Neural drives to breathing during exercise

An Improved Dynamic Model for the Respiratory Response to Exercise

Respiratory system modeling has been extensively studied in steady-state conditions to simulate sleep disorders, to predict its behavior under ventilatory diseases or stimuli and to simulate its interaction with mechanical ventilation. Nevertheless, the studies focused on the instantaneous response are limited, which restricts its application in clinical practice. The aim of this study is double: firstly, to analyze both dynamic and static responses of two known respiratory models under exercise stimuli by using an incremental exercise stimulus sequence (to analyze the model responses when step inputs are applied) and experimental data (to assess prediction capability of each model). Secondly, to propose changes in the models' structures to improve their transient and stationary responses. The versatility of the resulting model vs. the other two is shown according to the ability to simulate ventilatory stimuli, like exercise, with a proper regulation of the arterial blood gases, suitable constant times and a better adjustment to experimental data. The proposed model adjusts the breathing pattern every respiratory cycle using an optimization criterion based on minimization of work of breathing through regulation of respiratory frequency.

The impact of human pregnancy on perceptual responses to chemoreflex vs. exercise stimulation of ventilation: a retrospective analysis

We examined the impact of human pregnancy on breathlessness intensity at matched levels of ventilation (V˙E) during isoxic hyperoxic CO₂ rebreathing and incremental cycle exercise tests in 21 healthy women in the third trimester (TM₃) and again ∼5 months post-partum (PP). Pregnancy had no significant (P > 0.05) effect on the slope or threshold of the breathlessness intensity-V˙E relationship during both exercise and rebreathing. By contrast, the slope of the breathlessness intensity-V˙E relationship was significantly higher, while the threshold of this relationship was consistently lower during rebreathing vs. exercise (both P < 0.05), regardless of pregnancy status (P > 0.05). As a result, breathlessness intensity was markedly higher at any given V˙E (e.g., by ∼4 Borg units at 40 L/min) during rebreathing vs. exercise, regardless of pregnancy status. Inter-subject variation in breathlessness intensity-V˙E slopes during exercise was not associated with inter-subject variation in breathlessness intensity-V˙E slopes during rebreathing or with increased central chemoreflex responsiveness during pregnancy (both P > 0.05). In conclusion, the intensity of perceived breathlessness for a given V˙E depends, at least in part, on the nature and source of increased central respiratory motor command output, independent of pregnancy status; and pregnancy-induced increases in activity-related breathlessness cannot be easily explained by increased central chemoreflex responsiveness.

Adaptation of exercise ventilation during an actively-induced hyperthermia following passive heat acclimation

Hyperthermia-induced hyperventilation has been proposed to be a human thermolytic thermoregulatory response and to contribute to the disproportionate increase in exercise ventilation (V̇e) relative to metabolic needs during high-intensity exercise. In this study it was hypothesized that V̇e would adapt similar to human eccrine sweating (ĖSW) following a passive heat acclimation (HA). All participants performed an incremental exercise test on a cycle ergometer from rest to exhaustion before and after a 10-day passive exposure for 2 h/day to either 50°C and 20% relative humidity (RH) ( n = 8, Acclimation group) or 24°C and 32% RH ( n = 4, Control group). Attainment of HA was confirmed by a significant decrease ( P = 0.025) of the esophageal temperature (Tes) threshold for the onset of ĖSW and a significantly elevated ĖSW ( P ≤ 0.040) during the post-HA exercise tests. HA also gave a significant decrease in resting Tes ( P = 0.006) and a significant increase in plasma volume ( P = 0.005). Ventilatory adaptations during exercise tests following HA included significantly decreased Tes thresholds ( P ≤ 0.005) for the onset of increases in the ventilatory equivalents for O2 (V̇e/V̇o2) and CO2 (V̇e/V̇co2) and a significantly increased V̇e ( P ≤ 0.017) at all levels of Tes. Elevated V̇e was a function of a significantly greater tidal volume ( P = 0.003) at lower Tes and of breathing frequency ( P ≤ 0.005) at higher Tes. Following HA, the ventilatory threshold was uninfluenced and the relationships between V̇o2 and either V̇e/V̇o2 or V̇e/V̇co2 did not explain the resulting hyperventilation. In conclusion, the results support that exercise V̇e following passive HA responds similarly to ĖSW, and the mechanism accounting for this adaptation is independent of changes of the ventilatory threshold or relationships between V̇o2 with each of V̇e/V̇o2 and V̇e/V̇co2.

