Epidural

 

Epidural anesthesia is a neuraxial technique offering wide range of applications for procedures below the neck level. An epidural block can be performed at cervical, thoracic, or lumbar levels. Since  autonomic, sensory and motor block occur with an epidural , it can be used for surgical anesthesia where relaxation is required.

 

Indications

As a primary anesthetic epidural can be used for:

Abdominal procedures

Inguinal, urogenital, and rectal surgeries

Lower extremity procedures

Obstetric analgesia

Chronic pain management and post operative pain control

 

Combined with General Anesthesia it can be used for:

High abdominal surgery

Cardiac surgery

Thoracic surgery

 

Anatomy

The epidural space lies between the ligamentum flavum and the dura mater; extending from the foramen magnum to the sacral hiatus and contains emerging nerve roots of the spinal cord, fat and veins.

                  

 

Epidural Drugs

The epidural agent is chosen based on the desired clinical effect:

Primary anesthetic

Supplementation of GA

Analgesia

Only preservative free LA; or agents labeled clearly for epidural use are employed.

 

Bupivacaine

Chloroprocaine

Etidocaine

Lidocaine

Mepivacaine

Ropivacaine

  

Adjuncts

Epinephrine 1:200,000 (5mcq/ml)

Prolong duration of the block

Decrease bleeding

Decrease toxicity

 

Opioids

Fentanyl 5-100mcq

Morphine 2-20 mg

 

Test Dose

A test dose is designed to detect both subarachnoid and intravascular injection.

Typically  a test dose consists of: 1.5% Lidocaine 3ml with 1:200,000 epinephrine.

If intrathecal – a spinal anesthesia should be rapidly apparent.

If intravascular – a 20% or more increase in heart rate should be noted.

 

Epidural Space

The epidural needle advances through the skin, superficial fascia, deep fascia – then the  supraspinous ligament, interspinous ligament, and the ligamentum flavum when using the midline approach.

 

Upward spread of anesthetic is limited by the dural attachment, which prevents high brainstem levels of anesthesia.

  

 

Lateral Position for Epidural

Knees are drawn toward the chest and head flexed forward to provide anterior flexion of the vertebral column.

A pillow is placed under head to maintain spinal alignment.

A line drawn between the posterior iliac crests crosses at the L4-5 interspace.

A line drawn between the inferior tips of the scapulae crosses at the T-9 spinous process.

   

 

Sitting Position for Epidural

The legs are allowed to hang over the edge of the bed with the feet supported by a stool.

The shoulders are “hunched” forward and the patient is encouraged to hug a pillow in towards the abdomen to provide anterior flexion of the spine.

This position helps to identify the midline of the spine and mark the desired lumbar or thoracic level.

      

 

Site Selection

Adult spinal cord ends at approximately L1, making the lumbar catheter placement safer than the thoracic or cervical placement.

 

Site Preparation

After the site is selected and marked, the skin is typically prepared with a povidone – iodine solution.

This solution works by contact and does not require scrubbing to reduce the number of microorganisms at the site.

Chlorhexidine may be used in the event of an iodine allergy.

This a detergent that does require scrubbing and prolonged contact to work.

After preparing the skin with an iodine (Betadine) solution and placing a sterile drape, the site should be wiped free of iodine and gloves changed to prevent introducing this solution into the epidural/ subarachnoid space.

 

Epidural Tray

25 gauge needle for skin analgesia

4cm, 22 gauge needle for deep infiltration

18 gauge needle for drawing up epidural solutions and piercing skin prior to inserting epidural needle

Epidural needle (Tuohy, Crawford)

Epidural catheter

2 ml glass syringe for infiltration

10 ml glass syringe for loss of resistance tests

Normal saline

Local anesthetic

Filters and caps for epidural

 

 

Epidural and Spinal Needles

Epidural needles are typically larger gauge (16-17) than a spinal needle to permit better assessment of loss of resistance and passage of a catheter. 

  

 

Hand Position

During local infiltration, the depth of the ligamentum flavum and orientation of the interspinous space can be assessed .

The hub of the epidural needle is grasped using its flanges.

Other fingers are braced against the back to prevent unintentional movement.

The needle is advanced in this manner through the interspinous ligament until entry into the ligamentum flavum is identified at approximately 3.5 – 5 cm deep to the skin.

 

Lumbar Insertion/Thoracic Insertion

A midline or paraspinous / paramedian approach may be used

 

Bromage Grip

Loss of Resistance technique employed after entering the ligamentum flavum

The glass syringe with air, or saline with an air bubble, is attached to the needle

Needle is advanced by rotation of entire hand while continuously compressing the syringe with a “bouncing” movement; “Bromage Grip”

When a “loss of resistance “occurs, air or saline is injected without resistance being felt. This indicates that the epidural space has been entered.

