Postnatal maturation of vagal respiratory reflexes in preterm and full-term lambs

Glottic patency during noninvasive ventilation in patients with chronic obstructive pulmonary disease

BACKGROUND

Non-invasive ventilation (NIV) provides ventilatory support for patients with respiratory failure. However, the glottis can act as a closing valve, limiting effectiveness of NIV. This study investigates the patency of the glottis during NIV in patients with acute exacerbation of Chronic Obstructive Pulmonary Disease (COPD).

METHODS

Electrical activity of the diaphragm, flow, pressure and videolaryngoscopy were acquired. NIV was randomly applied in pressure support (PSV) and neurally adjusted ventilatory assist (NAVA) mode with two levels of support. The angle formed by the vocal cords represented glottis patency.

RESULTS

Eight COPD patients with acute exacerbation requiring NIV were included. No differences were found in median glottis angle during inspiration or peak inspiratory effort between PSV and NAVA at low and high support levels.

CONCLUSIONS

The present study showed that glottis patency during inspiration in patients with an acute exacerbation of COPD is not affected by mode (PSV or NAVA) or level of assist (5 or 15 cm H₂O) during NIV.

Bifurcation of the respiratory response to lung inflation in anesthetized dogs

Numerous studies have demonstrated the effect of lung volume on prolongation of duration of expiration (TE) with limited understanding of the TE shortening and termination of expiration as observed in newborn. In 14 dogs, the effects of varied onset of lung inflation during expiration on the TE were evaluated. When lung inflation was applied in the first part of expiration (20-60% of TE) TE was lengthened. However, in the second portion (60-80% of TE) of expiration, lung inflation either terminated or prolonged TE; whereas in the last portion of expiration (80-90% of TE), lung inflation tended to terminate expiration prematurely. The effects were abolished after bilateral vagotomy. We postulate that prolongation of TE relates to the Breuer-Hering inflation reflex, which increases the time needed for a passive expiration; whereas the ability to shorten TE could relate to Head's paradoxical reflex acting to initiate inspiration or to activate inspiratory motor activity to brake expiratory flow as occurs in the newborn.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Effects of caffeine and/or nasal CPAP treatment on laryngeal chemoreflexes in preterm lambs

Current knowledge suggests that laryngeal chemoreflexes (LCR) are involved in the occurrence of certain neonatal apneas/bradycardias, especially in the preterm newborn. While caffeine and/or nasal continuous positive airway pressure (nCPAP) are the most frequent options used for treating apneas in preterm newborns, their effects on LCR-related apneas/bradycardias are virtually unknown. The aim of the present study was to test the hypothesis that caffeine and/or nCPAP decreases LCR-related cardiorespiratory inhibition in a preterm ovine model. Seven preterm lambs were born vaginally on gestational day 133 (normal gestation: 147 days) after intramuscular injections of betamethasone and mifepristone. Five days after birth, a chronic surgical instrumentation was performed to record states of alertness, electrocardiogram, systemic arterial pressure, and electromyographic activity of a laryngeal constrictor muscle, as well as to insert a transcutaneous supraglottal catheter. LCR were induced in quiet sleep under four conditions: 1) control (without caffeine or nCPAP); 2) nCPAP (5 cmH2O, without caffeine); 3) caffeine (10 mg/kg infused intravenously for 30 min, without nCPAP); and 4) nCPAP + caffeine. Our results showed that nCPAP consistently blunted LCR-related cardiorespiratory inhibition vs. control condition, contrary to caffeine whose overall effect was nonsignificant. In addition, nCPAP condition was characterized by a more consistent and rapid arousal after HCl injection. No significant differences were observed between all tested conditions with regard to swallowing and cough. It is concluded that nCPAP should be further assessed for its usefulness in treating neonatal apneas linked to LCR.

Prenatal development of respiratory chemoreceptors in endothermic vertebrates

Respiratory chemoreceptors are neurons that detect PCO(2), PO(2), and/or pH in body fluids and provide sensory feedback for the control of breathing. They play a critical role in coupling pulmonary ventilation to metabolic demand in endothermic vertebrates. During birth in mammals and hatching in birds, the state change from placental or chorioallantoic gas exchange to pulmonary respiration makes acute demands on the neonatal lungs and ventilatory control system, including the respiratory chemoreceptors. Here we review the literature on prenatal development of carotid body chemoreceptors, central chemoreceptors, and airway chemoreceptors, with emphasis on the histology, histochemistry, and neurophysiology of chemosensory cells or their afferents, and their physiological genomics if known. In general, respiratory chemoreceptors develop prenatally and are functional but immature at birth or hatching. Each type of respiratory chemoreceptor has a unique prenatal developmental time course, and all studied to date require a period of postnatal maturation to express the full adult response.

