No 129-L'exercice physique pendant la grossesse et le postpartum.

OBJECTIF: Énoncer une directive canadienne visant à informer les fournisseurs de soins obstétricaux des répercussions, pour la mère, le fœtus et le nouveau-né, des exercices de conditionnement aerobique et musculaire pendant la grossesse. RéSULTATS ATTENDUS: Effets sur la morbidité maternelle, fœtale et néonatale et mesures de la forme physique maternelle. PREUVES: Une recherche sur MEDLINE des articles, publiés en anglais de 1966 à 2002, appartenant aux catégories suivantes : études sur le conditionnement aérobique et musculaire chez des femmes ne faisant pas jusque-là d'exercice et chez des femmes actives avant leur grossesse, ainsi que des études sur les répercussions du conditionnement aérobique et musculaire sur les issues précoces et tardives de la grossesse ou sur les issues néonatales; rapports de synthèse et méta-analyses portant sur l'exercice pendant la grossesse. VALEURS: Les résultats recueillis ont été revus par la Société des obstétriciens et gynécologues du Canada (Comité de la pratique clinique - obstétrique), avec la participation de la Société canadienne de physiologie de l'exercice, et ils ont été classés suivant les critères d'évaluation des preuves établis par le Groupe de travail canadien sur l'examen de santé périodique. RECOMMANDATIONS: VALIDATION: Cette directive a été approuvée par le Comité de pratique clinique - obstétrique de la SOGC, par le Comité exécutif et par le Conseil de la SOGC, ainsi que par le Conseil d'administration de la Société canadienne de physiologie de l'exercice. PARRAINé PAR: la Société des obstétriciens et gynécologues du Canada et par la Société canadienne de physiologie de l'exercice.

No. 129-Exercise in Pregnancy and the Postpartum Period.

OBJECTIVE: To design Canadian guidelines advising obstetric care providers of the maternal, fetal, and neonatal implications of aerobic and strength-conditioning exercises in pregnancy. OUTCOMES: Knowledge of the impact of exercise on maternal, fetal, and neonatal morbidity, and of the maternal measures of fitness. EVIDENCE: MEDLINE search from 1966 to 2002 for English language articles related to studies of maternal aerobic and strength conditioning in a previously sedentary population, maternal aerobic and strength conditioning in a previouslyactive population, impact of aerobic and strength conditioning on early and late pregnancy outcomes, and impact of aerobic and strength conditioning on neonatal outcomes, as well as for review articles and meta-analyses related to exercise in pregnancy. VALUES: The evidence collected was reviewed by the Society of Obstetricians and Gynaecologists of Canada (SOGC Clinical Practice Obstetrics Committee) with representation from the Canadian Society for Exercise Physiology, and quantified using the evaluation of evidence guidelines developed by the Canadian Task Force on the Periodic Health Exam. RECOMMENDATIONS: VALIDATION: This guideline has been approved by the SOGC Clinical Practice Obstetrics Committee, the Executive and Council of SOGC, and the Board of Directors of the Canadian Society for Exercise Physiology. SPONSORS: This guideline has been jointly sponsored by The Society of Obstetricians and Gynaecologists of Canada and the Canadian Society for Exercise Physiology.

Fetal heart rate response to maternal hypocapnia and hypercapnia in late gestation.

OBJECTIVE: To examine the effects of acute maternal hypocapnia and hypercapnia on electronic fetal heart rate (FHR) patterns in late gestation. METHODS: Thirty-five women with healthy singleton pregnancies performed a modified carbon dioxide (CO2) rebreathing procedure between 34 and 38 weeks of pregnancy. Prior to rebreathing, subjects hyperventilated for five minutes to reduce end-tidal CO2 tensions (PETco2) below 23 Torr (hypocapnia). During rebreathing, PETco2 progressively increased from hypocapnia to hypercapnia (PETco2 = 40-60 Torr) at a constant hyperoxic end-tidal O2 tension of 150 Torr. FHR responses were classified using standardized guidelines over four periods: 20 minutes before rebreathing (pretest), during hypocapnia and hypercapnia, and 20 minutes after rebreathing (post-test). RESULTS: Mean baseline FHR measures (SD) over the four test periods were 138(8), 144(10), 132(11), and 137(9) beats per minute (bpm). All pairwise comparisons were statistically significant except the pretest versus post-test comparison (P < 0.05, Tukey-Kramer multiple comparisons test). A single tachycardia episode of 170 bpm was recorded in the post-test period. In 20 subjects FHR variability changed from moderate in the pretest period to minimal during hypocapnia and/or hypercapnia. All but two returned to moderate FHR variability in the post-test period. One other fetus with minimal post-test variability had moderate values in the three preceding test periods. CONCLUSION: Electronic FHR parameters remained within normal limits for third-trimester fetuses with the exception of one fetus that experienced tachycardia. Acute maternal hypocapnia and hypercapnia over the range studied had no adverse effects on fetal well-being. These results support the safety of the modified CO2 rebreathing procedure for research in healthy, low-risk pregnancy.

