Independence of exercise hyperpnea and acidosis during high-intensity exercise in ponies

Physicochemical Analysis of Mixed Venous and Arterial Blood Acid-Base State in Horses at Core Temperature during and after Moderate-Intensity Exercise

The present study determined the independent contributions of temperature, strong ion difference ([SID]), total weak acid concentration ([Atot]) and PCO2 to changes in arterial and mixed venous [H+] and total carbon dioxide concentration ([TCO2]) during 37 min of moderate intensity exercise (~50% of heart rate max) and the first 60 min of recovery. Six horses were fitted with indwelling carotid and pulmonary artery (PA) catheters, had PA temperature measured, and had blood samples withdrawn for immediate analysis of plasma ion and gas concentrations. The increase in core temperature during exercise (+4.5 °C; p < 0.001) significantly (p < 0.05) increased PO2, PCO2, and [H+], but without a significant effect on [TCO2] (p > 0.01). The physicochemical acid-base approach was used to determine contributions of independent variables (except temperature) to the changes in [H+] and [TCO2]. In both arterial and venous blood, there was no acidosis during exercise and recovery despite significant (p < 0.05) increases in [lactate] and in venous PCO2. In arterial blood plasma, a mild alkalosis with exercise was due to primarily to a decrease in PCO2 (p < 0.05) and an increase in [SID] (p < 0.1). In venous blood plasma, a near absence of change in [H+] was due to the acidifying effects of increased PCO2 (p < 0.01) being offset by the alkalizing effects of increased [SID] (p < 0.05). The effect of temperature on PO2 (p < 0.001) resulted in an increased arterio-venous PO2 difference (p < 0.001) that would facilitate O2 transfer to contracting muscle. The simultaneous changes in the PCO2 and the concentrations of the other independent acid-base variables (contributions from individual strong and weak ions as manifest in [SID] and [Atot]) show complex, multilevel control of acid-base states in horses performing even moderate intensity exercise. Correction of acid-base variables to core body temperature presents a markedly different physiological response to exercise than that provided by variables measured and presented at an instrument temperature of 37 °C.

Neuromodulation of Limb Proprioceptive Afferents Decreases Apnea of Prematurity and Accompanying Intermittent Hypoxia and Bradycardia

BACKGROUND

Apnea of Prematurity (AOP) is common, affecting the majority of infants born at <34 weeks gestational age. Apnea and periodic breathing are accompanied by intermittent hypoxia (IH). Animal and human studies demonstrate that IH exposure contributes to multiple pathologies, including retinopathy of prematurity (ROP), injury to sympathetic ganglia regulating cardiovascular action, impaired pancreatic islet cell and bone development, cerebellar injury, and neurodevelopmental disabilities. Current standard of care for AOP/IH includes prone positioning, positive pressure ventilation, and methylxanthine therapy; these interventions are inadequate, and not optimal for early development.

OBJECTIVE

The objective is to support breathing in premature infants by using a simple, non-invasive vibratory device placed over limb proprioceptor fibers, an intervention using the principle that limb movements trigger reflexive facilitation of breathing.

METHODS

Premature infants (23-34 wks gestational age), with clinical evidence of AOP/IH episodes were enrolled 1 week after birth. Caffeine treatment was not a reason for exclusion. Small vibration devices were placed on one hand and one foot and activated in 6 hour ON/OFF sequences for a total of 24 hours. Heart rate, respiratory rate, oxygen saturation (SpO2), and breathing pauses were continuously collected.

RESULTS

Fewer respiratory pauses occurred during vibration periods, relative to baseline (p<0.005). Significantly fewer SpO2 declines occurred with vibration (p<0.05), relative to control periods. Significantly fewer bradycardic events occurred during vibration periods, relative to no vibration periods (p<0.05).

