What is spinal anesthesia?

Spinal Anesthesia: A Comprehensive Overview

What is Spinal Anesthesia?

Spinal anesthesia is a regional anesthetic technique where local anesthetic medication is injected directly into the cerebrospinal fluid in the subarachnoid space (the space around the spinal cord), producing rapid and profound numbness and muscle relaxation below the level of injection. 1

Basic Mechanism and Technique

• The procedure involves inserting a thin needle between the vertebrae into the subarachnoid space, where cerebrospinal fluid flows around the spinal cord. 1
• Small doses of local anesthetic are sufficient because the drug acts directly on spinal nerve roots, making it highly efficient. 1
• The block produces deep surgical anesthesia quickly, typically within 5-10 minutes of injection. 1
• A free flow of cerebrospinal fluid through the spinal needle confirms correct needle placement in the subarachnoid space. 2

Local Anesthetic Medications Used

Standard Agents

• Bupivacaine is the most commonly used long-acting local anesthetic for spinal anesthesia. 2, 1
• Lower doses of bupivacaine (less than 10 mg) significantly reduce the risk of hypotension while maintaining adequate anesthesia. 2, 3
• Levobupivacaine (the pure S-enantiomer) has lower cardiovascular and central nervous system toxicity compared to standard bupivacaine. 1
• Ropivacaine is another long-acting alternative with a favorable safety profile. 1

Short-Acting Agents for Day Surgery

• For outpatient procedures, shorter-acting agents like prilocaine 2% and 2-chloroprocaine are preferred to allow faster discharge. 2
• These agents provide adequate surgical anesthesia while minimizing prolonged motor blockade that delays mobilization. 2

Opioid Additives

• Intrathecal fentanyl is the preferred opioid additive for prolonging postoperative analgesia. 2, 3
• Fentanyl causes less respiratory depression and cognitive impairment compared to morphine or diamorphine. 2
• Morphine and diamorphine should be avoided due to greater risks of respiratory and cognitive depression, especially in elderly patients. 2

Dosing Strategies and Positioning

Dose Considerations

• The total dose of local anesthetic is the most important determinant of both therapeutic effects and side effects. 1
• Hyperbaric (heavy) bupivacaine can be used with specific patient positioning to target the anesthetic to one side or specific surgical area. 2
• For hip fracture surgery in elderly patients, positioning the patient laterally with the fractured hip down (inferior) may reduce hypotension. 2

Targeted Spinal Techniques

• Appropriate spinal dosing targeted to the surgical site minimizes side effects - for example, lateral positioning for unilateral knee arthroscopy or sitting position for perianal procedures. 2
• Small-dose spinal initiation techniques allow for adequate anesthesia with reduced cardiovascular effects. 2

Advantages Over General Anesthesia

Metabolic and Physiological Benefits

• Spinal anesthesia blunts the endocrine-metabolic stress response to surgery more effectively than general anesthesia. 4
• Blood loss is reduced during surgery, particularly for major lower limb orthopedic procedures. 4
• Thromboembolic complications (blood clots) are decreased compared to general anesthesia. 4

Specific Patient Populations

• For elderly patients undergoing major orthopedic surgery, short-term mortality may be reduced with spinal anesthesia, though long-term survival is not affected. 4
• The Scottish Intercollegiate Guidelines Network recommends that spinal or epidural anesthesia should be considered for all patients undergoing hip fracture repair unless contraindicated. 2
• For patients with severe pulmonary dysfunction who cannot tolerate general anesthesia and mechanical ventilation, spinal anesthesia provides an important alternative. 5

Drug Interaction Benefits

• Spinal anesthesia avoids potential proconvulsant effects of certain general anesthetic agents in patients with epilepsy. 3
• It eliminates concerns about drug interactions between general anesthetic agents and antiepileptic medications. 3

Comparison with Epidural Anesthesia

Advantages of Spinal Over Epidural

• Spinal anesthesia provides more rapid onset of anesthesia compared to epidural blockade. 4
• The level of analgesia is more predictable with spinal technique. 4
• Surgical anesthesia is more profound and reliable with spinal anesthesia. 4
• The needle-through-needle combined spinal-epidural technique is more reliable than epidural alone for operative delivery. 2

