Page 29 - ANZCP Gazette APRIL 2022

weeks gestation, which is too early for monitoring on CPB. It is possible to perform a fetal middle cerebral artery (MCA) doppler assessment from 18 weeks onwards, and then assessment of the umbilical artery and uterine artery, however these techniques require access to the abdomen, vagina, and groin respectively, which is impractical during a cardiac operation. From 22 weeks onwards a cardiotocograph (CTG) is an option. A CTG is more practical during surgery as it comprises a strap that sits around the abdomen, but there are issues around sterility, and it is positionally sensitive. Additionally, a CTG is difficult to interpret at less than 26 weeks and under a general anaesthetic, the fetus is also anaesthetised reducing fetal HR variability. Uterine contraction monitoring is also measured around the abdomen with the same associated sterility issues.
The patient was scanned pre and post-operatively to assess the fetal status. This may be the only feasible option depending on the expertise and equipment available as well as the gestational age of the fetus. During other types of non-obstetric surgery, a CS may be performed intraoperatively if the monitoring suggests the fetus may be under too much stress. However, during cardiac surgery an intraoperative CS would not be a favourable option, therefore optimisation of perfusion is the best approach. The optimisation of perfusion will be discussed in the following paragraphs.
Flow and pressure
During pregnancy maternal cardiac output increases dramatically. The sharpest rise can be seen at the beginning of the first trimester, with continued increases into the second trimester. By the end of the second trimester the increase can be up to 45% in a single pregnancy and 60% in a twin pregnancy (16).
It follows that a higher than normal flow rate on CPB will be required to provide adequate perfusion in a pregnant patient. Most authors recommend flow rates of at least 2.5 to 3.5 L/ min/m2 (17). Some authors also suggest the use of pulsatile flow on CPB to increase blood flow to the placenta using the roller pump (18) or an intra-aortic balloon pump (19)(20). If the option of pulsatile flow is not available, or it is not standard practice, then Mushtaq and Tufail (21) recommend a modified cardiac index of 3.0 L/min/m2 to sustain adequate fetoplacental gas exchange during nonpulsatile flow.
Fetal bradycardia is an indicator of fetal stress and is said usually to be reversible by increasing the perfusion pressure (19). Fetal monitoring was not possible during our case, so a cardiac index of at least 3.0 L/min/m2 was maintained throughout to keep perfusion pressures elevated.
Maintenance of a MAP above 70 mmHg is consistently recommended in the literature (22) (23) (24). When uterine contractions occur (seen more frequently with increasing gestational age) the blood pressure required to ensure adequate placental perfusion increases above 70 mmHg (9). If possible, it is useful to monitor for uterine contractions so that the requirement for higher pressures is known. If the maternal MAP is low, then CPB flow should be increased as vasoconstrictors reduce utero-placental blood flow and should ideally be avoided (25). If MAP is still low, then vasoconstrictors can be used to maintain perfusion pressure and to improve placental perfusion (10).
In the past, epinephrine has been used as the vasopressor of choice due to alpha agonists, such as phenylephrine and metaraminol, constricting uterine vessels. Although, recent studies suggest that this is not as important as initially thought (26). In 2019, Chao et al. (27) reviewed randomised controlled trials that compared the use of metaraminol with other vasopressors in spinal anaesthesia for CS. The results of this study indicated that metaraminol may be a more suitable vasopressor than ephedrine and its effects are at least not inferior to those of phenylephrine.
Tocolytic Therapy
As far back as the 70s and 80s, tocolytic therapy (the use of medications to suppress premature labour) has been used to stabilise the uterus in pregnant patients during CPB (14), (28). However, there are no proven benefits to routine administration of prophylactic perioperative tocolytic therapy in non-obstetric surgery (29). In fact, one category of tocolytic drugs, beta- agonists, has been shown to increase myocardial oxygen demand and myocardial work in a circulation that is already physiologically compromised by the burden of pregnancy and by the initiation of CPB (30). Volatile anaesthetics such as halothane, sevoflurane, desflurane and isoflurane, are shown to inhibit the uterine contractility, which may prove beneficial in preventing preterm contractions (31). The advantages and caution needed with a number of tocolytic agents are summarised in Table 2.
As mentioned previously, the measurement of uterine contractions may be problematic on CPB or it may be too early in pregnancy for measurements to be taken. However, tocometry during the postoperative period, with the improved access to the patient for monitoring, is useful as postoperative analgesia may mask awareness of mild early contractions and delay tocolysis (29).
The Case
Setup consisted of a Medtronic Affinity Fusion CPB circuit on a Stockert S5 heart-lung machine, standard central cannulation with a 24 fr Medtronic EOPA aortic cannula into the ascending aorta, and a 36/46 fr Medtronic two-stage venous cannula through the right atrial appendage. If the situation allows, it is best to avoid femoral cannulation. The fetal and uterine position may compress the inferior vena cava and therefore limit venous return. Femoral arterial cannulation can result in the hypoperfusion of the uterine blood vessels (33).

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