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ExtractPeriodic breathing (PB) is a respiratory pattern of young infants during the first few weeks of life and is due presumably to "immaturity" of the respiratory center. It has no pathologic significance and must thus be sharply distinguished from true apneic spells.Measurements and tests done on 141 infants exhibiting periodic breathing showed the following results. (/) The incidence of PB is 36.1 % in infants weighing more than 2,500 g at birth, and 94.5% in infants of low birth weight. (2) Periodic breathing begins after the first 1-2 days of life; it may persist for several weeks depending on the degree of the baby's prematurity. (3) Mean duration of apneic periods was 6.9 sec; mean number of apneic periods per minute varied from 5 to less than 1. (4) Endtidal P C o 2 showed a mean figure of 29.15 mm Hg during regular breathing; after the end of an apneic period it was 32.36 mm Hg. Corresponding values for end-tidal O2 were 113.58 and 98.33 mm Hg, respectively. (5) Ambient GO 2 in air tended to cause a gradual decrease in the number of apneic episodes as CO2 was raised. At 3.15% CO 2 , PB had practically disappeared in all patients. The CO 2 did not have an effect on the duration of individual apneic episodes, as long as such were present. (6) Ambient O2 had no effect on either duration or frequency of apneic episodes up to a certain characteristic level. If that particular O2 concentration in the environment of the baby was reached, PB abruptly turned to a regular breathing pattern ("turning point"). The "turning point" was a characteristic, and on any given day, reproducible finding in any given infant exhibiting PB. It would go down with time with a slope very similar for all infants, although absolute values varied considerably between different individuals. Speculation

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