Wo l f g a n g D o m e j , M i c h a e l Tr a p p , E v a - M a r i a M i g g i t s c h Hemma Tilz, Christoph Guger und GĂźnther Schwaberger
Autonomes Nervensystem, arterieller Blutdruck und HĂśhenexposition Autonomic nervous system, arterial blood pressure, and altitude exposure S U M M A RY The autonomic nervous system is a principal player in mediating many of the physiological adjustments to hypoxia. The first days at high altitude provoke an increase in both parasympathetic and sympathetic drive which is followed by a rapid decrease in parasympathetic and further increase in sympathetic activity; the latter attenuates with progressing acclimatization. Acute hypobaric hypoxia per se causes direct vasodilatation of peripheral vasculature, that is, however, offset by altitude-induced sympathetic vasoconstriction. The increase in sympathetic tone increases systemic vascular resistance, arterial blood pressure and heart rate, partly due to chemoreceptor reflexes and changes of the baroreceptor function. Increased heart rate in acute hypoxia is due to combined cardiac sympathetic excitation and decrease of parasympathetic/vagal tone; both mechanisms contribute almost to the same extent to the chronotropic response. Interindividual variations in heart rate and arterial blood pressure at altitude depend on systemic as well as cardiac sympathoadrenal activity, which, at sea level, are generally lower in subjects with high aerobic endurance capacity than in untrained persons. Generally, exposure to moderate altitude causes small increases in blood pressure and heart rate in healthy subjects. However, systemic vasoconstriction may also occur as a reflex response to a major increase in pulmonary pressure at altitude (HAPH). After several days at altitude and return to normoxic conditions the sympathetic drive may be maintained for some days; afterwards, heart rate and cardiac output return to a magnitude comparable with sea level values. Keywords: Autonomic nervous system, sympathetic and parasympathetic activity, arterial blood pressure, arterial hypertension, high altitudes
97
