CLINICAL OBSTETRICS AND GYNECOLOGY Volume 46, Number 2, 379–389 © 2003, Lippincott Williams & Wilkins, Inc. Physiologic Aspects of Exercise in Pregnancy MARY L. O’TOOLE, PhD Department of Obstetrics, Gynecology & Women’s Health and Women’s Exercise Research Laboratory, Saint Louis University, St. Louis, Missouri Exercise, regardless of type, intensity, or duration, requires an energy expenditure that is greater than resting energy expenditure. To increase energy expenditure, oxygen is needed by the working muscles to transform stored chemical energy (mainly fat and carbohydrate) into the mechanical energy of movement. Physiologic responses (ie, simultaneous cardiovascular and respiratory responses) occur at the onset of exercise to match energy expenditure and to ensure adequate oxygen availability to exercising muscle while maintaining the viability of other tissues. These cardiovascular and respiratory responses are mediated by hormonal influences and result in metabolic changes that provide energy for continued exercise. During pregnancy, endocrine changes occur that alter the regulation of metabolic and cardiopulmonary function and contribute to maternal responses to exercise. This chapter will describe the physiologic responses to acute exercise and exercise training during pregnancy. Correspondence: Mary L. O’Toole, PhD, Department of Obstetrics, Gynecology & Women’s Health, 6420 Clayton Rd., Suite 290, St. Louis, MO 63117. E-mail: [email protected] CLINICAL OBSTETRICS AND GYNECOLOGY / Physiologic Variables Altered by Exercise A number of physiologic variables are altered by exercise and are used to quantify various aspects of exercise-induced physi. ologic stress. Oxygen uptake (VO2) is used to quantify energy expenditure. Oxygen up. take can be reported as absolute VO2 reported in liters per minute (L/min) or as oxy. gen uptake indexed to body weight (ie, VO2 mL/kg/min). .Respiratory responses (minute ventilation [VE], tidal volume, and breathing frequency) are useful to describe the response of the respiratory system to exercise. Cardiovascular responses (heart rate, stroke volume, cardiac output, blood pressure, and systemic vascular resistance) can all be measured or calculated and are useful for quantifying the response of the cardiovascular system to exercise. Relationships between physiologic variables are used to describe systemic interactions during exercise. These include . . the ventilatory equivalent of oxygen equivalent of carbon (VE/VO2),. ventilatory . dioxide (VE/VCO2), respiratory exchange ratio (RER), ventilatory threshold, the respiratory. compensation point, and the heart rate–VO2 relationship. VOLUME 46 / NUMBER 2 / JUNE 2003 379 380 O’TOOLE Factors Influencing the Physiologic Responses to Exercise Factors that influence the physiologic responses to exercise include exercise mode, exercise intensity, whether the exercise stimulus is acute or chronic, and during pregnancy time of gestation. EXERCISE MODE Exercise can be broadly categorized as either weight-bearing or weight-supported. During weight-bearing exercise (eg, walking), energy expenditure includes that necessary to counteract the pull of gravity. Thus, body weight contributes to the intensity of the exercise. For example, the energy cost for a woman to walk a mile is increased from early to late pregnancy because her body weight is increased. During weightsupported exercise (eg, cycling or swimming), energy expenditure is independent of body weight. Energy expenditure for exercise in which body weight is supported is based on the amount of external work being done and the mechanical efficiency of the exerciser. For example, the energy cost to perform 100 Watts of work on a cycle ergometer is the same regardless of the exerciser’s weight. A special case of weightsupported exercise is swimming. In swimming, not only is body weight supported, but also external hydrostatic pressure is increased and may contribute to facilitated venous return, thus altering cardiodynamic responses to exercise. Body posture is also important, particularly during pregnancy. Supine exercise is not recommended after the second trimester because the gravid uterus may mechanically restrict venous return and resultant cardiac output. EXERCISE INTENSITY Exercise is also categorized by the amount of effort necessary to perform a given exercise task. The term “submaximal” is used to describe exercise that is performed with less than maximal effort. Submaximal exercise can be quantified either in absolute or relative terms. Absolute submaximal exercise reflects the amount of work being done. For example, walking at a pace of 20 minutes to the mile and cycling at a power output of 50 