Disadvantages of Spinal Compared to Epidural

• Spinal anesthesia is associated with greater degree of hypotension compared to epidural anesthesia. 4
• Epidural anesthesia uses higher local anesthetic doses, is slower to establish, and provides poorer sacral (lower spine) blockade. 2
• Epidural catheters allow for prolonged anesthesia through repeated dosing, while single-shot spinal has fixed duration. 2

Combined Spinal-Epidural Technique

Technique Description

• The needle-through-needle combined spinal-epidural technique uses the epidural needle as a guide for placing the spinal needle. 2
• This technique requires locating two anatomical spaces (epidural and subarachnoid), but the epidural needle guides the spinal placement. 2
• Free flow of cerebrospinal fluid through the spinal needle confirms correct midline placement within the epidural space. 2

Advantages and Disadvantages

• Combined spinal-epidural is chosen for its perceived reliability and the ability to "top up" the epidural catheter if needed. 2
• It allows small-dose spinal initiation with the safety net of epidural supplementation. 2
• The main disadvantage is that the epidural catheter cannot be tested until it may be too late to correct its position. 2
• This technique led to a number of failures in achieving the intended anesthetic effect. 2

Continuous Spinal Anesthesia

• Continuous spinal anesthesia involves placing a catheter in the subarachnoid space for repeated dosing or continuous infusion. 2
• This technique has theoretical advantages including ability to titrate anesthesia and prolong duration. 2
• It has more evidence base for surgery than for labor analgesia. 2
• Major limitations include lack of practitioner experience, unavailability of microspinal equipment in some countries, and risk of post-dural puncture headache with larger catheters. 2
• Continuous spinal anesthesia is currently considered a technique of last resort. 2

Side Effects and Management

Hypotension (Low Blood Pressure)

• Cardiovascular effects from sympathetic nervous system blockade are the most frequent side effects, but are successfully treated. 1
• Volume expansion with intravenous fluids and administration of vasoactive drugs (medications that raise blood pressure) effectively manage hypotension. 1
• Restricting intravenous fluids to no more than 500 ml reduces the incidence of urinary retention. 2
• Lower doses of local anesthetic (less than 10 mg bupivacaine) significantly reduce associated hypotension. 2, 3

Post-Dural Puncture Headache

• Concerns about post-dural puncture headache have historically limited use of spinal anesthesia in day surgery patients. 2
• Using smaller gauge needles (25 gauge or smaller) and pencil-point (non-cutting) needle designs has reduced the incidence to less than 1%. 2
• Information about post-dural puncture headache and management instructions should be included in discharge information. 2

Urinary Retention

• Limiting intravenous fluids helps reduce urinary retention risk. 2
• Patients should be encouraged to drink postoperatively to allow their body to self-correct fluid balance. 2
• Voiding before discharge is not always required, though high-risk patients (prolonged bladder instrumentation) should be identified. 2

Monitoring Requirements

Standard Monitoring

• Standard monitoring including continuous pulse oximetry, ECG, and blood pressure is sufficient for patients undergoing spinal anesthesia. 3
• Supplemental oxygen should always be provided during spinal anesthesia. 2
• Continuous core temperature monitoring is crucial to maintain normothermia (normal body temperature). 6

Special Considerations

• Invasive arterial pressure monitoring should be considered for high-risk cases with extensive surgery or significant associated medical conditions. 6
• Neuromuscular monitoring may be needed if any muscle relaxants are used as adjuncts. 6

Discharge Criteria After Spinal Anesthesia

Mobilization Criteria

• Nursing staff should follow strict criteria to enable safe mobilization after spinal anesthesia. 2
• Required criteria include: return of sensation to the perianal area (S4-5 nerve roots), plantar flexion of the foot at pre-operative strength levels, and return of proprioception (position sense) in the big toe. 2
• These criteria may be affected by supplementary local anesthetic infiltration or regional blocks used for longer-acting analgesia. 2

