Ventilatory response to hypoxia in unanesthetized newborn kittens

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Perinatal maturation of the mouse respiratory rhythm-generator: in vivo and in vitro studies

In vivo (plethysmography) and in vitro (en bloc preparations) experiments were performed from embryonic day 16 (E16) to postnatal day 9 (P9) in order to analyse the perinatal maturation of the respiratory rhythm-generator in mice. At E16, delivered foetuses did not ventilate and survive but at E18 they breathed at about 110 cycles/min with respiratory cycles of variable individual duration. From E18 to P0-P2, the respiratory cycles stabilised without changes in the breathing parameters. However, these increased several-fold during the next days. Hypoxia increased breathing frequency from E18-P5 and only significantly affected ventilation from P3 onwards. At E16, in vitro medullary preparations (pons resection) produced rhythmic phrenic bursts at a low frequency (about 5 cycles/min) with variable cycle duration. At E18, their frequency doubled but cycle duration remained variable. After birth, the frequency did not change although cycle duration stabilised. At E18 and P0-P2, the in vitro frequency decreased by around 50% under hypoxia, increased by 40-50% under noradrenaline or substance P and was permanently depressed by the pontine A5 areas. At E16 however, hypoxia had no effects, both noradrenaline and substance P drastically increased the frequency and area A5 inhibition was not expressed at this time. At E18 and P0-P2, electrical stimulation and electrolytic lesion of the rostral ventrolateral medulla affected the in vitro rhythm but failed to induce convincing effects at E16. Thus, a major maturational step in respiratory rhythmogenesis occurs between E16-E18, in agreement with the concept of multiple rhythmogenic mechanisms.

Development of the ventilatory response to hypoxia in Swiss CD-1 mice

We examined developmental changes in breathing pattern and the ventilatory response to hypoxia (7.4% O(2)) in unanesthetized Swiss CD-1 mice ranging in age from postnatal day 0 to 42 (P(0)-P(42)) using head-out plethysmography. The breathing pattern of P(0) mice was unstable. Apneas were frequent at P(0) (occupying 29 +/- 6% of total time) but rare by P(3) (5 +/- 2% of total time). Tidal volume increased in proportion to body mass ( approximately 10-13 ml/kg), but increases in respiratory frequency (f) (55 +/- 7, 130 +/- 13, and 207 +/- 20 cycles/min for P(0), P(3), and P(42), respectively) were responsible for developmental increases in minute ventilation (690 +/- 90, 1,530 +/- 250, and 2,170 +/- 430 ml. min(-1). kg(-1) for P(0), P(3), and P(42), respectively). Between P(0) and P(3), increases in f were mediated by reductions in apnea and inspiratory and expiratory times; beyond P(3), increases were due to reductions in expiratory time. Mice of all ages showed a biphasic hypoxic ventilatory response, which differed in two respects from the response typical of most mammals. First, the initial hyperpnea, which was greatest in mature animals, decreased developmentally from a maximum, relative to control, of 2.58 +/- 0.29 in P(0) mice to 1. 32 +/- 0.09 in P(42) mice. Second, whereas ventilation typically falls to or below control in most neonatal mammals, ventilation remained elevated relative to control throughout the hypoxic exposure in P(0) (1.73 +/- 0.31), P(3) (1.64 +/- 0.29), and P(9) (1. 34 +/- 0.17) mice but not in P(19) or P(42) mice.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Persistence of the biphasic ventilatory response to hypoxia in preterm infants

OBJECTIVE

To characterize postnatal maturation of the biphasic ventilatory response to hypoxia in order to determine whether it persists beyond the first weeks of life in preterm infants, and the contributions of respiratory frequency and tidal volume to this response.

METHODS

Stable preterm infants were studied at two postnatal ages, 2 to 3 weeks (n = 12) and 4 to 8 weeks (n = 12), before hospital discharge at 35 weeks (range, 33 to 38 weeks) of postconceptional age. Infants were exposed to 5 minutes of 15% (or 13%) inspired oxygen; ventilation, oxygen saturation, end-tidal partial pressure of carbon dioxide, and heart rate were simultaneously recorded.

RESULTS

Minute ventilation exhibited a characteristic biphasic response to hypoxia at both postnatal ages, regardless of the development of periodic breathing. At both ages there was a transient increase in tidal volume, which peaked at 1 minute, accompanied by a sustained decrease in respiratory frequency as a result of significant prolongation of expiratory time.

CONCLUSION

The characteristic biphasic ventilatory response to hypoxia persists into the second month of postnatal life in preterm infants. We speculate that this finding is consistent with the prolonged vulnerability of such infants to neonatal apnea.

