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Obstetric IAC > Pregnancy and Drugs used by Anaesthetists > Flashcards

Pregnancy and Drugs used by Anaesthetists Flashcards

1
Q

What are the effects of volatile agents in pregnancy?

A

Uterine

  • decrease in uterine tone clinically important >1.5-2 MAC
  • at delivery may contribute to PPH
  • resistant to oxytocin

General

  • MAC reduced in pregnancy
  • with controlled ventilation alveolar equilibration during inhaled concentration is slowed due to higher pulmonary blood flow

Neonatal

  • small and highly lipid soluble molecules of volatiles will cross the placenta freely but are rapidly exhaled by the newborn - no prolonged effects
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2
Q

What are the general effects of induction agents in pregnancy?

A
  • IV bolus is initially delviered to vessel-rich organs (CNS, heart, hepatic, renal and splanchnic bed)
  • this wears off due to redistribution of the agent to larger tissue masses with lower blood flow (skeletal muscles)
  • the uteroplacental unit has a high blood flow at tern - so recieves a significant proportion of the IV bolus
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3
Q

What are the uterine effects of IV induction agents in pregnancy?

A
  • no effect on uterine tone
  • placental perfusion is related to changes in maternal BP
    • propofol causes most maternal hypotension
    • ketamine causes least
    • thiopental is intermediate
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4
Q

What are the neonatal effects of IV induction agents in pregnancy?

A
  • thio and propofol are highly lipid soluble, cross the placenta freely
  • maternal redistribution results in rapidly decreasing blood concentrations in both mother and fetus before delivery
  • no major neonatal depression is seen beyond first few mins of life
  • fetal neurobehavioural scores have a subtle decrease for 48hrs
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5
Q

What happens to serum cholinesterase activity at term and postpartum?

A

Activity is decreased.

Usually not enough to affect duration of action of succinylcholine.

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6
Q

What effect does Mg have on the action of non-depolarizing agents?

A

Prolongs the action

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7
Q

Why would the duration of rocuronium be prolonged in pregnancy?

A

Due to it’s biliary clearance, which is inhibited by oestrogens

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8
Q

Can neuromuscular agents cross the placenta?

A

They’re highly ionized and only cross the placenta in low concentrations - no clinical effect in newborn

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9
Q

Regarding neuraxial opioids, what are the effects of lipophilic opioids?

A

Eg fentanyl

Short duration and rapid onset.

Dose requirements are closer to systemic doses and more segmental distribution of analgesia.

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10
Q

What are the side effects of opioids given neuraxial routes?

A

Profound analgesia.

Respiratory depression, pruritis, nausea, vomiting are dose-related.

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11
Q

Does epidural fentanyl impair breastfeeding?

A

No

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12
Q

What are the hydrophilic opioids and what are their properties?

A

Eg morphine and diamorphine.

They have opposite properties to lipophilic opioids - longer duration of action and increased incidence of side effects.

The risk period is prolonged and delayed resp depression can occur many hours post dose.

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13
Q

What is more likely to produce systemic side effects, lipophilic or hydrophilic neuraxial opioids?

A

Lipophilic - because the doses are closer to systemic levels and have rapid absorption.

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14
Q

What are the general effects of local anaesthetics in pregnancy?

A
  • neuraxial LAs spread further for a given dose/volume because the uterus compresses the IVC which diverts blood through the epidural venous plexus, causing a decrease in the volume of the spinal canal
  • systemic toxicity is increased because of decreased serum α-1 acid glycoprotein and albumin levels. This increases the toxic, unbound fraction, particularly at high serum levels. This is most significant for highly protein-bound agents such as bupivacaine
  • the speed of onset is dependent on the proportion of LA that is in unionised lipophilic form
    • LAs with pKa closer to body pH are more unionized
    • raising the pH of the solution with bicarbonate decreases ionization but increases possiblility of drug error/precipitation
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15
Q

What are the effects of LAs on the uterus?

A
  • no direct effects
  • changes in placental perfusion are due to the BP effects of the block used
  • neuraxial block results in sympathetic blockade, causing hypotension and compounds aortocaval compression
  • infusions of alpha-agonists (eg phenylephrine) are now used routinely to maintain placental perfusion
    • less hypotension/nausea
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16
Q

What are the direct neonatal effects of LAs on the foetus?

A
  • LAs are dose-dependent and agent-dependent
  • less protein bound and less ionized compounds cross placenta more
  • lidocaine has both of these proterties and high doses result in a sedated baby
  • bupivacaine is highly protein bound and has few neonatal effects, but will be highly tissue bound
17
Q

What are the indirect LA effects on the fetus?

