[The pneumology department]

The department of pneumology of the Erasme hospital exists since 25 years. The basic clinical activities include pulmonary function testing (7,500 patients per year), endoscopy, including interventional endoscopy (1,500 patients per year), thoracic oncology, allergology, rehabilitation and aid to smoking cessation. The following expertise fields have been largely developed: lung transplantation, treatment of cystic fibrosis in collaboration with the children's hospital Reine Fabiola, occupational.

Contribution of spindle reflexes to post-inspiratory activity in the canine external intercostal muscles

1. The external intercostal muscles have greater post-inspiratory activity than the parasternal intercostal muscles and are more abundantly supplied with muscle spindles. In the present study, the hypothesis was tested that spindle afferent inputs play a major role in determining this activity. 2. The electrical activity of the external and parasternal intercostal muscles in the rostral interspaces was recorded in anaesthetized spontaneously breathing dogs, and the ribs were manipulated so as to alter their normal caudal displacement and the normal lengthening of the muscles in early expiration. 3. Post-inspiratory activity in the external intercostal muscles showed a reflex decrease when the caudal motion of the ribs and the lengthening of the muscles was impeded, and it showed a reflex increase when the rate of caudal rib motion and muscle lengthening was increased. In contrast, the small post-inspiratory activity in the parasternal intercostal muscles remained unchanged. 4. When the two ribs making up the interspace investigated were locked to keep muscle length constant, post-inspiratory activity in the external intercostal muscles was reduced and no longer responded to cranial rib manipulation. 5. These observations confirm that afferent inputs from muscle receptors, presumably muscle spindles, are a primary determinant of post-inspiratory activity in the canine external intercostal muscles. In anaesthetized animals, the contribution of central control mechanisms to this activity is small.

Heterogeneity of metabolic activity in the canine parasternal intercostals during breathing

In the dog, the inspiratory mechanical advantage of the parasternal intercostals shows a marked spatial heterogeneity, whereas the expiratory mechanical advantage of the triangularis sterni is relatively uniform. The contribution of a particular respiratory muscle to lung volume expansion during breathing, however, depends both on the mechanical advantage of the muscle and on its neural input. To evaluate the distribution of neural input across the canine parasternal intercostals and triangularis sterni, we have examined the distribution of metabolic activity among these muscles in seven spontaneously breathing animals by measuring the uptake of the glucose tracer analog [(18)F]fluorodeoxyglucose (FDG). FDG uptake in any given parasternal intercostal was greatest in the medial bundles and decreased rapidly toward the costochondral junctions. In addition, FDG uptake in the medial parasternal bundles increased from the first to the second interspace, plateaued in the second through fifth interspaces, and then decreased progressively toward the eighth interspace. In contrast, uptake in the triangularis sterni showed no significant rostrocaudal gradient. These results overall strengthen the idea that the spatial distribution of neural input within a particular set of respiratory muscles is closely matched with the spatial distribution of mechanical advantage.

Respiratory effects of the external and internal intercostal muscles in humans

The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. Recent studies in dogs, however, have shown that this notion is only approximate. In the present studies, the respiratory actions of the human external and internal intercostal muscles were evaluated by applying the Maxwell reciprocity theorem. Thus the orientation of the muscle fibres relative to the ribs and the masses of the muscles were first assessed in cadavers. Five healthy individuals were then placed in a computed tomographic scanner to determine the geometry of the ribs and their precise transformation during passive inflation to total lung capacity. The fractional changes in length of lines with the orientation of the muscle fibres were then computed to obtain the mechanical advantages of the muscles. These values were finally multiplied by muscle mass and maximum active stress (3.0 kg cm-2) to evaluate the potential effects of the muscles on the lung. The external intercostal in the dorsal half of the second interspace was found to have a large inspiratory effect. However, this effect decreases rapidly in the caudal direction, in particular in the ventral portion of the ribcage. As a result, it is reversed into an expiratory effect in the ventral half of the sixth and eighth interspaces. The internal intercostals in the ventral half of the sixth and eighth interspaces have a large expiratory effect, but this effect decreases dorsally and cranially. The total pressure generated by all the external intercostals during a maximum contraction would be -15 cmH2O, and that generated by all the internal interosseous intercostals would be +40 cmH2O. These pressure changes are substantially greater than those induced by the parasternal intercostal and triangularis sterni muscles, respectively.

Response of the canine internal intercostal muscles to chest wall vibration

Although high-frequency mechanical vibration of the rib cage reduces dyspnea, its effects on the respiratory muscles are largely unknown. We have previously shown that in anesthetized dogs, vibrating the rib cage during inspiration elicits a marked increase in the inspiratory electromyographic (EMG) activity recorded from the external intercostal muscles but does not affect tidal volume (VT). In the present studies, we have tested the hypothesis that the maintenance of VT results from the concomitant contraction of the internal interosseous (expiratory) intercostals. When the rib cage was vibrated (40 Hz) during hyperventilation-induced apnea, a prominent activity was recorded from the external intercostals but no activity was recorded from the internal intercostals, including when the muscles were lengthened by passive inflation. The internal intercostals remained also silent when vibration was applied during spontaneous inspiration, and the phasic expiratory EMG activity recorded from them was unaltered when vibration was applied during expiration. Thus, the internal interosseous intercostals in dogs are much less sensitive to vibration than the external intercostals, and they do not interfere with the action of these latter during rib cage vibration. This lack of sensitivity might be the result of a reflex inhibition of the muscle spindle afferents by afferents from external intercostal muscle spindles.

Response of the rabbit diaphragm to tendon vibration

To evaluate the potential role of diaphragmatic muscle spindles in the act of breathing, we have recorded the electromyograms of the diaphragm and the external intercostal muscle in the third interspace during high-frequency mechanical vibration (50 Hz) of the central tendon in eight anesthetized, spontaneously breathing rabbits. Vibration induced a consistent, clear-cut increase in the inspiratory activity recorded from the external intercostal, thus indicating that the mechanical stimulus applied to the diaphragm was strong enough to trigger muscle spindles at distant sites. However, vibration did not elicit any alteration in costal or crural diaphragmatic activity in any animal. Similarly, when vibration was applied during hyperventilation-induced apnea, activity was recorded in the external intercostal but not in the diaphragm. These observations support the traditional view that the diaphragm is poorly endowed with muscle spindles and that these play little or no significant role in the act of breathing.

The canine parasternal and external intercostal muscles drive the ribs differently

1. In the dog, the elevation of the ribs during inspiration results from the combined actions of the parasternal and external intercostal muscles. In the present studies, the hypothesis was tested that co-ordinated activity among these two sets of muscles reduces the distortion of the rib cage. 2. During spontaneous inspiration before or after section of the phrenic nerves, the ribs moved cranially and outward in the same way as they did during passive inflation. However, whereas the sternum moved cranially during passive inflation, it was displaced caudally during spontaneous inspiration. 3. When the parasternal intercostal muscles were selectively denervated, both the sternum and the ribs moved cranially, but the rib outward displacement was markedly reduced. In contrast, when the external intercostals were excised and the parasternal intercostals were left intact, the sternum continued to move caudally and the outward displacement of the ribs was augmented relative to their cranial displacement. 4. These observations establish that the external intercostal muscles drive the ribs primarily in the cranial direction, whereas the parasternal intercostals drive the ribs both cranially and outward. They also indicate, in agreement with the hypothesis, that co-ordinated activity among these two sets of muscles displaces the ribs on their relaxation curve. 5. However, this co-ordinated activity also displaces the sternum caudally. Although this distortion requires an additional energy expenditure, it enhances the outward component of rib displacement which is more effective with respect to lung expansion.

