Control of breathing in man; insights from the 'locked-in' syndrome

Breathing control, brain, and bodily self-consciousness: Toward immersive digiceuticals to alleviate respiratory suffering

Breathing is peculiar among autonomic functions through several characteristics. It generates a very rich afferent traffic from an array of structures belonging to the respiratory system to various areas of the brain. It is intimately associated with bodily movements. It bears particular relationships with consciousness as its efferent motor control can be automatic or voluntary. In this review within the scope of "respiratory neurophysiology" or "respiratory neuroscience", we describe the physiological organisation of breathing control. We then review findings linking breathing and bodily self-consciousness through respiratory manipulations using virtual reality (VR). After discussing the currently admitted neurophysiological model for dyspnea, as well as a new Bayesian model applied to breathing control, we propose that visuo-respiratory paradigms -as developed in cognitive neuroscience- will foster insights into some of the basic mechanisms of the human respiratory system and will also lead to the development of immersive VR-based digital health tools (i.e. digiceuticals).

Brain-Computer Interface: Applications to Speech Decoding and Synthesis to Augment Communication

Damage or degeneration of motor pathways necessary for speech and other movements, as in brainstem strokes or amyotrophic lateral sclerosis (ALS), can interfere with efficient communication without affecting brain structures responsible for language or cognition. In the worst-case scenario, this can result in the locked in syndrome (LIS), a condition in which individuals cannot initiate communication and can only express themselves by answering yes/no questions with eye blinks or other rudimentary movements. Existing augmentative and alternative communication (AAC) devices that rely on eye tracking can improve the quality of life for people with this condition, but brain-computer interfaces (BCIs) are also increasingly being investigated as AAC devices, particularly when eye tracking is too slow or unreliable. Moreover, with recent and ongoing advances in machine learning and neural recording technologies, BCIs may offer the only means to go beyond cursor control and text generation on a computer, to allow real-time synthesis of speech, which would arguably offer the most efficient and expressive channel for communication. The potential for BCI speech synthesis has only recently been realized because of seminal studies of the neuroanatomical and neurophysiological underpinnings of speech production using intracranial electrocorticographic (ECoG) recordings in patients undergoing epilepsy surgery. These studies have shown that cortical areas responsible for vocalization and articulation are distributed over a large area of ventral sensorimotor cortex, and that it is possible to decode speech and reconstruct its acoustics from ECoG if these areas are recorded with sufficiently dense and comprehensive electrode arrays. In this article, we review these advances, including the latest neural decoding strategies that range from deep learning models to the direct concatenation of speech units. We also discuss state-of-the-art vocoders that are integral in constructing natural-sounding audio waveforms for speech BCIs. Finally, this review outlines some of the challenges ahead in directly synthesizing speech for patients with LIS.

Stroke and breathing

Stroke remains a leading cause of neurologic disability with wide ranging effects, including a variety of respiratory abnormalities. Stroke may influence the central control of the respiratory drive and breathing pattern, airway protection and maintenance, and the respiratory mechanics of inspiration and expiration. In the acute phase of stroke, the central control of breathing is affected by changes in consciousness, cerebral edema, and direct damage to brainstem respiratory centers, resulting in abnormalities in respiratory pattern and loss of airway protection. Common acute complications related to respiratory dysfunction include dysphagia, aspiration, and pneumonia. Respiratory control centers are located in the brainstem, and brainstem stroke causes specific patterns of respiratory dysfunction. Depending on the exact location and extent of stroke, respiratory failure may occur. While major respiratory abnormalities often improve over time, sleep-disordered breathing remains common in the subacute and chronic phases and worsens outcomes. Respiratory mechanics are impaired in hemiplegic or hemiparetic stroke, contributing to worse cardiopulmonary health in stroke survivors. Interventions to address the respiratory complications are under researched, and further investigation in this area is critical to improving outcomes among stroke survivors.

