Transient and steady state effects of CO-2 on mechanisms determining rate and depth of breathing

Vagal feedback is essential for breathing in unanesthetized ground squirrels

The roles of vagal afferent feedback in terminating inspiration and modulating breathing pattern and ventilatory responses to hypoxia and hypercapnia were assessed in the golden-mantled ground squirrel, Spermophilus lateralis, during wakefulness and urethane anesthesia. Hypoxia increased ventilation primarily through increases in breathing frequency (f(R)) while hypercapnia increased ventilation primarily through increases in tidal volume (V(T)) in both anesthetized and unanesthetized animals. Vagotomy resulted in an increase in tidal volume, a decrease in breathing frequency and ventilation, and depressed ventilatory responses to both hypoxia and hypercapnia in anesthetized animals. In unanesthetized animals vagotomy produced a transient 'gasp-like' breathing pattern that rapidly progressed to a non-obstructive central apnea. These data indicate that vagal feedback shapes ventilation on a breath-by-breath basis during anesthesia and is essential for ventilation in unanesthetized animals. The mechanisms that transform the influences of vagal input on breathing between anesthetized and unanesthetized states remain unclear. Changes in breathing pattern induced by the removal of vagal feedback compromise chemoreflexes.

Control of respiration in the isolated central nervous system of the neonatal opossum, Monodelphis domestica

Respiration represents an unusual motor activity with respect to its development. As newly born mammals enter the world, their limb movements are not coordinated; time and experience are required for effective performance to be achieved. Yet the rhythm of respiration is of necessity functionally perfected and unfailing at birth. Inspiratory and expiratory motor neurons are already able to fire at appropriate rates, under the command of rhythmically active neurons in the medulla. In this review, we discuss refinements of control present in the newborn opossum, particularly with respect to mechanisms that allow adaptation of respiration to changes in the level of activity or in the outside environment. Our own studies have been aimed at analyzing respiration at the earliest stages, and at establishing the way in which important variables influence inspiration and expiration. To this end, we have used the central nervous system (CNS) of a neonatal opossum, isolated in its entirety and maintained in culture. Although the opossum is unable to walk and highly immature at birth, its respiration is regular and unfailing. The isolated CNS survives, undergoes development, and maintains its neural activity and fine structure in vitro. Moreover, fictive respiration persists for over a day or longer at rates similar to those of the intact pup. The effects of altered pH, of increased temperature, and of drugs known to alter respiratory rhythm in intact animals can be measured directly, by electrical recordings made from medullary neurons or ventral roots. As in a slice, fluids of different composition can be applied focally, through micropipettes to the surface of the ventral medulla, or diffusely to the brainstem, With highly localized application of procaine hydrochloride (2%) to selected areas of the ventral medulla, the respiratory rhythm is reduced or abolished. As in adult mammals, both the rate and the amplitude of respiration simultaneously increase in response to lowered pH (6.5-.7.1) or to topical application of 1.0 microM carbachol. Conversely, as expected, the rate and amplitude decrease in response to increased pH (pH 7.5-7.7), or 100 microM scopolamine. Two characteristic features of the control of respiration in the neonatal opossum are evident from such tests. First, changes in rate are achieved by changes in the duration of the expiratory phase of respiration. This result suggests that the timing of the respiratory cycle in the neonatal opossum is controlled by an expiratory instead of an inspiratory "off-switch". Second, the rate and the amplitude of the respiratory excursions can be controlled independently, depending on the stimulus. For example, an increase in temperature increases the rate of fictive respiration without changing its amplitude, whereas noradrenaline decreases the rate while increasing the amplitude. Thus, changes of timing and amplitude need not go hand in hand. The opossum CNS offers a favorable preparation for the analysis of neural mechanisms that generate and modulate a motor rhythm, as the animal develops from embryonic to adult stages.

Chemosensitive medullary neurones in the brainstem--spinal cord preparation of the neonatal rat

