An inexpensive servo-respirator based upon regulation of a shunt resistance

Phasic pulmonary stretch receptor feedback modulates both eupnea and gasping in an in situ rat preparation

The perfused in situ juvenile rat preparation produces patterns of phrenic discharge comparable to eupnea and gasping in vivo. These ventilatory patterns differ in multiple aspects, including most prominently the rate of rise of inspiratory activity. Although we have recently demonstrated that both eupnea and gasping are similarly modulated by a Hering-Breuer expiratory-promoting reflex to tonic pulmonary stretch, it has generally been assumed that gasping was unresponsive to afferent stimuli from pulmonary stretch receptors. In the present study, we recorded eupneic and gasplike efferent activity of the phrenic nerve in the in situ juvenile rat perfused brain stem preparation, with and without phrenic-triggered phasic pulmonary inflation. We tested the hypothesis that phasic pulmonary inflation produces reflex responses in situ akin to those in vivo and that both eupnea and gasping are similarly modulated by phasic pulmonary stretch. In eupnea, we found that phasic pulmonary inflation decreases inspiratory burst duration and the period of expiration, thus increasing burst frequency of the phrenic neurogram. Phasic pulmonary inflation also decreases the duration of expiration and increases the burst frequency during gasping. Bilateral vagotomy eliminated these changes. We conclude that the neural substrate mediating the Hering-Breuer reflex is retained in the in situ preparation and that the brain stem circuitry generating the respiratory patterns respond to phasic activation of pulmonary stretch receptors in both eupnea and gasping. These findings support the homology of eupneic phrenic discharge patterns in the reduced in situ preparation and eupnea in vivo and disprove the common supposition that gasping is insensitive to vagal afferent feedback from pulmonary stretch receptor mechanisms.

Laryngeal muscle response to phasic and tonic upper airway pressure and flow

The hypothesis that respiratory modulation due to upper airway (UA) pressure and flow is dependent on stimulus modality and respiratory phase-specific activation was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. Negative pressure and flow applied to the isolated UA at room or body temperature during inspiration only enhanced posterior cricoarytenoid muscle activity from that present without UA pressure and flow (baseline) by 15--20%. Time shifting the onset of UA flow relative to tracheal flow decreased this enhancement. The same enhancement was observed with oscillatory or constant airflow. UA positive pressure and flow at room or body temperature applied during expiration only enhanced thyroarytenoid muscle activity from baseline by 50--160%. The same enhancement was observed with oscillatory or constant airflow at body temperature. Constant positive pressure and flow enhanced thyroarytenoid muscle activity more than oscillatory pressure and flow at room temperature. We conclude that the respiratory modulation of UA afferents is processed in a phase-specific fashion and is dependent on stimulus modality (tonic vs. phasic).

Modulation of laryngeal and respiratory pump muscle activities with upper airway pressure and flow

The hypothesis that upper airway (UA) pressure and flow modulate respiratory muscle activity in a respiratory phase-specific fashion was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. We generated negative pressure and inspiratory flow in phase with tracheal inspiration or positive pressure and expiratory flow in phase with tracheal expiration in the isolated UA. Stimulation of UA negative pressure receptors with body temperature air resulted in a 10--15% enhancement of phasic moving-time-averaged posterior cricoarytenoid electromyographic (EMG) activity above tonic levels obtained without pressure and flow in the UA (baseline). Stimulation of UA positive pressure receptors increased phasic moving-time-averaged thyroarytenoid EMG activity above tonic levels by 45% from baseline. The same enhancement of posterior cricoarytenoid or thyroarytenoid EMG activity was observed with the addition of flow receptor stimulation with room temperature air. Tidal volume and diaphragmatic and abdominal muscle activity were unaffected by UA flow and/or pressure, whereas respiratory timing was minimally affected. We conclude that laryngeal afferents, mainly from pressure receptors, are important in modulating the respiratory activity of laryngeal muscles.

