Spinal Anesthesia: Exploring Techniques Beyond the Routine

Introduction and Setting the Stage
What is the central question being explored in this presentation?
The presentation explores the question: "Spinal anesthesia, can it be different?" It challenges the routine practice of single-shot lumbar spinal anesthesia with hyperbaric bupivacaine and introduces alternative techniques.

Why is there often an over-reliance on traditional single-shot lumbar spinal anesthesia?
This over-reliance is often due to a combination of factors: ignorance about the modern anatomy of the spinal canal, unawareness of other drug solutions (isobaric, hypobaric), perceived difficulty in performing blocks in lateral or prone positions, difficulty in changing acquired habits, lack of knowledge about anterior and posterior nerve roots, unawareness of the safety of thoracic puncture, and a general lack of knowledge about continuous spinal anesthesia.

What are the alternative techniques to a standard single-shot spinal that were discussed?
The alternatives discussed include: segmental spinal anesthesia (also known as thoracic spinal), hypobaric spinal anesthesia, and continuous spinal anesthesia.


Segmental (Thoracic) Spinal Anesthesia
What is the evolving perspective on segmental spinal anesthesia?
The perspective has evolved from questioning its possibility and safety to recognizing it as a useful and trending technique, especially during the pandemic when avoiding general anesthesia was desirable. A 2022 article in the British Journal of Anaesthesia helped define its role in the 21st century, concluding it is a safe, feasible, and effective alternative for high-risk patients.

What is the ideal definition of segmental spinal anesthesia?
The ideal definition is blocking only the required dermatomes essential for the proposed surgical procedure using a very low volume of local anesthetic injected near the targeted nerve groups. Lower volumes of the drug make a true segmental block more likely.

What anatomical factors make segmental spinal anesthesia feasible at the thoracic level?
Several factors contribute: the spinal cord is positioned anteriorly, leaving a significant space behind it; the thoracic nerve roots are slender and thin; the amount of cerebrospinal fluid (CSF) is comparatively less, leading to less anesthetic dilution; and the natural thoracic kyphosis aids in the block.

What are the primary safety concerns associated with segmental spinal anesthesia?
The main concerns are: potential neurological injury from a dual puncture at low thoracic or high lumbar levels, ventilatory impairment from extensive thoracic nerve blocks, bradycardia and hypotension from blocking cardio-accelerator fibers, and unresolved medical-legal issues as the technique is not yet in standard textbooks.

How does the anatomy of the spinal canal protect against spinal cord damage during a thoracic puncture?
MRI and cadaveric studies show that in the thoracic segments, the spinal cord is positioned anteriorly, leaving a significant posterior space (e.g., 5-8mm). This distance is increased further when the patient is in a sitting position with the neck flexed, as the cord moves more anteriorly. The required needle angulation (40-45 degrees) also increases this distance.

Is there evidence supporting the safety of intentionally puncturing the dura at thoracic levels?
Yes, studies on accidental dural puncture during thoracic epidurals show a very low incidence of neurological complications. Additionally, many anesthesiologists unintentionally perform high lumbar or low thoracic spinals due to inaccuracies in palpating landmarks, with few reported issues.

What is the risk of ventilatory impairment with a high thoracic block?
The primary muscle of respiration, the diaphragm, is usually unaffected. While forceful expiration and coughing might be impaired due to paralysis of abdominal muscles, the low drug doses used in segmental spinal typically preserve coughing ability by causing minimal motor weakness and predominantly a sensory block.

How does segmental spinal anesthesia mitigate bradycardia and hypotension?
While the block may extend to T1-T4, affecting cardio-accelerator fibers, the lumbar spine and sacral sparing prevents significant venodilation in the lower limbs. This maintains right atrial filling, prevents the initiation of the Bezold-Jarisch reflex, and sustains heart rate. Hypotension is also less due to reduced venodilation.

What is the medico-legal standing of segmental spinal anesthesia?
Although it is still not widely covered in standard textbooks, there is now substantial evidence and references proving its utility, especially when general anesthesia is contraindicated. If a thoracic epidural (intentional dural puncture with a large needle) can be justified, then an intentional dural puncture with a fine spinal needle can also be justified with proper explanation and consent.

Is segmental spinal anesthesia technically and operationally feasible?
Technical Feasibility:
At low thoracic spaces, it is as easy as a lumbar spinal. At higher levels, it can be more difficult, but those experienced with thoracic epidurals may not find it very challenging.

Economic Feasibility:
It is a very economical technique.

