Studies of the pulmonary vagal control of central respiratory rhythm in the absence of breathing movements

Interaction between the pulmonary stretch receptor and pontine control of expiratory duration

Medial parabrachial nucleus (mPBN) neuronal activity plays a key role in controlling expiratory (E)-duration (TE). Pulmonary stretch receptor (PSR) activity during the E-phase prolongs TE. The aims of this study were to characterize the interaction between the PSR and mPBN control of TE and underlying mechanisms. Decerebrated mechanically ventilated dogs were studied. The mPBN subregion was activated by electrical stimulation via bipolar microelectrode. PSR afferents were activated by low-level currents applied to the transected central vagus nerve. Both stimulus-frequency patterns during the E-phase were synchronized to the phrenic neurogram; TE was measured. A functional mathematical model for the control of TE and extracellular recordings from neurons in the preBötzinger/Bötzinger complex (preBC/BC) were used to understand mechanisms. Findings show that the mPBN gain-modulates, via attenuation, the PSR-mediated reflex. The model suggested functional sites for attenuation and neuronal data suggested correlates. The PSR- and PB-inputs appear to interact on E-decrementing neurons, which synaptically inhibit pre-I neurons, delaying the onset of the next I-phase.

Breathing with the mind: Effects of motor imagery on breath-hold performance

We aimed at studying the effect of Motor Imagery (MI), i.e., the mental representation of a movement without executing it, on breath-holding performance. Classical guidelines for efficient MI interventions advocate for a congruent MI practice with regards to the requirements of the physical performance, specifically in terms of physiological arousal. We specifically aimed at studying whether an incongruent form of MI practice might enhance the breath-hold performance. In a counterbalanced design including three experimental sessions, participants engaged in maximal breath-hold trials while concomitantly performing i) MI of breathing, ii) MI of breath-hold, and iii) an "ecological" breath-hold trial, i.e., without specific instructions of MI practice. In addition to breath-hold durations, we measured the cardiac activity and blood oxygen saturation. Performance was improved during MI of breathing (73.06 s ± 24.53) compared to both MI of breath-hold (70.57 s ± 18.15) and the control condition (67.67 s ± 19.27) (p < 0.05). The mechanisms underlying breath-hold performance improvements during MI of breathing remain uncertain. MI of breathing might participate to decrease the threat perception associated with breath-holding, presumably due to psychological and physiological effects associated with the internal simulation of a breathing body state.

Right and left vagus nerves regulate breathing by multiplicative interaction

Although it has been recognized for more than a century, we still do not know how the two vagus nerves interact to produce Hering-Breuer reflex. In the current study, we tested the hypothesis that the vagus nerves interact via a multiplicative effect. We examined the Hering-Breuer reflex before and after unilateral (first) and then bilateral (second) vagotomies in the mouse. The lung is mostly innervated homolaterally. Since the right and left lung formed 68.2 and 31.8% of total lung weight, if the interaction is mediated by an additive mechanism, unilateral vagotomy would remove the reflex effects by 68.2 and 31.8%, respectively. Instead, unilateral vagotomy removed 85.4 ± 6.0% (>68.2%) or 52.8 ± 3.7% (>31.8%) of the reflex effects on respiratory rate (n=9, P<0.05); and removed 79.1 ± 4.5% (>68.2%) or 59.3 ± 9.1% (>31.8%) of the effect on expiratory pause induced by lung inflation (n=12, P<0.05). Since the first vagotomy removes more reflex effect than the second vagotomy, we conclude that the two vagus nerves exert their Hering-Breuer reflex effects by a multiplicative effect.

Non-verbal communication of compassion: measuring psychophysiologic effects

BACKGROUND

Calm, compassionate clinicians comfort others. To evaluate the direct psychophysiologic benefits of non-verbal communication of compassion (NVCC), it is important to minimize the effect of subjects' expectation. This preliminary study was designed to a) test the feasibility of two strategies for maintaining subject blinding to non-verbal communication of compassion (NVCC), and b) determine whether blinded subjects would experience psychophysiologic effects from NVCC.

METHODS

Subjects were healthy volunteers who were told the study was evaluating the effect of time and touch on the autonomic nervous system. The practitioner had more than 10 years' experience with loving-kindness meditation (LKM), a form of NVCC. Subjects completed 10-point visual analog scales (VAS) for stress, relaxation, and peacefulness before and after LKM. To assess physiologic effects, practitioners and subjects wore cardiorespiratory monitors to assess respiratory rate (RR), heart rate (HR) and heart rate variability (HRV) throughout the 4 10-minute study periods: Baseline (both practitioner and subjects read neutral material); non-tactile-LKM (subjects read while the practitioner practiced LKM while pretending to read); tactile-LKM (subjects rested while the practitioner practiced LKM while lightly touching the subject on arms, shoulders, hands, feet, and legs); Post-Intervention Rest (subjects rested; the practitioner read). To assess blinding, subjects were asked after the interventions what the practitioner was doing during each period (reading, touch, or something else).

RESULTS

Subjects' mean age was 43.6 years; all were women. Blinding was maintained and the practitioner was able to maintain meditation for both tactile and non-tactile LKM interventions as reflected in significantly reduced RR. Despite blinding, subjects' VAS scores improved from baseline to post-intervention for stress (5.5 vs. 2.2), relaxation (3.8 vs. 8.8) and peacefulness (3.8 vs. 9.0, P < 0.05 for all comparisons). Subjects also had significant reductions in RR (P < 0.0001) and improved HRV (P < 0.05) with both tactile and non-tactile LKM.

CONCLUSION

It is possible to test the effects of LKM with tactile and non-tactile blinding strategies; even with blinding in this small preliminary study, subjects reported significant improvements in well-being which were reflected in objective physiologic measures of autonomic activity. Extending compassion is not only good care; it may also be good medicine.

TRIAL REGISTRATION NUMBER

US National ClinicalTrials.gov registration number, NCT01428674.

