Ventilatory responses to muscular vibrations in healthy humans

Cardiopulmonary response to exercise in COPD and overweight patients: relationship between unloaded cycling and maximal oxygen uptake profiles

Cardiopulmonary response to unloaded cycling may be related to higher workloads. This was assessed in male subjects: 18 healthy sedentary subjects (controls), 14 hypoxemic patients with chronic obstructive pulmonary disease (COPD), and 31 overweight individuals (twelve were hypoxemic). They underwent an incremental exercise up to the maximal oxygen uptake (VO2max), preceded by a 2 min unloaded cycling period. Oxygen uptake (VO2), heart rate (HR), minute ventilation (VE), and respiratory frequency (fR) were averaged every 10 s. At the end of unloaded cycling period, HR increase was significantly accentuated in COPD and hypoxemic overweight subjects (resp., +14 ± 2 and +13 ± 1.5 min(-1), compared to +7.5 ± 1.5 min(-1) in normoxemic overweight subjects and +8 ± 1.8 min(-1) in controls). The fR increase was accentuated in all overweight subjects (hypoxemic: +4.5 ± 0.8; normoxemic: +3.9 ± 0.7 min(-1)) compared to controls (+2.5 ± 0.8 min(-1)) and COPDs (+2.0 ± 0.7 min(-1)). The plateau VE increase during unloaded cycling was positively correlated with VE values measured at the ventilatory threshold and VO2max. Measurement of ventilation during unloaded cycling may serve to predict the ventilatory performance of COPD patients and overweight subjects during an exercise rehabilitation program.

Effects of hypoxia on muscle response to tendon vibration in humans

Previous animal studies have shown that hypoxia markedly reduces the activation of muscle spindles. The present study was undertaken to determine whether a reduced oxygen supply to muscle affects the tonic vibration reflex (TVR) in humans. In resting healthy volunteers, the effects of inhalation of hypoxic gas, apnea, and total forearm ischemia produced by cuff inflation were studied on separate days. The TVR was recorded in flexor digitorum superficialis and the neuromuscular conduction time (CT) was measured from the compound muscle action potential; the latency and amplitude of the H reflex were also determined. TVR depression began during inhalation of the hypoxic gas, at the end of apnea, and during cuff inflation, and persisted during the recovery period. The H-reflex amplitude concomitantly increased or remained unchanged. Thus, hypoxia seems to directly alter muscle spindle reactivity. Such alterations of sensorimotor control may occur in patients suffering from respiratory or circulatory insufficiency and may contribute to their exercise limitation.

Influence of respiratory afferents upon the proprioceptive reflex of skeletal muscles in healthy humans

Relationships between respiratory afferents and the motor drive to skeletal muscles are well documented in animals, but human data are scarce. Tonic vibratory response (TVR) elicited by mechanical tendon vibrations were explored in an arm (extensor digitorum, ED) and a leg (vastus lateralis, VL) muscle, in healthy subjects. Tendon vibrations were delivered during unloaded breathing and after 10 breathing cycles while the subject breathed through an inspiratory or expiratory resistive load in order to activate respiratory afferents. Inspiratory loaded breathing significantly enhanced TVR in ED and VL muscles whereas the effects of expiratory loading depended on the vibrated muscle (increased TVR in ED; decreased TVR in VL). These results suggest that inspiratory muscle afferents activated during inspiratory loading facilitate the gamma motor drive to arm and leg muscles whereas the activation of pulmonary vagal afferents during expiratory loading can exert a facilitating or suppressive influence on the gamma motor drive, depending on the limb muscle group.

Acute hypoxemia depresses the cardiorespiratory response during phase I constant load exercise and unloaded cycling

The effects of acute inhalation of hypoxic gas mixtures on minute ventilation (VE), respiratory frequency (fR) and heart rate (HR) were studied in healthy subjects executing constant-load 100 W and 150 W hindlimb exercises (protocol 1) or unloaded (0 W) cycling (protocol 2). Attention was focussed on early changes in variables during phase I of constant load exercise, a period where neurogenic afferents from working muscles play a key role in adaptative cardiorespiratory response as they did also during 0 W cycling. In protocol 1, a 15% O2 gas mixture was used while in protocol 2, 15% and 10% O2 mixtures were tested. Compared to the variations of cardiorespiratory variables measured during room air breathing (normoxia), hypoxemia significantly and markedly depressed the rates of VE and fR changes during phase I exercise but did not affect the changes in HR. Reduced phase I ventilatory response was not accompanied by significant variations in rest values of PaCO2 and pHa associated with the response to hypoxia. The cardiorespiratory response to 0 W cycling was also lowered under hypoxemic conditions, the magnitude of VE and HR changes being inversely proportional to the fall in PaO2 level. Based on electrophysiological animal observations, the present results may be interpreted in terms of inhibitory influences of hypoxemia on proprioceptive muscle afferents.