CONCLUSIONS

In premature neonates, limb proprioceptive stimulation, simulating limb movement, reduces breathing pauses and IH episodes, and lowers the number of bradycardic events that accompany aberrant breathing episodes. This low-cost neuromodulatory procedure has the potential to provide a non-invasive intervention to reduce apnea, bradycardia and intermittent hypoxia in premature neonates.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02641249.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Lesions in the cerebellar fastigial nucleus have a small effect on the hyperpnea needed to meet the gas exchange requirements of submaximal exercise

The purpose of this study was to test the hypothesis that an intact cerebellar fastigial nucleus (CFN) is necessary for the hyperpnea to meet the gas exchange needs of submaximal exercise. Bilateral stainless steel microtubules were implanted in the cerebellum inside (n = 12) or outside (n = 2) the CFN for injection (0.5 to 10 microl) of the neurotoxin ibotenic acid. All goats had difficulty maintaining normal posture and walking for up to 1 mo after the implantation of the microtubules and again for hours or days after the neurotoxin was injected. Postmortem histology indicated there were 55% fewer living neurons (P < 0.001, n = 9, 3,720 +/- 553 vs. 1,670 +/- 192) in the CFN of the experimental goats compared with a control group of goats. As is typical for goats before implantation of the microtubules, the decrease in arterial Pco(2) from rest during mild and moderate treadmill exercise was 2.0 +/- 0.39 and 3.5 +/- 0.45 Torr, respectively. Implantation of the microtubules did not significantly change this exercise hyperventilation. However, neurotoxic lesioning with 10 mul ibotenic acid significantly (P < 0.05) attenuated the decrease in arterial Pco(2) by 1.3 and 2.8 Torr at the first and second workload, respectively. The modest attenuation of the exercise hypocapnia at both workloads in CFN-lesioned goats suggests that the CFN is part of the control system that enables the ventilatory response to meet the gas exchange requirements of submaximal exercise.

Human ventilatory responsiveness to hypoxia is unrelated to maximal aerobic capacity

Ventilatory responsiveness to hypoxia (HVR) has been reported to be different between highly trained endurance athletes and healthy sedentary controls. However, a linkage between aerobic capacity and HVR has not been a universal finding. The purpose of this study was to examine the relationship between HVR and maximal oxygen consumption (V̇o2 max) in healthy men with a wide range of aerobic capacities. Subjects performed a HVR test followed by an incremental cycle test to exhaustion. Participants were classified according to their maximal aerobic capacity. Those with a V̇o2 max of ≥60 ml·kg−1·min−1 were considered highly trained ( n = 13); those with a V̇o2 max of 50–60 ml·kg−1·min−1 were considered moderately-trained ( n = 18); and those with a V̇o2 max of <50 ml·kg−1·min−1 were considered untrained ( n = 24). No statistical differences were detected between the three groups for HVR ( P > 0.05), and the HVR values were variable within each group (range: untrained = 0.28–1.61, moderately trained = 0.23–2.39, and highly trained = 0.08–1.73 l·min·%arterial O2 saturation−1). The relationship between HVR and V̇o2 max was not statistically significant ( r = −0.1723; P > 0.05). HVR was also unrelated to maximal minute ventilation and ventilatory equivalents for O2 and CO2. We found that a spectrum of hypoxic ventilatory control is present in well-trained endurance athletes and moderately and untrained men. We interpret these observations to mean that other factors are more important in determining hypoxic ventilatory control than physical conditioning per se.

