The role of spinal cord transmission in the ventilatory response to exercise in man

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Differential control of respiratory frequency and tidal volume during exercise

The lack of a testable model explaining how ventilation is regulated in different exercise conditions has been repeatedly acknowledged in the field of exercise physiology. Yet, this issue contrasts with the abundance of insightful findings produced over the last century and calls for the adoption of new integrative perspectives. In this review, we provide a methodological approach supporting the importance of producing a set of evidence by evaluating different studies together-especially those conducted in 'real' exercise conditions-instead of single studies separately. We show how the collective assessment of findings from three domains and three levels of observation support the development of a simple model of ventilatory control which proves to be effective in different exercise protocols, populations and experimental interventions. The main feature of the model is the differential control of respiratory frequency (f_R) and tidal volume (V_T); f_R is primarily modulated by central command (especially during high-intensity exercise) and muscle afferent feedback (especially during moderate exercise) whereas V_T by metabolic inputs. Furthermore, V_T appears to be fine-tuned based on f_R levels to match alveolar ventilation with metabolic requirements in different intensity domains, and even at a breath-by-breath level. This model reconciles the classical neuro-humoral theory with apparently contrasting findings by leveraging on the emerging control properties of the behavioural (i.e. f_R) and metabolic (i.e. V_T) components of minute ventilation. The integrative approach presented is expected to help in the design and interpretation of future studies on the control of f_R and V_T during exercise.

Control of exercise hyperpnoea: Contributions from thin-fibre skeletal muscle afferents

NEW FINDINGS

What is the topic of this review? In this review, we examine the evidence for control mechanisms underlying exercise hyperpnoea, giving attention to the feedback from thin-fibre skeletal muscle afferents, and highlight the frequently conflicting findings and difficulties encountered by researchers using a variety of experimental models. What advances does it highlight? There has been a recent resurgence of interest in the role of skeletal muscle afferent involvement, not only as a mechanism of healthy exercise hyperpnoea but also in the manifestation of breathlessness and exercise intolerance in chronic disease.

ABSTRACT

The ventilatory response to dynamic submaximal exercise is immediate and proportional to metabolic rate, which maintains isocapnia. How these respiratory responses are controlled remains poorly understood, given that the most tightly controlled variable (arterial partial pressure of CO₂ /H⁺ ) provides no error signal for arterial chemoreceptors to trigger reflex increases in ventilation. This review discusses evidence for different postulated control mechanisms, with a focus on the feedback from group III/IV skeletal muscle mechanosensitive and metabosensitive afferents. This concept is attractive, because the stimulation of muscle mechanoreceptors might account for the immediate increase in ventilation at the onset of exercise, and signals from metaboreceptors might be proportional to metabolic rate. A variety of experimental models have been used to establish the contribution of thin-fibre muscle afferents in ventilatory control during exercise, with equivocal results. The inhibition of afferent feedback via the application of lumbar intrathecal fentanyl during exercise suppresses ventilation, which provides the most compelling supportive evidence to date. However, stimulation of afferent feedback at rest has no consistent effect on respiratory output. However, evidence is emerging for synergistic interactions between muscle afferent feedback and other stimulatory inputs to the central respiratory neuronal pool. These seemingly hyperadditive effects might explain the conflicting findings encountered when using different experimental models. We also discuss the increasing evidence that patients with certain chronic diseases exhibit exaggerated muscle afferent activation during exercise, resulting in enhanced cardiorespiratory responses. This might provide a neural link between the well-established limb muscle dysfunction and the associated exercise intolerance and exertional dyspnoea, which might offer therapeutic targets for these patients.

The control of ventilation during exercise: a lesson in critical thinking

Learning the basic competencies of critical thinking are very important in the education of any young scientist, and teachers must be prepared to help students develop a valuable set of analytic tools. In my experience, this is best achieved by encouraging students to study areas with little scientific consensus, such as the control mechanisms of the exercise ventilatory response, as it can allow greater objectivity when evaluating evidence, while also giving students the freedom to think independently and problem solve. In this article, I discuss teaching strategies by which physiology, biomedical science, and sport science students can simultaneously develop their understanding of respiratory control mechanisms and learn to critically analyze evidence thoroughly. This can be best achieved by utilizing both teacher-led and student-led learning environments, the latter of which encourages the development of learner autonomy and independent problem solving. In this article, I also aim to demonstrate a systematic approach of critical assessment that students can be taught, adapt, and apply independently. Among other things, this strategy involves: 1) defining the precise phenomenon in question; 2) understanding what investigations must demonstrate to explain the phenomenon and its underlying mechanisms; 3) evaluating the explanations/mechanisms of the phenomenon and the evidence for them; and 4) forming strategies to produce strong evidence, if none exists.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

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.

Are type III-IV muscle afferents required for a normal steady-state exercise hyperpnoea in humans?

When tested in isolation, stimuli associated with respiratory CO₂ exchange, feedforward central command and type III–IV muscle afferent feedback have each been shown to be capable of eliciting exercise-like cardio-ventilatory responses, but their relative contributions in a setting of physiological exercise remains controversial. We reasoned that in order to determine whether any of these regulators are obligatory to the exercise hyperpnoea each needs to be removed or significantly diminished in a setting of physiological steady-state exercise, during which all recognized stimuli (and other potential modulators) are normally operative. In the past few years we and others have used intrathecal fentanyl, a μ-opiate receptor agonist, in humans to reduce the input from type III–IV opiate-sensitive muscle afferents. During various types of intensities and durations of exercise a sustained hypoventilation, as well as reduced systemic pressure and cardioacceleration, were consistently observed with this blockade. These data provide the basis for the hypothesis that type III–IV muscle afferents are obligatory to the hyperpnoea of mild to moderate intensity rhythmic, large muscle, steady-state exercise. We discuss the limitations of these studies, the reasons for their disagreement with previous negative findings, the nature of the muscle afferent feedback stimulus and the need for future investigations.