Locked-In Syndrome: Practical Rehabilitation Management

Locked-in syndrome is a rare and devastating condition that results in tetraplegia, lower cranial nerve paralysis, and anarthria with preserved cognition, vertical gaze, and upper eyelid movements. Although acute management is much like that of any severe stroke, rehabilitation and recovery of these patients have not been previously described. Challenges relevant to this population include blood pressure management and orthostasis, timing and appropriateness of reinstating oral feeding, ventilatory support, decannulation after tracheostomy, bowel and bladder management, vestibular dysfunction, and eye care. Targeted rehabilitation of head, neck, and trunk stability to improve function, and proper fit in an appropriate wheelchair are essential to assist with mobility. Rehabilitation interventions should include a focus on distal motor control and upright tolerance training followed by balance and mobility exercises. In addition, special considerations must be given to developing early methods of communication through use of augmentative systems to call for help and express needs. These systems along with additional technology provide the basis to promote connectivity to family and friends through the use of social media and the internet. Establishment of communication, mobility, and connectivity is essential in promoting independence, autonomy, and improving quality of life. Overall, with specialized rehabilitative care and access to the proper equipment, long-term outcomes and quality of life in these patients can be favorable.

Slower Is Higher: Threshold Modulation of Cortical Activity in Voluntary Control of Breathing Initiation

Speech or programmed sentences must often be interrupted in order to listen to and interact with interlocutors. Among many processes that produce such complex acts, the brain must precisely adjust breathing to produce adequate phonation. The mechanism of these adjustments is multifactorial and still poorly understood. In order to selectively examine the adjustment in breath control, we recorded respiratory-related premotor cortical potentials from the scalp of human subjects while they performed a single breathing initiation or inhibition task. We found that voluntary breathing is initiated if, and only if, the cortical premotor potential activity reaches a threshold activation level. The stochastic variability in the threshold correlates to the distribution of initiation times of breathing. The data also fitted a computerized interactive race model. Modeling results confirm that this model is also as effective in respiratory modality, as it has been found to be for eye and hand movements. No modifications were required to account for respiratory cycle inhibition processes. In this overly simplified task, we showed a link between voluntary initiation and control of breathing and activity in a fronto-median region of the cerebral cortex. These results shed light on some of the physiological constraints involved in the complex mechanisms of respiration, phonation, and language.

Facing the challenge of mammalian neural microcircuits: taking a few breaths may help

Breathing in mammals is a seemingly straightforward behaviour controlled by the brain. A brainstem nucleus called the preBötzinger Complex sits at the core of the neural circuit generating respiratory rhythm. Despite the discovery of this microcircuit almost 25 years ago, the mechanisms controlling breathing remain elusive. Given the apparent simplicity and well-defined nature of regulatory breathing behaviour, the identification of much of the circuitry, and the ability to study breathing in vitro as well as in vivo, many neuroscientists and physiologists are surprised that respiratory rhythm generation is still not well understood. Our view is that conventional rhythmogenic mechanisms involving pacemakers, inhibition or bursting are problematic and that simplifying assumptions commonly made for many vertebrate neural circuits ignore consequential detail. We propose that novel emergent mechanisms govern the generation of respiratory rhythm. That a mammalian function as basic as rhythm generation arises from complex and dynamic molecular, synaptic and neuronal interactions within a diverse neural microcircuit highlights the challenges in understanding neural control of mammalian behaviours, many (considerably) more elaborate than breathing. We suggest that the neural circuit controlling breathing is inimitably tractable and may inspire general strategies for elucidating other neural microcircuits.

Canadian Association of Neuroscience Review: Respiratory Control and Behavior in Humans: Lessons from Imaging and Experiments of Nature

The purpose of this review is to demonstrate that respiration is a complex behavior comprising both brainstem autonomic control and supramedullary influences, including volition. Whereas some fundamental mechanisms had to be established using animal models, this review focuses on clinical cases and physiological studies in humans to illustrate normal and abnormal respiratory behavior. To summarize, central respiratory drive is generated in the rostroventrolateral medulla, and transmitted to both the upper airway and to the main and accessory respiratory muscles. Afferent feedback is provided from lung and muscle mechnoreceptors, peripheral carotid and aortic chemoreceptors, and multiple central chemoreceptors. Supramedullary regions, including cortex and subcortex, modulate or initiate breathing with volition, emotion and at the onset of exercise. Autonomic breathing control can be perturbed by brainstem pathology including space occupying lesions, compression, congenital central hypoventilation syndrome and sudden infant death syndrome. Sleep-wake states are important in regulating breathing. Thus, respiratory control abnormalities are most often evident during sleep, or during transition from sleep to wakefulness. Previously undiagnosed structural brainstem pathology may be revealed by abnormal breathing during sleep. Ondine's curse and 'the locked-in syndrome' serve to distinguish brainstem from supramedullary regulatory mechanisms in humans: The former comprises loss of autonomic respiratory control and requires volitional breathing for survival, and the latter entails loss of corticospinal or corticobulbar tracts required for volitional breathing, but preserves autonomic respiratory control.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

