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Simanonok, K. E.

Publications and source records attributed to Simanonok, K. E..

8 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Computer simulation of the effect of dDAVP with saline loading on fluid balance after 24-hour head-down tilt

Fluid loading (FL) before Shuttle reentry is a countermeasure currently in use by NASA to improve the orthostatic tolerance of astronauts during reentry and postflight. The fluid load consists of water and salt tablets equivalent to 32 oz (946 ml) of isotonic saline. However, the effectiveness of this countermeasure has been observed to decrease with the duration of spaceflight. The countermeasure's effectiveness may be improved by enhancing fluid retention using analogs of vasopressin such as lypressin (LVP) and desmopressin (dDAVP). In a computer simulation study reported previously, we attempted to assess the improvement in fluid retention obtained by the use of LVP administered before FL. The present study is concerned with the use of dDAVP. In a recent 24-hour, 6 degree head-down tilt (HDT) study involving seven men, dDAVP was found to improve orthostatic tolerance as assessed by both lower body negative pressure (LBNP) and stand tests. The treatment restored Luft's cumulative stress index (cumulative product of magnitude and duration of LBNP) to nearly pre-bedrest level. The heart rate was lower and stroke volume was marginally higher at the same LBNP levels with administration of dDAVP compared to placebo. Lower heart rates were also observed with dDAVP during stand test, despite the lower level of cardiovascular stress. These improvements were seen with only a small but significant increase in plasma volume of approximately 3 percent. This paper presents a computer simulation analysis of some of the results of this HDT study.

NASA Discipline Cardiopulmonary↗
NASA NTRS1994

The effect of dDAVP with saline loading on fluid balance during LBNP and standing after 24-hr head-down bedrest

Shuttle astronauts currently drink approximately a quart of water with eight salt tablets before reentry to restore lost body fluid and thereby reduce the likelihood of cardiovascular instability and syncope during reentry and after landing. However, the saline loading countermeasure is not entirely effective in restoring orthostatic tolerance to preflight levels. We tested the hypothesis that the effectiveness of this countermeasure could be improved with the use of a vasopressin analog, 1-deamino-8-D-arginine vasopressin (dDAVP). The rationale for this approach is that reducing urine formation with exogenous vasopressin should increase the magnitude and duration of the vascular volume expansion produced by the saline load, and in so doing improve orthostatic tolerance during reentry and postflight.

NASA Discipline Number 14-10↗
NASA NTRS1994

A comprehensive Guyton model analysis of physiologic responses to preadapting the blood volume as a countermeasure to fluid shifts

The Guyton model of fluid, electrolyte, and circulatory regulation is an extensive mathematical model capable of simulating a variety of experimental conditions. It has been modified for use at NASA to simulate head-down tilt, a frequently used analog of weightlessness. Weightlessness causes a headward shift of body fluids that is believed to expand central blood volume, triggering a series of physiologic responses resulting in large losses of body fluids. We used the modified Guyton model to test the hypothesis that preadaptation of the blood volume before weightless exposure could counteract the central volume expansion caused by fluid shifts, and thereby attenuate the circulatory and renal responses that result in body fluid losses. Simulation results show that circulatory preadaptation, by a procedure resembling blood donation immediately before head-down bedrest, is effective in damping the physiologic responses to fluid shifts and reducing body fluid losses. After 10 hours of head-down tilt, preadaptation also produces higher blood volume, extracellular volume, and total body water for 20 to 30 days of bedrest, compared with non-preadapted control. These results indicate that circulatory preadaptation before current Space Shuttle missions may be beneficial for the maintenance of reentry and postflight orthostatic tolerance in astronauts. This paper presents a comprehensive examination of the simulation results pertaining to changes in relevant physiologic variables produced by blood volume reduction before a prolonged head-down tilt. The objectives were to study and develop the countermeasure theoretically, to aid in planning experimental studies of the countermeasure, and to identify potentially disadvantageous physiologic responses that may be caused by the countermeasure.

