Epidural Anesthesia Student Notes PDF

EPIDURAL ANESTHESIA NRAN 80516 SUMMER 2024 RON ANDERSON, M.D. 1 KEY POINTS You must have a complete understanding of the anatomy, nature, and contents of the epidural space. Understand how epidural anesthesia differs from spinal anesthesia. The addition of epinephrine to an epidural produces...

EPIDURAL ANESTHESIA NRAN 80516 SUMMER 2024 RON ANDERSON, M.D. 1 KEY POINTS You must have a complete understanding of the anatomy, nature, and contents of the epidural space. Understand how epidural anesthesia differs from spinal anesthesia. The addition of epinephrine to an epidural produces significant reduction in peripheral vascular resistance. Successful, efficient epidural placement requires a bit more practice than spinal anesthesia, and you must maintain absolute control of needle depth. Epidural local anesthetics initially work at the segmental spinal nerves, but over time may gain access to the nerve roots. 2 KEY POINTS Understand the differences in speed of onset and duration of action of the local anesthetics discussed. Understand the purposes and mechanisms of the epidural additives discussed. Epidural anesthesia is more likely to result in systemic toxicity than spinal anesthesia. Be aware of the anticipated effects of incorrect epidural catheter placement. You must maintain a high index of suspicion for epidural hematoma, since delay in treatment will significantly worsen outcome. 3 EPIDURAL ANESTHESIA ANATOMY REVIEW PHYSIOLOGIC EFFECTS – Similarities to Spinal Anesthesia – Differences from Spinal Anesthesia TECHNIQUE PHARMACOLOGY CONTRAINDICATIONS COMPLICATIONS CONTROVERSY 4 ANATOMY OF THE EPIDURAL SPACE BOUNDARIES CONTENTS EXTERNAL LANDMARKS THE SPINAL COLUMN LIGAMENTS ANATOMY OF THE CAUDAL APPROACH 5 EPIDURAL SPACE IS BOUND: Cranially by the foramen magnum. Caudally by the sacrococcygeal ligament covering the sacral hiatus. Anteriorly by the posterior longitudinal ligament. Laterally by the vertebral pedicles. Posteriorly by both the ligamentum flavum and vertebral lamina. 6 EPIDURAL SPACE IS: A potential space Shallowest anteriorly – Dura may in some places fuse with the posterior longitudinal ligament Deepest posteriorly – Although the depth varies – Epidural space is intermittently obliterated posteriorly by contact between the dura mater and the ligamentum flavum or vertebral lamina. – Contact between the dura mater and the pedicles also interrupts the epidural space laterally. 7 EPIDURAL SPACE IS: A series of discontinuous compartments in the lumbar area Become continuous when the potential space separating the compartments is opened up by injection of air or liquid 8 EPIDURAL SPACE: Becomes less segmented in the thoracic area Depth of space – Decreases as it progresses more cephalad 9 CONTENTS OF EPIDURAL SPACE Fat – Primary component Venous plexus – Batson’s plexus Segmental Arteries Lymphatics – Laterally 10 EPIDURAL FAT Primarily in the posterior and lateral epidural space Tends to correlate with the general adiposity of the patient Decreases with age – May be the primary factor in decreased dose requirements with aging 11 EPIDURAL VENOUS PLEXUS Batson’s plexus – Extensive network of valveless veins – Anastomose freely with epidural veins, including: Intracranial venous sinuses Pelvic veins Azygous system – Communication with thoracic and abdominal veins results in: Abdominal and thoracic pressure changes being transmitted to the epidural venous system 12 EPIDURAL VENOUS PLEXUS – Increased abdominal pressure or a mass compressing the vena cava may dilate this venous plexus, increasing: Probability of puncturing a vein during epidural placement Spread of local anesthetic due to decreased effective volume of epidural space – Enough variability in dosing that this would be hard to quantify 13 EPIDURAL SPACE ALSO CONTAINS: Lymphatics – Near dural cuff Segmental arteries – Run between the aorta and the spinal cord. 14 VERTEBRAL LANDMARKS BROWN 15 VERTEBRAL ANATOMY Consider: – Angle of the spinous processes – Natural curvatures of the spine – Abnormal curvatures of the spine – Effect of positioning on above 16 LUMBAR VERTEBRA Interlaminar foramen – Spaces formed between the spinous processes – Triangular shaped Base formed by upper edge of the lower