Physiological mechanisms of hyperventilation during human pregnancy.

This study examined the role of pregnancy-induced changes in wakefulness (or non-chemoreflex) and central chemoreflex drives to breathe, acid-base balance and female sex hormones in the hyperventilation of human pregnancy. Thirty-five healthy women were studied in the third trimester (TM(3); 36.3+/-1.0 weeks gestation; mean+/-S.D.) and again 20.2+/-7.8 weeks post-partum (PP). An iso-oxic hyperoxic rebreathing procedure was used to evaluate wakefulness and central chemoreflex drives to breathe. At rest, arterialized venous blood was obtained for the estimation of arterial PCO(2) (PaCO(2)) and [H(+)]. Blood for the determination of plasma strong ion difference ([SID]), albumin ([Alb]), as well as serum progesterone ([P(4)]) and 17beta-estradiol ([E(2)]) concentrations was also obtained at rest. Wakefulness and central chemoreflex drives to breathe, [P(4)] and [E(2)], ventilation and V CO(2) increased, whereas PaCO(2) and the central chemoreflex ventilatory recruitment threshold for PCO(2) (VRTCO(2)) decreased from PP to TM(3) (all p<0.01). The reductions in PaCO(2) were not related to the increases in [P(4)] and [E(2)]. The alkalinizing effects of reductions in PaCO(2) and [Alb] were partly offset by the acidifying effects of a reduced [SID], such that arterial [H(+)] was still reduced in TM(3) vs. PP (all p<0.001). A mathematical model of ventilatory control demonstrated that pregnancy-induced changes in wakefulness and central chemoreflex drives to breathe, acid-base balance, V CO(2) and cerebral blood flow account for the reductions in PaCO(2), [H(+)] and VRTCO(2). This is the first study to demonstrate that the hyperventilation and attendant hypocapnia/alkalosis of human pregnancy results from a complex interaction of pregnancy-induced changes in wakefulness and central chemoreflex drives to breathe, acid-base balance, metabolic rate and cerebral blood flow.

Ventilatory control and acid-base regulation across the menstrual cycle in oral contraceptive users.

We examined the effect of menstrual cycle (MC) phase on acid-base regulation and ventilatory control at rest in monophasic oral contraceptive (OC) users. Twelve healthy women (25+/-1 years; mean+/-S.E.) were tested during the inactive (IP; 5.1+/-0.2 days) and active (AP; 21.1+/-0.7 days) pill phase of the MC. Central and peripheral chemoreflex responsiveness was examined using a modified CO(2) rebreathing procedure. Minute ventilation (V E), breathing pattern and metabolic rate were measured during 10 min of quiet, resting breathing. Blood for the determination of arterial P(CO2) (Pa(CO2)) and hydrogen ion concentration ([H(+)]); plasma concentrations of the strong ion difference ([SID]) and total weak acid ([A(tot)]); serum concentrations of progesterone ([P(4)]) and 17beta-estradiol ([E(2)]) were also obtained. Although [E(2)] (p<0.05) and [A(tot)] (p=0.05) were increased in the IP versus AP, MC phase had no significant effect on resting V E, breathing pattern, metabolic rate, [H(+)], Pa(CO2), [SID], [P(4)] and central or peripheral chemoreflex characteristics. Overall, OC had no significant physiological effect on acid-base regulation or ventilatory control at rest in healthy women. This may reflect suppression of endogenous fluctuations in circulating [P(4)] typically observed across the MC in healthy, eumenorrheic non-OC users.

