Craig Ramsdell - Academia.edu (original) (raw)

Papers by Craig Ramsdell

This synergy project was a one-year effort conducted cooperatively by members of the NSBRI Cardio... more This synergy project was a one-year effort conducted cooperatively by members of the NSBRI Cardiovascular Alterations and Neurovestibular Adaptation Teams in collaboration with NASA Johnson Space Center (JSC) colleagues. The objective of this study was to evaluate visual autonomic interactions on short-term cardiovascular regulatory mechanisms. Based on established visual-vestibular and vestibular-autonomic shared neural pathways, we hypothesized that visually induced changes in orientation will trigger autonomic cardiovascular reflexes. A second objective was to compare baroreflex changes during postural changes as measured with the new Cardiovascular System Identification (CSI) technique with those measured using a neck barocuff. While the neck barocuff stimulates only the carotid baroreceptors, CSI provides a measure of overall baroreflex responsiveness. This study involved a repeated measures design with 16 healthy human subjects (8 M, 8 F) to examine cardiovascular regulatory r...

AIP Conference Proceedings

ABSTRACT Alterations in cardiovascular regulation and function that occur during and after space ... more ABSTRACT Alterations in cardiovascular regulation and function that occur during and after space flight have been reported. These alterations are manifested, for example, by reduced orthostatic tolerance upon reentry to the earth’s gravity from space. However, the precise physiologic mechanisms responsible for these alterations remain to be fully elucidated. Perhaps as a result, fully effective countermeasures have yet to be developed. The National Space Biomedical Research Institute Cardiovascular Alterations Team is currently conducting a head-down tilt bed rest study in Boston. These studies involve the application of two powerful new methodologies developed at the NASA Center for Quantitative Cardiovascular Physiology, Modeling and Data Analysis at the Massachusetts Institute of Technology—cardiovascular system identification and T Wave Alternans analysis—for the study of the effects of simulated microgravity on the cardiovascular system. This study is being used as a basis for developing effective countermeasures against microgravity induced orthostatic hypotension and ventricular arrhythmias. © 2000 American Institute of Physics.

Journal of applied physiology (Bethesda, Md. : 1985), 2001

Many astronauts after being weightless in space become hypotensive and presyncopal when they assu... more Many astronauts after being weightless in space become hypotensive and presyncopal when they assume an upright position. This phenomenon, known as orthostatic intolerance, may interfere with astronaut function during reentry and after spaceflight and may limit the ability of an astronaut to exit a landed spacecraft unaided during an emergency. Orthostatic intolerance is more pronounced after long-term spaceflight and is a major concern with respect to the extended flights expected aboard the International Space Station and for interplanetary exploration class missions, such as a human mission to Mars. Fully effective countermeasures to this problem have not yet been developed. To test the hypothesis that alpha-adrenergic stimulation might provide an effective countermeasure, we conducted a 16-day head-down-tilt bed-rest study (an analog of weightlessness) using normal human volunteers and administered the alpha(1)-agonist drug midodrine at the end of the bed-rest period. Midodrine w...

Many astronauts after being weightless in space become hypotensive and presyncopal when they assu... more Many astronauts after being weightless in space become hypotensive and presyncopal when they assume an upright position. This phenomenon, known as orthostatic intolerance, may interfere with astronaut function during reentry and after spaceflight and may limit the ability of an astronaut to exit a landed spacecraft unaided during an emergency. Orthostatic intolerance is more pronounced after long-term spaceflight and is a major concern with respect to the extended flights expected aboard the International Space Station and for interplanetary exploration class missions, such as a human mission to Mars. Fully effective countermeasures to this problem have not yet been developed. To test the hypothesis that ␣-adrenergic stimulation might provide an effective countermeasure, we conducted a 16-day head-down-tilt bedrest study (an analog of weightlessness) using normal human volunteers and administered the ␣1-agonist drug midodrine at the end of the bed-rest period. Midodrine was found to significantly ameliorate excessive decreases in blood pressure and presyncope during a provocative tilt test. We conclude that midodrine may be an effective countermeasure for the prevention of orthostatic intolerance following spaceflight. cardiovascular system; vasovagal syncope

