eLFH - Pregnancy and Drugs used by Anaesthetists Flashcards by Thomas Eraifej

Categories of effects of of pregnancy on anaesthetic drugs used

General effects

Uterine effects (including placental blood flow)

Neonatal effects (of maternally administered drugs)

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General effects of volatile agents in pregnancy

MAC is reduced in pregnancy

With controlled ventilation, alveolar equilibration is slowed due to higher pulmonary blood flow

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Uterine effects of volatile agents in pregnancy

Reversible decrease in uterine muscle tone is clinically important at MAC 1.5 - 2

May contribute to PPH at delivery
Resistant to oxytocin

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Neonatal effects of volatile agents in pregnancy

Small, highly lipid soluble molecules of volatile agents cross the placenta freely

But are very rapidly exhaled by the newborn within first few minutes of life

No prolonged effects in newborn

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General effects of induction agents in pregnancy

IV bolus initially delivered to vessel rich group of organs causing initial effects.

Initial effects wear off due to redistribution to lower blood flow but larger tissues (eg skeletal muscle)

Uteroplacental unit has very high blood flow at term so receives high proportion of initial IV bolus

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Vessel rich group of organs

CNS
Heart
Hepatic
Renal
Splanchnic bed

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Uterine effects of induction agents in pregnancy

No effect on uterine tone

Placental perfusion is related to maternal BP

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Effect of induction agents on maternal BP

Propofol - most maternal hypotension

Ketamine - least hypotension

Thiopental - intermediate effects

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Neonatal effects of induction agents in pregnancy

Thiopental + Propofol are highly lipid soluble - cross placenta freely

Maternal redistribution causes rapid decreasing blood concentrations in mother and foetus before delivery

No major neonatal depression seen beyond first few minutes of birth

Foetal neurobehavioral scores reveal subtle decreases that last for up to 48 hours

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General effects of neuromuscular blockers in pregnancy

Serum cholinesterase activity is decreased at term and for 48 hours post partum - usually not enough to affect duration of suxamethonium clinically

Magnesium used for eclampsia prolongs action of non-depolarising neuromuscular blockers

Rocuronium has biliary clearance so may have prolonged effects due to cholestasis caused by oestrogens

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Uterine effects of neuromuscular blockers in pregnancy

No effect on uterine smooth muscle tone

Only affect skeletal muscle

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Neonatal effects of neuromuscular blockers in pregnancy

Neuromuscular agents are highly ionised so very low concentrations cross the placenta

No clinical effects in the newborn

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General effects of neuraxial opioid use in pregnancy

No impairment of breastfeeding

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Uterine effects of neuraxial opioid use in pregnancy

No significant uterine effects

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Neonatal effects of neuraxial opioid use in pregnancy

Lipophilic opioids (eg fentanyl) with doses closer to systemic levels and rapid absorption are more likely to produce effects (eg side effect profile inc respiratory depression) as more placental transfer occurs

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General effects of local anaesthetics in pregnancy

In late pregnancy, compression of IVC and subsequent engorgement of epidural venous plexus decreases volume of spinal canal
Therefore neuraxial LA spreads further

Decreased alpha1 acid glycoprotein and albumin so increased levels of unbound portion of LAs
Therefore potential greater risk of systemic toxicity

Systemic absorption also greater due to higher local blood flow

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Uterine effects of LAs in pregnancy

No direct effects

Changes in placental perfusion related to maternal BP drop

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Direct neonatal effects of local anaesthetics in pregnancy

Dose and agent dependent

Less protein bound and ionised agents (eg lidocaine) cross placenta more and can sedate baby

Bupivacaine highly protein bound and has few neonatal effects

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Indirect neonatal effects of local anaesthetics in pregnancy

Drop in maternal BP causing reduced placental perfusion

Spinal cause slightly lower foetal pH than epidural or GA

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Tocolytic agent effects

Decrease uterine tone and contractions

Often in context of premature labour

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Duration of tocolytic agent use and intended benefit

24-48 hours

Goal to give time to complete steroids for foetal lung maturity and counselling

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Tocolytic agents used for postponement of premature labour

Nifedipine

Atosiban

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Nifedipine mechanism of action