Use-dependent learning and memory of the Hering-Breuer inflation reflex in rats

The classic Hering-Breuer inflation reflex (HBIR) is a widely held tenet for understanding the lung volume-related vagal control of respiratory rhythm. Recent evidence, however, has revealed that the fictive HBIR elicited by electrical vagal stimulation in rats is not static but may be attenuated centrally by two forms of non-associative learning (habituation and desensitization) that continually mitigate the reflex effects with exponential adaptations like a differentiator or high-pass filter. Desensitization is analogous to habituation but exhibits an explicit short-term memory (STM) in the form of a rebound response with exponential decay during recovery from stimulation. To investigate whether such learning and memory effects are lung volume related and use-dependent (practice makes perfect), we compared the time-dependent changes in inspiratory and expiratory durations (t(I) and t(E)) during and after 1 or 8 min unilateral lung inflation or high-frequency, low-intensity vagal stimulation in anaesthetized, uni- or bi-vagotomized rats. Unilateral lung inflation and vagal stimulation both elicited abrupt shortening of t(I) and lengthening of t(E) (HBIR effects) and gradual habituation and desensitization throughout the 1 or 8 min test period, followed by rebound responses in t(I) and t(E) with exponential recovery (STM effects) in the post-test period. In both cases, the STM time constants for t(I) and t(E) were significantly longer with the 8 min test than with the 1 min test (17-45 versus 4-11 s, P < 0.01). We conclude that the HBIR and its central habituation and desensitization are mediated peripherally by lung volume-related vagal afferents, and that the STM of desensitization is use-dependent. The translational implications of these findings are discussed.

Modulation of breathing by phasic pulmonary stretch receptor feedback in an amphibian, Bufo marinus

This study examined the role of phasic pulmonary stretch receptor (PSR) feedback in ventilatory control, breath clustering and breath timing in decerebrate, paralysed and artificially-ventilated cane toads (Bufo marinus) under conditions designed to minimise tonic PSR feedback. Fictive breathing was recorded as trigeminal motor output to the buccal musculature. Artificial tidal ventilation, with hypercarbic gas mixtures, was either continuous or activated by the fictive breaths and was manipulated to provide differing amounts/patterns of phasic PSR feedback. The results demonstrate that increased amounts of phasic PSR feedback increase overall breathing frequency. Within multi-breath episodes there was an increase in the instantaneous breathing frequency during the later stages of the episode. The temporal relationship between a fictive breath and lung inflation influenced the duration of the pause between fictive breaths. The data indicate that phasic PSR feedback stimulates breathing by enhancing the occurrence of breathing episodes in this species but does not appear to modify the instantaneous breathing frequency during an episode.

Lung vagal afferent activity in rats with bleomycin-induced lung fibrosis

Bleomycin treatment in rats results in pulmonary fibrosis that is characterized by a rapid shallow breathing pattern, a decrease in quasi-static lung compliance and a blunting of the Hering-Breuer Inflation Reflex. We examined the impulse activity of pulmonary vagal afferents in anesthetized, mechanically ventilated rats with bleomycin-induced lung fibrosis during the ventilator cycle and static lung inflations/deflations and following the injection of capsaicin into the right atrium. Bleomycin enhanced volume sensitivity of slowly adapting stretch receptors (SARs), while it blunted the sensitivity of these receptors to increasing transpulmonary pressure. Bleomycin treatment increased the inspiratory activity, while it decreased the expiratory activity of rapidly adapting stretch receptors (RARs). Pulmonary C-fiber impulse activity did not appear to be affected by bleomycin treatment. We conclude that the fibrosis-related shift in discharge profile and enhanced volume sensitivity of SARs combined with the increased inspiratory activity of RARs contributes to the observed rapid shallow breathing of bleomycin-induced lung fibrosis.

An overview of the anatomy and physiology of slowly adapting pulmonary stretch receptors

Since the original work of by Hering and Breuer in 1868 numerous studies have demonstrated that slowly adapting pulmonary stretch receptors (SARs) are the lung vagal afferents responsible for eliciting the reflexes evoked by moderate lung inflation. SARs play a role in controlling breathing pattern, airway smooth muscle tone, systemic vascular resistance and heart rate. Both anatomical and physiological studies support the contention that SARs, by their close association with airway smooth muscle, continuously sense the tension within the myoelastic components of the airways caused by lung inflation, smooth muscle contraction and/or tethering of small intrapulmonary airways to the lung parenchyma. In addition, intrapulmonary SAR discharge activity is sensitive to changes in P(CO2) within the physiological range. Despite this extensive characterization of SARs, their role in determining breathing pattern and airway tone in individuals with respiratory diseases is only recently being appreciated.

Vagal feedback in the entrainment of respiration to mechanical ventilation in sleeping humans

We studied the capacity of four "normal" and six lung transplant subjects to entrain neural respiratory activity to mechanical ventilation. Two transplant subjects were studied during wakefulness and demonstrated entrainment indistinguishable from that of normal awake subjects. We studied four normal subjects and four lung transplant subjects during non-rapid eye movement (NREM) sleep. Normal subjects entrained to mechanical ventilation over a range of ventilator frequencies that were within +/-3-5 breaths of the spontaneous respiratory rate of each subject. After lung transplantation, during which the vagi were cut, subjects did demonstrate entrainment during NREM sleep; however, entrainment only occurred at ventilator frequencies at or above each subject's spontaneous respiratory rate, and entrainment was less effective. We conclude that there is no absolute requirement for vagal feedback to induce entrainment in subjects, which is in striking contrast to anesthetized animals in which vagotomy uniformly abolishes entrainment. On the other hand, vagal feedback clearly enhances the fidelity of entrainment and extends the range of mechanical frequencies over which entrainment can occur.

Phasic lung inflation shortens inspiration and respiratory period in the lung-attached neonate rat brain stem spinal cord

In intact mammals, lung inflation during inspiration terminates inspiration (Breuer-Hering inspiratory reflex, BHI) and the presence of lung afferents increases respiratory frequency. To test whether these responses could be obtained in vitro, a neonate rat brain stem/spinal cord preparation retaining the lungs and their vagal innervation was used. It was found that 1) the BHI could be replicated in vitro, 2) phasic lung inflation during inspiration caused increased respiratory frequency with declining efficacy as inflation delay increased, and 3) increased respiratory frequency did not require inspiratory shortening.