Journal of investigative medicine : the official publication of the American Federation for Clinical Research, 2005

Microgravity and simulated microgravity (SM) lead to important changes in orthostatic tolerance (... more Microgravity and simulated microgravity (SM) lead to important changes in orthostatic tolerance (OT), the autonomic nervous system (ANS), and the volume-regulating systems. After one is exposed to microgravity or SM, a period of readaptation to gravity is known to take place, but it is not certain if orthostatic function returns to baseline within the initial recovery and what mechanisms are involved. We hypothesized that after a period of recovery, OT, ANS, and volume-regulating systems would return to pre-SM levels. To test this hypothesis, 24 healthy men were placed on a constant diet for 3 to 5 days, after which a tilt-stand test (pre-TST) was performed. The TST was repeated after 14 to 16 days of head-down tilt bed rest (HDTB) (post-TST) and a 3-day period of recovery (rec-TST), at which times measurements of renal, cardioendocrine, and cardiovascular systems were conducted. Presyncope occurred in 46% of subjects pre-TST, in 72% post-TST, and in 23% during rec-TST. OT was signi...

Encyclopedia of Space Science and Technology, 2003

Neuropeptides, 1993

The distributions and extent of processing of four prodynorphin-derived peptides (dynorphin A (l-... more The distributions and extent of processing of four prodynorphin-derived peptides (dynorphin A (l-171, dynorphin A (l-81, dynorphin B, and a-neoendorphin) were determined in ten regions of the cortex as well as in the striatum of the guinea-pig. There were significant differences between concentrations of these peptides in most cortical regions, with aneoendorphin being several times more abundant than the other peptides, and dynorphin A (I-17) being present in the least amount. There were significant between-region differences in concentration for each peptide, although most regions had concentrations similar to those seen in the striatum. Concentrations of each peptide tended to be higher in piriform, entorhinal, motor, and auditory cortex than in other cortical regions. The extent of processing of prodynorphin varied across cortical regions as well, primarily due to the extent of processing to a-neoendorphin. Prodynorphin mRNA levels were not significantly different between cortical regions or from the amount observed in the striatum. Although specific regional variation exists, it appears that in general prodynorphin is expressed and processed in a similar manner in the cortex as in the striatum.

Journal Of Investigative Medicine, 2004

Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a var... more Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems--the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally. Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB). Sodium excretion increased initially (pre-HDTB = 182.8 +/- 10.4 mEq/total volume; early HDTB = 236.4 +/- 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 +/- 2.4/total volume; mid- to late HDTB = 89.4 +/- 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 +/- 0.21 ng/mL/h; end HDTB = 1.69 +/- 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed. We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Journal of Applied Physiology, 2004

Orthostatic intolerance (OI) is a major problem after spaceflight. Its etiology remains uncertain... more Orthostatic intolerance (OI) is a major problem after spaceflight. Its etiology remains uncertain, but reports have pointed toward an individual susceptibility to OI. We hypothesized that individual predisposition plays an important role in post-bed rest OI. Twenty-four healthy male subjects were equilibrated on a constant diet, after which they underwent tilt-stand test (pre-TST). They then completed 14-16 days of head-down-tilt bed rest, and 14 of the subjects underwent repeat tilt-stand test (post-TST). During various phases, the following were performed: 24-h urine collections and hormonal measurements, plethysmography, and cardiovascular system identification (a noninvasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness). Development of presyncope or syncope defined OI. During pre-TST, 11 subjects were intolerant and 13 were tolerant. At baseline, intolerant subjects had lower serum aldosterone ( P…