Characterization of hindlimb muscle afferents involved in ventilatory effects observed in decerebrate and spinal preparations

Neurogenic changes of phrenic activity have previously been observed during periodic passive motions of one hindlimb in decorticate, unanaesthetized and curarized rabbit preparations before and after high spinal transection (Palisses et al. 1988). In decerebrate and spinal preparations, we aimed to determine, through rhythmic electrical stimulation of hindlimb muscle nerves, which muscle afferents are involved in these effects. In decerebrate preparations, these electrical stimulations (trains of shocks at 80 Hz for 300 ms every second for 20 s) produced ventilatory effects when group I + II afferent fibres of either flexor or extensor nerves were stimulated together and more powerful changes as soon as group III fibres were recruited. Stimulation of group I fibres alone induced no such effects. When present, these changes in respiratory activity consisted of a maintained decrease of the respiratory period due to both inspiratory and expiratory time shortening; in addition, the amplitude of the phrenic bursts greatly increased at the onset of electrical stimulation. After spinal transection at C2 level and pharmacological activation by nialamide and DOPA, only short-lasting phrenic bursts developed spontaneously; the electrical stimulation of group II and mainly group III flexor afferent fibres induced large amplitude phrenic activity whereas the stimulation of the same extensor afferents was relatively ineffective. The activation of phrenic motoneurones during group III flexor afferent stimulation was closely linked to each 300 ms period of stimulation. While the phrenic effects obtained in the spinal preparations by natural and by electrical periodic stimulation are quite similar to each other, those produced in decerebrate preparations differ substantially.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory response to arm elevation in patients with chronic airflow obstruction

We have shown that patients with chronic airflow obstruction (CAO) complain of disabling dyspnea when performing seemingly trivial tasks with unsupported arms. Surprisingly little is known about the metabolic and ventilatory responses to unsupported upper extremity activity even though some of the muscles of the upper torso and shoulder girdle are used to perform simple and complex everyday tasks as well as partake in ventilation. To determine the effect of simple arm elevation in 20 patients with CAO we studied their lung function, VO2, VCO2, and VE, with arms down at the side (AD), during 2 min with arms extended forward up to shoulder level (AE), and during recovery. To determine the pattern of ventilatory muscle recruitment we also measured endoesophageal (Ppl), gastric (Pg), and transdiaphragmatic (Pdl) pressures. In five of the patients the electromyographic signal (EMG) of the sternocleidomastoid (Sm) muscle was recorded and analyzed in its time domain (amplitude) and power spectrum density (median frequency). Within 30 s of arm elevation VO2, VCO2, and VE rose and remained elevated for 1 min after the arms were lowered. The increase in VE resulted from increases in respiratory rate and minimal rise in tidal volume (VT). With AE, FEV1 decreased by 5% (p less than 0.02) but FRC increased by 2% (p less than 0.05). Peak inspiratory pressure (Pimax) dropped from 54 +/- 4 to 48 +/- 4 cm H2O (p less than 0.005); Pdimax remained unchanged. Immediately after raising the arms Pgi, inspiratory swing in Pdi (delta Pdi), end-expiratory Ppl, and end-expiratory Pg increased significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Contribution of central and reflex nervous activity to the rapid increase in pulmonary ventilation at the start of muscular exercise in man

To investigate the relative contributions of the central and peripheral neural drive to hyperventilation at the onset of muscular exercise, five volunteers were tested during the first ten breaths while performing both voluntary (VM) and passive (PM) ankle rotations with a frequency of 1 Hz and through an angle of 10 degrees. Resulting breathing patterns for the two movements were compared. Hypocapnic hyperventilation, found in both PM and VM, indicated its neural origin. Respiratory changes were higher in VM than in PM. In both experimental conditions, increases in ventilation (VE) depended more on respiratory frequency (f) than on tidal volume (VT). Moreover, increases in VT adapted, breath-by-breath, to values lower than the initial ones, while increases in f rose progressively. Expiratory time was reduced more than inspiratory time (TI); increases in inspiratory flow (VT/TI) depended to the same extent on changes in both TI and VT. Increases in expiratory tidal volume were initially higher than in inspiratory tidal volume, thereby producing a reduction in functional residual capacity. Because PM respiratory changes could be considered to be of nervous reflex origin only, the identical breathing patterns in PM and VM indicated that the hyperventilation found also in VM was mainly of reflex origin. The increase in VE was considered to be dependent on a greater stimulus from muscle proprioreceptors.

