Influence of peripheral O2 tension on the ventilatory response to CO2 in cats

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Hypercapnia attenuates inspiratory amplitude and expiratory time responsiveness to hypoxia in vagotomized and vagal-intact rats

A negative influence of central chemosensitivity on peripheral chemoreflex response has been demonstrated recently in a decerebrate-vagotomized rat preparation in situ with separate carotid body and brainstem perfusions. Here, we report similar negative influences of hypercapnia on the hypoxic respiratory response in anesthetized, spontaneously breathing rats before and after vagotomy and anesthetized, artificially ventilated rats after vagotomy. Baseline breathing patterns and responsiveness to hypercapnia and hypoxia varied widely between the three respiratory modes. Despite this, the responses in inspiratory amplitude and expiratory duration (and hence respiratory frequency and neural ventilation) to hypoxia varied inversely with the background CO2 level in all three groups. Results demonstrate a hypoadditive hypercapnic-hypoxic interaction in vivo that resembles the hypoadditive central-peripheral chemoreceptor interaction in situ for these respiratory variables in the rat, regardless of differences in vagal feedback, body temperature and ventilation method. These observations stand in contrast to previous reports of hyperadditive peripheral-central chemoreceptor interaction.

Peripheral chemoreceptors determine the respiratory sensitivity of central chemoreceptors to CO(2)

We assessed the contribution of carotid body chemoreceptors to the ventilatory response to specific CNS hypercapnia in eight unanaesthetized, awake dogs. We denervated one carotid body (CB) and used extracorporeal blood perfusion of the reversibly isolated remaining CB to maintain normal CB blood gases (normoxic, normocapnic perfusate), to inhibit (hyperoxic, hypocapnic perfusate) or to stimulate (hypoxic, normocapnic perfusate) the CB chemoreflex, while the systemic circulation, and therefore the CNS and central chemoreceptors, were exposed consecutively to four progressive levels of systemic arterial hypercapnia via increased fractional inspired CO(2) for 7 min at each level. Neither unilateral CB denervation nor CB perfusion, per se, affected breathing. Relative to CB control conditions (normoxic, normocapnic perfusion), we found that CB chemoreflex inhibition decreased the slope of the ventilatory response to CNS hypercapnia in all dogs to an average of 19% of control values (range 0-38%; n = 6), whereas CB chemoreflex stimulation increased the slope of the ventilatory response to CNS hypercapnia in all dogs to an average of 223% of control values (range 204-235%; n = 4). We conclude that the gain of the CNS CO(2)/H(+) chemoreceptors in dogs is critically dependent on CB afferent activity and that CNS-CB interaction results in hyperadditive ventilatory responses to central hypercapnia.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

An interdependent model of central/peripheral chemoreception: evidence and implications for ventilatory control

In this review we discuss the implications for ventilatory control of newer evidence suggesting that central and peripheral chemoreceptors are not functionally separate but rather that they are dependent upon one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the carotid body chemoreceptors and vice versa i.e., they are interdependent. We examine potential interactions of the interdependent central and carotid body (CB) chemoreceptors with other ventilatory-related inputs such as central hypoxia, lung stretch, and exercise. The limitations of current approaches addressing this question are discussed and future studies are suggested.

A negative interaction between brainstem and peripheral respiratory chemoreceptors modulates peripheral chemoreflex magnitude

Interaction between central (brainstem) and peripheral (carotid body) respiratory chemosensitivity is vital to protect blood gases against potentially deleterious fluctuations, especially during sleep. Previously, using an in situ arterially perfused, vagotomized, decerebrate preparation in which brainstem and peripheral chemoreceptors are perfused separately (i.e. dual perfused preparation; DPP), we observed that the phrenic response to specific carotid body hypoxia was larger when the brainstem was held at 25 Torr P(CO(2)) compared to 50 Torr P(CO(2)). This suggests a negative (i.e. hypo-additive) interaction between chemoreceptors. The current study was designed to (a) determine whether this observation could be generalized to all carotid body stimuli, and (b) exclude the possibility that the hypo-additive response was the simple consequence of ventilatory saturation at high brainstem P(CO(2)). Specifically, we tested how steady-state brainstem P(CO(2)) modulates peripheral chemoreflex magnitude in response to carotid body P(CO(2)) and P(O(2)) perturbations, both above and below eupnoeic levels. We found that the peripheral chemoreflex was more responsive the lower the brainstem P(CO(2)) regardless of whether the peripheral chemoreceptors received stimuli which increased or decreased activation. These findings demonstrate a negative interaction between brainstem and peripheral chemosensitivity in the rat in the absence of ventilatory saturation. We suggest that a negative interaction in humans may contribute to increased controller gain associated with sleep-related breathing disorders and propose that the assumption of simple addition between chemoreceptor inputs used in current models of the respiratory control system be reconsidered.

