Tilt Test is one of eleven International Bedrest Standard Measures conducted on healthy subjects at the :envihab facility in Cologne, Germany. The Tilt Test is used to assess orthostatic tolerance before and after bed rest. The purpose of this project is to populate a repository with data that enables high-level monitoring of countermeasure effectiveness and meaningful interpretation of health and performance outcomes. These standard measures will support future, hypothesis-driven research that enables planetary missions by providing an optimized minimal set of measures captured from subjects. These measures will be available to other studies under data sharing agreements as well as undergo analysis to understand risk posture.
APPROACH:
Subjects will be instrumented while in supine position and this position is maintained while baseline data is collected for five minutes. The table will then be titled to 80 degrees, placing subjects in a head-up tilt position at a rate of approximately 7 degrees per second. Subjects will remain in this position for 15 minutes or until they exhibit symptoms of presyncope. Total time at 80 degrees head-up tilt will be recorded.
During the test, blood pressure, electrocardiography (ECG) and Doppler ultrasound of blood flow velocity at the suprasternal notch will be continuously measured. Finger arterial blood pressure will be sampled at 200Hz using a photoplethysmography device. This device uses a hydrostatic adjustment routine to provide an accurate estimation of blood pressure independent of sensor location with respect to the heart. Oscillometric brachial artery pressure will be measured every minute by using a cuff placed around the upper arm. Systolic and diastolic blood pressure will be recorded and the mean arterial pressure (MAP) will be calculated. ECG data will be collected at 100 Hz using a 5-lead system. Heart rate measures are derived from the ECG and ECG collection is synchronized with the suprasternal notch Doppler ultrasound signals for stroke volume and cardiac output calculations. A two-dimensional echocardiogram will be used to obtain the aortic annulus diameter from the parasternal long axis during supine rest prior to data collection. The aortic blood velocity time integral will be measured for each beat during supine rest and during the period of 80 degrees head-up tilt. These Doppler measurements will be made at the suprasternal notch using a 1.9MHz pulsed wave Doppler probe. Images will be stored digitally for subsequent analyses. To ensure accuracy, images from at least three cardiac cycles during inspiration will be independently analyzed by two experienced sonographers. Stroke volume and cardiac output will be calculated.
The scheduled days of collection are as follows:
- Five days prior to mission
- The day mission ends
RESULTS:
The :envihab International Bedrest Standard Measures Test Battery is not a hypothesis-driven study, but rather a collection of data made available to future researchers and for trend analysis. As such, there are no specific hypotheses, outcome measures, or statistical analyses associated with the skeleton of the project. The questionnaire will capture a "snapshot" of optimized data that can be investigated by researchers in the future.
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