Aerobic fitness in women and responses to lower body negative pressure

High aerobic fitness may be associated with impaired responsiveness to orthostatic challenge. This could be detrimental to astronauts returning from spaceflight. Thus, we examined the cardiovascular responses of a group of 45 healthy women to graded lower body negative pressure (LBNP) through 5 min at -50 mm Hg or until they become presyncopal. The ages (range = 23-43 years, mean = 30.4) and peak aerobic capacities (range = 23.0-55.3 ml.kg-1.min-1, mean = 37.8) of these subjects paralleled those of the women astronauts. We monitored heart rate, stroke volume, cardiac output, Heather index of contractility, arterial pressure, peripheral resistance, change in calf circumference, and thoracic impedance (ZO)--a measure of fluid in the chest. The women in this study exhibited the same response pattern to LBNP as previously reported for male subjects. VO2peak of the six subjects who became presyncopal was not different from VO2peak of the tolerant subjects. At rest, only systolic and mean arterial pressures were significantly correlated with VO2peak. Percent changes in calf circumference (i.e. fluid accumulation in the legs) at -30 and -40 mm Hg were the only responses to LBNP significantly related to VO2peak. The greater pooling of blood in the legs during LBNP by women with higher aerobic fitness, and lower percent body fat may be related to more muscle tissue and vasculature in the legs of the more fit subjects. These data indicated that orthostatic tolerance is not related to aerobic capacity in women, and orthostatic tolerance need not be a concern to aerobically fit women astronauts.

Considerations in prescribing preflight aerobic exercise for astronauts

Many human responses to the weightless environment have been documented from actual spaceflights. These include physiological effects on the nervous system, cardiovascular system and fluid balance, and the musculoskeletal system, as well as psychological effects. Simulations on Earth have added to our knowledge about the physiology of weightlessness. Early data on orthostatic intolerance after real and simulated spaceflight led some scientists to discourage a high level of aerobic fitness for astronauts. They believed it was detrimental to orthostatic tolerance on return to Earth. However, most of the data available today do not support this contention. Furthermore, aerobic fitness is beneficial to cardiovascular function and mental performance. Therefore, it may be important in performing extra-vehicular activities during flight. Some astronauts claim exercise enhances their feeling of well-being and self image. And, although the cardiovascular system and exercise performance may recover more slowly after flight to preflight levels when fitness level prior to flight is high, the musculoskeletal system may recover more rapidly. Research is needed to determine optimal levels of aerobic training for performing tasks in flight, maintaining health and well-being during flight, and assuring satisfactory recovery on return to Earth.

Hysteresis in response to descending and ascending lower-body negative pressure

We have investigated the pattern of fluid redistribution and cardiovascular responses during graduated orthostatic stress. Twelve men, age 30-39 yr, underwent a 25-min lower-body negative pressure (LBNP) test protocol that involved sequential stages of LBNP at -8 mmHg (1 min), -16 mmHg (1 min), -30 mmHg (3 min), -40 mmHg (5 min), -50 mmHg (5 min), -40 mmHg (5 min), -30 mmHg (3 min), -16 mmHg (1 min), and -8 mmHg (1 min). Data were recorded at the end of each stage. For many measured variables values during the descending phase of LBNP (-8 to -40 mmHg) were significantly different from values during the ascending phase of (-40 to -8 mmHg). These differences appear to be due to a component of fluid translocation that occurs during LBNP and cannot be reversed within the duration of the procedure. We hypothesize that this slowly reversed component is sequestration of fluid in the interstitial and lymphatic compartments. In contrast, venous pooling is a rapidly reversible component of fluid movement during LBNP. A scheme describing fluid and cardiovascular responses to LBNP based on these data and the data of other investigators is presented.