Abdominal muscle strength in patients with tetraplegia

The abdominal muscles are completely paralyzed after traumatic transection of the cervical cord. To assess the residual pressure-generating capacity of these muscles, we first measured the changes in gastric pressure (DeltaPga) during paired bilateral stimulation of the lower thoracic nerve roots in eight chronic patients with C5-C7 tetraplegia and eight matched unaffected subjects in the seated posture. Stimulations were applied with a 90-mm circular magnetic coil positioned at the level of T10 and connected to a Magstim 250 stimulator. During relaxation at FRC, DeltaPga during maximal stimulation averaged (mean +/- SE) 76.0 +/- 11.7 cm H(2)O in the control subjects, whereas in the patients it was only 29.9 +/- 3.7 cm H(2)O (p = 0.002). Stimulations were next applied during the course of a forced expiration. All patients consistently demonstrated an abrupt increase in esophageal pressure (22.7 +/- 4.5 cm H(2)O), and six of them also showed an increase in expiratory flow. The cumulative thickness of the four abdominal muscles, as measured with an ultrasound probe, was 34% smaller in the patients than in the control subjects and correlated positively with maximal DeltaPga. We conclude that in patients with tetraplegia, muscle atrophy causes a marked reduction in abdominal muscle strength. However, magnetic stimulation of the abdominal muscles elicits increases in intrathoracic pressure that are greater than those required to initiate dynamic airway compression; it might, therefore, improve the clearing of airway secretions.

Response of the canine inspiratory intercostal muscles to chest wall vibration

High-frequency mechanical vibration of the rib cage reduces dyspnea, but the effect of this procedure on the respiratory muscles is largely unknown. In the present studies, we have initially assessed the electrical and mechanical response to vibration (40 Hz) of the canine parasternal and external intercostal muscles (third interspace) during hyperventilation-induced apnea. When the vibrator was applied to the segment investigated, prominent external intercostal activity was recorded in the seven animals studied, whereas low-amplitude parasternal intercostal activity was recorded in only four animals. Similarly, when the vibrator was applied to more rostral and more caudal interspaces, activity was recorded commonly from the external intercostal but only occasionally from the parasternal. The two muscles, however, showed similar changes in length. We next examined the response to vibration of the muscles in seven spontaneously breathing animals. Vibrating the rib cage during inspiration (in-phase) had no effect on parasternal intercostal inspiratory activity but induced a marked increase in neural drive to the external intercostals. For the animal group, peak external intercostal activity during the control, nonvibrated breaths averaged (mean +/- SE) 43.1 +/- 3.7% of the activity recorded during the vibrated breaths (p < 0.001). External intercostal activity during vibration also occurred earlier at the onset of inspiration and commonly carried on after the cessation of parasternal intercostal activity. Yet tidal volume was unchanged. Vibrating the rib cage during expiration (out-of-phase) did not elicit any parasternal or external intercostal activity in six animals. These observations thus indicate that the external intercostals, with their larger spindle density, are much more sensitive to chest wall vibration than the parasternal intercostals. They also suggest that the impact of this procedure on the mechanical behavior of the respiratory system is relatively small.

Effects of increased ventilatory drive on motor unit firing rates in human inspiratory muscles

This study was designed to determine whether increased neural drive increases firing rates of inspiratory motoneurons uniformly in humans. The discharge of single motor units in the diaphragm, parasternal intercostal and scalene muscles was recorded with monopolar electrodes. Ventilation was increased threefold with an external dead space. The discharge of 516 motor units was sampled in four subjects. All but 4 units increased their discharge rate during inspiration with only 46 discharging tonically during expiration. With increased dead space, discharge frequencies of diaphragmatic motor units increased from 11.0 +/- 2.7 to 17.7 +/- 3.3 Hz (mean +/- SD; p < 0.001). However, firing rates increased for parasternal intercostals from 10.0 +/- 1.6 to only 11.9 +/- 1.9 Hz (p < 0.001), and for scalenes from 8.7 +/- 1.8 to only 9.5 +/- 1.2 Hz (p < 0.05). Proportionate increases in rib cage and abdominal expansion accompanied the increased ventilation with added dead space. These results suggest that previously reported predominant increase in firing rates of diaphragmatic motor units in patients with chronic airflow limitation reflects the normal response of respiratory motor output to increased neural drive. The motoneuron pools of the parasternal intercostals and scalenes may show more prominent recruitment than frequency modulation.

Spatial distribution of external and internal intercostal activity in dogs

1. The observation that the external and internal interosseous intercostal muscles in the dog show marked regional differences in mechanical advantage has prompted us to re-examine the topographic distribution of electrical activity among these muscles during spontaneous breathing. 2. Inspiratory activity was recorded only from the areas of the external intercostals with an inspiratory mechanical advantage, and expiratory activity was recorded only from the areas of the internal intercostals with an expiratory mechanical advantage. The expiratory discharges previously recorded from the caudal external intercostals and the inspiratory discharges recorded from the rostral internal intercostals were probably due to cross-contamination. 3. Activity in each muscle area was also quantified relative to the activity measured during tetanic, supramaximal nerve stimulation (maximal activity). External intercostal inspiratory activity was consistently greater in the areas with a greater inspiratory advantage (i.e. the dorsal aspect of the rostral segments) than in the areas with a smaller inspiratory advantage, and internal intercostal expiratory activity was invariably greatest in the areas with the greatest expiratory advantage (i.e. the dorsal aspect of the caudal segments). 4. This topographic distribution of neural drive confers to the external intercostal muscles an inspiratory action on the lung during breathing and to the internal interosseous intercostals an expiratory action.

Respiratory mechanical advantage of the canine external and internal intercostal muscles

1. The current conventional view of intercostal muscle actions is based on the theory of Hamberger (1749) and maintains that as a result of the orientation of the muscle fibres, the external intercostals have an inspiratory action on the lung and the internal interosseous intercostals have an expiratory action. This notion, however, remains unproved. 2. In the present studies, the respiratory actions of the canine external and internal intercostal muscles were evaluated by applying the Maxwell reciprocity theorem. Thus the effects of passive inflation on the changes in length of the muscles throughout the rib cage were assessed, and the distributions of muscle mass were determined. The fractional changes in muscle length during inflation were then multiplied by muscle mass and maximum active stress (3.0 kg cm-2) to evaluate the potential effects of the muscles on the lung. 3. The external intercostals in the dorsal third of the rostral interspaces were found to have a large inspiratory effect. However, this effect decreases rapidly both toward the costochondral junctions and toward the base of the rib cage. As a result, it is reversed to an expiratory effect in the most caudal interspaces. The internal intercostals in the caudal interspaces have a large expiratory effect, but this effect decreases ventrally and rostrally, such that it is reversed to an inspiratory effect in the most rostral interspaces. 4. These observations indicate that the canine external and internal intercostal muscles do not have distinct inspiratory and expiratory actions as conventionally thought. Therefore, their effects on the lung during breathing will be determined by the topographic distribution of neural drive.

Determinants of rib motion in flail chest

We have previously developed a canine model of isolated flail chest to assess the effects of this condition on the mechanics of breathing, and these studies have led to the conclusion that the respiratory displacement of the fractured ribs is primarily determined by the fall in pleural pressure (Delta Ppl) and the action of the parasternal intercostal muscles. The present studies were designed to test the validity of this conclusion. A flail was induced in six supine anesthetized animals by fracturing both dorsally and ventrally the second to fifth ribs on the right side of the chest, after which the phrenic nerve roots were bilaterally sectioned in the neck. Sectioning the phrenic nerves caused a 34% decrease in Delta Ppl, associated with a 39% increase in parasternal intercostal inspiratory EMG activity (p < 0.05), and resulted in a marked reduction in the inspiratory inward displacement of the ribs. In three animals, the inward rib displacement was even reversed into a small outward displacement. When the airway was then occluded at end-expiration to increase Delta Ppl during the subsequent inspiration, all animals again showed a clear-cut inward rib displacement. These observations therefore confirm that in dogs with flail chest, the inspiratory displacement of the fractured ribs is set by the balance between the force related to pleural pressure and that generated by the parasternal intercostals. These observations also point to the critical importance of the pattern of inspiratory muscle activation in determining the magnitude of rib cage paradox in such patients.

