Ann Thorac Surg 1996;61:1865-1869
© 1996 The Society of Thoracic Surgeons
Current Review
Cardiopulmonary Bypass During Pregnancy
Andrew J. Parry, FRCS,
Stephen Westaby, FRCS
Oxford Heart Centre, John Radcliffe Hospital, Oxford, England
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Abstract
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Despite the incidence of heart disease during pregnancy falling to 1.5% over the last 25 years, when a cardiac operation is required the risk is obviously greater as two lives are at risk. The risk to the mother is now similar to that for nonpregnant female patients (3% overall) but the fetal mortality remains high (19%). Cardiac operation is ill advised except in extreme emergencies during the first two trimesters as the incidence of teratogenesis is high. During the third trimester, with improvements in the outcome for premature infants with modern neonatal intensive care, delivery of the child immediately before commencing cardiopulmonary bypass is a safe option. If this is inappropriate, high-flow, high-pressure, normothermic bypass for as brief a period as possible should be used. However, although it has theoretic advantages, the benefit of pulsatile perfusion is unproven. The fetal response to cardiopulmonary bypass is bradycardia thought to be due to hypoperfusion secondary to uterine contractions, and this dysrhythmia is reversible by increasing the perfusion rate. Fetal heart rate monitoring is therefore essential to allow these manipulations. The response of the fetoplacental unit is more complex, comprising two elements: an early vasoactive response is due to prostaglandin synthesis, whereas a more profound late acidosis appears to be related to a fetal stress response. Whether these responses can be modified by changes in our approach to cardiopulmonary bypass in pregnant women remains to be proven. Finally, uterine contractions occur in response to bypass, possibly due to a dilutional effect from the stabilizing influence of progesterone. Various techniques to modify this include the administration of progesterone, ß2-agonists, and intravenous alcohol, all with some effect. Uterine monitoring is essential to allow early control of these contractions as they are associated with significant fetal loss.
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Introduction
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In Europe and North America the incidence of maternal heart disease during pregnancy has fallen from 3.6% to 1.5% over the last 25 years [1]. Rheumatic valve disease accounts for 60% to 75% of this [2], with bacterial endocarditis occurring in 1 per 8,000 pregnancies [3]. However, when a cardiac problem requires an operation during pregnancy the risks are inevitably increased and substantial efforts must be made to reduce the risk to both mother and fetus. Closed mitral valvotomy has been used frequently with success to carry the mother through the pregnancy [4, 5], but although percutaneous balloon valvuloplasty may develop a role when adequate imaging can be performed by echocardiography, at present the radiation exposure necessary is prohibitive.
See also Ann Thorac Surg 1996;61:259–68.
Cardiopulmonary bypass was first used during pregnancy in 1959. The patient underwent a pulmonary valvotomy and closure of an atrial septal defect when the fetus was at 6 weeks' gestation [6]. The mother survived but the fetus spontaneously aborted 3 months later. Experience increased over the following decade, and two early reviews in 1965 [7] and 1969 [8] identified 24 cases performed with an overall maternal mortality of 4.2% and a fetal mortality of 29.2%. Table 1
summarizes the reported experience of open heart operations in pregnant women to date.
Between 1969 and 1994 there has been a steady improvement in outcome with maternal mortality falling to 3% (4 deaths in 133 patients; one death was late due tohepatitis). Maternal mortality for a particular operation is therefore now similar to that for nonpregnant women of the same age group. However, although fetal mortality has also fallen, now being 19%, there still remains a significant risk to the child. Fetal mortality is greatest after closure of intracardiac shunts, particularly ventricular septal defects, and after aortic valve replacement, particularly for infective endocarditis. This may reflect both longer bypass times and a worse clinical status at the time of operation, although the data available do not allow a meaningful analysis of these variables.
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Fetal Age and the Timing of Cardiac Operation
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Although there is no proven relationship between the gestational age of the fetus and mortality [4, 7, 8] congenital malformations occur more commonly when cardiopulmonary bypass is performed during the first trimester [8, 15, 26]. This risk of teratogenesis due to drug administration and possibly cardiopulmonary bypass during the first trimester of pregnancy is always present, and any surgical procedure is ill advised during this time. This does not imply a universally unfavorable outcome, and 6 entirely successful pregnancies have been reported when cardiopulmonary bypass was carried out early in the first trimester before the patients were aware they were pregnant [4]. There are now many reports of fetal survival to term after operations performed in the second or third trimesters [16, 17], but interpretation of these results must be tempered by the knowledge that successes are reported far more frequently than failures.
