Y. Jammes

Fatigue-induced changes in diaphragmatic afferents and cortical activity in the cat

The rationale for the present study was to test the hypothesis that changes in phrenic sensory activity during diaphragmatic fatigue may modify the transmission of phrenic afferent action potentials to the cortex and also the spontaneous EEG activity. This was performed in anesthetized cats. Diaphragmatic fatigue was produced by intermittent direct muscle stimulation for a 30 min period. Diaphragmatic metaboreceptors (tonically active afferents) and mechanoreceptors (phasic phrenic activity) were identified by their activation by intraarterial lactic acid injection or their discharge in phase with diaphragmatic contraction, respectively. Cortical phrenic evoked potentials (CPEPs) and spontaneous EEG activity were recorded from the left sensorimotor area. Diaphragmatic failure was shown from the 10th minute of stimulation. Then, the activity of tonic phrenic afferents increased markedly whereas, in parallel, the phasic discharge of mechanoreceptors decreased progressively. This was associated with progressive lengthening in onset and peak latencies of CPEPs. The main EEG changes (visual and fast Fourier transform analysis) were characterized by a transient increased energy in the delta frequency band during the first minutes of the fatigue run, followed by decreased energy in the theta frequency band after 11-25 min of stimulation. Denervation of the diaphragm suppressed the EEG changes during the fatigue run. The present observations suggest that the cortical integration of sensory information from the diaphragm may be altered during fatigue.

Combined electromyography--31P-magnetic resonance spectroscopy study of human muscle fatigue during static contraction.

Metabolic changes measured by 31P‐magnetic resonance spectroscopy and surface electromyograms were simultaneously recorded during isometric contraction of forearm flexor muscles sustained at 60% of maximal force until exhaustion. Throughout the fatigue trial, energy in the low‐frequency (L) band continuously increased whereas energy in the high‐frequency (H) band first increased and then fell only prior to exhaustion. PCr content decreased linearly. Intracellular pH (pHi) transiently increased during the first 22 s of trial. The triggering of acidosis was associated with critical PCr values (35–70% of initial content) and decreased electromyogram (EMG) energy in the H band. Linear relationships were only found between energy in the L band, pHi, and PCr content. The interindividual variability of metabolic and EMG changes was high despite standardized conditions of contraction. Maximal PCr consumption was correlated with the maximal pHi decrease measured at the end of the trial. Overall, there was no correlation between H/L EMG ratio and changes in muscle metabolism.

Vagal afferents and EEG rhythms in the Sl area in anesthetized cats : similarities between responses to electrical and chemical (phenyldiguanide) stimulations

In anesthetized, artificially ventilated cats with open chest, bilateral stimulation of all afferent vagal fibres (pulse duration: 800 microseconds, 30 Hz, train duration 30 to 40 s) produced marked changes in the spontaneous EEG activities in the primary somatosensory cortex (Sl area). They were characterized by depressed background rhythms, with a tendency to desynchronization, decreased amplitude and number of spindles, with altered pattern, and/or evoked sustained fast rhythmic activities. These effects occurred within 1 to 5 sec during vagal stimulation. On the contrary, the EEG response was weaker or absent when only myelinated vagal afferents were stimulated (100 microseconds). I.v. injection of phenyldiguanide (PDG), used for stimulation of unmyelinated vagal sensory fibres and mainly of pulmonary afferents, induced EEG changes within the first 30 s, similar to those observed during electrical vagal stimulation. These EEG responses were unrelated to the induced hypotension. Cervical bivagotomy produced persistent changes in EEG activity, with enhancement of the magnitude, duration and number of spindles, which resembled the delayed effects induced by PDG. The present results obtained with three test agents (electrical or chemical vagal stimulation and bivagotomy) demonstrated that, in cats, vagal afferent information interacted with the spontaneous EEG rhythms in the Sl area.

Respiratory effects of cold-gas breathing in humans under hyperbaric environment.