Pattern formation and rhythm generation in the ventral respiratory group

1. There is increasing evidence that the kernel of the rhythm-generating circuitry for breathing is located within a discrete subregion of a column of respiratory neurons within the ventrolateral medulla referred to as the ventral respiratory group (VRG). It is less clear how this rhythm is transformed into the precise patterns appearing on the varied motor outflows. 2. Two different approaches were used to test whether subregions of the VRG have distinct roles in rhythm or pattern generation. In one, clusters of VRG neurons were activated or inactivated by pressure injection of small volumes of neuroactive agents to activate or inactivate groups of respiratory neurons and the resulting effects on respiratory rhythm and pattern were determined. The underlying assumption was that if rhythm and pattern are generated by neurons in different VRG subregions, then we should be able to identify regions where activation of neurons predominantly alters rhythm with little effect on pattern and other regions where pattern is altered with little effect on rhythm. 3. Based on the pattern of phrenic nerve responses to injection of an excitatory amino acid (DL-homocysteate), the VRG was divided into four subdivisions arranged along the rostrocaudal axis. Injections into the three rostral regions elicited changes in both respiratory rhythm and pattern. From rostral to caudal the regions included: (i) a rostral bradypnoea region, roughly associated with the Bötzinger complex; (ii) a dysrhythmia/tachypnoea area, roughly associated with the pre-Bötzinger complex (PBC); (iii) a second caudal bradypnoea area; and, most caudally, (iv) a region from which no detectable change in respiratory motor output was elicited. 4. In a second approach, the effect of unilateral lesions of one subregion, the PBC, on the Breuer-Hering reflex changes in rhythm were determined. Activation of this reflex by lung inflation shortens inspiration and lengthens expiration (TE). 5. Unilateral lesions in the PBC attenuated the reflex lengthening of TE, but did not change baseline respiratory rhythm. 6. These findings are consistent with the concept that the VRG is not functionally homogenous, but consists of rostrocaudally arranged subregions. Neurons within the so-called PBC appear to have a dominant role in rhythm generation. Nevertheless, neurons within other subregions contribute to both rhythm and pattern generation. Thus, at least at an anatomical level resolvable by pressure injection, there appears to be a significant overlap in the circuitry generating respiratory rhythm and pattern.

Influence of volume dependency and timing of airway occlusions on the Hering-Breuer reflex in infants

Both end-inspiratory (EIO) and end-expiratory (EEO) airway occlusions are used to calculate the strength of the Hering-Breuer inflation reflex (HBIR) in infants. However, the influence of the timing of such occlusions is unknown, as is the extent to which changes in volume within and above the tidal range affect this reflex. The purpose of this study was to compare both techniques and to evaluate the volume dependency of the HBIR in healthy, sleeping infants up to 1 yr of age. The strength of the HBIR was expressed as the ratio of expiratory or inspiratory time during EIO or EEO, respectively, to that recorded during spontaneous breathing, i.e., as the "inhibitory ratio" (IR). Paired measurements of the EIO and EEO in 26 naturally sleeping newborn and 15 lightly sedated infants at approximately 1 yr showed no statistically significant differences in the IR according to technique: mean (95% CI) of the difference (EIO - EEO) being -0.02 (-0.17, 0.13) during the first week of life and 0.04 (-0.14, 0.22) at 1 yr. During tidal breathing, a volume threshold of approximately 4 ml/kg was required to evoke the HBIR. Marked volume and age dependency were observed. In newborn infants, occlusions at approximately 10 ml/kg during sighs always resulted in an IR > 4, whereas a similar response was only evoked at 25 ml/kg in older infants. Age-related changes in the volume threshold may reflect maturational changes in the control of breathing and respiratory mechanics throughout the first year of life.

Control of ventilation during lung volume changes and permissive hypercapnia in dogs

We investigated the effect changes in end-expiratory lung volume (EEVL) had on the response to progressive hypercapnia (CO2-response curve) in eight open-chest, anesthetized dogs, in order to clarify the role that vagal lung mechanoreceptors have in altered respiratory drive during permissive hypercapnia. The dogs were ventilated using a positive-pressure ventilator driven by phrenic neural activity. Systemic arterial CO2 tension (PaCO2) was elevated by increasing the fraction of CO2 delivered to the ventilator. EEVL was altered from approximated functional residual capacity ("FRC") to 1.5 and 0.5 "FRC" by changing positive end-expiratory pressure. Although the tidal volume (VT)-PaCO2 and inspiratory time (TI)-PaCO2 relationships were not affected, decreasing EEVL from 1.5 "FRC" to "FRC" and then to 0.5 "FRC" caused a significant (p < 0.01) upward shift in the CO2-response curves for minute ventilation (V I) and frequency (f ), and a significant (p < 0.01) downward shift in the CO2- response curve for expiratory time (TE). We conclude that these shifts were explained by a decrease in the inhibitory activity of slowly adapting pulmonary stretch receptors (PSRs) as EEVL was lowered. In addition, increases in EEVL from 0.5 "FRC" to 1.5 "FRC" caused a significant (p < 0.05) increase in the apneic threshold, which we attribute to an inhibitory effect on central drive caused by increased PSR activity.

The Hering-Breuer reflex in anesthetized infants: end-inspiratory vs. end-expiratory occlusion technique

Both end-inspiratory (EIO) and end-expiratory (EEO) occlusions have been used to measure the strength of the Hering-Breuer inflation reflex (HBIR) in infants. The purpose of this study was to compare both techniques in anesthetized infants. In each infant, HBIR activity was calculated as the relative prolongation of expiratory and inspiratory time during EIO and EEO, respectively. Respiratory drive was assessed from the change in airway pressure during inspiratory effort against the occlusion, both at a fixed time interval of 100 ms (P0.1) and a fixed proportion (10%) of the occluded inspiratory time (P10%). Twenty-two infants [age 14.3 +/- 6. 4 (SD) mo] were studied. No HBIR activity was present during EIO [-11.8 +/- 15.9 (SD) %]. By contrast, there was significant, albeit weak, reflex activity during EEO [HBIR: 27.2 +/- 17.4%]. A strong HBIR (up to 310%) was elicited in six of seven infants in whom EIO was repeated after lung inflation. P0.1 was similar during both types of occlusions, whereas mean +/- SD P10% was lower during EEO than during EIO: 0.198 +/- 0.09 vs. 0.367 +/- 0.15 kPa, respectively (P < 0.01). These data suggest a difference in the central integration of stretch receptor activity in infants during anesthesia compared with during sleep.

Mechanism of relief of tachypnea during pressure support ventilation

Pressure support ventilation (PSV) provides a range of ventilatory support from partial respiratory muscle unloading, where inspiratory work is shared between the patient and the mechanical ventilator, to total respiratory muscle unloading, where inspiratory work is performed solely by the ventilator. This study is designed to determine if minimizing work fully accounts for relief of tachypnea during PSV. We examined respiratory parameters over a range of PSV that includes the crossover from partial to total respiratory muscle unloading. Eight studies were obtained on seven intubated patients in respiratory failure. Ventilation, occlusion pressure (P0.1), and patient inspiratory work (WOBinsp) were measured while PSV was varied. In all patients, WOBinsp decreased as PSV increased. The level of PSV where WOBinsp was minimized was identified; this marked the crossover from partial to total respiratory muscle unloading. Frequency decreased with increasing PSV but remained elevated (range, 22 to 38 breaths/min) at the crossover. Frequency was normalized only at PSV levels 131 to 193% of the levels of pressure at the crossover. Tidal volume (VT) changed little during partial support and averaged 5.9 mL/kg at the crossover. VT increased only on PSV providing total unloading. Six of seven patients exhibited increasing static compliance with increasing VT suggesting alveolar recruitment. P0.1 tracked WOBinsp over the entire range of PSV (r = 0.95, p < 0.001). The normalization of frequency observed above the crossover coincided with increasing VT rather than decreasing work. These observations suggest that reflexes resulting from increased VT and/or alveolar recruitment may have contributed to the normalization of frequency.

