Role of chemoreception in cardiorespiratory acclimatization to, and deacclimatization from, hypoxia

During sojourn to high altitudes, progressive time-dependent increases occur in ventilation and in sympathetic nerve activity over several days, and these increases persist upon acute restoration of normoxia. We discuss evidence concerning potential mediators of these changes, including the following: 1) correction of alkalinity in cerebrospinal fluid; 2) increased sensitivity of carotid chemoreceptors; and 3) augmented translation of carotid chemoreceptor input (at the level of the central nervous system) into increased respiratory motor output via sensitization of hypoxic sensitive neurons in the central nervous system and/or an interdependence of central chemoreceptor responsiveness on peripheral chemoreceptor sensory input. The pros and cons of chemoreceptor sensitization and cardiorespiratory acclimatization to hypoxia and intermittent hypoxemia are also discussed in terms of their influences on arterial oxygenation, the work of breathing, sympathoexcitation, systemic blood pressure, and exercise performance. We propose that these adaptive processes may have negative implications for the cardiovascular health of patients with sleep apnea and perhaps even for athletes undergoing regimens of "sleep high-train low"!