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Biomedical subjects

A R Hargens

Publications and source records attributed to A R Hargens.

At least 19 recordsLinked to original sources
PubMed2001

Lumbar spine disc height and curvature responses to an axial load generated by a compression device compatible with magnetic resonance imaging.

STUDY DESIGN: Axial load-dependent changes in the lumbar spine of supine healthy volunteers were examined using a compression device compatible with magnetic resonance imaging. OBJECTIVE: To test two hypotheses: Axial loading of 50% body weight from shoulder to feet in supine posture 1) simulates the upright lumbar spine alignment and 2) decreases disc height significantly. SUMMARY OF BACKGROUND DATA: Axial compression on the lumbar spine has significantly narrowed the lumbar dural sac in patients with sciatica, neurogenic claudication or both. METHODS: Using a device compatible with magnetic resonance imaging, the lumbar spine of eight young volunteers, ages 22 to 36 years, was axially compressed with a force equivalent to 50% of body weight, approximating the normal load on the lumbar spine in upright posture. Sagittal lumbar magnetic resonance imaging was performed to measure intervertebral angle and disc height before and during compression. RESULTS: Each intervertebral angle before and during compression was as follows: T12-L1 (-0.8 degrees +/- 2.5 degrees and -1.5 degrees +/- 2.6 degrees ), L1-L2 (0.7 degrees +/- 1.4 degrees and 3.3 degrees +/- 2.9 degrees ), L2-L3 (4.7 degrees +/- 3.5 degrees and 7.3 degrees +/- 6 degrees ), L3-L4 (7.9 degrees +/- 2.4 degrees and 11.1 degrees +/- 4.6 degrees ), L4-L5 (14.3 degrees +/- 3.3 degrees and 14.9 degrees +/- 1.7 degrees ), L5-S1 (25.8 degrees +/- 5.2 degrees and 20.8 degrees +/- 6 degrees ), and L1-S1 (53.4 degrees +/- 11.9 degrees and 57.3 degrees +/- 16.7 degrees ). Negative values reflect kyphosis, and positive values reflect lordosis. A significant difference between values before and during compression was obtained at L3-L4 and L5-S1. There was a significant decrease in disc height only at L4-L5 during compression. CONCLUSIONS: The axial force of 50% body weight in supine posture simulates the upright lumbar spine morphologically. No change in intervertebral angle occurred at L4-L5. However, disc height at L4-L5 decreased significantly during compression.

Adult↗
PubMed2001

Ischemia causes muscle fatigue.

The purpose of this investigation was to determine whether ischemia, which reduces oxygenation in the extensor carpi radialis (ECR) muscle, causes a reduction in muscle force production. In eight subjects, muscle oxygenation (TO2) of the right ECR was measured noninvasively and continuously using near infrared spectroscopy (NIRS) while muscle twitch force was elicited by transcutaneous electrical stimulation (1 Hz, 0.1 ms). Baseline measurements of blood volume, muscle oxygenation and twitch force were recorded continuously, then a tourniquet on the upper arm was inflated to one of five different pressure levels: 20, 40, 60 mm Hg (randomized order) and diastolic (69 +/- 9.8 mm Hg) and systolic (106 +/- 12.8 mm Hg) blood pressures. Each pressure level was maintained for 3-5 min, and was followed by a recovery period sufficient to allow measurements to return to baseline. For each respective tourniquet pressure level, mean TO2 decreased from resting baseline (100% TO2) to 99 +/- 1.2% (SEM), 96 +/- 1.9%, 93 +/- 2.8%, 90 +/- 2.5%, and 86 +/- 2.7%, and mean twitch force decreased from resting baseline (100% force) to 99 +/- 0.7% (SEM), 96 +/- 2.7%, 93 +/- 3.1%, 88 +/- 3.2%, and 86 +/- 2.6%. Muscle oxygenation and twitch force at 60 mm Hg tourniquet compression and above were significantly lower (P < 0.05) than baseline value. Reduced twitch force was correlated in a dose-dependent manner with reduced muscle oxygenation (r = 0.78, P < 0.001). Although the correlation does not prove causation, the results indicate that ischemia leading to a 7% or greater reduction in muscle oxygenation causes decreased muscle force production in the forearm extensor muscle. Thus, ischemia associated with a modest decline in TO2 causes muscle fatigue.