Reflex inhibition of canine inspiratory intercostals by diaphragmatic tension receptors

1. Electrical stimulation of phrenic afferent fibres in the dog elicits a reflex inhibition of efferent activity to the inspiratory intercostal muscles. However, electrical stimulation has a poor selectivity, so the sensory receptors responsible for this inhibition were not identified. 2. In the present studies, cranial forces were applied during spontaneous inspiration to the abdominal surface of the central, tendinous portion of the canine diaphragm to activate tension mechanoreceptors in the muscle. Vagal afferent inputs were eliminated by vagotomy. 3. The application of force to the central tendon caused a graded, reflex reduction in inspiratory intercostal activity, especially in external intercostal activity. This reduction was commonly associated with a decrease in inspiratory duration and was invariably attenuated after section of the cervical dorsal roots. 4. In contrast, no change in inspiratory intercostal activity was seen when high frequency mechanical vibration was applied to the central tendon to stimulate diaphragmatic muscle spindles. 5. These observations provide strong evidence that tension receptors in the diaphragm, but not muscle spindles, induce reflex inhibition of inspiratory intercostal activity. The expression of this reflex probably involves supraspinal structures.

Mechanical advantage of the human parasternal intercostal and triangularis sterni muscles

1. Previous studies in dogs have demonstrated that the maximum change in airway pressure (DeltaPao) produced by a particular respiratory muscle is the product of three factors, namely the mass of the muscle, the maximal active muscle tension per unit cross-sectional area ( approximately 3.0 kg cm-2), and the fractional change in muscle length per unit volume increase of the relaxed chest wall (i.e. the muscle's mechanical advantage). In the present studies, we have used this principle to infer the DeltaPao values generated by the parasternal intercostal and triangularis sterni muscles in man. 2. The mass of the muscles and the direction of the muscle fibres relative to the sternum were first assessed in six cadavers. Seven healthy individuals were then placed in a computed tomographic scanner to determine the orientation of the costal cartilages relative to the sternum and their rotation during passive inflation to total lung capacity. The fractional changes in length of the muscles during inflation, their mechanical advantages, and their DeltaPao values were then calculated. 3. Passive inflation induced shortening of the parasternal intercostals in all interspaces and lengthening of the triangularis sterni. The fractional shortening of the parasternal intercostals decreased gradually from 7.7 % in the second interspace to 2.0 % in the fifth, whereas the fractional lengthening of the triangularis sterni increased progressively from 5.9 to 13.8 %. These rostrocaudal gradients were well accounted for by the more caudal orientation of the cartilages of the lower ribs. 4. Since these fractional changes in length corresponded to a maximal inflation, the inspiratory mechanical advantage of the parasternal intercostals was only 2.2-0. 6 % l-1, and the expiratory mechanical advantage of the triangularis sterni was only 1.6-3.8 % l-1. In addition, whatever the interspace, parasternal and triangularis muscle mass was 3-5 and 1-3 g, respectively. As a result, the magnitude of the DeltaPao values generated by a maximal contraction of the parasternal intercostals or triangularis sterni in all interspaces would be only 1-3 cmH2O. 5. These studies therefore confirm that the parasternal intercostals in man have an inspiratory action on the lung whereas the triangularis sterni has an expiratory action. However, these studies also establish the important fact that the pressure-generating ability of both muscles is substantially smaller than in the dog.

Neck and abdominal muscle activity in patients with severe thoracic scoliosis

Patients with severe chronic obstructive pulmonary disease (COPD) do not use the sternocleidomastoid muscles when breathing at rest, but have a greater than normal neural drive to the rib-cage inspiratory muscles, the abdominal muscles, and the diaphragm. Yet the increased activation of the abdominal muscles and diaphragm in such patients has only limited mechanical effects, and this has led to the suggestion that the overall increase in neural drive is simply an automatic response of the respiratory system to a greater than resting stimulation. To test this hypothesis, we examined the pattern of respiratory-muscle activation in eight patients with severe thoracic scoliosis (Cobb angle between 100 degrees and 136 degrees). We recorded electromyograms of the sternocleidomastoid, scalene, rectus abdominis, external oblique, and transversus abdominis muscles; esophageal (Pes) and gastric (Pga) pressures; and the anteroposterior (AP) diameter of the abdomen during resting breathing in the seated posture. All patients had invariable phasic inspiratory activity in the scalenes; and five patients had invariable phasic expiratory activity in the transversus; intermittent expiratory activity in the transversus was also recorded in three patients. In contrast, only one patient had invariable phasic inspiratory activity in the sternocleidomastoid, and only one patient had invariable phasic expiratory activity in the external oblique. The decrease in abdominal AP diameter during expiration was commonly associated with a rise in Pga. These observations therefore indicate that the pattern of respiratory-muscle activation in patients with severe thoracic scoliosis is essentially similar to that seen in patients with severe COPD. This supports the concept that the order of recruitment of the respiratory muscles during breathing is an automatic response of the central controller.

Rib cage muscle interaction in airway pressure generation

We have previously demonstrated in dogs that the change in airway opening pressure (DeltaPao) produced by isolated maximum activation of the parasternal intercostal or triangularis sterni muscle in a single interspace, the sternomastoids, and the scalenes is proportional to the product of muscle mass and the fractional change in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have assessed the interactions between these muscles by comparing the DeltaPao obtained during simultaneous activation of a pair of muscles (measured DeltaPao) to the sum of the DeltaPao values obtained during their separate activation (predicted DeltaPao). Measured and predicted DeltaPao values were compared for the following pairs of muscles: the parasternal intercostals in two interspaces, the parasternal intercostals in one interspace and either the sternomastoids or the scalenes, two segments of the triangularis sterni, and the interosseous intercostals in two contiguous interspaces. For all these pairs, the measured DeltaPao was within approximately 10% of the predicted value. We therefore conclude that 1) the pressure changes generated by the rib cage muscles are essentially additive; and 2) measurements of the mass of a particular muscle and of its fractional change in length during passive inflation can be used to estimate the potential pressure-generating ability of the muscle during coordinated activity as well as during isolated activation.

[Respiration mechanics in tetraplegia]

Patients with quadriplegia due to transection of the lower cervical cord show, on spirographic examination, a marked decrease in vital capacity and its two components, i.e. inspiratory capacity (i.c.) and expiratory reserve volume (ERV). The loss of IC results partly from the decreased inspiratory muscle strength consecutive to the intercostal muscle paralysis but mostly from a reduction in the distensibility of the lungs and the rib cage. The reduction in ERV is related to the paralysis of all the well-recognized muscles of expiration (abdominals, interosseous internal intercostals); however, the clavicular portion of the pectoralis major allows these patients to maintain a small ERV.

Mechanical advantage of the canine triangularis sterni

Recent studies on the canine parasternal intercostal, sternomastoid, and scalene muscles have shown that the maximal changes in airway opening pressure (delta Pao) obtained per unit muscle mass (delta Pao/m) during isolated contraction are closely related to the fractional changes in muscle length per unit volume increase of the relaxed chest wall. In the present study, we have examined the validity of this relationship for the triangularis sterni, an important expiratory muscle of the rib cage in dogs. Passive inflation above functional residual capacity (FRC) induced a virtually linear increase in muscle length, such that, with a 1.0-liter inflation, the muscle lengthened by 17.9 +/- 1.6 (SE) % of its FRC length. When the muscle in one interspace was maximally stimulated at FRC, Pao increased by 0.84 +/- 0.11 cmH2O. However, in agreement with the length-tension characteristics of the muscle, when lung volume was increased by 1.0 liter before stimulation, the rise in Pao amounted to 1.75 +/- 0.12 cmH2O. At the higher volume, delta Pao/m therefore averaged +.053 +/- 0.05 cmH2O/g, such that the coefficient of proportionality between the change in triangularis sterni length during passive inflation and delta Pao/m was the same as that previously obtained for the parasternal intercostal and neck inspiratory muscles. These observations, therefore, confirm that there is a unique relationship between the fractional changes in length of the respiratory muscles, both inspiratory and expiratory, during passive inflation and their delta Pao/m. Consequently, the maximal effect of a particular muscle on the lung can be predicted on the basis of its change in length during passive inflation and its mass. A geometric analysis of the rib cage also established that the lengthening of the canine triangularis sterni during passive inflation is much greater than the shortening of the parasternal intercostals because, in dogs, the costal cartilages slope downward from the sternum.

