Effect of head-down tilt on respiratory responses and human inspiratory muscle activity

The effect of a head-down tilt on the responses of the external respiration system and the functional capacity of the diaphragm and parasternal muscles were investigated in 11 healthy subjects. A 30-min head-down tilt posture (−30° relative to the horizontal) significantly increased the inspiratory time, decreased the respiration rate and the inspiratory and expiratory flow rates; and increased the airway resistance compared to these values in the vertical posture. There were no significant changes in tidal volume or minute ventilation. The electromyograms (EMGs) of the diaphragm and parasternal muscles showed that the constant values of tidal volume and minute ventilation during head-down tilt could be provided by an increase in the electric activity of the thoracic inspiratory muscles. It was established that the contribution of the thoracic inspiratory muscles increased, while the diaphragms’ contribution decreased, during patient, spontaneous breathing. The maximal inspiratory effort (Muller’s maneuver) during a head-down tilt evoked the opposite EMG-activity pattern: the contribution of inspiratory thoracic muscles was decreased and the diaphragm EMG activity was increased compared to the vertical posture. These results suggest that coordinated modulations in inspiratory muscle activity make it possible to preserve the functional reserve of human inspiratory muscles during a short-term head-down tilt.     

Parasternal and external intercostal responses to various respiratory maneuvers.

Recent studies suggest that the external intercostal (EI) muscles of the upper rib cage, like the parasternals (PA), play an important ventilatory role, even during eupneic breathing. The purpose of the present study was to further assess the ventilatory role of the EI muscles by determining their response to various static and dynamic respiratory maneuvers and comparing them with the better-studied PA muscles. Applied interventions included 1) passive inflation and deflation, 2) abdominal compression, 3) progressive hypercapnia, and 4) response to bilateral cervical phrenicotomy. Studies were performed in 11 mongrel dogs. Electromyographic (EMG) activities were monitored via bipolar stainless steel electrodes. Muscle length (percentage of resting length) was monitored with piezoelectric crystals. With passive rib cage inflation produced either with a volume syringe or abdominal compression, each muscle shortened; with passive deflation, each muscle lengthened. During eupneic breathing, each muscle was electrically active and shortened to a similar degree. In response to progressive hypercapnia, peak EMG of each intercostal muscle increased linearly and to a similar extent. Inspiratory shortening also increased progressively with increasing PCO2, but in a curvilinear fashion with no significant differences in response among intercostal muscles. In response to phrenicotomy, the EMG and degree of inspiratory shortening of each intercostal muscle increased significantly. Again, the response among intercostal muscles was not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of posture and breathing route on neural drive to upper airway dilator muscles during exercise.

Our purpose was to determine the influence of posture and breathing route on electromyographic (EMG) activities of nasal dilator (NDM) and genioglossus (GG) muscles during exercise. Nasal and oral airflow rates and EMG activities of the NDM and GG were recorded in 10 subjects at rest and during upright and supine incremental cycling exercise to exhaustion. EMG activities immediately before and after the switch from nasal to oronasal breathing were also determined for those subjects who demonstrated a clear switch point (n = 7). NDM and GG EMG activities were significantly correlated with increases in nasal, oral, and total ventilatory rates during exercise, and these relationships were not altered by posture. In both upright and supine exercise, NDM activity rose more sharply as a function of nasal inspired ventilation compared with total or oral inspired ventilation (P < 0.01), but GG activity showed no significant breathing-route dependence. Peak NDM integrated EMG activity decreased (P = 0.008), and peak GG integrated EMG activity increased (P = 0.032) coincident with the switch from nasal to oronasal breathing. In conclusion, 1) neural drive to NDM and GG increases as a function of exercise intensity, but the increase is unaltered by posture; 2) NDM activity is breathing-route dependent in steady-state exercise, but GG activity is not; and 3) drive to both muscles changes significantly at the switch point, but the change in GG activity is more variable and is often transient. This suggests that factors other than the breathing route dominate drive to the GG soon after the initial changes in the configuration of the oronasal airway are made.     