General Discharge Considerations

• Nurse-led discharge using agreed protocols should be the standard pathway. 2
• Protocols may be adapted to allow low-risk patients to be discharged without fulfilling all traditional criteria. 2
• Mild postoperative confusion in elderly patients is common and usually insignificant, and should not influence discharge if social circumstances permit. 2

Pain Management After Spinal Wears Off

• An analgesic plan is required for when the spinal block wears off, otherwise patients may experience significant pain. 2
• This should include premedication with oral analgesics before surgery, plus postoperative oral analgesics with written instructions about timing. 2
• Multimodal opioid-sparing analgesia combining paracetamol (acetaminophen) and NSAIDs is recommended unless contraindicated. 6
• Regional techniques such as peripheral nerve blocks should be considered for extended postoperative analgesia. 6

Contraindications

Absolute Contraindications

• Coagulopathy (bleeding disorders) or anticoagulation are standard contraindications to spinal anesthesia that must be verified before proceeding. 3
• Patient refusal is an absolute contraindication. 2
• Infection at the injection site prevents safe needle insertion. 1

Relative Contraindications and Special Considerations

• Anatomical abnormalities may require individualized assessment, including: availability of appropriately low interspace, possibility of spinal cord or cauda equina tethering, previous back surgery, absence of epidural space, short stature, and high body mass index. 2
• Risk of rapid onset or excessively high block must be considered in certain patients. 2
• Risk of cerebrospinal fluid leak in patients with raised intracranial pressure requires careful evaluation. 2

Sedation Considerations

• Sedation is seldom required during spinal anesthesia and should be used cautiously, especially in very elderly patients. 2
• Midazolam and propofol are commonly used when sedation is needed. 2
• Ketamine may be used theoretically to counteract hypotension, but may be associated with postoperative confusion. 2
• Avoiding oversedation is essential as it can mask important neurological signs, and patient pain perception is a safety mechanism that should be preserved. 3
• In arthroscopic procedures, patients may wish to observe the procedure without sedation, and surgeons can explain findings in real-time. 2

Special Clinical Contexts

Obstetric Use

• Regional analgesia including spinal techniques should be considered to aid labor management when planned, with ability to extend blockade for operative delivery. 2
• Factors associated with failure to extend labor regional analgesia for cesarean section include: increased number of analgesic boluses during labor, increased urgency category for surgery, and non-specialized obstetric anesthetist providing care. 2
• Spinal anesthesia for cesarean section in younger patients differs conceptually from use in elderly patients requiring different dosing strategies. 2

Orthopedic Surgery

• Spinal anesthesia is particularly beneficial for major lower limb orthopedic procedures due to reduced blood loss and thromboembolic complications. 4
• For hip fracture surgery, either spinal or general anesthesia is recommended, but simultaneous administration is associated with precipitous blood pressure drops and should be avoided. 2
• Peripheral nerve blockade should always be considered as an adjunct to extend non-opioid postoperative analgesia. 2

Day Surgery

• Modern short-acting local anesthetics make spinal anesthesia increasingly suitable for outpatient procedures. 2, 1
• Patients having spinal or regional anesthesia with minimal or no sedation can often bypass first-stage recovery and be fast-tracked. 2
• The key is matching the duration of spinal block to the surgical procedure and expected discharge time. 2, 1

Safety Profile

• Spinal anesthesia can be considered adequately safe, and severe complications are reasonably rare. 1
• A scientific rationale exists for the continued use of spinal anesthesia based on clinical studies showing benefits for certain patients and procedures. 4
• The technique has evolved significantly with development of new drugs and special techniques, improving clinical utility. 1

Emerging Practices

• While general anesthesia remains most common for spine surgery, regional anesthesia (especially non-neuraxial techniques like fascial plane blocks) is emerging practice. 7
• The popularity of fascial plane blocks for spinal surgery has significantly increased since 2017. 7
• Regional and neuraxial techniques may improve postoperative pain relief, quality of recovery, and patient satisfaction. 7
• More research is needed to determine superiority of regional techniques over conventional approaches for various surgical procedures. 8, 7