Calcium channel blocker

Causes smooth muscle relaxation

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Nifedipine initial dose for tocolysis

20 mg

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Side effects of nifedipine

Tachycardia Hypotension Headaches Dizziness

Atosiban mechanism of action

Oxytocin receptor antagonist

Atosiban administration

IV bolus followed by infusion

When is Atosiban preferred to Nifedipine

When there is CVS instability due to nifedipine side effects

When are tocolytic agents used for acute management of uterine hypertonicity

When causing foetal distress - eg during induction or augmentation of labour (when stopping uterotonics alone is insufficient)

Tocolytic agents used for acute management of uterine hypertonicity

Terbutaline Nitrates

Terbutaline mechanism of action

Beta agonist

Terbutaline dose and administration

0.2 to 0.5 mg subcutaneously can be repeated

Terbutaline side effects

Tachycardia

Nitrates mechanism of action

Directly acting smooth muscle relaxant

Nitrates administration

Sublingual Buccal

Tocolytic agents used for intraoperative uterine relaxation

Volatile agents Terbutaline

Volatile agent mechanism of action for uterine relaxation and dose

Directly actine smooth muscle relaxant Increasing MAC to 1.5 - 2 causes uterine relaxation

Other tocolytic agents

Magnesium Crystalloid IV fluid bolus 10-15 ml/kg

Magnesium mechanism of action of tocolysis

Directly acting smooth muscle relaxant Not licensed for tocolysis but often used for pre-eclampsia

Side effects of magnesium

Muscle weakness Prolongation of neuromuscular blockade

Crystalloid IV bolus mechanism of action of tocolysis

Unknown mechanism but may have posterior pituitary effect that reduces oxytocin secretion

Uterotonic agent effects

Increase uterine tone and contractions

Uterotonic agent uses

Induction and augmentation of labour Active management of third stage of labour Prophylaxis and management of PPH

Most common cause of PPH

Atonic uterus

Risk factors for PPH where prophylactic uterotonics should be considered

Prolonged or augmented labour Instrumental or operative delivery Multiple gestation Previous PPH

Uterotonic agents

Oxytocin Ergometrine Prostaglandins

Oxytocin mechanism of action

Syntocinon - synthetic polypeptide identical to human oxytocin Mediates uterine contraction in normal labour

Syntocinon presentation

Clear colourless solution 5 or 10 IU in 1 ml

Syntocinon administration

IV or IM Give slowly IV as bolus can cause hypotension and reflex tachycardia Use separate line if transfusing blood or plasma as oxytocin is rapidly metabolised but plasma oxytocinase

Syntocinon dose

5 IU by slow IV bolus Repeated once (total 10 IU) Followed by infusion Eg. 40 IU in 500 ml saline at 125 ml/hour (10 IU per hour)

Syntocinon side effect

Similar in structure to ADH Can cause water retention and hyponatraemia

Ergometrine mechanism of action

Alkaloid of the ergot fungus Potent smooth muscle constrictor

Ergometrine side effects

Vasoconstriction / HTN Potent emetic

Ergometrine contraindications

Pre-eclampsia Cardiac / vascular disease Porphyria

Ergometrine presentation

Clear colourless solution of 0.5 mg in 1 ml

Ergometrine dose and administration

0.5 mg IV or IM Usually given in combination with syntocinon as syntometrine

Syntometrine

Combination of Syntocinon 5 IU + Ergometrine 0.5 mg If ergometrine contraindicated then usually give syntocinon 5 IU alone

Prostaglandin examples and route of administration

Dinoprostone - PV Misoprostol - PV or PO or PR Carboprost - IM

Prostaglandin mechanism of action

Direct uterotonic effect Potent smooth muscle constrictors

Prostaglandin side effects

Bronchospasm

Relative contraindication of prostaglandins (especially carboprost)

Asthma

Carboprost dose

0.25 mg IM repeated every 15 mins up to a total of 2 mg

Typical dose of misoprostol

0.6 to 0.8 mg rectally

Order in which uterotonic medications should be administered for prophylaxis and management of PPH First line to fourth line treatments

Syntocinon - IV Ergometrine - IM / IV Carboprost - IM Misoprostol - PR