Role of the peripheral chemoreflex in the early stages of ventilatory acclimatization to altitude

This review of ventilatory acclimatization to altitude/hypoxia (VAH) emphasizes the widely differing timescales that VAH is considered to encompass. The review concludes: (1) that early (24-48h) VAH is unlikely to arise as a reaction to the respiratory alkalosis that is normally associated with exposure to hypoxia; (2) that changes in peripheral chemoreflex function may be sufficiently rapid to explain early VAH; (3) that alterations in gene expression induced by hypoxia through the hypoxia-inducible factor (HIF) signalling pathway may underlie a major component of VAH; and (4) that compensatory adjustments to acid-base balance in response to the initial respiratory alkalosis may have more significance for the slower changes observed later in VAH.

Prior sustained hypoxia attenuates interaction between hypoxia and exercise as ventilatory stimuli in humans

Both exercise and hypoxia increase pulmonary ventilation. However, the combined effects of the two stimuli are more than additive, such that exercise may be considered to potentiate the acute ventilatory response to hypoxia (AHVR), and vice versa. Exposure to sustained hypoxia of 8 h duration or more has been shown to increase the acute chemoreflex responses to hypoxia and hypercapnia. The purpose of this study was to determine whether sustained exposure to hypoxia also changed the stimulus interaction between the effects of exercise and hypoxia on ventilation. Ten subjects undertook two main protocols on two separate days. On one day, subjects were exposed to isocapnic hypoxia (IH) at an end-tidal partial pressure of O(2) of 55 mmHg and on the other day, subjects were exposed to air as a control (C). Before and after each exposure, the sensitivity of AHVR was assessed during both resting conditions and exercise at 35% of the subjects' maximal oxygen uptake capacity. Average values (means +/- s.d.) obtained for the sensitivity of AHVR from protocol IH were 0.85 +/- 0.35 (rest, prehypoxic exposure), 1.60 +/- 0.66 (exercise, prehypoxic exposure), 1.69 +/- 0.63 (rest, posthypoxic exposure) and 1.81 +/- 0.86 l min(-1) %(-1) (exercise, posthypoxic exposure). A non-dimensional variable, Phi, was used to quantify the interaction present between exercise and hypoxia. The variable Phi fell significantly following the sustained exposure to hypoxia (P < 0.02, ANOVA), indicating that the degree of stimulus interaction between acute hypoxia and exercise had declined. We suggest that the mechanisms by which sustained hypoxia modifies peripheral chemoreflex function may also modify the effects of exercise on the peripheral chemoreflex.

Brainstem PCO2 modulates phrenic responses to specific carotid body hypoxia in an in situ dual perfused rat preparation

Inputs from central (brainstem) and peripheral (carotid body) respiratory chemoreceptors are coordinated to protect blood gases against potentially deleterious fluctuations. However, the mathematics of the steady-state interaction between chemoreceptors has been difficult to ascertain. Further, how this interaction affects time-dependent phenomena (in which chemoresponses depend upon previous experience) is largely unknown. To determine how central P(CO2) modulates the response to peripheral chemostimulation in the rat, we utilized an in situ arterially perfused, vagotomized, decerebrate preparation, in which central and peripheral chemoreceptors were perfused separately (i.e. dual perfused preparation (DPP)). We carried out two sets of experiments: in Experiment 1, we alternated steady-state brainstem P(CO2) between 25 and 50 Torr in each preparation, and applied specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) under both conditions; in Experiment 2, we applied four 5 min bouts (separated by 5 min) of specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) while holding the brainstem at either 30 Torr or 50 Torr P(CO2). We demonstrate that the level of brainstem P(CO2) modulates (a) the magnitude of the phrenic responses to a single step of specific carotid body hypoxia and (b) the magnitude of time-dependent phenomena. We report that the interaction between chemoreceptors is negative (i.e. hypo-additive), whereby a lower brainstem P(CO2) augments phrenic responses resulting from specific carotid body hypoxia. A negative interaction may underlie the pathophysiology of central sleep apnoea in populations that are chronically hypocapnic.

