What are the recommendations for performing a subarachnoid (spinal) block?

Subarachnoid Block (Spinal Anesthesia): Clinical Recommendations

Primary Technique and Drug Selection

For lower extremity surgery, use hyperbaric bupivacaine 0.5% at 10-12.5 mg (2-2.5 mL) via the subarachnoid route, as hyperbaric solutions produce more predictable blocks with fewer high blocks compared to isobaric solutions. 1

Anatomical Approach

• The Taylor approach targeting the L5-S1 interspace is particularly useful when midline lumbar approaches are difficult due to calcified ligaments, previous back surgery, or anatomical abnormalities, as this represents the largest intervertebral space in the spine 1
• Use atraumatic (pencil-point) needles rather than cutting bevel needles to reduce complication rates including post-dural puncture headache 1

Drug Administration Principles

• Administer the smallest dose and concentration required to produce the desired result 2
• Use fractional (incremental) doses rather than rapid injection of large volumes 2
• Avoid rapid injection as it increases risk of high or total spinal block 2

Immediate Post-Injection Monitoring

Monitor blood pressure frequently, especially during the first 15-30 minutes after positioning supine, as hypotension is the most frequent complication with an incidence of approximately 1 in 4367 cases for high/total spinal. 1, 3

Block Height Assessment

• Assess block height at least once every 5 minutes until no further extension is observed 4, 3
• Signs of developing high block include increasing agitation, significant hypotension, bradycardia, upper limb weakness, dyspnoea, or difficulty speaking 4

Hemodynamic Management

• Support circulation with vasopressors and intravenous fluids if hypotension develops 4, 3
• Administer supplemental oxygen immediately if signs of high block appear 4, 3
• Prepare for tracheal intubation and ventilation if respiratory compromise occurs 4, 3

Recovery Assessment and Ambulation Criteria

Test for straight-leg raising at 4 hours from the time of injection; if the patient cannot perform straight-leg raise at 4 hours, immediate comprehensive evaluation is required. 1, 3

Sequence of Neurological Recovery

• The sequence of return following spinal block is: sympathetic nervous system activity, pinprick sensation, somatic motor function, followed by proprioception in the feet 5
• Safe ambulation criteria include: (1) return of pinprick sensation in the peri-anal area (S4-5), (2) plantar flexion of the foot at pre-anesthetic levels of strength, and (3) return of proprioception in the big toe 5

Motor Block Monitoring

• Use the Bromage Scale for detailed evaluation of motor block 3, 6
• During labor: evaluate motor block every hour using straight-leg raise test 3
• After cesarean or other procedures: evaluate straight-leg raise capacity at 4 hours from last local anesthetic dose 3

Critical Safety Considerations

High and Total Spinal Block

• High or total spinal block occurs in approximately 1 in 4367 cases, requiring cardiovascular and/or respiratory support 4, 1, 3
• Unintentional subarachnoid injection during epidural block can result in high or total spinal blocks, hypotension, and even respiratory or cardiac arrest 4, 2
• Signs and symptoms of subarachnoid block typically start within 2-3 minutes of injection 2

Dosing Errors and Prevention

• Clear labeling of catheters and good communication between healthcare professionals are essential to prevent inadvertent administration of epidural doses intrathecally 4, 3
• This is particularly critical during patient transfer to the operating theatre 4
• Cases of high and total spinal anesthesia have been reported even after smaller doses (3 mL chloroprocaine 3% and 1.6 mL hyperbaric bupivacaine 0.75% with 15 mcg fentanyl) 4

Contraindications and Special Populations

Absolute Contraindications

• Active infection at injection site 3
• Coagulopathy or severe thrombocytopenia 3
• Patient refusal 2

High-Risk Situations

• Patients in poor general condition due to aging or compromising factors such as partial or complete heart conduction block, advanced liver disease, or severe renal dysfunction require special attention and dose adjustment 2
• Multiple attempts (limit to 4 attempts maximum) significantly increase complication risk 1

Alternative Drug Options

Ropivacaine for Spinal Anesthesia

• Ropivacaine 0.5% and 0.75% are sufficient and effective for intrathecal subarachnoid block 6
• Ropivacaine demonstrates lower risk of cardiovascular and CNS toxicity compared to bupivacaine 7
• Doses of 15 and 22.5 mg of ropivacaine resulted in sensory levels as high as T5 and T4, respectively 2

Management of Neurological Complications

Early Warning Signs

• Persistent motor block beyond expected duration requires immediate evaluation 3
• Symptoms of epidural hematoma or abscess include localized back pain, radiculopathy, fever, and spinal cord syndromes progressing from paraparesis to paraplegia 3

Urgent Intervention Protocol

• MRI of the spine is the preferred imaging modality 3
• Neurology, neurosurgery, or radiology consultation per local protocols 3
• Surgical intervention should ideally occur within 8-12 hours to prevent permanent neurological damage 3

Common Pitfalls to Avoid

• Never assume negative CSF aspiration excludes subarachnoid catheter placement 3
• Do not leave flexible container bags in place for more than 24 hours 2
• Avoid barbotage technique, as it does not facilitate control of analgesia level during isobaric spinal block and increases unpredictability 8
• Do not use intra-articular infusions of local anesthetics, as this has been associated with chondrolysis 2