vertebra’s lamina – Flexing slides the articular processes upward enlarging the interlaminar foramen 17 POSITIONING – LUMBAR LORDOSIS BROWN BROWN 18 CURVATURES OF THE SPINAL COLUMN Cervical and lumbar curves are convex anteriorly Thoracic and sacral curves are convex posteriorly – Spinal curves have significant impact on spread of local anesthetics (LA) Supine position – High points of the cervical and lumbar curves are at C5 and L5 – Low points of the thoracic and sacral curves are at T5 and S2 19 BARASH LIGAMENTS OF THE SPINAL COLUMN Ligamentum Flavum Interspinous Ligament Supraspinous ligament MILLER MILLER 20 CROSS-SECTION AT L3-4 LEVEL KEY POINTS: Greatest thickness of ligamentum flavum in midline Greatest depth of epidural space in midline BROWN 21 MENINGES 22 CAUDAL ANATOMY BROWN 23 CAUDAL ANATOMY BROWN 24 CAUDAL ANATOMY MALE vs. FEMALE 25 BROWN PHYSIOLOGIC EFFECTS DIFFERENCES FROM SPINAL ANESTHESIA Larger Volume and Dosage of Drug Required Effect of Position Addition of Epinephrine 26 PHYSIOLOGIC EFFECTS OF EPIDURAL For the most part, the physiologic effects of epidural anesthesia do not differ from those of spinal anesthesia. As in spinal anesthesia, the effects are primarily a function of the height of the block and subsequent sympathectomy The differences that do exist are due to the much larger volume and dosage of drug required for epidural anesthesia. 27 DIFFERENCES FROM SPINAL ANESTHESIA Larger volume and dose of drug – Local anesthetic blood levels may be adequate to reduce narcotic requirements. – Excessive blood levels of local may produce toxic effects – Epidural fentanyl produces a blood level almost equivalent to an IV dose. 28 PATIENT POSITION WITH EPIDURAL DOSING Much less significant in determining block height than with spinal anesthesia Avoid extremes of positioning, such as sitting, unless desiring a lower block Maintaining a patient in the lateral decubitis position after epidural dosing will provide a somewhat better block on the “down” side, but you will still have effects on the “up” side – Effect much less pronounced than with spinal anesthesia 29 BARASH 30 EPIDURAL NEEDLES 31 EPIDURAL TECHNIQUE GENERAL PROCEDURE IDENTIFICATION OF EPIDURAL SPACE Hanging Drop Loss of Resistance EPIDURAL CATHETERS DIFFERENCES BY ANATOMIC LOCATION Lumbar Thoracic Cervical Caudal 32 GENERAL PROCEDURE Preparation – Assure this is an appropriate surgery for an epidural – Assure the patient is an acceptable candidate for epidural anesthesia Essentially the same contraindications we discussed with spinal anesthesia Position – Key element needed for success 33 LATERAL DECUBITIS POSITION BROWN 34 CAUDAL IN A CHILD BROWN 35 SITTING POSITION BROWN BROWN 36 POSITIONING Prone – Used most frequently for caudal epidural in adults – A pillow under the iliac crests rotates the pelvis making it easier to enter the sacral canal. – Legs spread slightly with toes pointed inwards MILLER 37 PREP AND DRAPE Monitors on Sedation as appropriate Oxygen as needed – Usually nasal cannula adequate Do a good, WIDE prep with betadine, chlorhexidine, etc – You may not be successful at your chosen interspace Betadine vs, Chlorhexidine – Effectiveness – Timing Drapes – Pay particular attention to maintaining a sterile field. – Once you lay the drape on, don’t pick it back up and move it Clear plastic Paper Sterile towels SKIN AND SUBQ ANALGESIA Identify the L3-4 or L4-5 interspace – Certainly no higher than L2-3 Starting near the bottom of the chosen interspace, create a skin wheal of 1% lidocaine with a 25ga or smaller needle Inject to a depth of 1-2” in direction of anticipated epidural needle travel. Base depth of injection on patient’s body habitus MIDLINE APPROACH Insert needle: – Midline – Nearer the bottom of the interspace – With a 100 -150 cephalad angle – Anchor styleted needle in ligament ADVANCING THE NEEDLE Many ways to hold needle, you will find what works best for you Goals: – Absolute control of needle depth – Tactile sensation of different tissues and perforation of the dura Distinction between ligament and paraspinous muscle which is entered if you deviate from midline STRUCTURES TO TRAVERSE BROWN 42 IDENTIFICATION OF THE EPIDURAL SPACE A potential space Ligamentum flavum is 3.5 – 6 cm deep in 80% of patients Two methods: – Hanging drop Subatmospheric pressure in epidural space will suck the drop of solution into the needle – Loss of resistance – MOST COMMON When entering the epidural space you will notice a loss of resistance on the syringe plunger and the contents can be injected. 