Jacopo P. Mortola

On the barometric method for measurements of ventilation, and its use in small animals.

The barometric method is a common technique for measurements of pulmonary ventilation in unrestrained animals. It basically consists of recording the changes in chamber pressure generated during breathing. In fact, as the air inspired is warmed and humidified from the ambient to the pulmonary values, the total pressure in the animal chamber increases; the opposite occurs in expiration. The present commentary is an introduction to this method, briefly reviewing its historical development, the conceptual pitfalls, and potential sources of errors during practical applications.

Ventilation and oxygen consumption during acute hypoxia in newborn mammals: a comparative analysis

We asked whether the lack of sustained hyperventilation during acute hypoxia, often reported to occur in the infant, is a common characteristic among newborn mammalian species, and to which extent inter-species differences may be accounted for by differences in metabolic responses. Ventilation (VE) and breathing pattern have been measured by flow-plethysmography or by the barometric method in normoxia and after 10 min of 10% O2 breathing in newborn mammals of 17 species over a 3 g to 20 kg range in body size. In 14 of these species oxygen consumption (VO2) has also been measured by a manometric technique or by calculation from the changes in chamber O2 pressure. VE and VO2 changed in proportion, among species, both in normoxia and hypoxia. In hypoxia, VE was higher, similar, or even lower than in normoxia, with some relation to the degree of maturity of the species at birth. In general, the small or absent VE responses to hypoxia resulted from small or no increase in tidal volume, while breathing frequency stayed elevated. The few departures from this pattern could be explained by interspecies differences in hypoxic sensitivity, since additional experiments in kittens and puppies indicated that, with more severe hypoxia, the pattern changed from rapid and shallow to deep and slow. In all cases, irrespective of the magnitude of the VE response, the VE/VO2 (and the mean inspiratory flow/VO2) increased during hypoxia, because the drop in VE, when present, was accompanied by an even larger drop in VO2. In fact, VO2 in hypoxia decreased in most species, although to variable degrees. Body temperature either did not change or decreased slightly, possibly indicating a trend toward a decrease of the set point of thermoregulation during hypoxia. In conclusion, the analysis gave further support to the concept that, during acute hypoxia, changes in metabolic rate play a paramount role in the ventilatory response of the newborn mammal.

Implications of hypoxic hypometabolism during mammalian ontogenesis.

During hypoxia, many newborn mammals, including the human infant, decrease metabolic rate, therefore adopting a strategy common to many living creatures of all classes, but usually not adopted by adult humans and other large mammals. In acute hypoxic conditions, hypometabolism largely consists in actively dropping mechanisms of thermoregulation. One implication is a decrease in body temperature. This is a safety mechanism, which favours hypoxic survival. Indeed, artificial warming during hypoxia can be counterproductive. Because carbon dioxide is an important stimulus for pulmonary ventilation, the drop in its metabolic production may tilt the balance of ventilatory control in favor of respiratory inhibition. Some experimental data support this view. In conditions of sustained hypoxia, the newborns hypometabolism also results from a depression of tissue growth and differentiation. Some organs are affected more than others. To what extent the blunted organ growth will be compatible with survival depends not only on the severity and duration of hypoxia, but also on the timing of its occurrence during development. Upon termination of hypoxia, the newborns metabolic rate recovers and growth resumes at higher rate. Even if body weight may be completely regained, alterations in the respiratory mechanical properties and in aspects of ventilatory control can persist into adulthood, a phenomenon not seen when the hypoxia was experienced at later stages of development. Some of the long-term respiratory effects of neonatal hypoxia are reminiscent of those observed in adult animals and humans native and living in high altitude regions.