Adult↗
PubMed2000

Supine lower body negative pressure exercise during bed rest maintains upright exercise capacity.

Bed rest and spaceflight reduce exercise fitness. Supine lower body negative pressure (LBNP) treadmill exercise provides integrated cardiovascular and musculoskeletal stimulation similar to that imposed by upright exercise in Earth gravity. We hypothesized that 40 min of supine exercise per day in a LBNP chamber at 1.0-1.2 body wt (58 +/- 2 mmHg LBNP) maintains aerobic fitness and sprint speed during 15 days of 6 degrees head-down bed rest (simulated microgravity). Seven male subjects underwent two such bed-rest studies in random order: one as a control study (no exercise) and one with daily supine LBNP treadmill exercise. After controlled bed-rest, time to exhaustion during an upright treadmill exercise test decreased 10%, peak oxygen consumption during the test decreased 14%, and sprint speed decreased 16% (all P < 0.05). Supine LBNP exercise during bed rest maintained all the above variables at pre-bed-rest levels. Our findings support further evaluation of LBNP exercise as a countermeasure against long-term microgravity-induced deconditioning.

Adult↗
PubMed2000

Supine lower body negative pressure exercise simulates metabolic and kinetic features of upright exercise.

Exercise within an artificial gravity environment may help prevent microgravity-induced deconditioning. We hypothesized that supine lower body negative pressure (LBNP) exercise simulates physiological and biomechanical features of upright exercise. Walking (4.5 +/- 0.3 km/h) and running (8.0 +/- 1.0 km/h) while supine within a LBNP exerciser were compared with walking and running while upright. Eight healthy subjects exercised for 5 min at each of the four posture/gait conditions. LBNP of 52 +/- 4 mmHg generated one body weight of supine ground reaction force (GRF). Gait parameters and GRFs were measured during the third minute of exercise, and heart rate and oxygen consumption were measured during the fifth minute. Oxygen consumption during supine LBNP treadmill exercise [walking: 14.6 +/- 0.9; running: 32.2 +/- 1.6 (SE) ml. min(-1). kg(-1)] was similar to that during upright treadmill exercise (walking: 15.1 +/- 0.9; running: 34.0 +/- 1.9 ml. min(-1). kg(-1)). Heart rate for supine LBNP exercise (grand mean: 133 +/- 11 beats/min) was also similar to that for upright exercise (136 +/- 11 beats/min). Footward forces integrated over each stride (330.5 +/- 34.4 vs. 319. 1 +/- 29.6 N. s) and rate of force generation (26,483 +/- 4,310 vs. 25,634 +/- 4,434 N/s) were similar for upright and LBNP exercise, respectively. Our collective results indicate that supine exercise within LBNP can simulate the physiological stress and GRFs that are generated during upright gait.

Adult↗
PubMed2000

Cerebrovascular responses during lower body negative pressure-induced presyncope.

BACKGROUND: Reduced orthostatic tolerance is commonly observed after spaceflight, occasionally causing presyncopal symptoms which may be due to low cerebral blood flow (CBF). It has been suggested that CBF decreases in early stages of exposure to orthostatic stress. The purpose of this study was to investigate cerebrovascular responses during presyncope induced by lower body negative pressure (LBNP). HYPOTHESIS: Although CBF decreases during LBNP exposure, blood pressure (BP) or heart rate (HR) contributes more to induce presyncopal conditions. METHODS: Eight healthy male volunteers were exposed to LBNP in steps of 10 mm Hg every 3 min until presyncopal symptoms were detected. Electrocardiogram (ECG) was monitored continuously and arterial BP was measured by arterial tonometry. CBF velocity at the middle cerebral artery was measured by transcranial Doppler sonography (TCD). Cerebral tissue oxygenation was detected using near-infrared spectroscopy (NIRS). We focused our investigation on the data obtained during the final 2 min before the presyncopal endpoint. RESULTS: BP gradually decreased from 2 min to 10 s before the endpoint, and fell more rapidly during the final 10 s. HR did not change significantly during presyncope. CBF velocity did not change significantly, while cerebral tissue oxygenation decreased prior to the presyncopal endpoint in concert with BP. Our results suggest that CBF is maintained in the middle cerebral artery during presyncope, while BP decreases rapidly. CONCLUSIONS: Cerebrovascular hemodynamics are relatively well maintained while arterial hypotension occurs just prior to syncope.