NASA Center JSC↗
NASA NTRS1993

Physiologic mechanisms of circulatory and body fluid losses in weightlessness identified by mathematical modeling

Central volume expansion due to fluid shifts in weightlessness is believed to activate adaptive reflexes which ultimately result in a reduction of the total circulating blood volume. However, the flight data suggests that a central volume overdistention does not persist, in which case some other factor or factors must be responsible for body fluid losses. We used a computer simulation to test the hypothesis that factors other than central volume overdistention are involved in the loss of blood volume and other body fluid volumes observed in weightlessness and in weightless simulations. Additionally, the simulation was used to identify these factors. The results predict that atrial volumes and pressures return to their prebedrest baseline values within the first day of exposure to head down tilt (HDT) as the blood volume is reduced by an elevated urine formation. They indicate that the mechanisms for large and prolonged body fluid losses in weightlessness is red cell hemoconcentration that elevates blood viscosity and peripheral resistance, thereby lowering capillary pressure. This causes a prolonged alteration of the balance of Starling forces, depressing the extracellular fluid volume until the hematocrit is returned to normal through a reduction of the red cell mass, which also allows some restoration of the plasma volume. We conclude that the red cell mass becomes the physiologic driver for a large 'undershoot' of the body fluid volumes after the normalization of atrial volumes and pressures.

Simanonok, K. E.↗
NASA NTRS1992

Space sickness predictors suggest fluid shift involvement and possible countermeasures

Preflight data from 64 first time Shuttle crew members were examined retrospectively to predict space sickness severity (NONE, MILD, MODERATE, or SEVERE) by discriminant analysis. From 9 input variables relating to fluid, electrolyte, and cardiovascular status, 8 variables were chosen by discriminant analysis that correctly predicted space sickness severity with 59 pct. success by one method of cross validation on the original sample and 67 pct. by another method. The 8 variables in order of their importance for predicting space sickness severity are sitting systolic blood pressure, serum uric acid, calculated blood volume, serum phosphate, urine osmolality, environmental temperature at the launch site, red cell count, and serum chloride. These results suggest the presence of predisposing physiologic factors to space sickness that implicate a fluid shift etiology. Addition of a 10th input variable, hours spent in the Weightless Environment Training Facility (WETF), improved the prediction of space sickness severity to 66 pct. success by the first method of cross validation on the original sample and to 71 pct. by the second method. The data suggest that WETF training may reduce space sickness severity.

Simanonok, K. E.↗
NASA NTRS1992

Computer simulation of preflight blood volume reduction as a countermeasure to fluid shifts in space flight

Fluid shifts in weightlessness may cause a central volume expansion, activating reflexes to reduce the blood volume. Computer simulation was used to test the hypothesis that preadaptation of the blood volume prior to exposure to weightlessness could counteract the central volume expansion due to fluid shifts and thereby attenuate the circulatory and renal responses resulting in large losses of fluid from body water compartments. The Guyton Model of Fluid, Electrolyte, and Circulatory Regulation was modified to simulate the six degree head down tilt that is frequently use as an experimental analog of weightlessness in bedrest studies. Simulation results show that preadaptation of the blood volume by a procedure resembling a blood donation immediately before head down bedrest is beneficial in damping the physiologic responses to fluid shifts and reducing body fluid losses. After ten hours of head down tilt, blood volume after preadaptation is higher than control for 20 to 30 days of bedrest. Preadaptation also produces potentially beneficial higher extracellular volume and total body water for 20 to 30 days of bedrest.

Simanonok, K. E.↗
NASA NTRS1992

Computer simulation analysis of the effects of countermeasures for reentry orthostatic intolerance

Fluid loading is a countermeasure currently in routine use to improve the g-tolerance of crewmembers during reentry and return of Shuttle flights. However, its effectiveness diminishes with mission duration. Countermeasures that will be effective on long-duration flights are needed and are presently under development. This paper discusses the application of computer simulation in the analysis of the effects of countermeasures for reentry orthostatic intolerance. The results suggest improvements upon the fluid loading countermeasure currently in use.

NASA Center JSC↗