Instability of spontaneous breathing patterns in patients with persistent vegetative state

We investigated the breathing patterns of 27 patients in a persistent vegetative state (PVS) and 15 normal control volunteers. During the baseline period breathing air, 15 patients (the PVS-IB) exhibited irregular breathing (IB), whereas the other 12 (the PVS-OB) displayed oscillatory breathing (OB). Both groups maintained an average value for tidal volume (V(T)), total breath duration (T(TOT)), minute ventilation (V (E)), oxygen saturation (SpO2) similar to the control, but the PVS-OB displayed significantly lower end-tidal CO2 tension (P(ET)CO2) than the control. The V(T), T(TOT), V (E) and P(ET)CO2 of the PVS-OB showed cyclic changes. The coefficients of variation of V(T), T(TOT) and V (I) were: PVS-OB>PVS-IB>control. Inhalation of 100% O2 significantly reduced the respiratory variability and prevented OB of the PVS-OB. We concluded that PVS patients display respiratory instability and that brain damage, hypocapnia, and/or increased loop gain of arterial chemoreceptors may contribute to the pathogenesis of OB, whereas brain damage presumably may be the cause of IB.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

The respiratory response to carbon dioxide in humans with unilateral and bilateral resections of the carotid bodies

The acute hypercapnic ventilatory response (AHCVR) arises from both peripheral and central chemoreflexes. In humans, one technique for identifying the separate contributions of these chemoreflexes to AHCVR has been to associate the rapid component of AHCVR with the peripheral chemoreflex and the slow component with the central chemoreflex. Our first aim was to validate this technique further by determining whether a single slow component was sufficient to describe AHCVR in patients with bilateral carotid body resections (BR) for glomus cell tumours. Our second aim was to determine whether the slow component of AHCVR was diminished following carotid body resection as has been suggested by studies in experimental animals. Seven BR subjects were studied together with seven subjects with unilateral resections (UR) and seven healthy controls. A multifrequency binary sequence in end-tidal PCO2 was employed to stimulate ventilation dynamically under conditions of both euoxia and mild hypoxia. Both two- and one-compartment models of AHCVR were fitted to the data. For BR subjects, the two-compartment model fitted significantly better on 1 out of 13 occasions compared with 22 out of 28 occasions for the other subjects. Average values for the chemoreflex sensitivity of the slow component of AHCVR differed significantly (P < 0.05) between the groups and were 0.95, 1.38 and 1.50 l min-1 Torr-1 for BR, UR and control subjects, respectively. We conclude that, without the peripheral chemoreflex, AHCVR is adequately described by a single slow component and that BR subjects have sensitivities for the slow component that are lower than those of control subjects.

Selected contribution: High-altitude natives living at sea level acclimatize to high altitude like sea-level natives

Sea-level (SL) natives acclimatizing to high altitude (HA) increase their acute ventilatory response to hypoxia (AHVR), but HA natives have values for AHVR below those for SL natives at SL (blunting). HA natives who live at SL retain some blunting of AHVR and have more marked blunting to sustained (20-min) hypoxia. This study addressed the question of what happens when HA natives resident at SL return to HA: do they acclimatize like SL natives or revert to the characteristics of HA natives? Fifteen HA natives resident at SL were studied, together with 15 SL natives as controls. Air-breathing end-tidal Pco(2) and AHVR were determined at SL. Subjects were then transported to 4,300 m, where these measurements were repeated on each of the following 5 days. There were no significant differences in the magnitude or time course of the changes in end-tidal Pco(2) and AHVR between the two groups. We conclude that HA natives normally resident at SL undergo ventilatory acclimatization to HA in the same manner as SL natives.

Is ventilatory acclimatization to hypoxia a phenomenon that arises through mechanisms that have an intrinsic role in the regulation of ventilation at sea level?

The purpose of this article is to set out the hypothesis that arterial PO2 may play a significant role in the regulation of breathing at sea level. The following points are made: 1) Although CO2 is clearly the dominant feedback signal in the acute setting, there is evidence, particularly clinical observation, that the ventilatory response to CO2 may adapt. 2) Although the ventilatory response to an acute variation in alveolar PO2 around sea-level values is feeble, studies at altitude have shown that over longer-time periods alveolar PO2 is a more powerful regulator of ventilation. 3) Recent evidence suggests that mechanisms associated with ventilatory acclimatization to hypoxia are active at sea-level values for PO2, and indeed affect the acute ventilatory response to hypoxia. 4) While most evidence suggests that the peripheral and central chemoreflexes are independent and additive in their contributions to ventilation, experiments over longer durations suggest that peripheral chemoreceptor afferents may play an important role in regulating central chemoreflex sensitivity to CO2. This is potentially an important mechanism by which oxygen can alter the acute chemoreflex responses to CO2. In conclusion, the mechanisms underlying ventilatory acclimatization to hypoxia may have an important role in regulating the respiratory system at sea level.

