Heart Rate and Cardiovascular Variability at High Altitude

Primary effect of hypobaric hypoxia on the circulation is a direct vasodilatory effect on the peripheral circulation, which is normally prevented by a sympathetic- induced vasoconstriction. Most of the clinical methods for testing the baroreflex sensitivity only evaluate the cardiac-vagal branch of the baroreflex, but at altitude it is also of importance to test the vascular effects of the baroreflex. This is possible by directly measuring sympathetic efferent activity (by microneurography) or by directly stimulating the carotid baroreceptors (by the neck suction). By cyclical stimulation of the carotid baroreceptors, neck suction-synchronous reflex oscillations could be observed in a large number of signals, including RR interval, blood pressure, microcirculation, muscle sympathetic nerve activity. An increase in fluctuations at the same frequency of the stimulus was considered an evidence of the ability of the carotid baroreceptors to modulate a given physiological signal. The sinusoidal neck suction was set at 0.10 Hz (low-frequency stimulation), or to a frequency close to- but distinct from- the respiratory signal (0.20 Hz, high frequency stimulation, whereas respiration was fixed to 0.25 Hz). The method is noninvasive, without side effects connected to use of drugs, and evaluates both the response to the heart and to the blood pressure of the baroreflex. The altitude-induced sympathetic activation was evidenced in sea level natives by a decrease in RR interval, an increase in blood pressure and in the 0.1 Hz components of cardiac and vascular signals. The arterial baroreflex was active on RR interval and also in blood pressure, even during acute exposure to high altitude, thus indicating that it was counteracting and modulating the increase in sympathetic tone. Signs of exaggerated sympathetic activation were evident in subjects with severe acute mountain sickness, while successful therapy was associated with a restoration of autonomic modulation. Conversely, sympathetic activation was reduced (and baroreflex enhanced) in himalayan high altitude natives. In conclusion, a comprehensive understanding of the mechanisms taking place during the adaptation to high altitude requires a multisignal approach, also integrated with equipment designed to provide specific provocative tests, such as those necessary to measure the cardiorespiratory interactions.

I. Introduction

Hypoxia(due to endogenous or atmospheric causes) primarily affects ventilatory control, but, due to strong reciprocal influences, also greatly influence autonomic cardiovascular regulation in normal as well as in pathologic conditions, such as chronic cardiac and respiratory failure. While in pathologic conditions the effects of chronic hypoxia superimpose and complicate other aspects of the underlying disease, exposure to high altitude of healthy subjects provides a unique model to study specific adaptations to acute and prolonged hypoxia independently from pre-existing pathology.