Defining the neurocircuitry of exercise hyperpnoea

One hundred years ago in this journal, Krogh and Lindhard published a seminal paper highlighting the importance of the brain in the control of breathing during exercise. This symposium report reviews the historical developments that have taken place since 1913, and attempts to place the detailed neurocircuitry thought to underpin exercise hyperpnoea into context by focusing on key structures that might form the command network. With the advent of enhanced neuroimaging and functional neurosurgical techniques, a unique window of opportunity has recently arisen to target potential circuits in humans. Animal studies have identified a priori sites of interest in mid-brain structures, in particular the subthalamic locomotor region (subthalamic nucleus, STN) and the periaqueductal grey (PAG), which have now been recorded from in humans during exercise. When all data are viewed in an integrative manner, the PAG, in particular the lateral PAG, and aspects of the dorsal lateral PAG, appear to be key communicating circuitry for 'central command'. Moreover, the PAG also fulfils many requirements of a command centre. It has functional connectivity to higher centres (dorsal lateral prefrontal cortex) and the basal ganglia (in particular, the STN), and receives a sensory input from contracting muscle, but, importantly, it sends efferent information to brainstem nuclei involved in cardiorespiratory control.

New perspectives concerning feedback influences on cardiorespiratory control during rhythmic exercise and on exercise performance

The cardioaccelerator and ventilatory responses to rhythmic exercise in the human are commonly viewed as being mediated predominantly via feedforward 'central command' mechanisms, with contributions from locomotor muscle afferents to the sympathetically mediated pressor response. We have assessed the relative contributions of three types of feedback afferents on the cardiorespiratory response to voluntary, rhythmic exercise by inhibiting their normal 'tonic' activity in healthy animals and humans and in chronic heart failure. Transient inhibition of the carotid chemoreceptors during moderate intensity exercise reduced muscle sympathetic nerve activity (MSNA) and increased limb vascular conductance and blood flow; and reducing the normal level of respiratory muscle work during heavier intensity exercise increased limb vascular conductance and blood flow. These cardiorespiratory effects were prevented via ganglionic blockade and were enhanced in chronic heart failure and in hypoxia. Blockade of μ opioid sensitive locomotor muscle afferents, with preservation of central motor output via intrathecal fentanyl: (a) reduced the mean arterial blood pressure (MAP), heart rate and ventilatory responses to all steady state exercise intensities; and (b) during sustained high intensity exercise, reduced O(2) transport, increased central motor output and end-exercise muscle fatigue and reduced endurance performance. We propose that these three afferent reflexes - probably acting in concert with feedforward central command - contribute significantly to preserving O(2) transport to locomotor and to respiratory muscles during exercise. Locomotor muscle afferents also appear to provide feedback concerning the metabolic state of the muscle to influence central motor output, thereby limiting peripheral fatigue development.

Group III and IV muscle afferents contribute to ventilatory and cardiovascular response to rhythmic exercise in humans

We investigated the role of somatosensory feedback on cardioventilatory responses to rhythmic exercise in five men. In a double-blind, placebo-controlled design, subjects performed the same leg cycling exercise (50/100/150/325 ± 19 W, 3 min each) under placebo conditions (interspinous saline, L(3)-L(4)) and with lumbar intrathecal fentanyl impairing central projection of spinal opioid receptor-sensitive muscle afferents. Quadriceps strength was similar before and after fentanyl administration. To evaluate whether a cephalad migration of fentanyl affected cardioventilatory control centers in the brain stem, we compared resting ventilatory responses to hypercapnia (HCVR) and cardioventilatory responses to arm vs. leg cycling exercise after each injection. Similar HCVR and minor effects of fentanyl on cardioventilatory responses to arm exercise excluded direct medullary effects of fentanyl. Central command during leg exercise was estimated via quadriceps electromyogram. No differences between conditions were found in resting heart rate (HR), ventilation [minute ventilation (VE)], or mean arterial pressure (MAP). Quadriceps electromyogram, O(2) consumption (VO(2)), and plasma lactate were similar in both conditions at the four steady-state workloads. Compared with placebo, a substantial hypoventilation during fentanyl exercise was indicated by the 8-17% reduction in VE/CO(2) production (VCO(2)) secondary to a reduced breathing frequency, leading to average increases of 4-7 Torr in end-tidal PCO(2) (P < 0.001) and a reduced hemoglobin saturation (-3 ± 1%; P < 0.05) at the heaviest workload (∼90% maximal VO(2)) with fentanyl. HR was reduced 2-8%, MAP 8-13%, and ratings of perceived exertion by 13% during fentanyl vs. placebo exercise (P < 0.05). These findings demonstrate the essential contribution of muscle afferent feedback to the ventilatory, cardiovascular, and perceptual responses to rhythmic exercise in humans, even in the presence of unaltered contributions from other major inputs to cardioventilatory control.

Exercise Capacity in Chronic Obstructive Pulmonary Disease: Mechanisms of Limitation

Patients with chronic obstructive pulmonary disease (COPD) are often caught in a downward spiral that progresses from expiratory flow limitation to poor quality of life and invalidity. Within this downward spiral, exercise tolerance represents a key intermediate outcome. As recently stated by the GOLD initiative, improvement in exercise tolerance is now rec ognized as an important goal of COPD treatment. This objective will be achieved only by a comprehensive understanding of the mechanism of exercise limitation in this disease. The objective of this paper is to review the mechanisms of exercise limitation in COPD and discuss their relative contribution to exercise intolerance in patients suffering from this disease.

Muscle sympathetic nerve activity and ventilation during exercise in subjects with and without chronic heart failure

BACKGROUND

Changes within skeletal muscle, including augmentation of its capacity to elicit reflex increases in both efferent muscle sympathetic nerve activity (MSNA) and ventilation during work, contribute significantly to exercise intolerance in heart failure (HF). Previously, we demonstrated that peak oxygen uptake (pVO(2)) in HF relates inversely to MSNA at rest and during exercise.