Watts are examples of absolute exercise intensities. These activities require submaximal effort from most healthy women of childbearing age. Absolute exercise intensities require a set amount of energy to perform. Many physiologic responses are responsive to changes in energy demands. For example, for a given energy demand, expected minute ventilation and cardiac output can be predicted. Submaximal exercise can also be expressed in relative terms as a percentage of maximal capacity (eg, 70% of peak walking speed or 70% of peak Watts). Exercise at the same relative intensity may require different absolute amounts of work and therefore a different magnitude of respiratory and cardiovascular response. The physiologic stress of exercise (ie, perception of effort) is more closely related to relative than absolute intensity. The duration of exercise also has an important effect on physiologic responses. For example, prolonged exercise often elicits a drift in response from what was seen during a short (10-minute) exercise bout. Maximal exercise refers to all-out exercise to exhaustion (ie, the highest-intensity, greatest-load, or longestduration exercise of which an individual is capable). It is important to understand the specifics of the exercise stimulus to fully interpret the physiologic responses and in particular to interpret differences in response that are attributable to pregnancy. ACUTE VERSUS CHRONIC EXERCISE Acute exercise refers to a single exercise session as a stimulus. The specific physiologic responses to acute exercise depend on the characteristics of the stimulus, such as mode, intensity, and duration. The responses may also be dependent on the characteristics of the exerciser, including fitness level. Chronic exercise refers to exercise training that results in physiologic changes Physiologic Aspects of Exercise in Pregnancy or adaptations that optimize response to an exercise stimulus. An individual may begin a pregnancy as a trained individual or as a sedentary person. Similarly, a pregnant woman may undertake an exercise training program during pregnancy. LENGTH OF GESTATION Pregnancy causes profound anatomic and physiologic changes. Most notably, weight is gained, resting energy expenditure is increased, and cardiovascular and respiratory homeostasis is disturbed. Most of these changes occur progressively but at varying rates from early gestation through term. Each of these may affect the physiologic responses to exercise in varying amounts at different times during pregnancy. Therefore, to fully interpret exercise responses, one should consider the length of gestation along with other exercise-related factors. Physiologic Responses to Acute Exercise Pregnancy results in physiologic changes that alter energy expenditure as well as cardiovascular and respiratory homeostasis during rest and exercise.1 ENERGY EXPENDITURE With the onset of exercise, energy expendi. ture (quantified as oxygen uptake or VO2) increases above resting energy expenditure in direct proportion to the work being done, such that physical activity and energy expenditure are linearly related. Total energy expenditure for a given task can be estimated as resting energy expenditure plus the energy expenditure necessary to perform a given amount of external work. During pregnancy, resting energy expenditure is significantly increased. The increased resting energy expenditure occurs early in pregnancy and is related to the metabolic demands of the uteroplacental unit as well as to the increased maternal body weight. There is also a slightly augmented work of breathing 381 during exercise while pregnant that contributes to an overall increase in energy expenditure. Therefore, a given amount of exer. cise requires a higher VO2 during pregnancy compared with that required during the nonpregnant state. The magnitude of the difference in energy expenditure between pregnancy and nonpregnancy depends on the exercise mode and intensity, the amount of maternal weight gain, and the age of gestation. During weight-supported exercise . such as stationary cycling, the increased VO2 for a given amount of submaximal exercise will be similar to the increase in resting energy expenditure. Khodiguian et al2 reported an . 