Adult↗
PubMed1999

Physiological response to submaximal isometric contractions of the paravertebral muscles.

STUDY DESIGN: Brief (30-second) isometric trunk extensions at 5%, 20%, 40%, 60%, and 80% of maximal voluntary contraction (MVC) and 3 minutes of prolonged trunk extension (20% MVC) in erect position were studied in nine healthy male subjects. OBJECTIVES: To investigate the intercorrelation between intramuscular pressure and tissue oxygenation of the paravertebral muscles during submaximal isometric contractions and further, to evaluate paravertebral electromyogram and intramuscular pressure as indicators of force development. SUMMARY OF BACKGROUND DATA: Local physiologic responses to muscle contraction are incompletely understood. METHODS: Relative oxygenation was monitored with noninvasive near-infrared spectroscopy, intramuscular pressure was measured with a transducer-tipped catheter, and surface electromyogram was monitored at three recording sites. RESULTS: The root mean square amplitudes of the paravertebral electromyogram (L4, left and right; T12, right) and intramuscular pressure measured in the lumbar multifidus muscle at L4 increased with greater force development in a curvilinear manner. A significant decrease in the oxygenation of the lumbar paravertebral muscle in response to muscle contraction was found at an initial contraction level of 20% MVC. This corresponded to a paravertebral intramuscular pressure of 30-40 mm Hg. However, during prolonged trunk extension, no further decrease in tissue oxygenation was found compared with the tissue oxygenation level at the end of the brief contractions, indicating that homeostatic adjustments (mean blood pressure and heart rate) over time were sufficient to maintain paravertebral muscle oxygen levels. CONCLUSION: At a threshold intramuscular pressure of 30-40 mm Hg during muscle contraction, oxygenation in the paravertebral muscles is significantly reduced. The effect of further increase in intramuscular pressure on tissue oxygenation over time may be compensated for by an increase in blood pressure and heart rate. Surface electromyogram amplitudes and intramuscular pressure can be used as indicators of paravertebral muscle force.

Adult↗
PubMed1999

Hoffmann-reflex is delayed during 6 degree head-down tilt with balanced traction.

BACKGROUND: Increased spinal height due to the lack of of axial compression on spinal structures in microgravity may stretch the spinal cord, cauda equina, nerve roots, and paraspinal tissues. HYPOTHESIS: Exposure to simulated microgravity causes dysfunction of nerve roots so that the synaptic portion of the Achilles tendon reflex is delayed. METHODS: Six healthy male subjects were randomly divided into two groups with three in each group. The subjects in the first group underwent horizontal bed rest (HBR) for three days. After a two week interval they underwent bed rest in a position of head-down tilt with balanced traction (HDT). So that each subject could serve as his own control, the second group was treated identically but in opposite order. Bilateral F waves and H-reflexes were measured daily (18:30-20:30) on all subjects placed in a prone position. RESULTS: By means of ANOVA, differences between HDT and HBR were observed only in M-latency and F-ratio, not in F-latency, central latency, and H-latency. Differences during the course of the bed rest were observed in M-latency and H-latency only. Tibial H latency was significantly lengthened in HDT group on day 2 and 3, although no significant difference between HDT and HBR was observed. CONCLUSION: The monosynaptic reflex assessed by H-reflex was delayed during 6 degree HDT with traction. The exact mechanism of this delay and whether the change was due to lengthening of the lower part of the vertebrae remain to be clarified.

Action Potentials↗
PubMed1999

Self-generated lower body negative pressure exercise.