OBJECTIVE

To test the hypothesis that there is an independent positive relationship between resting MSNA and the ratio of ventilation to carbon dioxide output during exercise (VE/VCO(2)) that is augmented in HF.

METHODS

MSNA at rest and VE/VCO(2)) during stationary cycling were measured in 30 patients (27 men) with HF (mean +/- SD ejection fraction 20+/-6%) and in 31 age-matched controls (29 men).

RESULTS

MSNA was higher in HF patients than in controls (51.5+/-14.3 bursts/min versus 33.0+/-11.1 bursts/min; P<0.0001). The VE/VCO(2) slope was also higher in HF patients than in controls (33.7+/-5.7 versus 26.0+/-3.5; P<0.0001), whereas pVO(2) was lower in HF patients than in controls (18.6+/-6.6 versus 31.4+/-8.4 mL/kg/min; P<0.0001). There were significant relationships between MSNA and VE/VCO(2) in both HF (r=0.50; P=0.005) and control subjects (r=0.36; P=0.046). The slope of this regression equation was steeper in HF (0.20 versus 0.11 x MSNA; P=0.001). An analysis of covariance for main effects, including age and pVO(2), identified a significant independent relationship between MSNA burst frequency and VE/VCO(2) (P=0.013) that differed between HF and controls (P<0.01).

CONCLUSIONS

The magnitude of resting sympathetic activity correlates positively with the VE/VCO(2) slope. Augmentation of this relationship in HF patients is consistent with the concept that enhanced mechanoreceptor reflex activity exaggerates their ventilatory response to exercise.

Métabosensibilité musculaire et adaptations physiologiques au cours de l’exercice

The first role played by group III (thin myelinated) and group IV (unmyelinated) afferent fibres from skeletal muscle is to transmit nociceptive information from muscle to the central nervous system. The second role of these free endings located in the interstitium of the muscle is to induce cardiovascular and respiratory adjustments during muscular exercise. These respiratory and circulatory responses during muscular exercise may be reflexly induced via muscular afferents. Indeed, static contraction of hindlimb muscles in anaesthetised mammals has been shown to reflexly increase the ventilatory function, the myocardial contractility and heart rate. The mechanical muscle deformation and the accumulation of metabolites in its intersitium are the cause of raised activity in small nerve fibres which in turn induces the physiological responses. It is also admitted that the central locomotor areas on the medullary and spinal neuronal pools control ventilatory and cardiovascular function during exercise. This mechanism is called "central command". Furthermore, adjustments of the locomotor activity during exercise is mediated by the thinly myelinated and unmyelinated fibres with endings in the working muscle. These fibres, also called "metaboreceptor" may be responsible of the coupling between the ventilation and the locomotion. Thickly myelinated muscle afferents (i.e. group I and II) appear to play little role in causing the reflex autonomic responses to contraction.

VE response to VCO2 during exercise is unaffected by exercise training and different exercise limbs

We designed two experiments to investigate the relationship between ventilation (VE) and CO2 output (VCO2) during exercise under the conditions of exercising different limbs, the arms as opposed to the legs (experiment 1), and of different physical training states after undergoing standard exercise training for 90 d (experiment 2). Six healthy young subjects underwent submaximal ramp exercise at an incremental work rate of 15 W/min for the arm and leg, and 11 healthy middle-aged subjects underwent an incremental exercise test at the rate of 30 W/3 min before and after exercise training. We measured pulmonary breath-by-breath VE, VCO2, oxygen uptake (VO2), tidal volume (VT), breathing frequency (bf), and end-tidal O2 and CO2 pressures (PETO2, PETCO2) via a computerized metabolic cart. In experiment 1, arm exercise produced significantly greater VE than did leg exercise at the same work rates, as well as significantly higher VO2, VCO2, and bf. The slopes of the regression lines in the VE-VCO2 relationship were not significantly different: the values were 27.8 +/- 2.1 (SD) during the arm exercise, and 25.3 +/- 3.9 during the leg exercise, with no differences in their intercepts. In experiment 2, the VO2, VCO2, and VE responses at the same work rates were similar in both before and after the 90-d exercise training, whereas the heart rate (HR) and mean blood pressure (MBP) were significantly reduced after training. Exercise training did not alter the VE-VCO2 relationship, the slope of which was 31.9 +/- 4.9 before exercise training and 34.2 +/- 4.4 after exercise training. We concluded that the VE-VCO2 relationship during exercise is unaltered, independent of not only working muscle regions but also exercise training states.

Repeated exercise paired with "imperceptible" dead space loading does not alter VE of subsequent exercise in humans

We employed an associative learning paradigm to test the hypothesis that exercise hyperpnea in humans arises from learned responses forged by prior experience. Twelve subjects undertook a "conditioning" and a "nonconditioning" session on separate days, with order of performance counterbalanced among subjects. In both sessions, subjects performed repeated bouts of 6 min of treadmill exercise, each separated by 5 min of rest. The only difference between sessions was that all the second-to-penultimate runs of the conditioning session were performed with added dead space in the breathing circuit. Cardiorespiratory responses during the first and last runs (the "control" and "test" runs) were compared for each session. Steady-state exercise end-tidal PCO(2) was significantly lower (P = 0.003) during test than during control runs for both sessions (dropping by 1.8 +/- 2 and 1.4 +/- 3 Torr during conditioning and nonconditioning sessions, respectively). This and all other test-control run differences tended to be greater during the first session performed regardless of session type. Our data provide no support for the hypothesis implicating associative learning processes in the ventilatory response to exercise in humans.

