4. Pregnancy Flashcards
Which general issues should be considered when planning to anaesthetise a pregnant woman?
General anaesthesia during pregnancy is associated
with an increased risk of intrauterine growth retardation,
early labour and early infant death.
For these reasons, elective surgery
is contraindicated during pregnancy. If
surgery cannot be delayed until after delivery,
ideally it should take place in the second trimester.
> Less than 2 weeks’ gestation: anaesthesia will have an ‘all or nothing effect’ on the pregnancy, i.e. it will continue apparently unaffected or will be lost.
> 3–8 weeks’ gestation: organogenesis takes place, and anaesthesia is best avoided.
> 8 weeks+: risk of growth retardation with anaesthesia.
> Third trimester: risk of precipitating premature labour is increased.
> Concerns are growing that exposure to anaesthesia may adversely affect neurodevelopment in the developing brain.
This may have implications for
the fetus, but at the time of publication there is not a great deal of data from this emerging field.
The following factors are of importance:
> There are two patients to consider,
though the mother’s needs must take priority.
- The case should be discussed with a senior anaesthetist.
- Surgery should be carried out by a senior surgeon who can complete the procedure in a timely manner.
> The type of surgery proposed.
• Consider whether regional anaesthetic techniques would be appropriate.
> The stage of the pregnancy and its effect on maternal physiology (for details see answer on ‘Pregnancy’ in Study Guide 1, Chapter 37).
• Stomach emptying may be delayed
from 12 weeks onwards, so do not use a laryngeal mask airway beyond the first trimester; patients
should be intubated.
• The most teratogenic period is thought to be 31–71 days.
From 24 weeks onwards, the fetus may potentially survive outside the uterus.
• The choice of anaesthetic drugs and their potential effects on the fetus must be taken into consideration (see list of drug effects at end of this
question).
How would you anaesthetise a woman who
is 24 weeks pregnant for an appendicectomy?
> Pre-operatively:
> Pre-operatively:
- Inform the anaesthetic consultant of the patient.
- Confirm the diagnosis and the need for the operation with the surgeons.
- Inform the obstetricians of the plan for surgery and arrange appropriate fetal monitoring (see below).
• Take consent from the woman for rapid sequence induction of general
anaesthesia.
> Induction:
> Induction:
• Place the table on a left lateral tilt, to reduce the risk of aorto-caval compression by the gravid uterus
(All pregnant patients should be ‘tilted’ from 20 weeks’ gestation onwards)
- Gain IV access with a large bore (≥16 G) cannula.
- Have a low threshold for inserting an arterial line, both to monitor BP and to allow regular blood gas analysis.
- Pre-oxygenate at 30° head up to maintain FRC.
- Ensure adequate preoxygenation, as pregnant women have a reduced FRC and increased oxygen consumption and consequently desaturate more rapidly.
In addition, intubation may be more difficult
because of enlarged breasts making insertion of the laryngoscope awkward, so consider using a short-handled laryngoscope.
• Induction drugs: thiopentone 5 mg/kg and suxamethonium 2 mg/kg.
> Intra-operative:
• Avoid hypoxia or hypercarbia as these will alter blood flow to the placental bed. The fetus can adapt to short periods of maternal hypoxia by increasing its oxygen extraction,
but in the face of longstanding hypoxia, the utero-placental bed will constrict and fetal hypoxia will result.
Maternal hyperoxia is not detrimental to the fetus
but maternal hypercarbia results in fetal respiratory acidosis as the CO2 passes freely across the placenta.
- MAC falls in pregnancy, but maintain adequate end-tidal volatile concentration to ensure that the patient is not aware.
- Maintain relaxation with atracurium, rocuronium or vecuronium, none of which cross the placenta in significant amounts.
- Provide analgesia with paracetamol and morphine.
Do not give NSAIDs in the third trimester as they promote closure of the ductus arteriosus.
- Give fluid, guided by clinical parameters: BP, heart rate and urine output.
- If vasopressors are needed, ephedrine has a long history of use in obstetric anaesthesia and is thought to have least effect on the placental blood flow, although phenylephrine and metaraminol are used as first-line therapies in many units.
- The issue of whether the fetus should be monitored with a CTG intra-operatively is contentious and depends on several factors:
CTG monitoring
> Monitoring is useful only if it will change management and so if the fetus is unable to survive outside of the uterus (traditionally accepted to be <24 weeks’ gestation) there is no useful reason to monitor it intra-
operatively.
> I f the fetus is viable, there may be an argument to monitor to allow delivery, should it show signs of distress; however, the fetus will also experience the effects of general anaesthesia administered to the mother and so its CTG trace will lose variability and become difficult/impossible to interpret, rendering monitoring useless.
> I f monitoring is applied, there must be someone present at all times who can interpret it and make the necessary decisions based on the evidence available.
> A s a primary candidate, you should have a grasp of the issues above. In reality, it is sensible for the pregnant woman to have a CTG performed
before the operation and after recovery from the anaesthetic to assess the condition of the fetus.
> Post-operatively:
> Post-operatively:
- Extubate the patient awake and sitting up. Check for an air leak with the cuff deflated to ensure no laryngeal oedema.
- Ensure sufficient analgesia; the patient must be able to cough and breathe easily to avoid chest infection, hyperventilation causing a respiratory alkalosis or hypoventilation causing acidosis.
- Discharge to an appropriate level of care, depending on patient’s condition.
THE EFFECT OF DRUGS ON THE MOTHER AND THE FETUS
At 24 weeks, the fetus is past the ‘teratogenic window’ (31–71 days).
There are few certainties about the use of
drugs in pregnancy, and although many anaesthetic agents are thought to be safe from anecdotal evidence, the manufacturers will not officially endorse their use in pregnancy.
NITROUS OXIDE
Inhibits methionine-synthase
+ tetrahydrofolate reductase
and has been shown to be teratogenic in rats.
However, this was when used at high
concentration for prolonged periods.
The clinical implication of this, when used
for a short period of time in humans, is questionable, and certainly it provides the mainstay of analgesia during labour.
VOLATILE AGENTS
There are many conflicting studies, some-showing potential teratogenicity in animals,
although the design of some of these is questionable. At best, the clinical relevance of these findings is not clear.
Most would advocate the use of
isoflurane or sevoflurane where necessary.
THIOPENTONE
Teratogenic in high doses in animals.
Causes a fall in uteroplacental blood flow
(up to 35%), which is not sustained.
This is the intravenous induction agent of
choice.
PROPOFOL
Does not alter uterine blood flow,
but studies show it decreases perinatal
survival
and so the manufacturers advise against using in pregnancy.
Despite this, it is used as the first line induction agent for pregnant women in many centres because of its familiarity amongst anaesthetists along with the decline in the use of thiopentone.
This trend is likely to continue, especially in light
of the finding of NAP 5, which showed a higher incidence of accidental awareness in pregnant women undergoing Caesarean section under
GA (1:670) using thiopentone.
NON-DEPOLARISING
MUSCLE RELAXANTS
Non-teratogenic, and these
bulky drugs do not cross the placenta well. Their
duration of action may be prolonged, presumably due to altered hepatic metabolism. This is not true of atracurium because of Hoffman degradation.
AMIDE LOCAL
ANAESTHETICS
Not teratogenic.
OPIOIDS
Considered safe in pregnancy at appropriate doses.
