[Central and peripheral mechanisms of difficult breathing in man].

The review is dedicated of phenomenology of the difficult breathing and analysis of the peripheral and central mechanisms of it. Analysis based on model investigations with resistive loads in awake or sleeping healthy humans, including results of experiments in the anesthetized animals. The recent data having principle importance in estimation of the central mechanisms of the control breathing are included too in the review. It concerned to the contemporary visualization of central neuronal activity on the awake person during dyspnoea. It became possible due to application of the unique precision equipment--functional magnetic--resonance and positron--emission tomography.

Respiratory responses to voluntary and reflexly-induced stepping movements in normal subjects and spinal patients.

In the present study, the mechanisms of interaction between the breathing and stepping movements were investigated. The investigations were carried out in 8 normal subjects and 4 individuals with complete spinal cord lesion. Additionally, experimental studies were performed in 7 decerebrate and 4 spinal cats. Involuntary reflex stepping movements in the air were induced by vibrators attached above a tendon of m. rectus femoris and m. biceps femoris. In the spinal patients, epidural stimulation was carried out with electrodes located in the posterior epidural space at the vertebral level Th11. It was found that voluntary-induced stepping, which are accompanied by visceral reactions, were mediated by the central mechanisms. Changes in breathing pattern in response to vibration-induced involuntary stepping movements point up to the peripheral component being predominant in such interactions. In the spinal patients under epidural spinal cord stimulation delivered below the site of injury, the reaction in the cardio-respiratory system was abolished. Vibrations to the leg muscles also did not evoke respiratory responses. However, in the condition of forced breathing such vibrations evoked the rhythmic bursting EMG activity in m. rectus femoris, which was in harmony with the breathing rhythm. The results confirmed the presence of an interaction between breathing and stepping generators. This interaction is underlain both by central and peripheral components.

Mechanisms of genioglossus responses to inspiratory resistive load in rabbits.

The purpose of the present study has been to determine whether pharyngeal dilator muscles participate in inspiratory load compensatory responses and if so, to elucidate role of upper airway mechanoreceptors in these responses. The experiments were performed on anaesthetized rabbits. Each animal was tested in three ways by the imposition of inspiratory resistive load: (1) at upper airways via face mask, (2) at the tracheostomic cannula placed below larynx (all upper airway receptors were 'bypassed') and (3) at the mouth after the section of the hypoglossus nerves (motor denervation of genioglossus muscle). The inspiratory load applied to the upper airways evoked significant increases in integrated genioglossus activity (to 129 +/- 14.7% of control) and its inspiratory duration (to 113 +/- 5% of control) already within the first loaded breath (P < 0.05). The increases in the inspiratory activity of musculius genioglossus were relatively greater than the simultaneous increases in the activity of the diaphragm. Motor denervation of the pharynx dilator muscles (including m. genioglossus) increased airway resistance to 184 +/- 19% of control (P < 0.05) and induced obstructive alterations in the breathing pattern during unloaded breathing: decrease in maximal inspiratory flow (-13%) and increase in the level of negative oesophageal pressure (+14%) and the peak diaphragm activity (+6%). After nervi hypoglossus sections additional increases in motor and pressure outputs were required in order to maintain unaltered ventilation at the same degree of loading as before denervation. The results indicate that the pharyngeal dilator muscles have a role in compensation of added inspiratory load. Activation of these muscles facilitate the load compensating function of 'pump' muscles by decreasing airway resistance. Tracheostomy did not reduce the genioglossus response to inspiratory loading, ruling out any role for upper airways receptors in the genioglossus response to inspiratory load compensations.

[Involvement of pharyngeal muscles in compensatory responses of the respiration system to inspiration resistance load]. / Mekhanizmy vovlecheniia faringeal'nykh myshts v kompensatornye reaktsii dykhatel'noi sistemy na inspiratornuiu rezistivnuiu nagruzku.

The present study was undertaken to test the hypothesis that recruitment of upper airway muscles in loaded breathing is a result of integration of peripheral chemoreceptor and pulmonary mechanoreceptor afferents. Experiments were performed in spontaneously breathing tracheostomized anesthetized rabbits. It had been studied the effects of inspiratory resistive loading to EMG activity of genioglossus muscle. In the intact rabbits the peak value and duration of inspiratory activity of genioglossus increased in loading. Imposition of resistive load in vagotomized animals did not evoke alteration in inspiratory activity of genioglossus in the first loaded breath. Hyperoxia decreased the response of genioglossus muscle to inspiratory loading and vagatomy. We conclude that hypoxic stimulation of peripheral chemoreceptors and decrease in volume-related afferent activity from pulmonary stretch receptors are major mechanisms of the upper airway muscle recruiment in inspiratory resistive loading.

[The role of pharyngeal muscles in the regulation of airway resistance during inspiration resistive load in rabbits]. / Rol' faringeal'nykh myshts v reguliatsii soprotivleniia dykhatel'nykh putei pri intspiratornoi rezistivnoi nagruzke u krolikov.