Cardiovascular responses during submaximal electrical stimulation-induced leg cycling in individuals with paraplegia

This study investigated the cardiovascular responses during electrical stimulation-induced leg cycling (ES-LCE) in people with paraplegia (PARA) compared with voluntary leg cycling (VOL) at similar levels of oxygen uptake in able-bodied (AB) individuals. Six PARA with sensory and motor complete spinal cord lesions (TS-T9) and six AB participated in this study. Oxygen uptake (VO2), stroke volume (SV), heart rate (HR) and cardiac output (Q) were measured at rest and during submaximal, steady-state leg cycling. At the highest power output achieved (9.2 +/- 2.4 W for PARA versus 42.8 +/- 1.0 W for AB), VO2 was augmented above resting levels to 0.75 +/- 0.11 min(-1) in PARA and to 0.74 +/- 0.071 min(-1) in AB. HR and SV were also increased during ES-LCE in PARA (92.1 +/- 8-6 beats min(-1) and 93.9 +/- 11.3 ml bea(-1), respectively) and during VOL in AB (83.9 +/- 9.2 beats min(-1) and 89.7 +/- 9.0 ml beat(-1), respectively). At an equivalent submaximal VO2, HR and SV were not different between the two groups, however, Q was higher in PARA (6.6 +/- 0.7 versus 4.1 +/- 0.9 1 min l(-1) deltaVO2). These data suggest that ES-LCE at relatively low power outputs elicits increases in several cardiovascular variables in PARA. Furthermore, it is possible that ES-LCE leads to a 'hyperkinetic circulation' (a greater Q for a given VO2).

Excessive ventilation after acute myocardial infarction and its improvement in 4 months

The relationship between ventilation (VE) and CO2 output (VCO2) is fitted linearly. The steeper gradient implies excessive ventilation. Through an evaluation of the VE-VCO2 slopes, this study investigated whether patients with acute myocardial infarction (AMI) have excessive ventilation and whether it improved in 4 months. The VE-VCO2 slopes were determined in exercise tests at 1 and 4 months in 131 patients with AMI. Patients were divided into 3 groups according to the 1 month VE-VCO2 slope value: (i) normal (<30); (ii) intermediate (30-32); and (iii) excessive (>32). In the normal group (n=76), at 4 months, the slope decreased in 10, increased in 5 and remained unchanged in 61 patients; in the intermediate (n=31) group, 9, 2 and 20; and in the excessive (n=24) group, 15, 3 and 6, respectively, showing that the slope reduction was greater in the excessive group (p<0.01). The slope correlated with age and acute phase heart failure. The percent reduction of the slope did not correlate with these parameters. In conclusion, a substantial fraction of patients with AMI have excessive ventilation that improves in 4 months. The improvement is greater in patients with greater excessive ventilation but is not associated with an improvement in exercise capacity nor hemodynamics.

Control of the exercise hyperpnoea in humans: a modeling perspective

Models of the exercise hyperpnoea have classically incorporated elements of proportional feedback (carotid and medullary chemosensory) and feedforward (central and/or peripheral neurogenic) control. However, the precise details of the control process remain unresolved, reflecting in part both technical and interpretational limitations inherent in isolating putative control mechanisms in the intact human, and also the challenges to linear control theory presented by multiple-input integration, especially with regard to the ventilatory and gas-exchange complexities encountered at work rates which engender a metabolic acidosis. While some combination of neurogenic, chemoreflex and circulatory-coupled processes are likely to contribute to the control, the system appears to evidence considerable redundancy. This, coupled with the lack of appreciable error signals in the mean levels of arterial blood gas tensions and pH over a wide range of work rates, has motivated the formulation of innovative control models that reflect not only spatial interactions but also temporal interactions (i.e. memory). The challenge is to discriminate between robust competing control models that: (a) integrate such processes within plausible physiological equivalents; and (b) account for both the dynamic and steady-state system response over a range of exercise intensities. Such models are not yet available.

Carotid baroreflex control of heart rate and blood pressure during ES leg cycling in paraplegics

This study investigated control of heart rate (HR) and mean arterial pressure (MAP) at rest and during electrical stimulation (ES) leg cycling exercise (LCE) in paraplegics (Para). Seven men with complete spinal lesions (T(5)-T(11)) and six able-bodied (AB) men participated in this study. Beat-to-beat changes in HR and MAP were recorded during carotid sinus perturbation. Carotid baroreflex function curves were derived at rest and during ES-LCE for Para and during voluntary cycling (Vol) for AB. From rest to ES-LCE, oxygen uptake (VO(2)) increased (by 0.43 l/min) and HR rose (by 11 beats/min), yet MAP remained unchanged. In AB, Vol increased VO(2) (by 0.53 l/min), HR (by 22 beats/min), and MAP (by 8 mmHg). ES-LCE did not alter the carotid sinus pressure (CSP)-MAP relationship, but it displaced the CSP-HR relationship upward relative to rest. No rightward shift was observed during ES-LCE. Vol by AB produced an upward and rightward displacement of the CSP-MAP and CSP-HR relationships relative to rest. These findings suggested that the carotid sinus baroreflex was not reset during ES-LCE in Para.

Heart rate during exercise with leg vascular occlusion in spinal cord-injured humans

Feed-forward and feedback mechanisms are both important for control of the heart rate response to muscular exercise, but their origin and relative importance remain inadequately understood. To evaluate whether humoral mechanisms are of importance, the heart rate response to electrically induced cycling was studied in participants with spinal cord injury (SCI) and compared with that elicited during volitional cycling in able-bodied persons (C). During voluntary exercise at an oxygen uptake of approximately 1 l/min, heart rate increased from 66 +/- 4 to 86 +/- 4 (SE) beats/min in seven C, and during electrically induced exercise at a similar oxygen uptake in SCI it increased from 73 +/- 3 to 110 +/- 8 beats/min. In contrast, blood pressure increased only in C (from 88 +/- 3 to 99 +/- 4 mmHg), confirming that, during exercise, blood pressure control is dominated by peripheral neural feedback mechanisms. With vascular occlusion of the legs, the exercise-induced increase in heart rate was reduced or even eliminated in the electrically stimulated SCI. For C, heart rate tended to be lower than during exercise with free circulation to the legs. Release of the cuff elevated heart rate only in SCI. These data suggest that humoral feedback is of importance for the heart rate response to exercise and especially so when influence from the central nervous system and peripheral neural feedback from the working muscles are impaired or eliminated during electrically induced exercise in individuals with SCI.