1. Using the isolated medulla and spinal cord of the neonatal rat, the response to CO2-induced changes in superfusate pH was examined in whole cell and perforated patch recordings from ventral medullary neurones which were identified by injection of Lucifer Yellow. The respiratory response to changing the CO2 concentration (from 2 to 8%) consisted of an increase in phrenic burst frequency, which could be accompanied by an increase, decrease or no change in burst amplitude. 2. Five classes of neurone - inspiratory, post-inspiratory, expiratory, respiration-modulated and ionic - were distinguished on the basis of their membrane potential and discharge patterns. Almost all (112 of 123) responded rapidly to 8% CO2 with a sustained change in membrane potential. Depolarizing responses (3-18 mV) occurred in inspiratory, respiration-modulated and 45% of tonic neurones. Hyperpolarizing responses (2-19 mV) occurred in expiratory and post-inspiratory neurones. The remaining tonic neurones were inhibited or showed no response. 3. In representatives of each class of neurone, membrane potential responses to 8% CO2 were retained when tested in the presence of tetrodotoxin (n = 7), low (0.2 mM) Ca(2+)-high (5 mM) Mg2+ (n = 23) or Cd2+ (0.2 mM) (n = 3)-containing superfusate, implying that they are mediated by intrinsic membrane or cellular mechanisms. 4. Neurones were distributed between 1200 microns rostral and 400 microns caudal to obex, and their cell bodies were located between 50 and 700 microns below the ventral surface (n = 104). Almost all responsive neurones (n = 78) showed dendritic projections to within 50 microns of the surface. 6. These experiments indicate that significant numbers of ventral medullary neurones, including respiratory neurones, are intrinsically chemosensitive. The consistency with which these neurones show surface dendritic projections suggests that this sensitivity may arise in part at this level.

Localization of chemosensitive structures in the isolated brainstem of adult guinea-pig

1. Central respiratory chemosensitivity has been intensively examined but some questions remain unsolved; namely, what is the nature of the stimulus (fixed acid and/or CO2) and where is the site of brainstem chemosensitivity (near the ventral medullary surface or structures deeper within the brainstem)? To examine these questions, we used the in vitro isolated brainstem of adult guinea-pig perfused independently through the basilar artery and the bath. 2. Respiratory motor output was recorded with a suction electrode from cranial hypoglossal (XII) roots. Changes in pH and CO2 in the Krebs perfusate were made by changing either the bicarbonate concentration or the PCO2 saturating the Krebs solution. 3. Changes in basilar artery perfusate consisting of (i) an acidifying increase in PCO2 (hypercapnic acidic Krebs solution), (ii) an increase in PCO2 with no change in pH (hypercapnic Krebs solution), or (iii) a decrease in pH with no change in PCO2 (acidic Krebs solution) evoked increases in respiratory frequency and a concomitant decrease in inspiratory burst amplitude. 4. Bath superfusion with hypercapnic acidic Krebs solution increased the inspiratory burst amplitude with no effect on respiratory burst frequency. 5. Bath superfusion with hypercapnic non-acidic Krebs solution increased the inspiratory burst amplitude and decreased the respiratory frequency, while normocapnic acidic Krebs solution increased the respiratory frequency with no change in burst amplitude. 6. These results show that respiratory responses to changes in CO2 and pH depend upon the sites of action. While a CO2 increase or a pH decrease affected the respiratory frequency in the deep brainstem structures (perfused through the basilar artery), CO2 respiratory chemosensitivity at the ventral surface could be differentiated from the hydrogen ion chemosensitivity. This suggests that different mechanisms mediated respiratory responses when deep versus superficial brainstem structures were stimulated.

Postinspiratory-ramp activity of diaphragm under inspiratory resistive load

Inspiratory ramp activity, postramp activity (PRA) of diaphragm and respiratory flow were studied in anesthetized rabbits and conscious humans under inspiratory resistive load. Under load with chloralose-urethane anesthesia neural inspiration lasted 124 +/- 12 msec more than under control conditions and end of mechanical inspiration lagged behind that of neural inspiration by 114 +/- 4 msec. With pentobarbital-urethane anesthesia corresponding changes under load were 67 +/- 18 msec and 44 +/- 6 msec; after SO2 block of slowly adapting stretch receptors in bronchi corresponding changes under load were nil and 65 +/- 7 msec. In humans corresponding changes under load were about 192 and 127 msec. Both in rabbits and humans time course of PRA under load was essentially unchanged. Hence, under load a considerable part of PRA (or all of it under pentobarbital-urethane anesthesia without SO2 block) occurred during inspiration and that occurring during expiration was smaller than under control conditions. Consequently, under load braking of expiratory flow was smaller and peak expiratory flow occurred earlier than under control conditions.

Effect of hypoxic, hyperoxic or hypercapnic ventilation on inspiratory termination in the cat

Intercostal nerve stimulation was used to examine the effects of altered concentrations of inspired oxygen or carbon dioxide on the termination of inspiration. Experiments were performed in decerebrate cats which were paralyzed, artificially ventilated and bilaterally vagotomized. The threshold current at which electrical stimulation of the T6 intercostal nerve terminated phrenic neural activity was determined at 10 different delays from the onset of phrenic nerve discharge. Ventilation with either hypercapnic normoxic gas (4% CO2) or hypoxic gas (17% O2) increased the threshold current required for inspiratory termination. Hyperoxic ventilation (45% O2), however, decreased the threshold for inspiratory termination. Bilateral section of the carotid sinus nerve abolished the response to hyperoxic ventilation, but did not alter the response to normoxic hypercapnia. These results demonstrate that an oxygen-related stimulus transduced by the peripheral chemoreceptors can influence the mechanism(s) responsible for inspiratory termination.

