Cardiovascular and respiratory adaptations in pregnancy

During exercise, there is a redistribution of blood flow away from the visceral organs and toward the exercising muscles. The redistribution of blood away from the uterus is related to the intensity and duration of exercise. However, in pregnancy there are corresponding adaptations that are characterized by an increase in blood volume, compensated for by increased venous capacity and decreased peripheral vascular resistance [27]. Although fetal oxygen and substrate availability could be counterbalanced by an increase in the amount of oxygen and substrate taken from the maternal blood supply, the question remains as to whether the redistribution of blood flow during regular or extended physical activity impacts transplacental transport of oxygen, carbon dioxide, and nutrients.

Exercise intensity is usually expressed in terms of demand on the cardiovascular system as percentage of maximal heart rate. The values for volume of oxygen consumed during exercise at maximum capacity (%VO2max), metabolic equivalents (METs), and maximal heart rate (%HRmax) for the average nonpregnant woman can be found in Table 3.1 [10]. Maternal heart rate response to strenuous exercise is blunted and does not follow a linear relationship; this is the reason why target heart rates cannot be used for exercise prescriptions in pregnancy. In order to track the level of intensity during physical activity, pregnant women may make use of the following easy-to-use methods. First, the talk test may be used to monitor level of intensity. A subject who is exercising at a moderate intensity (3-4 METs) should be able to comfortably hold a conversation; however, if winded or out of breath during the activity, she may be exercising too vigorously. Another helpful method for measuring intensity is the Borg Scale Rating of Perceived Exertion (RPE) [28]. The RPE is a subjective measure that correlates to a person's physical perception of exercise intensity including heart rate, respiration, perspiration, and muscle fatigue. The Borg RPE scale ranges from a level of 6, which is no exertion at all, to a level of 20, which is maximal exertion. An RPE level of 12-14 would be perceived as "somewhat hard," which corresponds to moderate activity. If exertion were reported as 19 or "extremely hard" on the Borg scale, decreasing to a lower intensity would be beneficial, thus modifying the intensity according to maternal symptoms. To estimate an individual's heart rate during exercise, RPE can be multiplied by 10 (i.e., an RPE of 12-14 x 10 = a heart rate of 120-140 beats per minute). Increased energy expenditure may be estimated using METs, a unit of resting oxygen uptake. One MET is equivalent to 1 kcal per kg of body weight per hour. For example, if a 70-kg woman walks at a brisk pace of 3-4 METs for a half hour, she would increase her caloric requirement by 70-105 kcal(2-3 MET increase over resting x 70 kcal x 0.5 hrs.

Cardiovascular response to body position should be considered. After the first trimester, the supine position results in relative obstruction of the venous return due to the enlarging uterus [29]. Pregnant women may experience a decrease in cardiac output reflective of symptoms associated with this supine hypotensive syndrome.

Table 3.1

Intensity of Exercise for the Nonpregnant Woman: %VO , METs, and %HR

2max max

Table 3.1

Intensity of Exercise for the Nonpregnant Woman: %VO , METs, and %HR

2max max






















Very heavy






Unduly heavy




From [10] % VO2max= percentage of aerobic capacity, METs = metabolic equivalent, % HRmax = percentage of maximal heart rate. Adapted from [10]

From [10] % VO2max= percentage of aerobic capacity, METs = metabolic equivalent, % HRmax = percentage of maximal heart rate. Adapted from [10]

Pregnancy is associated with profound respiratory changes: minute ventilation (tidal volume x breaths/minute) increases by approximately 50%, primarily as a result of increased tidal volume (volume of gas inhaled and exhaled during one respiratory cycle) [30, 31]. Because of the increased resting oxygen requirements and the increased work of breathing caused by pressure of the enlarged uterus on the diaphragm, there is decreased oxygen availability for performance of aerobic exercise during pregnancy. Thus, both workload and maximum exercise performance are decreased [31, 32].

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