Role of exercise ventilation in the limitation of functional capacity in patients with congestive heart failure

Patients with heart failure have, compared with normal subjects, an increased minute ventilation (VE) at matched workloads. This heightened ventilatory drive may contribute to their limitation of functional capacity through an increase in the work of breathing and further worsening in the lung ventilation-perfusion mismatch. To measure the ventilatory response to exercise, VE should not be assessed in absolute units but be related to one of its main determinants, e.g., carbon dioxide production (VCO2). Particularly, as VE is closely related to VCO2 during exercise, the ventilatory response to exercise has been assessed using the slope of the relation of VE versus VCO2. This slope is significantly increased in heart failure patients compared with normal subjects and is inversely related to other parameters of maximal exercise capacity, namely peak VO2. The mechanisms of exercise hyperpnea in heart failure patients are still unsettled. A first possibility is that it is a compensatory response to the abnormal exercise hemodynamics with secondary increase of the pulmonary dead space to tidal volume ratio. This mechanism should be aimed to maintain constancy of the arterial gas composition and acid-base balance. However, exercise-induced hypoxemia and/or hypercapnia do not generally develop in heart failure patients. This might imply that other mechanisms, such as an increased sensitivity of the arterial chemoreceptors and/or the activation of reflexes by the abnormal skeletal muscles, stimulate the ventilatory response in heart failure patients. Regardless of its mechanisms, exercise hyperpnea may be clinically relevant in the assessment of patients with chronic heart failure. In fact, it is inversely related with peak exercise capacity, and interventions known to improve peak functional capacity such as therapy with ACE inhibitors, physical training and heart transplantation, also tend to normalize exercise hyperpnea.

Hyperpnoea during and immediately after exercise in man: evidence of motor cortical involvement

1. The neurophysiological basis for the increase in breathing associated with exercise remains obscure. The present study uses positron emission tomography (PET) to measure relative regional cerebral blood flow (rCBF) in order to identify sites of increased neuronal activation during and immediately following exercise. 2. Male volunteers underwent H2(15)O PET scanning during two complementary studies. Firstly, six subjects performed right leg exercise, adequate to increase oxygen uptake 2.5-fold. Secondly, five different subjects were scanned immediately following bicycle exercise (adequate to increase oxygen uptake 5-fold) while breathing was still increased. In each study, as a control, scanning was also performed during matched passive isocapnic positive pressure ventilation; additionally, in the first study, passive right leg movement was performed. 3. Increases in relative rCBF were obtained in each individual and co-registered with their magnetic resonance image of the brain defining individual gyral morphology. 4. During exercise, individual and group analysis revealed significant relative rCBF increases in the left and right superomedial primary motor cortex (the motor cortical 'leg' areas) and also in the left and right superolateral primary motor cortex in areas previously shown to be associated with volitional breathing. After exercise, there was no significant increase in relative rCBF in the superomedial areas but such increases were still present bilaterally in the superolateral areas which had been activated during the exercise. Other relative rCBF increases were also found, both during and after exercise, in cortical and subcortical areas known to be involved in motor control. 5. The results from PET scans during and after exercise, taken together, provide evidence for motor cortical involvement in the exercise-related hyperpnoea in man.

Skeletal muscle and the control of ventilation on exercise: evidence for metabolic receptors

Patients with chronic heart failure have an increased ventilatory response to exercise, and have metabolically abnormal skeletal muscle. It has been proposed that a neural signal to ventilation arising from exercising muscle may be heightened in chronic heart failure. Our objective was to detect evidence for such a signal in normal subjects by studying ventilatory behaviour during exercise with muscles in different metabolic states. Fifteen normal subjects undertook treadmill exercise both with and without cuffs inflated around each thigh to suprasystolic pressure. In a second experiment, a group of 11 normal subjects undertook cycle exercise using arms or legs at the same absolute work load. Metabolic gas exchange was measured using mass spectrometry with indicator gas dilution. The ventilatory response was greater at a given workload when subjects exercised with inflated cuffs. Oxygen consumption was reduced in keeping with the isolation of the exercising muscle bulk from the circulation. The ventilation/carbon dioxide output relationship was described by a linear regression function, but the slope of the relationship was increased by 25% from 20.9 (0.46) to 25.43 (0.73) (P < 0.001). Arm exercise at the same load as leg exercise resulted in unchanged oxygen consumption indicating that the same external work was being performed. There was an increase in ventilation at a given workload. The ventilation/carbon dioxide output slope was increased by 25% (from 21.9 (0.9) to 26.3 (0.8)) (P < 0.001). There is a signal to ventilation arising from exercising skeletal muscle which is enhanced by the ischaemia induced by cuff inflation during exercise. This signal appears to be neural.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and ventilatory responses to electrically induced cycling with complete epidural anaesthesia in humans