Carotid body denervation in dogs: eupnea and the ventilatory response to hyperoxic hypercapnia

We assessed the time course of changes in eupneic arterial PCO(2) (Pa(CO(2))) and the ventilatory response to hyperoxic rebreathing after removal of the carotid bodies (CBX) in awake female dogs. Elimination of the ventilatory response to bolus intravenous injections of NaCN was used to confirm CBX status on each day of data collection. Relative to eupneic control (Pa(CO(2)) = 40 +/- 3 Torr), all seven dogs hypoventilated after CBX, reaching a maximum Pa(CO(2)) of 53 +/- 6 Torr by day 3 post-CBX. There was no significant recovery of eupneic Pa(CO(2)) over the ensuing 18 days. Relative to control, the hyperoxic CO(2) ventilatory (change in inspired minute ventilation/change in end-tidal PCO(2)) and tidal volume (change in tidal volume/ change in end-tidal PCO(2)) response slopes were decreased 40 +/- 15 and 35 +/- 20% by day 2 post-CBX. There was no recovery in the ventilatory or tidal volume response slopes to hyperoxic hypercapnia over the ensuing 19 days. We conclude that 1) the carotid bodies contribute approximately 40% of the eupneic drive to breathe and the ventilatory response to hyperoxic hypercapnia and 2) there is no recovery in the eupneic drive to breathe or the ventilatory response to hyperoxic hypercapnia after removal of the carotid chemoreceptors, indicating a lack of central or aortic chemoreceptor plasticity in the adult dog after CBX.

Selected contribution: chemoreflex responses to CO2 before and after an 8-h exposure to hypoxia in humans

The ventilatory sensitivity to CO2, in hyperoxia, is increased after an 8-h exposure to hypoxia. The purpose of the present study was to determine whether this increase arises through an increase in peripheral or central chemosensitivity. Ten healthy volunteers each underwent 8-h exposures to 1) isocapnic hypoxia, with end-tidal PO2 (PET(O2)) = 55 Torr and end-tidal PCO2 (PET(CO2)) = eucapnia; 2) poikilocapnic hypoxia, with PET(O2) = 55 Torr and PET(CO2) = uncontrolled; and 3) air-breathing control. The ventilatory response to CO2 was measured before and after each exposure with the use of a multifrequency binary sequence with two levels of PET(CO2): 1.5 and 10 Torr above the normal resting value. PET(O2) was held at 250 Torr. The peripheral (Gp) and the central (Gc) sensitivities were calculated by fitting the ventilatory data to a two-compartment model. There were increases in combined Gp + Gc (26%, P < 0.05), Gp (33%, P < 0.01), and Gc (23%, P = not significant) after exposure to hypoxia. There were no significant differences between isocapnic and poikilocapnic hypoxia. We conclude that sustained hypoxia induces a significant increase in chemosensitivity to CO2 within the peripheral chemoreflex.

Identification of fast and slow ventilatory responses to carbon dioxide under hypoxic and hyperoxic conditions in humans

1. Under conditions of both euoxia and hypoxia, it is generally accepted that the ventilatory response to CO2 has both rapid (peripheral chemoreflex) and slow (central chemoreflex) components. However, under conditions of hyperoxia, it is unclear in humans whether the fast component is completely abolished or merely attenuated in magnitude. 2. The present study develops a technique to determine whether or not a two-compartment model fits the ventilatory response to CO2 significantly better than a one-compartment model. Data were collected under both hypoxic (end-tidal PO2 = 50 Torr) conditions, when two components would be expected, and under hyperoxic (end-tidal PO2 = 200 Torr) conditions, when the presence of the fast compartment is under question. 3. Ten subjects were recruited, of whom nine completed the study. The end-tidal PCO2 of each subject was varied according to a multi-frequency binary sequence that involved 13 steps into and 13 steps out of hypercapnia lasting altogether 1408 s. 4. In four out of nine subjects in hypoxia, and six out of nine subjects in hyperoxia, the two-compartment model fitted the data significantly better than the one-compartment model (F ratio test on residuals). This improvement in fit was significant for the pooled data in both hypoxia (P < 0.05) and hyperoxia (P < 0.005). Mean ventilatory sensitivities for the central chemoreflex were (mean +/- s.e.m.) 1. 69 +/- 0.39 l min-1 Torr-1 in hypoxia and 2.00 +/- 0.32 l min-1 Torr-1 in hyperoxia. Mean ventilatory sensitivities for the peripheral chemoreflex were 2.42 +/- 0.36 l min-1 Torr-1 in hypoxia and 0.75 +/- 0.16 l min-1 Torr-1 in hyperoxia. 5. It is concluded that the rapid and slow components of the ventilatory response to CO2 can be separately identified, and that a rapid component persists under conditions of hyperoxia.