    

 

Thoracic Epidural

Needle is inserted one fingerbreadth lateral to the desired interspace

Needle is inserted perpendicular to the skin until the lamina is met

Needle is angled 45 degrees medially and 45 degrees cephelad and walked to the ligamentum flavum

Loss of resistance technique is then used to enter the epidural space

 

 

Catheter Placement

A test dose of local anesthetic and epi is used to exclude intravascular or subarachnoid injection.

The bevel of needle is directed cephalad.

Catheter is advanced 3 to 4 cm beyond needle tip.

The needle is then withdrawn while feeding the catheter through.

The length of the catheter remaining outside the skin is noted and if more than 4 cm has been introduced the excess is withdrawn.

The catheter is then secured and the dose injected.

 

Assessment of block efficacy

Each spinal segment supplies a specific region of the skin and a specific number of muscles. In the cervical, brachial and lumbosacral regions the anterior rami join to form nerve plexuses. In epidural blockade, motor paralysis affects the movement of various joints and muscles while also providing a sensory block. The innervation of these is shown in the following list:

 

Assessment of Motor Block

Shoulder C6-8

Elbow C5-8

Wrist C6-7

Hand and digits C7-8, T1

Intercostals T1-11

Diaphragm C3-5

Abdominal T7-12

Hip flexion L1-3

Hip extension L5, S1

Knee flexion L5-S1

Knee extension  L3-4

Ankle, flexion L4-5

 

The Bromage scale can be used to evaluate motor blockade:

 

No block (the ability to flex the knees and feet)

Partial block ( ability to flex the knees and resist gravity with full movement of the feet)

Almost complete block (inability to flex the knees but retained ability to flex the feet)

Complete block (inability to move the legs

 

 

Assessment of Sensory Block: Dermatome Chart

The sensory level achieved by a block can be assessed with a pinprick

The level of sympathectomy is assessed by measuring skin temperature

   

 

Onset of Action

The onset of epidural block with all local anesthetics can usually be detected within 5 minutes in the dermatomes immediately surrounding the injection site.

The time to peak effect differs somewhat among local anesthetics. Shorter-acting drugs usually reach their maximum spread in 15-20 minutes, whereas longer acting drugs required 20-25 minutes.

Increasing the dose of local anesthetic speeds the onset of both motor and sensory block.

Missed Segments

Unblocked segments ( patchy block) are usually one sided in an otherwise satisfactory field of anesthesia.

This occurs more often in obstetrical patients than in the general surgical population.

The occurrence of residual pain in one area means failure of the total block, since the pain in this area is just as agonizing as if no anesthesia existed in the neighboring segments.

 

Missed Segments

This complication seems to occur more often when small amounts of local anesthetics are used through a catheter versus a one – shot needle technique.

The reasons for this complication are not completely clear and several factors may be involved.

 

Speed of injection: Bromage tested the influence of speed of injection on the incidence of patchy sacral analgesia and found that slow injections (.5 ml/sec) decreased the incidence from 30% to 6% in human volunteers.

 

Local Anesthetic pH: The carbonated solutions of lidocaine and prilocaine were found to have a very low incidence of persistent unblocked segments, when compared to plain bupivicaine and tetracaine. This is attributed to the fact that that carbon dioxide penetrates nerve cell membranes extremely fast which was proven by studies in animals and humans.

 

Migration of the catheter into an intervertebral foramen is usually the cause of persistent one sided block. This is occurs with increasing frequency the farther the catheter is inserted. Retraction of the catheter and reinjection of additional dose with the patient positioned on the side of the unblocked segment may help. Lately, adding a narcotic, (Fentanyl or Morphine) to the local anesthetic has markedly reduced the incidence of patchy analgesia.

 

Complications

Epidural complications are very rare, but when they do occur they can be serious

 

Post Dural Puncture Headache

If the dura is punctured during the procedure, this is called a wet tap. If this is recognized at the time, then a different level is attempted. The headache occurs because the CSF around the spinal cord communicates with the CSF around the brain. A leak in the system causes the pressure to drop increasing tension on the supporting membranes of the brain. Treatment can be conservative or active. Treatment can consist of blood patch (15 -20cc of the autologous blood is injected into the epidural space at the level of the dural puncture), analgesics, caffeine, maintaining a horizontal position and fluid loading. Approximately 90% of patients respond to a single blood patch.

