The medical service systems at disasters have been thoroughly reexamined since the Hanshin Awaji Earthquake Disaster. The number of helicopters boarded by physicians has been increased and used for transportation of patients requiring urgent medical treatments. Since a helicopter usually flies at the altitude of 1,000-2,000 m above the sea level, respirators installed on it are required to have correcting function for the altitude. In the present study, the correcting function for the altitude was tested on 12 types of respirators provided by 11 makers in an artificial chamber. The tidal volume and respiratory frequency in each respirator were adjusted to 500ml and 15/min at 610m, respectively. The tidal volume was measured at 610m, 2,000m, and 3,000m to cover the altitudes where transportation by helicopters is frequently performed in Japan. The altitudes were attained at the three speeds; 150m/min, 300m/min, and 450m/min. Alveolar carbon dioxide (P_ACO_2) and oxygen pressures (P_AO_2) were estimated from the alveolar ventilation equation and the alveolar gas equation assuming the dead space; 150ml, VO_2; 250ml, and VCO_2; 200ml. The results were as follows. 1) Ten of 12 respirators were equipped with correcting function for altitude; the tidal volume increased with increasing altitude. In three respirators with the highest correcting function, tidal volume increased from the baseline of 500±53ml to 617±52ml at 2,000m, and 685±52ml at 3,000m on averages for three respirators. 2) The reduction in P_AO_2 with increasing altitude was attenuated in respirators with correction compared with that in those with no correction. In three respirators with the highest correcting function, P_AO_2 was 98±3mmHg at 610m, 86±2mmHg at 2,000m, and 75±5mmHg at 3,000m on average for three respirators. 3) P_ACO_2 decreased with increasing altitude in respirators with correction while that in those with no correction remained constant. In three respirators with the highest correcting function, P_ACO_2 was 34±3mmHg at 610m, decreased to 25±2mmHg at 2,000m, and 22±2mmHg at 3,000m, on averages for three respirators. These results suggest that since correction for increasing altitude was achieved by increasing tidal volume, which results in respiratory alkalosis due to increased expiration rate of CO_2. This is the reason that respirators are used below the altitude of 2,000m. It is necessary to develop a new respirator to maintain P_AO_2 at increased altitude without reduction in P_ACO_2.