Developmental changes in brainstem neurons regulating lower airway caliber

Premature infants are at risk for lower airway obstruction; however, maturation of reflex pathways regulating lower airway patency is inadequately studied. We hypothesized that postnatal maturation causes developmental change in brainstem efferent airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) that project to the airways and in pulmonary afferent fibers that terminate in the nucleus tractus solitarius (NTS). Ferrets aged 7, 14, 21, and 42 d received intrapulmonary injection of cholera toxin (CT)-beta subunit, a transganglionic retrograde tracer. Five days later, their brainstem was processed for dual immunolabeling of CT-beta and the cholinergic marker, choline acetyl transferase. CT-beta-labeled AVPNs and CT-beta-labeled afferent fiber optical density (OD) were analyzed. There was a significantly higher CT-beta-labeled cell number within the rNA at the youngest compared with older ages. All efferent CT-beta-labeled cells expressed choline acetyl transferase. OD of CT-beta-labeled afferent fibers was also higher at 7 d compared with 14 d. We conclude that the number of efferent AVPNs and afferent fiber OD both diminish over the second postnatal week. We speculate that exposure to injurious agents in early postnatal life may inhibit natural remodeling and thereby enhance later vulnerability to airway hyperreactivity.

Dyspnoea in children. Does development alter the perception of breathlessness?

Dyspnoea, the perception of an unpleasant and/or uncomfortable sensation of breathlessness, offers several physiological, anatomical and teleological analogies with pain. Pain perception has been shown to exist in the newborn, suggesting that dyspnoea may also occur from birth onwards. The perception of breathlessness will be subservient to developmental changes in the behaviour of sensors and lung and muscular receptors implicated in dyspnoea, some of which are known to be active at time of birth. For example, perinatal resetting of the arterial chemoreceptor could lead to transient depression of the dyspnoeic response to hypoxia. However, though early evoked ventilatory responses and peripheral receptor maturation do exist, dyspnoea will only occur if the corresponding central neural circuitry undergoes parallel maturation. Our knowledge of dyspnoea in later childhood is based on a small number of clinical or psychophysical studies, predominantly dealing with asthma and exercise. There is a thus a clear need for systematic assessment of the existence and severity of dyspnoea sensing in younger children that takes into account its role as an alarm mechanism for triggering adaptive and/or protective responses.

Brain-derived neurotrophic factor in arterial baroreceptor pathways: implications for activity-dependent plasticity at baroafferent synapses

Functional characteristics of the arterial baroreceptor reflex change throughout ontogenesis, including perinatal adjustments of the reflex gain and adult resetting during hypertension. However, the cellular mechanisms that underlie these functional changes are not completely understood. Here, we provide evidence that brain-derived neurotrophic factor (BDNF), a neurotrophin with a well-established role in activity-dependent neuronal plasticity, is abundantly expressed in vivo by a large subset of developing and adult rat baroreceptor afferents. Immunoreactivity to BDNF is present in the cell bodies of baroafferent neurons in the nodose ganglion, their central projections in the solitary tract, and terminal-like structures in the lower brainstem nucleus tractus solitarius. Using ELISA in situ combined with electrical field stimulation, we show that native BDNF is released from cultured newborn nodose ganglion neurons in response to patterns that mimic the in vivo activity of baroreceptor afferents. In particular, high-frequency bursting patterns of baroreceptor firing, which are known to evoke plastic changes at baroreceptor synapses, are significantly more effective at releasing BDNF than tonic patterns of the same average frequency. Together, our study indicates that BDNF expressed by first-order baroreceptor neurons is a likely mediator of both developmental and post-developmental modifications at first-order synapses in arterial baroreceptor pathways.

Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO(2) sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO(2) during early development, and, if so, how early? And, (2) does IPC CO(2) sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO(2) before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO(2) sensitivity when compared to adults (P< or =0.05). These results suggest that during early development phasic IPC CO(2) sensitivity is fully developed, yet tonic IPC CO(2) sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.

Postnatal development of right atrial injection of capsaicin-induced apneic response in rats

Apnea and respiratory failure often occur in infants with pulmonary disease. Bronchopulmonary C-fiber (PCF)-mediated apnea is an important component of respiratory dysfunction. This study was undertaken to define the postnatal development of PCF-mediated apnea. The experiments were conducted in five groups of anesthetized, tracheotomized, and spontaneously breathing rats with ages at postnatal days P1-3, P7-9, P14-16, P21-23, and P56-58. Right atrial bolus injection of three doses of capsaicin (Cap), equivalent to 2, 4, and 8 microg/kg used previously in 450-g rats, was applied to stimulate PCFs. We found that 1) Cap-induced apneic response [percent change from the baseline expiratory duration (Te) values (deltaTe%)] and the sensitivity of this response (deltaTe%.microg(-1)) were significantly greater in the rats <P10 than those >P10; 2) the Cap-induced apneas were vagally dependent in all rats tested; and 3) bivagotomy-induced prolongation of Te was much greater in the rats <P10 than those >P10. From these findings we concluded that, compared with the older rats (>P10), the newborn rats have a stronger PCF-mediated respiratory inhibition that may contribute to infants' vulnerability to respiratory failure.