In anesthetised spontaneously breathing rabbits, an increase in the inspiratoryactivity m.genioglossus was greater than a simultaneous increase in the respiratory activity of the diaphragm cluring loaded breathing. A bypass of the air flow via tracheostoma did not affect the response of the m.genioglossus to a resistive load. The difference seems to be due to different functional inputs from the lung mechanoreceptors to phrenic and hypoglossal neuronal pools.

Central and peripheral components of diaphragmatic fatigue during inspiratory resistive load in cats.

The development of fatigue was investigated in the diaphragm of anaesthetized, tracheostomized, spontaneously breathing cats during restricted air flow. Ventilation, transdiaphragmatic pressure (Pdi), integrated electrical activity of diaphragm (Edi) and phrenic nerve (Eph) were measured simultaneously and expressed as a percentage of values at unloaded breathing. Inspiratory loads were 60, 70 and 80% of Pdi max. The Pdi max was measured by airway occlusion at functional residual capacity. The duration of loads was 40-60 min. The diaphragmatic fatigue developed only during heavy inspiratory loading (80% Pdi max). During the first 10 min of heavy load Pdi, Edi and Eph increased to 905 +/- 60%, 248 +/- 20% and 229 +/- 24%, respectively (P < 0.01), and then began to fall gradually. Ventilation declined to 39 +/- 3% after 60 min of heavy load (P < 0.01), resulting in acute hypercapnia and hypoxia. Initial fatigue appeared as a decrease in Pdi (to 781 +/- 63%) and parallel decline in Edi (to 233 +/- 21%) after 30 min of load (P < 0.05). Phrenic nerve activity did not change during this stage. These data suggest a peripheral basis of diaphragmatic fatigue, related to disorders in neuromuscular transmission. After 60 min of heavy load, Pdi fell to 675 +/- 49%, Edi declined to 209 +/- 28% and Eph decreased to 189 +/- 25%. We interpret the decrease in phrenic nerve activity as a weakening of central inspiratory drive and development of the central component of diaphragmatic fatigue in the last stage.

[The regulation of breathing during the inhalation of different densities of gas mixtures under increasing muscle work in man]. / Reguliatsiia dykhaniia v usloviiakh ingaliatsii gazovykh smesei razlichnoi plotnosti pri vozrastaiushchei myshechnoi rabote u cheloveka.

The maximal working ability of the subjects did not depend on the composition of inhaled gas mixtures. The lung ventilation, oxygen consumption and CO2 release, as well as the heart rate were increasing along with the physical load increase, the changes being not dependent on the mixture composition. A significant decrease in the effort of the respiratory muscles, in the respiration work and in central inspiratory activity occurred in inhalation of a helium-oxygen mixture, whereas these parameters increased in breathing with an argon-oxygen mixture. The compensatory responses of the respiratory system seem to appear on the basis of afferents from the lungs and respiratory muscles mechanoreceptors as well as on account of the segmentary level reflexes and the properties of a muscular fibre itself.

[The mechanism of apnea during difficult breathing]. / Mekhanizm apnoé v usloviiakh zatrudnennogo dykhaniia.

The experiments on anesthetized cats demonstrated that only a combination of hypoxia and inspiratory resistive loading induced respiratory arrest. The findings suggest that both central and peripheral mechanisms of apnea exist due to functional insufficiency of respiratory muscles as a result of synaptic disorder and excitatory contractility disorder in diaphragmatic muscle fibers.

[Physical performance and function of the cardiorespiratory system in additional respiratory resistance]. / Fizicheskaia rabotosposobnost' i funktsii kardiorespiratornoi sistemy pri dobavochnom soprotivlenii dykhaniiu.

This paper presents results of examining respiration and circulation parameters of healthy volunteers pedalling a bicycle ergometer at different degrees of inspiratory-expiratory resistance. In this situation the performance-limiting factor is an increased load on respiratory muscles. An additional signal of performance problems is respiratory discomfort. The paper gives a formula for predicting maximally allowable workloads in response to increased respiration resistance.

[Chemoreceptor modulation of the respiratory reactions during muscle contractions in the cat]. / Khemoretseptornaia moduliatsiia dykhatel'nykh reaktsii pri myshechnykh sokrashcheniiakh u koshki.

Phasic contractions of the hindlimb muscles were induced in hypercapnia, hypoxia and hyperoxia. Absolute value of the lung ventilation and its kinetics depended on hypoxic and hypercapnic stimuli, reaching its maximum in their combination. Hypoxic stimulus decreased an increment of the lung ventilation and decelerated the latter's development. The interaction seems to be realized by neuronal mechanisms of dorsal respiratory nucleus. The latter is believed to play a major role in the mechanism of generation of central inspiratory activity based on respective chemoreceptor and proprioceptor afferent information.