Cardiovascular dynamics during the initial period of head-down tilt

The cardiovascular response to 1 h of 6 degrees head-down tilt was studied in 12 male subjects, ages 30-39 years, to simulate the early effects of weightlessness. Fluid shifts, hemodynamic variables, and indices of myocardial contractility were evaluated by utilizing electrocardiography, systolic time intervals, impedance cardiography, sphygmomanometry, and measurement of calf circumference. Most cardiovascular variables remained stable throughout the initial 30 min of the protocol, even though translocation of fluid from the legs to the thorax commenced immediately with the onset of head-down tilt. In contrast, minutes 30-60 were characterized by reduced stroke volume, cardiac output, mean stroke ejection rate, and Heather Index concomitant with an elevation in mean arterial pressure. Intrathoracic fluid volume continued to increase while leg volume continued to decrease. This latter physiological response suggests intrathoracic sequestration of fluid volume; blood was apparently redistributed to the pulmonary circulation rather than being retained in the great veins.

Cardiovascular responses of women to lower body negative pressure

Lower body negative pressure (LBNP) has provided a method for studying cardiovascular responses in men while simulating a return to the stresses of 1-G following space flight. In this study, we have monitored responses of women to the stresses provided by LBNP. There were 20 women, 23-43 years, each tested in the follicular and luteal phases of the menstrual cycle. Variables were recorded during supine control; at -30, -40, -50 mm Hg LBNP; immediately after pressure release; and after 5 min recovery. There were no significant differences in response to LBNP between the two menstrual phases. During LBNP calf circumference was enlarged; transthoracic impedance was increased; stroke volume, left ventricular ejection time, the Heather Index of contractility and systolic pressure were reduced; total peripheral resistance was elevated; and cardiac output fell despite a rise in heart rate. Differences in cardiovascular variables between 0 mm Hg LBNP and -50 mm Hg LBNP were generally similar to reported differences between supine and standing. The responses of these women to LBNP were qualitatively similar to those reported for the Apollo astronauts and other male subjects. These women appeared to compensate with a greater heart rate response; however, the net cardiovascular compensation as determined from arterial pressure appears to be similar in men and women.

Metabolic, hemodynamic, and respiratory responses to performing cardiopulmonary resuscitation

The purpose of this study was to evaluate the cardiorespiratory demands inherent to the maintenance of continuous one- and two-man cardiopulmonary resuscitation (CPR) for 10 min. Ten male paramedics (X age = 26 yrs) certified to perform CPR participated. Each subject assumed the three possible roles for administering CPR: ventilator, compressor, and one-man technique. Cardiorespiratory responses were determined by impedance cardiography and open-circuit spirometry at 2 min intervals while performing CPR with a resuscitation manikin. Left ventricular performance was evaluated by impedance ventricular function indices (VFI) and systolic time intervals (STI). All three roles elicited an increase in oxygen uptake compared to kneeling rest, with the roles of one-man and compressor being the most demanding. There were moderate increases in cardiac output and heart rate during the exercise roles, but stroke volume remained similar to resting values. Pulmonary ventilation increased during exercise, with the greatest increase occurring for the one-man role. STI and impedance VFI reflected an enhanced ventricular performance while performing the roles of one-man and compressor. These findings suggest that the energetic demands placed on the rescuer depend upon the specific role assumed. Properly trained and experienced individuals can perform CPR efficiently for at least 10 min while eliciting only moderate physiological stress.

Central hemodynamics during progressive upper- and lower-body exercise and recovery

The purpose of this study was to compare stroke volume (SV) and myocardial contractility responses during and immediately after upper- and lower-body exercise. Nine men (mean 28 yr, 78 kg) completed progressive intensity discontinuous tests on both an arm crank and cycle ergometer. Exercise for each power output (PO) was 7 min with 20-min rest periods interspersed. Impedance cardiography was used to measure cardiac output (Q), SV, and contractility on a beat-by-beat basis during exercise and a 15-s recovery period. Q increased linearly, and total peripheral resistance decreased exponentially with increasing PO levels. During recovery from exercise, the Q and heart rate (HR) values decreased immediately at all PO levels. When the exercise VO2 exceeded 1.0 1 X min-1, SV fell significantly during recovery for both exercise modes. In general, the recovery myocardial contractility indices remained similar to exercise values. It was concluded that immediately after low intensities of exercise, Q decreases because of a fall in HR. After moderate- and high-intensity exercise, Q decreases because of a fall in both HR and SV.

