Neuraxial Flashcards
The uptake and elimination of a drug in the subarachnoid space is largely influenced by?
Uptake
- More lipid soluble = faster uptake
Elimination
- More lipid soluble = faster elimination
Vascular absorption in both the epidural and subarachnoid space
UN-protein bound
How does protein binding effect duration of action of neuraxial drugs?
duration of action = increased with protein binding (only the unbound fraction is available for both action and excretion).
How does the addition of epinephrine affect drugs in the intrathecal space?
increases its duration of action - vasoconstriction —> decreased absorption
Epi also has activation of alpha-2 receptors within the dorsal horn of the spinal cord, providing additional anesthesia
Where can drugs injected into the epidural space diffuse?
lipophilic epidural fat
exit the intervertebral foramen to paraspinous muscles
diffuse into blood vessels –> systemic effects and elimination.
What opioid may induce delayed respiratory depression when givenvia epidural?
Hydrophilic = Morphine
- bi-modal respiratory depression = immediate and 6-12hrs later
What 3 factors determine the level and duration of an epidural block?
Location
Total drug dose
Volume administered
What are the 3 most important factors for determining the level of a spinal block?
Dose of anesthetic
Baricity
Position of patient during and after injection
What structure covers the sacral hiatus, allowing epidural anesthesia via the caudal route?
sacrococcygeal ligament
What are two methods for identifying the sacral hiatus?
- locate the coccyx, go superiorly, just above gluteal cleft, until the two sacral cornua are palpated. continues to palpate superiorly until the apex of the horseshoe-shaped sacral hiatus is identified
- two PSISs are palpated = form the base of an equilateral triangle, the apex of which is the cranial aspect of the sacral hiatus.
Name three membranes that make up the meningeal system of the central nervous system and describe their collective function.
dura mater, the arachnoid mater, and the pia mater
protection of the spinal cord and brain
Which intracranial artery is most commonly damaged in head injuries?
The middle meningeal artery is most commonly damaged in head injuries
At the level of the spinal cord, the epidural space is bordered by what two structures?
between the dura and the ligamentum flavum
The subdural space can be found between what two membranes?
less clearly defined potential space found between the dura and arachnoid mater
What is the name of the compartment that contains cerebrospinal fluid and is bound by the pia mater and the arachnoid mater?
subarachnoid space
Which meningeal membrane forms the venous sinuses in the brain?
dura mater
List three neuraxial techniques employed in the administration of local anesthesia.
continuous epidural
combined spinal-epidural
spinal blocks
List three risks associated with sedation administered before a neuraxial block.
respiratory depression
hemodynamic instability
uncontrolled movements
Describe the properties of midazolam that make it a useful sedative for premedication.
rapid onset
ease of titration
high clearance
minimal side effects
What block height is necessary for C-section?
T4
Name three physiological monitors that are used during administration of regional anesthetic.
pulse oximetry (SpO2)
electrocardiography (ECG)
arterial blood pressure (BP)
temperature measurement
What is the most common test dose preparation in adults?
3 mL of lidocaine 1.5% with epinephrine 5 mcg/ml (1:200,000)
100% sensitivity and specificity for intravascular injection by detecting an increase in HR of 20bpm or greater within 1 minute or an increase in systolic blood pressure 15 mmHg or greater (more reliable for those on beta-blockers) following the administration of 10-15 mcg of epinephrine
Describe the progression of cardiac anesthetic toxicity.
decrease HR and BP–> fatal ventricular arrhythmias
Name two potential causes of high or total spinal block.
- Inadvertent intrathecal placement of an epidural catheter
- large doses during spinal block
Signs and Sx
- difficulty in breathing and upper extremity weakness —> dysphonia, hypotension, bradycardia, loss of consciousness, and cardiopulmonary arrest
What block height is necessary for pelvic procedures?
T6-8
What are the CNS manifestations of LAST?
tinnitus circumoral numbness dizziness hallucinations seizures unconsciousness respiratory arrest
What are the two main types of spinal needles?
