History of space medicine: the formative years at NASA.

Almost nothing was known about the effects of spaceflight on human physiology when, in May of 1961, President John F. Kennedy committed the United States to land a man on the Moon and return him safely to Earth within the decade. There were more questions than answers regarding the effects of acceleration, vibration, cabin pressure, CO2 concentration, and microgravity.

Evidence for increased beta-adrenoreceptor responsiveness induced by 14 days of simulated microgravity in humans.

We studied hemodynamic responses to alpha- and beta-receptor agonists in eight healthy men before and after 14 days of 6 degrees head-down tilt (HDT) to test the hypothesis that increased adrenoreceptor responsiveness is induced by prolonged exposure to simulated microgravity. Steady-state infusions of isoproterenol (Iso) at rates of 0.005, 0.

Application of USAF G-suit technology for clinical orthostatic hypotension: a case study.

Introduction: The purpose of this study was to determine the effectiveness of a USAF anti-gravity suit (G-suit) on the stability of a patient with chronic orthostatic hypotension.

Methods: A 37-yr-old female with a history of insulin-dependent diabetes mellitus (IDDM) and symptomatic orthostasis was evaluated and the results were compared with those of non-diabetic controls, matched for age, height, and weight. Cardiac vagal tone was assessed by determination of standard deviation of 100 R-R intervals (R-R SD).

Increased beta-adrenergic responsiveness induced by 14 days exposure to simulated microgravity.

Increased sensitivity of end-organ responses to neuroendocrine stimuli as a result of prolonged exposure to the relative inactivity of microgravity has recently been hypothesized. This notion is based on the inverse relationship between circulating norepinephrine and beta-adrenoreceptor sensitivity. Beta-adrenoreceptor activity is reduced in individuals who have elevated plasma norepinephrine as as a result of regular exposure to upright posture and physical exercise.

Hemodynamic and hormonal responses to lower body negative pressure in men with varying profiles of strength and aerobic power.

Hemodynamic, cardiac, and hormonal responses to lower-body negative pressure (LBNP) were examined in 24 healthy men to test the hypothesis that responsiveness of reflex control of blood pressure during orthostatic challenge is associated with interactions between strength and aerobic power. Subjects underwent treadmill tests to determine peak oxygen uptake (VO2max) and isokinetic dynamometer tests to determine knee extensor strength. Based on predetermined criteria, subjects were classified into one of four fitness profiles of six subjects each, matched for age, height, and body mass: (a) low strength/average aerobic fitness, (b) low strength/high aerobic fitness, (c) high strength/average aerobic fitness, and (d) high strength/high aerobic fitness.

Cardiovascular physiology. Effects of microgravity.

Experiments during spaceflight and its groundbase analog, bedrest, provide consistent data which demonstrate that numerous changes in cardiovascular function occur as part of the physiological adaptation process to the microgravity environment. These include elevated heart rate and venous compliance, lowered blood volume, central venous pressure and stroke volume, and attenuated autonomic reflex functions. Although most of these adaptations are not functionally apparent during microgravity exposure, they manifest themselves during the return to the gravitational challenge of earth's terrestrial environment as orthostatic hypotension and instability, a condition which could compromise safety, health and productivity.

Effect of breakfast on selected serum and cardiovascular variables.

We compared high-density lipoprotein cholesterol (HDL-C) and other blood and cardiovascular variables of subjects when they had been fasting overnight to those values when they had eaten breakfast. The subjects were 47 men (24 to 70 years) and 34 women (23 to 63 years) who visited our laboratory on two occasions, once fasting and once after breakfast, at the same time of the morning, exactly 1 week apart. Systolic (SBP) and diastolic (DBP) pressures and heart rates (HR) were recorded, and 24-ml blood samples were obtained from an antecubital vein and analyzed for hemoglobin (Hgb), hematocrit (Hct), HDL-C, total cholesterol (T Chol), triglycerides (Trig), total calcium (T Ca), ionized calcium (Ca++), and phosphorus (P).

Impairment of carotid-cardiac vagal baroreflex in wheelchair-dependent quadriplegics.

The incidence of orthostatic hypotension can increase after prolonged exposure to chair rest and bedrest and is associated with post-bed rest impairment of the carotid-cardiac baroreflex response. We therefore hypothesized that the hypotension observed in humans confined to wheelchairs may be manifested by a reduced baroreflex sensitivity. We compared baroreflex responses of 16 wheelchair-dependent (WCD) quadriplegics with those of 15 able-bodied subjects (ABS) matched for age, height, and weight.

Effect of a central redistribution of fluid volume on response to lower-body negative pressure.

