“Bicaval venous cannulation with right atrial opening to scavenge the cardioplegia solution may be necessary to protect the fetal heart from depression by the high potassium solution.”
Perfusion of the Pregnant Patient
To provide cardiopulmonary bypass to patients which suffer clinical cardiac disease during their pregnancy. When this cardiac disease has become exacerbated and surgical intervention becomes necessary, it should be carried out as early in the second trimester as possible following: (1) the conclusion of first trimester organogenesis; (2) prior to maximal maternal hemodynamic load; and (3) before cardiac decompensation occurs.
1. Usual and customary disposable and capital equipment necessary for a routine cardiac procedure.
2. **An oxygenator capable of oxygenating 30-50% more blood than would normally be required for the patient if she were not pregnant.
3. Normal cardiac monitoring found in the O.R. for a routine procedure.
4. Fetal cardiac monitor (bilateral probe placement is suggested).
5. Abdominal doppler unit and proper personnel for placement.
1. Normal CPB parameters
2. Maternal core temperature (> 34 C)
3. Maternal blood temperature (> 32 C)
4. Circulating potassium levels.
5. Strict anticoagulation management per institutional protocol pre-CPB and frequent anticoagulation monitoring tests while on CPB.
6. In-line monitoring of blood gasses, Hb (if possible), and venous saturation’s.
B. Fetal Monitoring:
1. Fetal heart rate.
2. Uterine contractions.
A. Place the patient in a mild, left lateral decubitus position by elevating the right hip 15 degrees. In doing so, the uterus is rotated away from the inferior vena cava which will optimize venous return to the caval cannula.
B. THE SUPINE POSITION SHOULD ALWAYS BE AVOIDED DUE TO AORTOCAVAL COMPRESSION.
A. In the absence of an emergent situation, cannulation of the femoral artery and vein should be avoided due to the possibility of decreased venous return and obstruction of the vena cava by fetal and uterine position.
B. Additionally, femoral cannulation may result in hypo-perfusion of the uterine blood vessels due to the non-physiologic retrograde arterial blood flow, even in the presence of increased pump flows.
C. Bicaval venous cannulation with right atrial opening to scavenge the cardioplegia solution may be necessary to protect the fetal heart from depression by the high potassium solution.
CONDUCT OF PERFUSION:
A. While the CPB system is unchanged, the perfusionist must be prepared to perfuse the pregnant patient at flow rates 30-50% higher than would normally be expected.
B. The adequacy of fetal perfusion can be monitored via the fetal heart rate. Fetal bradycardia is an indication of hypoxic ischemia and is usually alleviated by an increase in flow rate.
C. Pump flow should be calculated utilizing a cardiac index (CI) of 2.6-3.0 L/min/m2 with flows adjusted up or down in response to the fetal heart rate.
D. The perfusionist must take into account the patients increased blood volume (BV) with a disproportionate plasma volume (PV) increase, to calculate anticoagulation requirements.
E Pregnant patients frequently present with an increased antithrombin III titer, and require substantially higher than usual heparin doses. (It should be noted that heparin does not cross the placental barrier due to its large molecular weight (MW20,000) and therefore the fetus is not in jeopardy of intra-abdominal or intra-cranial bleeding seen with the heparinization of neonates.)
F. Although successful anticoagulation is reported with a 30% increase in the heparin dosage, maternal heparinization is reported to carry increased risks of uterine hemorrhage and placental disruption.
G. Strict heparin monitoring prior to initiation of bypass should be followed according to normal hospital protocol with more frequent coagulation testing performed throughout the CPB period.
H. The circulating blood volume of the pregnant woman increases steadily through at least the first two trimesters, and the rate of increase outstrips simultaneous weight gain.
I. Circulating Blood Volume (CBV) of the pregnant patient may be 9-10% of the body weight.
J. As the CBV increases, Hct decreases to a normal value of 30-32% during the third trimester. These physiologic changes should be accounted for in estimating bypass Hct, which should be maintained above 20-25% to assure adequate oxygen transport to the fetus.
K. Fetal compromise through cardiac arrhythmia’s or fibrillation in association with exposure to moderate and deep hypothermia has been very well reported in the literature. Most authors also conclude that fetal rewarming results in decreased fetal HR, and induces uterine contractions and may precipitate premature labor.
L. Unless long aortic cross clamp times are anticipated, surgery should be carried out under normothermic to mild hypothermic conditions. During CPB, maternal core temperature should start near normothermic levels and be allowed to drift slowly to 34C. Maternal blood temperature should not be permitted to fall below 32C.
M. Selection of the vasoactive and inotropic agents should be carefully considered. The four major concerns are:
1. The effect on the utero-placental blood flow.
2. The effect on uterine muscle tone and labor.
3. The direct and indirect fetal effects.
4. Undesirable maternal and fetal side effects.
N. Placental blood flow is regulated primarily by alpha receptors, therefore, alpha agonist should be strictly avoided to pharmacologically treat hypotension during CPB with pregnant patients. Epinephrine is the vasopressor of choice because at low doses it has a primary beta stimulatory effect.
O. Nitroprusside should be strictly avoided to prevent cyanide toxicity in both the mother and the fetus, and the chance that blood flow is shifted away from the fetus. Hydralazine becomes the vasodilator of choice in a hypertensive crises during CPB because in low doses it will decrease BP in the mother while increasing renal and uterine blood flow.
P. As previously stated under the section on cannulation, scavenging cardioplegia from the right atrium at the coronary sinus can be used to prevent fetal cardiac dysrhythmia and/or arrest. Drugs should be given to the pregnant patient ONLY WHEN THE BENEFITS JUSTIFY THE RISK TO THE MOTHER AND THE FETUS.
Brief Excerpts on Drug Therapy:
– Used for edema and hypertension. Crosses the placenta and may decrease placental perfusion. May prevent normal plasma volume expansion. Avoid if possible.
– May increase risk of congenital defects as a result of placental transfer. Induces electrolyte imbalance (hypoglycemia, hyponatremia, hypokalemia, and thrombocytopenia). Secondary fetal bradycardia.
– Crosses the placenta without causing fetal harm. Considered drug of choice for persistent fetal tachycardia.
– No known adverse effects. Increases uterine blood flow and may stimulate uterine contractions.
– May cause constriction of uterine blood vessels and reduce blood flow. May stimulate uterine contraction. Primarily alpha. Avoid this drug.
– Same as norepinephrine.
– Crosses the placenta. In low doses will decrease BP in mother while increasing renal and uterine blood flow. No association with congenital defects. Drug of choice for hypertensive crisis.
8. Sodium Nitroprusside:
– Contraindicated in pregnant patients because of the risk of cyanide toxicity to both mother and fetus. May shift blood flow away from the uterus. Crosses the placenta.
– Not related to congenital defects. Crosses the placenta and related to fetal thrombocytopenia. Used with digoxin to treat fetal supraventircular tachycardia. Second drug of choice after digoxin for the treatment of persistent fetal tachycardia
– Rapidly crosses the placenta. No evidence found for an association with fetal malformations. No significant changes were noted in fetal heart rate. No effect on uterine activity.
– Readily crosses the placenta. Intrauterine growth retardation may be related to this drug. Fetal bradycardia and fetal toxicity found to suggest a relationship to malformations. Lowers umbilical and uterine blood flow. Avoid this drug.
– Crosses the placenta with no fetal malformations noted A decrease in fetal heart rate may be observed.
– Placenta passage was demonstrated with no relationship to congenital defects. May reduce uterine blood flow with fetal hypoxia a potential risk. Avoid this drug if possible.