8% increase in VO2 during cycle ergometry performed at 25 W and 50 W. A greater increase (11%) was observed when exercise intensity was higher (75 W). The comparison in this study was between responses at 33 weeks’ gestation and those measured in the same women at 12.5 weeks postpartum. Heenan et. al3 reported a similar increase (6%) in VO2 when responses of pregnant women were compared with a nonpregnant control group during submaximal cycle. ergometry. Pivarnik et al 4 reported V O 2 (L/min) increases of 10–15% for absolute work rates of 50 W and 75 W during the third trimester. Although a decrease in mechanical. efficiency could theoretically increase VO2 even during weight-supported cycle ergometry, the decrease in efficiency has been reported to be negligible.3 A related consideration is the apparent effect pregnancy has on substrate utilization. A preferential use of carbohydrate has been reported to occur during submaximal, weight-supported exercise.5 Also of interest is the report by Artal et al6 that pregnant women may safely engage in submaximal exercise at an altitude of 6,000 ft or less with no adverse effects. During weight-bearing exercise, energy expenditure during pregnancy can be expected to increase continually from early gestation to term in proportion to gains in body weight. Pivarnik et al4 estimated that O’TOOLE 382 . V O2 (L/min) for a given walking speed would increase by approximately 10% over the course of a pregnancy. During weightbearing activities, additional energy expenditure can be expected for activities that require agility, balance, or stabilization throughout the course of pregnancy. These differences in energy cost . during pregnancy are evident only when VO2 is reported in absolute units (ie, L/min) for absolute amounts of external work (eg, 25 W). . When V O 2 is. indexed to body weight (mL/kg/min), VO2 at absolute external work rates is expected to decrease as pregnancy progresses. For example, Artal et al7 re. ported a 17% lower VO2 mL/kg/min during moderate-intensity treadmill walking for pregnant women in comparison with nonpregnant controls. . Absolute VO2max (L/min) appears to be unaffected by pregnancy itself for most activities. Lotgering et al8 reported no difference in either treadmill or cycle ergometer . VO2max at 16, 25, and 35 weeks’ gestation in comparison with 7-week postpartum val9–11 ues. Sady reported no change in . et al cycle V O2max when midpregnancy (26 weeks’ gestation) maximal capacity was compared with values at up to 7 months 3 postpartum. Heenan . et al reported no difference in cycle VO2max between a group of pregnant women (35 weeks’ gestation) and a group of nonpregnant control subjects matched for physical and demographic char12 acteristics. Spinnewijn et . al reported on cycling and swimming VO2max in a small group of women at 30 to 35 weeks’ gestation and . again at 8 to 12 weeks postpartum. VO2max was not different for either swimming or cycling when comparing pregnancy and postpartum values, but cycling values were significantly higher than swimming values during both pregnancy and the postpartum. This is consistent with findings in other nonpregnant populations. . Although VO2max appears to be unaffected by pregnancy, the amount of external work that can be done during pregnancy is reduced when exercise is weight-bearing and unchanged for weight-supported activity. Thus, physiologic capacity to increase metabolism is unchanged, but the resultant mechanical output is reduced. When energy expenditure is reported indexed to body weight (mL/kg/min) during pregnancy, . there is a significant decrease in V O2 at maximal effort regardless of whether the exercise is weight-bearing or weightsupported. . Others have reported a decrease in VO2max during pregnancy and have postulated that the decrease may be the result of decreases in physical activity during the course of gestation. In habitually active women, Clapp et al13 reported a decrease in usual intensity of running and walking . from preconceptual intensities of 74% VO2max to 57% by gestational week 20 and 47% by . gestational week 32. The decrease in V O2max with advancing pregnancy may also be the result of factors associated with the type of activity. McMurray et al14 re. ported that swimming VO2max was 17% lower during pregnancy than postpartum. They suggested that maternal ventilation may have been the limiting factor secondary to limited diaphragm movement caused by cranial displacement of. the fetal mass. Thus, reported decreases in VO2max during pregnancy may result from the combination of a decrease in the intensity and amount of physical activity as well as from maternal weight gain. RESPIRATORY RESPONSES In general, respiratory activity is precisely matched to oxygen uptake and controlled by carbon dioxide. levels. During pregnancy, the increased VO2 at rest causes respiratory responses that are in the same direction as those seen during mild exercise in nonpregnant women—that is, minute ventilation increases mainly as a result of increased tidal volume with only small increases in breathing frequency until tidal volume reaches its highest point. The increased minute ventilation at rest continues to rise throughout pregnancy. Resting minute ventilations at the Physiologic Aspects of Exercise in Pregnancy end of the second and third trimesters have been reported to be 21% and 50% higher, respectively, than those postpartum. 