Effects of enhanced human chemosensitivity on ventilatory responses to exercise

It is not clear what the effects of different types of intermittent hypoxia have on human exercise ventilation. The purpose of this study was to determine whether short-duration intermittent hypoxia, and the subsequent augmentation of the hypoxic ventilatory response (HVR), would lead to an increase in ventilatory responses during exercise at sea level. It was hypothesized that subjects exposed to short-duration intermittent hypoxia would have a greater increase in the ventilatory response to exercise compared to those exposed to long-duration intermittent hypoxia. Subjects (n = 17, male) were randomly assigned to short-duration intermittent hypoxia (SDIH: 5 min of 12% O2 separated by 5 min of normoxia for 1 h) or long-duration intermittent hypoxia (LDIH: 30 min of 12% O2). Both groups had 10 exposures over a 12 day period. The HVR was measured on days 1 and 12. Maximal oxygen consumption (VO2max) was determined using a ramped cycle exercise test. Maximal exercise data were not different (P > 0.05) between SDIH and LDIH groups or following intermittent hypoxia. Minute ventilation, tidal volume and respiratory frequency were compared at 20, 40, 60, 80 and 100% of VO2max . There was no difference in the ventilatory responses at any intensity of exercise following the intermittent hypoxia period. The HVR was significantly increased following the intermittent hypoxia intervention (P < 0.05) but was not different between SDIH and LDIH (P > 0.05). The relationships between HVR and VO2max were non-significant on day 1 (r = 0.30) and day 12 (r = 0.47; P > 0.05). Our findings point to a lack of functional significance of increasing HVR via intermittent hypoxia on ventilatory responses to exercise at sea level.

Sex differences in respiratory exercise physiology

Respiratory exercise physiology research has historically focused on male subjects. In the last 20 years, important physiological and functional differences have been noted between the male and female response to dynamic exercise where sex differences have been reported for most of the major determinants of exercise capacity. Female participation in competitive and recreational sport is growing worldwide and it is universally accepted that participation in regular physical activity is of health benefit for both sexes. Understanding sex differences is of potential importance to both the clinician-scientist and the exercise physiologist since differences could impact upon exercise rehabilitation programmes for patient populations, exercise prescription for disease prevention in healthy individuals and training strategies for competitive athletes. Sex differences have been shown in resting pulmonary function, which may impact on the respiratory response to exercise. Women typically have smaller lung volumes and maximal expiratory flow rates even when corrected for height relative to men. Differences in resting and exercising ventilation across the menstrual cycle and relative to men have also been reported, although the functional significance remains unclear. Expiratory flow limitation and a high work of breathing are seen in women. Pulmonary system limitations, in particular exercise-induced arterial hypoxia, have been reported in both men and women; however, the prevalence in women is not yet known. From the available literature, it appears that there are sex differences in some areas of respiratory exercise physiology. However, detailed sex comparisons are difficult because the number of subjects studied to date has been woefully small.

Thermal drive contributes to hyperventilation during exercise in sheep

The etiology of exercise hypocapnia is unknown. The contributions of exercise intensity (ExInt), lactic acid, environmental temperature, rectal temperature (Tre), and physical conditioning to the variance in arterial CO2 tension (PaCO2) in the exercising sheep were quantified. We hypothesized that thermal drive contributes to hyperventilation. Four unshorn sheep were exercised at approximately 30, 50, and 70% of maximal O2 consumption for 30 min, or until exhaustion, both before and after 5 wk of physical conditioning. In addition, two of the sheep were shorn and exercised at each intensity in a cold (<15 degrees C) environment. Tre and O2 consumption were measured continuously. Lactic acid and PaCO2 were measured at 5- to 10-min intervals. Data were analyzed by multiple regression on PaCO2. During exercise, Tre rose and PaCO2 fell, except at the lowest ExInt in the cold environment. Tre explained 77% of the variance in PaCO2, and ExInt explained 5%. All other variables were insignificant. We conclude that, in sheep, thermal drive contributes to hyperventilation during exercise.

