Non-linear heart rate control in orthostatic tolerant cosmonauts after short-duration spaceflight

Spaceflight causes changes in the cardiovascular system. These changes contribute to the occurrence of orthostatic intolerance after spaceflight. The study of heart rate variations (HRV) provides a non-invasive means to study the autonomic modulation of heart rate. Non-linear control mechanisms have been suggested to be involved in normal heart rate variations. Non-linear control would be helpful in maintaining healthy cardiovascular values. However these methods have not yet been used in combination with data after spaceflight. In this study we examined the non-linear response of heart rate to standing before and after spaceflight. Methods: ECG was measured for at least 10 minutes in supine and standing position 30 days before launch; and at 1, 4, 9, and 25 days after return to earth. These measurements were performed in 5 cosmonauts who were in the ISS for 10 days. The Approximate entropy of HRV was calculated. Results: The non-linear stand response was slightly altered immediately after spaceflight but was restored after 25 days. Conclusion: Nonlinear control of heart rate was affected immediately after short duration spaceflight, but not to the extent that the astronauts experienced orthostatic intolerance.