Introduction
During pregnancy, several changes occur to the pregnant woman's body which would allow for the development and protection of the fetus, preparation for labor, and compensation for new demands. Numerous factors, such as maternal age and health conditions, multiple gestations, and genetic factors, can affect the ability of the mother to adapt to the increased demands of pregnancy. Pregnancy induces changes in the cardiovascular and respiratory systems that are necessary for meeting the increased requirements of the mother and the fetus.
Significant cardiovascular changes occur during pregnancy, including increased cardiac output, plasma volume, and reduced vascular resistance. Major respiratory system changes also occur in the upper airway, static lung volumes, chest wall, and ventilation and gas exchange. So an insight into the physiologic adaptations is pivotal for the clinician to distinguish the common physiological changes from disease states that occur during pregnancy.
What Physiological Changes Happen in the Cardiac System During Pregnancy?
Cardiovascular adaptations are highly markable during pregnancy. This is because the physical demands during pregnancy require physiologic changes that include increased heart rate, blood volume, cardiac output, and reductions in systemic vascular resistance and blood pressure.
Anatomical Changes:
The heart position will change as the pregnancy progresses toward the head and rotate to the left, and this is due to the elevation of the diaphragm and uterine growth. There will be growth in the four heart chambers, especially in the left atrium and left ventricle, and thickening of their walls. More than 90 percent of healthy pregnant women have mild tricuspid and pulmonary regurgitation, and more than one-third of these pregnant women present mild mitral regurgitation. However, no changes are seen in the ejection fraction or the left ventricular function. A non-pathologic systolic murmur is seen in 90 percent to 95 percent of pregnant women. Electrocardiography shows that P and T-wave inversion, small Q waves, and changes in ST may spike.
Blood Volume:
There is an increase of 10 percent in the blood volume as early as the seventh week of gestation, reaching the maximum level at around 32 weeks. This is similar to an increase of 45 percent to 50 percent (1500 to 1600 ml) when compared to nonpregnant women. The blood volume in pregnant women ranges between 73 ml/kg and 96 ml/kg, whereas in nonpregnant women, the value is about 60 ml/kg, and the increase is more noticeable in multiple pregnancies. With a rise in plasma volume, there will be better blood perfusion to the vital organs, including the fetus and uteroplacental unit. Additionally, the total body water rises by 6.5 to 8.5 liter by the end of pregnancy. This includes approximately 3.5 liter of total body water present in the placenta, fetus, and amniotic fluid. The expected blood loss at delivery will compensate for this expansion of water volume. Oncotic pressure depends on albumin, which is reduced in pregnant women; it is even more reduced in those with preeclampsia. Normal physiologic changes of plasma proteins and electrolytes during pregnancy can directly affect hydrostatic pressure and osmolarity.
Blood Pressure:
Blood pressure (BP) changes can occur as a result of altered cardiac output and systemic vascular resistance. Typically, the blood pressure decreases by approximately 10 percent around the seventh week of pregnancy, reaches the lowest level at the midpoint of the pregnancy, and returns to normal values around the end of pregnancy. The decrease in BP during mid-pregnancy is attributed to systemic vasodilation following changes in progesterone levels. Variations in BP can happen due to the position, the food consumed, and the activities of the pregnant women. Both systolic and diastolic blood pressures increase with uterine contractions, with the maximum increase appearing in the second stage of labor.
Cardiac Output:
Cardiac output (CO) starts increasing gradually at ten weeks of pregnancy and reaches its peak of 30 percent to 50 percent between 25 and 30 weeks. Stroke volume also escalates by 20 percent to 35 percent from the five weeks of pregnancy and reaches its peak at about 32 weeks and slightly decreases thereafter. Cardiac output is further increased during the time of labor. Each uterine contraction expels up to 500 ml of blood into the mother's circulation. This, in turn, increases venous return and cardiac output by another 30 percent.
Decreased cardiac output of 20 percent to 25 percent may be noted when the pregnant woman is placed in a supine position. This is due to the uterus compressing the inferior vena cava, which decreases the venous return. This is called supine hypotensive syndrome, which is noted in 0.5 percent to 11.2 percent of pregnant women. Supine hypotension causes tachycardia, diaphoresis, lightheadedness, nausea, vomiting, fatigue, and pallor. Women without this syndrome have compensatory increases in collateral flow through azygos and paravertebral systems leading to the maintenance of blood pressure.
Heart Rate:
Maternal heart rate increases as early as the fifth week of pregnancy, and in the third trimester of pregnancy, it is approximately 20 percent more than in the nonpregnant population.
Systemic Vascular Resistance:
Systemic vascular resistance (SVR) starts decreasing by about 10 percent as early as the fifth week of pregnancy and reaches its lowest at approximately 35 percent below baseline at 14 to 24 weeks of pregnancy. This is caused by the vasodilation linked with the low-resistance placental circuit. A drop in pulmonary vascular resistance can also be noted, but with no changes in pulmonary artery pressure. Until week 32 of pregnancy, the SVR is seen to remain constant, subsequently increasing and reaching normal prepregnancy values at the end of pregnancy.
What Physiological Changes Happen in the Respiratory System During Pregnancy?
During pregnancy, major changes happen in the upper respiratory tract, including hyperemia, glandular hypersecretion, and edema; this may lead to increased rhinitis, epistaxis (bleeding from the nose), and nasal congestion. These changes complicate air intake, obstruct airflow, and make intubation more complex. Minute ventilation and tidal volume will be increased during this period.
Functional residual capacity decreases between 16 and 24 weeks of pregnancy and continues until the end of the pregnancy. Spirometry studies conducted in pregnant women have not shown any changes in expiratory volume or in its ratio with vital functional capacity, as well as no changes in expiratory flow resistance.
Oxygen consumption rises by 30 to 50 ml/min, two-thirds of which comprise the mother's additional requirements that are prioritized for the kidney and one-third for the fetus and the placenta. Rising oxygen consumption and reduction in functional residual capacity make pregnant women more prone to hypoxemia during endotracheal intubation attempts and periods of apnea.
Conclusion
The changes in physiology during pregnancy occur in all maternal systems. However, the extent of changes varies by trimester and across every pregnant woman. So an awareness of the normal physiological changes during pregnancy will help healthcare professionals predict and anticipate possible complications.