Effects of abdominal strapping on forced expiration in tetraplegic patients

Patients with traumatic transection of the lower segments of the cervical cord contract the clavicular portion of the pectoralis major during forced expiration and cough, and the rise in intrathoracic pressure resulting from this contraction produces dynamic airway compression in many patients. Because the abdominal muscles are paralyzed, however, there is paradoxical expansion of the abdomen, which may reduce the rise in intrathoracic pressure and the degree of airway collapse. To evaluate the magnitude of this effect, we measured expiratory flow rate (Vexp) and esophageal pressure (Pes) during a series of forced expiratory vital capacity maneuvers and constructed isovolume-pressure flow (IVPF) curves before and after abdominal strapping in eight C5-8 tetraplegic subjects. Strapping produced small and inconsistent changes in maximal Vexp and Pes and resulted in the development of small flow plateaus in only four patients. In tetraplegic subjects, abdominal strapping thus has small effects on forced expiration and is unlikely, therefore, to improve the efficiency of cough.

Role of joint receptors in modulation of inspiratory intercostal activity by rib motion in dogs

1. Inspiratory activity in the canine external intercostal muscles is exquisitely sensitive to the direction and amplitude of the inspiratory displacement of the ribs. This study was designed to investigate the role of muscle receptors, in particular the muscle spindles, in mediating this phenomenon. 2. External intercostal inspiratory activity showed a reflex increase when the normal cranial motion of the ribs and the normal shortening of the muscles was reduced, and showed a reflex decrease when the cranial motion of the ribs and the shortening of the muscles was augmented. However, clamping the two ribs making up the interspace and maintaining muscle length constant only moderately attenuated these responses. 3. These persistent responses remained unchanged after section of the levator costae muscles. 4. The responses were attenuated but still present after section of the external intercostals in the contiguous segments and denervation of the internal intercostals. 5. These reflex responses are therefore mediated in part by non-muscular receptors, which most likely lie within the costovertebral joints. These joint receptors might be a primary determinant of the load-compensating reflex.

Mechanical advantage of sternomastoid and scalene muscles in dogs

Theoretical studies have led to the prediction that the maximal effect of a given respiratory muscle on airway opening pressure (Pao) is the product of muscle mass, the maximal active muscle tension per unit cross-sectional area, and the fractional change in muscle length per unit volume increase of the relaxed chest wall. It has previously been shown that the parasternal intercostals behave in agreement with this prediction (A. De Troyer, A. Legrand, and T. A. Wilson. J. Physiol. (Lond) 495: 239-246, 1996; A. Legrand, T. A. Wilson, and A. De Troyer. J. Appl. Physiol. 80: 2097-2101, 1996). In the present study, we have tested the prediction further by measuring the response to passive inflation and the pressure-generating ability of the sternomastoid and scalene muscles in eight anesthetized dogs. With 1-liter passive inflation, the sternomastoids and scalenes shortened by 2.03 +/- 0.17 and 5.98 +/- 0.43%, respectively, of their relaxation length (P < 0.001). During maximal stimulation, the two muscles caused similar falls in Pao. However, the sternomastoids had greater mass such that the change in Pao (delta Pao) per unit muscle mass was -0.19 +/- 0.02 cmH2O/g for the scalenes and only -0.07 +/- 0.01 cmH2O/g for the sternomastoids (P < 0.001). After extension of the neck, there was a reduction in both the muscle shortening during passive inflation and the fall in Pao during stimulation. The delta Pao per unit muscle mass was thus closely related to the change in length; the slope of the relationship was 3.1. These observations further support the concept that the fractional changes in length of the respiratory muscles during passive inflation can be used to predict their pressure-generating ability.

On the intercostal muscle compensation for diaphragmatic paralysis in the dog

1. Paralysis of the diaphragm in the dog is known to cause a compensatory increase in activation of the inspiratory intercostal muscles (parasternal intercostals, external intercostals, and levator costae). The present studies were designed to assess the mechanism(s) of that compensation. 2. Complete, selective diaphragmatic paralysis was induced by injecting local anaesthetic into small silicone cuffs placed around the phrenic nerve roots in the neck. 3. Paralysis produced a decrease in tidal volume and an increase in arterial P(CO2) (P(a,CO2)). The increased hypercapnic drive was a primary determinant of the increased inspiratory intercostal activity. 4. However, paralysis also produced an increased inspiratory cranial displacement of the ribs. When this increased rib displacement was reduced to that seen before paralysis, it appeared that the increase in external intercostal and levator costae inspiratory activity was commonly greater than anticipated on the basis of the increased P(a,CO2). 5. Diaphragmatic paralysis after bilateral vagotomy also elicited disproportionate increases in inspiratory intercostal activity, thus indicating that these increases are not caused by vagal afferent inputs. 6. These observations are consistent with the idea that the intercostal muscle compensation for diaphragmatic paralysis is, in part, due to the release of an inhibition originating from the contracting diaphragm. This inhibition might arise in the diaphragmatic tendon organs.

Neural drive to the diaphragm in patients with severe COPD

Patients with severe chronic obstructive pulmonary disease (COPD) have a greater neural drive to the parasternal intercostal and scalene muscles and greater inspiratory expansion of the rib cage than do healthy individuals. However, such patients also have a reduced outward displacement or a paradoxical inward displacement of the ventral abdominal wall during inspiration. This has led to the suggestion that they may have less use of the diaphragm, possibly secondary to chronic muscle fatigue. To assess the effect of COPD on the neural drive to the diaphragm, we inserted needle electrodes into the costal part of the right hemidiaphragm in eight patients with severe disease (mean [+/- SD] FEV1: 0.82 [+/- 0.27] L) and six control subjects of similar age, and measured the discharge frequencies of single motor units during resting breathing. A total of 115 diaphragmatic motor units were recorded in the control subjects and 122 in the patients. All motor units discharged rhythmically in phase with inspiration. However, whereas 95% of the units in the control subjects had a peak discharge frequency between 7 and 14 Hz, 79% of the units in the COPD patients had a peak discharge frequency greater than 15 Hz. As a result, the discharge frequency of all units averaged 10.5 [+/- 2.4] Hz in the control subjects, but 17.9 [+/- 4.3] Hz in the patients (p < 0.001). These observations indicate that patients with severe COPD have an increased neural drive not only to the rib cage inspiratory muscles, but also to the diaphragm. Consequently, the reduced inspiratory expansion of the abdomen in severe COPD results from mechanical factors alone.

Effect of hyperinflation on the diaphragm

Acute hyperinflation causes the inspiratory muscles to operate at shorter than normal lengths. The ability of these muscles, in particular the diaphragm, to lower intrathoracic pressure is therefore reduced. Skeletal muscles, however, adapt to chronic shortening, and animals models of emphysema have shown that with chronic hyperinflation, the diaphragmatic muscle fibres lose sacromeres. As a result, the force-generating ability of these fibres is relatively preserved. In patients with hyperinflation due to chronic obstructive pulmonary disease, the ability of the diaphragm to generate pressure is also better than anticipated on the basis of hyperinflation alone. However, the diaphragm in these patients is also lower in the chest wall than in healthy subjects. Consequently, even though the neural drive to the muscle is greater than normal, its ability to descend during inspiration is impaired. Its rib cage expanding action is also reduced; in patients with severe hyperinflation, contraction of the diaphragm even produces deflation, rather than expansion, of the rib cage. In such patients, therefore, the ability of the diaphragm to increase lung volume is reduced, and hence the act of breathing is more dependent on the rib cage inspiratory muscles.

