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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 (VT), augmented breaths were categorized as 'type A' (biphasic, VT > or = 2 x control VT), or 'type M' (multiphasic, VT < 2 x control VT). 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 is unclear; they appear to be characteristic of the newborn, and may represent the response of an immature respiratory system to load.

The ventilatory response (VR) of the full-term newborn to CO2 was studied during quiet sleep using rebreathing and steady-state methods. Rebreathing responses were obtained under normoxic (fractional inspired oxygen concentration, Fi,O2 = 0.21) and hyperoxic (Fi,O2 = 0.4) conditions. Ten infants were tested three to five times using each of the two rebreathing protocols and the results averaged. Overall, there was no significant difference between either the mean variability (coefficient of variation) or slope of the VR measured under normoxic and hyperoxic conditions. Four infants were studied using a steady-state technique. There was marked test-to-test variability in VRs measured by this method. The results appear to indicate that the variability of the VR of the newborn to CO2 is not a result of chemoreflex changes in ventilation elicited in response to fluctuations in arterial PO2.

1. Direct simultaneous recordings from chronically implanted electrodes in different parts of the diaphragm were made in young lambs in which laryngeal adductor (thyroarytenoideus) and intercostal electromyograms, airflow, tracheal pressure and electrocorticogram and electro-oculograms for behavioural state were also recorded.2. An asynchrony of diaphragmatic contraction occurred which was dependent on sleep state. The vertebral portion showed maximal post-inspiratory activity while the lateral paratendinous portion usually terminated abruptly with end-inspiration, reciprocating closely with the onset of expiratory laryngeal adductor activity during quiet sleep. The contraction of the sternal portion was similar to the vertebral portion. In active (rapid eye movement) sleep there was no expiratory laryngeal constriction and post-inspiratory activity occurred in all portions of the diaphragm. During the characteristic bursts of rapid breathing in active sleep all post-inspiratory activity disappeared and the diaphragm contracted synchronously.3. General anaesthesia (Halothane/N(2)O or Nembutal) abolished expiratory laryngeal adductor activity and the discharge pattern became similar in all parts of the diaphragm.4. Intrathoracic vagotomy of Xylocaine blockade below the recurrent laryngeal nerves abolished post-inspiratory activity in all parts of the diaphragm, in contrast to the effect on expiratory laryngeal adductor activity which increased.5. Sustained ;tonic' electromyographic activity was often recorded from the costal and to a lesser extent the paratendinous portion of the diaphragm. This activity related to adjacent intercostal activity:ipsilateral intercostal blockade with local anaesthetic (Xylocaine 1%) abolished both the intercostal and the ;tonic' activity of the costal margins of the diaphragm. Conversely ipsilateral phrenic nerve blockade abolished all but the ;tonic' activity which related to intercostal activity.6. Interpretation of the respiratory activity of the diaphragm could not be made adequately from conventionally placed electrodes (i.e. costal, sternal slip or surface) during spontaneous breathing in unanaesthetized lambs. Simultaneous recordings showed that while expiratory flow and duration were actively controlled by expiratory laryngeal adductor activity and the diaphragm, the latter performed asynchronously. While both laryngeal and diaphragmatic expiratory functions were substantially under vagal control, the former increased and the latter decreased with reduced vagal input.