Changes in total lung resistance (RL) during inhalation of cold gas mixtures were measured in 4 human volunteers during an experimental dive at 46 ATA. The subjects breathed helium-nitrogen-oxygen mixtures during the decompression schedule, and measurements were performed at 46, 36, 21, 12.5, 6 and 2 ATA (1 ATA = 100 kPa). RL was measured during eupneic ventilation when individuals inhaled either ambient gas at +30 to +33 degrees C (control condition), or cooled gas at +7 to +18 degrees C. RL values measured in control conditions increased with gas density. Thus, the changes in RL induced by cold gas breathing were expressed in percent of the corresponding control values. No cold-induced bronchospasm occurred at low ambient pressure, even at the lowest inspired temperature, +7 degrees C. However, the airway response was present at pressure up to 21 ATA and then occurred at higher level of inspired gas temperature. The convective respiratory heat loss (Cr), calculated at each pressure level and experimental condition, was linearly related to cold-induced changes in RL; the value of Cr inducing 20% increase in RL was around 1.4 kcal.min-1. The bronchomotor response was related to the increase in respiratory heat loss induced by the high thermal capacity of the gas mixture used in hyperbaric environment. The present observations confirm previous data obtained under hyperbaric conditions (25 ATA) as well some experiments performed at sea level in normal individuals breathing very cold air.

In vivo and in vitro studies on cold-induced airway response in normal and sensitized rabbits.

We measured the changes in lung resistance (RL) induced by cool inspired air under dry air conditions in anesthetized, paralyzed and artificially ventilated rabbits. We compared the airway response to cold air in non-sensitized (NS) animals with the response in rabbits sensitized (S) to bovine serum albumin. Using in vitro tracheal preparations from the same animals, we observed the effects of lowering the bath temperature on smooth muscle tension and also the response to acetylcholine (Ach). Dose-response curves to Ach were constructed and analyzed in terms of maximal contraction (delta Tmax) and pD2 coefficient (negative logarithm of the molar concentration of Ach producing 50% of maximum contraction). The magnitude of cold induced airway response tested in vivo was significantly greater in S (delta RL = +52% +/- 2) than in NS rabbits (+30% +/- 4; P less than 0.01). In vitro the decrease of temperature in the bathing medium induced a significant relaxation of tracheal spirals in both NS and S preparations (delta T = -2.42 +/- 0.34 g/mg wet tissue, -3.36 +/- 0.8 g/mg wet tissue, respectively). However, an adaptation of this response occurred after 8 min in NS rabbits whereas relaxation persisted in S rabbits. The tracheal contractile response to Ach was decreased by bath cooling in S ans NS animals (delta Tmax = 5.53 +/- 0.15 g/mg wet tissue and 5.03 +/- 0.27 g/mg wet tissue at 37 degrees C; 3.05 +/- 0.30 g/mg wet tissue and 1.93 +/- 0.35 g/mg wet tissue at 17 degrees C, respectively). Moreover, cooling also reduced the velocity of response. In all cases, S preparations displayed hyperresponsiveness to Ach and this was observed for maximal changes in tension as well as for pD2 values. Present observations show that cold induces different effects on airway smooth muscle with the production of a contractile response in in vivo preparations but relaxes isolated tracheal spirals. They also suggest that sensitization by foreign proteins may modify the intrinsic properties of tracheal smooth muscle.

Quantitative EEG changes under various conditions of hyperventilation in the sensorimotor cortex of the anaesthetized cat

