Ascent to altitude: an integrated cerebrovascular, ventilatory and acid-base response

Abstract

Intricate relationships exist between arterial blood gases, acid–base balance, cerebral blood flow and ventilatory variables. Central to this phenomenon is the basic carbonic anhydrase equation: CO2+ H2O →[H2CO3]→[H+]+[HCO3−]. As arterial CO2 increases in the brain and cerebrospinal fluid, it forms carbonic acid and dissociates to hydrogen ions [H+]. Respiratory chemoreflex and cerebral blood flow control systems act to protect cerebral and systemic [H+] as part of a feedback system, where CO2/[H+] stimulate (1) the respiratory chemoreceptors to increase ventilation, and (2) cerebral arteriolar vasodilatation. In this context, alterations in acid–base in any system will affect the relationship between and [H+] (i.e. a greater increase in arterial [H+] for a given increase in ), and thereby affect respiratory control. Furthermore, elevated cerebral blood flow allows a ‘washout’ of CO2 and thus [H+] ions, to reduce the ventilatory stimulus and stabilize breathing. Therefore, cerebrovascular responses are in place to adjust [H+], thereby affecting respiratory chemoreflex control.