43 HANGING DROP TECHNIQUE BROWN 44 LOSS OF RESISTANCE Epidural needle inserted with bevel facing laterally Attach glass syringe with saline, air , or saline with a small air bubble to needle. Slowly advance needle while applying constant, or bouncing pressure on plunger. When “loss of resistance” encountered, inject some of the saline and turn the bevel cephalad or caudad. 45 EPIDURAL CATHETERS 46 ADVANCING THE EPIDURAL CATHETER Injection of air or saline may facilitate advancing the catheter Catheter should advance easily without meeting resistance, if not: – You’re in the wrong place – You’re up against something If the catheter won’t advance out the end of the needle you may withdraw the catheter and reposition the needle If the catheter has advanced any distance out the end of the needle, but not far enough, you must remove the needle and catheter as a unit and start over. 47 ADVANCING THE EPIDURAL CATHETER How far? Once catheter is in position, the needle is withdrawn over the catheter, taking care not to remove the catheter with it. Hub is attached to the catheter and following aspiration a test dose is given to affirm that you are not: – Intravascular – Subarachnoid 48 MEASURING CATHETER DEPTH BROWN 49 CATHETER MALPOSITION Possible Locations: – – – – Subarachnoid Subdural Intravascular Foraminal Other Concens: – Septum – Adhesions BROWN 50 SECURING THE CATHETER Use benzoin or mastisol to improve adhesion Sterile occlusive dressing preferred for long-term use – e.g. pain management Tape generally adequate for surgery or labor analgesia 51 THE TEST DOSE Prior to dosing the epidural you must give a test dose to insure the catheter is not: – Intravascular Ringing in ears Increased heart rate >20bpm above baseline – Subarachnoid Rapidly developing block at a level higher than anticipated – Most common test dose is lidocaine with epinephrine Subsequent dosing should be done incrementally, and only after aspiration – Typically 5 ml at a time 52 CAUDAL ANATOMY BROWN 53 CAUDAL NEEDLE PLACEMENT Puncture sacrococcygeal membrane at ~ 450 angle until bone is contacted Withdraw slightly and redirect needle in a shallower plane Advance needle 1-2 centimeters into the caudal canal Rapidly inject 5 ml of saline, while palpating gently over the sacrum Absence of a midline bulge implies correct, or at least not subcutaneous placement of the needle 54 CAUDAL PRESSURE WAVE BROWN 55 PHARMACOLOGY SITE OF ACTION DRUGS DOSING ADDITIVES 56 SITE OF ACTION Early – At segmental spinal nerves traversing the epidural and paravertebral spaces Later – Subdural locations, including within the spinal cord BROWN 57 LOCAL ANESTHETICS FOR EPIDURAL USE Rapid Onset, Short Duration – 2-Chloroprocaine 2% and 3% Intermediate Onset, Intermediate Duration – Lidocaine 2% – Mepivacaine 2% Intermediate Onset, Longer Duration – Etidocaine 1% Slower Onset, Longer Duration – Bupivacaine 0.5 – 0.75% – Levobupivacaine 0.5 – 0.75% – Ropivacaine 0.75 – 1% 58 ONSET OF SENSORY ANALGESIA DRUG CONCENTRATION (%) ONSET (min) 2-Chloroprocaine 3 10-15 Lidocaine 2 15 Mepivacaine 2 15 Etidocaine 1 15 Ropivacaine 0.75-1.0 15-20 Levobupivacaine 0.5-0.75 15-20 Bupivacaine 0.5-0.75 20 59 DURATION OF SENSORY ANALGESIA DRUG 2-DERMATOME REGRESSION(min) COMPLETE RESOLUTION(min) PROLONGATION BY EPINEPHRINE (%) Chloroprocaine 3% 45-60 100-160 40-60 Lidocaine 2% 60-100 160-200 40-80 Mepivacaine 2% 60-100 160-200 40-80 Ropivacaine 0.5-1% 90-180 240-420 Minimal Etidocaine 1-1.5% 120-240 300-460 Minimal Bupivacaine 0.5-0.75% 120-240 300-460 Minimal 60 ADAPTED FROM STOELTING, MILLER FACTORS AFFECTING EPIDURAL DOSE Significant inter-patient variability Most important determinant is injection site Age – decreasing dose required with aging – Decreased epidural fat content – Less compliant epidural space – Decreased ability of drug to leak out of intervertebral foramina Height and weight – Mixed results from studies – Likely only of consequence at extremes of height or in morbid obesity Pregnancy – Studies vary – some suggest a reduced dosage requirement in pregnancy 61 EPIDURAL DOSING With a lumbar epidural injection site: 20 ml of local anesthetic will typically produce a mid-thoracic sensory block – Increasing volume at constant dose, or increasing volume and dose at constant concentration increases spread, but in a non-linear manner. – Position has little effect. 