The supplementary motor area exerts a tonic excitatory influence on corticospinal projections to phrenic motoneurons in awake humans

Introduction

In humans, cortical mechanisms can interfere with autonomic breathing. Respiratory-related activation of the supplementary motor area (SMA) has been documented during voluntary breathing and in response to inspiratory constraints. The SMA could therefore participate in the increased resting state of the respiratory motor system during wake (i.e. "wakefulness drive to breathe").

Methods

The SMA was conditioned by continuous theta burst magnetic stimulation (cTBS, inhibitory) and 5 Hz conventional rTMS (5 Hz, excitatory). The ensuing effects were described in terms of the diaphragm motor evoked response (DiMEPs) to single-pulse transcranial magnetic stimulation over the motor cortex. DiMEPs were recorded at baseline, and at 3 time-points ("post1", "post2", "post3") up to 15 minutes following conditioning of the SMA.

Results

cTBS reduced the amplitude of DiMEPs from 327.5±159.8 µV at baseline to 243.3±118.7 µV, 217.8±102.9 µV and 240.6±123.9 µV at post 1, post 2 and post 3, respectively (F = 6.341, p = 0.002). 5 Hz conditioning increased the amplitude of DiMEPs from 184.7±96.5 µV at baseline to 270.7±135.4 µV at post 3 (F = 4.844, p = 0.009).

Conclusions

The corticospinal pathway to the diaphragm can be modulated in both directions by conditioning the SMA. This suggests that the baseline respiratory activity of the SMA represents an equipoise from which it is possible to move in either direction. The resting corticofugal outflow from the SMA to phrenic motoneurones that this study evidences could putatively contribute to the wakefulness drive to breathe.

Challenges of developing communicative interaction in individuals with congenital profound intellectual and multiple disabilities

BACKGROUND

Physiological responses have been used in individuals with acquired disability to enable communicative interaction without motor movement. This study explored four autonomic nervous system (ANS) signals-electrodermal activity, skin temperature, cardiac patterns and respiratory patterns-to enable interaction with individuals born with profound intellectual and multiple disabilities (PIMD).

METHOD

A series of case studies were conducted to teach a 15-year-old individual with PIMD to voluntarily control his ANS signals for the purposes of communicative interaction. Training was carried out according to an alternating treatment single-subject research design.

RESULTS

Training was unsuccessful; however, ANS signal patterns revealed issues unique to people with PIMD: profound intrasubject variability, discrepancy between physiological responses and caregiver perspectives, and the participant's lack of contingency awareness.

CONCLUSIONS

These three priority areas unique to people with congenital PIMD must be addressed before ANS signals can be used to enable communicative interaction with this population.

Supramedullary influences on cough

The evidence for supramedullary influences on cough is largely indirect. Cough can be voluntarily induced or inhibited, functions usually thought to reside in the cerebral cortex. A sensation of 'urge-to-cough' usually precedes cough due to an airway irritant stimulus, and this may well involve the cerebral cortex. In conditions with interruption of the pathways between the cortex and the brainstem, such as strokes and Parkinson's disease, voluntary cough may be inhibited without disruption of reflex cough from the larynx or lower airways. 'Habit cough', like Tourette's syndrome, is assumed to be cortically mediated. Placebos and many treatments based on complementary medicine are effective in inhibiting clinical cough, and the site of action is likely to be the cerebral cortex. In sleep and in anaesthesia cough is depressed and, again, this seems likely to be at a cortical level. However there are few or no experimental or clinical observation as to the localization and functions of supramedullary areas responsible for cough. It is a field of research wide open for exploration.