Chemoreflex thresholds to CO2 in decerebrate cats

We used a modified rebreathing technique to measure chemoreflex thresholds to CO2 in decerebrate, paralyzed and ventilated cats. Cats were hyperventilated to neural apnea (PaCO2 < 15 mmHg) with one ventilator and then switched to a rebreathing circuit consisting of a balloon inside a bottle connected to a second ventilator. The volume of the circuit was approximately 110 ml. The balloon contained 5% CO2:95% O2 for hyperoxic rebreathing or approximately 5% CO2 with 11 or 6.5% O2 for moderately and severely hypoxic rebreathing. A plateau in CO2 concentration at the onset of rebreathing indicated equilibration of CO2 between the circuit, alveolar gas and venous and arterial blood. After rapid equilibration of CO2 between the cat and the circuit, CO2 increased linearly with time during rebreathing. Under hyperoxic conditions, phrenic activity began to increase at an end-tidal P(CO2) (PET(CO2)) of 35.1 +/- 6.1 (SD) mmHg (n = 8); during hypoxia, phrenic activity began to increase at a significantly lower PET(CO2) of 27.8 +/- 4.8 mmHg (P < 0.01, n = 6). We interpret these values as the central and peripheral chemoreflex thresholds to CO2, respectively. Persistent phrenic activity prevented determination of a threshold during severe hypoxic rebreathing. Our modified method of hyperoxic and hypoxic rebreathing allows detection of the effects of hypoxia on the central and peripheral chemoreflex thresholds and, within a cat, measurements of chemoreflex sensitivities.

The effect of halothane on phrenic and chemoreceptor responses to hypoxia in anesthetized kittens

We examined the effect of halothane on phrenic never and carotid sinus discharge during hypoxia in anesthetized kittens. In 12 animals, phrenic amplitude was measured during normoxia, during isocapnic hypoxia, and after a return to normoxia, both with and without halothane. Without halothane, all animals had an increase in phrenic amplitude during hypoxia. With halothane, half the animals showed an increase in phrenic amplitude followed by a decline. In a second group of animals, recordings were obtained from single or a few fiber strands of carotid sinus nerve. Without halothane, an increase in chemoreceptor discharge frequency during hypoxia was seen. With 1.0% halothane, frequency was decreased during normoxia and did not increase during hypoxia. Thus, halothane's effect on the ventilatory response to hypoxia, as measured by phrenic discharge, is at least partially explained by an effect on peripheral chemoreceptors.

An assessment of central-peripheral ventilatory chemoreflex interaction using acid and bicarbonate infusions in humans

1. The object of this study was to investigate the effect of central chemoreceptor stimulation on the ventilatory responses to peripheral chemoreceptor stimulation. 2. The level of central chemoreceptor stimulation was varied by performing experiments at two different levels of end-tidal CO2 pressure (PCO2). Variations in peripheral chemoreceptor stimulus were achieved by varying arterial pH (at constant end-tidal PCO2) and by varying end-tidal O2 pressure (PO2). 3. Two protocols were each performed on six human subjects. In one protocol ventilatory measurements were made during eucapnia, when the arterial pH was lowered from 7.4 to 7.3. The variation in pH was achieved by the progressive infusion of acid (0.1 M HCl). In the other protocol ventilatory measurements were made during hypercapnia, when the arterial pH was increased from 7.3 to 7.4. The variation in pH was achieved by the progressive infusion of 1.26% NaHCO3. In each protocol ventilatory responses were measured during euoxia (end-tidal PO2, 100 Torr), hypoxia (end-tidal PO2, 50 Torr) and hyperoxia (end-tidal PO2, 300 Torr), with end-tidal PCO2 held constant. 4. The increase in ventilatory sensitivity to arterial pH induced by hypoxia (50 Torr) was not significantly different between protocols (acid protocol, -104 +/- 31 l min-1 (pH unit)-1 vs. bicarbonate protocol, -60 +/- 44 l min-1 (pH unit)-1; mean +/- S.E.M.; not significant (n.s.)). The ventilatory sensitivity to hypoxia at an arterial pH of 7.35 was not significantly different between protocols (acid protocol, 14.7 +/- 3.3 l min-1 vs. bicarbonate protocol, 15.6 +/- 2.4 l min-1; mean +/- S.E.M.; n.s.). The results provide no evidence to suggest that peripheral chemoreflex ventilatory responses are modulated by central chemoreceptor stimulation.

Ventilatory interaction between hypoxia and hypercapnia in piglets shortly after birth