Caffeine effect on respiratory muscle endurance and sense of effort during loaded breathing

The present study examined respiratory muscle endurance and the magnitude of the sense of effort during inspiratory threshold loading following a dose of caffeine (600 mg) previously observed to increase diaphragm strength. Experiments were performed on 12 normal subjects. Respiratory muscle endurance at a given level of load was assessed from the time of exhaustion and from the time course of the change in the power spectrum (centroid frequency) of the diaphragm electromyogram (EMG). The intensity of the sense of effort during loaded breathing was evaluated using a category (Borg) scale. Increasingly severe loads were associated with more rapid onset of fatigue. At a given load, caffeine prolonged the time to exhaustion and decreased the rate of fall of the centroid frequency of the diaphragm EMG. Caffeine also decreased the sense of effort during loaded breathing in 9 of 11 subjects. Changes in respiratory muscle endurance after caffeine administration were not explained by changes in the pressure-time index of the respiratory muscles or the pattern of thoracoabdominal movement. We conclude that caffeine enhances inspiratory muscle endurance, while concomitantly reducing the sense of effort associated with fatiguing inspiratory muscle contractions.     

Respiratory muscle responses elicited by dorsal periaqueductal gray stimulation in rats

The periaqueductal gray matter is an essential neural substrate for central integration of defense behavior and accompanied autonomic responses. The dorsal half of the periaqueductal gray matter (dPAG) is also involved in mediating emotional responses of anxiety and fear, psychological states that often are associated with changes in ventilation. However, information regarding respiratory modulation elicited from this structure is limited. The present study was undertaken to investigate the relationship between stimulus frequency and magnitude on ventilatory pattern and respiratory muscle activity in urethane-anesthetized, spontaneously breathing rats. Electrical stimulation in the dPAG-recruited abdominal muscle activity increased ventilation and increased respiratory frequency by significantly shortening both inspiratory time and expiratory time. Ventilation increased within the first breath after the onset of stimulation, and the respiratory response increased with increasing stimulus frequency and magnitude. dPAG stimulation also increased baseline EMG activity in the diaphragm and recruited baseline external abdominal oblique EMG activity, normally quiescent during eupneic breathing. Significant changes in cardiorespiratory function were only evoked by stimulus intensities >10 microA and when stimulus frequencies were >10 Hz. Respiratory activity of both the diaphragm and abdominal muscles remained elevated for a minimum of 60 s after cessation of stimulation. These results demonstrate that there is a short-latency respiratory response elicited from the dPAG stimulation, which includes both inspiratory and expiratory muscles. The changes in respiratory timing suggest rapid onset and sustained poststimulus dPAG modulation of the brain stem respiratory network that includes expiratory muscle recruitment.     

Changes in cardiac output during sustained maximal ventilation in humans

To determine the increment in cardiac output and in O2 consumption (Vo2) from quiet breathing to maximal sustained ventilation, Vo2 and cardiac output were measured using an acetylene rebreathing technique in five subjects. Cardiac output and Vo2 were measured multiple times in each subject at rest and during sustained maximal ventilation. During maximal ventilation subjects breathed 5% CO2 to prevent hypocapnia. The increase in cardiac output from rest to maximal breathing was taken as an estimate of respiratory muscle blood flow and was used to calculate the arteriovenous O2 content difference across the respiratory muscles from the Fick equation. Cardiac output increased by 4.3 +/- 1.0 l/min (mean +/- SD), from 5.6 +/- 0.7 l/min at rest to 9.9 +/- 1.1 l/min, during maximal ventilations ranging from 127 to 193 l/min. Vo2 increased from 312 +/- 29 to 723 +/- 69 ml/min during maximal ventilation. O2 extraction across the respiratory muscles during maximal breathing was 9.6 +/- 1.0 vol% (range 8.5 to 10.7 vol%). These values suggest an upper limit of respiratory muscle blood flow of 3-5 l/min during unloaded maximal sustained ventilation.     

Exercise-induced respiratory muscle fatigue: implications for performance.

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O(2) and CO(2) transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.     

Relative contributions of rib cage and abdomen to breathing in normal subjects.

By use of the method of Konno and Mead and the respiratory magnetometer, the partition of respired gas volumes into rib cage and diaphragm-abdomen components was accomplished in 81 normal subjects including 32 young and middle-aged men, 29 young and middle-aged women, and 20 elderly men. Studied were isovolume maneuvers and the relaxation configuration over the inspiratory capacity range, quiet tidal breathing, increased amplitudes of slow breathing, rapid inspirations and expirations, and both quiet and forceful phonation. No major differences were noted between men and women or between the young and the elderly during any respiratory acts. During quiet breathing most normal subjects are abdominal breathers when supine and thoracic breathers when upright. Rapid respiratory maneuvers were accomplished mostly through rib cage displacement suggesting that rib cage muscles are capable of more rapid action than diaphragm and abdominal muscles. Data from deep breathing and rapid maneuvers supported the view that abdominal and rib cage muscles often act to optimize the mechanical (length-tension) advantage of the diaphragm.     