Causes of Cheyne-Stokes respiration

Cheyne-Stokes respiration (CSR) is one of several types of unusual breathing with recurrent apneas (dysrhythmias). Reported initially in patients with heart failure or stroke, it was then recognized both in other diseases and as a component of the sleep apnea syndrome. CSR is potentiated and perpetuated by changing states of arousal that occur during sleep. The recurrent hypoxia and surges of sympathetic activity that often occur during the apneas may have serious health consequences. Heart failure and stroke are risk factors for sleep apnea. The recurrent apneas and intermittent hypoxia occurring with sleep apnea further damage the heart and brain. Although all breathing dysrhythmias do not have the same cause, instability in the feedback control involved in the chemical regulation of breathing is the leading cause of CSR. Mathematical models have helped greatly in the understanding of the causes of recurrent apneas.

Physiological and pathophysiological down-regulation of cough

Recent clinical studies have emphasized the up-regulation (sensitization) of cough in pathological conditions of the airways. However there are also many situations where voluntary and reflex cough can be down-regulated. These include: (1) chemical stimulation of breathing by hypercapnia or hypoxia or both, establishing that cough sensitivity can be inversely related to drive to breathing; (2) voluntary inhibition of cough, probably similar in mechanism to the depression of cough that can be induced by hypnosis and other branches of alternative medicine; (3) the placebo effect of many antitussive treatments; (4) sleep; (5) general anaesthesia; (6) central nervous disorders such as coma, stroke, Parkinson's disease and several other conditions where the defect in the protective reflexes may lead to aspiration pneumonia; (7) increased activity in various afferent inputs from viscera in the thorax and abdomen; (8) a number of bronchopulmonary clinical disorders. The list is long, but regrettably the nervous mechanisms of these down-regulations have been little studied. In addition there are a number of situations, such as exercise, coitus, talking and singing which, while important to coughing humans, have been not investigated in relation to cough. Most of the studies have been with experimental animals, and their extension to human research is desirable. In view of the importance of cough and other defensive reflexes in maintaining human well-being, far more research is needed. The field is wide-open.

Learning in respiratory control

In this article, it is argued that learning participates to fulfill the metabolic requirements by adapting respiratory control to changing internal and external states. Recent classical-conditioning experiments in newborn mice or adult rats show the close link between conditioned respiratory and arousal responses. The conditioned fear model may be a suitable and largely unexplored model of emotionally induced hyperventilation. The parabrachial nucleus and periacqueducal grey may play a pivotal role in the ventilatory component of conditioned fear. The sensitivity of breathing to conditioning in newborn and adult animals suggests that learning processes may shape breathing pattern throughout life.

Neuroimaging evidence implicating cerebellum in the experience of hypercapnia and hunger for air

Recent neuroimaging and neurological data implicate cerebellum in nonmotor sensory, cognitive, vegetative, and affective functions. The present study assessed cerebellar responses when the urge to breathe is stimulated by inhaled CO(2). Ventilation changes follow arterial blood partial pressure CO(2) changes sensed by the medullary ventral respiratory group (VRG) and hypothalamus, entraining changes in midbrain, pons, thalamus, limbic, paralimbic, and insular regions. Nearly all these areas are known to connect anatomically with the cerebellum. Using positron emission tomography, we measured regional brain blood flow during acute CO(2)-induced breathlessness in humans. Separable physiological and subjective effects (air hunger) were assessed by comparisons with various respiratory control conditions. The conjoint physiological effects of hypercapnia and the consequent air hunger produced strong bilateral, near-midline activations of the cerebellum in anterior quadrangular, central, and lingula lobules, and in many areas of posterior quadrangular, tonsil, biventer, declive, and inferior semilunar lobules. The primal emotion of air hunger, dissociated from hypercapnia, activated midline regions of the central lobule. The distributed activity across the cerebellum is similar to that for thirst, hunger, and their satiation. Four possible interpretations of cerebellar function(s) here are that: it subserves implicit intentions to access air; it provides predictive internal models about the consequences of CO(2) inhalation; it modulates emotional responses; and that while some cerebellar regions monitor sensory acquisition in the VRG (CO(2) concentration), others influence VRG to adjust respiratory rate to optimize partial pressure CO(2), and others still monitor and optimize the acquisition of other sensory data in service of air hunger aroused vigilance.

