Strength and cycle time of high-altitude ventilatory patterns in unacclimatized humans.

Respiration was monitored with magnetometers in 12 healthy supine young adults at sea level and in an altitude chamber at simulated high altitudes of 8,000, 9,000, 11,000, and 14,000 ft. Periodic breathing that was strong enough to include apnea at the time of minimum ventilation was seen in all subjects at high altitude. Cycle time of periodic breathing ranged from 12 to 34 s. On average across the population the incidence of periodic breathing increased with altitude. Cycle time of the periodic pattern increased as strength of the pattern increased. After normalizing to a standard pattern strength, cycle time decreased as altitude increased. The study included two series of experiments, the second occurring 3 wk after the first and involving seven of the same subjects. The standard cycle time at 14,000 ft for each subject in the second series was the same as in the first series to within, on the average, 6%. Each subject studied at 11,000 ft in both series reproduced his cycle time to within, on the average, 9%. The variation of standard cycle time for a given subject is less than the variation across the population, indicating characteristic cycle times for some individuals (one-way analysis of variance, P less than 0.025).