300 head up improves low-lumbar and sacral analgesia with no decrease in thoracic block height 62 ANATOMIC LANDMARKS Anatomic landmarks used to describe level of block: – – – – T10 umbilicus T 6 xiphoid process T4 nipple line C8 little finger 63 DOSING A LABOR EPIDURAL Two basic schools of thought: – Establish block with 0.25% Bupivacaine – Infusion of 0.0625 – 0.125% Bupivacaine Or – Establish block with 0.0625% - 0.125% Bupivacaine – Infusion of same * Most will add a narcotic to either of these 64 CONTINUOUS INFUSION FOR LABOR There are a wide variety of mixtures in use ranging from ~ 0.05% - 0.125% Bupivacaine And containing 1-5 mcg/ml of Fentanyl Infusion rates vary from 10 – 18 ml/hr Like so many things in anesthesia, there are lots of ways to get this done. 65 “PERINEAL OR “DELIVERY” DOSE ~ 10 ml of local anesthetic – Patient uncomfortable near delivery, but still needing to push Infusion concentration 2% 2-chloroprocaine 1% lidocaine – Surgeon planning instrumented delivery 3% 2-chloroprocaine 2% lidocaine 66 PLATEAU EFFECT IN EPIDURAL DOSING Unique feature of epidural dosing is that, having given a dose of local anesthetic, after a period of time, subsequent doses may be ineffective in raising block height. 67 ADDITIVES Adrenergic agonists – Epinephrine (5mcg/ml = 1/200,000) Significant prolongation of: – Lidocaine, Mepivacaine, 2-Chloroprocaine Only slight prolongation of others Reduces peak plasma levels of local anesthetics – Phenylephrine Rarely used with epidurals Lesser effect on reduction of peak plasma levels Sodium bicarbonate – 1 meq to 10 ml local anesthetic Speeds onset May improve density of block 68 DURATION OF SENSORY ANALGESIA DRUG 2-DERMATOME REGRESSION(min) COMPLETE RESOLUTION(min) PROLONGATION BY EPINEPHRINE (%) Chloroprocaine 3% 45-60 100-160 40-60 Lidocaine 2% 60-100 160-200 40-80 Mepivacaine 2% 60-100 160-200 40-80 Ropivacaine 0.5-1% 90-180 240-420 Minimal Etidocaine 1-1.5% 120-240 300-460 Minimal Bupivacaine 0.5-0.75% 120-240 300-460 Minimal 69 ADAPTED FROM STOELTING, MILLER REDOSING Any time you wish to bolus dose an epidural catheter you should first aspirate, as a catheter can migrate into a blood vessel or the subarachnoid space. – This includes a catheter with a continuous infusion running as it may be at a low enough level that intravascular injection goes unnoticed. – A test dose may be of limited usefulness if significant block already established. 70 CONTRAINDICATIONS APPROPRIATE FOR PLANNED SURGERY? PATIENT’S INABILITY TO REMAIN STILL INFECTION AT SITE/ SEPSIS AORTIC OUTLET OBSTRUCTION INCREASED INTRACRANIAL PRESSURE PATIENT REFUSAL COAGULOPATHY PRE-EXISTING NEUROLOGIC DISEASE? SEVERE HYPOVOLEMIA CONTRAINDICATIONS- SAME AS SPINAL Appropriate for planned surgery? Patient refusal Infection at site/ sepsis Pre-existing neurologic disease Increased intracranial pressure Severe hypovolemia 72 CONTRAINDICATIONS – SOME DIFFERENCES Coagulopathy – If catheter left in place must consider timing of removal in conjunction with anticoagulant dosing Patient’s inability to remain still – Particularly an issue with thoracic and cervical epidural placement Aortic outlet obstruction – Some controversy here Obviously preferable to spinal With patience you may produce a more gradual onset of block and subsequent decrease in peripheral vascular resistance 73 COMPLICATIONS INTRAVASCULAR INJECTION SUBARACHNOID INJECTION BACK PAIN POST-DURAL PUNCTURE HEADACHE NEUROLOGIC INJURY Needle Trauma Epidural Hematoma 74 SYSTEMIC TOXICITY Keys to Prevention – An adequate test dose – Incremental injection of the local anesthetic solution 75 INTRAVASCULAR INJECTION Most likely via an epidural vein Central Nervous System – Initially excitation due to: Inhibition of inhibitory neurons in cortex – At higher concentrations you see CNS depression due to: Inhibition of both inhibitory and excitatory pathways Cardiovascular – Narrower margin of safety with bupivacaine due to: Slower dissociation from Na+ channels – Ropivacaine and Levobupivacaine have a lower cardiotoxic potential than bupivacaine which may be advantageous for epidural anesthesia 76 SUBARACHNOID INJECTION Leading to high, or possibly total spinal Prevention – Test dose – Incremental injection Management – Blood pressure and heart rate support – Support of ventilation, intubation as needed Also consider a subdural location if an unexpected, but less severe, and more slowly developing high block occurs. 