Heart rate fluctuates constantly. These fluctuations are called heart rate variability and they are modulated by the autonomic nervous system. In a similar way blood pressure also varies on a beat-to-beat basis and its fluctuations are defined as blood pressure variability. It has become clear over the years that analysis of these fluctuations provides a window on the action of the autonomic nervous system of the cardiovascular system. Continuous feedback is established through pressure sensors in the baroreflex system. The objectives of this study were:
The hypothesis underlying this study is that cardiovascular control and adaptation mechanisms undergo significant but reversible changes due to long-term space flights. These changes will also influence post-flight cardiovascular function. These hypotheses will be confirmed or rejected by comparing ECG, blood pressure variability and flow data pre-, in flight and postflight. These mechanisms are also related to the ageing process to pathologies on earth (i.e. syncope) and to fluid shift resulting from heart disease.
The objectives of this study were - after obtaining continuous, short recordings (+ or - 15 minutes) of ECG and blood pressure (non-invasive) during normal gravity (control baseline measurements before and after the space flight in 1g) in supine, sitting and standing positions:
1. To obtain short time recordings in space during a Soyuz mission starting from the 3rd day at the station (awake) using Cardioscience equipment (ECG and blood pressure).
2. Compute heart rate variability (HRV) and blood pressure variability (BPV) parameters from the ECG and blood pressure signals on an almost beat-to-beat resolution with time- and frequency domain methods and non-linear analysis.
3. From HRV and BPV deduct information about cardiovascular function and adaptation and also baroreflex response during long-term space flight.
4. Compare the evolution of autonomic function during pre-, in flight and postflight periods, to monitor a possible restoration of autonomic function.
ECG, BP (Portapres) and respiration: The Portapres allows recording of continuous blood pressure. Also ECG (with skin electrodes) and non-invasive respiration were recorded. In normal gravity 20 minutes in supine position, 10 minutes sitting, and 20 minutes in standing position were recorded.
24h Holter: Measurement of 1-day ECG recording to obtain beat-to-beat heart rate. The subject wore a portable unit of 300 gr connected with electrodes on the chest-wall. During the day the subject was able to perform normal daily activities. The 24h Holter monitor was provided by the experimenters.
Tilt test: The tilt test is a protocol to test orthostatic intolerance. Before tilt test the subject laid down on a tilt table for 10 minutes. During this period the ECG recorder, respiratory sensor and Portapres were attached. Ten minutes of resting data was recorded. The subject was then passively tilted up to 60 degrees and kept in this position without moving for 45 minutes. During tilt, ECG, respiration, and blood pressure was continuous recorded. The tilt test was aborted in case of fainting of the subject .These measurements wiere performed with the assistance of the investigators.
RHYTHM: 20 minutes recordings of ECG, BP and respiration starting from the third day on the ISS. The subject was allowed to breath at his own pace, but movements and talking were avoided.
Same measurements as pre-flight were performed: ECG, respiration and BP (20 minutes supine, 10 minutes of sitting and 20 minutes of standing).
24h Holter monitoring.
Recording the electrocardiogram, respiratory frequency, and blood pressure during space flight allowed investigators to monitor the progress of cardiovascular deconditioning. Measurements prior to space flight served as reference. Post-flight measurements gave insight in the amount and timing of the recovery.
This investigation found post-flight differences in heart rate when standing or lying down, even in orthostatic tolerant astronauts, or those who don’t see an increase in heart rate upon standing. Differences disappeared immediately after space flight. In fact, 25 days after flight, the stand response was again similar to pre-flight, indicating that the astronauts had recovered rather quickly post-flight. Previously, no methods had been successful in either predicting or detecting the sudden onset of orthostatic intolerance. Given the changes seen in orthostatic-tolerant individuals, it may be possible to study these control mechanisms in orthostatic-intolerant astronauts.