20120619

Spinal Anesthesia The Subarachnoid Block

Indications 
Anesthesia for any surgical procedure below the level of T4 requiring sensory loss with or without motor blockade not requiring a secured airway or mechanical ventilation Anesthesia for a vaginal or cesarean delivery 
Physiology of Spinal Anesthesia 
Local anesthetic solution injected into the subarachnoid space blocks conduction of impulses along all nerves with which it comes in contact, although some nerves are more easily blocked than others. There are three classes of nerve: motor, sensory and autonomic. The motor convey messages for muscles to contract and when they are blocked, muscle paralysis results. Sensory nerves transmit sensations such as touch and pain to the spinal cord and from there to the brain, whilst autonomic nerves control the caliber of blood vessels, heart rate, gut contraction and other functions not under conscious control. Generally, autonomic and pain fibers are blocked first and motor fibers last. This has several important consequences. For example, vasodilation and a drop in blood pressure may occur when the autonomic fibers are blocked and the patient may be aware of touch and yet feel no pain when surgery starts. 
Anatomy Ligaments
Supraspinous/Interspinous Ligaments
Connect adjacent spinous processes


Ligamentum flavum
Connects the lamina of the adjacent vertebrae
Anterior/Posterior Longitudinal Ligaments
The primary supportive ligaments of the vertebral column
Binds vertebral bodies and provides stability

Blood Supply
Arterial Supply 
The two posterior spinal arteries arise from the vertebrals and supply the posterior 1/3 of the cord. The anterior spinal artery arises from the vertebrals and supplies the anterior 2/3 of the cord. The radicular arteries enter every intervertebral foramen and supply the spinal nerve roots The radiculospinal branches arise from the vertebral arteries and the aorta. Of these, the largest is the Artery of Adamkiewicz. It supplies much of blood flow to anterior spinal artery. 
Venous drainage 
Anterior spinal vein 
Posterior spinal vein 


Positions
Sitting Position
No torque
Chin on chest
Arms resting on knees
Footstool/Table to support feet





Lateral Position
Shoulders perpendicular to bed
Positioned with hips on edge of bed
Hugging pillow/knee chest position







Approaches for Spinal Anesthesia
Median Approach.
The most common approach, the needle or introducer is placed midline, perpendicular to spinous processes, aiming slightly cephalad. Paramedian Approach. Indicated in patients who cannot adequately flex because of pain or whose ligaments are ossified, the spinal needle is placed 1.5 cm laterally and slightly caudad to the center of the selected interspace. The needle is aimed medially and slightly cephalad and passed lateral to the supraspinous ligament. If the lamina is contacted, the needle is redirected and "walked off" in a medial and cephalad direction.
Taylor or Lumbosacral Approach.
This approach is useful in patients with calcified or fusion of higher intervertebral spaces. The injection site is 1cm medial and 1cm caudad of the posterior iliac spine. The needle is directed 45 degrees medial and 45 degrees caudad, after contacting the lamina the needle is walked upward and medially to enter the L5-S1 interspace. 

Procedure
Anatomic landmarks for the desired level of the block are first identified.
Superior Iliac crests palpated and L4 is identified.
The spine is palpated to ensure spine position with relation to the plane of the floor.
A sterile field is established with povidone-iodine applied with three basic sponges, the solution is applied starting from the injection site moving outward in a circular fashion.
A fenestrated drape is applied, and using a sterile gauze, wipe the iodine from the injection site to avoid initiation into the subarachnoid space.
A skin wheal is raised with 2cc of 1% lidocaine using a 25G needle to the selected space.


A 17G introducer is passed through the skin wheal, angled slightly cephalad through the epidermis, dermis, sub Q, supraspinous ligament, interspinous ligament, stopping in the ligamentum flavum.
A 25G choice needle is inserted into the introducer, passing through the epidural space, dura, and arachnoid to the sub arachnoid space stopping when the presence of CSF is determined.
CSF is aspirated and mixing lines are identified as a change in baricity and temperature as the local anesthetic and CSF mix in the syringe.
The dose is slowly injected, aspirating after instillation.
All needles are removed intact and the patient is positioned.

Monitoring
It is essential to monitor the respiration, pulse and blood pressure closely. The blood pressure can fall precipitously following induction of spinal anesthesia, particularly in the elderly and those who have not been adequately preloaded with fluid. Warning signs of falling blood pressure include pallor, sweating or complaining of nausea or feeling generally unwell.
For example, a moderate fall in systolic blood pressure to 80mmHg in a young fit patient or 100mmHg in an older patient is acceptable, provided the patient looks and feels well and is adequately oxygenated.
Bradycardia is quite common during spinal anesthesia particularly if the surgeon is manipulating the bowel or uterus. If the patient feels well, and the blood pressure is maintained, then it is not necessary to give atropine. If, however, the heart rate drops below 50 beats per minute or there is hypotension, then atropine 300-600mcg should be given intravenously.
It is generally considered good practice for all patients undergoing surgery under spinal anesthesia to be given supplemental oxygen by face mask at a rate of 2-4 liters/minute, especially if sedation has also been given.