The pattern of breathing in man in response to sine waves of alveolar carbon dioxide and hypoxia

Sine waves of alveolar CO2 at constant high alveolar O2, and sine waves of alveolar hypoxia (1/(PA, O2 -C) where C congruent to 32 torr) at constant alveolar CO2 have been administered to three subjects in each case. Six different periods of the sine waves were studied, ranging between 900 and 30 s for the CO2 sine waves and 300 and 20 s for the hypoxic sine waves. The sinusoidal variations in inspiratory and expiratory volumes (VT, I, VT, E), durations (TI, TE) and mean flows (vI, vE) produced by these manoeuvres were calculated, and the results analysed from the phase shifts of the responses. For the CO2 sine-wave results, the peak in the TI oscillation generally appeared after the minima for VT, I and vI, but before their maxima. The peak in the TE oscillation was variable. For the hypoxic sine-wave results, the peak in the TI oscillation showed no over-all tendency to lead or lag the peaks of VT, I and vI. The peak in the TE oscillation generally appeared after the maxima for VT, E and vE but before their minima. For the CO2 sine-wave results, expiratory mean flow led inspiratory mean flow, with the volumes showing no significant difference. For the hypoxic sine-wave results expiratory volumes and mean flows led inspiratory volumes and mean flows. The results are discussed in relation to the transient responses of the components of breathing pattern obtained from other perturbations of chemical drive.

Thoracoabdominal motion during progressive isocapnic hypoxia in conscious man

Respiratory frequency and separate rib-cage and abdomen-diaphragm compartmental tidal volume (VT) responses to progressive isocapnic hypoxia to a level of 75% O2 saturation (PO2 approximately 40 torr) were measured in eight healthy subjects in both seated and supine postures. Hypoxia was induced by a rebreathing technique and compartmental contributions to VT were measured by inductive plethysmography. The ventilatory, frequency and spirometric VT responses to hypoxia did not differ significantly between sitting and supine trials. The range of VT responsiveness to hypoxia among sitting individuals (0.71 +/- S.D. = 0.56% VC X % fall in Sa,O2(-1), where VC is vital capacity and Sa,O2 is arterial O2 saturation) was determined by both rib-cage (0.507 +/- S.D. = 0.378% VC X % fall in Sa,O2(-1) ) and abdomen-diaphragm (0.194 +/- S.D. = 0.178% VC X % fall fall in Sa,O2(-1) ) compartments. Among supine individuals the range of VT responsiveness to hypoxia (0.49 +/- S.D. = 0.45% VC X % fall in Sa,O2(-1) was determined equally by both rib-cage (0.27 +/- S.D. = 0.31% VC X % fall in SA,O2(-1) ) and abdomen-diaphragm (0.23 +/- S.D. = 0.29% VC X % fall in Sa,O2(-1) ) compartments. Thus, the VT response to progressive isocapnic hypoxia in conscious man is determined by both rib-cage and abdomen-diaphragm compartments regardless of posture.

The human ventilatory response to stimulation by transient hypoxia

1. The detailed pattern of transient changes in breathing pattern has been studied following a brief hypoxic stimulus (three breaths of nitrogen) in nine healthy subjects. All showed an increase in ventilation of which the magnitude and relative contributions of volume and frequency varied between subjects. 2. Ventilation, tidal volume, inspiratory, expiratory and total breath time were recorded or derived breath-by-breath; for each of these variables, several test sequences were time-averaged at half-second intervals for each individual; similarly, time-averages were obtained for percentage changes from base line over all nine subjects. 3. There ws an increase in inspiratory time accompanying the increasing tidal volume, in all but two subjects. This was statistically significant over all subjects, and in five individuals. Frequency changes were the resultant of alterations in the two phases; when total breath duration decreased it was always linked to a decrease in expiratory time. 4. Further analysis of the initial part of the response suggests that an increase of the duration of an inspiration may be the first change allowing an increase in tidal volume, before the 'drive' increases; this may be a dynamic feature of the control system whatever the nature and site of action of the stimulus.

Time course of intercostal afferent termination of the inspiratory process

The influence exerted by several somatic nerves on the inspiratory off-switch mechanism has been assessed in decerebrate cats. These animals were paralyzed, artificially ventilated, and bilaterally vagotomized. Inspiratory activity was monitored by a phrenic neurogram. Brief stimulation of either the superficial radical nerve or the sciatic nerve had an inconsistent effect on both the depth of inspiration and the timing of the respiratory cycle. However, stimulation of the T6 intercostal nerve during inspiration elicited a premature phase switch to expiration. Distinct, repeatable thresholds were determined for 10 delays in 100 msec increments after the onset of inspiration. As the delay increased, the threshold current was observed to decrease in all 30 decerebrate cats studied. An increase in the end-expiratory %CO2 caused an elevation of the stimulus threshold. These results correspond to the known characteristics of the inspiratory off-switch. Also, since the intercostal afferents are not normally a major determinant of respiratory rhythmicity in eupnea, this work establishes intercostal nerve stimulation as a very useful technique in the study of inspiratory to expiratory phase switching mechanisms.

