1. 
   

   What are normal physiologic adaptations during pregnancy?

   
   
2. 
   

   What are abnormal, concerning findings during pregnancy?

   
   
3. 
   

   How does pregnancy affect laboratory values?

Pregnancy and delivery require amazing maternal adaptation, with nearly every organ system altered by hormonal and/or mechanical means. Physiologic changes begin at conception and continue throughout pregnancy and the postpartum period. Such changes may impact the manifestation and course of a disease process as well as have implications for treatment. Maternal vulnerabilities, such as increased risk of thrombosis, hypertension, or diabetes, may also be revealed during pregnancy. Since any woman of reproductive age has the potential to be pregnant, their care must occur in the context of an understanding of normal and abnormal pregnancy physiology.

Cardiovascular

A healthy heart adapts well to the marked demands of pregnancy, but congenital or acquired heart disease may present or worsen in the gravid woman due to the cardiovascular changes associated with increased cardiac workload. Estrogen mediates an increase in cardiac output by six weeks gestation via increased preload and stroke volume. Blood volume rises by 30% to 40% during pregnancy, peaking at the end of the second trimester. Cardiac output (CO) peaks in the third trimester, typically 30% to 50% above baseline. Total peripheral resistance declines by 20%. Systolic and diastolic blood pressures drop by 10 to 15 mm Hg in the first trimester then return to baseline in the second half of pregnancy; diastolic blood pressure declines more than systolic, so pulse pressure widens. The cardiac axis is shifted leftward, anterior and cephalad.

Blood flow distribution changes during pregnancy such that up to 25% of CO is directed to the uteroplacental unit (a new “end-organ”) and up to 20% to the kidneys. An increased proportion of CO supplies breast tissue, but cerebral blood flow remains at baseline proportions.

Cardiovascular adaptations lead to common complaints in pregnant women: palpitations, decreased exercise tolerance, and dizziness. A shift of the heart toward the chest wall may contribute to the experience of palpitations by the gravida, but palpitations may also represent increased sensitivity to mild sinus tachycardia, premature atrial or ventricular systoles, or, less likely, supraventricular tachycardia. It is postulated that the increase in blood volume may be associated with stretching of the myocardium, thereby potentially increasing myocardial irritability and predisposing to arrhythmias. While this is not clearly proven, both atrial and ventricular arrhythmias may occur in pregnancy just as they occur in the nonpregnant patient. By late pregnancy there is attenuated ability to increase cardiac output with exercise, and this combined with normal weight gain of pregnancy can lead to decreased exercise tolerance. Beyond midpregnancy the gravid uterus causes aortocaval compression in the supine position, decreasing CO by 30%, and venous return, leading to dizziness and dyspnea (termed supine hypotensive syndrome) in some women. Pertinent exam findings include systolic flow murmur by midpregnancy, mammary soufflé, point of maximal impulse (PMI) displaced leftward and cephalad, and mild bilateral lower-extremity edema. Heart rate increases but is not generally above 100 beats per minute. While the neck veins may appear full, jugular venous pulsation is not elevated. Electrocardiograms of pregnant women often will show left axis deviation, atrial enlargement and nonspecific ST-T wave changes, but such findings should be carefully interpreted within clinical context. The cardiac silhouette may be generous on chest radiograph. Increased cardiac output and volume of distribution in pregnant women necessitate careful attention to timing of intravenous contrast dye bolus for computed tomography pulmonary angiography (CT-PA), in order to avoid radiation exposure for an inconclusive study. Care should be taken to position pregnant woman in the left lateral decubitus (or at least with hip elevation to displace the pregnant uterus off the vena cava) when performing medical investigations or treating ill pregnant women to minimize such adverse hemodynamic effects.

Respiratory

Pregnancy contributes to mechanical changes as well as progesterone-mediated physiologic adaptations in the respiratory system. In the upper airway, hyperemia, glandular hyperactivity, increased edema, and friability occur, related to elevated plasma volume and estrogen. Chest wall circumference increases by 5 to 7 cm due to elevated relaxin levels, and, eventually, the uterus elevates the diaphragm by about 4 cm though diaphragmatic excursion remains unchanged or even increased.

During pregnancy, oxygen consumption increases by 30% to 60% due to increased metabolic demands. Elevating progesterone levels increase respiratory drive early in pregnancy with continued effect throughout. Minute ventilation (tidal volume multiplied by breaths per minute) increases by up to 50% through increased tidal volume; respiratory rate should remain unchanged. Vital capacity remains unchanged while functional residual capacity declines by up to 25%. Overall, pulmonary function testing interpretation remains unchanged, as forced expiratory volume in 1 second (FEV1), FEV1/forced vital capacity (FVC) ratio, and peak expiratory flows remain stable.

Compensated respiratory alkalosis is expected on arterial blood gas testing, with PaO2 elevated to 100 to 105 mm Hg, and PaCO2 expected at 28 to 32 mm Hg. Clinically, pregnant women may complain of mild exertional dyspnea because of increased respiratory drive and physiologic anemia; however, this should not limit activities of daily living nor worsen with progressing pregnancy. Pertinent chest radiographic findings include generous cardiac silhouette, increased anteroposterior diameter, and prominent pulmonary vasculature. Clinical implications of respiratory anatomic changes in pregnancy include common complaints of gestational rhinitis (nasal congestion), epistaxis, and snoring, increased difficulty with intubation, and effect on needle insertion site for thoracentesis by the elevated diaphragm.

It is recommended that the most experienced provider available perform intubation since failure rates are far higher in pregnant women. Maternal oxygen saturation of 95% or greater is desirable to maintain PaO2 greater than 70 mm Hg for optimal placental oxygen diffusion.