Pharmacology and Neuraxial Anesthesia in Pregnancy Flashcards
What law/ principle explains how drugs traverse a biologic membrane?
Fick prinicple
What is the Fick principle? (formula)
Rate of diffusion = [diffusion coefficient x surface area x concentration gradient (between mom and fetus)] / membrane thickness
What are the most important variables in determining rate of diffusion of a drug?
Diffusion coefficient
Concentration gradient between mother and fetus
How does fetal acidosis affect drug transfer from mom to fetus?
It can increase the concentration gradient, leading to ion trapping
What drug characteristics favor placental transfer of the drug?
Low molecular weight (< 500 Daltons)
High lipid solubility
Non- ionized
Non- polar
What drugs have significant placental transfer?
Local anesthetics (except choloroprocaine)
IV anesthetics (usually not a problem)
Volatile anesthetics
Opioids
Benzodiazepines
Atropine
Beta- blockers
Magnesium (not lipophilic, but small)
What are special considerations for bupivicaine administration in pregnancy?
Low placental transfer d/t high protein binding and ionization
Greater sensory block relative to other locals
Risk of cardiac toxicity > cardiac sx before seizures in toxic doses
How does neuraxial ropivacaine compare to bupivacaine?
Less cardiac toxicity
Less potent
Less motor block
Why is lidocaine unpopular for labor analgesia?
Strong motor block
Risk of neurotoxicity if given in the subarachnoid space
How is 2-Chloroprocaine helpful for neuraxial labor analgesia?
Useful for emergency C-section when epidural is already in place d/t fast onset
How does 2-Chloroprocaine affect neuraxial opioids?
Antagonizes opioid (mu and kappa) receptors and reduces the efficacy of epidural morphine
When administered in a neuraxial block, what is the relationship between opioids and local anesthetics?
Synergistic
How are neuraxial opioids beneficial when administered alone?
No loss of sensation or proprioception
No sympathectomy
Do not impair ability to push
What opioid has local anesthetic properties?
Meperidine
What are side effects of neuraxial opioids?
Pruruitus (most common)
Nausea/ vomiting
Sedation
Respiratory depression
What are downsides of administering neuraxial opioids alone?
Lack of perineal relaxation
Less analgesia vs locals
Why is lidocaine not used as a continuous epidural infusion?
Tachyphlylaxis is more likely to develop
Crosses the placenta to a greater degree than other locals
What are three ways a patient can develop a total spinal?
An epidural dose injected into the subarachnoid space
An epidural dose injected into the subdural space
A single shot spinal after a failed epidural block
What is the patient management when an epidural dose is injected into the subarachnoid space?
Intubate the patient and stabilize her
How is subdural placement of an epidural catheter ruled out?
It is rare, but cannot be ruled out. Neither a test dose or catheter aspiration will rule out subdural placement.
What are the symptoms of an epidural catheter placement in the subdural space?
Symptoms will occur 10- 25 minutes after epidural placement
The space is very low volume > higher block height than expected for the given dose in the epidural space
Sensory and autonomic block is usually more intense than a motor block
What is the presentation of a total spinal?
Usually rapid progression of sensory and motor block (prolonged onset in subdural injection)
Dyspnea, difficulty phonating, hypotension
Loss of consciousness occurs as a result of cerebral hypoperfusion sectondary to severe hypotension
What is the anesthetic management for a total spinal?
Vasopressors, IVF, LUD, leg elevation to manage hemodynamics
Intubate trachea if pt loses consciousness or is unable to protect airway
What are differential diagnoses for total spinal?
anaphylactic shock, eclampsia, amniotic fluid embolism