Journal of Applied Physiology, 2004

Microgravity-induced orthostatic intolerance (OI) continues to be a primary concern for the human... more Microgravity-induced orthostatic intolerance (OI) continues to be a primary concern for the human space program. To test the hypothesis that exposure to simulated microgravity significantly alters autonomic nervous control and, thus, contributes to increased incidence of OI, we employed the cardiovascular system identification (CSI) technique to evaluate quantitatively parasympathetic and sympathetic regulation of heart rate (HR). The CSI method analyzes second-to-second fluctuations in noninvasively measured HR, arterial blood pressure, and instantaneous lung volume. The coupling mechanisms between these signals are characterized by using a closed-loop model. Parameters reflecting parasympathetic and sympathetic responsiveness with regard to HR regulation can be extracted from the identified coupling mechanisms. We analyzed data collected from 29 human subjects before and after 16 days of head-down-tilt bed rest (simulated microgravity). Statistical analyses showed that parasympath...

Journal of Applied Physiology, 2004

Orthostatic intolerance (OI) is a major problem following spaceflight, and, during flight, astron... more Orthostatic intolerance (OI) is a major problem following spaceflight, and, during flight, astronauts also experience sleep restriction. We hypothesized that sleep restriction will compound the risk and severity of OI following simulated microgravity and exaggerate the renal, cardioendocrine, and cardiovascular adaptive responses to it. Nineteen healthy men were equilibrated on a constant diet, after which they underwent a tilt-stand test. They then completed 14–16 days of simulated microgravity [head-down tilt bed rest (HDTB)], followed by repeat tilt-stand test. During HDTB, 11 subjects were assigned to an 8-h sleep protocol (non-sleep restricted), and 8 were assigned to a sleep-restricted protocol with 6 h of sleep per night. During various phases, the following were performed: 24-h urine collections, hormonal measurements, and cardiovascular system identification. Development of presyncope or syncope defined OI. There was a significant decrease in time free of OI ( P = 0.02) and...

Brain Research Bulletin, 2000

Although the orthostatic cardio-respiratory response is primarily mediated by the baroreflex, stu... more Although the orthostatic cardio-respiratory response is primarily mediated by the baroreflex, studies have shown that vestibular cues also contribute in both humans and animals. We have demonstrated a visually mediated response to illusory tilt in some human subjects. Blood pressure, heart and respiration rate, and lung volume were monitored in 16 supine human subjects during two types of visual stimulation, and compared with responses to real passive whole body tilt from supine to head 80°upright. Visual tilt stimuli consisted of either a static scene from an overhead mirror or constant velocity scene motion along different body axes generated by an ultra-wide dome projection system. Visual vertical cues were initially aligned with the longitudinal body axis. Subjective tilt and self-motion were reported verbally. Although significant changes in cardio-respiratory parameters to illusory tilts could not be demonstrated for the entire group, several subjects showed significant transient decreases in mean blood pressure resembling their initial response to passive head-up tilt. Changes in pulse pressure and a slight elevation in heart rate were noted. These transient responses are consistent with the hypothesis that visual-vestibular input contributes to the initial cardiovascular adjustment to a change in posture in humans. On average the static scene elicited perceived tilt without rotation. Dome scene pitch and yaw elicited perceived tilt and rotation, and dome roll motion elicited perceived rotation without tilt. A significant correlation between the magnitude of physiological and subjective reports could not be demonstrated.