We studied cardiovascular responses to lower-body negative pressure (LBNP) following 1 hour (h) of 6 degrees head-down tilt to determine whether a redistribution of blood volume toward the central circulation modifies the subsequent response to orthostatic stress. Responses of 12 men, ages 30-39 years, were evaluated by electrocardiography, impedance cardiography, sphygmomanometry, and measurement of calf circumference. During the LBNP that followed head-down tilt--as compared with control LBNP (no preceding head-down tilt)--subjects had smaller stroke volume and cardiac output, greater total peripheral resistance, and less calf enlargement.

Association of sex and age with responses to lower-body negative pressure.

Responses of 21 women and 29 men (29-56 yr of age) to -50 Torr lower body negative pressure (LBNP) were examined for differences due to sex or age. Responses to LBNP were normal, including fluid shift from thorax to lower body, increased heart rate and peripheral resistance, and decreased stroke volume, cardiac output, and Heather index of ventricular function. Mean arterial blood pressure did not change.

Leg size and muscle functions associated with leg compliance.

Leg compliance is "causally related with greater susceptibility" to orthostatic stress. Since peak O2 uptake (peak VO2) and muscle strength may be related to leg compliance, we examined the relationships between leg compliance and factors related to muscle size and physical fitness. Ten healthy men, 25-52 yr, underwent tests for determination of vascular compliance of the calf (Whitney mercury strain gauge), peak VO2 (Bruce treadmill), calf muscle strength (Cybex isokinetic dynamometer), body composition (densitometry), and anthropometric measurements of the calf.

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.

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.

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.

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.

Haptoglobin and serum enzymatic response to maximal exercise in relation to physical fitness.

This study investigated the potential value of specific cellular responses to acute and chronic exercise as indices of physical fitness. Muscle enzymatic and hemolytic responses following a progressive cycle-ergometer test to maximal aerobic capacity were studied in 12 women and 12 men, aged 27 to 55 yr, who had been previously assigned to "high" and "low" fitness groups. Venous blood samples were obtained at rest prior to the cycle test, immediately following maximal effort, and 1, 2, 4, 6 and 24 h post exercise.

Pre- and postflight systolic time intervals during LBNP: the second manned Skylab mission.

After space flight of 59 d, Skylab 3 astronauts were stressed with lower body negative pressure (LBNP). During this stress procedure vectorcardiograms, pneumograms, phonocardiograms, and carotid pulse tracings were monitored and recorded onto analog tape. Accepted techniques were used to measure the intervals of systole.

Lower body negative pressure: the second manned Skylab mission.

Results of orthostatic evaluations of the crew of Skylab 3 with lower body negative pressure (LNBP) stress tests during their 59-d mission are reported. The test protocol was identical to that used in the first manned Skylab mission and the latter Apollo flights. Except for an inflight increase (rather than a decrease) in resting heart rates, results were essentially parallel to those observed in crewmen of the shorter Skylab 2 mission.

Measurement of a single tendon reflex in conjunction with a myogram: the second manned Skylab mission.

A generalized hyperreflexia was observed in Skylab 2 crewmembers during the immediate postflight clinical evaluations. Unfortunately, no provisions had been made to quantitate the observed phenomenon. The cause of the hyperreflexia was not readily apparent at the time of the observation.

Determination of cardiac size following space missions of different durations: the second manned Skylab mission.

A simple method to estimate cardiac size from single frontal plane chest roentgenograms has been described. Pre- and postflight chest X-rays from Apollo 17, and Skylab 2 and 3 have been analyzed for changes in the cardiac silhouette size. The data obtained from the computed cardiothoracic areal ratios compared well with the clinical cardiothoracic diametral ratios (r = .

VECTAN II: A computer program for the spatial analysis of the vectorcardiogram.

This paper presents the operation of a digital computer program, VECTAN II, for the spatial analysis of the vectorcardiogram (VCG). The program incorporates a unique waveform recognition algorithm based on the spatial vector length which has been shown to perform better than previous algorithms. The waveform analysis employed by the program considers the vectorcardiogram as a three dimensional entity rather than as scalar or planar representations.

Skylab experiment M-092: results of the first manned mission.

Blood pressure at 30-sec intervals, heart rate, and percentage increase in leg volume continuously were recorded during a 25-min protocol in the M092 Inflight Lower Body Negative Pressure (LBNP) experiment carried out in the first manned Skylab mission. These data were collected during six tests on each crewman over a 5-month preflight period. The protocol consisted of a 5-min resting control period, 1 min at -8, 1 min at -16, 3 min at -30, 5 min at -40, and 5 min at -50 mm Hg LBNP.

Effect of potassium depletion in normal males: an Apollo 15 simulation.

In the course of Apollo 15, physiologic abnormalities, manifested by ectopic activity on the ECG and unusual alterations in excerise tolerance, occurred in the crew of the Lunar Excursion Module. These were associated with decreases in total body potassium, measured by 42K, of 10% and 15%. The possibility of inadequate potassium (K plus) intake existed.