Operational Feasibility:
It can be performed in any setup, from freelance to corporate, with a good learning curve.


For what types of surgeries can segmental spinal anesthesia be used?
It can be used for practically all intra-abdominal surgeries (upper or lower), breast surgeries, superficial thoracic surgeries, some awake thoracoscopic surgeries (bullectomy, thymectomy, lung volume reduction), and some prone or lateral position musculoskeletal spine surgeries.

What are the different modes of using segmental spinal anesthesia?
  1. Single-shot spinal: For short to mid-duration surgeries (up to 120 minutes).
  2. Combined spinal-epidural (CSE): For longer surgeries, or in morbid patients where a low intrathecal dose is used, with the epidural acting as a backup or for post-op analgesia.
  3. Continuous spinal anesthesia (CSA): Using spinal catheters for titrated doses, especially in morbid cases or after accidental dural puncture.

What are the drug options for segmental spinal anesthesia, and how do they compare?
Isobaric Drugs:
Preferred for most procedures. They are less sensitive to positioning, provide good hemodynamic stability, and in low doses, offer selective sensory block (sparing motor function). They can be given in the operating position. Disadvantages include an inability to modify the block level by repositioning, slower onset at lumbar levels, and slightly less muscle relaxation in muscular patients.

Hyperbaric Drugs:
Can be a better choice for open surgeries in muscular patients when good relaxation is needed. Onset time to reach T3 levels is reduced. Disadvantages include gravity-dependent spread that must be carefully managed.

Hypobaric Drugs:
Not very useful alone due to unpredictability, but can be combined with isobaric or hyperbaric drugs to achieve desired levels.


How does the site of injection affect the spread of isobaric drugs?
The principle is that 1 mL of isobaric drug spreads 2-3 segments above and below the site of injection. For most abdominal surgeries, injecting 2-2.5 mL at the T10-L1 space (the center of the surgical field) is sufficient to block segments from T2 to S1.

What levels of block are required for different types of breast surgery?

What is the recommended approach for locating thoracic intervertebral spaces?
Landmarks include counting down from the prominent C7 spinous process, the root of the spine of the scapula (T3), the inferior angle of the scapula (T7), or the intercristal line (L4-L5). However, palpation is inaccurate (only 29-41% correct). Ultrasound is a much more accurate tool for pre-procedure scanning to identify the level, depth, and midline.

What are the anatomical hurdles for a thoracic spinal and how can they be overcome?
Thoracic spinous processes are sharply angled (between T4-T9), and the interlaminar spaces are narrow. A midline approach can be challenging. A paramedian or a paraspinous approach is often preferred to access the interlaminar space.

What are the key 'Do's and Don'ts' for performing segmental spinal anesthesia safely?
Do's:
Know your technique, drugs, and surgeon. Know the patient's comorbidities and willingness. Use a position you are comfortable with. Perform a pre-procedural ultrasound scan if possible. Use a CSE kit if combining with epidural. Always use multimodal analgesia and have a backup plan ready.

Don'ts:
Don't force an unwilling surgeon. Don't advance the needle millimeter by millimeter without checking for CSF flow. Don't proceed if there is any paresthesia. Don't use high doses of local anesthetics at mid/high thoracic spaces. Don't add too many additives if you are not accustomed to their effects. Don't use excessive sedation, especially with high blocks, as it can change respiratory patterns.



Hypobaric Spinal Anesthesia
How is a hypobaric solution prepared for spinal anesthesia?
A hypobaric solution is created by diluting a plain (isobaric) local anesthetic with distilled water. Hyperbaric drugs cannot be made hypobaric, as any amount of dextrose will keep them hyperbaric. Adding fentanyl, which is hypobaric, can also help make a solution hypobaric.

What are the clinical applications and advantages of hypobaric spinal anesthesia?
Applications:
It is useful for anorectal surgeries in the jackknife position, and for unilateral spinal anesthesia for lower limb surgeries (by positioning the operative side up). It can also produce a "posterior spinal hemi-anesthesia" for superficial back surgeries when given in the prone position.

Advantages:
It provides profound sensory block with minimal motor involvement, leading to negligible hemodynamic fluctuations, early ambulation, and suitability for daycare surgeries in high-risk patients.

Disadvantages:
The need for on-table preparation increases the chance of contamination. The spread can be unpredictable due to the marginal density difference between the drug and CSF, and muscle relaxation is less, which surgeons must be accustomed to.