7,14 During pregnancy, there is an increased ventilatory sensitivity to carbon dioxide (CO2) that is mediated by higher circulating progesterone. The increased sensitivity to CO2 is reflected in the increased ventilatory . . equivalent of CO2 (VE/VCO2) that is observed throughout gestation at rest and during both submaximal and maximal exercise. Dyspnea is also common in pregnant women both at rest and during exercise. At light to moderate exercise intensities, the increase in minute ventilation is linearly related to oxygen uptake. During pregnancy, minute ventilation is increased by 23–26% during a given level of submaximal exercise in comparison to nonpregnant controls.2 With advancing gestational age, minute ventilation for a given level of submaximal cycle ergometry continues to increase. Wolfe et al15 reported that minute ventilation at the end of the third trimester was significantly higher than at the end of the second trimester for intensities of 30, 60, and 90 W (Fig. 1). At the end of the third trimester, the minute ventilation during submaximal exercise (30, 60, and 90 W) was greater (36%, 29%, and 27%, respectively) than postpartum values.15 As exercise intensity increases to maximal effort, the augmentation in minute ventilation is reduced. In the nonpregnant individual, . the . ventilatory equivalent for oxygen (VE/VO2) during light exercise is expected.to be . less than 26:1, but during pregnancy VE/VO2 is frequently greater than 32:1 (Fig. 2).7 FIG. 1. Minute ventilation at the end of the second and third trimesters and postpartum during light, moderate, and peak exercise intensities. (Wolfe LA, Walker RMC, Bonen A, et al. Effects of pregnancy and chronic exercise on respiratory responses to graded exercise. J Appl Physiol. 1994;76:1928–1936) 383 Another respiratory response of interest is the respiratory exchange ratio (RER = ra. tio of CO . 2 produced [VCO2] to O2 metabolized [VO2]). Below the ventilatory threshold, RER can be used as an estimate of the relative utilization of fat and carbohydrate in nonpregnant individuals. During moderate exercise, RER is not different at different times during pregnancy, nor are pregnancy values different from those during the postpartum period.15,16 However, the relationship of RER to substrate utilization during pregnancy is unclear. During vigorous exercise, the slope . more . of the VE/VO2 relationship is increased at a point designated as the . ventilatory threshold. At this point,. V CO2 increases more steeply relative to VO2, mainly as a result of bicarbonate buffering of lactic acid. Above . this threshold, V E is closely coupled to . VCO2. As exercise intensity continues to increase, a respiratory compensation point is reached that marks the onset of metabolic acidosis. In sedentary individuals, the ventilatory threshold and the respiratory compensation point occur at exercise intensities of approximately 50–60% and 80–90% of peak . VO2, respectively. Pregnancy does not appear to have a marked effect on the identification of. these points despite its marked effect on VE and on ventilatory .sensitivity to CO . 2. However, the slope of VCO2 versus VO2 above the ventilatory threshold is more shallow during pregnancy than postpartum, suggesting that the buffering of lactic acid is reduced.17 At maximal exercise, minute ventilation during cycle ergometry has been reported to 384 O’TOOLE FIG. 2. . .Ventilatory equivalent of oxygen (VE/VO2) before and during symptom-limited treadmill walking to maximal effort in pregnant versus nonpregnant controls. (Artal R, Wiswell R, Romem Y, et al. Pulmonary responses to exercise in pregnancy. Am J Obstet Gynecol. 1986;154:378–383, with permission) be higher8 or not different12,15 during pregnancy than postpartum. Lotgering et al8 . found VEmax to be 7–11% higher during both cycle ergometry and treadmill exercise during pregnancy compared with the postpartum. In contrast, others have reported no . change in cycling or swimming VEmax in 12,15 pregnancy compared . with postpartum. No differences in VEmax based on length of gestation have been reported for treadmill, cycle, or swimming exercise. At maximal exercise, neither the ventilatory equivalent of oxygen or carbon dioxide is different during pregnancy in comparison with values for matched controls, but RER is significantly lower (1.19 vs. 1.25).3 This is consistent with a lower peak lactate concentration and may reflect a