Ventilatory effects of prolonged systemic (CNS) hypoxia in awake goats

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.

Determinants of ventilation and pulmonary artery pressure during early acclimatization to hypoxia in humans

Pulmonary ventilation and pulmonary arterial pressure both rise progressively during the first few hours of human acclimatization to hypoxia. These responses are highly variable between individuals, but the origin of this variability is unknown. Here, we sought to determine whether the variabilities between different measures of response to sustained hypoxia were related, which would suggest a common source of variability. Eighty volunteers individually underwent an 8-h isocapnic exposure to hypoxia (end-tidal P(O2)=55 Torr) in a purpose-built chamber. Measurements of ventilation and pulmonary artery systolic pressure (PASP) assessed by Doppler echocardiography were made during the exposure. Before and after the exposure, measurements were made of the ventilatory sensitivities to acute isocapnic hypoxia (G(pO2)) and hyperoxic hypercapnia, the latter divided into peripheral (G(pCO2)) and central (G(cCO2)) components. Substantial acclimatization was observed in both ventilation and PASP, the latter being 40% greater in women than men. No correlation was found between the magnitudes of pulmonary ventilatory and pulmonary vascular responses. For G(pO2), G(pCO2) and G(cC O2), but not the sensitivity of PASP to acute hypoxia, the magnitude of the increase during acclimatization was proportional to the pre-acclimatization value. Additionally, the change in G(pO2) during acclimatization to hypoxia correlated well with most other measures of ventilatory acclimatization. Of the initial measurements prior to sustained hypoxia, only G(pCO2) predicted the subsequent rise in ventilation and change in G(pO2) during acclimatization. We conclude that the magnitudes of the ventilatory and pulmonary vascular responses to sustained hypoxia are predominantly determined by different factors and that the initial G(pCO2) is a modest predictor of ventilatory acclimatization.

Quantifying hypoxia-induced chemoreceptor sensitivity in the awake rodent

We evaluated several methods for characterizing hypoxic chemosensitivity in the conscious rat. Adult Sprague-Dawley rats (n = 30) were exposed to normobaric hypoxia [inspired oxygen fraction (Fio2) 0.15, 0.12, and 0.09]. We measured ventilation (V̇e; barometric plethysmography), arterial oxygen saturation (SpO2; pulse oximeter), and oxygen consumption and carbon dioxide production (V̇o2 and V̇co2; analysis of expired air). Linear regression analysis was used to define stimulus-response relationships. Testing was performed on 2 days to assess day-to-day reproducibility. Exposure to graded, steady-state hypoxia caused progressive reductions in SpO2 that were, for any given Fio2, quite variable (SpO2 range, 20-30%) among individuals. Hypoxia produced progressive increases in V̇e caused by increases in both tidal volume (VT) and breathing frequency. Hypoxia also increased the VT:inspiratory time (Ti) ratio, an indicator of central respiratory "drive." Hypoxia caused consistent, progressive declines in V̇o2, V̇co2, and core temperature (>20% at the lowest SpO2). We propose that optimal quantification of carotid chemoreceptor hypoxic sensitivity in the unanesthetized rodent should employ SpO2 [a surrogate for arterial Po2 (PaO2 )] as the stimulus variable and the ventilatory equivalent for V̇co2 (V̇e/V̇co2) and/or mean inspiratory flow rate (VT/Ti) normalized for V̇co2 as the response variables. Both metrics take into account not only the important influence of a falling metabolic rate, but also SpO2, which represents the hypoxic stimulus at the carotid body. Because of the somewhat curvilinear nature of these responses, exposure to multiple levels of graded hypoxia provides the most complete characterization of hypoxic chemosensitivity.

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.

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.

The influence of chronic hypoxia upon chemoreception

Carotid body chemoreceptors are essential for time-dependent changes in ventilatory control during chronic hypoxia. Early theories of ventilatory acclimatization to hypoxia focused on time-dependent changes in known ventilatory stimuli, such as small changes in arterial pH that may play a significant role in some species. However, plasticity in the cellular and molecular mechanisms of carotid body chemoreception play a major role in ventilatory acclimatization to hypoxia in all species studied. Chronic hypoxia causes changes in (a) ion channels (potassium, sodium, calcium) to increase glomus cell excitability, and (b) neurotransmitters (dopamine, acetylcholine, ATP) and neuromodulators (endothelin-1) to increase carotid body afferent activity for a given PO(2) and optimize O(2)-sensitivity. O(2)-sensing heme-containing molecules in the carotid body have not been studied in chronic hypoxia. Plasticity in medullary respiratory centers processing carotid body afferent input also contributes to ventilatory acclimatization to hypoxia. It is not known if the same mechanisms occur in patients with chronic hypoxemia from lung disease or high altitude natives.