In 12 piglets aged 0-1.5 days we assessed the relative contribution of the peripheral and central chemoreceptors in mediating the ventilatory response to CO2 at three levels of arterial O2 tension using the dynamic end-tidal forcing technique. With this technique the ventilatory response is separated into a peripheral and a central component using a two-compartment model. Each component is described by a CO2 sensitivity, a time constant, a transport time and a single apnoeic threshold. The results showed that the sensitivity of the peripheral chemoreceptors significantly (P < 0.01) increased from 25.0 +/- 23.6 ml.min-1.kPa-1.kg-1 (mean +/- SD) during normoxia (PaO2 = 12.8 +/- 0.3 kPa) to 42.5 +/- 29.4 ml.min-1.kPa-1.kg-1 during moderate hypoxia (PaO2 = 8.8 +/- 0.4 kPa) and to 80.2 +/- 44.4 ml.min-1.kPa-1.kg-1 at severe hypoxia (PaO2 = 5.1 +/- 0.3 kPa). There was no significant effect of the level of PaO2 on the other parameters. The results were compared with those obtained in a previous study in piglets aged 2-11 days. It showed that the interaction strength at the level of the peripheral chemoreceptors, defined as the negative ratio of the change in the peripheral CO2 sensitivity to the changes in PaO2 was greater in the younger piglets. From these results we conclude that in the newborn piglet the positive ventilatory interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors is already developed shortly after birth and becomes smaller during development.

The effects of hypoxia on the ventilatory response to sudden changes in CO2 in newborn piglets

1. The ventilatory response to square-wave challenges in end-tidal partial pressure of CO2 (PCO2) was investigated at three levels of arterial PO2 (Pa,O2) in nineteen anaesthetized 2- to 11-day-old piglets. 2. The ventilatory responses, measured on a breath-to-breath basis, were separated into a peripheral and a central component using a two-compartment model. Both components were described by a CO2 sensitivity, a time constant, a time delay and a single offset. 3. Fifty-six responses were analysed against a background of normoxaemia (Pa,O2 = 12.70 +/- 0.72 kPa, mean +/- S.D.), fifty-three against a background of moderate hypoxaemia (Pa,O2 = 8.63 +/- 0.34 kPa) and fifty-one against a background of severe hypoxaemia (Pa,O2 = 4.98 +/- 0.30 kPa). 4. The sensitivity of the peripheral chemoreceptors in mediating the response to CO2 increased from 38.3 +/- 17.0 ml min-1 kPa-1 kg-1 during normoxaemia to 48.8 +/- 15.3 ml min-1 kPa-1 kg-1 during moderate hypoxaemia and to 72.9 +/- 24.0 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 5. As compared with the central CO2 sensitivity during moderate hypoxaemia and normoxaemia (104.0 +/- 39.0 and 100.8 +/- 41.6 ml min-1 kPa-1 kg-1, respectively) it decreased to 85.9 +/- 54.1 ml min-1 kPa-1 kg-1 at severe hypoxaemia. 6. We conclude that in newborn piglets there is a positive interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors while severe hypoxaemia reduced the CO2 sensitivity centrally.

An assessment of central-peripheral ventilatory chemoreflex interaction in humans

The independence of the central and peripheral chemoreflexes has been tested in humans. Acute metabolic acidosis generated by a prior bout of brief, hard exercise was used to stimulate primarily the peripheral chemoreceptors, and respiratory acidosis generated by inhaled CO2 was used to stimulate both central and peripheral chemoreceptors. Seven healthy young men were studied. Ventilation and arterial pH, PCO2 and PO2 were recorded. Peripheral chemoreflex sensitivity to hypoxia during acute metabolic acidosis was repeatedly determined by measuring ventilation in euoxia (PETO2 = 100 Torr) and hypoxia (PETO2 = 50 Torr) as the subject recovered from exercise-induced acidosis. Peripheral chemoreflex sensitivity to hypoxia during CO2 inhalation was repeatedly determined by measuring ventilation in euoxia and hypoxia at two levels of hypercapnia (PETCO2 = 45 Torr and PETCO2 = 50 Torr). The ventilatory sensitivity to hypoxia at matched arterial pH values was not significantly different between conditions of high (CO2 inhalation) and low (metabolic acidosis) central chemoreceptor activity. We therefore conclude that interaction between central and peripheral chemoreflexes was non-significant in all subjects.

Mechanical properties of the rabbit upper airway during hypoxia and hypercapnia

It has been suggested that the response of upper airway muscles to hypoxia may be different from the response of these muscles to hypercapnia. We therefore measured pulmonary ventilation and the mechanical properties of the isolated upper airway in 9 anesthetised rabbits during respiration of hypoxic and hypercapnic gas mixtures. Each animal was exposed to several levels of elevated inspiratory CO2 fraction, FICO2 (0.03 to 0.17) and depressed inspiratory O2 fraction, FIO2 (0.19 to 0.09). The steady-state ventilatory response, the tidal pressure in the upper airway (PTUA) and the upper airway elastance were measured under each condition. Straight lines were calculated by least squares regression relating pulmonary VT to FICO2 and FIO2 and PTUA to FICO2 and FIO2. The PTUA was estimated graphically at two levels of hypoxia and hypercapnia producing equal augmentation of VT. The ratio of PTUA during hypoxia to PTUA during hypercapnia was 1.06 +/- 0.21 (mean +/- 95% C.I.) at low VT and 1.15 +/- 0.25 at high VT. Elastance of the upper airway rose from 6.25 +/- 1.13 cmH2O/ml under control conditions to a maximum of 7.95 +/- 1.24 cmH2O/ml (P less than 0.05) during hypercapnia and to a maximum of 8.02 +/- 1.17 cmH2O/ml (P less than 0.05) during hypoxia. There was no difference between the mean (+/- 95% C.I.) change associated with hypercapnia (1.64 +/- 1.08 cmH2O/ml) and the mean change associated with hypoxia (1.77 +/- 1.26 cmH2O/ml). We concluded that hypoxia did not result in a greater change in upper airway mechanical properties than hypercapnia.