Cardiovascular and ventilatory responses to electrically induced dynamic exercise were investigated in eight healthy young males with afferent neural influence from the legs blocked by epidural anaesthesia (25 ml 2% lidocaine) at L3-L4. This caused cutaneous sensory anaesthesia below T8-T9 and complete paralysis of the legs. Cycling was performed for 22.7 +/- 2.7 min (mean, SE) (fatigue) and oxygen uptake (VO2) increased to 1.90 +/- 0.13 1 min-1. Compared with voluntary exercise at the same VO2, increases in heart rate (HR) (135 +/- 7 vs. 130 +/- 9 beats min-1) and cardiac output (16.9 +/- 1.1 vs. 17.3 +/- 0.91 min-1) were similar, and ventilation (54 +/- 5 vs. 45 +/- 41 min-1) was higher (P < 0.05). In contrast, the rise in mean arterial blood pressure during voluntary exercise (93 +/- 4 (rest) to 119 +/- 4 mmHg (exercise)) was not manifest during electrically induced exercise with epidural anaesthesia [93 +/- 3 (rest) to 95 +/- 5 mmHg (exercise)]. As there is ample evidence for similar cardiovascular and ventilatory responses to electrically induced and voluntary exercise (Strange et al. 1993), the present results support the fact that the neural input from working muscle is crucial for the normal blood pressure response to exercise. Other haemodynamic and/or humoral mechanisms must operate in a decisive manner in the control of HR, CO and VE during dynamic exercise with large muscle groups.

Increased hypoxic ventilatory sensitivity during exercise in man: are neural afferents necessary?

1. The acute ventilatory response to 3 min periods of hypoxia (AHR) was examined in nine patients with clinically complete spinal cord transection (T4-T7) during (a) rest and (b) electrically induced leg exercise (EEL). 2. EEL was produced by surface electrode stimulation of the quadriceps muscles so as to cause the legs to extend at the knee against gravity. End-tidal PCO2 was held constant 1-2 mmHg above resting values throughout both protocols. 3. On exercise, the average increase in metabolic CO2 production (VCO2 +/- S.E.M.) was 41 +/- 5 ml min-1. Venous lactate levels did not rise with exercise. 4. Baseline euoxic ventilation did not increase significantly with EEL, but there was a consistent and highly significant increase in the ventilatory response to hypoxia during EEL (mean delta AHR +/- S.E.M. of 1.6 +/- 0.21 min-1). 5. We conclude that an increase in hypoxic sensitivity during exercise can occur in the absence of volitional control of exercise and in the absence of afferent neural input from the limbs.

Acute ventilatory responses to hypoxia during voluntary and electrically induced leg exercise in man

1. The acute ventilatory response to a brief period of hypoxia (AHVR) was measured in six subjects (a) at rest, (b) during electrically induced leg exercise (EEL), (c) during voluntary leg exercise at an external work rate matched to electrical exercise (EV1) and (d) during voluntary leg exercise at an internal work rate (i.e. metabolic rate) matched to electrical exercise (EV2). The end-tidal PO2 during hypoxia was 50 mmHg and the end-tidal PCO2 was held constant at 1-2 mmHg above resting values throughout each of these four protocols. 2. EEL was produced by surface electrode stimulation of the quadriceps muscles so as to cause the legs to extend at the knee and lift a set of weights via a pulley system. During EV1, each subject lifted the same weight through the same height and at the same frequency as during his EEL protocol. During EV2, the weight, the height through which it was lifted and the frequency of voluntary contractions were altered to produce a similar O2 consumption and CO2 production as during EEL. 3. In each subject, end-tidal PCO2 values showed no change between the four protocols, and in three subjects in whom they were measured, arterial PCO2 values were also similar between the protocols. Venous lactate levels did not increase after EEL or EV2. 4. The AHVR during EEL (14.1 +/- 1.42 l min-1; mean +/- S.E.M) was significantly increased (Student's paired t test) compared with rest (7.55 +/- 1.10 l min-1; P < 0.003).(ABSTRACT TRUNCATED AT 250 WORDS)

Neural control of cardiovascular responses and of ventilation during dynamic exercise in man

1. Nine subjects performed dynamic knee extension by voluntary muscle contractions and by evoked contractions with and without epidural anaesthesia. Four exercise bouts of 10 min each were performed: three of one-legged knee extension (10, 20 and 30 W) and one of two-legged knee extension at 2 x 20 W. Epidural anaesthesia was induced with 0.5% bupivacaine or 2% lidocaine. Presence of neural blockade was verified by cutaneous sensory anaesthesia below T8-T10 and complete paralysis of both legs. 2. Compared to voluntary exercise, control electrically induced exercise resulted in normal or enhanced cardiovascular, metabolic and ventilatory responses. However, during epidural anaesthesia the increase in blood pressure with exercise was abolished. Furthermore, the increases in heart rate, cardiac output and leg blood flow were reduced. In contrast, plasma catecholamines, leg glucose uptake and leg lactate release, arterial carbon dioxide tension and pulmonary ventilation were not affected. Arterial and venous plasma potassium concentrations became elevated but leg potassium release was not increased. 3. The results conform to the idea that a reflex originating in contracting muscle is essential for the normal blood pressure response to dynamic exercise, and that other neural, humoral and haemodynamic mechanisms cannot govern this response. However, control mechanisms other than central command and the exercise pressor reflex can influence heart rate, cardiac output, muscle blood flow and ventilation during dynamic exercise in man.

Ventilatory responses during arm and leg exercise at varying speeds and forces in untrained female humans

1. Involvement of neural stimuli, central and/or peripheral in origin, in exercise ventilatory control was ascertained by examining the ventilatory responses to varying mechanical conditions of arm and leg cycle ergometries. Twelve untrained women underwent each of two modes of exercise at three levels of loading (0, 5 and 10 N), each at three levels of speed (30, 50 and 72 r.p.m.), during which steady-state values of minute ventilation (VE), tidal volume (VT), respiratory frequency (f) and CO2 excretion (VCO2) were measured. 2. Using the data obtained at the aerobic work intensities, the relationship of ventilatory responses (VE, VT and f) to the metabolic (VCO2) and mechanical (speed and load) variables were studied by multiple linear regression analysis. Coefficient of determination (r2) of the regression model was lowest (0.84) for f in the arm exercise and highest (0.99) for VE in the leg exercise. 3. Standardized partial regression coefficients of the model indicated that VE response is related to VCO2 at the rate of 94 +/- 3% (mean +/- S.E.M.) and to the pedal rate at 8 +/- 3% during the leg exercise, while it is closely related to VCO2 in the arm exercise. For f response, influence of the rate of limb movement was seen in the leg exercise but not in the arm exercise. The different effects of the rate of limb movement between the two exercise modes may be related to familiarity with the exercise modes, suggesting that a familiarity-related mechanism is involved in exercise ventilatory control. 4. A heavier load imposed on the limb muscles elicited a greater VT both in the arm and leg exercise and a lower f in the arm exercise. Postural control in the upper torso during increased limb muscle tension seems to affect VT and f.