C-fiber blockade influence on non-nutritive swallowing in full-term lambs

Systemic C-fiber blockade (CFB) has been reported to inhibit induced swallowing in adult guinea pigs. The aim of this study was to investigate the effects of CFB on spontaneous, non-nutritive swallowing (NNS) frequency and NNS-respiration coordination in the neonatal period. Seven CFB lambs and seven control lambs aged 2+/-1 days were chronically instrumented for recording electroencephalogram, eye movements, diaphragm EMG, thyroarytenoid muscle EMG, nasal airflow and electrocardiogram. Polysomnographic recordings were performed in non-sedated lambs, using radiotelemetry transmission. CFB lambs spent more time in active sleep than controls (p=0.02). Frequency of non-nutritive swallowing was not different in CFB and control lambs, whatever the state of alertness. In addition, CFB did not disrupt the overall respiratory-swallowing coordination, inspiratory-related NNS being the most frequent and expiratory-related NNS the least in both CFB and control lambs. Further analyses revealed that CFB had no effect on baseline respiratory and heart rate, and apnea and sigh frequency, whatever the state of alertness. Our results suggest that, in the neonatal period, C-fibers are not involved in NNS frequency and have no influence on the overall respiratory-swallowing coordination.

Development of mechanics and pulmonary reflexes

The mechanical properties of the respiratory system are paramount in converting neural output into ventilation. The highly compliant chest wall of the newborn results in chest distortion and volume loss during inspiration and, as the chest is also unable to resist the inward recoil of the lung, there is a reduction in lung volume at end expiration (functional residual capacity) and a tendency for alveoli to collapse. Vagal innervation of the lungs and airways is responsible for eliciting various reflexes that result in the dynamic modification of respiratory mechanics and an improvement in ventilation. From the first breath, the newborn increases the frequency of augmented breaths to improve lung compliance and prolongs the expiratory time constant in order to increase the amount of air remaining in the lung at end expiration and help prevent lung collapse. This review examines the respiratory mechanics of the mammalian neonate at birth and during early development together with the vagal reflexes that are responsible for the dynamic modification of respiratory mechanics in order to ensure that effective gas exchange occurs from birth.

Development of adaptive behaviour of the respiratory network: implications for the pontine Kolliker-Fuse nucleus

Breathing is constantly modulated by afferent sensory inputs in order to adapt to changes in behaviour and environment. The pontine respiratory group, in particular the Kolliker-Fuse nucleus, might be a key structure for adaptive behaviours of the respiratory network. Here, we review the anatomical connectivity of the Kolliker-Fuse nucleus with primary sensory structures and with the medullary respiratory centres and focus on the importance of pontine and medullary postinspiratory neurones in the mediation of respiratory reflexes. Furthermore, we will summarise recent findings from our group regarding ontogenetic changes of respiratory reflexes (e.g., the diving response) and provide evidence that immaturity of the Kolliker-Fuse nucleus might account in neonates for a lack of plasticity in sensory evoked modulations of respiratory activity. We propose that a subpopulation of neurones within the Kolliker-Fuse nucleus represent command neurones for sensory processing which are capable of initiating adaptive behaviour in the respiratory network. Recent data from our laboratory suggest that these command neurones undergo substantial postnatal maturation.

Respiratory responses to intermittent hypoxia in unsedated piglets: relation to substance P binding in brainstem

Respiratory responses to single intermittent hypoxia (5 min 21% O(2), 5 min 8% O(2) X6) in 5-6, 10-11, 21-22 and 26-27 day-old piglets, and to recurrent six daily intermittent hypoxia in 10-11 and 26-27 day-old piglets were assessed. Substance P binding in the piglets' brainstem immediately after the last hypoxic episode was measured. All piglets hyperventilated during hypoxia. Weight adjusted inspired ventilation, tidal volume and instantaneous flow decreased with age. The oldest piglets uniquely displayed attenuated ventilation and tidal volume during the sixth versus first hypoxic episode with single intermittent hypoxia, and reduced inspired ventilation and tidal volume during the first hypoxic episode on the sixth daily hypoxia compared to single hypoxia. By contrast, substance P binding was greatly reduced in the solitary, hypoglossal, paraambigual and lateral reticular brainstem nuclei of both younger and older piglets following either single or recurrent intermittent hypoxia. Thus, the reduction in membrane-bound neurokinin receptors by intermittent hypoxia, presumably consequent to endogenously released substance P, does not exclusively determine whether the ventilatory response to that hypoxia will be attenuated or not.