Annals of Noninvasive Electrocardiology, 2005

Background: There are numerous anecdotal reports of ventricular arrhythmias during spaceflight; h... more Background: There are numerous anecdotal reports of ventricular arrhythmias during spaceflight; however, it is not known whether spaceflight or microgravity systematically increases the risk of cardiac dysrhythmias. Microvolt T wave alternans (MTWA) testing compares favorably with other noninvasive risk stratifiers and invasive electrophysiological testing in patients as a predictor of sudden cardiac death, ventricular tachycardia, and ventricular fibrillation. We hypothesized that simulated microgravity leads to an increase in MTWA. Methods: Twenty-four healthy male subjects underwent 9 to 16 days of head-down tilt bed rest (HDTB). MTWA was measured before and after the bed rest period during bicycle exercise stress. For the purposes of this study, we defined MTWA outcome to be positive if sustained MTWA was present with an onset heart rate ≤125 bpm. During various phases of HDTB, the following were also performed: daily 24-hour urine collections, serum electrolytes and catecholamines, and cardiovascular system identification (measure of autonomic function). Results: Before HDTB, 17% of the subjects were MTWA positive [95%CI: (0.6%, 37%)]; after HDTB, 42% of the subjects were MTWA positive [95%CI: (23%, 63%)] (P = 0.03). The subjects who were MTWA positive after HDTB compared with MTWA negative subjects had an increased versus decreased sympathetic responsiveness (P = 0.03) and serum norepinephrine levels (P = 0.05), and a trend toward higher potassium excretion (P = 0.06) after bed rest compared to baseline. Conclusions: HDTB leads to an increase in MTWA, providing the first evidence that simulated microgravity has a measurable effect on electrical repolarization processes. Possible contributing factors include loss in potassium and changes in sympathetic function.

Journal of Clinical Anesthesia, Jan 12, 2012

Study Objective: To evaluate three evening insulin glargine dosing strategies for achievement of ... more Study Objective: To evaluate three evening insulin glargine dosing strategies for achievement of target (100-179 mg/dL; 5.5-9.8 mmol/L) and widened (80-249 mg/dL; 4.4-13.7 mmol/L) preoperative fasting blood glucose (FBG) ranges on the day of surgery. Design: Prospective, randomized, open trial. Setting: Preoperative units at two sites of a suburban hospital system. Patients: 401 adult, ASA physical status 3 and 4 patients with type 1 and type 2 diabetes, undergoing elective noncardiac surgery. Interventions: Patients were divided into two groups according to absence of daily rapid-acting/shortacting insulin (insulin glargine-only group) or presence of daily rapid-acting/short-acting insulin (insulin glargine plus bolus group). Subjects were then randomized to three evening insulin glargine dosing strategies: (a) take 80% of usual dose, (b) call physician for dose, or (c) refer to dosing table, based on ☆ Study funding was provided from Sanofi-aventis US,

AIP Conference Proceedings

Journal of applied physiology (Bethesda, Md. : 1985), 2001

Many astronauts after being weightless in space become hypotensive and presyncopal when they assu... more Many astronauts after being weightless in space become hypotensive and presyncopal when they assume an upright position. This phenomenon, known as orthostatic intolerance, may interfere with astronaut function during reentry and after spaceflight and may limit the ability of an astronaut to exit a landed spacecraft unaided during an emergency. Orthostatic intolerance is more pronounced after long-term spaceflight and is a major concern with respect to the extended flights expected aboard the International Space Station and for interplanetary exploration class missions, such as a human mission to Mars. Fully effective countermeasures to this problem have not yet been developed. To test the hypothesis that ␣-adrenergic stimulation might provide an effective countermeasure, we conducted a 16-day head-down-tilt bedrest study (an analog of weightlessness) using normal human volunteers and administered the ␣1-agonist drug midodrine at the end of the bed-rest period. Midodrine was found to significantly ameliorate excessive decreases in blood pressure and presyncope during a provocative tilt test. We conclude that midodrine may be an effective countermeasure for the prevention of orthostatic intolerance following spaceflight. cardiovascular system; vasovagal syncope

Journal of investigative medicine : the official publication of the American Federation for Clinical Research, 2005

Encyclopedia of Space Science and Technology, 2003

Neuropeptides, 1993

Journal Of Investigative Medicine, 2004

Journal of Applied Physiology, 2004

Brain Research Bulletin, 2000

Annals of Noninvasive Electrocardiology, 2005

Journal of Clinical Anesthesia, Jan 12, 2012