Vestibular and cardiac reactions to open-sea exposure

Nineteen healthy male volunteers were exposed to a 72-hour open-sea stay in a lifeboat in order to study whether randomized protracted stimulation of different parts of the vestibular system would cause a decrease of vestibular activity as assessed by unilateral calorization of the horizontal vestibular canal. The influence on the autonomic nervous system was investigated by calculating the heart rate by the R-R intervals at ECG at rest and at maximum speed of the nystagmus slow phase. Calorization and ECG investigations were performed immediately before and after the open-sea exposure. Seven healthy males who were not exposed to the open sea served as controls. They were investigated in the same manner as the test subjects, that is twice, 3 days apart. All test subjects experienced seasickness. After the sea stay, the caloric nystagmus response had decreased significantly more than in the controls. In 7 subjects with the most pronounced vestibular habituation, the resting heart rate decreased significantly after the sea exposure. Caloric stimulation of the labyrinths did not cause any specific change of the heart rate. It is concluded that prolonged open-sea exposure causes vestibular habituation to unilateral labyrinthine stimulation. Furthermore, there are some indications that the mechanism that is responsible for vestibular habituation to some extent influences the function of the autonomic nervous system, resulting in a decreased pulse rate.