Continuous Spinal Anesthesia (CSA)
What is continuous spinal anesthesia (CSA) and when should it be considered?
CSA involves producing and maintaining spinal anesthesia with small, intermittent doses of local anesthetic injected into the subarachnoid space via an indwelling catheter. It is useful in difficult clinical situations like difficult airways, prolonged surgeries, morbid obesity, severe cardiac disease, and patients with previous spine surgery or deformities. It can also be used unintentionally after an accidental dural puncture.

What equipment is used for continuous spinal anesthesia?
Equipment ranges from standard epidural macro catheters to specially designed micro catheter sets (e.g., catheter-over-needle sets). These kits often include a 22G or 24G catheter and a fine Quincke needle. Pediatric epidural catheters can also be used.

What are the key considerations and potential complications with CSA?
Technical Considerations:
Only insert the catheter 2-3 cm into the subarachnoid space to avoid coiling or a caudally directed catheter. Never withdraw the catheter back through the needle to prevent shearing. Clearly label the catheter as "SPINAL CATHETER" to avoid accidental misuse.

Complications/Risks:
Risks include infection, post-dural puncture headache (PDPH), and potential for catheter breakage. Catheters should not be left in place for more than 24 hours. To reduce PDPH risk, it is advised to leave the catheter in for 24 hours and consider injecting preservative-free saline before removal.


How is continuous spinal anesthesia managed, particularly for labor analgesia?
Management can be through: For an emergency C-section, an initial bolus of 5-7.5 mg of hyperbaric bupivacaine can be given, followed by 2.5 mg increments every 3-5 minutes to achieve the desired level.

Is placing an intrathecal catheter after an accidental dural puncture during an epidural beneficial?
Yes, it can be beneficial. While the initial reflex is to pull the needle out, the dural damage is already done. Placing a catheter and leaving it in situ for 24 hours can actually lower the risk of PDPH, especially in morbidly obese and obstetric patients, by sealing the dural hole.


Q&A and Discussion Highlights
How do you prepare a hypobaric solution from an isobaric one, and can a hyperbaric drug be made hypobaric?
A hypobaric solution is made by adding distilled water to a plain isobaric drug. For example, adding 3 mL of distilled water to 2 mL of plain bupivacaine creates a hypobaric solution. Hyperbaric drugs, even with a tiny amount of dextrose, will remain hyperbaric regardless of further dilution and cannot be made hypobaric.

Does temperature affect the baricity of local anesthetics?
Yes, but gross differences matter. A drug at room temperature injected into the body (37°C) can have a different baricity. However, the small volume of injectate typically used warms up quickly to body temperature, so this effect is minimal in practice.

Does kyphosis or scoliosis affect the level of blockade with isobaric drugs?
It doesn't usually affect the level of blockade itself, but it can make identifying and accessing the correct intervertebral space difficult. Isobaric drugs depend more on the site and volume of injection than gravity, so careful space selection is key.

What causes diaphragmatic movement that can disturb the surgeon during laparoscopic surgery under segmental spinal?
This is often due to heavy sedation, which changes the patient's natural breathing pattern, not the block level itself. It's advised to avoid heavy sedation unless the airway is secured. If the block levels are adequate, patients usually do not exhibit such movements.

Is dexmedetomidine FDA-approved for intrathecal use, and what is its effect?
It is not FDA-approved for intrathecal use, but there are many reports of its use. It provides excellent results, prolonging both sensory and motor block significantly. However, it must be dosed cautiously (in micrograms), as a small error can lead to prolonged motor blockade for several hours.

How do you determine the correct intervertebral space without ultrasound?
Traditional anatomical landmarks are used: the prominent spinous process of C7, the root of the spine of the scapula (T3), and the inferior angle of the scapula (T7). However, these are arbitrary, and palpation can be off by 3-4 spaces, especially in obese patients.

What is the longest surgical case you have managed with segmental spinal?
Cases lasting around 5-6 hours have been managed. For longer surgeries, the mode of anesthesia is chosen accordingly. An initial single-shot spinal might cover 2 hours, and then an epidural component or continuous spinal catheter is used to supplement for the remaining duration.

How do you manage a patient with prior spine surgery for a subsequent surgery requiring spinal anesthesia?
The surgical area is avoided. The advantage of segmental spinal is that the drug and the space can be chosen according to the desired effect. For example, if a higher level is needed, a lower space can be used with an isobaric or hypobaric drug.

Why are morbidly obese patients thought to have a lower incidence of PDPH?
It is hypothesized that the increased intra-abdominal and epidural fat may compress the thecal sac, helping to seal the dural puncture site and thus reducing CSF leak and the subsequent headache.