Mechanical action of the intercostal muscles on the ribs.

The external and internal interosseous intercostal muscles were separately stimulated at end-expiratory lung volume in anesthetized dogs. These muscles were all found to elevate the ribs into which they insert. By attaching weights to the ribs, it was determined that the nonlinear compliance of the ribs was responsible for this phenomenon.

The diaphragm: two muscles.

The costal and crural parts of the diaphragm were separately stimulated in anesthetized dogs. Stimulation of the costal part increased the dimensions of the lower rib cage, whereas stimulation of the costal part decreased the dimensions of the lower rib cage. It is concluded that the diaphragm consists of two muscles that act differently on the rib cage.

Mechanics of intercostal space and actions of external and internal intercostal muscles.

It is conventionally considered that because of their fiber orientations, the external intercostal muscles elevate the ribs, whereas the internal interosseous intercostals lower the ribs. The mechanical action of the intercostal muscles, however, has never been studied directly, and the electromyographic observations supporting this conventional thinking must be interpreted with caution. In the present studies, the external and internal interosseous intercostal muscles have been separately stimulated in different interspaces at, above, and below end-expiratory rib cage volume in anesthetized dogs. The axial (cephalo-caudal) displacements of the ribs were measured using linear displacement transducers. The results indicate that when contracting in a single interspace and other muscles are relaxed, both the external and internal intercostals have a net rib elevating action at end-expiratory rib cage volume. This action increases as rib cage volume decreases, but it progressively decreases as rib cage volume increases such that at high rib cage volumes, both the external and internal intercostals lower the ribs. Stimulating the intercostal muscles in three adjacent intercostal spaces simultaneously produced similar directional rib motion results. We conclude that (a) in contrast with the conventional thinking, the external and internal interosseous intercostals acting alone have by and large a similar effect on the ribs into which they insert; (b) this effect is very much dependent on rib cage (lung) volume; and (c) intercostal muscle action is primarily determined by the resistance of the upper ribs to caudad displacement relative to the resistance of the lower ribs to cephalad displacement. The lateral intercostals, however, might be more involved in postural movements than in respiration. Their primary involvement in rotations of the trunk might account for the presence of two differently oriented muscle layers between the ribs.

Disturbance of respiratory muscle function in patients with mitral valve disease.

A reduced total lung capacity associated with a normal or decreased lung recoil pressure at full inflation (Pel max) has been noted in patients with valvular heart lesions. In order to investigate the mechanism underlying this inappropriately low Pel max, we measured respiratory mechanics in a group of 15 patients with mitral valve disease uncomplicated by other illness. The total lung capacity was 81 percent of control. The static pressure-volume curve of the long intersected the normal one in the vicinity of functional residual capacity (i.e., the recoil pressure was increased at large lung volumes and diminished at low lung volumes), and both expiratory compliance and Pel max were significantly decreased. In 13 of the 15 patients, the minimal (inspiratory) pleural pressure-volume curve was shifted so that the pressures generated by the inspiratory muscles were less negative than normal at any given lung volume. The decrease in Pel max was proportional to the alteration in muscle pressures. These findings indicate (1) that patients with mitral valve disease have compromised function of the inspiratory muscles, and (2) that this alteration is responsible for the low Pel max. Respiratory muscle weakness contributes to the restriction of lung volume in patients with pulmonary vascular congestion and is probably implicated in cardiac dyspnea.

Mechanism of relief of dyspnea after thoracocentesis in patients with large pleural effusions.

In an attempt to understand the mechanism underlying the relief of dyspnea that follows thoracocentesis in patients with large pleural effusions, we measured respiratory mechanics in nine patients before and two hours after removal of 600 to 2,750 ml (mean = 1,818 ml) of pleural fluid. Thoracocentesis resulted in only small changes in pulmonary mechanics: Mean vital capacity and functional residual capacity increased by 300 and 460 ml, respectively, lung recoil pressure slightly decreased, and mean static expiratory compliance increased by 0.021 liter/cm H2O. These changes were inconsistent and could not explain the immediate and remarkable relief of dyspnea noted by the patients. By contrast, thoracocentesis invariably resulted in a shift of the minimal (inspiratory) pleural pressure-volume curve so that the pressures generated by the inspiratory muscles were markedly more negative at any comparable lung volume. This shift was entirely due to the decrease in thoracic cage volume. We suggest that the relief of dyspnea following thoracocentesis results primarily from reduction in size of the thoracic cage, which allows the inspiratory muscles to operate on a more advantageous portion of their length-tension curve.

Phrenic nerve function after pneumonectomy.