How newborn mammals cope with hypoxia

The most immediate response to acute hypoxia in newborn mammals is hyperventilation, like in the adult. However. hyperventilation is often achieved by a reduction in metabolic rate (hypometabolism), rather than by an increase in ventilation (hyperpnea). This response is a regulated phenomenon largely based on inhibition of thermogenesis in all its forms, shivering, non-shivering and behavioural, with a resetting of the thermocontrol at a lower value of body temperature (Tb). Forcing Tb to the normoxic value in an hypoxic newborn can therefore provoke responses that are disadvantageous to the general strategy against hypoxia. The small or absent hyperpnea in the hypoxic newborn is the expected response to the decrease in metabolic rate; therefore, it should not be necessarily regarded as an expression of inadequate ventilatory control. However, during hypoxia the low metabolic rate can enhance the relative efficacy of inputs inhibitory on breathing, and this could be a mechanism contributing to ventilatory irregularities and apneas. The advantages of the hypometabolic strategy are numerous, and are at the basis of the extraordinary ability of newborn mammals to survive periods of severe hypoxia. The disadvantages become apparent with chronic hypoxia, because the reduced growth of tissues and organs may be incompatible with survival, or could lead to long-lasting structural and functional alterations.

Effect of CO2 on the metabolic and ventilatory responses to ambient temperature in conscious adult and newborn rats.

1. In newborn and adult rats, hypoxia decreases metabolic rate, especially at low ambient temperatures (Ta). We examined whether a similar effect can occur during hypercapnia. 2. We measured metabolism (oxygen consumption, Vo2, open flow‐through method), and expiratory ventilation (VE; barometric method (adults), airflow plethysmograph (newborns)) in air and 2% or 5% CO2 in normoxia. 3. In adults, Vo2 was higher at Ta = 10 degrees C than 25 degrees C. At each Ta, CO2 breathing did not change Vo2, but increased VE, less at 10 degrees C (up to +100%) than at 25 degrees C (+161%). Blood pressure was maintained at both values of Ta and CO2, while pulse rate and body temperature were decreased in 5% CO2 at 10 degrees C. 4. In newborns, the metabolic response to lowering Ta (from 40 to 20 degrees C) much depended on behavioural responses, being larger in groups of two or four pups than in individual animals. In no case did CO2 influence the response. VE increased during 5% CO2 exposure, more so at Ta = 33 percent C (+69%) than at 25 degrees C (+49%). 5. In both adults and newborns, hypoxia (10% O2) always decreased metabolic rate. 6. We conclude that hypercapnia has no appreciable effects on metabolic rate in rats (both newborns and adults) even at low Ta, a result quite different from the hypometabolic response to hypoxia.

STATICS OF THE RESPIRATORY SYSTEM IN NEWBORN MAMMALS

The static mechanical properties of the respiratory system have been studied in newborn mammals within the first two weeks of life, ranging in body size from rat to piglet, and compared to the corresponding adult. Animals were supine, paralyzed, and passively ventilated. The pressure-volume curves of the respiratory system and of the lung have been constructed by changing volume a known amount and measuring the tracheal pressure before and after opening the rib cage. In the newborns, the mean elastic recoil pressure of the lung at functional residual capacity (PLFRC) is 1.9 cm H2O with no clear correlation with body weight. The dry lung weight (LW)/body weight (BW) ratio is greater in newborns than in adults and progressively decreases with age. FRC (measured directly with the saline displacement method) is proportional to LW1.09 and BW1.03. In newborns, lung compliance per lung weight (CL/LW) is similar between species (Cl alpha LW1.07). Chest wall compliance per body weight is also constant (CW alpha BW1.00). By comparing the newborns with the corresponding adults we found that PLFRC is 35-57% of the adult value. FRC/LW in the newborn is less than or equal to the adult value, while FRC/BW is generally larger in the newborn. CL/LW is usually smaller in the newborn than in the adult while CW/BW is larger.