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.

The ventilatory response to carbon dioxide and sustained hypoxia is enhanced after episodic hypoxia in OSA patients

Our primary hypothesis was that the acute ventilatory response to carbon dioxide in the presence of sustained hypoxia {VRCO2 (hypoxia)} or hyperoxia {VRCO2 (hyperoxia)} would increase in subjects with obstructive sleep apnea (OSA) after exposure to episodic hypoxia. Secondarily, we hypothesized that chronic (i.e. years) exposure to episodic hypoxia, a hallmark of OSA, would facilitate persistent augmentation of respiratory activity (i.e. long-term facilitation) after acute (i.e. minutes) exposure to episodic hypoxia. Nine healthy males with OSA that were healthy otherwise completed a series of rebreathing trials before and after exposure to eight 4 min episodes of hypoxia. On a separate occasion, the rebreathing trials were repeated before and after exposure to atmospheric air for a duration equivalent to the episodic hypoxia protocol (i.e. sham episodic hypoxia). During the rebreathing trials, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (P(ET)CO2) below 25 Torr. Subjects then rebreathed from a bag containing a normocapnic (42 Torr), low (50 Torr) or high oxygen gas mixture (140 Torr). During the trials, P(ET)CO2 increased while the selected level of oxygen was maintained. The point at which ventilation began to rise in a linear fashion as P(ET)CO2 increased was the ventilatory threshold. The ventilatory response below and above the threshold was determined. The results showed that the VRCO2 (hypoxia) and the VRCO2 (hyperoxia) was increased after exposure to episodic hypoxia {VRCO2 (hypoxia): 7.9 +/- 1.3 versus 10.5 +/- 1.3, VRCO2 (hyperoxia): 5.9 +/- 1.1 versus 6.7 +/- 1.1 L/min/Torr}. However, only the increase in the VRCO2 (hypoxia) after episodic hypoxia was greater than the increase measured after exposure to sham episodic hypoxia. Long-term facilitation of ventilation, tidal volume and breathing frequency was not evident after episodic hypoxia. We conclude that the VRCO2 (hypoxia) is enhanced after exposure to acute episodic hypoxia and that enhancement of the VRCO2 (hypoxia) occurs even though long-term facilitation is not evident.

Effect of a repeated series of intermittent hypoxic exposures on ventilatory response in humans

The purpose of this study was to elucidate the magnitude and the time course of ventilatory changes resulting from a repeated series of hypoxic exposures. Eight healthy males participated in the present study. The subjects spent 1 h/day in normobaric hypoxia (12% inspired oxygen). Inspired minute ventilation (V(I)), end-tidal partial pressure of carbon dioxide (P(ET(CO2))), and arterial oxygen saturation (SaO2) were measured in a hypoxic tent. These measurements were taken for 10 consecutive days (series 1), and were taken again after the subjects had been away from hypoxic exposure for 1 month (series 2). P(ET(CO2)) decreased and SaO2 increased progressively in the hypoxic tent during the 10 days of intermittent hypoxia in series 1. At the onset of series 2 (days 1 to 3), P(ET(CO2)) was significantly lower and SaO2 was significantly higher than those on day 1 during series 1. These results suggest that humans who have had previous hypoxic exposure adapt sooner to hypoxic condition due to an increase in the magnitude of hyperventilation in the first few days of a series of reexposures to hypoxia.

Ventilatory responses to carbon dioxide at low and high levels of oxygen are elevated after episodic hypoxia in men compared with women