Skeletal muscle chemoreflex and pHi in exercise ventilatory control

To determine whether skeletal muscle hydrogen ion mediates ventilatory drive in humans during exercise, 12 healthy subjects performed three bouts of isotonic submaximal quadriceps exercise on each of 2 days in a 1.5-T magnet for 31P-magnetic resonance spectroscopy (31P-MRS). Bilateral lower extremity positive pressure cuffs were inflated to 45 Torr during exercise (BLPPex) or recovery (BLPPrec) in a randomized order to accentuate a muscle chemoreflex. Simultaneous measurements were made of breath-by-breath expired gases and minute ventilation, arterialized venous blood, and by 31P-MRS of the vastus medialis, acquired from the average of 12 radio-frequency pulses at a repetition time of 2.5 s. With BLPPex, end-exercise minute ventilation was higher (53.3 +/- 3.8 vs. 37.3 +/- 2.2 l/min; P < 0.0001), arterialized PCO2 lower (33 +/- 1 vs. 36 +/- 1 Torr; P = 0.0009), and quadriceps intracellular pH (pHi) more acid (6.44 +/- 0.07 vs. 6.62 +/- 0.07; P = 0.004), compared with BLPPrec. Blood lactate was modestly increased with BLPPex but without a change in arterialized pH. For each subject, pHi was linearly related to minute ventilation during exercise but not to arterialized pH. These data suggest that skeletal muscle hydrogen ion contributes to the exercise ventilatory response.

Ventilatory effects of specific carotid body hypocapnia and hypoxia in awake dogs

Specific carotid body (CB) hypocapnia in the-10-Torr (less than eupneic) range reduced ventilation in the awake and sleeping dog to the same degree as did CB hyperoxia [CB PO2 (PCBO2); > 500 Torr; C.A. Smith, K.W. Saupe, K. S. Henderson, and J. A. Dempsey. J. Appl. Physiol. 79:689-699, 1995], suggesting a powerful inhibitory effect of hypocapnia at the carotid chemosensor over a range of PCO2 encountered commonly in physiological hyperpneas. The primary purpose of this study was to assess the ventilatory effect of CB hypocapnia on the ventilatory response to concomitant CB hypoxia. The secondary purpose was to assess the relative gains of the CB and central chemoreceptors to hypocapnia. In eight awake female dogs the vascularly isolated CB was perfused with hypoxic blood (mild, PCBO2 approximately equal to 50 Torr or severe, PCBO2 approximately equal to 36 Torr) in a background of normocapnia or hypocapnia (10 Torr less than eupneic arterial PCO2) in the perfusate. The systemic (and brain) circulation was normoxic throughout, and arterial PCO2 was not controlled (poikilocapnia). With CB hypocapnia, the peak ventilation (range 19-27 s) in response to hypoxic CB perfusion increased 48% (mild) and 77% (severe) due to increased tidal volume. When CB hypocapnia was present, these increases in ventilation were reduced to 21 and 27%, respectively. With systemic hypocapnia, with the isolated CB maintained normocapnic and hypoxic for > 70 s, the steady-state poikilocapnic ventilatory response (i.e., to systemic hypocapnia alone) decreased 15% (mild CB hypoxia) and 27% (severe CB hypoxia) from the peak response, respectively. We conclude that carotid body hypocapnia can be a major source of inhibitory feedback to respiratory motor output during the hyperventilatory response to hypoxic carotid body stimulation.

Contribution of peripheral chemoreceptor drive in exercise hyperpnea in humans

The peripheral chemoreceptors play a dominant role in the respiratory compensation of lactic acidosis during heavy exercise of humans. Our object was to determine the contribution of peripheral chemoreceptors to exercise hyperpnea during mild to moderate and heavy exercise above the anaerobic threshold. We used a hyperoxic suppression test in six normal male subjects. Inspired gas was abruptly changed without the subject's knowledge from air to pure oxygen for 5 to 6 breaths. The maximal ventilatory depression after O2 breathing was 5.5 +/- 1.7 L/min (BTPS) at mild exercise, and the depression increased with increasing exercise intensity up to 12.8 +/- 4.1 L/min (BTPS). The relative contribution of the peripheral chemoreceptors to ventilation in terms of percentage of the maximal ventilatory depression was maintained, being 20% throughout the entire work ranges studied. The contribution of the peripheral chemoreceptors to total ventilation is hardly altered by lactic acidosis caused by heavy exercise above the anaerobic threshold according to our data. These results suggested that the peripheral chemoreceptors may not be solely responsible for excessive hyperventilation, or residual activities of peripheral chemoreceptors still exist after O2 breathing especially during heavy exercise above the anaerobic threshold.