Correlations between ejection times measured from the carotid pulse contour and the impedance cardiogram

Systolic time interval (STI) analysis is a commonly employed noninvasive technique for evaluating myocardial function. It requires simultaneous recording of an electrocardiogram, phonocardiogram, and the carotid pulse contour, from which left ventricular ejection time is measured. The carotid pulse contour may be difficult to record when there is subject movement, such as with exercise or other stresses utilized in aerospace medical research. Impedance cardiography is a relatively new noninvasive technique for measuring stroke volume. It also provides a measure of systolic ejection time without the necessity of recording a carotid pulse contour. The purpose of this study was to determine the correlation between left ventricular ejection time (LVET) determined from conventional STI analysis and systolic ejection time (T) obtained with impedance cardiography. The electrocardiogram, phonocardiogram, carotid pulse contour, and impedance cardiogram were monitored simultaneously in 17 male subjects 39-63 years of age (6 normotensive, 7 with established hypertension, and 4 with labile hypertension). Subjects were monitored at seated rest and during submaximal and maximal cycle ergometer exercise. Beat-by-beat analysis revealed high intrasubject correlations between LVET and T for each subject during all three activity levels. Correlations between LVET and T for the combined group of 17 subjects were: rest r = 0.990, submaximal exercise r = 0.976, maximal exercise r = 0.986; p less than 0.01. These results indicate impedance cardiography can be used in the determination of STIs for the evaluation of ventricular function, as well as for the noninvasive determination of stroke volume and cardiac output.

Cardiovascular response to a mentally stressful stimulus

The purpose of this study was to investigate multivariable cardiovascular responses to a non-mathematical mental task. Fifty-two subjects, 8 to 69 years of age, were monitored at rest and while attempting to solve a Raven's matrix test without prodding or pressure from the experimentors. Adults (greater than or equal to 18 years) had higher blood pressures (BP) and longer pre-ejection periods (PEP) at rest than did the children. Women had higher resting heart rates (HR) than men. The task induced significant increases in systolic and diastolic BP and HR in adults and children, with the adults exhibiting larger BP responses. During the stressful stimulus significant decreases in left ventricular ejection time occurred in men and women, and significant increases in forearm blood flow occurred in men. The stability in PEP during the stressful period when both BP and HR were increased is evidence of enhanced contractility brought on by the stress. In general, men and women responded similarly. Thus, even a mild, non-mathematical stress of short duration elicits the multiple cardiovascular responses, including increases in BP, HR, muscle blood flow, and contractility, which are observed with more threatening tasks.

Cardiac function monitored by impedance cardiography during changing seatback angles and anti-G suit inflation

Impedance cardiography (IC) appears to be a promising noninvasive technique for monitoring small changes in pilot cardiovascular status during conditions simulating flight. Heart rate (HR), stroke volume (SV), cardiac output (CO), ventricular ejection time (VET), and thoracic impedance (Zo) were monitored in ten volunteers for 5 min at each of four seatback angles from vertical: 12 degrees, 30 degrees, 45 degrees, and 60 degrees. Data were also obtained at three seatback angles (12 degrees, 30 degrees, 60 degrees) for 6 min each before, during, and after inflation of the standard USAF anti-G suit to 1.5 psi. Significant differences (p less than 0.05) in HR, SV, CO, VET and Zo were observed among the four positions. Inflation of the standard anti-G suit to 1.5 psi at 1.0 +Gz did not significantly alter HR, SV, or CO; whereas, 1 min of deflation of the anti-G suit significantly altered HR, SV, CO compared to inflation values. The results suggest IC can detect small differences in HR, SV, CO, VET, and Zo within subjects as a function of minor changes in body position.