Effect of hyperinflation on the diaphragm

Acute hyperinflation causes the inspiratory muscles to operate at shorter than normal lengths. The ability of these muscles, in particular the diaphragm, to lower intrathoracic pressure is therefore reduced. Skeletal muscles, however, adapt to chronic shortening, and animals models of emphysema have shown that with chronic hyperinflation, the diaphragmatic muscle fibres lose sacromeres. As a result, the force-generating ability of these fibres is relatively preserved. In patients with hyperinflation due to chronic obstructive pulmonary disease, the ability of the diaphragm to generate pressure is also better than anticipated on the basis of hyperinflation alone. However, the diaphragm in these patients is also lower in the chest wall than in healthy subjects. Consequently, even though the neural drive to the muscle is greater than normal, its ability to descend during inspiration is impaired. Its rib cage expanding action is also reduced; in patients with severe hyperinflation, contraction of the diaphragm even produces deflation, rather than expansion, of the rib cage. In such patients, therefore, the ability of the diaphragm to increase lung volume is reduced, and hence the act of breathing is more dependent on the rib cage inspiratory muscles.

Actions of the inspiratory intercostal muscles in flail chest

We have previously shown in dogs that the ribs in flail chest move paradoxically inward during inspiration but continue to move cranially. We have also shown that flail elicits, probably via an increased activation of the muscle spindles, a threefold to fourfold increase in external intercostal inspiratory EMG activity without inducing any changes in parasternal intercostal activity. Therefore, the present studies were undertaken to test the hypothesis that the persistent cranial motion of the fractured ribs resulted primarily from the action of the external intercostals. A flail was induced in seven supine anesthetized animals by fracturing both dorsally and ventrally ribs 3 to 6 on the right side of the chest, after which the external intercostal muscles in interspaces 1 to 7 were severed. Severing the external intercostals caused a small increase in the inspiratory inward displacement of the fractured ribs, from 2.76 +/- 0.31 to 3.25 +/- 0.38 mm (p < 0.05), but it did not affect the parasternal intercostal EMG activity or the cranial rib displacement (before, 3.61 +/- 1.03 mm; after, 3.22 +/- 1.43 mm; NS). However, when the parasternal intercostals in interspaces 1 to 7 were also denervated, the inspiratory inward displacement of the ribs increased markedly to 5.95 +/- 0.48 mm (p < 0.01), and their inspiratory cranial displacement was reversed into a 1.05 +/- 0.58 mm inspiratory caudal displacement (p < 0.01). We conclude, therefore, that in dogs with flail chest the respiratory displacements of the ribs are still primarily determined, besides pleural pressure, by the action of the parasternal intercostals. These observations also suggest that in anesthetized dogs, spindle-induced excitation of the external intercostals has little impact on the mechanical behavior of the ribs.

Rostrocaudal gradient of electrical activation in the parasternal intercostal muscles of the dog

1. Because the inspiratory mechanical advantage of the canine parasternal intercostal muscles is greatest in the third interspace and decreases gradually in the caudal direction, the electromyograms of these muscles in interspaces 3, 5 and 7 have been recorded in anaesthetized, spontaneously breathing dogs. Each activity was expressed as a percentage of the activity measured during tetanic, supramaximal stimulation of the internal intercostal nerve (maximal activity). 2. Parasternal inspiratory activity during resting, room air breathing was invariably greater in the third than in the fifth interspace (62.0 +/- 6.0 vs. 41.3 +/- 4.6% of maximal activity; P < 0.001) and smallest in the seventh interspace (22.8 +/- 2.7% of maximal activity; P < 0.001). This distribution of activity persisted during hyperoxic hypercapnia and during breathing against increased inspiratory airflow resistance. 3. This rostrocaudal distribution of activity also persisted after complete paralysis of the diaphragm as well as after deafferentation of the ribcage. 4. Studies of the distribution of the muscle fibre types indicated that the parasternal intercostals in all interspaces had a higher proportion of slow-twitch oxidative (SO; type I) fibres than fast-twitch oxidative-glycolytic (FOG; type II a) fibres. 5. Thus the topographic distribution of parasternal inspiratory activity along the rostrocaudal axis of the ribcage is precisely matched with the topographic distribution of mechanical advantage. This extraordinarily effective pattern of activation probably results from the unequal distribution of central inputs throughout the parasternal motoneurone pool.

Rostrocaudal gradient of mechanical advantage in the parasternal intercostal muscles of the dog

1. Previous theoretical studies have led to the predictions that, in the dog, the parasternal intercostal muscles in the rostral interspaces shorten more during passive inflation than those in the caudal interspaces and have, therefore, a greater inspiratory mechanical advantage. The present studies were undertaken to test these predictions. 2. The effects of passive inflation on the length of the parasternal intercostals interspaces 1 to 7 were evaluated with markers implanted in the costal cartilages. Although the muscles in all interspaces shortened with passive inflation, the fractional shortening increased from the first to the second and third interspaces and then decreased continuously to the seventh interspace. 3. To understand this peculiar distribution, a geometric model of the parasternal area was then developed and a relation was obtained between muscle shortening and the angles that describe the orientation of the muscle and costal cartilage relative to the sternum. Measurement of these angles indicated that the rostrocaudal gradient of parasternal shortening resulted from the different orientations of the costal cartilages and their different rotations during passive inflation. 4. The changes in airway pressure generated by the parasternal intercostals in interspaces 3, 5 and 7 were finally measured during selective, maximal stimulation. The fall in pressure was invariably greatest during contraction of the third interspace and smallest during contraction of the seventh. 5. These observations indicate that, in the dog, the rostrocaudal gradient in rib rotation induces a rostrocaudal gradient of mechanical advantage in the parasternal intercostals, which has its climax in the second and third interspaces. These observations also support the concept that the respiratory effect of a given respiratory muscle can be computed from its behaviour during passive inflation.

Mediolateral gradient of mechanical advantage in the canine parasternal intercostals

Previous theoretical studies have postulated that the potential effect of a given respiratory muscle on lung volume or pleural pressure (i.e., its respiratory effect) is proportional to the change in length of the muscle during inflation of the passive chest wall (T. A. Wilson and A. De Troyer J. Appl. Physiol. 73: 2283-2288, 1992). To test this prediction, we have studied the parasternal intercostals in 18 interspaces in 8 supine anesthetized dogs. In each interspace, we have measured the changes in length of the medial and lateral portions of the parasternal during passive inflation and we have also assessed the changes in airway opening pressure (delta Pao) generated by these portions during isolated bilateral stimulation of the internal intercostal nerve. The results showed that 1) the medial fibers shorten more than the lateral fibers during passive inflation (P < 0.001); 2) when stimulated, the medial portion generated a larger fall in Pao than the lateral portion (P < 0.001); and 3) delta Pao was closely related to change in length (r = 0.81; P < 0.001). These observations thus imply that the medial portion of the parasternal intercostals contributes much more to lung expansion during breathing than the lateral portion. These observations also suggest, in agreement with the theoretical prediction, that measurements of the changes in length of the different respiratory muscles during passive inflation can be used to predict the potential respiratory effect of these muscles and to compare their mechanical advantages.

Respiratory muscle response to flail chest

We have previously shown that flail chest in the dog causes an inspiratory inward displacement of the ribs and an increased inspiratory activity in the external intercostal muscles, and we have speculated that this increased activity is due to an increased spindle afferent activity. The present studies were designed to test this hypothesis. Twenty-nine supine anesthetized dogs were studied, and flail was produced surgically by fracturing ventrally and dorsally two to four contiguous ribs on the right side of the chest. Although flail elicited an increased inspiratory activity in the external intercostal and levator costae muscles in the disconnected segment of the rib cage, it did not alter the inspiratory activity in the diaphragm and parasternal intercostals. Expiratory activity in the triangularis sterni, internal intercostals, and transversus abdominis remained unchanged also, as did the inspiratory activity in the external intercostals on the left side of the chest. After flail, the normal inspiratory shortening of the external intercostal muscles in the disconnected segment was also reversed into an inspiratory muscle lengthening. However, when the fractured ribs were connected to the adjacent ribs so that the external intercostals were prevented from lengthening during inspiration, external intercostal and levator costae inspiratory activity was unaltered. These observations support the hypothesis that the increased external intercostal muscle activity seen in flail chest results primarily from an increased activation of the muscle spindles.