The influence of oxygen on the ventilatory response to carbon dioxide in man

1. The ventilatory response to isoxic square-wave challenges in end-tidal PCO2 was investigated at three levels of end-tidal PO2 (PET, O2) in nine healthy male subjects. 2. Twenty-seven responses against a background of mild hypoxia (PET, O2 approximately 10 kPa), sixty-seven against a background of normoxia (PET, O2 approximately 14.5 kPa) and seventy-six against a background of hyperoxia (PET, O2 approximately 70 kPa) were collected. 3. The breath-to-breath data were partitioned into a fast and a slow ventilatory component using a two-compartment model. 4. In the normoxic and hypoxic experiments the CO2 sensitivity of the fast component averaged to about 30 and 40% of the total CO2 sensitivity, respectively. In the hyperoxic experiments three subjects had no fast component in their response while in three others the CO2 sensitivity of the fast component averaged to about 24% of the total CO2 sensitivity. In the remaining three subjects the presence of a fast component was doubtful. 5. We argue that the fast component is due to the peripheral chemoreflex loop and the slow component to the central chemoreflex loop. 6. The central CO2 sensitivity and the apnoeic threshold (extrapolated end-tidal CO2 at zero ventilation in the steady state) were 15% smaller in hyperoxia than those in normoxia and hypoxia. In normoxia and mild hypoxia the central CO2 sensitivities were not significantly different. 7. We argue, that apart from peripheral oxygen-carbon dioxide interaction, there is evidence for central oxygen-carbon dioxide interaction in human subjects. 8. We conclude that in general there is a contribution to ventilation of the peripheral chemoreceptors during hyperoxia in man.

Almitrine and the peripheral ventilatory response to CO2 in hyperoxia and hypoxia

The effects of almitrine bismesylate (initial intravenous dose 0.6 mg.kg-1 followed by continuous infusion of 0.4 mg.kg-1.h-1) on the ventilatory response to CO2 during hyperoxia and hypoxia were determined in 6 anaesthetized cats with the use of the dynamic end-tidal CO2 forcing technique. It was found that almitrine almost doubled the peripheral ventilatory sensitivity to CO2 during hyperoxia (mean PETO2 45.6 kPa) and also during mild hypoxia (mean PETO2 8.7 kPa). The apnoeic threshold (B) was in both cases shifted to substantially lower values than those of the control measurements. No significant effects of almitrine were found on the central ventilatory sensitivity to CO2 either during hyperoxia or during hypoxia. It is argued that the decrease of the apnoeic threshold may be due to an inhibitory effect of almitrine on the carotid body dopaminergic activity, and that the increase of the sensitivity to CO2 stems from a "hypoxia mimetic" mechanism.

Central-peripheral chemoreceptor ventilatory interaction in awake goats

This study was designed to characterize the ventilatory interaction between central and carotid body (CB) chemoreceptor stimulation in awake goats undergoing selective CB perfusion. This model allowed us to expose central and CB chemoreceptors to separate blood gas conditions in an animal that is conscious and not systemically hypoxic. Systemic CO2 ventilatory response curves, performed by progressively increasing FICO2 in systemic hyperoxia, were completed in 7 goats during CB perfusion with hypercapnic-hypoxic blood and normocapnic-normoxic blood, and in 3 goats without CB perfusion. The slopes of the curves done with perfusion were not significantly different (P greater than 0.05) in CB hypercapnic hypoxia and CB normocapnic normoxia for VE, VT, f and VT/TI, and the coefficients of variation of slopes generated with and without perfusion were similar. Our data indicate there is addition of central and CB chemoreceptor input in respiratory control, and we conclude that the previously demonstrated stimulus interaction at the CB is the primary source of the hyperadditive hypercapnic-hypoxic ventilatory interaction in an animal unaffected by anesthetics or brain hypoxia.