BACKGROUND: Exercise during spaceflight helps prevent musculoskeletal and cardiovascular deconditioning to Earth gravity. This report evaluates the aerobic and anaerobic exercise stimulus provided by self-generated lower body negative pressure. METHODS: A lower body negative pressure cylinder expands and collapses longitudinally, but not radially. As the legs push footward to expand the cylinder, the air pressure in the cylinder decreases, increasing the force required to continue expanding the cylinder. In addition, valves control air flow into and out of the cylinder, and thus workload. In seven supine subjects, knee bend exercise was performed at 19 cycles per minute for 6 min. Footward force was measured with load cells, cylinder pressure with a transducer, heart rate from ECG, and oxygen consumption with turbine volumetry and gas analysis. RESULTS: Maximum footward force at the peak of the exercise cycle averaged 1120+/-88 N (114+/-9 kg), and pressure within the cylinder concomitantly decreased 26+/-3 mmHg below ambient. Heart rate and oxygen consumption increased 75+/-4 bpm and 26.3+/-1.4 ml O2/kg x min(-1) from supine resting values, respectively. CONCLUSIONS: With the air inlet valve nearly closed, exercise with this device approximates a resistance-type leg press. With more inflow of air, more rapid, aerobic knee bends can be performed. This exercise device/concept provides simultaneous dynamic musculoskeletal and cardiovascular stresses without an external power source.

Adult↗
PubMed1998

Noninvasive measurement of pulsatile intracranial pressure using ultrasound.

The present study was designed to validate our noninvasive ultrasonic technique (pulse phase locked loop: PPLL) for measuring intracranial pressure (ICP) waveforms. The technique is based upon detecting skull movements which are known to occur in conjunction with altered intracranial pressure. In bench model studies, PPLL output was highly correlated with changes in the distance between a transducer and a reflecting target (R2 = 0.977). In cadaver studies, transcranial distance was measured while pulsations of ICP (amplitudes of zero to 10 mmHg) were generated by rhythmic injections of saline. Frequency analyses (fast Fourier transformation) clearly demonstrate the correspondence between the PPLL output and ICP pulse cycles. Although theoretically there is a slight possibility that changes in the PPLL output are caused by changes in the ultrasonic velocity of brain tissue, the decreased amplitudes of the PPLL output as the external compression of the head was increased indicates that the PPLL output represents substantial skull movement associated with altered ICP. In conclusion, the ultrasound device has sufficient sensitivity to detect transcranial pulsations which occur in association with the cardiac cycle. Our technique makes it possible to analyze ICP waveforms noninvasively and will be helpful for understanding intracranial compliance and cerebrovascular circulation.

Brain↗
PubMed1998

Leg intramuscular pressures during locomotion in humans.

To assess the usefulness of intramuscular pressure (IMP) measurement for studying muscle function during gait, IMP was recorded in the soleus and tibialis anterior muscles of 10 volunteers during treadmill walking and running by using transducer-tipped catheters. Soleus IMP exhibited single peaks during late-stance phase of walking [181 +/- 69 (SE) mmHg] and running (269 +/- 95 mmHg). Tibialis anterior IMP showed a biphasic response, with the largest peak (90 +/- 15 mmHg during walking and 151 +/- 25 mmHg during running) occurring shortly after heel strike. IMP magnitude increased with gait speed in both muscles. Linear regression of soleus IMP against ankle joint torque obtained by a dynamometer produced linear relationships (n = 2, r = 0.97 for both). Application of these relationships to IMP data yielded estimated peak soleus moment contributions of 0.95-1.65 N . m/kg during walking, and 1.43-2.70 N . m/kg during running. Phasic elevations of IMP during exercise are probably generated by local muscle tissue deformations due to muscle force development. Thus profiles of IMP provide a direct, reproducible index of muscle function during locomotion in humans.

Adult↗
PubMed1998

Influence of hydrostatic pressure gradients on regulation of plasma volume after exercise.