Effect of pulmonary lymphatic obstruction on respiratory rate and airway rapidly adapting receptor activity in rabbits

1. The effects on respiratory rate of obstruction of pulmonary lymph flow, reduction of plasma protein concentration and a combination of the two procedures were examined in anaesthetized rabbits. The former was achieved by raising the pressure in a pouch created from the right external jugular vein and the latter by batch plasmapheresis. 2. In spontaneously breathing rabbits, neither pulmonary lymphatic obstruction (n = 6) nor plasmapheresis (n = 5) produced a significant change in respiratory rate. However, their combination (n = 8) produced a significant increase in respiratory rate (P < 0.05). 3. Cooling of the cervical vagi to 8-9 degrees C (n = 4) and vagotomy (n = 7) abolished this response. 4. There was a significant increase in the activity of the airway rapidly adapting receptors (RARs; n = 9) during pulmonary lymphatic obstruction, plasmapheresis and their combination (P < 0.05). 5. It is concluded that in the rabbit, obstruction of lymphatic drainage from the lung after plasmapheresis causes a reflex increase in respiratory rate. The afferent pathway for this reflex response lies in the vagus nerve and the RARs are likely to be the receptors involved in this response.

Vagally mediated modification of inspiratory activity by changes in airway pressure

The impact of varying the end-expiratory transpulmonary pressure (PE'tp) on phrenic (Phr) and parasternal intercostal (IC) activity early in inspiration was studied in 12 spontaneously breathing, anaesthetized cats. PE'tp was varied in a stepwise fashion by rapidly changing the tracheal pressure (Ptr) from ambient pressure to a pressure in the range between -1.2 and +1.2 kPa relative to ambient pressure, and vice versa. The amplitudes of integrated Phr and IC activity were determined in each breath at three particular points in time (t), corresponding to 0.1, 0.2 and 0.3 sec after the onset of Phr activity, respectively. For all three values of t, Phr and IC activity increased with a decrease in Ptr and decreased with an increase in Ptr. The relationships between Phr activity and PE'tp and between IC activity and PE'tp could be described satisfactorily by mono-exponential functions. An increase in PE'tp of 0.1 kPa resulted in a decrease in Phr activity of 16%, 13% and 12% for the 3 aforementioned values of t, respectively. For IC activity the corresponding values were 42%, 42% and 36%, respectively. After bilateral vagotomy no changes in Phr activity in response to changes in Ptr were observed any more, whereas the changes in IC activity had decreased to about one tenth of the corresponding values obtained before vagotomy. Apparently, extra-vagal mechanisms contribute for a small part to the effect of changes in PE'tp on IC activity. Our results indicate that relatively small changes in PE'tp may have a large impact on central inspiratory output.

Effect on breathing of raising end-expiratory lung volume in sleeping laryngectomized man

In animals, tonic vagal activity from lung receptors provides a means by which changes in end-expiratory lung volume can influence respiratory timing. We wished to examine whether increasing the end-expiratory lung volume within the tidal volume range had a similar effect in man. In order to minimize behavioral influences on breathing, the study was performed in subjects during deep non-rapid eye movement sleep. Five laryngectomized subjects were chosen for the study since their permanent tracheal stomata allow simple, airtight connection to respiratory apparatus and avoided problems with glottic closure. During EEG-documented sleep, end-expiratory volume was increased by up to 350 ml with the addition of expiratory threshold loads of 1 to 10 cm H2O. End-expiratory volume increased linearly with expiratory pressure. Inspiratory and expiratory times (TI and TE) were not affected by increases in end-expiratory volume. Tidal volume (VT) was decreased such that end-inspiratory volume remained unchanged. The decrease in VT may result from a reduction in inspiratory muscle efficiency at a higher lung volume. The results of the study provide no evidence that tonic vagal afferent information from the lungs is important in controlling respiratory rhythm within the tidal volume range in man when behavioral control of breathing is minimized.

Phrenic nerve responses to lung inflation and hypercapnia in decerebrate dogs

The effects of changes in static airway pressure (Paw) and arterial PCO2 (PaCO2) on phrenic nerve activity were studied in unanesthetized, decerebrate dogs and compared with previous results from chloralose/urethane anesthetized dogs using the same experimental preparation (Mitchell et al. 1982; Mitchell and Selby 1987). In ten mid-collicular decerebrate dogs, the lungs were independently ventilated while the left pulmonary artery was occluded and the right vagus nerve was transected. Changes in left lung Paw, therefore, exerted effects on pulmonary stretch receptors without altering blood gases; changes in the inspired gas ventilating the right lung controlled blood gas composition, without altering lung volume feedback. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cmH2O at various constant levels of PaCO2 between 31 and 69 mmHg. The major findings of this study are: (1) hypercapnia decreases the slope of the relationship between expiratory duration (tE) and Paw in both decerebrated and anesthetized dogs; (2) hypercapnia increases the inspiratory duration (tI) in decerebrated, but not anesthetized dogs; and (3) hypercapnia decreases the slope of the relationship between f and Paw due to these effects on tE and tI. These results support previous studies indicating that vagal and suprapontine mechanisms exert independent effects on respiratory timing. It is concluded that neither suprapontine influences nor anesthesia are necessary in the mechanism underlying interactions between stretch receptors and CO2-chemoreceptors in modulating tE. Furthermore, decerebration reveals a unique effect of CO2-chemoreceptors on tI, an effect found in anesthetized dogs only after carotid denervation.

Responses of inspiratory neurons of the dorsal respiratory group to stimulation of expiratory muscle and vagal afferents

In decerebrate, paralyzed and ventilated cats, we monitored the intracellular responses of 30 inspiratory neurons of the dorsal respiratory group (DRG) to stimulation of vagal and expiratory muscle (internal intercostal and abdominal) afferents. We hypothesized that the inhibitory effects of stimulation of expiratory muscle afferents, previously reported, would block the excitatory responses of inspiratory neurons of the DRG to vagal stimulation. Although prolonged stimulus trains to expiratory muscle afferents caused respiratory phase-switching, single shocks or short trains elicited no responses in 17 bulbospinal neurons, excitatory responses in 6, and inhibitory responses in 2. Of the 4 propriobulbar neurons tested, 2 had inhibitory responses and 2 did not respond. In only 2 neurons, both bulbospinal, did conditioning stimuli to expiratory muscle afferents block or reduce the excitatory effects of vagal stimulation. These results suggest that interaction of vagal and expiratory muscle afferents, which might account for the absence of a change in inspiratory duration despite increased vagal afferent feedback at elevated end-expiratory lung volumes, does not occur within the DRG.