Ventilatory response to hypoxia in rats: gender differences

The ventilatory response to hypoxia of adult conscious rats, measured during sleep as change in ventilation-oxygen consumption ratio (VE/VO2) while breathing 10% O2 for 15-30 min, was found to be approximately 23% larger in females (female) than in males (male, p < 0.001). Also arterial PCO2 during hypoxia decreased more in female. The gender difference occurred at all metabolic levels, and was not related to normoxic VE/VO2 or the hypometabolic response to hypoxia; it was solely due to greater hyperpnea in female. With hypoxia, changes in blood pressure, heart rate and body temperature were similar between male and female; hence, gender differences in baro- or thermal-stimuli were not a contributing factor. Hematocrit, hemoglobin, lung, heart and diaphragm mass/body weight were also similar, whereas respiratory system compliance was higher, and resistance lower, in female. Ovariectomy did not change the female response. In prepubertal rats the VE/VO2 hypoxic response was also larger in female (approximately 12%, p < 0.05). After prolonged (approximately 4 months) hypoxia, the gender difference in the response to acute hypoxia was no longer statistically significant. In conclusion, conscious adult female rats have a higher ventilatory response to hypoxia than male. The reason for the difference remains elusive, but is probably not due to ovarian hormones.

Gas exchange in avian embryos and hatchlings.

The avian egg has been proven to be an excellent model for the study of the physical principles and the physiological characteristics of embryonic gas exchange. In recent years, it has become a model for the studies of the prenatal development of pulmonary ventilation, its chemical control and its interaction with extra-pulmonary gas exchange. Differently from mammals, in birds the initiation of pulmonary ventilation and the transition from diffusive to convective gas exchange are gradual and slow-occurring events amenable to detailed investigations. The absence of the placenta and of the mother permits the study of the mechanisms of embryonic adaptation to prenatal perturbations in a way that would be impossible with mammalian preparations. First, this review summarises the general aspects of the natural history of the avian egg that are pertinent to embryonic metabolism, growth and gas exchange and the characteristics of the structures participating in gas exchange. Then, the review focuses on the embryonic development of pulmonary ventilation, its regulation in relation to the embryos environment and metabolic state, the effects that acute or sustained changes in embryonic temperature or oxygenation can have on growth, metabolism and ventilatory control.

Metabolism and ventilation in hypoxic rats: effect of body mass.

Oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured by the flow-through method, and ventilation (VE) by the barometric technique in post-weaning age rats of 50, 100, 250 and 400 g, (5 males and 5 females in each group), at ambient temperature congruent to 24 degrees C. In normoxia, VO2, VCO2 and VE decreased with the increase in body weight (BW), whether normalization was by BW or by BW minus the weights of fat and skeleton; VE/VO2 and rectal temperature remained constant. In hypoxia (10% inspired O2), VE VO2 increased in all groups, to 2-2.5 times the normoxic values, because of a significant increase in VE (hyperpnea) and decrease in VO2 (hypometabolism); arterial PCO2, measured in some 100 g and 400 g rats, dropped similarly. However, the hyperpnea was about twice as large, and metabolism and body temperature decreased significantly less, in the 400 g than in the 50 g rats. The cost (ml O2) of breathing, computed in the paralysed animal artificially ventilated, averaged approximately 0.7% (normoxia) and 2% of VO2 (hypoxia), with no systematic differences with BW. The results agree with the concept that the metabolic response to hypoxia can be an important determinant of the magnitude of the hyperpnea.

Metabolic control of pulmonary ventilation in the developing chick embryo

In birds, during the period from the breaking of the air cell by the beak (internal pipping) to hatching, pulmonary ventilation (VE) begins and gas exchange is jointly provided by the lungs and the chorioallantoic membrane (CAM). We asked to what extent, during this phase of two concurrent gas exchange organs, changes in the embryos metabolic needs were accompanied by changes in VE. The carbon dioxide and oxygen exchange rates (VCO2, VO2) through lungs and CAM were separately, but simultaneously, measured in chicken embryos at 20-21 days of incubation, while VE was calculated from the measurements of pressure oscillations in the air cell during breathing. During the last 24 h of incubation, lung VO2 and VCO2 gradually progressed as the corresponding CAM values declined. An increase in egg temperature (T) from 33 to 39 degrees C increased the embryos total metabolic rate, especially when the lungs were the predominant gas exchange route. Whether metabolism increased because of the embryos development or because of the increase in T, VE was linearly proportional to lung VO2 and VCO2, and not to the embryos total metabolic rate. Hence, in the developing chick embryo, VE control mechanisms sense the peripheral tissue requirements via the gaseous component of cellular metabolism.