The impact of posture on the immediate recovery of intravascular fluid and protein after intense exercise was determined in 14 volunteers. Forces which govern fluid and protein movement in muscle interstitial fluid pressure (PISF), interstitial colloid osmotic pressure (COPi), and plasma colloid osmotic pressure (COPp) were measured before and after exercise in the supine or upright position. During exercise, plasma volume (PV) decreased by 5.7 +/- 0.7 and 7. 0 +/- 0.5 ml/kg body weight in the supine and upright posture, respectively. During recovery, PV returned to its baseline value within 30 min regardless of posture. PV fell below this level by 60 and 120 min in the supine and upright posture, respectively (P < 0. 05). Maintenance of PV in the upright position was associated with a decrease in systolic blood pressure, an increase in COPp (from 25 +/- 1 to 27 +/- 1 mmHg; P < 0.05), and an increase in PISF (from 5 +/- 1 to 6 +/- 2 mmHg), whereas COPi was unchanged. Increased PISF indicates that the hydrostatic pressure gradient favors fluid movement into the vascular space. However, retention of the recaptured fluid in the plasma is promoted only in the upright posture because of increased COPp.

Adult↗
PubMed1998

Intracranial pressure dynamics during simulated microgravity using a new noninvasive ultrasonic technique.

It is believed that intracranial pressure (ICP) may be elevated in microgravity because a fluid shift toward the head occurs due to loss of gravitational blood pressures. Elevated ICP may contribute to space adaptation syndrome, because as widely observed in clinical settings, elevated ICP causes headache, nausea, and projectile vomiting, which are similar to symptoms of space adaptation syndrome. However, the hypothesis that ICP is altered in microgravity is difficult to test because of the invasiveness of currently-available techniques. We have developed a new ultrasonic technique, which allows us to record ICP waveforms noninvasively. The present study was designed to understand postural effects on ICP and assess the feasibility of our new device in future flight experiments.

Adult↗
PubMed1998

Development of a noninvasive technique for the measurement of intracranial pressure.

Intracranial pressure (ICP) dynamics are important for understanding adjustments to altered gravity. Previous flight observations document significant facial edema during exposure to microgravity, which suggests that ICP is elevated during microgravity. However, there are no experimental results obtained during space flight, primarily due to the invasiveness of currently available techniques. We have developed and refined a noninvasive technique to measure intracranial pressure noninvasively. The technique is based upon detecting skull movements of a few micrometers in association with altered intracranial pressure. We reported that the PPLL technique has enough sensitivity to detect changes in cranial distance associated with the pulsation of ICP in cadavera. In normal operations, however, we place a transducer on the scalp. Thus, we cannot rule out the possibility that the PPLL technique picks up cutaneous pulsation. The purpose of the present study was therefore to show that the PPLL technique has enough sensitivity to detect changes in cranial distance associated with cardiac cycles in vivo.

Blood Pressure↗
PubMed1998

Plasma colloid osmotic pressure increases in humans during simulated microgravity.

BACKGROUND: On exposure to microgravity, astronauts lose up to 12% of their plasma volume which may contribute to post-flight orthostatic intolerance. HYPOTHESIS: Whole-body dehydration during prolonged microgravity, simulated by 6(0) head-down tilt (HDT), may increase plasma colloid osmotic pressure (COP). METHODS: There were seven healthy male subjects (30-55 yr of age) were placed in 6(0) HDT for 16 d. Plasma COP was measured from blood samples drawn immediately before HDT, on day 14 of HDT, and 1 h following bed rest termination using a 20 muL colloid osmometer. Plasma volume was determined before HDT, on day 16 of HDT, and 1 h following bed rest termination using a modified Evans blue dye technique. RESULTS: Plasma COP on day 14 of bed rest (29.9 +/- 0.7 mm Hg) was higher (p = 0.01) than pre-HDT value (23.1 +/- 0.8 mm Hg), coinciding with a decrease of plasma volume. At 1 h of upright recovery following HDT, plasma volume stayed below baseline and plasma COP remained elevated (26.6 +/- 0.6 mm hg; p = 0.003) as compared with the pre-HDT value. CONCLUSION: Our results indicate that reduced plasma volume and significantly elevated plasma COP probably reflect an overall loss of extracellular fluids during simulated microgravity.

Adult↗
PubMed1998

Current concepts in the pathophysiology, evaluation, and diagnosis of compartment syndrome.