The effects on the EEG rhythms recorded from the sensorimotor cortex (post-sigmoid gyrus) of anaesthetized cats were studied under 4 conditions of artificial mechanical hyperventilation (HV) before and after cervical bilateral vagotomy. In animals with intact vagus nerves, using visual examination, EEG changes were only observed within the 2nd min during HV produced by increased stroke volume (delta V) with associated hypocapnia. Quantitative EEG (qEEG) showed that, for the same increase in minute ventilation and the same degree of hypocapnia, delta V induced a greater and earlier relative decrease (2nd min) in the power density of delta, theta and alpha bands, than increased pump frequency (delta F). The delta F tests produced a fall only in the theta band and within the 3rd min. With constant paCO2, transient modifications occurred only with delta V and were limited to the first 30 sec. In bivagotomized cats, moderate EEG responses to delta V plus associated hypocapnia persisted partly in the alpha band. Finally, no changes appeared with delta V or delta F when the vagus nerves were cut and paCO2 was maintained constant. The present data suggest strongly that, in anaesthetized cats, peripheral vagal afferents from the respiratory system play a major role in the EEG changes caused by artificial hyperventilation.

Le tonus du muscle lisse trachéobronchique et son controle réflexe

Airway smooth muscle tone is reinforced during the inspiratory phase of the breathing cycle and depends largely from neurogenic motor drive carried by the vagus nerve. This muscle tone seems to be produced mostly by a vago-vagal reflex loop initiated by the tonic discharge of tracheo-bronchial and/or alveolar receptors connected to thin sensory vagal fibres (non-myelinated or C-fibres). Inhibitory influences carried by large myelinated vagal fibres connected to tracheobronchial stretch receptors and also numerous afferents from the upper airways, systemic and pulmonary circulation, digestive tract and skeletal and respiratory muscles participate to the modulation of airway tone. The identification of neurotransmitters specific of the motor or sensory pathways helps to understand the peripheral modulation of airway motor drive and also the central integration of some peripheral informations.

Relationship between inspired and expired gas temperatures in a hyperbaric environment

When breathing room air at sea level the expired gas temperature (TE) increases in proportion to the inspired one (TI). Previous studies conducted under hyperbaric conditions have assumed that the TE vs TI relationship was the same when humans breathed room air at atmospheric pressure or helium-oxygen mixture under hyperbaric conditions. We hypothesized that the use of dilutant gases, as helium (He) or hydrogen (H2), having low density but high specific heat compared to nitrogen, could change the TE vs TI regression. The present study was conducted on 3 professional divers participating in the COMEX Hydra IX experiment. Three conditions were studied: A, (23.5 ATA, He-H2-O2 mixture); B, (21 ATA, H2-O2 mixture); and C, (21 ATA, He-O2 mixture). In each condition six different inspired temperatures were tested, while minute ventilation, TI and TE values were measured simultaneously. In all cases a linear relationship was found between TE and TI, but the slopes of the regression lines obtained in conditions A and B (gas mixture containing H2) were significantly lower than in condition C (He-O2 mixture). Computation of the convective respiratory heat loss (Cr) revealed that, when the subjects breathed the coldest gas mixtures (+10 degrees C), Cr value was 1.6 times higher in condition B than in C. These data are consistent with theoretical considerations and they demonstrate that a single equation cannot be used to predict the TE vs TI relationship in all environmental circumstances.

étude de la fonction cardio-vasculaire chez I'Homme au cours de plongées expérimentales à 26 Ata (mélange hélium-azote-oxygéne)

AbstractDuring two human experimental dives at 26 Ata (helium-nitrogen-oxygen gaz mixture; Pio2 = 400 mbar), the cardiac frequency (Fc) and radial arterial pulse were continously recorded, at rest and during periods of maximal expiratory (Valsalva) or inspiratory (Muller) manoeuvres, used to increase or decrease the intrathoracic pressure, respectively. Cardiovascular variables were measured at 1 and 26 Ata in resting individuals and during the maximal respiratory manoeuvres. Discontinuous measurement of arterial blood pressure using a sphygomanometer allowed to calculate the mean arterial pressure. The value of mean arterial pressure was maintained against the membrane of a radial pulse sensor. This procedure, proposed by Posey et al. (1969), gives a continuous approximation and recording of arterial blood pressure and its components.The present results did not show significant variation in the values of Fc nor systolic or diastolic blood pressures measured at rest or during Muller manoeuvres performed a...