Coupling between the ribs and the lung in dogs

Investigation of inspiratory intercostal muscle activity in patients with spinal cord injury: a pilot study using electromyography, ultrasonography, and respiratory inductance plethysmography

[Purpose] The respiratory function in patients with cervical spinal cord injury is influenced by inspiratory intercostal muscle function. However, inspiratory intercostal muscle activity has not been conclusively evaluated. We evaluated the inspiratory intercostal muscle activity in patients with cervical spinal cord injury by using inspiratory intercostal electromyography, respiratory inductance plethysmography, and ultrasonography. [Participants and Methods] Three patients with cervical spinal cord injury were assessed. The change in mean amplitude (rest vs. maximum inspiration) was calculated by using intercostal muscle electromyography. Changes in intercostal muscle thickness (resting expiration and maximum inspiration) were also evaluated on ultrasonography. The waveform was converted to spirometry ventilation with respiratory inductance plethysmography, and the waveform at the xiphoid was considered to determine the rib cage volume. Each index was compared with the inspiratory capacities in each case. [Results] Intercostal muscle electromyography failed to measure the notable myoelectric potential in all the patients. The rib cage volume was higher at higher inspiratory capacities. The changes in muscle thickness were not significantly different between the patients. [Conclusion] The rib cage volume (measured with inductance plethysmography) was greater in the patients with cervical spinal cord injury when inspiratory intercostal muscle activity was high. Respiratory inductance plethysmography can capture inspiratory intercostal muscle function in patients with cervical spinal cord injury.

Relationship between costovertebral joint kinematics and lung volume in supine humans

This study investigates the relationship between the motion of the first ten costovertebral joints (CVJ) and lung volume over the inspiratory capacity (IC) using detailed kinematic analysis in a sample of 12 asymptomatic subjects. Retrospective codified spiral-CT data obtained at total lung capacity (TLC), middle of inspiratory capacity (MIC) and at functional residual capacity (FRC) were analysed. CVJ 3D kinematics were processed using previously-published methods. We tested the influence of the side, CVJ level and lung volume on CVJ kinematics. In addition, the correlations between anthropologic/pulmonary variables and CVJ kinematics were analysed. No linear correlation was found between lung volumes and CVJ kinematics. Major findings concerning 3D kinematics can be summarized as follows: 1) Ranges-of-motion decrease gradually with increasing CVJ level; 2) rib displacements are significantly reduced at lung volumes above the MIC and do not differ between CVJ levels; 3) the axes of rotation of the ribs are similarly oriented for all CVJ levels.

Mechanism of the increased rib cage expansion produced by the diaphragm with abdominal support

When the abdomen in quadriplegic subjects is given a passive mechanical support, the expansion of the lower rib cage during inspiration is greater and the inward displacement of the upper rib cage is smaller. These changes have traditionally been attributed to an increase in the appositional force of the diaphragm, but the mechanisms have not been assessed. In this study, the inspiratory intercostal muscles in all interspaces were severed in anesthetized dogs, so that the diaphragm was the only muscle active during inspiration, and the displacements of the ribs 10 and 5 and the changes in pleural and abdominal pressure were measured during unimpeded breathing and during breathing with a plate applied on the ventral abdominal wall. In addition, external forces were applied to the 10th rib pair in the cranial and lateral directions, and the rib trajectories thus obtained were used as the basis for a vector analysis to estimate the relative contributions of the insertional and appositional forces to the rib 10 displacements during breathing. Application of the abdominal plate caused a marked increase in the inspiratory cranial and outward displacement of rib 10 and a decrease in the inspiratory caudal displacement of rib 5. Analysis of the results showed, however, that 1) the insertional and appositional forces contributed nearly equally to the increased inspiratory displacement of rib 10 and 2) the decrease in the expiratory displacement of rib 5 was the result of both the greater displacement of the lower ribs and the decrease in pleural pressure.