 

Total Spinal Injection

If the dura is punctured and the epidural dose is injected intrathecally, a total spinal may occur. As the local anesthetic ascends the spine towards the brain stem and cortex, severe hypotension, bradycardia, apnea, and unconsciousness are likely. With proper support (artificial ventilation, IV fluids, and vasopressors), the total spinal effects are reversible after 1-2 hours depending on the local anesthetic used.

 

Seizures

Since the epidural space has a rich supply of veins, 10% of all epidural injections may be placed into a vein rather than the epidural space. If a large volume of epidural solution is injected intravenously, a seizure may occur. This can be prevented by careful aspiration on the needle before and during the injection. Benzodiazepines and hyperventilation raise the threshold of local anesthetic induced seizures. Thiopental 1 – 2mg/kg reliably terminates seizures. Respiratory support must be provided.

 

Anaphylaxis

Rarely a massive allergic reaction can occur during or shortly after an epidural injection. It usually causes severe hypotension, severe wheezing, urticaria (itchy skin rash), and rapid swelling of the face and extremities.  The treatment of anaphylaxis includes vasopressors and fluids to support the blood pressure, and drugs to reduce the magnitude of the allergic response (steroids).

 

Backache

The etiology of backache is not clear, although needle trauma, local anesthetic irritation, and ligamentous strain secondary to muscle relaxation have been offered as explanations.

 

Neurologic injury

Persistent paresthesias and limited motor weakness are the most common injuries, although paraplegia and diffuse injury to the cauda equina (cauda equina syndrome) do occur rarely. Injury may be related to: direct needle trauma to the spinal cord or spinal nerves, spinal cord ischemia, injection of neurotoxic drugs, introduction of bacteria into the subarachnoid/ epidural space, or very rarely epidural hematoma.

 

Cauda Equina Syndrome

The mechanism by which local anesthetics produce cauda equina syndrome is not yet clear; however, in vitro evidence suggests that local anesthetics produce excitotoxic damage by depolarizing neurons and increasing intracellular calcium concentrations. It is also unclear as yet whether adjuncts, such as epinephrine, added to local anesthetics contribute to cauda equina syndrome. However, based on animal studies, it has been argued that epinephrine should not be added to intrathecal lidocaine. Rather, if prolonged duration of spinal anesthesia is necessary, a longer-acting drug like bupivacaine should be used.

 

Spinal Hematoma

May occur if an epidural vein is inadvertently punctured in the presence of a severe blood coagulation defect. Epidural injections should not be performed in this situation, as an unrecognized hematoma can compress the spinal cord, producing paralysis and incontinence. Patients with the following should alert their doctor prior to treatment – warfarin and heparin therapy, low platelets, severe liver disease, hemophilia, leukemia and other bone marrow disorders. There is no problem with low dose aspirin therapy for the prevention of strokes.

 

Spinal Abscess

Formation may occur if bacteria enter the epidural space during the injection. This can be greatly reduced by proper sterile technique using gloves and anti-septic skin preparation. In some patients the cause of the abscess is due to the formation of an epidural clot which then becomes secondarily infected via the blood route. The caudal route is a potentially more dirty area because of it’s proximity to the perineum. Diabetics have a reported greater incidence of staphylococcal abscess formation. Left untreated an abscess can cause paralysis and incontinence. A persistently high temperature, feeling unwell, with increasing back ache may suggest an abscess, and requires urgent spinal surgical investigation and treatment.

 

 

No pain relief

Probably the most common late complication. The causes are non-nerve root sciatica, the needle missing the epidural space, or a large disc prolapse causing nerve root compression rather than irritation.

 

Hypotension

Is usually believed to be related to the inhibition of the sympathetic arterial vasoconstrictors and cardiac accelerator fibers. However, the capacitance vessels, i.e., the veins, are also under the influence of the sympathetic nervous system, blockade of which removes the ability of the veins to constrict.

 

Cardiac arrest

This is a rare complication of epidural block but may result from severe hypotension and consequent myocardial ischemia. The hypotension may be the result of a severe vasovagal response, or due to a high block in a severely hypovolemic patient. In such cases the patient should respond to normal cardiopulmonary resuscitation with added vasopressor and fluids as appropriate.

 

Accidental subdural injection

An injection of local anesthetic into this space can spread a considerable distance and will only be separated from the spinal nerves by the relatively thin arachnoid and pia mater. A block similar to a high or total spinal block may ensue.

 

Trauma

Involves a single spinal nerve. The needle or catheter can enter a nerve and if an injection is made directly into that nerve then some disruption of the fibers can occur. This may lead to a degree of neuritis which can last for weeks or months.

 

Adhesive arachnoiditis

This is a serious condition and may result in permanent loss of spinal cord function. It usually follows injection of an irritant solution and several compounds have been implicated, such as detergent used for cleaning syringes.

 

References

 

Questions