Using surface electrodes over the lower chest wall, we measured the phrenic nerve conduction time, ie, the time interval between the stimulation of the phrenic nerve in the neck and the onset of the diaphragmatic muscle action potential, in ten patients who had undergone pneumonectomy several years before (mean, eight years) and in 31 control subjects (bilaterally in 19 of them). The mean (+/- SD) value in the control subjects was 7.0 (+/- 0.9) msec, and no value exceeded 10 msec. All the values obtained in the patients were within the control range. We concluded that phrenic nerve function remains normal after pneumonectomy. This method appears to be the most effective means of identifying phrenic nerve involvement in these patients.

Bloody pleural effusion in a patient with sarcoidosis.

A 50-year old man was evaluated for pleuritic pain.. Chest roentgenogram showed diffuse parenchymal infiltrates and bilateral effusion that, on thoracocentesis, was found to be a bloody fluid. Biopsy of paratracheal nodes demonstrated abundant noncaseating granulomas consistent with sarcoidosis. Prednisone therapy resulted in rapid disappearance of the pleural effusion, progressive clearing of parenchymal infiltrates, and marked improvement of pulmonary function tests. Sarcoidosis should be included in the differential diagnosis of bloody pleural effusion.

Impairment of pulmonary function in acute pancreatitis.

Arterial blood gas levels, lung volumes, and diffusing properties for carbon monoxide were measured in 22 patients with uncomplicated acute pancreatitis who had no clinical or radiographic evidence of pulmonary involvement. Mild arterial hypoxemia (less than 75 mm Hg) was present in four patients. The mean values of inspiratory lung volumes were clearly reduced, and the diffusing properties were sharply altered; the mean value for the diffusing capacity of carbon monoxide per unit of lung volume (Krogh's constant [KCO]) was 78 percent of predicted. Four patients with a low KCO in the first four days after an acute episode had normal values when reevaluated one week later. These findings suggest the occurrence, even in mild acute pancreatitis, of transient pulmonary injury mainly localized at the level of the capillaries, leading to decreased gas transfer.

Radiology of the diaphragm as two muscles.

In order to investigate the postulate that the diaphragm consists of two muscles, cineradiographic studies and plain roentgenograms were taken of the diaphragmatic movement in eight dogs. A characteristic pattern of contraction was seen following direct electric stimulation of each of the sections of the diaphragm, namely the costal muscle, mainly anterior to the central tendon, and the smaller, posterior crural muscle. Stimulation of the roots of the phrenic nerves in the neck showed that, with C5 root stimulation, contraction of the costal area occurred, but that with C6 and C7 root stimulation, contraction of the crural area occurred, although the movement was larger with C6 than with C7 root stimulation. Downward displacement of the entire diaphragm followed stimulation of the intrathoracic phrenic nerves. These results support the concept that the canine diaphragm actually consists of two muscles with a different segmental innervation of each part.

Inspiratory elevation of the ribs in the dog: primary role of the parasternals.

To assess the relative contributions of the different groups of inspiratory intercostal muscles to the cranial motion of the ribs in the dog, we have measured the axial displacement of the fourth rib and recorded the electromyograms of the parasternal intercostal, external intercostal, and levator costae in the third interspace in 15 anesthetized animals breathing at rest. In eight animals, the parasternal intercostals were denervated in interspaces 1-5. This procedure caused a marked increase in the amount of external intercostal and levator costae inspiratory activity, and yet the inspiratory cranial motion of the rib was reduced by 55%. On the other hand, the external intercostals in interspaces 1-5 were sectioned in seven animals, and the reduction in the cranial rib motion was only 22%; the amount of parasternal and levator costae activity, however, was unchanged. When the parasternals in these animals were subsequently denervated, the levator costae inspiratory activity increased markedly, but the inspiratory cranial motion of the rib was abolished or reversed into an inspiratory caudal motion. These studies thus confirm that, in the dog breathing at rest, the parasternal intercostals have a larger role than the external intercostals and levator costae in causing the cranial motion of the ribs during inspiration. A quantitative analysis suggests that the parasternal contribution is approximately 80%.

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.

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.

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.

Respiratory function of intercostal muscles in supine dog: an electromyographic study.

It is traditionally considered that the difference in orientation of the muscle fibers makes the external intercostals elevate the ribs and the internal interosseous intercostals lower the ribs during breathing. This traditional view, however, has recently been challenged by the observation that the external and internal interosseous intercostals, when contracting alone in a single interspace, have a similar effect on the ribs into which they insert. This view has also been challenged by the observation that the external and internal intercostals in a given interspace often change their length in the same direction during breathing. In an attempt to clarify the respiratory function of these muscles, we studied eight supine lightly anesthetized dogs during quiet breathing and during static inspiratory efforts. In each animal electromyographic (EMG) recordings from the external and internal interosseous intercostals were obtained in all interspaces from the second to the eighth, and selective denervations were systematically performed to ensure with complete certainty the origin of the recorded EMG activities. The external intercostals were only activated in phase with inspiration, whereas the internal interosseous intercostals were only activated in phase with expiration. These phasic EMG activities, however, were generally small in magnitude, and the muscles were often silent. Indeed, activation of the externals was always confined to the upper portion of the rib cage, whereas activation of the internals was limited to the lower portion of the rib cage. Internal intercostal activation always occurred sequentially along a caudocephalic gradient. These observations are thus compatible with the traditional view of intercostal muscle action.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.