The ventilatory response to sine wave variation in exercise loads and limb movement frequency

The current study's experiments tested the hypothesis that limb movement frequency is a significant determinant of exercise hyperpnoea. To this end, 19 healthy participants walked on a treadmill, where work was varied sinusiodally by alterations in either treadmill speed or grade. Measured responses were fitted with sine waves to determine their amplitudes and phase angles. Walking pace amplitude was greater during speed tests than grade tests, and phase lag relative to the treadmill smaller, as expected. Ventilation, carbon dioxide production, and oxygen uptake amplitudes were higher during speed tests than grade tests. Further, phase angle lags relative to the treadmill for these measures were shorter during speed tests than grade tests. We concluded that these findings demonstrate the presence of changes in breathing during exercise that can be attributed to changes in limb movement frequency.

Sensing vascular distension in skeletal muscle by slow conducting afferent fibers: neurophysiological basis and implication for respiratory control

This review examines the evidence that skeletal muscles can sense the status of the peripheral vascular network through group III and IV muscle afferent fibers. The anatomic and neurophysiological basis for such a mechanism is the following: 1) a significant portion of group III and IV afferent fibers have been found in the vicinity and the adventitia of the arterioles and the venules; 2) both of these groups of afferent fibers can respond to mechanical stimuli; 3) a population of group III and IV fibers stimulated during muscle contraction has been found to be inhibited to various degrees by arterial occlusion; and 4) more recently, direct evidence has been obtained showing that a part of the group IV muscle afferent fibers is stimulated by venous occlusion and by injection of vasodilatory agents. The physiological relevance of sensing local distension of the vascular network at venular level in the muscles is clearly different from that of the large veins, since the former can directly monitor the degree of tissue perfusion. The possible involvement of this sensing mechanism in respiratory control is discussed mainly in the light of the ventilatory effects of peripheral vascular occlusions during and after muscular exercise. It is proposed that this regulatory system anticipates the chemical changes that would occur in the arterial blood during increased metabolic load and attempts to minimize them by adjusting the level of ventilation to the level of muscle perfusion, thus matching the magnitudes of the peripheral and pulmonary gas exchange.

Selective brain cooling seems to be a mechanism leading to human craniofacial diversity observed in different geographical regions

Selective brain cooling (SBC) can occur in hyperthermic humans despite the fact that humans have no carotid rete, a vascular structure that facilitates countercurrent heat exchange located at the base of the skull in some mammals. Emissary and angular veins, upper respiratory tract, tympanic cavity and cerebrospinal fluid are major components of SBC system in humans. The efficiency of SBC is increased by evaporation of sweat on the head and by ventilation through the nose, but it is surprising to find out that mammals do not display SBC during exercise hyperthermia. What is the explanation then for the SBC at high body temperatures? Our hypothesis is that selective brain cooling protects the brain from thermal damage in a long-standing manner by allowing adaptive mechanisms to change the craniofacial morphology appropriate for different environmental conditions. Since the brain can only be as big that can cool, it is not surprising to find a lower (below 1300 cm(3)) cranial volume in Australian Aborigines with respect to the one (over 1450 cm(3)) in Eskimos. In addition to lower brain volume, other craniofacial features such as thick everted lips, broader nasal cavity and bigger paranasal sinuses that provide more evaporating surfaces seem to be anatomical variations developed in time for an effective SBC in hot climates. It was reported previously that these biological adaptations result from the tissues of neural crest origin. Among the crest derivatives, leptomeninges (pia and arachnoid mater), skeletal and connective tissues of the face and much of the skull seem to be structures upon which environment operates to produce more convenient craniofacial morphology for an effective SBC. In conclusion, selective brain cooling seems to be a mechanism leading to adaptive craniofacial diversity observed in different geographical regions. Thus, SBC is necessary for long-term biological adaptation, not for protecting the brain from acute thermal damage.