Cutting-tip
- sharp, beveled cutting tip, hole at the tip
- Quincke needle
Pencil-point
- conical tip, holes positioned laterally to the tip
- Sprotte, Whitacre, and Gertie-Marx needles
- significantly reduced the incidence of post-dural puncture headache (PDPH).
List three common surgeries where spinal anesthesia would be appropriate.
Cesarean delivery
total knee arthroplasty
total hip arthroplasty
What are five absolute contraindications to spinal anesthesia?
patient refusal severe coagulopathy severe hypovolemia increased intracranial pressure due to the presence of a space-occupying mass infection at the site of injection
severe aortic and mitral stenosis
List the time between spinal and the last dose of the following medications:
- Aspirin
- Clopidogrel
- Warfarin
- Heparin
- Enoxaprin
- None
- 7 days
- Normal INR
- Prophylactic dose – 1 hours; Full dose – normalization of PTT. If taking > 4 days, check platelet count (risk of HIT)
- Prophylactic dose – 12 hours/ Full dose – 24 hours
Define pKa.
pH at which the fraction of non-ionized molecules = the fraction of ionized molecules
pKa close to physiologic pH (7.4) = faster onset of action than those that have a pKa further from physiologic pH (because more molecules are non-ionized).
Name four factors that influence the height of a spinal block.
dose and baricity of the local anesthetic
neuraxial level of injection
posture/positioning of the patient
volume and density of the cerebrospinal fluid
Name three additives that can increase the duration of a spinal block.
highly protein-bound = longer duration of action (bupivacaine and ropivacaine)
epinephrine, phenylephrine, and clonidine - only in lidocaine or tetracaine
What are three risk factors for cauda equina syndrome?
spinal micro catheter w/ continuous infusion of LA
repeated injections of LA in the subarachnoid space
lithotomy position
lidocaine as the LA
What patient position(s) exacerbates a PDPH?
sitting or standing
List four complications of spinal block.
Hypotension (dec SVR) Bradycardia Shortness of breath Weak handgrip = cervical nerves are affected Paralysis of the diaphragm = C3-5 Patient unresponsiveness
What are some risk factors for developing a PDPH?
younger age female gender pregnancy history of PDPH multiple dural punctures type of needle used (with larger diameter needles increasing risk and cutting tip needles posing a greater risk than pencil point needles)
What is the treatment for a PDPH?
hydration
caffeine (cerebral vessel vasoconstrictor)
analgesics
adrenocorticotropic hormone (ACTH)
then —> blood patch
Blood patches performed after the first 24 hours post dural puncture are often more effective than those performed earlier.
TNS will most often occur with which LA?
Lidocaine
Describe the start and end of the epidural space and how its size changes by location.
foramen magnum superiorly–> sacral hiatus inferiorly
progressively increases in size from the cervical –> lumbar spine (most voluminous)
thickness of the ligamentum flavor = thin in the cervical region and thickest in the lumbar
List some common indications for epidural analgesia
Orthopedic surgeries involving the pelvis / lower extremities. Abdominal surgeries. Thoracic surgeries. Gynecologic procedures. Urologic procedures. Cesarean delivery. Labor analgesia. Vascular surgeries.
List three absolute contraindications to epidural anesthesia.
Patient refusal. Infection at the insertion site. Documented allergy to local anesthetics. Severe bleeding diathesis. Therapeutic anticoagulation. Increased intracranial pressure.
What is the primary meningeal barrier to drug diffusion from the epidural space to the spinal cord?
arachnoid mater = ~95% of the resistance
List some of the complications of epidural anesthesia.
Accidental dural puncture and post dural puncture headache.
Accidental subarachnoid injection of local anesthetic leading to high or total spinal anesthesia.
Systemic toxicity of local anesthetics.
Bladder dysfunction, especially if sacral segments are blocked.
Major neurologic injury.
Direct trauma to the spinal cord.
Epidural hematoma.
Infectious complications, including epidural abscess formation.
Hypotension and hemodynamic instability.
What is the role of a test dose and what is the common medication used?