Novel whole body plethysmography system for the continuous characterization of sleep and breathing in a mouse

Sleep is associated with marked alterations in ventilatory control that lead to perturbations in respiratory timing, breathing pattern, ventilation, pharyngeal collapsibility, and sleep-related breathing disorders (SRBD). Mouse models offer powerful insight into the pathogenesis of SRBD; however, methods for obtaining the full complement of continuous, high-fidelity respiratory, electroencephalographic (EEG), and electromyographic (EMG) signals in unrestrained mice during sleep and wake have not been developed. We adapted whole body plethysmography to record EEG, EMG, and respiratory signals continuously in unrestrained, unanesthetized mice. Whole body plethysmography tidal volume and airflow signals and a novel noninvasive surrogate for respiratory effort (respiratory movement signal) were validated against simultaneously measured gold standard signals. Compared with the gold standard, we validated 1) tidal volume (correlation, R(2) = 0.87, P < 0.001; and agreement within 1%, P < 0.001); 2) inspiratory airflow (correlation, R(2) = 0.92, P < 0.001; agreement within 4%, P < 0.001); 3) expiratory airflow (correlation, R(2) = 0.83, P < 0.001); and 4) respiratory movement signal (correlation, R(2) = 0.79-0.84, P < 0.001). The expiratory airflow signal, however, demonstrated a decrease in amplitude compared with the gold standard. Integrating respiratory and EEG/EMG signals, we fully characterized sleep and breathing patterns in conscious, unrestrained mice and demonstrated inspiratory flow limitation in a New Zealand Obese mouse. Our approach will facilitate studies of SRBD mechanisms in inbred mouse strains and offer a powerful platform to investigate the effects of environmental and pharmacological exposures on breathing disturbances during sleep and wakefulness.     

Relative strengths of the chest wall muscles

We hypothesized that during maximal respiratory efforts involving the simultaneous activation of two or more chest wall muscles (or muscle groups), differences in muscle strength require that the activity of the stronger muscle be submaximal to prevent changes in thoracoabdominal configuration. Furthermore we predicted that maximal respiratory pressures are limited by the strength of the weaker muscle involved. To test these hypotheses, we measured the pleural pressure, abdominal pressure (Pab), and transdiaphragmatic pressure (Pdi) generated during maximal inspiratory, open-glottis and closed-glottis expulsive, and combined inspiratory and expulsive maneuvers in four adults. We then determined the activation of the diaphragm and abdominal muscles during selected maximal respiratory maneuvers, using electromyography and phrenic nerve stimulation. In all subjects, the Pdi generated during maximal inspiratory efforts was significantly lower than the Pdi generated during open-glottis expulsive or combined efforts, suggesting that rib cage, not diaphragm, strength limits maximal inspiratory pressure. Similarly, at high lung volumes, the Pab generated during closed-glottis expulsive efforts was significantly greater than that generated during open-glottis efforts, suggesting that the latter pressure is limited by diaphragm, not abdominal muscle, strength. As predicted, diaphragm activation was submaximal during maximal inspiratory efforts, and abdominal muscle activation was submaximal during open-glottis expulsive efforts at midlung volume. Additionally, assisting the inspiratory muscles of the rib cage with negative body-surface pressure significantly increased maximal inspiratory pressure, whereas loading the rib cage muscles with rib cage compression decreased maximal inspiratory pressure. We conclude that activation of the chest wall muscles during static respiratory efforts is determined by the relative strengths and mechanical advantage of the muscles involved.     