Assessment of the central chemosensitivity in man under transient or progressive hypercapnia

In healthy man, the central chemosensitivity to CO2 was studied after depression of the arterial chemoreflex drive by inhalation of pure oxygen. The effectiveness of the functional decrease of arterial chemoreceptor function was assessed by the delayed hyperventilation which followed transient inhalation of hypercapnic gas mixtures for 3 or 5 breaths in hyperoxic conditions. In such a case the first significant increase in tidal volume (VT) occurred 13.9 +/- 3.2 (SE) sec later than the early change in this variable measured in normoxic conditions. The stimulus strength was estimated by the change in CO2 partial pressure in end-tidal alveolar gas (delta PETCO2). The central chemosensitivity (SCO2), defined as the ratio between change in ventilation (delta V) and delta PETCO2, was assessed either by transient inhalation of gas mixtures containing 5 to 8% CO2 in pure O2 ("varying transients") or by progressive hypercapnia (rebreathing in pure O2). In both cases, the first significant change in ventilation was due to an increase in VT, but, for a given delta PETCO2, VT changes were higher during rebreathing than after transient hypercapnia; (2) The respiratory frequency (fR) was progressively enhanced during rebreathing (shortening of expiratory duration in all cases and of inspiratory time in some subjects) but the ventilatory rhythm diminished after transient stimulation as soon as delta PETCO2 reached one kPa, and this was due to an increase in inspiratory duration; (3) The associated changes in VT and fR during rebreathing could explain that SCO2 values given by this method were 5.2 times greater than after transient hypercapnia ("varying tests"). The differences are discussed in terms of, (1) isolated changes in arterial PCO2 or associated decrease in pH of the cerebrospinal fluid; (2) changes in brain blood flow, and (3) stimulation of lung stretch receptors by the important increase in VT during rebreathing.

The pattern of breathing following step changes of alveolar partial pressures of carbon dioxide and oxygen in man

1. The pattern of breathing during the approach to the steady state following step changes of end-tidal P(CO2) and P(O2) has been determined in normal conscious human subjects. Three types of step were studied: (a) steps of P(A, CO2) against a constant background of hyperoxia (P(A, O2) approximately 200), an almost pure intracranial chemoreceptor stimulus, (b) steps of P(A, O2) between approximately 50 and 80 torr against a background of constant mild hypercapnia, an arterial chemoreceptor stimulus, and (c) steps of P(A, CO2) against a background of constant hypoxia (P(A, O2) approximately 50), a mixed stimulus. Steps were small and the responses barely detectable by the subjects.2. Steps of CO(2) in hyperoxia produced the slowest approach to the steady state. A single exponential fitted the ventilation response up to about 4 min (mean half time 83 sec for the ;up' and 69 sec for the ;down' transients). During the transient the pattern of change of tidal volume (V(T)) and expiratory time (T(E)) was the same as in the steady state. Inspiratory time (T(I)), however, in the early part of the transient, changed in the opposite direction to T(E), returning to its steady value only after 1(1/2)-3 min. This effect occurred in both ;up' and ;down' transients and resulted in a smaller change of respiratory frequency than would have been predicted from the steady-state response.3. Hypoxic steps produced the fastest approach to the steady state with mean half-times for ventilation of 10.9 sec for the ;up' transients and 6.6 sec for the ;down'. T(I) followed the same pattern during the transient as in the steady state, whereas T(E), following the step out of hypoxia, lengthened to far beyond its final steady value within five breaths of the step, only returning to its steady-state value 3-4 min after the step. This resulted in an exaggerated change of frequency during the early part of the transient.4. Steps of CO(2) in hypoxia, a mixed peripheral and central chemoreceptor stimulus, showed a ventilation response which was best fitted by two exponentials, the half-times of which were consistent with those obtained for the separate responses. The patterning was also consistent with a mixed response, more so for T(I) than for T(E).5. The steady-state pattern derived from the pre-switch means was consistent with the pattern previously described.6. Possible mechanisms are discussed. It is suggested that these results could explain the different patterns seen in the past by those using re-breathing and steady-state techniques.7. The validity of using one or two breath oxygen or nitrogen tests (or other similar tests) as a quantitative measure of the hypoxic response in man is questioned.