Transthoracic impedance: differences between men and women with implications for impedance cardiography

Impedance cardiography (IC) is a reliable noninvasive technique for monitoring stroke volume (SV) and cardiac output. Transthoracic impedance (Zo) is one variable in the equation used for the calculation of SV. Thoracic impedance reflects the resistivity offered by tissues and air and the length and cross-sectional area of the thoracic volume. The purpose of this study was to evaluate possible differences in Zo between men and women. Measurements (Mean +/- S.D.) of Zo in 29 men (age 25.6 +/- 4.6 yr) and 35 women (24.2 +/- 6.0 yr) in the seated posture revealed Zo values were significantly (p less than 0.05) greater for the women (31.5 +/- 3.3 ohm) than the men (23.5 +/- 1.84 ohm). The observed differences in Zo cannot be attributed to thoracic length; distances between the monitoring electrodes were similar for the men (25.1 cm) and the women (25.7 cm). We hypothesize that the greater Zo observed for women is due to a smaller thoracic cross-sectional area and greater resistivity resulting primarily from relatively more fat tissue, smaller heart size, and lesser central blood volume as compared with men.

Exercise does not change high-density lipoprotein cholesterol in women after ten weeks of training

Effects of a 10 wk, three times per wk individualized bicycle ergometer training program were investigated in 16 healthy sedentary women 19-29 yr-old who were not taking oral contraceptives or other medications. Twelve women were in an interval type program, 6 in a continuous program, all performing 30 min exercise per session at 70% maximum heart rate reserve. Conditioning responses did not differ between the training regimens. Training produced increases in maximum oxygen uptake and physical work capacity. Percent body fat determined by underwater weighing was significantly reduced as was resting heart rate, after the training program. Maximum heart rate was unchanged. Despite changes in "fitness" variables, post-training values of high density lipoprotein cholesterol and triglycerides did not differ from pretraining. High-density lipoprotein cholesterol was significantly reduced at 2 and 5 wk of training and returned to control levels at 10 wk. Exercise conditioning leading to improved physical fitness in healthy women may not be associated with increments in high density lipoprotein cholesterol levels.

Computer-assisted method for performing impedance cardiography calculations

Impedance cardiography provides a noninvasive technique to monitor stroke volume on a beat-by-beat basis. It correlates well with other techniques at rest, with both cycle and arm-ergometer exercise, and during head-up tilt. It has the advantage that it does not require active subject participation, as does the CO2-rebreathing technique; yet it is still noninvasive. The method described herein facilitates the measurement and calculation of stroke volume, cardiac output, and indices of ventricular function when this technique is used - whether a few or many beats are recorded. This method utilizes a recording of the impedance cardiogram, which may include a simultaneously recorded electrocardiogram and phonocardiogram; a digitizer for reading the coordinates from the recording; a CRT terminal with keyboard for additional data input and for monitoring output; a printer; and a minicomputer for calculation of variables and data analysis. Data related to approximately 100 cardiac beats can be read, calculated, printed, and statistically analyzed in about an hour.

Influence of respiration on stroke volume determined by impedance cardiography

The conventional method of calculating stroke volume (SV) with impedance has been to measure changes in thoracic impedance for only those beats of the impedance cardiogram (IC) which cross on a specific baseline. This severely limits the number of usable beats since respiration produces oscillation of the IC around this baseline. This study investigated the influence of respiration on SV calculated independent of the baseline in seven women, 20-44 years old, in each of three postures: seated, supine, and standing. SV was determined in three continuous respiratory cycles from the following beats: 1. all; 2. all inspiratory (I); 3. I on baseline; 4. all expiratory (E); 5. E on baseline; 6. all end-E; 7. end-E on baseline (conventional). The absolute values for SV, heart rate (HR), and cardiac output (Q) were posture dependent. SVs did not differ among the respiratory phases in the standing and seated postures; in the supine posture, the smallest SV occurred at end-E on baseline. HR varied with respiration in both the standing and seated postures but not in the supine posture. Despite significant changes in HR in the seated and standing postures, Qs did not differ among the respiratory phases. Supine Qs were significantly smaller during end-E due primarily to a small SV. Consequently, when reporting Q values for supine subjects, it is important to designate the respiratory phase and baseline criteria. Our results indicate that SVs calculated independent of the baseline in general agree with the more conventional method of calculation. Therefore, we recommend the calculation of SV independent of the baseline. The reliability of the data will be increased by increasing the number of observations and SVs can be determined under circumstances where it is difficult to obtain end-E beats on the baseline.