Effect of head-down tilt on respiratory responses and human inspiratory muscles activity

The effect of head-down tilt on respiration and diaphragmal and parasternal muscles activity was investigated in 11 healthy subjects. The short-time (30 min) head-down tilt posture (-30 degrees relatively horizont) increased the inspiratory time (P < 0.05), decreased breathing frequency (P < 0.05), inspiratory and expiratory flow rate (P < 0.05) and increased the airway resistance (P < 0.05) compared with values in vertical posture. There were no significant changes in tidal volume and minute ventilation. Constant values of tidal volume and minute ventilation during head-down tilt were provided by increasing in EMG activity of parasternal muscles more then twice. It was established that the contribution of chest wall inspiratory muscles increased while the diaphragm's contribution decreased during head-down spontaneous breathing. Maximal inspiratory effort (Muller's maneuver) during head-down tilt evoked the opposite EMG-activity pattern: the contribution of inspiratory thoracic muscles was decreased and diaphragm's EMG-activity was increased compared with vertical posture. These results suggest that coordinate modulations in inspiratory muscles activity allows to preserve the functional possibility of human inspiratory muscles during short-time head-down tilt.     

Activation of the parasternal intercostals during breathing efforts in human subjects

We have tested the possibility that the electromyographic (EMG) activity present in the parasternal intercostal muscles during quiet inspiration was reflexive, rather than agonistic, in nature. Using concentric needle electrodes we measured parasternal EMG activity in four normal subjects during various inspiratory maneuvers. We found that 1) phasic inspiratory activity was invariably present in the parasternal intercostals during quiet breathing, 2) the parasternal EMG activity was generally increased during attempts to perform the tidal breathing maneuver with the diaphragm alone, 3) parasternal EMG activity was markedly decreased or suppressed in the presence of rib cage distortion during diaphragmatic isovolume maneuvers, and 4) that EMG activity could not be voluntarily suppressed during breathing unless the inspired volume was trivial. We conclude that the parasternal EMG activity detected during quiet inspiration in the normal subjects depends on a central involuntary mechanism and is not related to activation of intercostal mechanoreceptors.     

Triangularis sterni muscle use in supine humans

The electrical activity of the triangularis sterni (transversus thoracis) muscle was studied in supine humans during resting breathing and a variety of respiratory and nonrespiratory maneuvers known to bring the abdominal muscles into action. Twelve normal subjects, of whom seven were uninformed and untrained, were investigated. The electromyogram of the triangularis sterni was recorded using a concentric needle electrode, and it was compared with the electromyograms of the abdominal (external oblique and rectus abdominis) muscles. The triangularis sterni was usually silent during resting breathing. In contrast, the muscle was invariably activated during expiration from functional residual capacity, expulsive maneuvers, "belly-in" isovolume maneuvers, static head flexion and trunk rotation, and spontaneous events such as speech, coughing, and laughter. When three trained subjects expired voluntarily with considerable recruitment of the triangularis sterni and no abdominal muscle activity, rib cage volume decreased and abdominal volume increased. These results indicate that unlike in the dog, spontaneous quiet expiration in supine humans is essentially a passive process; the human triangularis sterni, however, is a primary muscle of expiration; and its neural activation is largely coupled with that of the abdominals. The triangularis sterni probably contributes to the deflation of the rib cage during active expiration.     

Modulation of laryngeal and respiratory pump muscle activities with upper airway pressure and flow.

The hypothesis that upper airway (UA) pressure and flow modulate respiratory muscle activity in a respiratory phase-specific fashion was assessed in anesthetized, tracheotomized, spontaneously breathing piglets. We generated negative pressure and inspiratory flow in phase with tracheal inspiration or positive pressure and expiratory flow in phase with tracheal expiration in the isolated UA. Stimulation of UA negative pressure receptors with body temperature air resulted in a 10--15% enhancement of phasic moving-time-averaged posterior cricoarytenoid electromyographic (EMG) activity above tonic levels obtained without pressure and flow in the UA (baseline). Stimulation of UA positive pressure receptors increased phasic moving-time-averaged thyroarytenoid EMG activity above tonic levels by 45% from baseline. The same enhancement of posterior cricoarytenoid or thyroarytenoid EMG activity was observed with the addition of flow receptor stimulation with room temperature air. Tidal volume and diaphragmatic and abdominal muscle activity were unaffected by UA flow and/or pressure, whereas respiratory timing was minimally affected. We conclude that laryngeal afferents, mainly from pressure receptors, are important in modulating the respiratory activity of laryngeal muscles.     

Influence of nasal airflow temperature and pressure on alae nasi electrical activity.