The biphasic ventilatory response to hypoxia in preterm infants is not due to a decrease in metabolism

The mechanism underlying the biphasic ventilatory response to hypoxia in neonates is poorly understood. Because alveolar PCO2 (PaCO2) decreases and remains low during hypoxia, it has been argued that a decrease in metabolism may occur. We hypothesized that if the late decrease in ventilation during hypoxia is due to a decrease in CO2 production, an increase in PACO2 should abolish it. We studied 27 preterm infants [birth weight, 1,700 +/- 41 g (mean +/- SEM); study weight, 1,760 +/- 36 g; gestational age 32 +/- 0.2 weeks; postnatal age, 17 +/- 1 days]. A flow-through system and Beckman analyzers were used to measure ventilation and alveolar gases. Metabolism was expressed as changes in oxygen consumption. Infants were studied randomly during hypoxia alone (15% O2 + N2, n = 55) and during hypoxia plus CO2 (0.5% CO2, n = 30; 2% CO2, n = 10). Each experiment consisted of 2 minutes of control measurements (21% O2), 5 minutes of measurements during hypoxia alone or hypoxia plus CO2, followed by 2 minutes of recovery (21% O2). We found a biphasic response to hypoxia with or without CO2 supplementation, the percent change in ventilation from initial peak hyperventilation to late hypoventilation at 5 minutes being -16 +/- 2 on 15% O2; -9 +/- 3 on 15% O2 + 0.5% CO2; and -15 +/- 9 on 15% O2 + 2% CO2 (P < 0.05). The decrease in ventilation was primarily due to a significant decrease in frequency; tidal volume increased. Oxygen consumption decreased similarly with the various inspired gas mixtures during hypoxia. These findings indicate that the decrease in ventilation during hypoxia is unlikely to be solely due to a decrease in metabolism since the late decrease in ventilation following initial hyperventilation still occurred despite the elimination of a fall in PACO2. We speculate that the mechanism underlying the late decrease in ventilation is likely of central origin, probably mediated through the release of inhibitory neurotransmitters.

Characterization of ventilatory responses to hypoxia in neonatal rats

Newborn animals exhibit a biphasic response to hypoxia, with ventilation increasing and then declining. Our purpose was to define if this response could be supported by the pontile and medullary respiratory centers. Spontaneously breathing and paralyzed and ventilated decerebrate or anesthetized, vagotomized rats were studied from birth to 13 days thereafter. Peak integrated phrenic activity, or tidal volume, and frequency initially increased and then declined after inspired oxygen was reduced from hyperoxic to hypoxic levels; most animals became apneic in hypoxia. Apnea occurred in a greater proportion of animals and more quickly with more severe hypoxia. Following sectioning of the carotid sinus nerves, ventilatory activity declined with a change from hyperoxia to normoxia. We conclude that the biphasic ventilatory response to hypoxia represents a balance between synaptically-induced augmentations and reductions of brainstem neuronal activities. The carotid chemoreceptors play a fundamental role in the augmentations, and reductions appear dependent upon actions of hypoxia upon brainstem mechanisms.

Influence of vagal afferents on diphasic ventilatory response to hypoxia in newborn lambs

The effect of vagal afferents on the ventilatory response to hypoxia was studied in eleven awake newborn lambs. Tests were repeated before and after vagotomy in the same lambs in two conditions: with intact upper airways and after intubation. During hypoxia, a diphasic pattern of ventilatory response was observed in both vagotomized and intact lambs. However, face mask-breathing vagotomized lambs had a blunted increase in ventilation (VI) to hypoxia as compared with intact lambs (P = 0.0001) and they showed an expiratory braking during all hypoxic time. Furthermore, the normal increase in frequency (f) to hypoxia was abolished after vagotomy. After intubation, expiratory braking disappeared and, consequently, magnitude of the VI response to hypoxia was similar in intact and vagotomized lambs. These changes were due to improved tidal volume response in vagotomized intubated lambs (P < 0.002) with no significant change in f response. We concluded that, in awake newborn lambs, vagal afferents are essential for maintaining the pattern and the magnitude of the ventilatory response to hypoxia, the latter by controlling the motor output to the larynx.

