Functional mechanism of expiratory pattern generator

Tidal expiratory activity and the time to its peak (TEa, i.e., time of active expiration) were measured from the integration curve of external oblique muscle activity in rabbits. The rabbits were spontaneously breathing under urethan-chloralose anesthesia in the prone position. CO2 rebreathing was performed, and the changes in the temporal profile of the integrated expiratory activity were studied. Hypercapnia increased the tidal expiratory activity before and after bilateral vagotomy. Before vagotomy, the rate of rise (tidal expiratory activity/TEa) increased significantly and TEa was shortened. On the other hand, after vagotomy, the rate of rise remained unchanged and TEa was prolonged. An expiratory off-switch model is postulated according to the quantitative relationship between the time and amplitude of the tidal expiratory activity. The expiratory activity pattern in vagotomized rabbits was similar to that in humans.     

Respiratory responses in reversible diaphragm paralysis

The effects of diaphragm paralysis on respiratory activity were assessed in 13 anesthetized, spontaneously breathing dogs studied in the supine position. Transient diaphragmatic paralysis was induced by bilateral phrenic nerve cooling. Respiratory activity was assessed from measurements of ventilation and from the moving time averages of electrical activity recorded from the intercostal muscles and the central end of the fifth cervical root of the phrenic nerve. The degree of diaphragm paralysis was evaluated from changes in transdiaphragmatic pressure and reflected in rib cage and abdominal displacements. Animals were studied both before and after vagotomy breathing O2, 3.5% CO2 in O2, or 7% CO2 in O2. In dogs with intact vagi, both peak and rate of rise of phrenic and inspiratory intercostal electrical activity increased progressively as transdiaphragmatic pressure fell. Tidal volume decreased and breathing frequency increased as a result of a shortening in expiratory time. Inspiratory time and ventilation were unchanged by diaphragm paralysis. These findings were the same whether O2 or CO2 in O2 was breathed. After vagotomy, no significant change in phrenic or inspiratory intercostal activity occurred with diaphragm paralysis in spite of increased arterial CO2 partial pressure. Ventilation and tidal volume decreased significantly, and respiratory timing was unchanged. These results suggest that mechanisms mediated by the vagus nerves account for the compensatory increase in respiratory electrical activity during transient diaphragm paralysis. That inspiratory time is unchanged by diaphragm paralysis whereas the rate or rise of phrenic nerve activity increases suggest that reflexes other than the Hering-Breuer reflex contribute to the increased respiratory response.     

Effects of expiratory loading on respiration in humans

We examined the effects of expiratory resistive loads of 10 and 18 cmH2O.l-1.s in healthy subjects on ventilation and occlusion pressure responses to CO2, respiratory muscle electromyogram, pattern of breathing, and thoracoabdominal movements. In addition, we compared ventilation and occlusion pressure responses to CO2 breathing elicited by breathing through an inspiratory resistive load of 10 cmH2O.l-1.s to those produced by an expiratory load of similar magnitude. Both inspiratory and expiratory loads decreased ventilatory responses to CO2 and increased the tidal volume achieved at any given level of ventilation. Depression of ventilatory responses to Co2 was greater with the larger than with the smaller expiratory load, but the decrease was in proportion to the difference in the severity of the loads. Occlusion pressure responses were increased significantly by the inspiratory resistive load but not by the smaller expiratory load. However, occlusion pressure responses to CO2 were significantly larger with the greater expiratory load than control. Increase in occlusion pressure observed could not be explained by changes in functional residual capacity or chemical drive. The larger expiratory load also produced significant increases in electrical activity measured during both inspiration and expiration. These results suggest that sufficiently severe impediments to breathing, even when they are exclusively expiratory, can enhance inspiratory muscle activity in conscious humans.     