Evidence for interaction between the contributions to ventilation from the central and peripheral chemoreceptors in man

1. The question of whether there is any interaction between the peripheral and central chemoreceptor contributions to ventilation in man has been addressed. 2. Subjects were exposed to an end-tidal PCO2 of ca. 10 Torr above resting for 8 min at an end-tidal PO2 of 100 Torr. The end-tidal PCO2 was then reduced to near eucapnia. This provided a period of time when the PCO2 at the peripheral chemoreceptors would be near eucapnia, but would still be raised at the central chemoreceptors. 3. Against the background above, the effect of an hypoxic end-tidal step from a PO2 of 100 Torr to a PO2 of 50 Torr was studied, and compared with the effect of the same step when both sets of chemoreceptors were near eucapnia. 4. Three subjects were studied, each contributing twelve sets of data to each of the three protocols required for the comparisons. 5. In two of the three subjects, the ventilatory response to hypoxia was augmented when central PCO2 was high. 6. The results support the idea that there is an interaction between the central and peripheral chemoreceptors in man. The consequences of this and other possible interpretations of the results are discussed.

Hypoxic-hypercapnic ventilatory interaction at the carotid body of awake goats

Neurophysiological studies have demonstrated that a positive interaction between hypoxic and hypercapnic stimuli occurs at the carotid body (CB). The present study was designed to confirm that this interaction at the CB was translated into a similar interaction in the ventilatory response. By utilizing an awake goat model in which the CB could be selectively perfused using an extracorporeal circuit we avoided confounding central effects. In six goats the CB was stimulated by progressively decreasing PcbO2 from 160 to 40 Torr at two constant levels of PcbCO2, 36 and 61 Torr. The animals breathed room air with supplemental CO2 to maintain systemic isocapnia. The response to CB hypoxia was significantly greater in CB hypercapnia than in CB normocapnia for minute ventilation, tidal volume, respiratory frequency, and mean inspiratory flow rate. We conclude that the hypercapnic-hypoxic interaction at the CB is reflected in the ventilatory responses of the animal.

Hopf bifurcations and the stability of the respiratory control system

A simple model of a feedback loop controlling ventilation is analysed. This model is intended to describe the response of the system, initially at equilibrium, to a sudden fall in CO2 concentration in the lung, brought about by a deep sigh. A previous paper described the model in detail and the general method of analysis. Here we continue the discussion of stability, first in terms of local stability after a small displacement from equilibrium and then by computer simulation to illustrate the behaviour after large displacements. The local analysis is used to select representative sets of system parameters to illustrate the different types of trajectory obtained by computer simulation. When the equilibrium point is stable the response to a disturbance is overdamped, underdamped or critically damped. When the equilibrium point is unstable the system responds by going into a limit cycle. The transition between these two cases proceeds via a Hopf Bifurcation. The limit cycle type of ventilatory pattern, i.e. a periodic, underdamped waxing and waning of ventilation is commonly seen in premature infants and in term infants between 1 and 6 months of age.

Effects of brain stem hypoxaemia on the regulation of breathing

In 22 cats, anaesthetized with chloralose-urethane, the brain stem was artificially perfused with their own blood via a gas exchanger in which the central PaO2 and PaCO2 were imposed independently from the peripheral PaO2 and PaCO2 in the systemic arterial blood. The effects of brain stem hypoxaemia on ventilation and on the ventilatory responses to central and peripheral chemoreceptor stimulation were investigated. When the central PaO2 was lowered from 375 mm Hg to 100 and 50 mm Hg, keeping all other blood gas tensions constant, ventilation decreased on the average by 0.22 L X min-1 and 0.54 L X min-1, respectively. The increase in ventilation due to peripheral hypoxaemia and the sensitivities to central and peripheral CO2 (delta VE/delta PaCO2) were independent of the central PaO2, despite the depression of ventilation. The sensitivity to central CO2 was also not influenced when central hypoxaemia was combined with peripheral hypoxaemia. The linear VE-VT relation was not affected by central hypoxaemia. Our findings suggest that the functioning of respiratory neurons in the brain stem is unaltered during moderate central hypoxaemia.

Central respiratory CO2 sensitivity at extreme hypocapnia

In 7 cats anaesthetized with chloralose-urethane the ponto-medullary region was artificially perfused with blood having PaCO2 values (central PaCO2) in the range of 0.3-4.5 kPa. The ventilatory response to changes in central PaCO2 was measured at constant hypercapnic and hypoxic conditions in the systemic circulation. Ventilation decreased upon lowering the central PaCO2 down to values of 0.5 kPa. There was no threshold for the effect of the central PaCO2 on ventilation. The CO2 sensitivity was undiminished at extreme hypocapnia compared to eucapnia. Under extreme central hypocapnic conditions the breathing pattern became irregular. It is concluded that there is still central CO2 sensitivity related to ventilation at extreme hypocapnia. Our findings suggest that central chemosensitive structures have a neural threshold below a PaCO2 of 0.5 kPa.