Effects of human pregnancy and advancing gestation on respiratory discomfort during exercise.

This study examined the effects of human pregnancy and advancing gestation on the intensity of respiratory discomfort (dyspnea) during cycle exercise. Fourteen pregnant women (PG) performed a progressive cycle ergometer exercise test involving 20 W/min increases in work rate to symptom limitation and/or a heart rate of 170-175 beats/min at 19.7+/-1.2 weeks (ENTRY), 28.2+/-0.3 weeks (TM2) and 36.3+/-0.3 weeks (TM3) gestation. Eight, age-matched, sedentary non-pregnant women (CG) were also studied for comparison purposes. Measurements included dyspnea intensity (Borg scale), minute ventilation (VE), breathing pattern and other cardiorespiratory parameters. At peak exercise, neither pregnancy nor advancing gestation had an effect on dyspnea, VE, breathing pattern, oxygen uptake or work rate (p>0.05). VE was significantly greater (by 11 L/min at 100 W) in the PG at TM3 versus CG (p<0.05) at all submaximal work rates. VE also increased progressively from ENTRY to TM2 and TM3 during submaximal exercise. Dyspnea was not significantly different at any submaximal work rate in the PG at TM3 versus CG or with advancing gestation in the PG. In addition, dyspnea at a standardized exercise VE of 40 L/min was not different at TM3 versus ENTRY or in the PG at TM3 versus CG. Neither pregnancy nor advancing gestation were associated with increased respiratory discomfort during strenuous non-weight bearing cycle ergometer exercise, despite substantial increases in VE and progressive mechanical adaptations of the respiratory system to accommodate the increasing size of the gravid uterus.

Cold pressor test protocol to evaluate cardiac autonomic function.

The primary objective of this study was to develop a cold pressor test (CPT) protocol to evaluate cardiac autonomic function. Secondary objectives were to assess CPT protocol reliability and to examine gender differences in response to orthostatic stress and the CPT. Healthy, normotensive men and women (n = 12 per group) completed 2 trials on different days in the left lateral decubitus and standing postures and during a 6 min CPT (hand immersion while seated). Measurements included R-R interval, blood pressure, ventilatory responses, spontaneous baroreflex sensitivity, and heart rate variability indices. During the CPT, blood pressure and the sympathetic nervous system (SNS) indicator increased significantly and low-frequency power, high-frequency power (normalized for tidal volume), and the parasympathetic nervous system (PNS) indicator decreased significantly. Standing caused significant increases in the SNS indicator and decreases in the R-R interval in both genders. The PNS indicator was higher in women than in men in the left lateral decubitus posture. The 6 min hand-immersion CPT provoked cardiac sympathetic activation and parasympathetic withdrawal; however, it is best suited to studies with a repeated measures design, as analysis of reliability suggests that responses are highly variable between individuals. Performing the CPT in the left lateral decubitus position may prevent vasovagal responses.

Physiological adaptation in early human pregnancy: adaptation to balance maternal-fetal demands.

After conception, the corpus luteum, placenta, and developing embryo release hormones, growth factors, and other substances into the maternal circulation. These substances trigger a cascade of events that transform the functioning of the maternal cardiovascular, respiratory, and renal systems, which in turn alter the physicochemical determinants of [H(+)]. Following implantation, maternal adaptations fulfill 4 important functions that support fetal growth. Increased availability of substrates and precursors for fetal-placental metabolism and hormone production is mediated by increases in dietary intake, as well as endocrine changes that increase the availability of glucose and low-density lipoprotein (LDL) cholesterol. Transport capacity is enhanced by increases in cardiac output, facilitating the transport of substrates and precursors to the placenta, and fetal waste products to maternal organs for disposal. Maternal-fetal exchange is regulated by the placenta after 10-12 weeks gestation, but it may occur through histiotrophic mechanisms before this time. Disposal of additional waste products (heat, carbon dioxide, and metabolic byproducts) occurs through peripheral vasodilation and increases in skin blood flow, ventilation, and renal filtration. The maternal physiological adaptations described above must meet the combined demands of maternal exercise and fetal growth. More research is needed to formulate evidence-based guidelines for healthy physical activity in early pregnancy.

