In Rotterdam in 1996 with invasive neurological monitoring systems, such as brain tissue oxygen monitoring (PbtiO2), jugular bulb oxygen catheters (SavO2j), intracranial pressure (ICP), exhaled CO2, and arterial blood gases in patients with severe cranial lesions, we designed a test to measure the CO2 reactivity. This test produced moderate hyperventilation, CO2 decreased from 35 ± 2 mm Hg to 31 ± 3 mm Hg. Since hyperventilation is a common therapeutic routine in severe head trauma, which decreases ICP, it is actually a controversial maneuver since that hyperventilation decreases CBF and oxygen delivery. CO2 reactivity is a physiological mechanism that allows for blood flow regulation in the brain according to metabolical requirements. For each mmHg of CO2 change, the blood flow is modified by 4ml. Hyperventilation results in vasoconstriction, while hypoventilation causes vasodilation. This auto regulation mechanism acts in the adaptation or failure to adapt to high altitude, and therefore can cause consciousness alterations when adaptation periods are too short. During hyperventilation at high altitude, PaCO2 resulted in decreased brain oxygenation. It was more accentuated in later days than in the earlier ones. Near-infrared spectroscopy (NIRS) a noninvasive technique, measures the relationship between oxygen consumption and delivery from a mix of arterial and venous oxygen. The light passes through the skin, the skull, the cerebral parenchyma and makes it 4 to 5 cm into the dura mater with values of about 65 to 70% and the extreme values is less than 20%. The Zubietas published tables to calculate changes in CO2 as environmental O2 decreases at increasing heights above sea level. These tables are quite useful; however, we do not have measurements determining the changes in brain blood flow inferred through studies of noninvasive multiparametric monitoring of oxygen, or other techniques such as bispectral index (BIS), let alone changes in blood flow following hyper- and hypoventilation at high altitude. We also lack data comparing healthy unadapted patients to native and adapted patients at different altitudes above sea level, nor do we have comparisons between adapted and unadapted patients at great heights exhibiting pathologies in which the relationship between supply and consumption of O2 is crucial. CO2 reactivity is the last mechanism lost in traumatic brain injury resulting in a dramatic outcome. Investigating this approach in noninvasive monitoring could be useful in HACE or HAPE cerebral edema and other pathologies presented by patients who either ascent to high altitude or descend from them abruptly, because CO2 reactivity could be used to prevent further damage.
Patients living at high altitudes have a CO2 autoregulation which is adapted to a certain level of O2 supply and consumption which therefore allows the patient to carry out multiple activities without manifesting clinical alterations. On the other hand, when unadapted individuals are abruptly exposed to high altitude, compensation between cerebral blood flow and CO2 levels is not regulated properly, which may result in abrupt changes in blood flow, which, depending on susceptibility, may in turn result in clinical manifestation that could be picked up by monitoring systems. Last year, in Cuzco (3200 m above sea level), we compared hyperventilation during 3 to 5 minutes between locals and unadapted foreigners. We measured exhaled CO2 levels up to 16 mm Hg without inducing clinical or NIRS manifestations in those who were adapted to the height. In comparison, not being adapted to such heights, I presented signs of drowsiness and voice tone changes. It is with this antecedent that we have decided to present the same workshop in this symposium with different altitudes. We will have three objectives: To demonstrate the reliability of our monitoring systems. To describe the physiological changes measured with different monitoring systems—BIS, NIRS and exhaled CO2—and their correlations in locals and foreigners adapted to different altitudes. To invite you to partake in a research project with this protocol, in which industries, universities or tourism could provide substantial support given the benefits they would obtain from the applications of these measurements.