glycopyrrolate.

Even in patients with ischemic heart disease, cardiac output appears maintained.
It is important but as yet unclear what level of blood pressure is appropriate under subarachnoid block. As this remains unclear, practitioners will invariably choose to support the patient’s blood pressure through use of vasopressor medications (the mixed alpha- and beta-agonist ephedrine and/or the alpha-agonist phenylephrine) and intravenous fluids. However, the value of intravenous fluid resuscitation in supporting blood pressure has been in dispute.
Perhaps because of the rapid redistribution of crystalloid out of the intravascular space, preloading the patient with these solutions may have minimal benefit for prevention hypotension. Prehydration with colloid solutions may be more effective. In a study of pregnant patients undergoing spinal anesthesia for cesarean section, having patients sit up for 5 minutes before placing them supine reduced requirements for intravenous fluids andephedrineanddecreased nausea, vomiting and dyspnea.

Tidal volumes tend to remain unchanged during subarachnoid block, although expiratory reserve is diminished secondary to paralysis of abdominal musculature. The gut is contracted due to unopposed parasympathetic activity. Hyperperistalsis may contribute to nausea and vomiting but, perhaps more commonly, nausea and vomiting are indicators of hypotension. Renal function is preserved.