Effects of hypercapnia on inspiratory and expiratory muscle activity during expiration

Persistence of inspiratory muscle activity during the early phase of expiratory airflow slows the rate of lung deflation, whereas heightened expiratory muscle activity produces the opposite effect. To examine the influence of increased chemoreceptor drive and the role of vagal afferent activity on these processes, the effects of progressive hypercapnia were evaluated in 12 anesthetized tracheotomized dogs before and after vagotomy. Postinspiratory activity of inspiratory muscles (PIIA) and the activity of expiratory muscles were studied. During resting breathing, the duration of PIIA correlated with the duration of inspiration but not with expiration. Parasternal intercostal PIIA was directly related to that of the diaphragm. Based on their PIIA, dogs could be divided into two groups: one with prolonged PIIA (mean 0.57 s) and the other with brief PIIA (mean 0.16 s). Hypercapnia caused progressive shortening of the PIIA in the dogs with prolonged PIIA during resting breathing. The electrical activity of the external oblique and internal intercostal muscles increased gradually during CO2 rebreathing in all dogs both pre- and postvagotomy. After vagotomy, abdominal activity continued to increase with hypercapnia but was less at all levels of PCO2. The internal intercostal response to hypercapnia was not affected by vagotomy. The combination of shorter PIIA and augmented expiratory activity with hypercapnia might, in addition to changes in lung recoil pressure and airway resistance, hasten exhalation.     

Inspiratory neuron activity in the ventrolateral medulla of the dog

The purpose of this study was to describe the distribution and activity pattern of respiratory neurons located in the ventrolateral medulla (VLM) of the dog. Spike activity of 129 respiratory neurons was recorded in 23 ketamine-anesthetized spontaneously breathing dogs. Pontamine blue dye was used to mark the location of each neuron. Most VLM neurons displaying respiratory related spike patterns were located in a column related closely to ambigual and retroambigual nuclei. Both inspiratory and expiratory neurons were present with inspiratory units being grouped more rostrally. The predominant inspiratory neuron firing pattern was "late" inspiratory, although eight "early" types were located. All expiratory firing patterns were the late expiratory variety. Each neuron burst pattern was characterized by determining burst duration (BD), spikes per burst (S/B), peak frequency (PF), time to peak frequency (TPF), rate of rise to peak frequency (PF/TPF), and mean frequency. CO2-induced minute ventilation increases were associated with decreases in BD and TPF and increases in PF, S/B, and PF/TPF. In 11 experiments the relative influences of vagotomy and tracheal occlusion on late inspiratory units were compared. Tracheal occlusion increased late inspiratory BD and S/B but did not alter PF/TPF. Vagotomy increased BD and S/B beyond those obtained by tracheal occlusion and, in some neurons, decreased the PF/TPF. We conclude that the location of respiratory units in the VLM of the dog is similar to that in other species, the discharge pattern of VLM respiratory units is similar to those in cat VLM, and vagotomy and tracheal occlusion affect discharge patterns differently.     

Abdominal muscle use during breathing in unanesthetized dogs

The pattern of abdominal muscle use during breathing in unanesthetized dogs is unknown. Therefore, we have recorded the electromyograms of the rectus abdominis, external oblique, and transversus abdominis in eight conscious animals breathing quietly in the sitting, standing, and prone postures. During quiet breathing in the sitting posture, all animals invariably had a large amount of phasic expiratory activity in the transversus abdominis. In contrast, only four animals showed some expiratory activity in the external oblique, and only one animal had expiratory activity in the rectus abdominis. A similar pattern was observed when the animals were standing or lying prone, although the amount of expiratory activity was less in this posture. Bilateral cervical vagotomy in four animals did not affect the degree of transversus abdominis expiratory activation or the influence of posture. We conclude that in conscious dogs 1) the abdominal muscles play an important role during breathing and make spontaneous quiet expiration a very active process, 2) the transversus abdominis is the primary respiratory muscle of the abdomen, and 3) unlike in anesthetized animals, extrapulmonary receptors play a major role in promoting abdominal expiratory contraction.     

The breathing pattern after breath-holding: the influence of chest position and the drive to breathe.

The pattern of breathing following the breaking-point of sixty breath-holds has been studied in five healthy adults and compared with the pattern during recovery from CO2-rebreathing. The volume and direction of the first respiratory movement, and the VT, V relation for the first four complete breaths was measured. Only when breath-holds were terminated with an inspiration was the accumulated drive to breathe reflected in an increased volume of the first respiratory movement: terminating expirations simply returned the chest to the resting respiratory level. The volume of the first inspiration was not influenced by the intervention of a terminating expiration, suggesting that expiratory movements do not dissipate the non-chemical component of the drive to breathe. In three of the five subjects the tidal volumes for given levels of ventilation were greater following breath-holding than following rebreathing. This altered pattern of breathing has been interpreted in terms of an insiratory-augmenting reflex.     