Role of adenosine in the hypoxic ventilatory response of the newborn piglet

The role of intracerebral adenosine levels in the control of ventilatory response to hypoxia was explored in 15 spontaneously breathing intubated piglets, 1-5 days old, sedated with chloral hydrate. Respiration was recorded via by a pneumotachograph. In all animals exposed to hypoxia (12% O2) for 10 minutes. There was an initial increase in ventilation followed by a late decrease (the biphasic ventilatory response). Both intravenous caffeine citrate (20 mg/kg) and an FiCO2 of 0.05 separately abolished or attenuated the late ventilatory depression associated with hypoxia. In the same piglets, the administration of 10 micrograms dipyridamole, a competitive inhibitor of adenosine receptors, directly into the cerebral ventricles abolished the hyperventilatory response to hypoxia. Conversely, the use of 20 micrograms intraventricular 8-phenyltheophylline abolished the late ventilatory depression associated with hypoxia. Neither drug had a direct effect on ventilation at the time of injection. These results suggest that adenosine is a part of the diphasic ventilatory response to hypoxia.

Effect of dichloroacetate on ventilatory response to sustained hypoxia in normal adults

In adult humans, the ventilatory response to acute sustained hypoxia is biphasic, characterized by an initial brisk increase followed by a decline to an intermediate plateau. Recently, it has been shown that hypoxic lactate formation in the brain depresses ventilation in peripherally chemodenervated animals, and postulated that this formation might mediate the hypoxic ventilatory decline observed in adult humans. To investigate this hypothesis, the ventilatory response to 25 min of acute isocapnic hypoxia (SaO2 = 80%) was evaluated in adult humans after pretreatment with intravenous dichloroacetate (DCA), a drug that crosses the blood-brain barrier and reduces lactate formation. Ten subjects were pretreated with DCA (50 mg.kg-1.h-1) or normal saline infusion on two days in a double blind manner. The infusion started 35 min before the institution of hypoxia and continued throughout the experiment. Independent of pretreatment, the ventilatory response to acute sustained hypoxia was biphasic; an increase followed by a decline. Ventilation during hypoxia declined significantly and the magnitude of the decline did not differ between the DCA and placebo pretreatments, averaging 3.32 +/- 0.45 and 3.17 +/- 0.58 L/min, respectively (mean +/- SE). With and without DCA infusion the hypoxic ventilatory decline was due to significant decrease in tidal volume and mean inspiratory flow without changes in breathing frequency. We conclude that brain lactic acidosis is unlikely to be involved in the ventilatory response to sustained hypoxia of adult humans, at least in the range of hypoxia studied.

Efferent phrenic nerve and respiratory neuron activities in the developing kitten: spontaneous discharges and hypoxic responses

Efferent phrenic nerve and medullary respiratory neuron discharges were examined for age-dependent changes of activities during normocapnic hyperoxia and hypoxia in anesthetized and decerebrate kittens (22-150 days old). In animals less than 39 days of age, phrenic power spectra during hyperoxia were dominated by components in the medium-frequency band (20-50 Hz), whereas spectra of animals of at least 39 days of age were dominated by components in the high-frequency band (50-100 Hz). Such high-frequency oscillations were also observed in the power spectra of some inspiratory neurons in animals of at least 43 days old. In hypoxia, the amplitude of phrenic discharge exhibited an initial facilitation followed by a diminution (i.e. biphasic response) in animals 39 days old or younger. In animals older than 39 days, however, hypoxia elicited a sustained facilitation of phrenic discharge amplitude. In contrast, no such age-dependent change in response pattern to hypoxia was observed for neuronal discharges; rather, responses of most neurons consisted of either decreases of discharge frequency, or complete abolishment of discharges.

External intercostal muscle activity during acute hypoxia in the kitten

The effects of acute hypoxia on the recruitment of external intercostal muscle activity were determined in 12 kittens, aged 14 to 36 days. The animals were anesthetized with 1.23 +/- 0.23% halothane and bipolar electrodes were placed in the costal and crural diaphragm and in dorsal external intercostal muscles. Acute hypoxia was induced by the animals breathing 13% oxygen; arterial gases were sampled during baseline conditions and at 1 and 5 min after induction of hypoxia. Peak-moving average (PA) and minute electromyogram (EMG) activity (PA x f) were recorded during baseline conditions and at 1 and 5 min after onset of acute hypoxia. At 1 min of acute hypoxia, PA and PA x f of the costal diaphragm, crural diaphragm, and external intercostal muscles were significantly increased above baseline values (P less than 0.01). After 5 min of acute hypoxia, PA of all three muscles remained elevated above baseline values (P less than 0.05) but PA x f returned toward baseline levels. Respiratory frequency remained unchanged during the hypoxic stimulus. These data document that the newborn is capable of increasing inspiratory external intercostal muscle EMG activity during acute hypoxia. We speculate that this phasic recruitment could be of physiologic benefit to the newborn by stabilizing the complaint chest wall and by increasing the contribution of rib cage expansion to tidal breathing.