Experimentally induced Cheyne-Stokes breathing

We have studied the propensity for periodic breathing to occur in cats anaesthetized with pentobarbitone breathing either spontaneously or with the aid of a 'servo-respirator' governed continuously by the efferent phrenic nerve activity. Sustained periodic breathing could be induced increasing 'controller gain', either by increasing the gain of the respirator, or by lung deflation, which reflexly increased controller responses to both hypoxia and hypercapnia. Periodic breathing was potentiated both by hypoxia and by diminishing the central (CO2, H+)-drive by focal cooling at the ventral surface of the medulla, two procedures which increase the relative influence of hypoxic drive. Less hypoxia was needed to produce periodic breathing at high rather than low controller gains. Reducing controller gain to zero by constant artificial respiration always abolished periodic breathing. Periodic breathing was also eradicated when the relative importance of CO2 drive was enhanced by breathing the cats with CO2-enriched gas mixtures or with 100% O2. The results are consistent with theoretical predictions for the occurrence of oscillations in the mechanisms for the chemical control of breathing and indicate that increasing controller gas can produce periodic breathing. The results further emphasize the importance of the (CO2, H+)-drive in preserving ventilatory stability.

Transient hypoxia: ventilatory response after vagotomy and during artificial phasic inflation

The early ventilatory response to transient hypoxia was examined in the anaesthetized rabbit. In intact spontaneously breathing animals, an increase in tidal volume (VT) with an accompanying slight increase in inspiratory duration (TI) and a decrease in the expiratory duration (TE) was observed. After vagotomy, the ventilatory response was distinguished by a greater increase in VT and a significant decrease in TI and TE. In another group of artificially ventilated rabbits, an increase in inspiratory volume with a simultaneous decrease in breathing frequency was found to involve a smaller reflex increase in phrenic inspiratory discharge after onset of transient hypoxia. These observations suggest that afferents from pulmonary vagal stretch receptors inhibit those from arterial chemoreceptors.

An analysis of respiratory frequency alterations in vagotomized, decerebrate cats

Changes in inspiratory (TI), expiratory (TE) and total respiratory cycle (TTOT) durations with hypercapnia- of hypoxia-induced tidal volume (VT) elevations were evaluated in midcollicular decerebrate cats. As VT increased, TI, TE, and TTOT decreased for most animals having intact vagi. Following vagotomy, TI, TE, and TTOT increased with hypercapnia for cats which had TI, TE and TTOT values shorter than 1.6, 3.7 and 5.2 sec respectively while breathing 100% O2; values longer than these forecast hypercapnia-induced decreases in each parameter. Similar systematic changes were not evident for hypoxia-induced responses. Varying the midbrain transection level or pentobarbital administration altered TI, TE and TTOT values while breathing 100% O2; however, the predictability of hypercapnia-induced responses, based on data analysis from midcollicular decerebrate cats, was maintained. It is concluded that the vagally-independent brainstem frequency controller is sensitive to hypercapnia and hypoxia. The predictability of hypercapnia-induced TI, TE and TTOT changes in vagotomized animals is considered in the context of previous models for respiratory rhythm generation.

Influence of rate of induction of hypoxia on the ventilatory response

1. We have studied the influence of rate of induction of hypoxia on the duration of the phases of the respiratory cycle in seven conscious healthy subjects.2. Nine hypoxic procedures were performed on each subject, comprising three different rates of induction of hypoxia each at three different levels of P(CO2). Ventilation vs. arterial O(2) saturation plots were constructed and patterns of response were analysed in terms of ventilation vs. tidal volume and tidal volume vs. inspiratory duration (T(I)) and vs. expiratory duration (T(E)).3. The highest ventilatory responses were observed during steady-state hypoxia when P(CO2) was held at a level similar to the control mixed venous level; progressive hypoxia at this P(CO2) failed to stimulate ventilation to comparable levels. The ventilatory response was not influenced by rate of induction of hypoxia when P(CO2) was maintained at an end-tidal P(CO2) close to the control level.4. As tidal volume increased in response to hypoxia, T(I) and T(E) shortened. The progressive decrease in cycle duration occurred at all levels of P(CO2) and at all rates of induction of hypoxia, but was most marked under eucapnic conditions during steady state.5. It is concluded that steady-state hypoxia may produce a higher ventilatory response than progressive, non-steady-state hypoxia. Inspiratory time shortens as tidal volume increases irrespective of the rate of induction of hypoxia.