Pattern of breathing in the rat.

The basic ventilation values - tidal volume (VT), breathing frequency (f), minute ventilation (VE) and the duration of inspiration (TI) and expiration (TE) -- were determined in adult male rats. The range of these values is given and the pattern of breathing is defined as the relationship between VE and VT, which in the rat is linear throughout its entire range. The role of TI and TE in changing f in the rat were evaluated. The breathing pattern of the rat was compared with data for the rabbit and man, using percentual expression of the basic values. A shift of the breathing pattern to higher f values was observed in rats with experimental lung diseases. In these rats, the inhalation of 100% O2 shifted the pattern of breathing markedly to lower VE values, though not to values comparable with the controls. Bilateral cervical vagotomy was followed by a pronouced decrease in f, an increase in VT and T1 persisted even after vagotomy, however; it can be assumed that this relationship is effected either by means of receptors in the chest muscles, or by the direct action of CO2 which is used to stimulate breathing, on the bulbopontine pacemaker.     

Interaction between upper airway negative pressure pulses and CO2 on breathing pattern

The interaction between CO2 and negative pressure pulses on breathing pattern was investigated in 10 anesthetized, spontaneously breathing rabbits. The upper airway was functionally isolated into a closed system. A servo-respirator triggered by the inspiratory activity of the diaphragm was used to apply pressure pulses of -15 cmH2O to the isolated upper airway in early inspiration while the animal was breathing room air, 100% O2, 6% CO2 in O2, or 9% CO2 in O2. The negative pressure pulses produced a reversible inhibition of inspiration in most trials with resultant increase in inspiratory duration (TI); no change was observed in peak diaphragmatic electromyogram (Dia EMG) or expiratory duration, whereas a decrease was seen in mean inspiratory drive (peak Dia EMG/TI). This prolongation of inspiratory duration and decrease in mean inspiratory drive with negative pressure pulses persisted at higher levels of CO2; the slopes of the test breaths were not significantly different from that of control breaths. These results suggest that upper airway negative pressure pulses are equally effective in altering the breathing pattern at all levels of CO2.     

Breathing pattern and CO2 response in newborn rats before and during anesthesia

We have studied the breathing pattern (minute ventilation VE, tidal volume VT, and respiratory rate f) in newborn rats before and during barbiturate (20-30 mg/kg ip) or ketamine anesthesia (40-80 mg/kg ip). Animals were intact and prone in a flow plethysmograph in thermoneutral conditions. Before anesthesia, CO2 breathing (5 min in 5% and 5 min in 10% CO2 in O2) resulted in a substantial increase in VE (169 and 208%, respectively), which was maintained throughout the entire CO2 breathing period. This indicates that, despite the extremely large VE per kilogram at rest, in these small animals there is still a large reserve for a sustained increase in VE. During barbiturate, the resting VE dropped to 45% of control, due to a reduction in VT (83%) and f (59%). This latter result was due to a prolongation of the expiratory time (214%) with no significant changes in inspiratory time. CO2 response was also much depressed, to approximately 63% of the control. The late portion of the expiratory flow-volume curves, the slope of which represents the expiratory time constant of the system, was similar before and during anesthesia in approximately 50% of the animals, whereas it increased during anesthesia in the remaining animals. Although compliance of the respiratory system was generally unaltered, the increased impedance during anesthesia probably reflected an increased resistance. Qualitatively similar results were obtained during ketamine anesthesia. Therefore, as observed in adult mammals, anesthesia in newborn rats has a marked depressant effect on resting breathing pattern and CO2 response, occasionally accompanied by an increase in the expiratory impedance of the respiratory system.     