Brain responses associated with consciousness of breathlessness (air hunger)

Little is known about the physiological mechanisms subserving the experience of air hunger and the affective control of breathing in humans. Acute hunger for air after inhalation of CO(2) was studied in nine healthy volunteers with positron emission tomography. Subjective breathlessness was manipulated while end-tidal CO(2-) was held constant. Subjects experienced a significantly greater sense of air hunger breathing through a face mask than through a mouthpiece. The statistical contrast between the two conditions delineated a distributed network of primarily limbic/paralimbic brain regions, including multiple foci in dorsal anterior and middle cingulate gyrus, insula/claustrum, amygdala/periamygdala, lingual and middle temporal gyrus, hypothalamus, pulvinar, and midbrain. This pattern of activations was confirmed by a correlational analysis with breathlessness ratings. The commonality of regions of mesencephalon, diencephalon and limbic/paralimbic areas involved in primal emotions engendered by the basic vegetative systems including hunger for air, thirst, hunger, pain, micturition, and sleep, is discussed with particular reference to the cingulate gyrus. A theory that the phylogenetic origin of consciousness came from primal emotions engendered by immediate threat to the existence of the organism is discussed along with an alternative hypothesis by Edelman that primary awareness emerged with processes of ongoing perceptual categorization giving rise to a scene [Edelman, G. M. (1992) Bright Air, Brilliant Fire (Penguin, London)].

Corticospinal pathway and exercise hyperpnea: lessons from a patient with Arnold Chiari malformation

The study of a patient with an Arnold Chiari malformation gave us the opportunity to test the hypothesis that the motor cortex contributes significantly to respiratory control during muscular exercise through the corticospinal pathway. The patient was a 25 years old woman who exhibited a severe impairment of the 'automatic' ventilatory control due to a type I Arnold Chiari malformation. Since she never complained of being breathlessness even on exertion, the breath-by-breath ventilatory (VE) and pulmonary gas exchange responses to a three minute bout of constant work rate exercise at 60 W, 90 W and 120 W were studied before then 16 and 23 months after posterior fossa decompression. The VE response to the three different levels of exercise was dramatically blunted so that the expected vertical relationship between PET(CO(2)) and VE during moderate exercise was replaced by an almost horizontal relationship with a slope ranging from 0.15 to 0.17 l/min/Torr. The reduced VE response was associated with a total lack of respiratory sensation during and following the exercise bouts. This abnormal ventilatory response to exercise persisted despite posterior fossa decompression. There was however no evidence of an alteration of the corticospinal pathway. Indeed, not only was there no sign of motor deficit but the patient was able both to mobilize 96% of her expected vital capacity and to voluntarily increase her ventilation to the level expected in a normal subject during exercise. This observation suggests that during exercise, motor control of respiratory muscles via a direct corticospinal pathway does not play a major role in adjusting phrenic motoneuron activity to the magnitude of the motor inputs to the exercising skeletal muscles.

Respiratory control and respiratory sensation in a patient with a ganglioglioma within the dorsocaudal brain stem

We encountered a young woman with severe central sleep apnea caused by a medullary glioma located slightly dorsal to and to the right of the midline, a region not generally associated with CO(2) chemosensitivity. The patient had normal spirometric readings, lung volumes, diffusing capacity, maximal inspiratory pressure, and alveolar-arterial oxygen difference. While awake, she displayed marked irregularity in her breathing pattern; her end-tidal CO(2) (FET(CO(2))) ranged from 5.3 to 10.9%. During voluntary hyperpnea, she could quickly reduce her FET(CO(2)) to 4.2%, but her PCO(2) did not change after administration of acetazolamide or progesterone. Like patients with congenital central hypoventilation syndrome (CCHS), our patient had a relatively intact ventilatory response to exercise; her PCO(2) was high at the start of exercise and increased slightly thereafter. In contrast to CCHS patients, however, our patient had an intact hypoxic ventilatory response (DeltaVE/ DeltaSa(O(2)) = -0.37 L/min/Sa(O(2))). In further contrast to CCHS patients, our patient had a very short breathholding time and described a sensation of air hunger as the factor limiting her breathholding ability. Her heart rate and blood pressure responses to the Valsalva maneuver were normal.