Control of expiratory duration by arterial CO2 oscillations in vagotomized dogs

The role of respiratory oscillations of PaCO2 (CO2 oscillations) in the regulation of expiratory duration (TE) was examined in eight anesthetized dogs by measuring instantaneous changes of arterial pH during the respiratory cycle with a catheter-tip ISFET (ion-sensitive field effect transistor) pH electrode. Steady-state changes in arterial pH oscillation were induced by vagotomy, which increased the amplitude of pH oscillation from 0.014 +/- 0.002 (mean +/- SEM) to 0.058 +/- 0.006 units (P less than 0.001), and prolonged TE from 5.12 +/- 0.56 to 9.99 +/- 1.11 sec (P less than 0.005) with little change in average pH (0.021 +/- 0.011 units, P = 0.12). Vagotomy also reversed the phase relationship between arterial pH oscillation at the carotid bodies and tidal ventilation, such that pH rose during early expiration, rather than falling as occurred in the intact state. When the within-breath oscillation of arterial pH was transiently reduced by having the vagotomized dogs inspire a single breath of CO2 enriched air, TE of the same breath was shortened in proportion to the reduction in amplitude of pH oscillation (r = 0.72, P less than 0.001), rather than in proportion to changes in the average pH of the test breath (r = 0.44). The results indicate that the profile of within-breath oscillation of PaCO2 (phase relationship and amplitude) can exert an important influence on TE, and may in part account for the prolongation of TE following vagotomy.

Activities of pulmonary stretch receptors during ventilatory cycles without lung inflation

When lung inflation is temporarily withheld in paralyzed, ventilated cats with intact vagi, the activities of inspiratory motor nerves are greater during the second cycle without inflation than during the first. This response is not easily attributable to increasing drive from chemoreceptors as it is abolished by vagotomy. We examined the hypothesis that the increasing inspiratory activity is the result of decreasing inhibitory feedback from pulmonary stretch receptors (PSRs). Decerebrate, paralyzed cats were ventilated by a servo-respirator in accordance with their own phrenic nerve activity. Afferent activities from individual PSRs were recorded from a few cut fibers of one vagus nerve; the vagi were otherwise intact. When lung inflation was withheld, phrenic and hypoglossal nerve activities and the durations of inspiration and expiration all increased and were significantly greater during the second cycle without inflation than during the first. The frequency of PSR discharge was also greater during the second cycle and thus did not account for the responses recorded from the motor nerves. We conclude that the latter responses probably reflect neural processes within the brain stem, involving a persistent inhibitory influence from lung inflation, which outlasts the inflation itself.

Effects of prolonged inflation on pulmonary stretch receptor discharge in turtles

The tonic and phasic discharge characteristics of single, slowly adapting pulmonary stretch receptors (SAR) were examined before and after 1 h periods of constant pressure inflation to normal resting (VLr, pressure = 0 cm H2O) and elevated (VLe, pressure = 10 cm H2O) lung volumes in turtles (Chrysemys sp.). Based on their discharge at VLr, SAR were classified as either low (n = 13) or high threshold (n = 4) receptors. Inflations were performed with both air and 5% CO2 in air. Lung gas composition and arterial PCO2 and pH were measured during the maintained inflations. In animals ventilated with air, low and high threshold receptors adapted by 57 and 30% respectively over the first 3 min at VLe. During the remainder of the 1 h period, the discharge of low threshold SAR fell an additional 20% while that of the high threshold SAR remained relatively constant. There were significant increases in both alveolar and arterial PCO2 during the maintained inflations. Ventilation with 5% CO2 reduced the static discharge levels of low and high threshold SAR by 10 and 25% respectively, suggesting that a part of the apparent adaptation of these receptors to maintained inflation for 1 h with air was due to the accumulation of metabolic CO2. Following 1 h of maintained inflation, the phasic responses to pump ventilation were decreased in low threshold SAR but remained unchanged in high threshold SAR. The static discharge associated with step inflation was unchanged in both receptor groups. The data suggest that increased SAR discharge is sustained indefinitely during increased lung volume and may account for persistent changes in breathing pattern previously observed during chronic changes in lung volume.

Effect of pulmonary venous congestion on respiratory rate in dogs

1. The effect of pulmonary venous congestion on the respiratory rate was examined in dogs anaesthetized with alpha-chloralose. The study was done on both spontaneously breathing and artificially ventilated animals. Pulmonary venous congestion was produced by partial obstruction of the mitral valve sufficient to raise the left atrial pressure by 5 mmHg. 2. In artificially ventilated dogs, pulmonary venous congestion increased significantly the activity in phrenic nerves. Both the number of bursts/min and the total number of impulses/min increased. However, there was no significant change in the number of impulses/burst. 3. In spontaneously breathing dogs, pulmonary venous congestion produced a significant increase in the frequency of breathing with a significant shortening of the inspiratory and expiratory durations. 4. Cooling of the cervical vagi to 8-9 degrees C abolished both the above responses. 5. Pulmonary venous congestion (left atrial pressure +5 mmHg) stimulated the rapidly adapting receptors of the airways. This effect was abolished by cooling the ipsilateral vagus proximally to 8-9 degrees C. 6. It is concluded that pulmonary venous congestion increases the respiratory rate reflexly in dogs. The afferent pathway for this reflex response resides in the vagus and the rapidly adapting receptors are likely to be the receptors involved in this response.

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.

Effects of carotid denervation on interactions between lung inflation and PaCO2 in modulating phrenic activity

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration. We examined the role of carotid chemoreceptors in this response using an experimental preparation that allowed independent control of lung inflation and CO2 reflexes. Experiments were conducted in intact (n = 6) and carotid denervated (CBX; n = 12) chloralose/urethane anesthetized dogs. Integrated phrenic amplitude (Phr), f, and the inspiratory (TI) and expiratory durations (TE) were measured as a function of Paw (2-12 cm H2O) at levels of PaCO2 between 30 and 80 mm Hg. In intact dogs: (1) f decreased as Paw increased, and elevated PaCO2 decreased the slope of this relationship; (2) neither PaCO2 nor Paw affected TI; and (3) TE increased hyperbolically with Paw, and elevated PaCO2 attenuated this relationship. In CBX dogs: (1) f decreased as Paw increased, but this relationship was not affected by PaCO2; (2) TI increased as PaCO2 increased but was unaffected by Paw; and (3) TE increased as Paw increased but was unaffected by PaCO2. The results indicate that carotid chemoreceptors are necessary in the mechanism whereby hypercapnia attenuates the effects of Paw on f and TE. Furthermore, carotid denervation reveals an effect of hypercapnia on TI, an effect that is not evident in dogs with functional carotid chemoreceptors.