Reflex effect of esophageal distension on respiratory muscle activity and pressure

The electrical activity of the respiratory skeletal muscles is altered in response to reflexes originating in the gastrointestinal tract. The present study evaluated the reflex effects of esophageal distension (ED) on the distribution of motor activity to both inspiratory and expiratory muscles of the rib cage and abdomen and the resultant changes in thoracic and abdominal pressure during breathing. Studies were performed in 21 anesthetized spontaneously breathing dogs. ED was produced by inflating a balloon in the distal esophagus. ED decreased the activity of the costal and crural diaphragm and external intercostals and abolished all preexisting electrical activity in the expiratory muscles of the abdominal wall. On the other hand, ED increased the activity of the parasternal intercostals and expiratory muscles located in the rib cage (i.e., triangularis sterni and internal intercostal). All effects of ED were graded, with increasing distension exerting greater effects, and were eliminated by vagotomy. The effect of increases in chemical drive and lung inflation reflex activity on the response to ED was examined by performing ED while animals breathed either 6.5% CO2 or against graded levels of positive end-expiratory pressure (PEEP), respectively. Changes in respiratory muscle electrical activity induced by ED were similar (during 6.5% CO2 and PEEP) to those observed under control conditions. We conclude that activation of mechanoreceptors in the esophagus reflexly alters the distribution of motor activity to the respiratory muscles, inhibiting the muscles surrounding the abdominal cavity and augmenting the parasternals and expiratory muscles of the chest wall.     

Importance of vagal afferents in determining ventilation in newborn rats

We studied the effect of acute bilateral vagotomy on ventilation and ventilatory pattern in rats. In 1- to 6-day-old unanesthetized rats, vagotomy resulted in a substantial decrease (38%) in ventilation during air breathing. After vagotomy there was a threefold increase in tidal volume (VT), inspiratory time (TI) doubled, and expiratory time (TE) was six times longer. When studied under isoflurane anesthesia, newborn rats showed decreases in ventilation similar to that observed without anesthesia, whereas anesthetized adult rats had no consistent changes in ventilation. Adult and newborn rats had nearly identical proportionate increases in VT and TI after vagotomy, but TE lengthened to a greater extent in the newborns. Additionally, we demonstrated a significant decrease in ventilation when 100% O2 rather than air was supplied to nonvagotomized unanesthetized newborn rats. Ventilation decreased by 19% after vagotomy under hyperoxic conditions. We conclude that vagal afferent input, probably of pulmonary mechanoreceptor origin, provides positive feedback to respiration in newborn rats and that newborn rats greater than 24 h old also have a degree of peripheral chemoreceptor drive during air breathing.     

Effects of bronchoconstriction on respiratory muscle activity during expiration

The effect of methacholine-induced bronchoconstriction on the electrical activity of respiratory muscles during expiration was studied in 12 anesthetized spontaneously breathing dogs. Before and after aerosols of methacholine, diaphragm, parasternal intercostal, internal intercostal, and external oblique electromyograms were recorded during 100% O2 breathing and CO2 rebreathing. While breathing 100% O2, five dogs showed prolonged electrical activity of the diaphragm and parasternal intercostals in early expiration, postinspiratory inspiratory activity (PIIA). Aerosols of methacholine increased pulmonary resistance, decreased tidal volume, and elevated arterial PCO2. During bronchoconstriction, when PCO2 was varied by CO2 rebreathing, PIIA was shorter at low levels of PCO2, and external oblique and internal intercostal were higher at all levels of PCO2. Vagotomy shortened PIIA in dogs with prolonged PIIA. After vagotomy, methacholine had no effects on PIIA but continued to increase external oblique and internal intercostal activity at all levels of PCO2. These findings indicate that bronchoconstriction influences PIIA through a vagal reflex but augments expiratory activity, at least in part, by extravagal mechanisms.     

Intrapulmonary Co2 receptors and ventilatory response to lung Co2 loading

The possible role of intrapulmonary CO2 receptors (IPC) in arterial CO2 partial pressure (PaCO2) homeostasis was investigated by comparing the arterial blood gas and ventilatory responses to CO2 loading via the inspired gas and via the venous blood. Adult male Pekin ducks were decerebrated 1 wk prior to an experiment. Venous CO2 loading was accomplished with a venovenous extracorporeal blood circuit that included a silicone-membrane blood oxygenator. The protocol randomized four states: control (no loading), venous CO2 loading, inspired CO2 loading, and venous CO2 unloading. Intravenous and inspired loading both resulted in hypercapnic hyperpnea. Comparison of the ventilatory sensitivity (delta VE/delta PaCO2) showed no significant difference between the two loading regimes. Likewise, venous CO2 unloading led to a significant hypocapnic hypopnea. Sensitivity to changes in PaCO2 could explain the response of ventilation under these conditions. The ventilatory pattern, however, was differentially sensitive to the route of CO2 loading; inspired CO2 resulted in slower deeper breathing than venous loading. It is concluded that IPC play a minor role in adjusting ventilation to match changes in pulmonary CO2 flux but rather are involved in pattern determination.     