Ventilatory and P0.1 response to hypercapnia in quadriplegia

Unlike individuals with comparable degrees of respiratory muscle weakness from other causes, quadriplegic patients have a blunted ventilatory and P0.1 response to hypercapnia. This suggests that the diminished response in quadriplegia is due, in part, to an alteration in respiratory drive. We measured the hypercapnic response in 9 subjects with chronic quadriplegia (Q) and 8 normal controls (N). Ventilatory muscle strength, maximum voluntary ventilation (MVV), and lung volumes were measured in all subjects. The ventilatory response (HCVR) in Q was significantly less than in N (0.73 +/- 0.37 vs 2.95 +/- 0.4 L.min-1.mmHg-1; P less than 0.001), even when normalized for indices of respiratory muscle performance (e.g., vital capacity, MVV). There was no significant change in the HCVR in Q after the administration of naloxone. We also serially studied 2 subjects with acute quadriplegia, and found that despite progressive improvement in respiratory muscle performance, there was no accompanying increase in the response to hypercapnia. These data suggest that muscle weakness alone cannot explain the blunted hypercapnic response in quadriplegia, and are consistent with the hypothesis that these subjects have a reduced ventilatory drive.

Carotid chemoreceptor response to increases in CO2 output

The effect on carotid chemoreceptor activity of alterations in PaCO2 oscillations produced by venous CO2 loading via the small intestines was studied in anaesthetised cats, which were paralysed and ventilated at constant frequency. Changes in PaCO2 oscillations were assessed from continuously recorded oscillations of arterial pH. Chemoreceptor activity was averaged over 20-50 pH cycles to determine the amplitude (csnd ampl) and mean level (csnd mean) of the respiratory fluctuation in discharge frequency during control and CO2 loaded periods. Two groups of 5 cats were studied. In both groups, the amplitude of the PaCO2 oscillations increased by an overall average of 74.5% (standard error, SE = 9.2) with minimal change in mean PaCO2 (average increase 0.7 mm Hg), in response to an increase in VCO2 of 77.5% (SE = 7.4). When isoxia was maintained (Group 1) csnd ampl increased by 51.3% (SE = 11.1) and csnd mean by 17.3% (SE = 7.8). In Group 2, PaO2 rose by 16.7 mm Hg (SE = 1.5) during CO2 loading and both csnd ampl and csnd mean decreased by 7.4% (SE = 9.3) and 2.3% (SE = 13.3) respectively. The results demonstrate a sensitivity of chemoreceptors to changes in VCO2 which is influenced by small changes in PaO2.

Ventilatory and circulatory responses at the onset of exercise in man following heart or heart-lung transplantation

1. Ventilatory and cardiovascular responses to the onset of voluntary and electrically induced leg exercise were studied in six patients following heart transplantation and five following heart-lung transplantation; the results were compared between the patient groups and also with responses from a group of normal subjects. 2. Oxygen consumption, carbon dioxide production and ventilation and its components were measured over two 30 s periods prior to, and two 30 s periods following, the onset of exercise. Relative changes in stroke volume and cardiac output were derived from ensemble-averaged Doppler measurements of ascending aortic blood velocity over the same 30 s periods. 3. None of the groups of subjects showed any significant differences in responses to voluntary exercise compared to electrically induced exercise of similar work pattern and intensity. 4. Compared to normal controls, the transplanted subjects showed higher resting heart rates which did not increase at the onset of exercise; stroke volume increased, but less than in the normal subjects. The resulting cardiac output increases in the transplanted subjects were minimal compared to the normal subjects. 5. Ventilation and oxygen uptake increased immediately and with similar magnitude in all three groups. 6. These results show that in the same individual it is possible to have an appropriate ventilatory response to the onset of exercise in the presumed absence of a normal corticospinal input to the exercising muscles (electrically induced exercise) and afferent neural information from the lungs and heart, and in the absence of a normal circulatory response to exercise. The mechanisms underlying this ventilatory response remain undetermined.