On the mechanism of the mediolateral gradient of parasternal activation

Recent studies have shown that in spontaneously breathing dogs the parasternal intercostals are activated according to a mediolateral gradient. To assess the mechanism of this regionalization of activity, we assessed the pattern of activation of these muscles after section of the dorsal roots and examined the topographic distribution of the muscle fiber types from the sternum to the chondrocostal junctions. The pattern of parasternal activity after dorsal rhizotomy was similar in all respects to that previously observed in intact animals. Thus activity in the medial parasternal bundles at the onset of inspiration frequently preceded activity in the middle bundles, and no activity was recorded from the lateral bundles. The amount of medial activity, when expressed as a percentage of the activity recorded during supramaximal tetanic stimulation of the internal intercostal nerve (maximal activity), was also consistently greater than the amount of middle activity (52.6 +/- 4.6 vs. 23.1 +/- 2.6% maximal activity; P < 0.001). Furthermore, the medial, middle, and lateral parasternal bundles had a higher proportion of slow-twitch oxidative fibers than of fast-twitch oxidative-glycolytic fibers; no topographic difference in fiber type distribution was observed. We conclude, therefore, that the mediolateral gradient of parasternal activity is probably due to the unequal distribution of central inputs throughout the pool of alpha-motoneurons.

Rib motion modulates inspiratory intercostal activity in dogs

1. A test was performed of the hypothesis that the motion of the ribs during inspiration modulates, via changes in spindle afferent activity, the activation of the inspiratory intercostal muscles. The electrical activity of the parasternal intercostal, external intercostal, and levator costae muscles in anaesthetized spontaneously breathing dogs was thus recorded during manipulation of the inspiratory displacement of the ribs over a wide range of rib motion. 2. In agreement with the hypothesis, the external intercostal and levator costae muscles lengthened and showed increased inspiratory activities when the normal inspiratory cranial motion of the lower rib was reduced or reversed into an inspiratory caudal motion. Conversely, the inspiratory activities decreased when the inspiratory cranial motion of the rib and the inspiratory shortening of the muscles was augmented. The inspiratory activity of the parasternal intercostal remained unchanged throughout. 3. However, when the two ribs making up the interspace were linked together so that the external intercostal muscle was constant in length, the relationship of muscle activity to rib motion was maintained. 4. In addition, when the upper rather than the lower rib of the interspace was manipulated, the relationship between the change in muscle length and inspiratory activity was reversed, so that activity decreased when the muscle was lengthened and increased when the muscle was shortened. The relationship of muscle activity to lower rib motion, however, was still maintained. 5. These observations thus indicate that rib motion triggers proprioceptive reflexes which, regardless of the changes in length of the individual muscles, make the external intercostal inspiratory activity exquisitely sensitive to the direction of rib displacement.

Discharge frequencies of parasternal intercostal and scalene motor units during breathing in normal and COPD subjects

To determine whether patients with chronic obstructive pulmonary disease (COPD) contract the inspiratory muscles of the rib cage more strongly than do healthy subjects, we recorded the discharge frequencies of single motor units in the scalene and second parasternal intercostal muscles of seven patients with stable COPD (FEV1 = 33 +/- 13% predicted, mean +/- SD) and seven control subjects. Recordings were made with insulated monopolar electrodes during resting breathing, and single motor-unit discharges were identified with a customized method based on "template" matching. A total of 211 motor units were recorded in the control subjects and 260 in the patients. The inspiratory discharge frequencies were greater in the COPD patients than in the control subjects for both the parasternal (13.4 versus 10.1 Hz, p < 0.05) and scalene (11.4 versus 8.5 Hz, p < 0.02) muscles. Recording sites at which no motor units were recruited were more common in the control subjects than in the patients (p < 0.001). The sternomastoid muscle was silent in both subject groups. Therefore, effective central neural drive is increased to both the scalene and parasternal intercostal muscles but not to the sternomastoid muscle in patients with COPD.

Sternomastoid muscle size and strength in patients with severe chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) imposes a major strain on the respiratory muscle pump, and it is conventionally thought that the inspiratory muscles of the neck adapt to this chronic overload by developing hypertrophy. Yet previous anthropometric studies have shown atrophy of the sternomastoid muscles. To solve this discrepancy, we have measured the cross-sectional area of these muscles by computed tomography. Ten stable patients with severe airflow obstruction (FEV1 = 0.76 +/- 0.12 L) and hyperinflation (FRC = 210 +/- 29% of predicted) and 10 control subjects matched for age, sex, and height were studied. The sternomastoid cross-sectional area in the patients averaged (mean +/- SD) 4.29 +/- 1.48 cm2, and that in the control subjects was 3.96 +/- 0.82 cm2. This small difference could be entirely accounted for by hyperinflation, and it was not statistically significant. Sternomastoid muscle torque in patients was also similar to that in the control subjects. In patients with severe COPD, therefore, the sternomastoid muscles are essentially normal. As a corollary, their frequent prominence on clinical examination is only apparent.

Response of the inspiratory intercostal [correction of intercoastal] muscles to increased inertial loads

To test the hypothesis that the external intercostals and levator costae constitute an inspiratory reserve system, we have examined the response of these muscles to increased inertial loads. Weights were t hus attached sequentially to the ribs in ten lightly anesthetized, spontaneously breathing dogs. As weights were attached, the ribs were progressively displaced caudally at end-expiration, so that the external intercostal muscles were lengthened. In addition, the cranial motion of the ribs during inspiration was gradually reduced, the inspiratory shortening of external intercostal disappeared, and the external intercostal and levator costae inspiratory EMG activities increased. The parasternal intercostal inspiratory activity, however, remained unchanged. Studies also showed that: (1) the increases in external intercostal activity appeared with the first loaded breath and disappeared as soon as the load was removed; (2) these increases were related to the suppression of the inspiratory muscle shortening, rather than to the increase in precontraction muscle length or to vagal inputs; and (3) denervation of the external intercostal caused inspiratory muscle lengthening but had little effect on the inspiratory motion of the ribs. These observations thus indicate that increased inertial loads on the ribs trigger reflexes, possibly spindle reflexes, which cause selective increases in external intercostal and levator costae inspiratory EMG activities. In that sense, the present findings are consistent with the idea that these two muscles constitute an inspiratory reserve system. However, it appears that the major effect of these increased activities is simply to prevent the muscles from lengthening during inspiration.

Inhomogeneous activation of the parasternal intercostals during breathing

Recent computations of the mechanical advantage of the canine intercostal muscles have suggested that the inspiratory advantage of the parasternal intercostals is not uniform. In the present studies, we have initially tested this hypothesis. Using a caliper and markers implanted in the costal cartilages, we have thus measured, in four supine paralyzed dogs, the length of the medial, middle, and lateral parasternal fibers at functional residual capacity and after a 1-liter mechanical inflation. With inflation, the medial fibers always shortened more than did the middle fibers (-9.8 +/- 0.8 vs. -6.0 +/- 0.8%; P < 0.001), whereas the lateral fibers remained virtually constant in length (-0.2 +/- 0.8%). This gradient of mechanical advantage agreed well with the gradient of orientation of the muscle fibers. Therefore, we have also recorded the electromyograms of the medial, middle, and lateral parasternal bundles during spontaneous breathing in nine anesthetized animals (20 interspaces); each activity was expressed as a percentage of the activity recorded during tetanic, supramaximal stimulation of the internal intercostal nerve (maximal activity). The medial bundle was invariably more active than was the middle bundle during resting breathing (57.3 +/- 3.3 vs. 25.5 +/- 3.4% of maximum; P < 0.001), and in 10 interspaces, medial activity consistently preceded middle activity at the onset of inspiration. These differences persisted during hypercapnia, during inspiratory resistive loading, as well as after phrenicotomy. Activity was never recorded from the lateral bundle.(ABSTRACT TRUNCATED AT 250 WORDS)