Mechanical role of expiratory muscles during breathing in prone anesthetized dogs

The purpose of the present study was to assess the mechanical role of the expiratory muscles during spontaneous breathing in prone animals. The electromyographic (EMG) activity of the triangularis sterni, the rectus abdominis, the external oblique, and the transversus abdominis was studied in 10 dogs light anesthetized with pentobarbital sodium. EMGs were recorded during spontaneous steady-state breathing in supine and prone suspended animals both before and after cervical vagotomy. We also measured the end-expiratory lung volume [functional residual capacity (FRC)] in supine and prone positions to assess the mechanical role of expiratory muscle activation in prone dogs. Spontaneous breathing in the prone posture elicited a significant recruitment of the triangularis sterni, the external oblique, and the transversus abdominis (P less than 0.05). Bilateral cervical vagotomy eliminated the postural activation of the external oblique and the transversus abdominis but not the triangularis sterni. Changes in posture during control and after cervical vagotomy were associated with an increase in FRC. However, changes in FRC, on average, were 132.3 +/- 33.8 (SE) ml larger (P less than 0.01) postvagotomy. We conclude that spontaneous breathing in prone anesthetized dogs is associated with a marked phasic expiratory recruitment of rib cage and abdominal muscles. The present data also indicate that by relaxing at end expiration the expiratory muscles of the abdominal region are directly responsible for generating roughly 40% of the tidal volume.     

Human breathing patterns on mouthpiece or face mask during air, CO2, or low O2

Steady-state breathing patterns on mouthpiece and noseclip (MP) and face mask (MASK) during air and chemostimulated breathing were obtained from pneumotachometer flow. On air, all 10 subjects decreased frequency (f) and increased tidal volume (VT) on MP relative to that on MASK without changing ventilation (VE), mean inspiratory flow (VT/TI), or mean expiratory flow (VT/TE). On elevated CO2 and low O2, MP exaggerated the increase in VE, f, and VT/TE due to profoundly shortened TE. On elevated CO2, MASK exaggerated VT increase with little change in f. Increased VE and VT/TI were thus due to increased VT. During low O2 on MASK, both VT and f increased. During isocapnia, shortened TE accounted for increased f; during hypocapnia, increased f was related primarily to shortened TI. Thus the choice of a mouthpiece or face mask differentially alters breathing pattern on air and all components of ventilatory responses to chemostimuli. In addition, breathing apparatus effects are not a simple consequence of a shift from oronasal to oral breathing, since a noseclip under the mask did not change breathing pattern from that on mask alone.     

Cervical spinal cord injury alters the pattern of breathing in anesthetized rats.

The mechanisms by which chronic cervical spinal cord injury alters respiratory function and plasticity are not well understood. We speculated that spinal hemisection at C(2) would alter the respiratory pattern controlled by vagal mechanisms. Expired volume (V(E)) and respiratory rate (RR) were measured in anesthetized control and C(2)-hemisected rats at 1 and 2 mo postinjury. C(2) hemisection altered the pattern of breathing at both postinjury time intervals. Injured rats utilized a higher RR and lower V(E) to maintain the same minute ventilation as control rats. After bilateral vagotomy, the pattern of breathing in injured rats was not different from controls. The frequency of augmented breaths was higher in injured rats at 2 mo postinjury before vagotomy; however, the V(E) of augmented breaths was not different between groups. In conclusion, C(2) hemisection alters the pattern of breathing at 1 and 2 mo postinjury via vagal mechanisms.     

Calcium and avian intrapulmonary chemoreceptor response to CO2.