Influence of lung volume on respiratory responses to spontaneous bladder contractions

Spontaneous bladder contractions (SBCs) in decerebrate, vagotomized, paralyzed, ventilated cats have been shown to decrease phrenic and hypoglossal inspiratory nerve activities, as well as the activities of other respiratory motor nerves. To determine whether vagal afferents from the lung influence the respiratory inhibition associated with SBCs, we recorded phrenic and hypoglossal nerve activities in decerebrate, paralyzed, vagally intact cats. The animals were ventilated by a servo-respirator, which inflated the lungs in accordance with integrated phrenic nerve activity. Maintained increases in end-expiratory lung volume were produced by the application of 2-10 cm H2O positive end-expiratory pressure (PEEP). SBCs were accompanied by decreases in both phrenic and hypoglossal peak integrated nerve activities, as well as by marked decreases in respiratory frequency. The reduction of respiratory frequency was greater with higher levels of PEEP, a few animals becoming apneic during SBCs. After bilateral vagotomy, SBCs continued to decrease phrenic and hypoglossal peak integrated nerve activities as previously reported, but the reduction of respiratory frequency was much less striking than when the vagi were intact. These results indicate that activity of vagal afferents from the lung augments the respiratory influence of SBCs. Furthermore, SBCs in vagally intact animals can induce periodic breathing.

Alteration of ventilatory activity by intralaryngeal CO2 in the cat

1. We investigated the responses of phrenic and hypoglossal nerve activities to the addition of 3, 5 and 10% CO2 to a constant flow of warm, humidified air through the isolated upper airway in decerebrate, paralysed, artificially ventilated cats. 2. In bilaterally vagotomized animals, intralaryngeal CO2 caused a dose-related decrease in peak integrated phrenic activity. This response became attenuated with time, but was still discernible after 3 min of continuous intralaryngeal CO2. In the same experiments, intralaryngeal CO2 caused a gradual increase in peak integrated hypoglossal nerve activity. 3. Intermittent pulsing of intralaryngeal CO2 during neural inspiration or expiration resulted in similar, but smaller decreases in the phrenic activity of some animals. Hypoglossal activity was not influenced appreciably by this procedure. 4. Systemic hypercapnia attenuated the phrenic responses to intralaryngeal CO2. The hypoglossal responses were greatly reduced or abolished. 5. In vagally intact cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, intralaryngeal CO2 resulted in only a trace of reduction in phrenic discharge. After bilateral vagotomy, the same animals showed typical responses, as described above. 6. All responses to intralaryngeal CO2 were abolished after bilateral section of the superior laryngeal nerves (SLNs). 7. We conclude that intralaryngeal CO2 acts by way of receptors with afferents in the SLNs to decrease phrenic and increase hypoglossal nerve activities. The responses are not importantly gated during neural inspiration or expiration. The responses to intralaryngeal CO2 are most clearly demonstrable after bilateral vagotomy, suggesting that vagal mechanisms serve to stabilize respiratory motor neural activity in intact animals.

Discharge of vagal pulmonary receptors differentially alters neural activities during various stages of expiration in the cat

1. The purpose was to evaluate the hypothesis that neural expiration is composed of two phases: I, a post inspiratory period; and II, the period at which expiratory activities of spinal nerves reach peak values. We hypothesized that the discharge of pulmonary stretch receptors might differentially alter neural activities during these two phases. 2. Activities of the phrenic nerve, intercostal nerve and nerves innervating the thyroarytenoid muscle of the larynx and triangularis sterni muscle of the chest wall were recorded in decerebrate and paralysed cats. 3. The experimental animals were ventilated with a servo-respirator which produced changes in tracheal pressure, and lung volume, in parallel with alterations in integrated activity of the phrenic nerve. 4. In order to assess the influence of the discharge of slowly adapting pulmonary stretch receptors upon neural activities during expiration, lung volume was held at end-expiratory or end-inspiratory levels for individual respiratory cycles. 5. When pulmonary inflation was prevented, phrenic activity increased, as did activity of the thyroarytenoid nerve during early expiration. In contrast, activities of the triangularis sterni and intercostal nerves during mid- to late expiration declined. 6. Holding the lungs at end-inspiratory levels caused a reduction of thyroarytenoid activity and increases in peak triangularis sterni and intercostal activities. Neural expiration typically continued as long as the lungs were maintained at the end-inspiratory level. 7. Responses were qualitatively similar in hypocapnia, normocapnia and hypercapnia, but the magnitude of changes in neural activities was typically augmented with elevations in end-tidal fractional concentrations of CO2. 8. We conclude that the discharge of slowly adapting pulmonary stretch receptors inhibits neural activities during early expiration and augments activities during mid-to late expiration. Hence, our data support the concept that neural expiration is composed of two stages in which neural activities may be differentially controlled.