77 BACK PAIN Back pain is a relatively common complaint following all types of anesthesia, but more common with regional techniques – Spinal 11% – Epidural 30% Potential etiologies – Needle trauma – Local anesthetic irritation – Ligamentous strain 78 POST-DURAL PUNCTURE HEADACHE Occurs in up to 50% of young patients following dural puncture with a large diameter epidural needle. Typically resolves in several days with conservative treatment – Bed rest, analgesics, caffeine-containing fluids Definitive treatment – Epidural blood patch Orientation of the needle bevel is important 79 NEUROLOGIC INJURY Extremely rare, but potentially devastating problem Possible etiologies: – Direct needle trauma – Vascular injury with subsequent neural ischemic insult – Epidural hematoma Neurosurgical emergency Early detection is key to good outcome Outcome significantly worse if treatment delayed more than 8 hours 80 INCIDENCE OF EPIDURAL HEMATOMA Overall rate: – Spinal: – Epidural: 1/220,000 1/150,000 1993-1998 in the U.S. – Spinal – Epidural – Post-op Epidural 1/40,800 1/6,600 1/3,100 81 CANT GET CATHETER OUT An hour or so after delivery the labor nurse calls you and says she’s unable to remove the epidural catheter. What do you do? – A. Tell her to pull harder – B. Pull it as tight as possible, wash with alcohol at the skin, cut it, and let it retract into the epidural space – C. Send the patient for an MRI – D. None of the above 82 CONTROVERSY EPIDURALS IN ANESTHETIZED PATIENTS VALUE OF EPINEPHRINE IN TEST DOSE FOLLOWING CSEA DO HIGH LEVELS OF DRUG REACH THE SAS OUTCOMES Intraop Postop 83 CONTROVERSY Is it safe to place an epidural in a patient who is under general anesthesia? – Possibly not quite as safe due to lack of patient feedback – Removes the potential of the patient moving – Definitely a liability exposure What is the value of epinephrine in the test dose? – Is the heart rate change notable in labor? – Could it harm the fetus? – Is it reliable in beta-blocked patients? 84 CSEA POTENTIAL RISK Do high levels of local anesthetic from the epidural space reach the subarachnoid space following dural puncture? Meningeal hole made by the spinal needle may allow dangerously high concentrations of subsequently administered epidural drugs to reach the subarachnoid space Anecdotal case reports and in vitro animal studies suggest that this may be a legitimate concern 85 CONTROVERSY - OUTCOMES Are outcomes improved following regional anesthesia or following regional analgesia for postoperative pain relief? In many cases, yes. If the risk/ benefit ratio is favorable, what does the cost/ benefit ratio look like? How do we reapportion money to provide this? 86 CHARTING YOUR EPIDURAL Sterile prep and drape with betadine. Skin wheal and deep infiltration with 25 gauge, 1 ½” needle. 17 gauge Touhy to epidural space x 1 with loss of resistance technique. Catheter threaded easily to depth of 6 cm. Needle removed. Negative aspiration for heme or CSF. Negative test dose with 3ml of 1.5% xylocaine with epinephrine 1/200,000. Catheter secured. Patient placed supine with left uterine displacement. Injected incrementally as above. Patient tolerated procedure well. Bupivacaine lot# ---------. Expiration 10/2019 Fentanyl lot#-------- Expiration 10/2019 SP&D c betadine. Skin wheal & deep infilt. c 25 ga I ½”. 17ga Touhy to epid space x1 c LOR. Cath to 6cm easily. – asp heme/CSF - test dose. Secured.→sup c LUD Tol. Well. Bup lot------exp10/2019 Fent lot----- exp 10/2019 87 SOURCE MATERIAL MILLER’S ANESTHESIA – 9th edition. Miller CLINICAL ANESTHESIA – 8th edition. Barash, Cullen, Stoelting BASICS OF ANESTHESIA – 6th edition. Stoelting, Miller ATLAS OF REGIONAL ANESTHESIA - Brown 88