Contribution of acid-base changes to control of breathing during exercise

The mechanisms mediating the exercise hyperpnea remain controversial; there is no unequivocal evidence that any of numerous proposed mechanisms mediates the hyperpnea. However, a great deal has been learned including the potential role of changes in PCO2, [H+], strong ion differences (SID), weak acids, or any other acid-base component. The contribution of acid-base changes to the hyperpnea during exercise is likely through known or postulated chemoreceptors. Two of these, pulmonary and intracranial chemoreceptors, do not appear critical for the ventilatory adjustments to meet the metabolic demands of exercise. A third, the carotid chemoreceptors, appear to fine-tune alveolar ventilation during exercise to minimize disruptions in arterial blood gases. The role of the fourth chemoreceptors, those within skeletal muscles, is least clear. However, there is evidence that they do contribute to the hyperpnea, and it is quite clear that a muscle chemoreflex contributes to the exercise muscle pressor reflex; thus the contribution of these chemoreceptors to the exercise hyperpnea requires additional study.

Hypoxic response is inversely related to degree of exercise hyperventilation

The Dejours hyperoxic test has been used to quantitate peripheral chemoreceptor contribution to the hyperpnea of exercise. The strength of this drive, measured by the percent reduction in ventilation, varies among individuals and is lacking in chemodenervated humans, who also fail to manifest a hyperventilatory response in heavy exercise. We reasoned that greater hyperventilation in exercise above the anaerobic threshold ought to be associated with greater hypoxic (carotid body) drive. The present study tested this hypothesis. In 17 naive subjects, carotid body O2 chemosensitivity was tested repeatedly during exercise above the ventilatory anaerobic threshold (VAT) using 2 breaths of O2. The response to these transients was quantitated by the percentage change in ventilation, and exercise hyperventilation was quantitated by VE in excess of VCO2 predicted from the slope of delta VE/delta VCO2 below VAT in incremental exercise. Contrary to expectations, there was an inverse relation between the degree of exercise hyperventilation and the percentage reduction in exercise ventilation in response to O2. The significance of this observation and its integration with current thinking of the role of the peripheral chemoreceptor in mediating hyperventilation of heavy exercise is discussed.

Do carotid chemoreceptors inhibit the hyperventilatory response to heavy exercise?

In this paper two types of evidence are presented which question the commonly presumed role of carotid chemoreceptor stimulation as the primary mediator of the hyperventilatory response to heavy exercise. First, carotid-body denervation in ponies increases their hyperventilatory response to heavy exercise. Second, the awake dog and the goat at rest show an immediate and substantial depression of tidal volume and of ventilation when their isolated carotid chemoreceptors are made hypocapnic. Accordingly, it is proposed that during heavy exercise the carotid chemoreceptors are inhibitory to respiratory motor output and that the cause of the hyperventilatory response originates from extrachemoreceptor, locomotor-linked, feed-forward stimuli.

Adaptations and limitations in the pulmonary system during exercise

In most circumstances in health, efficient alveolar ventilation and alveolar-to-arterial exchange of O2 and CO2 are among the strongest of links in the gas-transport chain during maximal exercise. Indeed, in most instances, the metabolic cost of ventilation represents the only significant contribution of the pulmonary system to the limitation of O2 transport of locomotor muscles and thus to the limitation of maximum performance. Of the "weaknesses" inherent in the healthy pulmonary system response to exercise, the most serious one may well be its absence of structural adaptability to physical training or to the trained state. Thus, the lung's diffusion capacity and pulmonary capillary blood volume remain unaltered in the highly trained human or horse, while maximum pulmonary blood flow rises linearly with the enhanced max VO2. Similarly, ventilatory requirement rises markedly, with no alteration in the capability of the airways to produce higher flow rates or of the lung parenchyma to stretch to higher tidal volumes, and little or no change in the pressure-generating capability of inspiratory muscles. The case of the elderly athlete who remains capable of achieving high maximum pulmonary blood flows and ventilatory requirements and whose lung undergoes a normal aging process underscores the importance of deficits (from "normal") on the capacity end of this continuum of cost versus capacity in the pulmonary system. The asthmatic athlete may represent another such example of limited flow-generating capacity; and the healthy, young, highly fit athlete who shows marked reductions in SaO2 and in max VO2 at even moderately high altitudes demonstrates that, in many situations, precious little room can be added to the demand side or removed from the capacity side before signs of failure can be seen.