A new ventricular-performance variable using electrocardiogram and carotid pulse contour derivative

The pre-ejection period (PEP) of systole is a widely used, non-invasive index of ventricular performance. For precise measurements, simultaneous recordings are required from which the Q-wave on the electrocardiogram, second heart sound on the phonocardiogram, and upstroke and incisura of the carotid pulse contour are discretely identifiable and temporally accurate. This is frequently difficult to accomplish. We, therefore, have compared 2 other noninvasive indices of ventricular performance for correlation with PEP in 17 female and 18 male subjects while supine and during head-up tilt. We also examined similarity of these indices to PEP in correlations with heart rate and blood pressure and compared them for ease in monitoring and precision in measurement. The ratio of the peak amplitude of the carotid pulse contour derivative to its total amplitude (DAR) is easier to monitor and more reliably measured than PEP; however, it is poorly correlated with PEP. Time from electrocardiographic Q-wave to peak dD/dt (Q-MAX) is also easier to monitor and somewhat more reliably measured than PEP. Although correlations of PEP and Q-MAZ with subject age are dissimilar, Q-MAX is correlated with PEP in supine and tilted subjects; the 2 variables are similarly correlated with heart rate and arterial pressure; and in 6 subjects both variables were prolonged after intravenous administration of propranolol-hydrochloride. We, therefore, propose Q-MAX as an alternative measure of ventricular performance.

Effect of a controlled exercise program on serum lipoprotein levels in women on oral contraceptives

The effects of a 10-wk individualized bicycle ergometer interval training program were investigated in a group of 13 sedentary women on a specific oral contraceptive (50 micrograms mestranol, 1 mg norethisterone) to determine if, like men on physical training, their high-density lipoprotein cholesterol levels would increase. Six additional women on the oral contraceptive served as a nonexercising control group. All subjects were between 18 and 30 yr and were nonsmokers. Diet and alcohol intake were stable throughout the study period, and were monitored throughout. The interval training program was three times per week, 30-min exercise per session, at 70% maximum heart rate reserve (resting heart rate + 0.7 x [maximum heart rate-resting heart rate]). Training produced increases (p < 0.05) in maximum oxygen uptake. Body weight was unchanged. However, unlike similar training programs with males, there were no significant changes in plasma high-density lipoprotein cholesterol or triglycerides. In women receiving estrogen progestin oral contraceptives, consistent exercise programs may fail to elevate high density lipoprotein cholesterol levels, consistent with an interaction of sex hormone-exercise effects and/or with the less marked effect of exercise on high-density lipoprotein cholesterol in women.

Reflex cardiovascular responses to cold exposure of the face or foot

Six subjects performed a breathholding maneuver during facial cooling and immersed their foot in cold water, without drugs and after the intravenous administration of propranolol plus atropine (P + A). Cardiac interval (INT), mean interval for longest consecutive 5 cycles (L5INT/5); systolic time intervals including electromechanical systole (EMS), left ventricular ejection time (LVET), pre-ejection period (PEP), and PEP/LVET; and systolic (SP) and diastolic pressures (DP) were monitored during supine rest, during apnea with a plastic bag of ice water on the face, and from 16-30th and 46-60th sec of 1-min periods of foot immersion in 4 degrees C water. P+A administration induced reduction in INT, L5INT/5, and LVET and increase in PEP, PEP/LVET, and DP. INT, L5INT/5, PEP, SP, and DP increased during facial cooling without drugs. Only the increases in INT and L5INT/5 were abolished by P+A and thus considered to result from reflexes mediated by vagal or sympathetic outflow to the heart. Reductions in INT, L5INT/5, EMS, PEP, and PEP/LVET at 16-30 sec of foot immersion without drugs were not observed after P+A; 46-60 sec responses neared resting values, however, with and without P+A. These results indicate an initial cardiac reflex response to foot immersion may be overpowered by the ventricular afterload and the foot immersion may be overpowered by the ventricular afterload and the baroreceptor response due to the increased arterial pressure.

