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).
The respiration of 16 supine subjects was monitored at sea level and at an altitude of 3,050 m. Breath shape was defined by inspiratory and expiratory durations [TI, TE] and volumes [VI, VE], total breath duration [Tt], and ventilation [.VE(=VE/Tt)]. Dynamic breath pattern analysis revealed that the chronological sequences of these variables often display large sustained oscillations. Depending on their relative phase the simultaneous oscillations in VE and Tt are "compensating" or "reinforcing." The first typifies the traditional view that breath-to-breath changes in VE and Tt oscillate in phase, i.e., deeper breaths are longer thereby reducing ventilatory fluctuations. For the reinforcing oscillations breath-to-breath changes in VE and Tt oscillate out-of-phase, i.e., deeper breaths tend to be shorter, thereby enhancing ventilatory fluctuations. The observed amplitudes of the reinforced .VE oscillations are as large as 75% of the mean .VE level. In five subjects strong reinforcing oscillations with characteristic period of 20.6 +/- 3.5 (SD) s accompanied high-altitude exposure (most prominently after acclimatization) and did not appear to be a function of acid-base balance.
A very important problem in the analysis of biological data sequences is the detection of oscillations in the presence of random variations (noise). If the oscillations are not stationary, i.e., if they drift in frequency and amplitude, or occur in bursts, traditional analysis techniques utilizing the power spectrum or its time-domain equivalent, the autocorrelation function, can be both misleading and insensitive. Temporal filtering by a "comb" or se of band-pass filters is very effective for identifying and describing nonstationary oscillations. The basic procedures for interpreting the output of a comb filter are presented here, illustrated by examples using predefined input test sequences.
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