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.