Effects of end-expired pressure on phrenic output in servo-ventilated dogs

The pattern of breathing induced by increases in end-expired lung volume (EEVL) was determined in 9 anesthetized dogs. The pulmonary and systemic circulations were separately pump-perfused and the lungs were ventilated with a servo-ventilator actuated from the phrenic neurogram. EEVL was increased as a continuous ramp by slowly raising end-expired transpulmonary pressure from 1.5 to 12 cm H2O. Tidal volume (VT), inspiratory time (TI), and expiratory time (TE) were measured at vagal temperatures of 39 degrees C and 7 degrees C and following vagotomy. At a vagal temperature of 39 degrees C, increasing EEVL produced significant reductions in VT and TI while greatly prolonging TE. Vagal cooling to 7 degrees C, substantially altered the reflex response to increased EEVL. At 7 degrees C, VT decreased as EEVL increased, but the reduction was not so pronounced as at 39 degrees C. In addition, both TI and TE shortened. Increasing EEVL following vagotomy had no consistent effects on breathing pattern. We conclude that increasing EEVL stimulates tachypneic promoting pulmonary afferent nerves, most likely pulmonary C-fibers, but at normal vagal temperature their effect is masked by the stronger reflex inhibition of slowly adapting pulmonary stretch receptors.

Effects of hypoxemia on phrenic nerve responses to static lung inflation in anesthetized dogs

To study interactions between hypoxemia and lung stretch in modulating ventilatory activity, an experimental preparation was used that allows independent control of static airway pressure (Paw) and arterial PO2 in anesthetized dogs. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cm H2O at levels of PaO2 between 30 and 200 mm Hg. Experiments were repeated in intact (n = 8) and carotid denervated dogs (CBX; n = 7). In intact dogs, f decreased with increasing Paw through an effect on the expiratory duration (TE). Hypoxia increased f by decreasing both the inspiratory duration (TI) and TE. Hypoxia had no effect on the slope of the f vs Paw relationship, but attenuated the effect of Paw on TE. Phr was increased by hypoxia, but Paw had little effect. After CBX, f was still inhibited by Paw, but PaO2 had no consistent effect on f, TI or TE at any level of Paw. Phr was inhibited by hypoxia after CBX, but Paw had no effect. The results indicate that Paw and PaO2 exert additive effects on f in anesthetized dogs. Hypoxia attenuates the effect of Paw on TE, which alone would attenuate the slope of the f vs Paw relationship. However, the effect of hypoxia on TI enhances the slope of the f vs Paw relationship, restoring a parallel shift. These effects are abolished by carotid denervation.

Non-invasive technique for examining the respiratory proprioceptor system in man

Our technique enables non-invasive experiments to be conducted on the proprioceptor part of respiratory control, while eliminating misleading responses due to interaction with the chemoreceptor system; interaction was prevented by stabilizing arterial PO2 and PCO2 with the aid of an optimal regulator based on a mini-computer which controlled the inspired gas mixture. The proprioceptor system in a human was disturbed by applying positive pressure pulses at the mouth, responses were derived from continuous air-flow measurement. The classical inflation inhibiting reflex and an effect akin to Head's paradoxical reflex were demonstrated.

The pattern of sympathetic neurone activity during expiration in the cat

The properties of sympathetic preganglionic neurone activity during expiration were studied in pentobarbitone-anaesthetized (n = 26) and in non-anaesthetized, mid-collicular decerebrate (n = 5), paralysed, artificially ventilated cats in which the electrical activity of the phrenic nerve and of the cervical sympathetic trunk was recorded. In control conditions (end-tidal PCO2 between 35 and 40 mmHg, zero end-expiratory pressure) sympathetic activity during expiration was either steady at a low level (n = 11) or showed a modest progressive increase from a low level in early expiration (n = 17). Very infrequently (n = 3), it showed a transient increase during the second half of expiration. Artificial ventilation with positive end-expiratory pressures in the range from 2.1 +/- 0.4 (mean +/- S.D.) to 6.7 +/- 0.6 cmH2O caused, in cats with intact vagus nerves, an increase in sympathetic neurone activity during the second half of expiration. Within this range of pressures, the magnitude of the increase was related to the magnitude of the positive end-expiratory pressure. This effect reversed at higher positive end-expiratory pressures. Pressures in excess of 10.2 +/- 1.8 cmH2O caused inhibition of sympathetic activity. The sympatho-excitatory effect of positive end-expiratory pressure disappeared after bilateral cervical vagotomy. With intact vagus nerves, it also disappeared at levels of systemic hypocapnia (end-tidal PCO2 less than or equal to 15 mmHg) which abolished phrenic nerve activity. In hypocapnia, artificial ventilation with peak tracheal pressures greater than 7.2 +/- 1.1 cmH2O caused inhibition of sympathetic activity, while ventilation with lower end-expiratory pressures had no effect on sympathetic activity. It may be concluded that the sympatho-excitatory effect of positive end-expiratory pressure is mediated by vagal afferents and requires a certain level of brain-stem respiratory neurone activity. Sympatho-excitation during expiration was also observed, in normocapnic conditions, during short-duration static lung inflation with tracheal pressures in the range from 2.5 +/- 0.3 to 7.0 +/- 0.8 cmH2O as well as during artificial ventilation with zero end-expiratory pressure when lung inflation occurred in expiration. These responses were abolished by bilateral cervical vagotomy and during systemic hypocapnia. Sympatho-excitation during expiration was also observed when systemic hypercapnia was produced in vagotomized cats by artificial ventilation with gas mixtures containing 5 or 10% CO2.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachypnoeic response to amyl nitrite inhalation in the rabbit

Intratracheal inhalation of amyl nitrite, a non-specific smooth muscle relaxant, in the pentobarbitone/urethane anaesthetized rabbit caused reductions in tidal volume and both inspiratory and expiratory times, without a preceding apnoea, that were independent of the associated hypotension and of reflex influences from the carotid sinus region but dependent on supra-abdominal vagal integrity. In artificially ventilated, paralyzed rabbits amyl nitrite caused a pronounced sensitization of pulmonary stretch receptors (PSRs) during the inflation phase, typically with a reduction in the level of activity during the deflation phase. The time course of the change in the pattern of PSR activity paralleled that of the tachypnoeic response. The sensitization of a small sample of rapidly adapting 'irritant' receptors was of a significantly shorter duration. A unitary analysis of non-myelinated vagal afferents was not attempted. The sensitization of these vagal afferents cannot be attributed to the smooth muscle relaxant properties of amyl nitrite since other relaxants (sodium nitrite, sodium nitroprusside, phentolamine) did not possess this capacity, and such properties would be expected to diminish their activities.