A
  • effects due to maternal hypotension and placental hypoperfusion most important
  • spinal anaesthesia results in slightly lower fetal pH than epidural or GA
  • maternal BP should be maintained at maternal baseline and alpha agonist infusions are best
  • ephedrine results in more hypotension and increases fetal metabolism - contributes to acidosis
18
Q

What are tocolytic agents used for?

A

Decrease uterine tone and contractions.

Can be used for acute management of uterine hypertonicity and to relax the uterus intraoperatively (eg in a difficult C-section with abnormal fetal lie/presenting part or a sustained contraction causing fetal distress).

Can also be used in preterm labour (pre 37 weeks) for 24-48hrs to buy time while completing antenatal corticosteroids for lung maturity.

19
Q

What drugs are used to postpone premature labour?

A

Nifedipine (calcium channel blocker)

  • relaxes smooth muscle
  • dose 20mg
  • SEs: tachycardia, hypotension, headaches, dizziness

Atosiban

  • oxytocin receptor antagonist
  • IV bolus then infusion
  • used if CVS intability
  • more expensive
20
Q

What drugs are used in the acute management of uterine hypertonicity?

A

Stopping uterotonics (eg oxytocin) can be sufficient.

Terbutaline

  • beta agonist
  • doses of 0.2 - 0.5mg SC can be repeated
  • SEs: tachycardia

Nitrates

  • direct acting smooth muscle relaxant
  • given SL or buccal
21
Q

What would you do for intraoperative uterine relaxation?

A

Volatiles - increasing MAC >1.5-2 causes uterine relaxation by directly acting on smooth muscle to relax it.

Terbutaline - beta agonist, 0.2-0.5mg SC

22
Q

What are uterotonic agents used for?

A
  • to initiate labour in women not yet contracting
  • to augment contractions in labour that is not progressing satisfactorily
23
Q

Which uterotonics are used at induction and augmentation of labour?

A

Prostaglandins (eg dinoprostone) vaginally.

Misoprostol, given vaginally or orally, for induction.

Synthetic oxytocin (syntocinon) can be used as IV infusion to induce/augment labour.

24
Q

What uterotonics are given at the 3rd stage of labour?

A

To augment contractions and speed delivery of placenta.

Syntometrine (syntocinon 5IU and ergometrine 0.5mg in 1ml) IM is used.

Or syntocinon 5IU IV or IM alone if ergometrine is contraindicated.

25
Q

What is the commonest cause of PPH?

A

Atonic uterus

26
Q

When should prophylaxis for an atonic uterus be given?

A
  • prolonged/augmented labour
  • instrumental/operative delivery
  • multiple gestation
  • previous PPH
27
Q

What is oxytocin?

A
  • synthetic polypeptide identical to human oxytocin (natural posterior pituitary hormone that mediates uterine contractions in normal labour)
  • clear colourless solution (5 to 10IU in 1ml)
  • given IV or IM
  • bolus causes transient hypotension with reflex tachycardia
  • structurally similar to vasopressin(ADH) - can cause water retention and hyponatraemia
  • use a separate line if transfusing blood/plasma because oxytocin is rapidly metabolised by plasma oxytocinase
28
Q

What is ergometrine?

A

An alkaloid of the ergot fungus and potent smooth muscle constrictor.

Clear colourless solution, 0.5mg in 1ml IV/IM.

Can cause vasoconstriction. Potent emetic.

29
Q

What are the contraindictations to ergometrine?

A
  • pre-eclampsia
  • cardiac/vascular disease
  • porphyria

(causes vasoconstriction)

30
Q

What are prostaglandins?

A

Eg carboprost and misoprostil

  • direct uterotonic effect
  • potent smooth muscle constrictors
  • SE: bronchopasm - relatively CI in asthmatics
31
Q

What is the recommended sequence of uterotonic drugs in PPH?

A
  1. Syntocinon 5IU bolus (repeated x1 then infusion)
  2. Ergometrine 0.5mg IM
  3. Carboprost (hemabate) 250mcg IM every 15 minutes, up to 8 doses
  4. Misoprostol (600-800 mcg rectally)
32
Q

Does more or less of an IV induction bolus reach the fetus if given during a contraction?

A

Less.

Placental blood flow and therefore drug transfer are reduced during contractions.

33
Q

How is the ED50 of local anaesthetics affected by pregnancy?

A

It is reduced.

34
Q
A

Obstetric IAC (9 decks)

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