CO2 does not affect passive exercise ventilatory decline

Breathing increases abruptly at the start of passive exercise, stimulated by afferent feedback from the moving limbs, and declines toward a steady-state hyperpnea as exercise continues. This decline has been attributed to decreased arterial CO2 levels and adaptation in afferent feedback; however, the relative importance of these two mechanisms is unknown. To address this issue, we compared ventilatory responses to 5 min of passive leg extension exercise performed on 10 awake human subjects (6 men and 4 women) in isocapnic and poikilocapnic conditions. End-tidal Pco2 decreased significantly during poikilocapnic (Delta = -1.5 +/- 0.5 Torr, P < 0.001), but not isocapnic, passive exercise. Despite this difference, the ventilatory responses to passive exercise were not different between the two conditions. Using the fast changes in ventilation at the start (5.46 +/- 0.40 l/min, P < 0.001) and end (3.72 +/- 0.33 l/min, P < 0.001) of passive exercise as measures of the drive to breathe from afferent feedback, we found a decline of 68%. We conclude that the decline in ventilation during passive exercise is due to an adaptation in the afferent feedback from the moving limbs, not a decline in CO2 levels.

Direct cooling of the human brain by heat loss from the upper respiratory tract

This study is the first report on human intracranial temperature in conscious patients during and after an upper respiratory bypass. Temperatures were measured in four subjects subdurally between the frontal lobes and cribriform plate (T(cr)) and on the vault of the skull (T(sd)). Further measurements were taken in the esophagus (T(es)) and on the tympanic membrane. Reinstitution of airflow in the upper respiratory tract under conditions of mild hyperthermia gave a rapid drop in T(cr) of 0.4-0.8 degrees C. In three patients the intracranial temperature at the basal aspect of the frontal lobes fell below T(es). Thus local selective cooling of the brain surface below that of the trunk temperature was shown to occur. Intensive breathing by the patients after extubation for a 3-min period produced a cooling at the site of T(cr) measurement at a rate of up to 0.1 degrees C/min, and this response could be evoked on demand. The results support the view that cooling of the upper airway can directly influence human brain temperature.

The effect of exercise duration on the fast component of exercise hyperpnoea at work rates below the first ventilatory threshold

We examined the effect of exercise duration on the fast component of exercise hyperpnoea for light and moderate work rates [mean oxygen uptakes (SD) = 1.00 (0.27) 1.min-1 and 1.77 (0.53) 1.min-1, respectively]. Ten subjects exercised on a motor-driven treadmill while ventilation and end-expiratory partial pressures of carbon dioxide and oxygen were recorded on a breath-by-breath basis. The magnitude of the fast component of exercise hyperpnoea was determined by quantifying the abrupt changes in ventilation at the beginning and end of exercise. Five exercise tests with durations ranging from 1 min to 8 min were completed at each of the two periods of exercise at different work rates. Subsequent statistical analysis revealed that the rapid changes in ventilation at the end of exercise were significantly smaller than those at the start [pooled means (SE) = 6.27 (0.48) and 13.05 (1.06) 1.min-1 for light and moderate exercises respectively] regardless of exercise duration. Further statistical analysis failed to find a relationship between the fast ventilatory changes present at the end of exercise, expressed as a proportion of those at the start of exercise, and either exercise duration or work rate (73% and 62% for light and moderate exercises respectively). We conclude that the fast component of exercise hyperpnoea declines rapidly in the first minute of exercise, and interpret this decline as an indication that the fast neural drive to ventilation, proportional to limb movement frequency, adapts quickly at the start of exercise.

Role of decreased carbohydrate oxidation on slower rises in ventilation with increasing exercise intensity after training

In these studies, we examined whether the rightward shift in steady-state minute ventilation (VE) versus O2 uptake curves after training is more closely linked to the reduced CO2 production from carbohydrate oxidation (CHOOX) after training than to the attenuated increase in blood lactate concentration. Steady state VE values and gas exchange were measured in eight previously sedentary men who underwent exercise tests of 60 W + 40 W every 6 min before and after a 9 week training programme of cycling approximately 40 min a day. Following training, the slower rises in VE with increasing exercise intensities were associated with a reduced reliance on CHOOX, (P < 0.01). Both before and after training, VE values in litres per minute rose as a linear VE = 18.CHOOX + 14, function of rates of CHOOX in grams per minute (r = 0.99), irrespective of a marked shift to the right in arterialized venous blood lactate concentration versus CHOOX curves following training (P < 0.01). Thus, slower increases in steady-state VE values with increasing exercise intensities following endurance training appeared to be more closely linked to the decreased reliance on CHOOX than to the attenuated increase in blood lactate concentration.

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.