Effects of single breath lung inflation on the pattern of subsequent breaths

In rabbits, on release of lung inflations 0.4 to 3.3 times control VT and lasting 1 to 30 sec, VT, peak diaphragmatic activity (Ep) and inspiratory duration (Ti) increased, whereas expiratory duration (Te) decreased relative to pre-inflation values. Similar changes occurred between pre- and postinflation occluded breaths. These changes lasted from a few breaths up to 30 sec, and were positively correlated with magnitude and duration of inflations. Postinflation changes of pulmonary stretch receptor activity were relatively small and limited to 1-3 breaths. At chemical drive close to control: (a) postinflation VT vs Ti relationship moved to the right without changing its slope, Ti occluded eventually exceeding Ti after vagotomy; (b) the Te vs Ti relationship moved downwards, its slope being decreased and eventually abolished; (c) the average rate of rise of E was decreased. An increase of VT, Ep and Ti, and a decrease Te also occurred on release of stimulation of the central ends of the cut vagi producing apnea at FRC in mono- and bilateral vagotomized rabbits. Postinflation effects were mainly of central origin and tentatively explained as rebound phenomena within the respiratory center.

The relation between hypoxia and CO2-induced reflex alternation of breathing in man

Four healthy young volunteers, selected for the responsiveness and steadiness of their breathing, were studied in rest and mild exercise while receiving alternate inspirates of low and high PCO2 (0 and 8.6 kPa). PACO2, oscillated between ca. 6 and 7.5 kPa (45-55 torr). PAO2 was held steady at 4-7 levels between 6 and 28 kPa (45-210 torr). Thirteen separate inspiratory and expiratory variables (volumes, times, flows) were recorded and tested for reflex alternation. Matched controls were performed. Responses were generally small in relation to the scatter. Reflex alternation of any one variable was not always evident. The incidences of the responses were, in descending order, inspiratory flows and volumes, expiratory flows and volumes, expiratory duration; inspiratory duration alternated seldom, and then with only small amplitude. Reflex alternation was more likely to be observed in hypoxia than in euoxia or hyperoxia. A tendency for the incidences to be greater in exercise than at rest was not significant, but the amplitudes of alternation showed a significant difference in favour of exercise. In a substantial minority of experiments the amplitude of reflex alternation was significantly and positively correlated with hypoxia (1/(PAO2--C)). Alternation also occurred frequently in another substantial minority of experiments in which, however, there was no significant amplitude-hypoxia correlation. It was concluded that these two groups probably differed not so much in the form of the amplitude-hypoxia relation as in respect of the extent of the scatter in the observations. The results are consistent with interaction of non-steady-state with steady-state signals at the arterial chemoreceptors.

Separation of the inspiratory and expiratory reflex effects of alternate-breath oscillation of PACO2 during hypoxia

Four healthy young men and women, selected for the responsiveness and steadiness of their breathing, were studied in rest and mild exercise (58 runs) while receiving alternate inspirates of low and high PCO2 (0 and 8.6 kPa). PACO2 oscillated between ca. 6 and 7.5 kPa (45-55 torr); PACO2 was held steady at more than one level between 6 and 9.6 kPa (45-72 torr). Using cross-correlation analysis, the phase relations were determined between the alternating PACO2 and the following reflex outputs: mean inspiratory and expiratory flows (VI and VE) and the reciprocal of the duration of expiration (1/TE), the two expiratory variables being lumped together for purposes of expression, but not of calculation. T1, being relatively unaffected alternating PACO2, was not re-studied (see companion paper). The common patterns of significant reflex alternation were: VI alone, usually in phase (with PACO2), 24%; VE alone, usually in phase, 17%; both inspiratory and expiratory variables, in phase with CO2 and each other, 15%; both inspiratory and expiratory variables, the expiratory being out of phase with both CO2 and with the inspiratory, 23%. Some runs showed a mixture of phase relations. In 71%, end-expiratory lung volume (VL,E' formerly called FRC) alternated significantly. It is concluded that expiratory events can be influenced by peripheral chemoreceptors independently of inspiration.

Interaction between vagal and chemoreceptors afferents in ventilatory response to transient hypercapnia (anaesthetized rabbit)

In rabbits anaesthetized with ethyl-carbamate, stimulation of chemoreceptors afferents was allowed by transient hypercapnia, before and after vagal blockade by DC current. In these relatively fast breathing animals, the transient hypercapnia produced light changes of inspiratory tidal volume (VI), inspiratory (TI) and expiratory durations (TE). Despite the identity of transient hypercapnia, it ensued that: (1) the higher the spontaneous VI and the lower the respiratory frequency (fR), the greater their respective changes (deltaVI and deltafR) during the ventilatory response; (2) after vagal blockade, greater changes in VI, TI, TE and mean inspiratory flow rate (VI/TI) occurred than in control state, while the relation between deltafR and fR was more significant than in control state. Respective roles played by vagal and chemoreceptors afferents in the ventilatory response to transient hypercapnia are discussed.