Rib cage distortion in a canine model of flail chest

Although blunt chest injuries frequently lead to respiratory failure, the effects of flail chest on the mechanics of breathing have not been evaluated. In the present studies, we have measured the respiratory displacements of the ribs and sternum and the electromyograms (EMG) of the parasternal and external intercostal muscles in eight supine, anesthetized, spontaneously breathing dogs before and after the third to sixth ribs on the right side of the chest were fractured both dorsally and ventrally. After flail, the fractured ribs moved inward, rather than outward, during inspiration, but their inspiratory cranial displacement remained unchanged. The inspiratory outward and caudal displacement of the sternum, the inspiratory EMG activity of the parasternal intercostals, the pattern of breathing, and the arterial blood gases were also unaltered. However, the inspiratory EMG activity recorded from the external intercostals increased consistently to 327 +/- 101% of control (p < 0.05). These observations indicate that with flail chest, the disconnected segment of the rib cage shows paradoxical motion exclusively along the lateral axis; the increased external intercostal activation may account, at least in part, for the persistent inspiratory cranial motion of the ribs. These observations also suggest that the harmful effects of blunt chest injuries are related to pulmonary contusion and pain, rather than to flail chest per se.

Evidence of dynamic airway compression during cough in tetraplegic patients

Although all the well-recognized muscles of expiration are paralyzed after traumatic transection of the lower cervical cord, tetraplegic subjects can still empty their lungs actively by contracting the clavicular portion of the pectoralis major. It is not known, however, whether contraction of this muscle bundle may raise pleural pressure enough to cause dynamic compression of the intrathoracic airways, which is critical for the production of an effective cough. To investigate this question, we measured expiratory flow rate and esophageal pressure during a series of forced expiratory vital capacity (VC) maneuvers in twelve subjects with C5-8 traumatic tetraplegia and constructed isovolume-pressure flow (IVPF) curves. The curves were interpretable with certainty in nine patients. Three of them did not show any plateau of flow. On the other hand, six patients had clearcut plateaus of flow on all IVPF curves between 80-60 and 20% VC, suggesting they had dynamic airway compression. Videoendoscopic recordings in two patients confirmed trachea and main bronchi collapse during forced expiration and cough. We conclude, therefore, that contraction of the pectoralis major causes dynamic airway compression during expiratory efforts in a substantial proportion of tetraplegic subjects. Increasing the pressure-generating capacity of this muscle might thus improve the effectiveness of cough and reduce the prevalence of bronchopulmonary infections.

Intercostal muscle compensation for parasternal paralysis in the dog: central and proprioceptive mechanisms

1. Denervation of the parasternal intercostal muscles in the dog is known to cause a substantial reduction in the inspiratory cranial displacement of the ribs and a compensatory increase in the activation of the other inspiratory intercostal muscles, namely the external intercostals and the levator costae. The present studies were designed to assess the mechanism(s) of that compensation. 2. Denervating the parasternal intercostals bilaterally caused a reduction in tidal volume and an increase in arterial PCO2 (Pa, CO2). Severing the parasternal intercostals selectively produced similar changes. The concomitant increases in external intercostal and levator costae activity, however, were much greater than predicted on the basis of the increased Pa, CO2. 3. Denervating the parasternal intercostals on one side of the chest produced large increases in ipsilateral, but not contralateral external intercostal activity. 4. Manipulating the ribs after the parasternal intercostals were inactivated so as to reproduce the normal inspiratory cranial displacement of the ribs elicited immediate, clear-cut reductions in external intercostal and levator costae activities. 5. The increases in external intercostal and levator costae activities that occur after inactivation of the parasternal intercostals thus result partly from the increased hypercapnic drive but mostly from proprioceptive reflexes, presumably muscle spindle reflexes.

Neck muscle activity in patients with severe chronic obstructive pulmonary disease

The present studies were designed to assess the pattern of activity and the frequency of activation of the neck muscles in patients with chronic obstructive pulmonary disease (COPD). Using concentric needle electrodes, we thus recorded the electromyograms of the scalene, sternocleidomastoid, and trapezius muscles during resting breathing in 40 stable patients with severe chronic airflow obstruction (FEV1 = 0.69 +/- 0.18 L) and hyperinflation (FRC = 228 +/- 40% of predicted); 17 patients were hypercapnic at rest. When breathing in the seated posture, all patients (100%) had strong inspiratory contraction of the scalenes. In contrast, no patient showed inspiratory activity in the trapezius, and only four patients (10%) showed definite, invariable inspiratory activity in the sternocleidomastoid. These two muscles were silent in the supine posture as well, even though the adoption of this posture was associated with an increase in dyspnea in most patients. We conclude, therefore, that in contrast to conventional thinking, most stable patients with severe COPD do not use the sternocleidomastoids or the trapezii when breathing at rest. Additional measurements indicated that the sternocleidomastoid inspiratory activity previously recorded in such patients was in general caused by a cross-contamination from surrounding muscles.

Contribution of the rib cage inspiratory muscles to breathing in baboons

We have measured the electromyograms of the rib cage inspiratory muscles, including the neck muscles, in five lightly anesthetized baboons breathing at rest in the supine and head-up postures. When supine, the animals did not have any activity in the scalene (three heads) or sternomastoid muscles. In contrast, a phasic inspiratory electrical activity was invariably recorded from the parasternal intercostals, external intercostals, and levator costae. Measurements of the changes in length of the parasternal intercostals indicated that these muscles also shortened during inspiration, and they further showed that this inspiratory shortening was eliminated after selective muscle denervation. Similar observations were made in the head-up posture, although the inspiratory shortening of the parasternal intercostals was smaller in this posture. These observations thus indicate that: (1) the inspiratory expansion of the rib cage in baboons results entirely from the actions of the inspiratory intercostal muscles and mostly from the action of the parasternal intercostals; and (2) the load imposed on these muscles is greater in the head-up posture, presumably because of the action of gravity on the chest wall.

Do canine scalene and sternomastoid muscles play a role in breathing?

To assess the respiratory function of the scalene and sternomastoid muscles in the dog, we studied the effect of graded increases in inspiratory airflow resistance and single-breath airway occlusion on the electrical activity of these muscles in 18 supine anesthetized spontaneously breathing animals. The sternomastoids never showed any activity, and the scalenes showed some inspiratory activity during occlusion in only two animals. The adoption of the prone position and bilateral cervical vagotomy did not affect this pattern. Hypercapnia also did not elicit any sternomastoid activity and induced scalene inspiratory activity during occlusion in only four of nine animals. On microscopic examination, however, both muscles were found to contain large numbers of spindles, suggesting that they have the capacity to respond to stretch. In addition, with increases in inspiratory resistance, both the sternum and ribs were displaced in the caudal direction. As a result, the scalenes demonstrated a gradual inspiratory lengthening and the normal inspiratory lengthening of the sternomastoids was accentuated. Additional studies in three unanesthetized animals showed consistent activity in the scalene and sternomastoid muscles during movements of the trunk and neck but no activity during breathing, including occluded breathing. These observations thus indicate that the alpha-motoneurons of the scalene and sternomastoid muscles in the dog have very small central respiratory drive potentials with respect to their critical firing threshold. In this animal, these muscles do not have a significant respiratory function.