Rule out intravascular or subarachnoid injection
3 ml of 1.5 % Lidocaine w/ 1:200,000 epinephrine = 15 mcg of epinephrine and 45 mg of lidocaine
Where is the main site of action of local anesthetics injected into the epidural space?
Nerve roots in the dorsal horn of the spinal cord
Below what dermatome does the caudal block provide reliable analgesia?
T10 - groin, genitals and extremity surgery
What are the boundaries of the sacral hiatus?
laterally - sacral cornea
inferiorly - coccyx
posteriorly - sacrococcygeal membrane
sacral hiatus - the lower corner of an equilateral triangle, the upper two corners of which are the posterior superior iliac spines
What are some contraindications to caudal block?
Infection or abnormalities of the spinal cord, pilonidal cyst, septicemia, anticoagulation, sacral dimple, intracranial hypertension, allergy to local anesthetics (rare).
Describe the position and technique for a caudal epidural in kids
Left lateral with hips flexed
needle inserted b/w the sacral cornu at a 30-60 degree angle.
needle passes through the sacrococcygeal membrane –> “pop” or “loss of resistance” = To avoid piercing the dura, do no insert needle any further
What is the most sensitive sign of intravascular injection of local anesthetic in children?
Peaked T waves
Describe the relative size of the volume of distribution of local anesthetics in children when compared to adults.
larger volume of distribution of local anesthetics –> reduce peak plasma concentration in single shot blocks and increase the risk of drug accumulation after prolonged infusion
What are the initial signs of local anesthetic toxicity in the awake patient?
twitching, tinnitus, perioral numbness, and altered mental status
What is the max dose of ropivacaine, levobupivacaine, bupivacaine?
3mg/kg
How does the uptake of local anesthetics in children differ when compared to adults?
Higher CO increases uptake in kids –> higher initial plasma concentrations and reduced duration
How does plasma protein in children differ when compared to adults and how does this affect plasma concentration os local anesthetics?
lower plasma [protein] –> inc plasma levels of protein bound drug
Name at least 3 risk factors that increase the likelihood of a spinal hematoma formation after neuraxial block.
◾Advanced age.
◾Underlying coagulopathy.
◾Simultaneous administration of multiple medications affecting coagulation.
◾Abnormalities of the spinal cord or vertebral column.
◾Difficulty during needle placement.
◾Indwelling catheter during sustained anticoagulation.
What test can be used to monitor heparin administration?
activated partial thromboplastin time (aPTT)
therapeutic anticoagulation = aPTT to >1.5 times the baseline value
Platelet count should be monitored in patients who are receiving heparin for > 4 days –> heparin-induced thrombocytopenia (HIT)
LMWH having greater activity against factor Xa.
LMWH has a half-life that is 2-4x longer than that of UFH. LMWH NOT reversible with Protamine.
How soon after the last dose of therapeutic LMWH can a neuraxial block be performed?
12hrs after last prophylactic dose of enoxaparin (0.5mg/kg).
24hrs after last therapeutic dose of enoxaparin (e.g. 1mg/kg every 12hrs or 1.5 mg/kg daily), dalteparin (120 U/kg every 12 hours or 200 U/kg daily) or tinzaparin (175 U/kg daily).
Name 2 reversal agents for warfarin
vitamin K
FFP
What is the mechanism of heparin?
binds to antithrombin III, causing a conformational change in antithrombin that facilitates its ability to inactivate thrombin (factor IIa), factor Xa, and factor IXa
What are the ASRA guidelines for neuraxial block with subcutaneous UFH?
5,000U BID dosing = no contraindication
TID dosing = caution, safety not established
What are the ASRA guidelines for neuraxial block with warfarin?
INR ≤ 1.4 for block placement and catheter removal
According to the ASRA 2010 guidelines, how many days should clopidogrel be held before a neuraxial block can be considered?
discontinue clopidogrel for 7 days and ticlopidine for 10 to 14 days before a neuraxial injection
How long should you wait to perform regional techniques with GPIIb/IIIa inhibitors?
Abciximab - 48 hours
Etifibitide - 8 hours
Tirofiban - 8 hours