Recent improvements in neonatal care have improved survival of premature infants greater than 28 weeks' gestation. For this reason, we and others have reported cesarean section to deliver the infant after heparinization and cannulation of the mother but before commencing cardiopulmonary bypass [25, 27, 34, 38]. Our patient did not experience hemorrhage from the placental bed. Martin and associates [34] performed an emergency cesarean section for fetal distress during bypass, with survival of both mother and child. We were prepared for this in a different patient but were misled by fetal monitoring with death of the fetus after aortic valve replacement (1979, unreported). We would consequently advocate delivery before cardiopulmonary bypass during the third trimester. In this situation the abdominal wall is best left open to allow access to the uterus to ensure that hemorrhage and a hematoma do not develop.
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Physiologic Changes With Pregnancy and Their Implications for Bypass
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When the operation cannot be delayed until the middle of the third trimester, an understanding of the physiologic changes that occur in the mother during pregnancy and the physiology of the fetoplacental unit is of paramount importance. In the mother, from the 12th to the 36th weeks of pregnancy cardiac output gradually increases to a peak of 50% above resting levels. Mean red cell mass increases, but this is outweighed by the increase in blood volume, which increases by 30%; hematocrit therefore falls. Heart rate correspondingly increases and oxygen consumption is elevated to 15% to 18% above nonpregnant levels [2, 3, 18]. Placental perfusion is dependent on mean arterial pressures of 70 mm Hg or greater [35] when the uterus is relaxed, but the required pressure necessarily increases when uterine contractions occur, these being more frequent with increasing gestational age.
The usual approach to bypass using hemodiluted, nonpulsatile, hyperoxygenated, hypotensive flow is therefore potentially detrimental to both mother and fetus. Theoretically therefore the approach to perfusion management should instead be to use high-flow (5 L/min or greater), high-pressure (70 to 75 mm Hg), normothermic bypass for as brief a period as possible [4, 16, 35]. However, despite the reasoned theoretic basis for this approach there have been anecdotal reports of successful outcomes for both mother and fetus after profoundly hypothermic circulatory arrest [39], but experience to date is insufficient to determine which factors in the management of this form of circulatory support are important in effecting a good outcome.* Normothermic perfusion may lead to problems with myocardial protection through early rewarming of the left ventricle; in one report a total of 3,500 mL of cardioplegia was required for satisfactory myocardial protection [35]. This may be avoided by a continuous cold pericardial irrigation circuit or, alternatively, continuous warm blood cardioplegia is effective [40] even with long cross-clamp times [9]. The latter may be the method of choice to prevent the crystalloid load imposed by frequently repeated boluses of cold crystalloid cardioplegia.
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Fetal Response to Cardiopulmonary Bypass
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No detailed experimental studies have yet been performed on the effects of maternal cardiopulmonary bypass on the fetus. Since the first report of the use of fetal heart recording during bypass by Koh and associates [31] in 1975, it has been known that fetal bradycardia almost invariably occurs at the onset of maternal cardiopulmonary bypass [31, 36, 41]. The reason for this dysrhythmia is unknown, but it does not appear to be related to maternal oxygenation or acid-base balance. It has been suggested that it is due to hypothermia at the onset of bypass [4, 31], but it has also been reported with the use of normothermic perfusion [35]. Some more recently, however, have claimed that high-flow, normothermic bypass can abolish this bradycardia [16], but this has not been confirmed by all groups [35]. However, all agree that an increase in perfusion rate corrects the bradycardia. The most convincing theory regarding its etiology is that it reflects placental hypoperfusion and subsequent fetal hypoxia. Koh and associates [31] reported the occurrence of fetal bradycardia with the onset of cardiopulmonary bypass and its cessation or even over-compensation when the maternal circulation was restored. It has therefore been suggested that uterine contractions occurring on a background of decreased perfusion and relative hypotension associated with cardiopulmonary bypass may produce inadequate placental perfusion with the fetal response of bradycardia [35]. This would explain why increasing the perfusion rate abolishes the bradycardia. Theoretically, therefore, pulsatile flow may be ideal during bypass to counteract the effects of uterine contractions, but to date there has only been 1 case reported in which pulsatile flow was used during pregnancy, and this case, despite high-flow, normothermic pulsatile perfusion, resulted in an intrauterine death [25]. At present, the best use of fetal heart rate monitoring is to allow adjustment of flow rates should fetal bradycardia occur.