We hypothesized that the acute ventilatory response to carbon dioxide in the presence of low and high levels of oxygen would increase to a greater extent in men compared with women after exposure to episodic hypoxia. Eleven healthy men and women of similar race, age, and body mass index completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the end-tidal partial pressure of carbon dioxide (PetCO2) below 25 Torr. Subjects then rebreathed from a bag containing a normocapnic (42 Torr), low (50 Torr), or high oxygen gas mixture (150 Torr). During the trials, PetCO2 increased while the selected level of oxygen was maintained. The point at which minute ventilation began to rise in a linear fashion as PetCO2 increased was considered to be the carbon dioxide set point. The ventilatory response below and above this point was determined. The results showed that the ventilatory response to carbon dioxide above the set point was increased in men compared with women before exposure to episodic hypoxia, independent of the oxygen level that was maintained during the rebreathing trials (50 Torr: men, 5.19 +/- 0.82 vs. women, 4.70 +/- 0.77 l x min(-1) x Torr(-1); 150 Torr: men, 4.33 +/- 1.15 vs. women, 3.21 +/- 0.58 l x min(-1) x Torr(-1)). Moreover, relative to baseline measures, the ventilatory response to carbon dioxide in the presence of low and high oxygen levels increased to a greater extent in men compared with women after exposure to episodic hypoxia (50 Torr: men, 9.52 +/- 1.40 vs. women, 5.97 +/- 0.71 l x min(-1) x Torr(-1); 150 Torr: men, 5.73 +/- 0.81 vs. women, 3.83 +/- 0.56 l x min(-1) x Torr(-1)). Thus we conclude that enhancement of the acute ventilatory response to carbon dioxide after episodic hypoxia is sex dependent.

Peripheral chemoreflex responsiveness is increased at elevated levels of carbon dioxide after episodic hypoxia in awake humans

We hypothesized that the acute ventilatory response to hypoxia is enhanced after exposure to episodic hypoxia in awake humans. Eleven subjects completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (Pet(CO(2))) below 25 Torr. Subjects then breathed from a bag containing normocapnic (42 Torr), low (50 Torr), or high oxygen (140 Torr) gas mixtures. During the trials, Pet(CO(2)) increased while a constant oxygen level was maintained. The point at which ventilation began to rise in a linear fashion as Pet(CO(2)) increased was considered to be the ventilatory recruitment threshold. The ventilatory response below and above the recruitment threshold was determined. Ventilation did not persist above baseline values immediately after exposure to episodic hypoxia; however, Pet(CO(2)) levels were reduced compared with baseline. In contrast, compared with baseline, the ventilatory response to progressive increases in carbon dioxide during rebreathing trials in the presence of low but not high oxygen levels was increased after exposure to episodic hypoxia. This increase occurred when carbon dioxide levels were above but not below the ventilatory recruitment threshold. We conclude that long-term facilitation of ventilation (i.e., increases in ventilation that persist when normoxia is restored after episodic hypoxia) is not expressed in awake humans in the presence of hypocapnia. Nevertheless, despite this lack of expression, the acute ventilatory response to hypoxia in the presence of hypercapnia is increased after exposure to episodic hypoxia.

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.

Respiratory control in humans after 8 h of lowered arterial PO2, hemodilution, or carboxyhemoglobinemia

In humans exposed to 8 h of isocapnic hypoxia, there is a progressive increase in ventilation that is associated with an increase in the ventilatory sensitivity to acute hypoxia. To determine the relative roles of lowered arterial PO2 and oxygen content in generating these changes, the acute hypoxic ventilatory response was determined in 11 subjects after four 8-h exposures: 1) protocol IH (isocapnic hypoxia), in which end-tidal PO2 was held at 55 Torr and end-tidal PCO2 was maintained at the preexposure value; 2) protocol PB (phlebotomy), in which 500 ml of venous blood were withdrawn; 3) protocol CO, in which carboxyhemoglobin was maintained at 10% by controlled carbon monoxide inhalation; and 4) protocol C as a control. Both hypoxic sensitivity and ventilation in the absence of hypoxia increased significantly after protocol IH (P < 0.001 and P < 0.005, respectively, ANOVA) but not after the other three protocols. This indicates that it is the reduction in arterial PO2 that is primarily important in generating the increase in the acute hypoxic ventilatory response in prolonged hypoxia. The associated reduction in arterial oxygen content is unlikely to play an important role.

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.

PDGF-beta receptor expression and ventilatory acclimatization to hypoxia in the rat