This article reviews present knowledge of the pathophysiology and diagnosis of acute compartment syndromes. Recent results using compression of legs in normal volunteers provide objective data concerning local pressure thresholds for neuromuscular dysfunction in the anterior compartment. Results with this model indicate that a progression of neuromuscular deficits occurs when IMP increases to within 35 to 40 mm Hg of diastolic blood pressure. These findings provide useful information on the diagnosis and compression thresholds for acute compartment syndromes. Time factors are also important, however, and usually are incompletely known in most cases of acute compartment syndrome. Although the slit catheter is a very good technique for monitoring IMP during rest, these catheters and their associated extracorporeal transducer systems are not ideal. Recently developed miniature transducer-tipped catheters and, perhaps, future development of noninvasive techniques may provide accurate recordings of IMP in patients with acute compartment syndromes.

Acute Disease↗
PubMed1998

Venoconstrictive thigh cuffs impede fluid shifts during simulated microgravity.

BACKGROUND: This study determined the efficacy of venoconstrictive thigh cuffs, inflated to 50 mmHg, on impeding fluid redistributions during simulated microgravity. METHODS: There were 10 healthy male subjects who were exposed to a 2-h tilt protocol which started in the standing position, and was followed by 30 min supine, 30 min standing, 30 min supine, 30 min of -12 degrees head down tilt (HDT, to simulate microgravity), 15 min of HDT with venoconstrictive thigh cuffs inflated, a further 10 min of HDT, 5 min supine, and 10 min standing. To increase the sensitivity of the techniques in an Earth-based model, 12 degrees HDT was used to simulate microgravity effects on body fluid shifts. Volume changes were measured with anthropometric sleeve plethysmography. RESULTS: Transition to the various tilt positions resulted in concomitant decrements in leg volume (Stand [STD] to Supine [SUP], -3.0%; SUP to HDT, -2.0%). Inflation of the venoconstrictive thigh cuffs to 50 mmHg, during simulated microgravity, resulted in a significant 3.0% increase in leg volume from that seen in HDT (p < 0.01). No significant changes in systemic cardiovascular parameters were noted during cuff inflation. CONCLUSIONS: We conclude that venoconstrictive thigh cuffs, inflated to 50 mmHg for 15 min during 12 degrees HDT, can create a more Earth-like fluid distribution. Cuffs could potentially be used to ameliorate the symptoms of cephalad edema seen with space adaptation syndrome and to potentiate existing fluid volume countermeasure protocols.

Adult↗
PubMed1997

Forearm muscle oxygenation decreases with low levels of voluntary contraction.

The purpose of our investigation was to determine if the near infrared spectroscopy technique was sensitive to changes in tissue oxygenation at low levels of isometric contraction in the extensor carpi radialis brevis muscle. Nine subjects were seated with the right arm abducted to 45 degrees, elbow flexed to 85 degrees, forearm pronated 45 degrees, and wrist and forearm supported on an armrest throughout the protocol. Altered tissue oxygenation was measured noninvasively with near infrared spectroscopy. The near infrared spectroscopy probe was placed over the extensor carpi radialis brevis of the subject's right forearm and secured with an elastic wrap. After 1 minute of baseline measurements taken with the muscle relaxed, four different loads were applied just proximal to the metacarpophalangeal joint such that the subjects isometrically contracted the extensor carpi radialis brevis at 5, 10, 15, and 50% of the maximum voluntary contraction for 1 minute each. A 3-minute recovery period followed each level of contraction. At the end of the protocol, with the probe still in place, a value for ischemic tissue oxygenation was obtained for each subject. This value was considered the physiological zero and hence 0% tissue oxygenation. Mean tissue oxygenation (+/-SE) decreased from resting baseline (100% tissue oxygenation) to 89 +/- 4, 81 +/- 8, 78 +/- 8, and 47 +/- 8% at 5, 10, 15, and 50% of the maximum voluntary contraction, respectively. Tissue oxygenation levels at 10, 15, and 50% of the maximum voluntary contraction were significantly lower (p < 0.05) than the baseline value. Our results indicate that tissue oxygenation significantly decreases during brief, low levels of static muscle contraction and that near infrared spectroscopy is a sensitive technique for detecting deoxygenation noninvasively at low levels of forearm muscle contraction. Our findings have important implications in occupational medicine because oxygen depletion induced by low levels of muscle contraction may be directly linked to muscle fatigue.

Adult↗