Respiratory effect of the intercostal muscles in the dog

In a previous paper (J. Appl. Physiol. 73: 2283-2288, 1992), respiratory effect was defined as the change in airway pressure produced by active tension in a muscle with the airway closed, mechanical advantage was defined as the respiratory effect per unit mass per unit active stress, and it was shown that mechanical advantage is proportional to muscle shortening during the relaxation maneuver. Here, we report values of mechanical advantage and maximum respiratory effect of the intercostal muscles of the dog. Orientations of the intercostal muscles in the third and sixth interspaces were measured. Mechanical advantages of the muscles in these interspaces were computed by computing their shortening from these data and data in the literature on rib displacement. We found that parasternal internal intercostals and dorsal external intercostals of the upper interspace have large inspiratory mechanical advantages and that dorsal internal intercostals of the lower interspace and triangularis sterni have large expiratory mechanical advantages. Mass distributions in the two interspaces were also measured, and maximum respiratory effects of the muscles were calculated from their mass, mechanical advantage, and the value for maximum stress in skeletal muscle. Estimated maximum respiratory effects of the inspiratory and expiratory muscle groups of the entire rib cage were tested by measuring the maximum inspiratory pressures that were generated by the parasternal and external intercostals acting alone. Measured pressures, -13 cmH2O for the parasternals and -11 cmH2O for the external intercostals, agreed well with the computed values.

Sternum dependence of rib displacement during breathing

The parasternal intercostals are the primary determinant of the inspiratory cranial displacement of the ribs in the dog. When they contract, however, these muscles also cause a caudal displacement of the sternum, presumably an expiratory motion. The present studies were designed to assess the effects of this sternal displacement on the cranial displacement of the ribs and on lung volume. Twelve supine anesthetized animals were studied. We first measured, in four paralyzed animals, the displacement of the ribs and sternum produced by known external forces applied to the ribs, the sternum, or both simultaneously. From these measurements, the elastic coupling between the ribs and sternum was determined. We then studied, in eight animals, the effect of sternal motion on rib motion and tidal volume during spontaneous breathing. Rib and sternal displacements and tidal volume were measured first with the sternum free to move caudally during inspiration and then with the sternum constrained to prevent caudal motion. Preventing the sternum from moving caudally caused a 24% increase in the inspiratory cranial displacement of the ribs; this increased displacement of the ribs agreed well with the elastic coupling between the sternum and the ribs as determined from the force-displacement observations. Tidal volume, however, remained unchanged. These observations indicate that the caudal displacement of the sternum produced by the parasternal intercostals reduces the cranial displacement of the ribs but probably increases the lateral expansion of the rib cage.

Lung volume restriction in patients with chronic respiratory muscle weakness: the role of microatelectasis

BACKGROUND

It is well established that patients with longstanding weakness of the respiratory muscles have a reduction in lung distensibility. Although this occurs in most patients without any radiographic changes suggesting parenchymal lung disease, it has been attributed to the development of microatelectasis.

METHODS

A high resolution computed tomographic (CT) scanner was used in eight patients with traumatic tetraplegia and six patients with generalised neuromuscular disorders to look for areas of atelectasis. With the patient in the supine posture scans of 1 mm thickness were obtained at total lung capacity at intervals of 1 cm from the apex to the base of the lung.

RESULTS

Vital capacity, total lung capacity, and inspiratory muscle strength were reduced to a mean of 59.5%, 73.9%, and 51.1% of predicted values, respectively. Static expiratory lung compliance was decreased in 12 of the 14 patients and averaged 69.1% of the predicted value. The CT scans revealed only small areas of atelectasis in one tetraplegic patient and in one patient with a generalised neuromuscular disorder; no parenchymal abnormality was seen in the other 12 patients.

CONCLUSIONS

In many patients with chronic weakness of the respiratory muscles the reduced lung distensibility does not appear to be caused by microatelectasis. It might be related to alterations in elasticity of the lung tissue.

Mechanics of the parasternal intercostals in prone dogs: statics and dynamics

It is well established that the parasternal intercostal muscles in supine dogs play a major role in causing the inspiratory elevation of the ribs. This posture, however, is not physiological in the dog. In the present study, we measured the electromyographic (EMG) activity and the respiratory changes in length of these muscles in the prone (standing) and supine postures in seven anesthetized spontaneously breathing dogs. With a change from the supine to the prone posture, the parasternal intercostals showed a 3.2% reduction in their relaxation length (Lr), but their mechanical behavior was essentially unchanged. Thus, the muscles continued to shorten below Lr during inspiration and to lengthen beyond Lr during expiration. With the adoption of the prone posture, the amount of parasternal inspiratory EMG activity and the amount of inspiratory muscle shortening each increased by 30-35%. Furthermore, when the parasternal intercostal in a single interspace was selectively denervated, the shortening of the muscle during inspiration in both postures was virtually eliminated. These observations indicate that in the dog the parasternal intercostals still play a major role in causing the inspiratory elevation of the ribs in the prone posture. These observations also suggest that these muscles in prone animals continue to operate on the descending limb of their length-tension curve.

Respiratory response to abdominal and rib cage muscle paralysis in dogs

To assess the respiratory response to abdominal and rib cage muscle paralysis, we measured tidal volume, esophageal and gastric pressures, arterial blood gases, and the electromyogram (EMG) of the diaphragm during progressive epidural anesthesia (lidocaine 2%) in 35 supine anesthetized dogs. The EMG activity of the diaphragm was measured with fine-wire electrodes; the abdominal cavity was thus left intact. Paralysis of the abdominal muscles alone did not produce any alterations. In contrast, when all rib cage muscles were also paralyzed, there were substantial increases in the peak height and the rate of rise of diaphragmatic EMG activity that were associated with a decrease in tidal volume and an increase in arterial PCO2 (PaCO2); swings in transdiaphragmatic pressure, however, were unchanged. The increased diaphragmatic activation due to rib cage muscle paralysis persisted after bilateral cervical vagotomy and was well explained by the increased PaCO2. These observations indicate that in the dog 1) the rib cage muscles contribute significantly to tidal volume, and their paralysis causes, through the increased hypercapnic drive, a compensatory increase in diaphragmatic activation; and 2) the rib cage inspiratory muscles enhance the diaphragm's ability to generate pressure during breathing.

Effect of respiratory muscle tension on lung volume

The chest wall is modeled as a linear system for which the displacements of points on the chest wall are proportional to the forces that act on the chest wall, namely, airway opening pressure and active tension in the respiratory muscles. A standard theorem of mechanics, the Maxwell reciprocity theorem, is invoked to show that the effect of active muscle tension on lung volume, or airway pressure if the airway is closed, is proportional to the change of muscle length in the relaxation maneuver. This relation was tested experimentally. The shortening of the cranial-caudal distance between a rib pair and the sternum was measured during a relaxation maneuver. These data were used to predict the respiratory effect of forces applied to the ribs and sternum. To test this prediction, a cranial force was applied to the rib pair and a caudal force was applied to the sternum, simulating the forces applied by active tension in the parasternal intercostal muscles. The change in airway pressure, with lung volume held constant, was measured. The measured change in airway pressure agreed well with the prediction. In some dogs, nonlinear deviations from the linear prediction occurred at higher loads. The model and the theorem offer the promise that existing data on the configuration of the chest wall during the relaxation maneuver can be used to compute the mechanical advantage of the respiratory muscles.

Abdominal muscle use during breathing in patients with chronic airflow obstruction

To assess the pattern of abdominal muscle contraction in stable patients with chronic obstructive pulmonary disease (COPD), we studied electromyograms of the rectus abdominis, external oblique, and transversus abdominis muscles in 40 patients with variable degrees of chronic airflow obstruction (FEV1 between 17 and 82% of predicted); 12 control subjects with normal pulmonary function tests were studied for comparison. The subjects were studied during resting breathing in the supine posture, and the electromyograms were recorded with concentric needle electrodes implanted with the aid of a high-resolution ultrasound. The rectus abdominis and external oblique were silent in virtually all patients. In contrast, 17 patients had invariable phasic expiratory activity in the transversus abdominis, and 11 additional patients had intermittent transversus expiratory activity. Expiratory contraction of the transversus was related to the degree of airflow obstruction (p less than 0.005), and when present, it persisted in the seated posture. We conclude that (1) when breathing at rest, many stable patients with severe chronic airflow obstruction contract the abdominal muscles during expiration, and (2) this expiratory contraction is usually confined to the transversus muscle. These observations also indicate that the physiology of dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEP) in such patients should be reevaluated.