reduced carbohydrate oxidation at high-intensity exercise. CARDIOVASCULAR RESPONSES Cardiovascular responses to exercise during pregnancy follow the same general pattern as nonpregnant responses. In both cases, cardiovascular responses are most meaningful when related to changes in absolute ex. ercise intensity (ie, V O L/min). Cardiac 2 . output (Q) increases in a linear and fixed re. lationship with oxygen uptake (VO2). Dur-. ing pregnancy, however, the increase in Q . per unit increase. in. VO2 is greater than when not pregnant. Q/VO2 during pregnancy is approximately 25% higher than that expected at rest when not pregnant. Both stroke volume .and heart rate contribute to the increase in Q at rest and during exercise. During exercise, stroke volume increases in untrained individuals until exercise intensity reaches 40–50% of maximal capacity and then plateaus at that level. Highly trained individuals may have the capacity to continue to increase stroke volume. Heart rate increases linearly with oxygen uptake up to maximal capacity. With increasing exercise intensity, systolic blood pressure rises, mainly the result of the increased cardiac output. Diastolic blood pressure may slightly increase, decrease, or stay the same. Systemic vascular resistance decreases because of generalized vasodilation, thereby preventing an exaggerated rise in blood pressure in response to the increased cardiac output. Distribution of cardiac output is a function of both neural and hormonal input such that blood is selectively directed toward areas of higher metabolic activity and away from splanchnic and renal vascular beds. Most studies have shown cardiac output, heart rate, and stroke volume to be increased at submaximal exercise intensities during pregnancy in comparison with nonpregnant responses. For example, during cycle ergometry at 25, 50, and 75 W, heart rates have been reported to be 18%, 16%, and 12% higher, respectively, during pregnancy than during the postpartum period (Fig. 3).2 Physiologic Aspects of Exercise in Pregnancy 385 FIG. 3. Submaximal exercise heart rates during cycle ergometry during pregnancy versus nonpregnancy. (Khodiguian N, Jaque-Fortunato SV, Wiswell RA, et al. A comparison of cross-sectional and longitudinal methods of assessing the influence of pregnancy on cardiac function during exercise. Semin Perinatol. 1996;20:232–241) During cycle ergometry with exercise intensities up to 100 W, Sady et al9–11 reported that cardiac outputs were 2.2 to 2.8 L/m in higher during pregnancy than postpartum. Interestingly, the cardiac output–oxygen uptake relationship was unchanged, suggesting that cardiac output response to exercise was not altered by pregnancy. Using direct Fick methodology, Pivarnik et al18 reported similar findings in response to both cycle ergometry and treadmill walking. Differences in cardiac output for treadmill walking during pregnancy in comparison to responses postpartum are greater than those for cycle ergometry. This is to be expected since postpartum weight loss markedly reduces the exercise load for weight-bearing exercise in comparison with that during pregnancy. Blood pressure responses to exercise during pregnancy are unchanged or slightly decreased in comparison to those in the nonpregnant state. The same linear relationship between systolic blood pressure and exercise intensity has been observed during pregnancy as expected in the nonpregnant state. Since cardiac output is increased to a greater extent during submaximal exercise during pregnancy, systemic vascular resistance during exercise must decrease more than during the nonpregnant state. There appears to be a dose-response effect of maternal exercise intensity on uteroplacental blood flow. Although limits have been extrapolated from animal studies, Mottola suggested that a maternal. exercise intensity of approximately 80% VO2max seems to be the threshold above which fetal blood flow is significantly diminished.19 The magnitude of cardiovascular responses to exercise during pregnancy is affected by length of gestation as well as by exercise mode and intensity. Several studies have demonstrated that stroke volumes and cardiac outputs are reduced as pregnancy advances, with little change in exercise heart rates. McMurray et al14 demonstrated higher stroke volumes when exercise was done in water rather than on land. They also demonstrated that stroke volumes during water exercise did not decrease as gestation advanced. These data support the hypothesis that decreases in stroke volume as pregnancy advances can be attributed to increased venous pooling and to decreased venous return. Veille et al20 studied maternal cardiodynamics via m-mode and