The direct effect on pulmonary stretch receptor discharge produced by changing lung carbon dioxide concentration in dogs on cardiopulmonary bypass and its action on breathing

1. Single fibre pulmonary stretch receptor discharge was recorded in dogs on cardiopulmonary bypass. 2. Inhalation of CO2 depressed pulmonary stretch receptor discharge despite the absence of changes in arterial PCO2. This effect was particularly marked with airway CO2 levels below 5%. 3. Changing arterial PCO2, without changing airway CO2, had only small and insignificant effects on pulmonary stretch receptor discharge. 4. The effect of changes in airway CO2 on pulmonary stretch receptor discharge was rapid and correlated well in time with the reflex tachypnoea produced when CO2 was inhaled in conditions of cardiopulmonary bypass. 5. Stimulation of the central end of the cut vagus nerve was triggered from simultaneously recorded action potentials from a single pulmonary stretch receptor. 6. In these conditions, the reflex response to CO2 could be simulated provided that the pulmonary stretch receptor had an end-expiratory discharge. 7. It is suggested that the vagally mediated tachypnoeic response to changes in airway CO2 seen in conditions of cardiopulmonary bypass is due to the effect of CO2 on the end-expiratory discharge of pulmonary stretch receptors.

Respiratory frequency control during hypercapnia in vagotomized, anesthetized cats

There is a small, but significant, increase in frequency during hypercapnia in vagotomized, anesthetized animals, indicating involvement of an extravagal mechanism in the response. The intent of this study was to determine the source of this second mechanism regulating frequency during hypercapnia. Experiments were performed on 22 vagotomized, anesthetized (Dial) cats. Frequency (f), inspiratory time (ti) and expiratory time (te) responses to CO2 were monitored before and after sectioning of afferent nerves from the carotid bodies (carotid sinus nerve section), chest wall (dorsal rhizotomies, T1-T12) and diaphragm (dorsal rhizotomies. C4-C7). Most vagotomized animals responded to 6% CO2 with an increased frequency, decreased ti and no consistent change in te. The responses to CO2 were essentially unaltered following chest wall and diaphragm deafferentation. Sodium cyanide stimulation of the carotid bodies produced similar respiratory pattern changes as CO2; furthermore, the f and ti changes with CO2 were still present following carotid body deafferentiation. The results of this study suggest that: (1) afferents from chest wall and diaphragm mechanoreceptors are not responsible for the vagal-like effects on ti and f during hypercapnia, (2) afferents from lung mechanoreceptors, via the vagus nerves, are the only inputs from respiratory mechanoreceptors causing an increased f during hypercapnia, (3) the extravagal mechanism responsible for the decreased ti and increased f during hypercapnia is inherent to the medullary-pontine rhythm generator, and (4) input from the chemoreceptors can elicit the response.

The effect of lung reflexes on the pattern of breathing in cats

Tidal volume (VT), and inspiratory (tI) AND EXPIRATORY (TE) durations have been measured in anaesthetized cats on stimulation of alveolar type J (nociceptive) and lung irritant receptors by intravenous injections of phenyl diguanide and of histamine acid phosphate respectively. The reflexes were studied during eupnoea, hypercapnic and hypoxic hyperpnoea, during rebreathing from hyperventilation apnoea and at different body temperatures. In all conditions the drugs caused rapid shallow breathing with reduction in VT, tI and tE. The VT/tI relationship for injection of the drugs was different from that caused by hypercapnic stimulation of breathing, but the tI/tE relationship was proportionally similar for all conditions. Recording single unit activity from phrenic motor fibres showed that the lung reflexes had little action on the initial frequency of discharge of the fibres, but cut short the discharge earlier than for the controls. The results are interpreted in terms of the ways in which lung reflexes can modify the pattern of breathing.

The effect of limb movements on the regulation of depth and rate of breathing

In anesthetized dogs and rabbits passive or active limb movements (1) shifted to the left the relationship between tidal volume (Vt) and inspiratory time (Ti), (2) lowered the relationship between expiratory time (Te) and Ti and decreased its slope, and (3) increased the output to the inspiratory muscles (Vt/Ti). These effects increased with increasing the frequency of movements. Similar effects were obtained after vagotomy. When the stimulus was started during expiration. Te was shorted in spite of the previous unaffected Ti, Arterial PCO2 during exercise was similar (active movements) or below (passive movements) control value. Since other chemical and physical humoral factors do not seem involved, the whole increase of ventilation should be produced by neurogenic stimuli. The time course of Te, Ti and VT/Ti at the onset and at the offset of limb movements indicates an abrupt and a slow component in the neurogenic drive. A single contraction of the limb during expiration of inspiration affected the timing and the VT/Ti of 3-8 breaths.

