O2 consumption (VO2), CO2 production (VCO2), and minute ventilation (VE) have been measured during normoxia and hypoxia (10-20 min in 10% O2) in specimens of 27 species from 6 mammalian orders, ranging in body mass (M) from a few grams to several kilograms. In normoxia, both metabolism and VE scaled close to M3/4, VE/VO2 and VE/VCO2 therefore being independent of M. In hypoxia, VE/metabolism increased in all species (on average greater than 100%), mostly because of a drop in VO2. On average, VE was 23% above the normoxic value but in some species decreased below normoxia. VO2 dropped in all but one species, on average 35%. Body temperature decreased by variable amounts, usually more in the smallest species. The decrease in metabolism during hypoxia was positively correlated with the resting metabolic rate of the species in a manner very similar to what can be calculated from data of previously studied newborn mammals. Hence hypoxia may decrease metabolic rate by decreasing thermogenesis, with larger effects in smaller animals, whether newborns or adults, because of their higher thermogenic requirements. We conclude that 1) hypoxic hypometabolism is a general characteristic of the mammalian response to hypoxia and cannot be neglected in the interpretation of ventilatory and cardiovascular responses and 2) its magnitude is inversely related to the resting VO2 of the species and therefore could be less prominent or possibly absent in adults of larger species.

Newborn mammals have a high O2 consumption (per unit body weight), which implies a high ventilation. The choice between an increase in volume, frequency, or both is probably dictated by energetic factors, including the likelihood of chest distortion with large inspirations. Data on ventilatory pattern of unanesthetized newborns of eight mammalian species, ranging in size from mice to infants, have been collected. Minute ventilation was linked to the O2 consumption and increased progressively less with the body weight of the species (BW0.86) due to a drop in frequency with size (BW-0.15), whereas tidal volume varied in proportion with body weight (BW1.01). Mean inspiratory flow per unit body weight was more than twice as large in newborn mice and rats than in piglets or infants, whereas the inspiratory time-to-total breath duration ratio was approximately constant among species, averaging 37%. During expiration occasional interruptions of the flow were observed in most newborns; measurements of esophageal pressure and diaphragmatic electromyogram pointed toward upper airways closure and not active breath holding as the explanation of this phenomenon.

This article reviews human and animal studies about the circadian patterns of physiological variables involved with the respiratory function. Some measures reflecting the mechanical properties of the lungs, such as functional residual capacity, forced expiratory volumes and airway resistance, change periodically with the time of the day. Also resting pulmonary ventilation (V(E)), tidal volume, and breathing rate follow circadian patterns. In humans, these patterns occur independently of the daily changes in activity, whereas, to some extent, they are linked to changes in the state of arousal. Differently, in some rodents, the circadian oscillations of the breathing pattern occur independently of the daily rhythms of either activity or state of arousal. Recent measurements of the breathing pattern for unlimited periods of time in undisturbed animals have indicated that the circadian changes occur in close temporal phase with those of oxygen consumption, carbon dioxide production, and body temperature. However, none of these variables can fully explain the circadian pattern of breathing, the origin of which remains unclear. Both in humans and in rats the V(E) responses to hypercapnia or hypoxia differ at various times of the day. In rats, the daily differences in V(E) responses are buffered by changes in metabolic rate, such that, unlike humans, the hyperventilation (defined as the increase in ventilation-metabolism ratio) remains constant throughout the 24 h. The presence of a biological clock is a major advantage in the adaptation to the environment, although it forces some variables to deviate periodically from their mean value. In humans, these deviations become apparent in conditions of hypoxia. Hence, a daily time-window exists in which the respiratory system is less capable of responding to challenges, a factor which may contribute to the findings that some cardio-respiratory symptoms and diseases peak at particular times of the day.

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