Modulation of the corticospinal control of ventilation by changes in reflex respiratory drive

We have determined whether changes in PCO(2) above and below eucapnia modulate the precision of the voluntary control of breathing. Twelve trained subjects performed a compensatory tracking task in which they had to maintain the position of a cursor (perturbed by a variable triangular forcing function) on a fixed target by breathing in and out of a spirometer (ventilatory tracking; at 10 l/min). Before each task, subjects hyperventilated for 5 min, and the end-tidal PCO(2) (PET(CO(2))) was controlled; tracking was then performed separately at hypocapnia, eucapnia, and hypercapnia (PET(CO(2)) approximately 25, 37, and 43 Torr, respectively). Ventilatory tracking error was unchanged during hypocapnia (P > 0.05) but was significantly worse during hypercapnia (P < 0.003), compared with eucapnia; arm tracking error, performed as a control, was not significantly affected by PET(CO(2)) (P > 0. 05). In conclusion, ventilatory tracking performance is unaffected by the eucapnic PCO(2). From this, we suggest that resting breathing in awake humans may be independent of chemical drives and of the prevailing PCO(2).

Effect of resistive loading on variational activity of breathing

To examine the effect of resistive loading on variational activity of breathing, we studied 18 healthy subjects breathing at rest and with inspiratory resistive loads of 3 and 6 cm H2O/L/s, applied randomly for 1 h each. Compared with resting breathing, a resistive load of 3 cm H2O/L/s decreased the total variational activity of expiratory time (TE) and minute ventilation (V I), whereas a load of 6 cm H2O/L/s increased the total variational activity of inspiratory time (TI). Compared with the load of 3 cm H2O/L/s, the load of 6 cm H2O/L/s increased total variational activity of tidal volume (VT), TI, TE, and V I. Partitioning of the total variational activity revealed that these alterations were due to changes in the random uncorrelated fraction. Compared with rest, both the resistive loads of 3 and 6 cm H2O/L/s increased the number of breath lags displaying significant serial correlations ("short-term memory") of TI. Compared with rest, the load of 3 cm H2O/L/s increased the autocorrelation coefficient at a lag of one breath for VT and the load of 6 cm H2O/L/s increased the correlated fraction of variational activity of VT. Thus, three measures of correlated behavior-autocorrelation coefficient at a lag of 1 breath, "short-term memory," and the correlated fraction of total variational activity- increased with loading. In conclusion, resistive loading changed total variational activity according to the size of the load: the random fraction decreased with the smaller load but increased with the larger load; in contrast, correlated behavior increased with both loads. The different behaviors of random and correlated variability with loading may reflect different physiologic influences on respiratory control.

Brain, breathing and breathlessness

This review attempts to summarize: (i) evidence on how man voluntarily or behaviourally (as in speech) alters breathing; and (ii) evidence on how the breathlessness induced by CO2 inhalation, is perceived. The application of new methods to study these problems, e.g. functional brain imaging and transcranial focal brain stimulation, is summarized. Studies of patients with specific neurological lesions have shed considerable light in this area. The key requirement for the ponto-medullary respiratory oscillator to be both 'intact' and 'responsive' for the perception of CO2-induced air hunger is emphasized. We are ignorant as to how the voluntary/behavioural control system interacts with the automatic system at any site above the final common pathway of the respiratory anterior horn cells in the cervical and thoracic spinal cord. The opportunities for further work are outlined.

The effect of hypoxia on the ventilatory response to carbon dioxide in man

We used rebreathing with prior hyperventilation to measure ventilatory responses to CO2 at iso-oxic PO2's of 100, 80, 60 and 40 mmHg in seven subjects. The mean sub-threshold ventilation (S.E.) of 7.60 (1.31) L min-1 did not vary with iso-oxic PO2. The mean peripheral-chemoreflex threshold of 41 (0.6)) mmHg PCO2 at an iso-oxic PO2 of 100 was greater than 39 (1.2) and 39 (0.6) at 60 and 40, respectively. The mean peripheral-chemoreflex sensitivity of 11.5 (5.2) L min-1 mmHg-1 at an iso-oxic PO2 of 40 was significantly greater than 3.0 (1.3), 2.7 (1.2) and 2.4 (1.2) at 60, 80 and 100, respectively. The mean central-chemoreflex threshold of 45 (1.5) mmHg PCO2 at an iso-oxic PO2 of 40 was significantly less than 48 (0.4) and 48 (0.7) at 80 and 100, respectively. The mean central-chemoreflex sensitivity of 5.0 (1.1) L min-1 mmHg-1 did not vary with iso-oxic PO2. These findings provide insights into the control of breathing in humans, including the implication that CO2 must exceed its peripheral-chemoreflex threshold before hypoxia can effectively increase ventilation.