Triangularis sterni: a primary muscle of breathing in the dog.

The isolated action, pattern of neural activation, and mechanical contribution to eupnea of the triangularis sterni (transversus thoracis) muscle were studied in supine anesthetized dogs. Linear displacement transducers were used to measure the axial displacements of the ribs and sternum. Tetanic stimulation of the triangularis sterni in the apneic animal caused a marked caudal displacement of the ribs, a moderate cranial displacement of the sternum, and a decrease in lung volume. During quiet breathing, there was invariably a rhythmic activation of the muscle in phase with expiration that was independent of the presence or absence of activity in the abdominal and internal interosseous intercostal muscles. This phasic expiratory activity in the triangularis sterni was of large amplitude and caused the ribs to be more caudal and the sternum to be more cranial during the spontaneous expiratory pause than during relaxation. Additional studies on awake animals showed that rhythmic activation of the triangularis sterni occurs in all body positions and is not caused by anesthesia. These findings indicate that expiration in the dog is not a passive process and that the end-expiratory volume of the rib cage is not determined by an equilibrium of static forces alone. Rather, it is actively determined and maintained below its relaxation volume by contraction of the triangularis sterni throughout expiration. The use of this muscle is likely to facilitate inspiration by increasing the length of the parasternal intercostals and taking on a portion of their work.

Mechanics of parasternals and triangularis sterni in upright vs. supine dogs.

The parasternal intercostals in supine dogs are activated and shorten during inspiration, whereas the triangularis terni is activated and shortens during expiration (J. Appl. Physiol. 61: 539-544, 1986). How the two muscles respond to posture, however, is not known. Thirteen vagotomized, phrenicotomized, spontaneously breathing animals were thus studied during multiple postural changes from supine to 80 degrees head up and 20 degrees head down. Head-up tilting elicited a gradual increase in the electrical activation of both the triangularis sterni and the parasternals. Recruitment of the triangularis sterni promoted an increase in the amount of expiratory muscle shortening, but recruitment of the parasternals was invariably associated with a considerable reduction in the amount of inspiratory muscle shortening. This reduction was abolished after sectioning of the abdominal wall. We conclude that 1) the contribution of the canine triangularis sterni to rib movement increases with the assumption of the upright posture, whereas the contribution of the parasternals decreases and 2) this decrease results primarily from the load imposed on the rib cage by gravitational forces. Thus assuming the upright posture adversely affects the rib cage inspiratory muscles as well as the diaphragm.

Inspiratory function of the levator costae and external intercostal muscles in the dog.

The shortening of the canine parasternal intercostals during inspiration may have a passive component, and we have previously speculated that this might result from the actions of the levator costae and external intercostals (J. Appl. Physiol. 66: 1421-1429, 1989). The present studies were designed, therefore, to evaluate the pattern of activation of these muscles in the dog and to define their action on the rib cage during breathing. The results indicate that 1) the levator costae and external intercostals in the cranial part of the rib cage are active during inspiration, both in the supine and in the prone posture; 2) the inspiratory activation of the two muscles is increased after bilateral phrenicotomy; 3) it is increased even more when the parasternal intercostals in the different interspaces are also denervated; and 4) when the levator costae and external intercostals are the only muscles active during inspiration, the ribs continue to move cranially, and the sternum, rather than moving caudally as it does in the intact animal, moves cranially as well. Therefore, we conclude that the levator costae and external intercostals in the dog have a true inspiratory function. When needed, they are capable of causing a significant expansion of the rib cage and the lung during breathing.

Interaction between left and right intercostal muscles in airway pressure generation.

The interactions between the different rib cage inspiratory muscles in the generation of pleural pressure remain largely unknown. In the present study, we have assessed in dogs the interactions between the parasternal intercostals and the interosseous intercostals situated on the right and left sides of the sternum. For each set of muscles, the changes in airway opening pressure (DeltaPao) obtained during separate right and left activation were added, and the calculated values (predicted DeltaPao) were then compared with the DeltaPao values obtained during symmetric, bilateral activation (measured DeltaPao). When the parasternal intercostals in one or two interspaces were activated, the measured DeltaPao was commonly greater than the predicted value. The difference, however, was only 10%. When the interosseous intercostals were activated, the measured DeltaPao was nearly equal to the predicted value. These observations strengthen our previous conclusion that the pressure changes produced by the rib cage inspiratory muscles are essentially additive. As a corollary, the rib cage can be considered as a linear elastic structure over a wide range of distortion.