Phasic menstrual cycle effects on the control of breathing in healthy women.

This study examined the effects of menstrual cycle phase on ventilatory control. Fourteen eumenorrheic women were studied in the early follicular (FP; 1-6 days) and mid-luteal (LP; 20-24 days) phase of the menstrual cycle. Blood for the determination of arterial PCO(2) (PaCO(2)) , plasma strong ion difference ([SID]), progesterone ([P(4)]), and 17beta-estradiol ([E(2)]) concentrations were obtained at rest. Subjects performed a CO(2) rebreathing procedure that included prior hyperventilation and maintenance of iso-oxia to evaluate central and peripheral chemoreflex, and nonchemoreflex drives to breathe. Resting PaCO(2) and [SID] were lower; minute ventilation (V (E)), [P(4)] and [E(2)] were higher in the LP versus FP. Within the LP, significant correlations were observed for PaCO(2) with [P(4)], [E(2)] and [SID]. Menstrual cycle phase had no effect on the threshold or sensitivity of the central and/or peripheral ventilatory chemoreflex response to CO(2). Both (V (E)) and the ventilatory response to hypocapnia (representing nonchemoreflex drives to breathe) were approximately 1L/min greater in the LP versus FP accounting for the reduction in PaCO(2) . These data support the hypothesis that phasic menstrual cycle changes in PaCO(2) may be due, at least in part, to the stimulatory effects of [P(4)], [E(2)] and [SID] on ventilatory drive.

Serial respiratory adaptations and an alternate hypothesis of respiratory control in human pregnancy.

This study determined the time course of changes in resting and exercising respiratory responses during the first half of human pregnancy, and examined the potential roles of plasma osmolality and the strong ion difference ([SID]) as mediators of pregnancy-induced increases in ventilation. Healthy active women (n = 11) were studied serially from 7 to 22 weeks gestation. Responses were compared with preconception data from 14 subjects (six of whom were tested in early pregnancy), and with late-gestation resting data from 10 additional women. Resting and exercising measurements included ventilation, PaCO2, progesterone, osmolality and [SID]. Results were analyzed using mixed-model linear regression. By 7 weeks gestation, increased ventilation resulted in a very large decrease in PaCO2 at rest and during moderate-intensity exercise. Large correlations (r > 0.5) between resting progesterone and PaCO2 support the traditional theory that circulating progesterone stimulates ventilation during pregnancy. The similar time course of changes and large correlations between raw and delta values of PaCO2 and each of plasma osmolality and [SID] also suggest that both variables may influence respiratory control at rest and during exercise in the first half of pregnancy. Future experiments should continue to explore the hypothesis that osmolality and [SID] contribute to pregnancy-induced respiratory changes.

Exercise in the prevention and treatment of maternal-fetal disease: a review of the literature.

Evidence-based guidelines indicate that regular prenatal exercise is an important component of a healthy pregnancy. In addition to maintaining physical fitness, exercise may be beneficial in preventing or treating maternal-fetal diseases. Women who are the most physically active have the lowest prevalence of gestational diabetes (GDM), and prevention of GDM may decrease the incidence of obesity and type 2 diabetes in both mother and offspring. However, few studies have investigated the effectiveness of exercise in delaying or preventing GDM in at-risk women, and exercise prescriptions that optimize outcomes for women with GDM are lacking. Physically active women are also less likely to develop pre-eclampsia, and we have proposed the following 4 mechanisms that may explain this protective effect: enhanced placental growth and vascularity, reduced oxidative stress, reduced inflammation, and correction of disease-related endothelial dysfunction. Exercise may also prevent reproductive complications associated with maternal obesity. Obesity increases the risk of infertility and miscarriage, and weight loss programs that incorporate diet and exercise are a cost-effective fertility treatment that may also reduce the probability of obesity-related complications during pregnancy. Regular exercise following conception may prevent excessive gestational weight gain and reduce post-partum weight retention.

Physicochemical analysis of acid-base responses to prolonged moderate exercise in late gestation.

