Variability of the breathing pattern in newborn rats: Effects of ambient temperature in normoxia or hypoxia

We hypothesized that the inter-breath variability of the breathing pattern in newborn rats varied with temperature and oxygenation. Breathing pattern was recorded in 4-day-old rats by airflow plethysmography, during normoxia in warm (control) and cold conditions, or during hypoxia (inspired O-2 = 10%) in warm or cold conditions, each lasting 15 min. The warm phase (36 degrees C) either preceded or followed the cold (24 degrees C). Time-domain analysis was applied to 500 continuous breaths recorded toward the end of each phase. All parameters describing the breathing pattern (instantaneous ventilation, tidal volume, and inspiratory and expiratory time) had lower variability when the condition differed from control i.e. in cold or hypoxia, with no correlation with the absolute level of ventilation. The difference in variability between warm-normoxia and the other conditions was reduced when cold preceded the warm phase. Gaseous metabolism was increased in cold because of thermogenesis. When the cold preceded the warm phase the increased thermogenesis partly persisted into the warm phase, raising the metabolic level. We conclude that the variability of the breathing pattern in newborn rats 1) does not depend on the absolute level of ventilation, and 2) is reduced by the increased chemical stimuli occurring during cold-hypermetabolism or hypoxia. In normoxia in warm condition metabolic and chemical stimuli are low, and the variability is the highest. The results are in agreement with the clinical observations of a higher incidence of apneic episodes in infants during warm conditions.