Activation of platelet-derived growth factor-beta (PDGF-beta) receptors in the nucleus of the solitary tract (nTS) modulates the late phase of the acute hypoxic ventilatory response (HVR) in the rat. We hypothesized that temporal changes in PDGF-beta receptor expression could underlie the ventilatory acclimatization to hypoxia (VAH). Normoxic ventilation was examined in adult Sprague-Dawley rats chronically exposed to 10% O(2), and at 0, 1, 2, 7, and 14 days, Northern and Western blots of the dorsocaudal brain stem were performed for assessment of PDGF-beta receptor expression. Although no significant changes in PDGF-beta receptor mRNA occurred over time, marked attenuation of PDGF-beta receptor protein became apparent after day 7 of hypoxic exposure. Such changes were significantly correlated with concomitant increases in normoxic ventilation, i.e., with VAH (r: -0.56, P < 0.005). In addition, long-term administration of PDGF-BB in the nTS via osmotic pumps loaded with either PDGF-BB (n = 8) or vehicle (Veh; n = 8) showed that although no significant changes in the magnitude of acute HVR occurred in Veh over time, the typical attenuation of HVR by PDGF-BB decreased over time. Furthermore, PDGF-BB microinjections did not attenuate HVR in acclimatized rats at 7 and 14 days of hypoxia (n = 10). We conclude that decreased expression of PDGF-beta receptors in the dorsocaudal brain stem correlates with the magnitude of VAH. We speculate that the decreased expression of PDGF-beta receptors is mediated via internalization and degradation of the receptor rather than by transcriptional regulation.

Changes in respiratory control during and after 48 h of isocapnic and poikilocapnic hypoxia in humans

Ventilatory acclimatization to hypoxia is associated with an increase in ventilation under conditions of acute hyperoxia (VEhyperoxia) and an increase in acute hypoxic ventilatory response (AHVR). This study compares 48-h exposures to isocapnic hypoxia (protocol I) with 48-h exposures to poikilocapnic hypoxia (protocol P) in 10 subjects to assess the importance of hypocapnic alkalosis in generating the changes observed in ventilatory acclimatization to hypoxia. During both hypoxic exposures, end-tidal PO2 was maintained at 60 Torr, with end-tidal PCO2 held at the subject's prehypoxic level (protocol I) or uncontrolled (protocol P). VEhyperoxia and AHVR were assessed regularly throughout the exposures. VEhyperoxia (P < 0.001, ANOVA) and AHVR (P < 0.001) increased during the hypoxic exposures, with no significant differences between protocols I and P. The increase in VEhyperoxia was associated with an increase in slope of the ventilation-end-tidal PCO2 response (P < 0.001) with no significant change in intercept. These results suggest that changes in respiratory control early in ventilatory acclimatization to hypoxia result from the effects of hypoxia per se and not the alkalosis normally accompanying hypoxia.

Human ventilatory response to CO2 after 8 h of isocapnic or poikilocapnic hypoxia

During ventilatory acclimatization to hypoxia (VAH), the relationship between ventilation (VE) and end-tidal PCO2 (PETCO2) changes. This study was designed to determine 1) whether these changes can be seen early in VAH and 2) if these changes are present, whether the responses differ between isocapnic and poikilocapnic exposures. Ten healthy volunteers were studied by using three 8-h exposures: 1) isocapnic hypoxia (IH), end-tidal PO2 (PETO2) = 55 Torr and PETCO2 held at the subject's normal prehypoxic value; 2) poikilocapnic hypoxia (PH), PETO2 = 55 Torr; and 3) control (C), air breathing. The VE-PETCO2 relationship was determined in hyperoxia (PETO2 = 200 Torr) before and after the exposures. We found a significant increase in the slopes of VE-PETCO2 relationship after both hypoxic exposures compared with control (IH vs. C, P < 0.01; PH vs. C, P < 0.001; analysis of covariance with pairwise comparisons). This increase was not significantly different between protocols IH and PH. No significant changes in the intercept were detected. We conclude that 8 h of hypoxia, whether isocapnic or poikilocapnic, increases the sensitivity of the hyperoxic chemoreflex response to CO2.

A new aspect of the carotid body function controlling hypoxic ventilatory decline in humans

Ventilatory response to eucapnic sustained mild hypoxia was measured in one patient with unilateral and three patients with bilateral carotid body (CB) resection (defined UR and BR, respectively). The profile of ventilatory response in UR patient was initially augmented then gradually declined (biphasic pattern) as generally seen in normal subjects although the absolute magnitude was substantially low. On the other hand, biphasic pattern was disappeared in all three BRs. Lack of hypoxic ventilatory decline (HVD) in the late period of sustained hypoxia was in marked contrast to that reported in the anaesthetized and CB-denervated animals whose ventilation was severely depressed lower than the pre-hypoxic control level. In view of recent knowledge that the analogous mild hypoxia in normal animals and humans elicits an useful adaptation to economize energy expenditure with maintaining reversible excitability in control of respiration, BR patients were considered to have lost this ability. We conclude that in awake humans the CB not only stimulates ventilation but also controls the degree of subsequent HVD during sustained hypoxia.

