Cardiac disease in pregnancy pathophysiology
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There are significant hemodynamic changes associated with pregnancy that begin early, reach their peak during the second trimester, and persist through delivery. These changes include a 40% increase in blood volume expansion, reductions in both the systemic vascular resistance and pulmonary vascular resistance, a 30% rise in cardiac output and little change in the blood pressure. These changes can have a significant impact on both the mother and the fetus, particularly when there are maternal cardiac disorders.
Effect of Pregnancy on Maternal Physiology
I. Hormonal Changes
Increased Progesterone Levels
Increased Estrogen Levels
- Elevated estrogen levels may increase myocardial contractility.
Increased Renin and Aldosterone Levels
- Peripheral vasodilation subsequently causes renal vasodilation and activation of the renin-angiotensin-aldosterone system, which results in:
- Increased sodium and water retention.
- Compensation for the decreased systemic vascular resistance.
II. Plasma Volume Expansion
- Plasma volume expansion starts as early as 6-weeks of gestation and is increased to approximately 40-45% by the mid trimester.
- Plasma volume expansion → hemodilution → anemia
- Despite the development of anemia, the total red cell mass is not decreased because the rate of rise in plasma volume is more than rate of rise in red cell mass. This occurs until 30-weeks of gestation and is referred to as the physiologic anemia of pregnancy.
- The hematocrit may drop to 33-38%.
- A greater increase in blood volume may be observed among multigravidas.
- An increase in atrial natriuretic peptide levels is observed in response to changes in intravasular volume.
III. Cardiac Output
- There is a higher volume of more dilute blood to circulate.
- There is approximately a 50% increase in cardiac output which is required to oxygenate the fetus.
- The increase in cardiac output begins as early as the 5th week of gestation and steadily increases up to the 24th week of gestation after which time it plateaus.
- Several factors influence the changes observed in cardiac output during pregnancy. Serial hemodynamic measurements performed in the supine position may be erroneous secondary to the compression of the inferior vena caval by the enlarging uterus which subsequently decreases the venous return from the lower extremities. Therefore, owing to the caval compression, cardiac output has been shown to decline in the supine position whereas the cardiac output increases in the left lateral position.
IV. Blood Pressure
- A drop in the blood pressure is caused by a decline in systemic vascular resistance secondary to reduced vascular tone. This is mediated by gestational hormone activity, increased circulating levels of prostaglandins and atrial natriuretic peptides, as well as endothelial nitric oxide.
- The blood pressure remains relatively unchanged when measured in the left lateral recumbent position. However, the supine hypotensive syndrome of pregnancy occurs in approximately 11% of pregnant women and is often associated with weakness, lightheadedness, nausea, dizziness and even syncope. Acute compression of the inferior vena cava by the gravid uterus is a possible explanation for this syndrome. Symptoms usually subside when the patient changes from the supine position.
V. Respiratory Rate
VI. Gastrointestinal Changes
- Reduced gastric emptying secondary to reduced gastrointestinal motility is observed during pregnancy.
- Intra-gastric pressure increases during the last trimester.
VII. Hematologic Changes in Pregnancy
VIII. Other Changes in Pregnancy
- Flared ribs.
- Breast hypertropy which may impede effective resuscitation.
Physiology of Labor and Delivery
Hemodynamic Changes During Labor and Delivery:
- Hemodynamics are altered substantially during labor and delivery secondary to increased sympathetic tone caused by anxiety, pain, and uterine contractions.
- These changes include:
- Oxygen consumption increases by threefold.
- Cardiac output rises progressively during labor owing to an increase in both stroke volume and heart rate. The work of labor has also shown to increase cardiac output by 60% over the baseline.
- Blood pressure is higher in the left lateral position. During uterine contractions, a marked increase in both the systolic and diastolic blood pressure is observed with a greater augmentation during the second stage of labor. The form of anesthesia impacts the blood pressure.
- During the second stage of labor, an increase in venous stasis, heart rate (greater than 120 beats/min) and blood pressure (more than 150 mmHg) is observed.
- By the time of delivery, cardiac output has increased by 50%, the plasma volume has increased by 40% and the red cell mass has increased by 25% to 30%.
- Immediately following delivery, the uterus contracts and delivers a sudden bolus of 500-750 cc of blood to the circulatory system which may result in pulmonary edema in patients with coronary heart disease.
Hemodynamic effects of Cesarean Section:
- To avoid the hemodynamic changes associated with vaginal delivery, cesarean section is frequently recommended for women with cardiovascular disease.
- Cesarean section is also associated with hemodynamic fluctuations that are related to intubation, analgesia and the choice of anesthesia. Most common form of anesthesia used is epidural which significantly reduces pain but is also associated with significant peripheral vasodilation and subsequent hypotension.
- Cesarean section is associated with greater extent of blood loss as well as provides relief of caval compression.
Hemodynamic changes Postpartum:
- There can be a temporary increase in venous return immediately after delivery due to relief of caval compression in addition to blood shifting from the contracting uterus into the systemic circulation.
- Effective increase in venous return and blood volume, despite the blood loss occurred during delivery, results in a substantial increase in ventricular filling pressures, stroke volume, and cardiac output that may lead to clinical deterioration.
- Both heart rate and cardiac output return to pre-gestational values by an hour after delivery. However, it takes about 24-hours for the blood pressure and stroke volume to return to baseline.
- Hemodynamic adaptation of pregnancy persists postpartum and gradually returns to pre-gestational values within 12-24 weeks after delivery.
- Uterine blood flow increases by a factor of 50 during pregnancy.
- The uterine blood vessels remain dilated throughout pregnancy.
- Transfer of oxygen across the placenta is flow-limited.
- Fetal oxygen tension is normally quite low (30 to 40 mmHg).
- Supplemental oxygen to the mother is quite effective in increasing fetal oxygen, particularly with fetal distress.
- Normal fetal pH is 7.35. Fetal scalp pHs <7.25 are abnormal.
- Labor can precipitate fetal distress because during uterine contractions, uterine blood flow is nearly occluded.
- In a mother with cyanosis, it is easier for problems to arise during labor because of the reduced reserve in oxygen delivery.
- With contractions, there may normally be a reduction or deceleration in the fetal heart rate, but this rapidly returns to normal.
- In fetal distress, the decelerations are later in the contraction and persist, i.e. late decelerations.
- Fetuses do not die suddenly during labor, and there are many minutes or hours of fetal distress before death so that there is time to intervene.
- Placing the mother in the left lateral recumbent position and oxygen will relieve many cases of fetal distress.
- Fetal monitoring should be used in the presence of maternal heart disease, cardiac surgery, cardioversion.
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