Stewart's physicochemical approach was employed to investigate the safety of an average recreational and occupational activity (prolonged moderate exercise) on maternal acid-base homeostasis. The responses of 10 healthy, physically active pregnant women (PG, gestational age 34-38 weeks) were compared with those of 10 non-pregnant female controls (CG). Subjects cycled for 40 min at 85% of their measured ventilatory threshold (VT). During the transition from rest to exercise, hydrogen ion concentration ([H+]) increased significantly and bicarbonate concentration ([HCO3-]) decreased significantly in both groups. Total weak acid ([Atot]) increased significantly with exercise in both groups, whereas the strong ion difference ([SID]) and CO2 tension (PaCO2) did not change significantly with exercise. Values for [H+], [HCO3-], PaCO2, [Atot] and [SID] were significantly lower in the PG vs. CG under all experimental conditions. Acid-base responses to prolonged moderate exercise are quantitatively similar in the pregnant vs. non-pregnant state. However, pregnant women maintain a lower plasma [H+] (approximately 3 neq/L (1 neq/L=1 nmol/L)) throughout rest, exercise, and recovery, as a result of lower values for PaCO2 and [Atot], which is partly offset by a lower [SID]. The results indicate that prolonged moderate exercise appears to be well tolerated by healthy recreationally and occupationally active pregnant women.

Effects of human pregnancy and aerobic conditioning on alveolar gas exchange during exercise.

This study examined the effects of aerobic conditioning during the second and third trimesters of human pregnancy on ventilatory responses to graded cycling. Previously sedentary pregnant women were assigned randomly to an exercise group (n = 14) or a nonexercising control group (n = 14). Data were collected at 15-17 weeks, 25-27 weeks and 34-36 weeks of pregnancy. Testing involved 20 W.min-1 increases in work rate to a heart rate of 170 beats.min-1 and (or) volitional fatigue. Breath-by-breath ventilatory and alveolar gas exchange measurements were compared at rest, a standard submaximal .VO2 and peak exercise. Within both groups, resting .V(E), .V(A), and V(T)/T(I) increased significantly with advancing gestation. Peak work rate, O2 pulse (.VO2/HR), .V(E), .V(A) respiratory rate, V(T)/T(I), .VO2, .VCO2, and the ventilatory threshold (T(vent)) were increased after physical conditioning. Chronic maternal exercise has no significant effect on pregnancy-induced changes in ventilation and (or) alveolar gas exchange at rest or during standard submaximal exercise. Training-induced increases in T(vent) and peak oxygen pulse support the efficacy of prenatal fitness programs to improve maternal work capacity.

Effects of pregnancy on maternal work tolerance.

This review summarizes current information on the tolerance of healthy pregnant women and their fetuses to acute strenuous exertion. Maximal aerobic power, expressed in L x min(-1), is not significantly affected in women who maintain an active lifestyle, whereas values expressed in ml x kg(-1) x min(-1) decline with advancing gestational age in relation to maternal/fetal weight gain. Efficiency during standard exercise testing and the ventilatory anaerobic threshold (Tvent) also appear to be unaffected by pregnancy, but the ability to utilize carbohydrate and exercise anaerobically during brief strenuous exercise may be reduced. Fetal responses to short strenuous exercise are usually moderate and revert to baseline values within approximately 30 min postexercise. Future studies should examine alveolar gas exchange kinetics at the start of exercise and during recovery from both moderate and strenuous exertion. Existing studies of the responses of pregnant women to prolonged exercise have focused primarily on substrate utilization and glucose homeostasis. Other maternal responses to prolonged exercise that should be examined include acid-base regulation, temperature regulation, fluid and electrolyte balance, and perception of effort. Fetal reactions should also be examined in relation to maternal responses. Until evidence-based, occupation-specific guidelines become available, it is recommended that pregnant women use the Joint SOGC/CSEP Clinical Practice Guideline: Exercise in Pregnancy and the Postpartum Period, published by the Society of Obstetricians and Gynaecologists of Canada and the Canadian Society for Exercise Physiology (2003).

Effects of human pregnancy on the ventilatory chemoreflex response to carbon dioxide.