Reflex partitioning of inputs from stretch receptors of bronchi and thoracic trachea

A selective block of slowly adapting stretch receptors in anesthetized rabbits was induced by exposing to SO2 all thoracic airways (T) or the carina and bronchi alone (B). Increment of inspiratory time (TI) relative to control was 61% greater under B than T. The reverse would have happened if input responsible for Breuer-Hering inflation reflex originated from both bronchi and trachea. Hence, bronchial input activates inspiratory off-switch, while tracheal input delays its activation. During single inspiratory efforts with airways closed at end expiration diaphragm activity (Adi) decreased and TI0 increased relative to control equally under B and T. Hence, the input facilitating central inspiratory activity at end expiratory volume does not stem from trachea. At end of inspiratory ramp Adi stopped within 43 msec at control and 57 msec under B and T. Hence, bronchial input speeds up off-switching of inspiration. Postinspiratory Adi was greater under B than T, and nearly nil at control. Hence, bronchial input inhibits postinspiratory Adi, while tracheal input facilitates it. Inspiratory and expiratory flows were more damped under B than T, and under T than at control.

Role of airway receptors in the breathing pattern of patients with chronic obstructive lung disease

To determine whether airway receptors are responsible for the rapid, shallow breathing pattern seen in hypercapnic chronic obstructive lung disease, 10 patients underwent upper airway anaesthesia with inhaled lignocaine in a placebo controlled study. There was a significant reduction in breathing frequency after lignocaine (p less than 0.001) that was due to an increase in expiratory time (p less than 0.001). The inspiratory time remained unchanged, but tidal volume increased significantly (p less than 0.02). It is concluded that, while airway receptors may have a role in determining the frequency of breathing in chronic obstructive lung disease, other factors are responsible for the reduced inspiratory time.

Verification of a model for the mechanisms controlling expiratory duration in rabbits under various conditions

A mathematical model for the mechanisms controlling expiratory (E) duration was verified through comparison of its prediction with the relationships between TE and waveform parameters of various input patterns obtained by bilateral, simultaneous electrical stimulation of the largest vagal afferent fibers in vagotomized, paralyzed rabbits at normal or elevated chemical drive under light or deep barbiturate anesthesia. The model accurately reproduced experimental results in each animal and condition; it suggests that in the absence of vagal input (v.i.) an exponentially decaying central activity (phi) prevents inspiratory (I) onset until a time independent threshold (phi thr) is reached; the vagal component of phi results from fixed gain, long time constant (tau = 1.07 +/- 0.03 sec; mean +/- SE for N = 37 for N = 37 animals) temporal summation of expiratory v.i., being unaffected by inspiratory v.i.; central and vagal components share a common mechanism, the same value of tau applying to v.i. integration and to phi decay in the absence of v.i. under all conditions; the amplitude of the central component is proportional to tidal changes in central I activity, independently of the condition studied; and phi thr increases at elevated chemical drive, is unaffected by v.i., and decreases under deep anesthesia.

Gallamine and vagotomy enhance respiratory modulation of reticular units

Respiration related units ( RRU ) were recorded in the brainstem of cats with spinal transection at the C7-Th 1 level and breathing N2O. The proportion of RRU in several structures was compared in control and in 3 experimental groups: (1) paralyzed with gallamine triethiodide; (2) vagotomized; and (3) both vagotomized and paralyzed. After gallamine, RRU percentage was multiplied by 3 in the bulbo-pontine reticular formation (RF) and as much as 20 in the mesencephalic RF. Vagotomy multiplied RRU proportions by 2 in the bulbo-pontine RF, by 12 in the mesencephalic RF and by 3 in the pneumotaxic complex (nucleus parabrachialis medialis ( NPBM ) and K olliker -Fuse (KF) nucleus). The effects of gallamine and vagotomy were not additive. Gallamine still increased proportion of RRU in the RF after vagotomy showing that the gallamine effect is not vagally mediated; this was in contrast to the previously reported suppression by vagotomy of phrenic discharge facilitation induced by gallamine. The total number of firing units in the RF was not modified by gallamine or vagotomy. It is concluded that the respiratory modulation of reticular neurons is selectively enhanced by gallamine and vagotomy through two independent mechanisms.

Bilateral reflex effects on phrenic nerve activity in response to single-shock vagal stimulation

The bilateral reflex actions of vagus nerve afferent signals on phrenic efferent activity have been tested by unilateral graded single shock electrical stimulation. An early excitation (latency 3-5 msec) was more prominent in the phrenic nerve contralateral to the stimulated vagus. Spinal cord hemisection at C3 eliminated both contralateral and ipsilateral responses: thus, both were mediated via descending tracts in the contralateral cord. A bilaterally symmetrical early inhibition (latency 8-12 msec) followed the early excitation. The electrical thresholds for evoking the early responses and the temperature for blocking these responses during graded vagal cooling were closely similar to the threshold and blocking temperature for pulmonary stretch receptor afferents. Higher stimulus strengths evoked a strong, bilaterally similar, late excitation (latency 12-20 msec) followed by a late inhibition. At very high stimulus strengths a third excitation (latency 25-30 msec) could appear. Sometimes these responses were followed by lowered phrenic activity for the remainder of inspiration. Single shock stimulation of the intact vagus nerve or of the peripheral end of the cut recurrent laryngeal nerve provoked, by the contraction of laryngeal muscles, a strong, short latency (12 msec) inhibition of phrenic activity mediated by superior laryngeal nerve afferents. The implications of these results with respect to the reflex pathways of the different responses and their possible integration in the central respiratory control mechanisms are discussed.

