Aortic repair with fetus in utero
If delivery of the fetus is not possible— in general, this is at less
than 28 weeks— there are limited studies and evidence regarding CPB
and pregnancy, bar the consensus that it is a high-risk, with high fetal
and maternal mortality43. Fetal mortality is a
significant risk during CPB, with estimates of around 3 - 20%, with
highest risk in early gestational age8,40. If the
decision to perform aortic surgery with the foetus in-utero is made, it
is necessary to take steps to maximise chances of foetal survival.
Therefore, intra-operative conditions must be optimised to facilitate
this, the focus of which will be maintaining placental blood flow and
foetal circulation, and greater studies needed to focus on which
strategies are best.
Foetuses have limited ability to modify their stroke volume thus cardiac
output is highly dependent on heart rate44.
Consequently, it is important to minimise foetal bradycardia. Foetal
bradycardia has long been associated with poor placental perfusion as
described by Koh et al45. Early case studies have also
reported resolution of foetal bradycardia on increasing flow
rate46–48. This lead to some authors recommending
that CPB flow rate should be maintained over
2.5L/min/m2, with a MAP of
70-75mmHg49.
Haemodilution should be minimised, and there should be an
intra-operative target of a haematocrit above 25%50.
The proposed benefit is twofold; a high haematocrit increases the oxygen
carrying capacity of the blood, and delivery to the foetus, whilst
preventing significant reduction in the concentration of circulating
progesterone which can trigger uterine contractions and disrupt
placental blood flow9. Experimental evidence suggests
that use of pulsatile flow should be used when available as this can
increase uterine and placental perfusion by stimulating nitric oxide
production and local vasodilatation51.
Analysis of case studies has shown a link between hypothermia during CPB
and increased foetal mortality4,8. One hypothesis for
this increase in mortality is due to induced foetal bradycardia.
Hypothermia can also induce uterine contractions and labours,
particularly in the rewarming phase50. Therefore, many
authors recommend avoiding temperatures below 35 degrees. However there
have been case reports of successful deep hypothermic circulatory arrest
performed with foetus in-utero, and some bigger retrospective
studies8,28,41,52,53. Normothermic perfusion was
recommended by Becker in 1983, or antegrade cerebral perfusion with
moderate hypothermia, used by Bachet and Guilmet; no single technique
has been proven to greater efficacy than the other28.