The influence of nasal airflow, temperature, and pressure on upper airway muscle electromyogram (EMG) was studied during steady-state exercise in five normal subjects. Alae nasi (AN) and genioglossus EMG activity was recorded together with nasal and oral airflows and pressures measured simultaneously by use of a partitioned face mask. At constant ventilations between 30 and 50 l/min, peak inspiratory AN activity during nasal breathing (7.2 +/- 1.4 arbitrary units) was greater than that during oral breathing (1.0 +/- 0.3 arbitrary units; P less than 0.005). In addition, the onset of AN EMG activity preceded inspiratory flow by 0.38 +/- 0.03 s during nasal breathing but by only 0.17 +/- 0.04 s during oral breathing (P less than 0.04). When the subject changed from nasal to oral breathing, both these differences were apparent on the first breath. However, peak AN activity during nasal breathing was uninfluenced by inspiration of hot saturated air (greater than 40 degrees C), by external inspiratory nasal resistance, or by changes in the expiratory route. The genioglossus activity did not differ between nasal and oral breathing (n = 2). Our findings do not support reflex control of AN activity sensitive to nasal flow, temperature, or surface pressure. We propose a centrally controlled feedforward modulation of phasic inspiratory AN activity linked with the tonic drive to the muscles determining upper airway breathing route.     

Influence of upper airway sensory receptors on respiratory muscle activation in humans

We reasoned that neural information from upper airway (UA) sensory receptors could influence the relationship between UA and diaphragmatic neuromuscular responses to hypercapnia. In this study, the electromyographic (EMG) activities of the alae nasi (AN), genioglossus (GG), and chest wall (CW) or diaphragm (Di) to ventilatory loading were assessed in six laryngectomized, tracheostomized human subjects and in six subjects breathing with an intact UA before and after topical UA anesthesia. The EMG activities of the UA and thoracic muscles increased at similar rates with increasing hypercapnia in normal subjects, in subjects whose upper airways were anesthetized, and in laryngectomized subjects breathing with a cervical tracheostomy. Furthermore, in the laryngectomized subjects, respiratory muscle EMG activation increased with resistive inspiratory loading (15 cmH2O X l-1 X s) applied at the level of a cervical tracheostomy. At an average expired CO2 fraction of 7.0%, resistive loading resulted in a 93 +/- 26.3% (SE) increase in peak AN EMG activity, a 39 +/- 2.0% increase in peak GG EMG activity, and a 43.2 +/- 16.5% increase in peak CW (Di) EMG activity compared with control values. We conclude that the ventilatory responses of the UA and thoracic muscles to ventilatory loading are not substantially influenced by laryngectomy or UA anesthesia.     

Blood flow index using near-infrared spectroscopy and indocyanine green as a minimally invasive tool to assess respiratory muscle blood flow in humans

Near-infrared spectroscopy (NIRS) in combination with indocyanine green (ICG) dye has recently been used to measure respiratory muscle blood flow (RMBF) in humans. This method is based on the Fick principle and is determined by measuring ICG in the respiratory muscles using transcutaneous NIRS in relation to the [ICG] in arterial blood as measured using photodensitometry. This method is invasive since it requires arterial cannulation, repeated blood withdrawals, and reinfusions. A less invasive alternative is to calculate a relative measure of blood flow known as the blood flow index (BFI), which is based solely on the NIRS ICG curve, thus negating the need for arterial cannulation. Accordingly, the purpose of this study was to determine whether BFI can be used to measure RMBF at rest and during voluntary isocapnic hyperpnea at 25, 40, 55, and 70% of maximal voluntary ventilation in seven healthy humans. BFI was calculated as the change in maximal [ICG] divided by the rise time of the NIRS-derived ICG curve. Intercostal and sternocleidomastoid muscle BFI were correlated with simultaneously measured work of breathing and electromyography (EMG) data from the same muscles. BFI showed strong relationships with the work of breathing and EMG for both respiratory muscles. The coefficients of determination (R(2)) comparing BFI vs. the work of breathing for the intercostal and sternocleidomastoid muscles were 0.887 (P < 0.001) and 0.863 (P < 0.001), respectively, whereas the R(2) for BFI vs. EMG for the intercostal and sternocleidomastoid muscles were 0.879 (P < 0.001) and 0.930 (P < 0.001), respectively. These data suggest that the BFI closely reflects RMBF in conscious humans across a wide range of ventilations and provides a less invasive and less technically demanding alternative to measuring RMBF.     