Intrapulmonary chemoreceptors (IPC) are highly responsive respiratory chemoreceptors that innervate the lungs of birds and diapsid reptiles. IPC are stimulated by low levels of lung Pco(2), inhibited by high levels of lung Pco(2), and their vagal afferents serve as a sensory limb for reflex adjustments of breathing depth and rate. Most IPC exhibit both phasic and tonic sensitivity to CO(2), and spike frequency adaptation (SFA) contributes to their phasic CO(2) responsiveness. To test whether CO(2) responsiveness and SFA in IPC is modulated by a Ca(2+)-linked mechanism, we quantified the role of transmembrane Ca(2+) fluxes and Ca(2+)-related channels on single-unit IPC function in response to phasic changes in inspired Pco(2). We found that 1) broad-spectrum blockade of Ca(2+) channels using cadmium or cobalt and blockade of L-type Ca(2+) channels using nifedipine increased IPC discharge; 2) activation of L-type Ca(2+) channels using BAY K 8644 reduced IPC discharge; 3) blockade of Ca(2+)-activated potassium channels using charybdotoxin (antagonist of large-conductance Ca(2+)-dependent K(+) channel) increased IPC discharge, but neither charybdotoxin nor apamin affected SFA; and 4) blockade of chloride channels, including Ca(2+)-activated chloride channels, with niflumic acid decreased IPC discharge at low Pco(2) and increased IPC discharge at high Pco(2), resulting in a net attenuation of the IPC CO(2) response. We conclude that Ca(2+) influx through L-type Ca(2+) channels has an inhibitory effect on IPC afferent discharge and CO(2) sensitivity, that spike frequency adaptation is not due to apamin- or charybdotoxin-sensitive Ca(2+)-activated K(+) channels in IPC, and that chloride channels blocked by niflumic acid help modulate IPC CO(2) responses.     

Acute effects of cigarette smoke on breathing in rats: vagal and nonvagal mechanisms

The acute ventilatory response to inhalation of cigarette smoke was studied in anesthetized Sprague-Dawley rats. Cigarette smoke (6 ml, 50%) generated by a machine was inhaled spontaneously via a tracheal cannula. Within the first two breaths of smoke inhalation, a slowing of respiration resulting from a prolonged expiratory duration (173 +/- 6% of the base line; n = 32) was elicited in 88% of the rats studied. This initial inhibitory effect on breathing was not affected either by an increase (410%) in the nicotine content of the cigarette smoke or by pretreatment with hexamethonium (33 mg/kg iv). However, bilateral vagotomy completely eliminated the initial ventilatory inhibition. Cooling both vagi to 5.1 degrees C blocked the reflex apneic response to lung inflation, but it did not abolish the inhibitory effect of smoke. After the initial response, a rapid shallow breathing pattern developed and reached its peak 5-12 breaths after inhalation of high-nicotine cigarette smoke; this delayed response could not be prevented by vagotomy and was undetectable after inhalation of low-nicotine smoke. We conclude that the initial inhibitory effect of smoke on breathing is mediated by vagal bronchopulmonary C-fiber afferents, which are stimulated by smoke constituents other than nicotine, whereas the delayed tachypneic response to smoke is caused by the absorbed nicotine.     

Pulmonary receptors in reptiles: discharge patterns of receptor populations in snakes versus turtles

This study examines the effects of lung inflation/deflation with and without CO2 on the entire population of pulmonary receptors in the vagus nerve in two species of snakes and two species of turtles. We asked the question, "how does the response of the entire mixed population of pulmonary stretch receptors (PSR) and intrapulmonary chemoreceptors (IPC) in species possessing both differ from that in species with only PSR"? This was studied under conditions of artificial ventilation with the secondary goal of extending observations on the presence/absence of IPC to a further three species. Our results indirectly illustrate the presence of IPC in the Burmese python and South American rattlesnake but not the side necked turtle, adding support to the hypothesis that IPC first arose in diapsid reptiles. In both species of snake, CO2-sensitive discharge (presumably from IPC) predominated almost to the exclusion of CO2-insensitive discharge (presumably arising from PSR) while the opposite was true for both species of turtle. The data suggest that for animals breathing air under conditions of normal metabolism there is little to distinguish between the discharge profiles of the total population of receptors arising from the lungs in the different groups. Interestingly, however, under conditions of elevated environmental CO2 most volume-related feedback from the lungs is abolished in the two species of snakes, while under conditions of elevated metabolic CO2, it is estimated that volume feedback from the lungs would be enhanced in these same species.