The characteristics and frequency of augmented breaths during CO2-induced hyperpnoea of newborn infants

1. Hypercapnic ventilatory responses of thirty-five full term infants aged 1-7 days were evaluated, and the characteristics and incidence of augmented breaths were determined. 2. According to the inspiratory airflow profile and tidal volume (V-T), augmented breaths were categorized as 'type A' (biphasic, V-T greater than or equal to 2 x control V-T), or 'type M' (multiphasic, V-T < 2 x control V-T). 3. Steady-state inhalation of 2-4% CO2 resulted in a 3-fold increase in the frequency of augmented breaths after 8 min; 80 s rebreathing a mixture containing 5-6% CO2 resulted in a 20-fold increase in the frequency of these breaths. 4. During eupnoea, 85% of spontaneously occurring augmented breaths were type A. With increasing respiratory drive, there was a disproportionate increase in type M compared with type A breaths: after 80 s rebreathing, 86% of augmented breaths were type M and only 14% were type A. 5. These findings indicate that the hypercapnic hyperpnoea facilitates an inspiration-augmenting reflex in newborn infants. The significance of type M breaths in unclear; they appear to be characteristic of the newborn, and may represent the response of an immature respiratory system to load.