Lack of long-term facilitation of ventilation after exposure to hypoxia in goats

Episodic hypoxia has been shown to induce augmented normoxic ventilatory drive or long-term facilitation (LTF, continued hyperventilation after termination of hypoxic stimulation) in awake dogs and awake goats. The main objective of these experiments was to examine whether continuous isocapnic hypoxia in awake goats elicits LTF and additionally, to determine if goats exhibit hypoxic ventilatory decline (roll-off) during the hypoxic exposure. Goats were exposed to either 4 h of isocapnic hypoxia (n = 10) or 30 min of isocapnic hypoxia (n = 7). Ventilation (VE), tidal volume and frequency were measured before, during and following the end of the isocapnic hypoxia (PaO2 40 Torr) exposure. During the 4 h period of hypoxia, VE increased in a time-dependent manner in a typical pattern of acclimatization, reaching a mean of 40.8 +/- 3.6 L/min at the end of 4 h. Five minutes after return to normoxia, VE was 13.0 +/- 0.8 L/min, not different than control VE (13.1 +/- 0.9 L/min) measured prior to the hypoxic exposure and remained unchanged from this value for another 30 min. During the 30 min hypoxic exposure, VE increased upon exposure to hypoxia, remained significantly elevated throughout the hypoxic exposure, but promptly returned to control levels upon return to normoxia. These results indicate that continuous isocapnic hypoxia elicits neither long term facilitation of ventilation nor hypoxic ventilatory decline in awake goats.

Human ventilatory response to acute hyperoxia during and after 8 h of both isocapnic and poikilocapnic hypoxia

During 8 h of either isocapnic or poikilocapnic hypoxia, there may be a rise in ventilation (VE) that cannot be rapidly reversed with a return to higher PO2 (L. S. G. E. Howard and P. A. Robbins. J. Appl. Physiol. 78:1098-1107, 1995). To investigate this further, three protocols were compared: 1) 8-h isocapnic hypoxia [end-tidal PCO2 (PETCO2) held at prestudy value, end-tidal PO2 (PETO2) = 55 Torr], followed by 8-h isocapnic euoxia (PETO2 = 100 Torr); 2) 8-h poikilocapnic hypoxia followed by 8-h poikilocapnic euoxia; and 3) 16-h air-breathing control. Before and at intervals throughout each protocol, the VE response to eucapnic hyperoxia (PETCO2 held 1-2 Torr above prestudy value, PETO2 = 300 Torr) was determined. There was a significant rise in hyperoxic VE over 8 h during both forms of hypoxia (P < 0.05, analysis of variance) that persisted during the subsequent 8-h euoxic period (P < 0.05, analysis of variance). These results support the notion that an 8-h period of hypoxia increases subsequent hyperoxic VE, even if acid-base changes have been minimized through maintenance of isocapnia during the hypoxic period.

Effects of specific carotid body and brain hypoxia on respiratory muscle control in the awake goat

1. We assessed the effects of specific brain hypoxia on the control of inspiratory and expiratory muscle electromyographic (EMG) activities in response to specific carotid body hypoxia in seven awake goats. We used an isolated carotid body perfusion technique that permitted specific, physiological, steady-state stimulation of the carotid bodies or maintenance of normoxia and normocapnia at the carotid bodies while varying the level of systemic, and therefore, brain oxygenation. 2. Isolated brain normocapnic hypoxia of up to 1.5 h duration increased inspired minute ventilation (VI) by means of increases in both tidal volume (VT) and respiratory frequency (fR). Electromyographic activities of both inspiratory and expiratory muscles were augmented as well. These responses were similar to those produced by low levels of whole-body normoxic hypercapnia. We conclude that moderate levels of brain hypoxia (Pa,O2 approximately 40 mmHg) in awake goats caused a net stimulation of ventilatory motor output. 3. Hypoxic stimulation of the carotid bodies alone caused comparable increases in VT and fR, and EMG augmentation of both inspiratory and expiratory muscles whether the brain was hypoxic or normoxic. These responses were quite similar to those obtained over a wide range of whole-body normoxic hypercapnia. We conclude that the integration of carotid body afferent information is not affected by moderate brain hypoxia in awake goats. 4. We found no evidence for an asymmetrical recruitment pattern of inspiratory vs. expiratory muscles in response to carotid body hypoxia or in response to brain hypoxia alone. 5. Our data support the concept that moderate brain hypoxia results in a net stimulation of respiratory motor output. These findings question the significance of 'central hypoxic depression' to the regulation of breathing under physiological levels of hypoxaemia in the awake animal.