This study examined the effects of human pregnancy on the central chemoreflex control of breathing. Subjects were two groups (n=11) of pregnant subjects (PG, gestational age, 36.5+/-0.4 wk) and nonpregnant control subjects (CG), equated for mean age, body height, prepregnant body mass, parity, and aerobic fitness. All subjects performed a hyperoxic CO2 rebreathing procedure, which includes prior hyperventilation and maintenance of iso-oxia. Resting blood gases and plasma progesterone and estradiol concentrations were measured. During rebreathing trials, end-tidal Pco2 increased, whereas end-tidal Po2 was maintained at a constant hyperoxic level. The point at which ventilation (Ve) began to rise as end-tidal Pco2 increased was identified as the central chemoreflex ventilatory recruitment threshold for CO2 (VRTco2). Ve levels below (basal Ve) and above (central chemoreflex sensitivity) the VRTco2 were determined. The VRTco2 was significantly lower in the PG vs. CG (40.5+/-0.8 vs. 45.8+/-1.6 Torr), and both basal Ve (14.8+/-1.1 vs. 9.3+/-1.6 l/min) and central chemoreflex sensitivity (5.07+/-0.74 vs. 3.16+/-0.29 l.min-1.Torr-1) were significantly higher in the PG vs. CG. Pooled data from the two groups showed significant correlations for resting arterial Pco2 with basal Ve, central chemoreflex sensitivity, and the VRTco2. The VRTco2 was also correlated with progesterone and estradiol concentrations. These data support the hypothesis that pregnancy decreases the threshold and increases the sensitivity of the central chemoreflex response to CO2. These changes may be due to the effects of gestational hormones on chemoreflex and/or nonchemoreflex drives to breathe.

Chemoreflex control of breathing during wakefulness in healthy men and women.

This study used a modified CO(2) rebreathing procedure to examine the effect of gender on the chemoreflex control of breathing during wakefulness in healthy men (n = 14) and women (n = 14). Women were tested in the follicular phase of the menstrual cycle. During rebreathing trials, subjects hyperventilated to reduce the partial pressure of end-tidal CO(2) (Pet(CO(2))) below 25 Torr and were then switched to a rebreathing bag containing a normocapnic hypoxic or hyperoxic gas mixture. During the trial, Pet(CO(2)) increased, while O(2) was maintained at a constant level. The point at which ventilation began to rise as Pet(CO(2)) increased was identified as the ventilatory recruitment threshold (VRT). Ventilation below the VRT was measured, and the slope of the ventilatory response above the VRT was determined. Gender had no effect on the hyperoxic or hypoxic VRT for CO(2). Central chemoreflex sensitivity was significantly greater in men than women but not after correction for forced vital capacity. Measures of peripheral chemoreflex sensitivity were similar between genders. However, the slope of the tidal volume (Vt) response to hyperoxic and hypoxic CO(2) rebreathing (corrected and uncorrected) was greater in men than women, respectively. We conclude that central chemoreflex sensitivity is greater in men compared with women as reflected by differences in ventilatory (uncorrected) and Vt (corrected and uncorrected) responses to CO(2). However, gender has no significant effect on the central chemoreflex VRT for CO(2). The peripheral chemoreflex control of breathing during wakefulness is similar between men and women.

The role of regular physical activity in preeclampsia prevention.

Preeclampsia affects 2-7% of pregnancies and is a leading cause of maternal and fetal morbidity and mortality. Despite extensive study, the etiology of preeclampsia is poorly understood. Abnormal placental development, predisposing maternal constitutional factors, oxidative stress, immune maladaptation, and genetic susceptibility have all been hypothesized to contribute to the development of preeclampsia. Physical conditioning and preeclampsia have opposite effects on critical physiological functions. This suggests that regular prenatal exercise may prevent or oppose the progression of the disease. Epidemiologic studies show that occupational and leisure-time physical activity is associated with a reduced incidence of preeclampsia. We hypothesize that this protective effect results from one of more of the following mechanisms: 1) stimulation of placental growth and vascularity, 2) reduction of oxidative stress, and 3) exercise-induced reversal of maternal endothelial dysfunction. Future research should include prospective epidemiological case-control studies that accurately measure occupational and leisure-time physical activity. Controlled randomized clinical trials examining the effects of prenatal exercise on biochemical markers for endothelial dysfunction, placental dysfunction, and oxidative stress are also needed. If future research supports the idea that exercise effectively protects against preeclampsia, this would provide a low-cost intervention that could dramatically improve prenatal care for women at risk of this disease.