twodimensional Doppler echocardiography and concluded that the increased stroke volumes seen during exercise early in pregnancy are the result of increased ventricular contractility, with increased preload becoming more important as gestation advances. This work lends support to the hypothesis of a blunted response to catecholamines in the third trimester. Most studies suggest that maximal cardiac outputs and stroke volumes9–11 are increased during pregnancy, but that maximal heart rates are unchanged.3 Some have suggested, however, that heart rates during pregnancy may be reduced because of a blunted response to sympathetic stimulation.2,8,15 They reported peak heart rate for cycle ergometry and treadmill exercise to be slightly (1–7%) but statistically signifi- 386 O’TOOLE cantly lower during pregnancy than postpartum. Because of the linear relationship that exists between heart rate and oxygen uptake, heart rates can be used to quantify exercise. Recently Pivarnik et al 21 provided data documenting changes . in the slope and intercept of heart rate–VO2 regression lines as pregnancy advances and into the postpartum. Slopes were approximately 10% flatter during pregnancy. This means that a greater . change in VO2 occurs per change in exercise heart rate during pregnancy compared with the postpartum. At a given submaximal heart rate, energy expenditure is less during pregnancy than during the postpartum. Thus, if exercise during pregnancy is to be used to control caloric balance, heart rate– . VO2 regression lines appropriate for gestational age should be constructed to predict caloric expenditures. Physiologic Response to Exercise in Well-Trained Pregnant Women Well-defined physiologic responses are expected in individuals engaged in regular exercise training. Whether these are preserved during pregnancy in women who continue to train is unclear. Cross-sectional studies22 suggest that women who continue aerobic exercise training during pregnancy have lower resting heart rates and higher stroke volumes than matched sedentary controls at 25 and 36 weeks’ gestation as well as at 12 weeks postpartum. No differences were observed in resting cardiac output, a-vO2diff, mean arterial pressure, or systemic vascular resistance between trained and sedentary women during gestation or at 12 weeks postpartum.22 During submaximal cycling exercise at a heart rate of approximately 140 bpm, physiologic responses of trained and sedentary pregnant women differed in virtually all measured variables.22 Caution should be used in interpreting these differences, since the trained group was exercising at a much higher absolute exercise intensity. Others have also compared the responses of active women (all regularly exercising three to six times per week) who were pregnant with those of similarly active nonpregnant women. In response to a single submaximal exercise intensity, no significant effect of pregnancy was evident on heart rate, oxygen uptake, or minute ventilation. Additionally, ventilatory thresholds during cycle ergometry were not different between pregnant and nonpregnant groups, nor were the respiratory compensation points or work efficiency different.3 The ventilatory equivalents for oxygen and carbon dioxide were significantly increased, and end-tidal and arterial carbon dioxide levels were significantly decreased in the pregnant women.3 Physiologic Response to Exercise Training During Pregnancy A number of studies have investigated physiologic responses of pregnant women to exercise training. Although specific training has varied, most has conformed to the same general structure: aerobic exercise (cycling, swimming, or varied aerobic exercise) at moderate intensity (exercise target heart rates 140–150 bpm) done three times per week for 25 to 60 minutes per session has been the training stimulus. Conditioning programs have started early in the second trimester and continued throughout pregnancy. Various outcomes have been used to assess the effectiveness of the training program. Most have documented an increase in aerobic capacity. However, some of the variables used to assess the effectiveness of training in nonpregnant populations may not show training-induced changes during pregnancy. For example, in nonpregnant individuals, a decrease in resting heart rate is a common result of training. When a training program is begun during pregnancy, resting bradycardia is usually not observed. An- Physiologic Aspects of Exercise in Pregnancy other cardiovascular adaptation to aerobic conditioning in nonpregnant individuals is an increase in left ventricular dimension. With exercise training during pregnancy, cardiac dimensions have been reported to be unchanged