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Fetoplacental Unit Response to Cardiopulmonary Bypass
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Placental function is dependent not only on the maternal side of the circulation but also on the fetal side with its responses to interventions. Much has been learned regarding the response of the fetoplacental unit to cardiopulmonary bypass from experiments into fetal cardiopulmonary bypass, and this information should affect the management of perfusion in pregnant women. Thirty to 60 minutes after the fetus is removed from bypass a severe, progressive respiratory acidosis develops [42]. This is a vasoactive phenomenon as sodium nitroprusside infusion reverses it by preventing the underlying elevation in placental vascular resistance [43]. The underlying cause is probably activation of eicosanoid products as further experiments have shown that inactivation of these products (particularly prostaglandin E2 and possibly thromboxane) [44, 45] using indomethacin and corticosteroids can prevent this respiratory acidosis. However a later, more intractable metabolic acidosis then develops 6 to 8 hours after bypass is discontinued [44]. Both of these profound acidoses can be associated with fetal death. Evidence is growing that this late metabolic acidosis is due to low cardiac output secondary to an increase in systemic vascular resistance. This may be due to an increase in catecholamine levels, which occurs with fetal manipulation and is part of the fetal stress response. The subsequent increase in systemic vascular resistance is poorly tolerated by the immature fetal myocardium. The most commonly used anesthetic agent, halothane, crosses the placenta and anesthetizes both mother and fetus, but it is known not to inhibit this stress response [46]. The fetus is therefore still vulnerable to excessive catecholamine release and the subsequent systemic vascular resistance changes. In the experimental setting of fetal cardiopulmonary bypass a spinal anesthetic to the fetus has been shown to prevent the stress response and subsequent metabolic acidosis [47], but this is clearly impractical in the clinical setting of maternal cardiopulmonary bypass. As clinical results to date show that maternal cardiopulmonary bypass is safe for the mother but puts the fetus at significant risk, experiments need to be performed to delineate the contribution of both of these forms of fetoplacental malperfusion to fetal loss during maternal cardiopulmonary bypass. More specifically, it should first be determined whether maternal premedication can reduce this high mortality by preventing placental malperfusion. Although both indomethacin and steroids have been shown to be beneficial in the experimental setting, the detrimental effects of indomethacin on platelet function with the subsequent risk of bleeding may place the mother at unreasonable risk, particularly in the perinatal period. Steroids would therefore be preferable in this context. Second, alternative forms of anesthesia should be studied that block the fetal stress response to prevent fetal systemic vascular resistance changes from causing the late metabolic acidosis.
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Uterine Response to Cardiopulmonary Bypass
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Uterine contractions occur frequently during cardiopulmonary bypass and are considered to be the most important predictor of fetal death [1, 17]. They are particularly common during the rewarming phase after moderate or profound hypothermia [4], further reinforcing the argument for the use of normothermic bypass, and they occur more frequently with increasing gestational age. The cause is not known, but it has been suggested that the dilutional effect of bypass causes a decrease in hormonal levels, particularly progesterone, which produces increased uterine excitability. Supportive evidence for this theory may be the fact that direct progesterone supplementation has been successfully used to stabilize the uterus around the time of bypass [35], although in the single case reported progesterone administration was only started postoperatively, and its efficacy in preventing labor perioperatively therefore remains questionable. Others have used ß2-agonists [10, 35] or intravenous alcohol infusions [35, 41] with similar good results, including in 1 patient in whom ß2-agonists were successfully used after the patient went into labor during bypass [10]. As discussed previously, uterine contractions may cause placental insufficiency and secondary fetal hypoxia during bypass. Gaining control of them is therefore crucial. The use of uterine monitoring is therefore recommended to allow early identification of contractions so that they can be adequately treated before their secondary effects of placental hypoperfusion and fetal bradycardia occur.
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Conclusions and Recommendations
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Much progress has been made in the management of cardiopulmonary bypass in pregnant women as evidenced by the decrease in maternal mortality to levels consistent with similar operations in nonpregnant women. However, the rate of fetal loss remains high. Questions remain on how best to manage the fetoplacental response to maternal cardiopulmonary bypass, and some answers are being provided from the fetal bypass research laboratory. No evidence has yet been provided, however, on the efficacy of similar manipulations in the pregnant mother, and future work must address this issue. However, from the evidence currently available the following guidelines may be recommended:
- Open heart operation should be avoided, if at all possible, during the first trimester. If this is not feasible, precautions as for the later stages of pregnancy should be taken.
- High-flow, high-pressure, normothermic bypass offers the least risk to the fetus.
- Fetal heart and uterine monitoring should be used to allow adjustments to the flow and pharmacologic manipulations to ensure adequate placental perfusion.
- When the fetus is more than 28 weeks' gestation, it is a safe option to deliver the child by cesarean section immediately before, and at the same operation as, the cardiac operation.
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Top
Abstract
Introduction