The effect of transient stimulation of lung irritant receptors on the pattern of breathing in rabbits

1. 100 ms pulses of inflation and deflation were applied to the lungs of anaesthetized rabbits before and during inactivation of pulmonary stretch receptors. 2. Pulses of either sign given in inspiration often produced augmented breaths, whether or not stretch receptors were inactivated. Inflation pulses were more effective than deflation pulses. 3. After an augmented breath it was impossible to produce another for at least 1 min (refractoriness). 4. Pulses of deflation always shortened expiration. Pulses of inflation early in expiration shortened expiration. Later in expiration they lengthened expiration when stretch receptors were active and shortened expiration when stretch receptors were blocked. No refractoriness was observed for the effects on expiratory time. 5. Pulses in paralysed animals caused a burst of phrenic activity of fixed duration, usually with brief latency. There was no refractoriness. 6. We suggest that the duration of inspiration is governed by the activity of pulmonary stretch receptors, except during an augmented breath, and that the duration of expiration is governed by a balance of stretch and irritant receptor activity.

The summation of left and right lung volume information in the control of breathing in dogs

1. Reflex respiratory responses to unilateral and bilateral changes of lung volume have been studied in anaesthetized paralysed, open-chest dogs.2. The two lungs were separately ventilated by two phrenic-driven respirators via a specially designed double-lumen tube. Respiratory motor output was measured from the phrenic motoneurone activity recorded from the C(5) root. Expiratory lung volumes were set by the use of expiratory threshold loads (e.t.l.s).3. The reflex changes of expiratory time (t(e)) were used to study the summation of left and right expiratory lung volume information. Changes in the peak amplitude of the phrenic ;integral' (Phr) and inspiratory time (t(i)) were used to assess summation of left and right tidal volume information.4. Summation in the reflex responses to bilateral lung volume changes was estimated by comparing the measured responses to these manoeuvres with the sum of the component unilaterally evoked responses. If simple addition were present, response (measured)/response (predicted) would equal 1.0 Mutual facilitation would give a value higher than this; mutual inhibition, a lower value.5. The responses of t(e) to changes of e.t.l. on the right side were always greater than for changes confined to the left, and in each animal the response of t(e) to bilateral changes of e.t.l. were greater than for either of the unilaterally evoked responses. In six out of eight animals this was shown to be due to simple addition of the responses evoked from the two lungs individually. In the remaining two animals, slight mutual inhibition was seen.6. The tidal volume V(T) was changed in one or both lungs. During the bilateral V(T) changes, the volumes were changed simultaneously, either in the same direction (;same' V(T) changes) or in one direction in one lung and in the opposite direction in the other (;opposite' V(T) changes).7. In the bilateral ;same' V(T) changes, mutual facilitation was seen in the response of Phr; Phr (measured)/Phr (predicted) = 1.60 +/- 0.42 (s.d.), n = 8. There was only slight facilitation in the response of t(i); t(i) (measured)/t(i) (predicted) = 1.18 +/- 0.17, n = 8.8. With the bilateral ;opposite' V(T) changes, responses of t(i) and Phr were markedly and significantly reduced compared to those for the bilateral ;same' V(T) changes. During these manoeuvres significant mutual inhibition was seen in the response of t(i), and the predicted responses of Phr in general could not be correlated with the measured response.9. The responses of the phrenic ;integrals' were the same in both right and left phrenic nerves.10. Unilateral vagotomy abolished both the responses of Phr and t(i) to ipsilateral V(T) changes in the range +/- 100% of control V(T), and also the response of t(e) to ipsilateral expiratory volume changes.

Control of breathing during acute hemorrhage in anesthetized cats

We studied in anesthetized cats the ventilatory response to acute blood loss under hyperoxic iso-capnic conditions. From a blood pressure, of approximately 150 down to approximately 70 mm Hg ventilation increased on the average by 2.5 times. The ventilatory response was characterized by two phases: there was an initial phase (down to a PB approximately 100 mm Hg) of increase in frequency due to an excitatory effect on bulbo-pontine respiratory timing which was estimated from the duration of breaths following occlusion of the airways at the end expiratory volume. This effect was mainly due to the withdrawal of inhibitory afferents from the baroreceptors of the aortic arch. To this phase corresponded the phase of vasomotor compensation. Subsequently there was a phase of excitatory effect on the respiratory output which was estimated from the rate of change of the pressure developed in the airways during occluded breaths. This effect was mainly due to afferents from carotid sinuses; a minor role was due to the decrease in inhibitory afferents from carotid baroreceptors while the greater part was likely to be due to the hypoxic stimulation of glomus cells due to reduced blood flow. Following vagotomy above the superior laryngeal and sinus denervation the excitatory effect on respiratory timing and output were reduced to about 30% of that observed in intact cats.

Effects of intrapulmonary CO2 and airway pressure on phrenic activity and pulmonary stretch receptor discharge in dogs

Intrapulmonary CO2 and stretch sensitivity were studied in anesthetized, thoracotomized dogs in which both lungs were independently ventilated. The left pulmonary artery was occluded so that changes in left (Test) lung CO2 did not alter systemic arterial PCO2. Adequate gas exchange was maintained in the right lung, which was vagally denervated. In one series of experiments, neural activity in the C5 root of the phrenic nerve was integrated to assess ventilatory drive at various levels of Test lung CO2 and airway pressure both during cyclic ventilation and static lung inflation. In a second series, single unit activity in vagal afferents from pulmonary stretch receptors (PSR) was recorded. Both phrenic activity and PSR discharge were strongly affected by changes in airway pressure but not by changes in Test lung CO2 between 2 and 7%. However, when test lung CO2 was decreased below 2%, phrenic activity decreased and PSR activity increased. These changes were invariably accompanied by an increase in peak airway pressure during cyclic ventilation suggesting that lung mechanics had been altered. The results indicate that, in this preparation, the effects of lung stretch on ventilatory drive spans a wide range whereas intrapulmonary CO2 exerts an effect only at very low levels. It appears that the reflex effects of both stimuli can be accounted by an effect on PSR activity.

The role of vagal afferent information during inspiration in determining phrenic motoneurone output

Dogs were anaesthetised, paralysed and artifically ventilated using ramp inflations triggered by the onset of the 'integrated' phrenic signal. VT was initially adjusted to maintain a constant PaCO2 and the duration of inflation (TI(resp)) was adjusted to match the phrenic TI. With these control values of VT and TI(resp) a variety of experimental manoeuvres were performed to alter the pattern of volume information delivered during inspiration. These manoeuvres included: (1) alterations in VT while maintaining a constant TI(resp); (2) alterations in TI(resp) alone, with and without obstruction to expiration; (3) delays in the onset of inflation with and without a simultaneous reduction in TI(resp) and (4) the imposition of two inflations, each with the same reduced TI(resp) within the same inspiration. The results suggest that the pattern of vagally-mediated volume information during inspiration determines both the shape and the duration of phrenic motoneurone output. The importance of volume at inspiratory 'offswitch' as the sole determinant of TI is therefore denied. The rate of inflation was not found to contribute to the responses obtained.