Blood pressure, electrolytes, and body size: their relationships in young relatives of men with essential hypertension

In 154 white chldren aged 8 to 18 years from four large kindreds, relationships among blood pressure (BP), age, sex, body size, and electrolyte excretion were studied. Each kindred was ascertained through one male aged 35-58 years with essential hypertension, namely, a diastolic blood pressure (DBP) over 95 mm Hg. Weight, relative weight (relative to NCHS median for age, sex, and stature), subcutaneous fatfolds, various indices of obesity, and other measures of body size were significantly correlated with systolic blood pressure (SBP) and DBP in each sex (r = 0.3 to 0.7). Sodium and potassium excretion in 24-hour urine was also positively correlated with some measures of body size, and tended to increase with body size at a slightly more rapid rate in boys than in girls. In addition, there was a strong correlation between electrolyte excretion and BP in boys (r = 0.2 to 0.6); however, when the effects of age, body size and fatness were statistically removed, the correlations between BP and electrolyte excretion were not significant, except for 4th phase diastolic pressure (DBP4). These data, therefore, while not strongly supporting a relationship between sodium excretion and BP in children, do not rule out such a relationship, especially in families with a history of hypertension. In addition, these data provide further evidence of a very strong association between BP and body size and fatness in boys and girls.

Blood pressure, electrolytes, and body size: their relationships in young relatives of men with essential hypertension

In 154 white chldren aged 8 to 18 years from four large kindreds, relationships among blood pressure (BP), age, sex, body size, and electrolyte excretion were studied. Each kindred was ascertained through one male aged 35-58 years with essential hypertension, namely, a diastolic blood pressure (DBP) over 95 mm Hg. Weight, relative weight (relative to NCHS median for age, sex, and stature), subcutaneous fatfolds, various indices of obesity, and other measures of body size were significantly correlated with systolic blood pressure (SBP) and DBP in each sex (r = 0.3 to 0.7). Sodium and potassium excretion in 24-hour urine was also positively correlated with some measures of body size, and tended to increase with body size at a slightly more rapid rate in boys than in girls. In addition, there was a strong correlation between electrolyte excretion and BP in boys (r = 0.2 to 0.6); however, when the effects of age, body size and fatness were statistically removed, the correlations between BP and electrolyte excretion were not significant, except for 4th phase diastolic pressure (DBP4). These data, therefore, while not strongly supporting a relationship between sodium excretion and BP in children, do not rule out such a relationship, especially in families with a history of hypertension. In addition, these data provide further evidence of a very strong association between BP and body size and fatness in boys and girls.

Cardiovascular response to cooling of limbs determined by noninvasive methods

Cold, even local exposure to a limited portion of the body, is a stress to man which elevates arterial pressure, thereby intensifying cardiac workload. The sequence of cardiac events following local cooling was noninvasively studied by observation of changes in cardiac interval, left ventricular ejection time, time from A wave of electrocardiogram to the peak of the dD/dt of the carotid pulse wave (which includes pre-ejection period), and amplitude of the pulse wave from a photoelectric cell on the earlobe, along with arterial pressures. Twelve subjects, aged 22--41 years, exposed a hand or foot to cold water for 1 min while seated and while supine (four experiements each). Results indicate that arterial pressure is monotonically elevated throughout the minute of exposure. Cardiac intervals are initially abbreviated, then return towards control. This may include an initial response to the cold, followed by a baroreflex at the heart. Subject posture and limb exposed also affect cardiac responses.