Control of blood-gas and acid-base status during isometric exercise in humans

1. At a given level of pulmonary gas exchange, ventilation (VE) is appreciably higher during isometric exercise than during isotonic exercise. It is presently not clear whether the resultant hypocapnia represents a compensatory hyperventilation for an arterial metabolic acidaemia or whether it might reflect a primary respiratory alkalaemia. 2. To resolve this issue, five subjects performed isometric leg exercise designed to induce exhaustion in ca. 5 min and, on a separate occasion, ca. 8 min. VE, CO2 output (VCO2), O2 uptake (VO2) and end-tidal gas tensions (PET,CO2, PET,O2) were measured breath-by-breath during exercise and recovery; arterialized venous blood (drawn from the dorsum of the heated hand) was sampled frequently and analysed for PCO2, PO2, pH, bicarbonate and lactate. These response profiles were compared with those resulting from exhausting bouts of isotonic leg exercise (cycle ergometry) of similar duration. 3. The isotonic exercise induced a metabolic (lactic) acidaemia with partial respiratory compensation. In contrast, isometric exercise consistently resulted in a respiratory alkalaemia, with little or no increase of blood [lactate]. At the end of the isometric exercise, VE fell abruptly and then rose again after a short interval (20 s, on average). This secondary stimulation presumably reflected the acid-base consequences of the increased blood [lactate] (3-5 mM, on average) which occurred in the recovery phase. 4. We therefore conclude that a primary respiratory alkalaemia occurs during isometric exercise, and that this results from ventilatory stimulation at a time when the 'exercise' metabolites are trapped within the contracting muscles as a consequence of impeded blood flow. The initial rapid reduction of ventilation which occurred at the cessation of the isometric exercise is consistent with a washing-out of 'hyperpnoea-inducing' metabolites from the muscles. Allowing for transit to the central circulation, the reduced ventilation is subsequently supplemented by a powerful humoral drive to breathe which results in a further hyperpnoea and secondary hypocapnia. Because of its latency, we hypothesize that this secondary hypocapnia is of peripheral chemoreceptor origin. 5. The ventilatory response profile for isometric exercise, and the subsequent recovery phase, supports the contention that both the exercising muscles and the peripheral chemoreceptors can be important sites for inducing hyperpnoea in humans.

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)

Studies on arterial chemoreceptors in man

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Influence of inspired oxygen concentration on the dynamics of the exercise hyperpnoea in man

In order to determine the role of the carotid bodies on the ventilatory control characteristics during the non-steady-state phase of exercise in man, six normal males performed cycle ergometry with four repetitions of a 6 min, constant-load work bout at inspired O2 fractions (FI,O2) of 0.12, 0.15, 0.21, 0.30 and 1.00. Each test began with unloaded pedalling; this was followed by a constant load which was 90% of the subject's anaerobic threshold at FI,O2 = 0.12. Ventilation (VE), CO2 output (VCO2) and O2 uptake (VO2) were determined breath-by-breath during the test and the time constants of response (tau VE, tau VCO2 and tau VO2) were established by least-squares techniques, following interpolation (1 s), temporal alignment and averaging of the four responses. In each subject, tau VE and tau VCO2 increased as functions of increasing FI,O2, and were inverse functions of the proportional contribution to VE of peripheral chemoreceptor drive (as estimated from hyperoxic-transition or 'Dejours' tests). tau VE averaged 40 s at FI,O2 = 0.12 and 112 s at FI,O2 = 1.00, each response being well fitted by a single exponential. However, tau VO2 was not significantly affected by the alterations in FI,O2. Although there was no discernible peripheral chemosensitivity at FI,O2 = 0.30 or 1.00, the tau VE increased appreciably between these inspirates. We therefore conclude that the peripheral chemoreceptors are important, but not exclusive determinants of the exponential response characteristics during the non-steady-state phase of the exercise hyperpnoea in man. This supports the contention of a component of the control being humorally mediated even during moderate exercise.

Factors determining symptoms in heart failure: comparison of fast and slow exercise tests

Factors determining the symptoms of breathlessness and fatigue in patients with congestive heart failure were investigated by comparing the response to slow and fast exercise. Symptom limited oxygen consumption (maximal); minute ventilation, mean pulmonary capillary wedge pressure; and arterial blood gases, pH, and lactate concentrations were measured during treadmill exercise using a slow protocol in 25 men (age 34-67 years) with congestive heart failure (New York Heart Association class II-III). Ten of these patients were also exercised according to a rapid protocol. Exercise was terminated by fatigue in 23/25 patients after the slow test and by breathlessness in all patients after the rapid test. Exercise capacity (maximal oxygen consumption and exercise duration) was not related to resting or exercise pulmonary capillary wedge pressure or the change in pulmonary capillary wedge pressure during exercise, nor was there any difference in pulmonary capillary wedge pressure at the end of exercise within individuals between the fast and slow tests. Minute ventilation was greater (51 vs 43 1/min), peak exercise lactate concentration higher (3.7 vs 2.2 mmol/l), and the change in pH from the resting state was greater (0.06 vs 0.02) during the rapid test than during the slow test. The sensation of breathlessness in congestive heart failure is not simply related to raised pulmonary capillary wedge pressure, but may in part be due to stimulation of peripheral chemoreceptors in response to metabolic acidosis.

Is the voluntary control of exercise in man necessary for the ventilatory response?

The ventilatory response to electrically induced exercise (EEL) was studied in eighteen normal subjects and compared with the response to performing the same exercise voluntarily (EV). EEL was produced by surface electrode stimulation of the quadriceps and hamstring muscles so as to cause a pushing movement at 1 HZ against a spring load; this produced no pain or discomfort. Matching of EV to EEL was achieved by subjects copying a tension signal recorded during EEL and displayed on a storage oscilloscope. There were no differences between the resting states measured before either form of exercise. The ventilatory response (change in ventilation as a ratio of the change in CO2 elimination) was similar in the two types of exercise. The increases in ventilation and CO2 elimination were greater with EEL. Small but significant increases in the gas exchange ratio and serum lactate were found for EEL but not for EV, suggesting an increase in anaerobic metabolism in EEL. End-tidal PCO2 showed little change in either form of exercise. In some runs end-tidal PCO2 rose, but insufficiently to account for the ventilatory response as judged by the response to inhaled CO2. In two subjects arterial blood samples showed small and inconsistent changes in both Pa,CO2 and PaO2 for EV and EEL. pH and base excess changes also were consistent with more anaerobiosis with EEL compared to EV. The first ten breaths of exercise were used to study the on transient. In EV, expiratory duration shortened and ventilation increased significantly on the first breath but CO2 elimination did not increase until the second breath; in EEL, these variables did not change significantly until the second breath. For the remainder of the on transient the pattern of the ventilatory response was similar for EV and EEL. By the end of the on transient both EV and EEL had reached approximately 80% of their final steady-state values. These results suggest that a normal ventilatory response can occur in the absence of a drive to exercise from the cortex.