The respiratory sensation at high altitude.

The respiratory sensation was studied in Nepal at four different altitudes, 1377 m before and after the ascension, 2800 m, 3900 m and 530 m. Dyspnea was noted at each altitude for the nine subjects. They had to rate 4 external resistive loads between 2.5 and 13 cm H2O.l-1.s, presented in 2 pairs, a low and a high one. The discrimination between the loads i.e. the subject's sensitivity was obtained from Sensory Decision Analysis. These subjects were compared to six normal ones observed at sea level while breathing air, an hypoxic mixture (FIP2:11%) and air in a cold environment (-6 degrees C). In these protocols, the load perception was not modified. The 2 populations exhibited a similar sensitivity when observed in normal conditions. At exertion and with altitude, the nine subjects demonstrated a progressive increase in dyspnea, rated with visual analog scales. At rest, the perception of the loads was not altered but slightly improved with altitude for 6 subjects. The other 3 subjects (2 subjects with clinical impairment, important dyspnea and pulmonary oedema) showed an impairment of the perception. The sensitivity to the loads was similar before and after the ascension for the well adapted subjects to altitude. In conclusion, the respiratory sensation is not impaired with altitude in well adapted subjects and transient hypoxia does not result in change in load perception. An impairment in load perception observed in some subjects is probably related to the secondary effects of chronic hypoxia, i.e. cerebral and/or pulmonary suboedema.