Previous studies have shown that after 1-2 weeks of bed rest, ingestion of a 1-liter salt water solution during 4 hours of continuous exposure to 30 mmHg lower body negative pressure (LBNP) will protect orthostatic function and blood volume for up to 24 hours. It was hypothesized that a similar countermeasure would reduce the effects of fluid loss induced by headward fluid shift during space flight. The inflight countermeasure, LBNP combined with salt and fluid ingestion, is intended as a single application, end-of-mission treatment to pre-adapt the cardiovascular system to orthostatic stress exposure during and after reentry, landing and cabin egress. The objective of this experiment was to evaluate the efficacy of the proposed countermeasure in reversing the effects of microgravity in the cardiovascular system. This investigation also documented additional cardiovascular responses to graded simulated orthostasis in space flight, including leg volume and total peripheral resistance changes.
Flight simulation baseline data collection sessions, at 6 and 3 months preflight, consisted of two 3-day repetitive sessions in which inflight LBNP procedures were followed. During these sessions, subjects performed the complete inflight experiment. No blood or urine collection occurred during these sessions.
The preflight supplemental baseline data collection occurs at L-30, L-10, and L-5. This required all participating subjects to undergo a single response test on each specified day. For each test, subjects provided a 24-hour urine specimen for analysis of electrolytes (Na, K, Cl) and hormones (ADH, aldosterone, epinephrine, norepinephrine) important in fluid regulation. Additionally, a blood sample was drawn, pre- and post- ramp for blood volume (hematocrit, hemoglobin), electrolyte, and hormone level determinations.
Inflight, LBNP permitted both testing and countermeasure protocols to be employed. A collapsible LBNP device (LBNPD) was used for all inflight decompressions. The American Flight Echocardiograph (AFE) was used to obtain and electrocardiogram (ECG) and generate a 2-dimensional image of the heart using ultrasound. The Automatic Blood Pressure System (ABPS) measured heart rate and blood pressure and provided output consisting of ECG, Korotoff sounds, cuff pressure, and the vacuum signal from the LBNP controller. The Central Venous Pressure Device (CVPD), an ultrasound probe which is pressed lightly over the jugular vein, was used to document blood flow and estimate central venous pressure. Calf measurements were obtained during LBNP using the Ultrasonic Limb Plethysmograph in which electrodes are attached to the crewmember's leg. The Stocking (Leg) Plethysmograph was used to determine leg circumference that was later calculated into leg volume.
LBNP response tests or "ramps", were conducted three times inflight over four flight days (4,5,6, and 7) for each subject. The control subjects performed a single ramp (flight day 4). During this test, 6 minutes of baseline data were collected, after which the pressure was decreased in 10 mmHg steps every 3 minutes until a pressure of -50 mmHg was attained. After 3 minutes at -50 mmHg, the vacuum was released and 3 minutes of recovery data were collected. ECG, cardiac dimensions, and calf dimensions were monitored continuously, and BP was taken every minute of the ramp. Leg volume was measured immediately before and after each ramp.
The LBNP countermeasure protocol, called "soak", was conducted once inflight ( on either flight day 4 or 5) for each participating crewmember. After 6 minutes of baseline data collection, the pressure was decreased in 10 mmHg steps every 3 minutes until -50 mmHg was attained. The vacuum was then reduced to -30 mmHg for the soak period and maintained for 225 minutes. During the first hour, the subjects ingested salt tablets (8 grams total) and four beverage containers of water (128 ounces total). ECG and BP were continuously monitored for 10 minutes at scheduled times every hour. In addition, one BP measurement was taken every 15 minutes throughout the soak. After the 4-hour treatment, the pressure was reduced in 10 mmHg steps every 3 minutes until -50 mmHg was attained. After 3 minutes at this pressure, the vacuum was released and 6 minutes of recovery data was collected. ECG, cardiac dimensions and calf dimensions were monitored continuously; BP measurements were made every minute.
Postflight data collection occurred at R+0 (landing day), R+1, R+3, and R+5 days. Each collection session included one ramp test on both inflight countermeasure subjects and the two control subjects. Urine and blood sample collection on participating subjects followed the same protocol as the preflight supplemental data collection.
The results present a summary of 12 subjects from six shuttle missions (STS-32, STS-43, STS-44, STS-42, STS-50, and STS-47). Valuable data on the time course of the loss of orthostatic tolerance were collected on almost all crewmembers. However, data applicable to the evaluation of the countermeasure were obtained on only 4 (including both Spacelab -J participants) on the 12 subjects.
HR increased significantly between preflight and early inflight ramp tests, but remained unchanged between the early inflight and pretreatment tests. One day after treatment, HR was significantly decreased compared to the pretreatment test and not significantly different from the preflight value. Two days after treatment, HR was returning toward the pretreatment value, but significantly different from the preflight value.
In conclusion, LBNP treatment restored heart rate responses to simulated orthostatic stress one day after treatment toward preflight values. Increased leg volume after treatment was no greater than after the brief LBNP tests, indication that fluid retention in the legs was probably not a major contributor to the treatment's effectiveness. Partial cardiovascular protection was still present two days after the treatment. Incomplete treatments (LBNP without fluid loading, or fluid loading without four hours of LBNP) are not effective (confirming bed rest findings).
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