Effect of unsupported arm exercise on ventilatory muscle recruitment in patients with severe chronic airflow obstruction

We have proposed that unsupported arm exercise alters ventilatory muscle recruitment and precipitates dyspnea in patients with severe chronic airflow obstruction (CAO). To test this hypothesis, we studied 11 patients with CAO during symptom-limited, unsupported arm exercise (UAE) and compared it with supported arm cycling (SAE). During each exercise period, we recorded endoesophageal (PpI), gastric (Pg), and transdiaphragmatic (Pdi) pressures along with heart rate, respiratory rate, and endurance time. Expired gas was collected to determine oxygen uptake (VO2) and minute ventilation (VE). Exercise endurance was shorter for UAE than for SAE (210 +/- 114 versus 270 +/- 120 s, p less than 0.05), even though peak exercise heart rate (113 +/- 5 versus 122 +/- 7 beats/min, p less than 0.05), VO2 (5.9 +/- 0.5 versus 7.1 +/- 0.8 ml/kg/min, p less than 0.05) and VE (16.5 +/- 1.2 versus 19.8 +/- 1.3 L/min, p less than 0.05) were lower for UAE. Mean (+/- SD) values for changes in pleural (delta PpI) and gastric (delta Pg) pressures during either type of arm exercise were significantly greater than at rest (p less than 0.02). In eight of 11 patients during UAE, the changes between end-inspiratory and end-expiratory transdiaphragmatic pressure (delta Pdi) were observed to develop in a similar pattern. In these patients, end-inspiratory Pg was more positive and end-inspiratory PpI was less negative during UAE than during SAE (p less than 0.02). In addition, PpI at end expiration was markedly positive when performing UAE (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Reflex modulation of phrenic activity through hindlimb passive motion in decorticate and spinal rabbit preparation

The neurogenic effect of passive hindlimb movement on phrenic nerve discharge was compared in decorticate unanaesthetized and curarized rabbit preparations prior to and after spinal transection. The question of how and where sensory information has access to the central respiratory network was addressed in each case. All passive motions, performed using a mechanical device, were of constant amplitude in a given preparation. The results clearly differed in decorticate and spinal preparations. In the decorticate vagotomized preparation, periodic passive motions led to an immediate shortening of the respiratory period which lasted throughout the periodic stimulation and stopped with its cessation; it did not depend on the frequency of the natural stimulation and was entirely due to a 20% shortening of the expiration time. Maintained full flexion or full extension both induced the same expiration time shortening, but limited to the first two to three respiratory cycles after onset and interruption of stimulation. After spinal transection at the C2 level, and moderate activation with DOPA, no phrenic activity developed in the absence of proprioceptive stimulation. Periodic hindlimb movements evoked simultaneous large bursts in both phrenic nerves during each extension; a 1:1 coordination of phrenic activity with the external imposed period (P) was observed for various P values. A strong phrenic activation could also be elicited through maintained full hindlimb extension but not through full flexion: this activation appeared as rhythmic discharge as long as extension was maintained. It is concluded that proprioceptive inputs act upon the medullary respiration generator and reset its own rhythm whereas, at the spinal level, they elicit an amplitude modulation at phrenic motoneuronal level without acting upon the rate of the spinal "respiration" generator itself; on the same phrenic motoneurons, a subthreshold central activation added to a subthreshold proprioceptive activation probably accounts for the phrenic bursting during maintained extension. Finally, the proprioceptive control from the hindlimb on phrenic activity is processed at different sites of the central respiratory network at medullary and at spinal level, and may depend on different input signals.

Dyssynchronous breathing during arm but not leg exercise in patients with chronic airflow obstruction

Some patients with chronic airflow obstruction experience dyspnea with mild arm exercise but not with more-intense leg exercise. To investigate why these patients have limited endurance during arm exertion, we studied ventilatory responses to exercise with unsupported arms in 12 patients with chronic airflow obstruction (mean [+/- SD] forced expiratory volume in one second, 0.68 +/- 0.28 liters). Unloaded leg cycling was also studied for comparison. In the five patients who had the most severe airflow obstruction, arm exercise was limited by dyspnea after 3.3 +/- 0.7 minutes, and dyssynchronous thoracoabdominal breathing developed. In the other seven patients, arm exercise was limited by the sensation of muscle fatigue after 6.1 +/- 2.0 minutes (P less than 0.05), and dyssynchronous breathing did not occur. None of the 12 patients had dyssynchronous breathing during unloaded leg cycling. Maximal transdiaphragmatic pressure, a measure of diaphragmatic fatigue, declined similarly after arm and leg exercise in both groups. During unsupported arm work, the accessory muscles of inspiration help position the torso and arms. We hypothesize that the extra demand placed on these muscles during arm exertion leads to early fatigue, an increased load on the diaphragm, and dyssynchronous thoracoabdominal inspirations. This sequence may contribute to dyspnea and limited endurance during upper-extremity exercise.

Ventilatory effects of biceps vibration during leg exercise in healthy humans

The ventilatory effects of biceps tendon vibration were studied in healthy human subjects at rest and at two levels of light leg exercise. This was performed with intent to add the ventilatory effects of selective stimulation of muscle spindles to nervous and humoral respiratory inputs from contracting muscles. Tendon vibration performed in individuals at rest elicited a marked increase in respiratory frequency and in the ratio between inspiratory time and total breath duration with variable changes in tidal volume; this was in agreement with previous results (Jammes et al., 1981). When stimulation of biceps proprioceptors was performed during steady state exercise, the changes in ventilatory timing were attenuated, but variations in tidal volume often occurred. These results suggest that, when respiratory centers are being entrained by performance of work, further activation of muscle receptors exerts complex effects on the breathing pattern with a lack of facilitatory influences in ventilation and gas exchange.