Effect of chronic altitude hypoxia on the behavior of the respiratory center. Study on normal subjects living at the altitude of Mexico City (2,240 meters)

Respiratory center (RC) output has been shown to be increased in hypoxemic Chronic Obstructive patients at sea level. In order to asses the separate role of chronic hypoxia on the RC output we studied 30 normal subjects all of them native and residents of Mexico City (altitude: 2,240 m, PaO2: 65-70 torr.). The parameters studied were: occlussion pressure (P0.1), mean inspiratory flow (Vi), and the ratio inspiratory time to total duration of the respiratory cycle (Ti/Ttot). The inspiratory impedance of the respiratory system as well as the minute ventilation (VE) and lastly to ensure isocapnic conditions, the end-tidal CO2 (PECO2), were also measured. These parameters were determined: 1) While breathing room air (RA), 2) after 30 min of breathing an inspired oxygen fraction (FiO2) of 30% and again 3) after 30 min of breathing and FiO2 of 100%. Fifteen of the subjects were studied on supine and the other 15 in the seated position. In most of the subjects the baseline (RA) values of P0.1 were found to be higher than those reported for normals at sea level. In every case, independent of body position, the P0.1 decreased (less than 0.01) to normal sea level values after 30 min of breathing O2 30%. Likewise, Vi and mechanical impedance also decreased (p less than 0.01) and no changes in Ti/Ttot were noted at this FiO2. No further changes occurred after breathing 100%. The above results show that: 1) The RC output in normal people at altitude (i.e. without mechanical abnormalities but with chronic hypoxia) is increased as compared to sea level.(ABSTRACT TRUNCATED AT 250 WORDS)