Spinal Needles
 
Pencil Point Needles (Sprotte)
Designed to spread the dural fibers and help reduce the occurrence of post dural puncture headache
Yields a distinct "pop" as the pencil point penetrates the dura
Offers increased "tip strength" to minimize bending or breakage
Precision-formed side hole enables directional flow of anesthetic and reduces the possibility of straddling the dura
Tracks straight when advancing through ligaments toward the dura
 
 

Cutting Needle (Quincke)
Dural "pop" is less likely to be appreciated due to the sharper tip
Increased risk of Postdural Puncture Headache due to increased trauma to the dura
Introducer may not be necessary depending on patient size 

Common Local Anesthetics 
 
Local Anesthetic
+/-  Fentanyl 0.25mcg/kg or 0.25-0.5mg Duramorph
Concentration
T10 level
Lower
Abdomen
T4 level
Upper Abdomen
Duration
Plain
Duration
With Epinephrine
Procaine
10%
125mg
200mg
45min
60min
Bupivicaine
0.75% in 8.25% dextrose
12-14mg
12-18mg
90-120min
100-150min
Tetracaine
1% in 10% glucose
10-12mg
10-16mg
90-120min
120-240min
Lidocaine
5% in 7.5% glucose
50-75mg
75-100mg
60-75min
60-90min
Ropivicaine
0.2-1%
12-16mg
16-18mg
90-120min
90-120
  
Factors Affecting the Spread of the Local Anesthetic Solution
A number of factors affect the spread of the injected local anesthetic solution within the CSF and the ultimate extent of the block obtained. Among these are:
the baricity of the local anesthetic solution
the position of the patient
the concentration and volume injected
the level of injection
the speed of injection
The specific gravity of the local anesthetic solution can be altered by the addition of dextrose. Concentrations of 7.5% dextrose make the local anesthetic hyperbaric (heavy) relative to CSF and also reduce the rate at which it diffuses and mixes with the CSF. Isobaric and hyperbaric solutions both produce reliable blocks. The most controllable blocks are probably produced by injecting hyperbaric solutions and then altering the patient's position.

Assessing the Block 
Some patients are very poor at describing what they do or do not feel; therefore, objective signs are valuable. If, for example, the patient is unable to lift his legs from the bed, the block is at least up to the mid-lumbar region. Sensory loss can best be assessed by testing temperature sensation using an alcohol swab. First touching the patient with the damp swab on the chest or arm (where sensation is normal), so that they appreciate that the swab feels cold. Then work up from the legs and lower abdomen until the patient again appreciates that the swab feels cold. The level of sympathectomy can be best assessed with light pin pricks moving from nipple line down. 

Practical Problems
The spinal needle feels as if it is in the right position but no CSF flows.
Wait at least 30 seconds, then try rotating the needle 90 degrees and wait again. If there is still no CSF, attach an empty 2ml syringe and inject 0.5-1ml of air to ensure the needle is not blocked then use the syringe to aspirate while slowly withdrawing the spinal needle. Stop as soon as CSF appears in the syringe.
 
Blood flows from the spinal needle.
Wait a short time. If the blood becomes pinkish and finally clear, proceed with the spinal. If blood only continues to drip, then it is likely that the needle tip is in an epidural vein and it should be advanced a little further or angled more medially to pierce the dura.
 
The patient complains of sharp, stabbing leg pain.
The needle has hit a nerve root because it has deviated laterally. Withdraw the needle and redirect it more medially away from the affected side.
Wherever the needle is directed, it seems to strike bone.
Make sure the patient is still properly positioned with as much lumbar flexion as possible and that the needle is still in the mid-line. If you think that you are not in the midline check with the patient which side they feel the needle. Alternatively, if the patient is elderly and cannot bend very much or has heavily calcified interspinous ligaments, it might be better to attempt a lateral approach to the dura.
 
Common Complications
Postdural Puncture Headache – incidence related to use of larger needles (22G), cutting needles. Occurrence can also be reduced by rotating the needle so that the bevel is pointed to the side, this decreases trauma to the dura.
Transient Radicular Syndrome/Transient Neurological Syndrome – self resolving pain related to the use of Lidocaine, lithotomy position, and early ambulation post-op.
Backache
Hypotension
Itching
 
Less Common Complications
Cauda Equina Syndrome
Total Spinal
Urinary Retention
Cardiac Arrest
Spinal/Epidural Hematoma
Aseptic Meningitis
Bacterial Meningitis
Cranial Nerve Palsies
Cranial Subdural Hematoma


Contraindications

Relative Contraindications

Absolute Contraindications

Hypovolemia

Patient refusal

Preexisting neurologic disorders

Infection at puncture site

Chronic back pain

Generalized sepsis

Localized infection peripheral to the regional technique site

Severe coagulation abnormalities

Patients taking ASA, NSAIDS, dipyridamole

Raised ICP

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