Reflex inhibition of crural diaphragmatic activity by esophageal distention in cats

Distention of the esophagus has been shown to result in selective inhibition of phasic inspiratory activity in the crural portion of the diaphragm, with no effect on costal diaphragmatic activity. The purpose of this study was to determine rigourously the afferent pathways that mediate this response. Bipolar EMG electrodes were placed in the costal and crural portions of the diaphragm in decerebrate, spontaneously breathing cats. Distention of the esophagus by inflation of a Foley catheter balloon with 20 ml of air resulted in a selective inhibition of crural hiatal EMG activity, while costal EMG activity was maintained at predistention levels. The distention was accompanied by a reduction in respiratory frequency. Transection of the spinal cord at the C8-T1 level did not obliterate the crural inhibition produced by inflation. Section of the C4-C8 dorsal roots also failed to abolish the response. However, after bilateral cervical vagotomy, esophageal distention no longer influenced diaphragmatic EMG activity. These results indicate that the crural inhibition observed with esophageal distention is vagally mediated and is not influenced importantly by intercostal or phrenic afferents. Records of activity of the phrenic nerve branch innervating the crural portion of the diaphragm showed a similar response pattern, confirming that the inhibition is central in origin and that the crural fibers inhibited by distention are only a fraction of the total population of crural phrenic motoneurons.

Characterization of respiratory-related activity of the facial nerve

Activities of the facial, hypoglossal and phrenic nerves were recorded in decerebrate and paralyzed cats. These animals were ventilated with a servo-respirator which produced lung inflations in parallel with phrenic activity. Peak inspiratory phrenic, hypoglossal and facial activities increased in hypercapnia or hypoxia. When pulmonary inflation was prevented, hypoglossal and facial activities increased more than phrenic. Responses to withholding lung inflation differed from those following vagotomy. These differences were observed in expiratory facial and hypoglossal activities and in hypercapnia- and hypoxia-induced changes in facial activity. Administration of pentobarbital or hyperventilation to hypocapnia caused greater suppressions of hypoglossal than facial activity; the latter declined more than phrenic activity. The results support the hypothesis that influences from the brainstem reticular formation and from pulmonary stretch receptors are differentially distributed to motoneurons innervating upper airway muscles compared to those of the bulbospinal-phrenic system. The concept that ventilatory activity is influenced by tonic, as well as phasic discharge of pulmonary receptors is discussed.

Respiratory-modulated activities of motor units of the facial nerve

The purpose of this work was to characterize the influence of activity of vagal pulmonary receptors upon the discharge pattern of motor units of the facial nerve. Decerebrate and paralyzed cats were ventilated with a servo-respirator which produced pulmonary inflations in parallel with activity of the phrenic nerve. At normocapnia, facial units discharged phasically during neural inspiration, expiration or across both phases or discharged tonically throughout the respiratory cycle. When pulmonary inflation was withheld, the tonic discharge of some units became phasic; others changed the pattern of phasic discharge. In hypercapnia, the number of tonic fiber activities increased and, again, some phasic discharge patterns were altered. Withholding inflation caused similar alterations as in normocapnia. Activities of facial fibers in vagotomized animals differed in that no tonic activities were recorded, and no change in phasic discharge patterns was induced by hypercapnia. We conclude that afferents from pulmonary stretch receptors influence ventilatory activity throughout the entire respiratory cycle. The concept is discussed that the tonic, as well as phasic discharge of these receptors, is important for the regulation of activity of motoneurons to upper airway muscles.

Influence of lung volume on phrenic, hypoglossal and mylohyoid nerve activities

In decerebrate, paralyzed cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, we examined the influence of lung volume on the activities of the phrenic, hypoglossal and mylohyoid nerves. When lung inflation was briefly withheld, the durations of inspiration (TI) and expiration (TE) and the activities of all three nerves increased. The relative increase in hypoglossal activity greatly exceeded that of phrenic activity and was apparent earlier in the course of inspiration. This hypoglossal response was enhanced by hypercapnia and isocapnic hypoxia. The responses of mylohyoid activity were quite variable: withholding lung inflation augmented inspiratory activity in some cats, but expiratory discharge in others. Sustained increases in end-expiratory lung volume were induced by application of 3-4 cm H2O of positive end-expiratory pressure (PEEP). Steady-state PEEP did not influence nerve activities or the breathing pattern. Bilateral vagotomy increased TI, TE, and the activities of all three nerves. No response to withoholding lung inflation could be discerned after vagal section. The results provide further definition of the influence of vagally mediated, lung volume dependent reflexes on the control of upper airway muscles. These reflexes are well suited to relieve or prevent upper airway obstruction.