Respiratory frequency response to progressive isocapnic hypoxia

1. Ventilatory, tidal volume and frequency responses to progressive isocapnic hypoxia have been measured in twenty-nine healthy subjects by a rebreathing technique. 2. A strong correlation was found between ventilatory response to hypoxia (deltaVI/DELTASaO2) and frequency response to hypoxia (deltaf/deltaSaO2) (r=0-82, P less than 0-001). There was a lesser correlation between deltaV1/deltaSaO2 and tidal volume response (deltaVT/deltaSaO2) (r=0-50, P less than 0-01). These findings suggest that the wide range of ventilatory response to hypoxia among subjects is mainly determined by differences in frequency response and contrast with previous findings in studies of the response to progressive hypercapnia. 3. The breathing pattern during progressive hypoxia and hypercapnia was compared in ten subjects. Ventilation/tidal volume plots were constructed and patterns of response were further analysed in terms of inspiratory duration (TI), expiratory duration (TE) and mean inspiratory flow rate (VI). 4. Increments in ventilation during hypoxia were achieved with a greater respiratory frequency and a smaller tidal volume than during hypercapnia in eight of the ten subjects studied. In two subjects no difference in breathing pattern during hypoxia and hypercapnia was observed. 5. Changes in respiratory frequency during progressive hypoxia were achieved in all subjects by a progressive shortening of TI and TE. By contrast, TI remained constant during hypercapnia until VT had increased to 3-5 times the eupnoeic value; during hypercapnia the increase in frequency was achieved mainly by a progressive shortening of TE. 6. It is concluded that different mechanisms may be involved in altering respiratory frequency when ventilation is driven progressively by these different chemical stimuli.

Excitability changes of the inspiratory "off-switch" mechanism tested by electrical stimulation in nucleus parabrachialis in the cat

The time course of the excitability of the inspiratory "off-switch" mechanism with and without phasic vagal stretch receptor feedback has been studied in cats under light pentobarbitone anesthesia by electrical stimulation in the rostral pons using brief tetanic stimulation (300 Hz for 0.2 s). The threshold strength required just to elicit inspiratory "off-switch" was high early in inspiration and fell steeply with time. The threshold curves were steeper with than without phasic vagal feedback, and the difference reflects the phasic vagal contribution to the excitability of the inspiratory "off-switch" the absence of phasic vagal vagal feedback the time course of this threshold curve usually corresponded closely to that of the "integrated" phrenic activity at all PCO2 levels and body temperatures tested indicating that the "integrated" phrenic activity can be used as an index of the centrally generated inspiratory activity. In response to a rise in PCO2 both the rate of change of excitability of the inspiratory "off-switch" mechanism and its initial threshold level was increased. Changes in body temperature caused no change in the initial threshold but produced marked changes in the rate of rise of the "off-switch" excitability; Following an "augmented breath" the inspiratory "off-switch" threshold was markedly reduced

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.

Role of peripheral and central chemosensitive afferents in the control of depth and frequency of breathing

In anaesthetized cats the response to hypercapnia was studied during normoxia, hypoxia (increased peripheral chemosensitive afferents, IPA) and after carotid sinus denervation (decreased peripheral chemosensitive afferents, DPA) in terms of: (a) central output to the inspiratory muscles, (b) bulbopontine respiratory activity and (c) threshold-inhibition curve for termination of inspiration (Vr/Ti relationship). IPA increased, whereas DPA decreased the central output, as estimated from the rate of change of the subatmospheric pressure developed in the trachea during occlusion of the airways. IPA and DPA did not modify the bulbo-pontine rhythm, as estimated from the timing of occluded breaths (phasic vagal afferents nil), nor the Vt/Ti relationship, both of which have been shown to vary as a function of PACO2. It is concluded that peripheral chemosensitive afferents influence only the output of the respiratory centres, whereas central chemosensitive afferents influence also the bulbo-pontine respiratory rhythm and the threshold inhibition curve for termination of inspiration.

Contribution of hypercapnic stimuli and of vagal afferents to the timing of breathing in anesthetized cats

In tracheotomized, anesthetized cats, the authors studied the role of phasic and tonic vagal discharge and of hypercapnic stimuli on the timing of breathing. The effect of tonic vagal discharge was separated from that of the other two parameters by comparing the duration of inspiration and of expiration of control breaths with those obtained during occlusions of the airways at FRC (bulbo-pontine activity). The effect of tonic vagal discharge was then separated from that of hypercapnic stimuli by comparing the bulbo-pontine activity before and after vagotomy. Hypercapnia caused shortening of inspiratory and expiratory duration set by the bulbo-pontine pacemaker and increased sensitivity of the respiratory centers for a given phasic vagal input (displacement to the left of the tidal volume vs inspiratory duration relationship). Vagotomy did not modify the bulbo-pontine duration of inspiration nor its possibility to shorten in hypercapnia; by contrast it caused a lengthening of the expiratory time which did not shorten any more in hypercapnia, suggesting that tonic vagal discharge mainly influences the expiratory duration.