beyond those attributable to pregnancy itself.23 During submaximal exercise by nonpregnant individuals, heart rates, minute ventila. . tion, VE/VO2, and RER are lower in conditioned versus sedentary individuals. During pregnancy, no differences in these variables were observed between exercise and control groups participating in a standard submaximal intensity of 75 W.24 However, during heavier . . exercise, submaximal heart rates, VE/VO2, and perceived exertion were reported to be lower in exercising versus sedentary pregnant women.25 Several investigators have suggested that the conditioning effects of exercise training normally seen at rest and during light exercise may be obscured by the physiologic effects of pregnancy (eg, increased blood volume) but are evident at higher exercise intensities. Wolfe et al15 reported . .an increase in ventilatory sensitivity (VE/VCO2) with advancing gestation in both control and exercise groups at submaximal levels, but no difference between groups. These data are in agreement with other reports of a well-documented increase in respiratory sensitivity during pregnancy and demonstrate that the increased sensitivity continues to be operative during submaximal exercise even in wellconditioned pregnant women. Wolfe et al15 also reported that the exercise intensity that represented the onset of blood lactate accumulation did not change with advancing gestation in the sedentary control group, but increased with exercise training during pregnancy and then decreased during the postpartum, when the physical conditioning program was discontinued. 15 Thus, the changes in ventilatory thresholds during pregnancy appear to be affected by exercise training, but not by pregnancy itself. Exercise training during pregnancy can result in improvement of aerobic fitness 387 similar in magnitude to that expected in a a nonpregnant group. Wolfe et al15 reported . 17% increase in oxygen pulse (VO2/HR) from early in the second trimester to late in the third trimester in a group that followed an exercise training program compared with a control group. Peak heart rates were lower at the end of the third trimester in comparison with postpartum in both the exercise and the control group, but there were no differences between groups. Peak postexercise lactate levels are expected to increase with exercise training in nonpregnant individuals. No peak postexercise increase was observed in this study,15 perhaps suggesting a decreased availability of carbohydrate late in gestation. Summary and Conclusions Both pregnancy and exercise alter energy expenditure as well as cardiovascular and respiratory homeostasis in comparison with that expected at rest for a nonpregnant individual. The magnitude of the differences in physiologic response to exercise between pregnancy and nonpregnancy depend on the exercise mode and intensity, the amount of maternal weight gain, and the age of gestation. During submaximal exercise, energy expenditure, minute ventilation, cardiac output, and heart rates are greater during pregnancy in comparison with responses during the nonpregnant state. During maximal exercise, the ability to transform energy appears to be unaffected by pregnancy, but maximal energy expenditure may decrease because of decreased regular activity during pregnancy. Maximal responses of respiratory and cardiovascular systems during pregnancy are less clear. Some studies have reported minute ventilation and cardiac output to be higher during pregnancy, while others have reported no change. Most studies suggest that maximal heart rates are unchanged, while others suggest that maximal heart rates may be reduced. Physiologic responses to exercise training during pregnancy appear to be similar to those expected in a nonpreg- 388 O’TOOLE nant population. However, caution is suggested in interpreting markers, such as resting heart rate, that are commonly used to assess the efficacy of exercise training. Many of these markers may be obscured during rest or light exercise by the physiologic effects of pregnancy (eg, increased blood volume) but will be evident at higher exercise intensities. References 1. Artal R, Wiswell RA, Drinkwater BL, eds. Exercise in Pregnancy, 2nd ed. Baltimore: Williams & Wilkins, 1991. 2. Khodiguian N, Jaque-Fortunato SV, Wiswell RA, et al. A comparison of crosssectional and longitudinal methods of assessing the influence of pregnancy on cardiac function during exercise. Semin Perinatol. 1996;20:232–241. 3. Heenan AP, Wolfe LA, Davies GA. Maximal exercise testing in late gestation: maternal responses. Obstet Gynecol. 2001;97: 127–134. 4. Pivarnik JM, Lee W, Miller JF. 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