Breathing during exercise: demands, regulation, limitations

In humans alveolar ventilation (VA) is adjusted almost perfectly to the metabolic demands of mild and moderate exercise. For example, in exercise transitions and in the steady state, PaCO2 rarely deviates by more than 1 to 3 mmHg from the value at rest. This near-homeostasis contrasts to most other mammalian species; equines for example, demonstrate a progressive hypocapnia and alkalosis as exercise intensity is increased to moderate levels. In equines, the control systems seem programmed for a specific hyperventilation that contributes to maintenance of PaO2 homeostasis. Generally, during heavy exercise all species hyperventilate creating hypocapnia, increased PAO2, widened A-a O2 gradient, and PaO2 homeostasis. The origin of the metabolic ventilatory stimulus remains controversial. Evidence exists for: a) "neural" mediation, either central command or peripheral afferent in nature; and b) "humoral" mediation with an intra-thoracic metabolite receptor being a possibility. The mechanism of the species differences in hyperventilation during exercise does not appear to be due to species variation in chemoreceptor "fine tuning". Contrary to traditional thinking, recent findings suggest that the hyperventilation during heavy exercise might not be mediated by lactacidosis stimulation of chemoreceptors. The increase in VA during exercise is achieved efficiently in that airway diameter is modulated and the pattern of breathing and the recruitment of respiratory muscles are set to minimize the O2 cost of breathing. It has been postulated that mechanoreceptors in airways, lung parenchyma and the chest wall are important to efficient breathing. Their role and contribution to the exercise hyperpnea has been shown by reductions in respiratory neural output within breath when respiratory impedance is reduced via helium breathing. Hilar nerve afferents do not appear to be critical to this response. However, carotid chemoreceptors appear essential for "fine tuning" of VA when respiratory impedance is reduced. In most healthy exercising mammals, the efficiency component of the exercise stimulus does not compromise VA. There are two known major exceptions. One is the extremely fit human athlete during very high workloads when atypically there is minimal or no hyperventilation resulting in arterial hypoxemia. That indeed the high O2 cost of breathing compromises VA is indicated by hyperventilation and alleviation of hypoxemia with resistance unloading through helium breathing. A second example of a compromise of VA is that of a galloping racehorse at very high workloads.(ABSTRACT TRUNCATED AT 400 WORDS)

The contribution of peripheral chemoreceptors to ventilation during heavy exercise

The purpose of this study was to determine, in man, the contribution of peripheral chemoreceptors to ventilation during constant-load, heavy exercise above anaerobic threshold at sea level, using hyperoxic suppression of peripheral chemoreceptor drive which was obtained by abrupt and surreptitious replacement of inspired air with 100% oxygen for a period of 20-30 sec during the exercise. There was a delay of at least 1 sec from the time of peripheral chemoreceptor blockade to the initial change in ventilation, suggesting the operation of a central neural reverberatory mechanism after the cessation of peripheral chemoreceptor drive. In contrast to Wasserman (1976), whose results indicated a 25% decrease in ventilation within two breaths, in the present study no significant drop was observed until some 4-6 breaths after the air-to-oxygen switch. Furthermore, the drop in ventilation, magnitude of which was of the order of 15%, was transient in 5 out of 8 subjects. In one subject, the ventilation increased following oxygen administration. It is concluded that the peripheral chemoreceptors are not the sole mediators of hyperventilation of heavy exercise above anaerobic threshold in man.