Thixotropy of rib cage respiratory muscles in normal subjects.

In this study, we searched for signs of thixotropic behavior in human rib cage respiratory muscles. If rib cage respiratory muscles possess thixotropic properties similar to those seen in other skeletal muscles in animals and humans, we expect resting rib cage circumference would be temporarily changed after deep rib cage inflations or deflations and that these aftereffects would be particularly pronounced in trials that combine conditioning deep inflations or deflations with forceful isometric contractions of the respiratory muscles. We used induction plethysmography to obtain a continuous relative measure of rib cage circumference changes during quiet breathing in 12 healthy subjects. Rib cage position at the end of the expiratory phase (EEP) was used as an index of resting rib cage circumference. Comparisons were made between EEP values of five spontaneous breaths immediately before and after six types of conditioning maneuvers: deep inspiration (DI); deep expiration (DE); DI combined with forceful effort to inspire (FII) or expire (FEI); and DE combined with forceful effort to inspire (FIE) or expire (FEE), both with temporary airway occlusion. The aftereffects of the conditioning maneuvers on EEP values were consistent with the supposition that human respiratory muscles possess thixotropic properties. EEP values were significantly enhanced after all conditioning maneuvers involving DI, and the aftereffects were particularly pronounced in the FII and FEI trials. In contrast, EEP values were reduced after DE maneuvers. The aftereffects were statistically significant for the FEE and FIE, but not DE, trials. It is suggested that respiratory muscle thixotropy may contribute to the pulmonary hyperinflation seen in patients with chronic obstructive pulmonary disease.     

Reproducibility and responsiveness of a noninvasive EMG technique of the respiratory muscles in COPD patients and in healthy subjects.

In the present study, we assessed the reproducibility and responsiveness of transcutaneous electromyography (EMG) of the respiratory muscles in patients with chronic obstructive pulmonary disease (COPD) and healthy subjects during breathing against an inspiratory load. In seven healthy subjects and seven COPD patients, EMG signals of the frontal and dorsal diaphragm, intercostal muscles, abdominal muscles, and scalene muscles were derived on 2 different days, both during breathing at rest and during breathing through an inspiratory threshold device of 7, 14, and 21 cm H2O. For analysis, we used the logarithm of the ratio of the inspiratory activity during the subsequent loads and the activity at baseline [log EMG activity ratio (EMGAR)]. Reproducibility of the EMG was assessed by comparing the log EMGAR values measured at test days 1 and 2 in both groups. Responsiveness (sensitivity to change) of the EMG was assessed by comparing the log EMGAR values of the COPD patients to those of the healthy subjects at each load. During days 1 and 2, log EMGAR values of the diaphragm and the intercostal muscles correlated significantly. For the scalene muscles, significant correlations were found for the COPD patients. Although inspiratory muscle activity increased significantly during the subsequent loads in all participants, the COPD patients displayed a significantly greater increase in intercostal and left scalene muscle activity compared with the healthy subjects. In conclusion, the present study showed that the EMG technique is a reproducible and sensitive technique to assess breathing patterns in COPD patients and healthy subjects.     

Posterior cricoarytenoid and diaphragm activities during tidal breathing in neonates

To investigate airflow regulation in newborn infants, we recorded airflow, volume, diaphragm (Di), and laryngeal electromyogram (EMG) during spontaneous breathing in eight supine unsedated sleeping full-term neonates. Using an esophageal catheter electrode, we recorded phasic respiratory activity consistent with that of the principal laryngeal abductors, the posterior cricoarytenoids (PCA). Sequential activation of PCA and Di preceded inspiration. PCA activity typically peaked early in inspiration followed by either a decrescendo or tonic EMG activity of variable amplitude during expiration. Expiratory airflow retardation, or braking, accompanied by expiratory prolongation and reduced ventilation, was commonly observed. In some subjects we observed a time interval between PCA onset and a sudden increase in expiratory airflow just before inspiration, suggesting that release of the brake involved an abrupt loss of antagonistic adductor activity. Our findings suggest that airflow in newborn infants is controlled throughout the breathing cycle by the coordinated action of the Di and the reciprocal action of PCA and laryngeal adductor activities. We conclude that braking mechanisms in infants interact with vagal reflex mechanisms that modulate respiratory cycle timing to influence both the dynamic maintenance of end-expiratory lung volume and ventilation.