Heart rate variability following coronary artery bypass graft surgery as a function of recovery time, posture, and exercise.

This study examined the claim made by Niemela et al. (1992) that the decline in heart rate variability after coronary artery bypass graft surgery is irreversible. We tested six women and 16 men six and 12 weeks postoperative in three postures: in the supine position, in the standing position, and during low-intensity steady-state exercise. Beat-by-beat arterial blood pressure and electrocardiographic R-R interval data were collected continuously for 10 min in each condition. R-R interval data were analyzed with spectral analysis; baroreflex data were analyzed using the sequence method. Our results show that the indices of parasympathetic modulation improved over time, as seen by an increased spontaneous baroreflex sensitivity and parasympathetic indicator, that both indices were affected by posture, and that spontaneous baroreflex sensitivity was also affected by low-intensity exercise. The effects of posture are consistent with attenuated responses of healthy older subjects to orthostatic stress. Similarly, the effects of low-intensity exercise are consistent with findings in healthy subjects. We found that spontaneous baroreflex sensitivity declined during exercise, whereas, in healthy subjects, this is maintained during low-intensity steady-state exercise. Our results of significant functional recovery between six and 12 weeks postoperative suggest that at least some of the autonomic dysfunction following surgery is temporary. Previously, no such duration of study has lasted longer than four or six weeks following cardiac surgery, which may not have been long enough to show significant functional restoration in heart rate variability.

Spontaneous baroreflex sensitivity after coronary artery bypass graft surgery as a function of gender and age.

The effects of coronary artery bypass graft (CABG) surgery on spontaneous baroreflex (SBR) sensitivity and heart rate variability were examined in 11 women and 23 men preoperatively and 5 days postoperatively. Electrocardiograph R-R interval and beat-by-beat arterial blood pressure data were collected continuously for 20 min in the supine and standing postures. Coarse graining spectral analysis was performed on the heart rate variability data. Spontaneous baroreflex sensitivity declined after surgery with a differential influence of gender. Men showed a decrease in SBR slope following surgery, with a greater decrease in the standing posture; the parasympathetic (PNS) indicator was lower postoperatively and in the standing posture; the reduction in low-frequency (LF) power was greater for the younger men. In women, the PNS indicator was lower in the standing posture. Both men and women showed a decrease in high-frequency power following CABG surgery, which decreased the sensitivity of the short-term cardiac control mechanisms that modulate heart rate, with the greater effects occurring in men. The reduction in SBR sensitivity indicates that the ability of the cardiovascular system to respond rapidly to changing stimuli was compromised. The decline in the PNS indicator implies that patients were vulnerable to the risks of myocardial ischemia, sympathetically mediated cardiac dysrhythmias, and sudden cardiac death.

Effects of human pregnancy on fluid regulation responses to short-term exercise.

This study tested the hypothesis that human pregnancy alters fluid and electrolyte regulation responses to acute short-term exercise. Responses of 22 healthy pregnant women (PG; gestational age, 37.0 +/- 0.2 wk) and 17 nonpregnant controls (CG) were compared at rest and during cycling at 70 and 110% of the ventilatory threshold (VT). At rest, ANG II concentration was significantly (P < 0.05) higher in PG vs. CG, whereas plasma osmolality and concentrations of AVP, sodium, and potassium were significantly lower. Atrial natriuretic peptide concentration at rest was similar between groups. ANG II and AVP concentrations increased significantly from rest to 110% VT in CG only, whereas increases in atrial natriuretic peptide concentration were similar between groups. Increases in osmolality, and total protein and albumin concentrations from rest to both work rates were similar between the two groups. PG and CG exhibited similar shifts in fluid during acute short-term exercise, but the increases in ANG II and AVP were absent and attenuated, respectively, during pregnancy. This was attributed to the significantly augmented fluid volume state already present at rest in late gestation.