Recruitment and plasticity in diaphragm, intercostal, and abdominal muscles in unanesthetized rats

Although rats are a frequent model for studies of plasticity in respiratory motor control, the relative capacity of rat accessory respiratory muscles to express plasticity is not well known, particularly in unanesthetized animals. Here, we characterized external intercostal (T2, T4, T5, T6, T7, T8, T9 EIC) and abdominal muscle (external oblique and rectus abdominis) electromyogram (EMG) activity in unanesthetized rats via radiotelemetry during normoxia (Nx: 21% O2) and following acute intermittent hypoxia (AIH: 10 × 5-min, 10.5% O2; 5-min intervals). Diaphragm and T2-T5 EIC EMG activity, and ventilation were also assessed during maximal chemoreceptor stimulation (

MCS

7% CO2, 10.5% O2) and sustained hypoxia (SH: 10.5% O2). In Nx, T2 EIC exhibits prominent inspiratory activity, whereas T4, T5, T6, and T7 EIC inspiratory activity decreases in a caudal direction. T8 and T9 EIC and abdominal muscles show only tonic or sporadic activity, without consistent respiratory activity. MCS increases diaphragm and T2 EIC EMG amplitude and tidal volume more than SH (0.94 ± 0.10 vs. 0.68 ± 0.05 ml/100 g; P < 0.001). Following AIH, T2 EIC EMG amplitude remained above baseline for more than 60 min post-AIH (i.e., EIC long-term facilitation, LTF), and was greater than diaphragm LTF (41.5 ± 1.3% vs. 19.1 ± 2.0% baseline; P < 0.001). We conclude that 1) diaphragm and rostral T2-T5 EIC muscles exhibit inspiratory activity during Nx; 2) MCS elicits greater ventilatory, diaphragm, and rostral T2-T5 EIC muscle activity vs. SH; and 3) AIH induces greater rostral EIC LTF than diaphragm LTF.

Pleural function and lymphatics

The pleural space plays an important role in respiratory function as the negative intrapleural pressure regimen ensures lung expansion and in the mean time maintains the tight mechanical coupling between the lung and the chest wall. The efficiency of the lung-chest wall coupling depends upon pleural liquid volume, which in turn reflects the balance between the filtration of fluid into and its egress out of the cavity. While filtration occurs through a single mechanism passively driving fluid from the interstitium of the parietal pleura into the cavity, several mechanisms may co-operate to remove pleural fluid. Among these, the pleural lymphatic system emerges as the most important one in quantitative terms and the only one able to cope with variable pleural fluid volume and drainage requirements. In this review, we present a detailed account of the actual knowledge on: (a) the complex morphology of the pleural lymphatic system, (b) the mechanism supporting pleural lymph formation and propulsion, (c) the dependence of pleural lymphatic function upon local tissue mechanics and (d) the effect of lymphatic inefficiency in the development of clinically severe pleural and, more in general, respiratory pathologies.

Respiratory effect of the lower rib displacement produced by the diaphragm

The diaphragm acting alone causes a cranial displacement of the lower ribs and a caudal displacement of the upper ribs. The respiratory effect of the lower rib displacement, however, is uncertain. In the present study, two sets of experiments were performed in dogs to assess this effect. In the first, all the inspiratory intercostal muscles were severed, so that the diaphragm was the only muscle active during inspiration, and the normal inspiratory cranial displacement of the lower ribs was suppressed at regular intervals. In the second experiment, the animals were given a muscle relaxant to abolish respiratory muscle activity, and external, cranially oriented forces were applied to the lower rib pairs to simulate the action of the diaphragm on these ribs. The data showed that 1) holding the lower ribs stationary during spontaneous, isolated diaphragm contraction had no effect on the change in lung volume during unimpeded inspiration and no effect on the fall in pleural pressure (Ppl) during occluded breaths; 2) the procedure, however, caused an increase in the caudal displacement of the upper ribs; and 3) pulling the lower rib pairs cranially induced a cranial displacement of the upper ribs and a small fall in Ppl. These observations indicate that the force applied on the lower ribs by the diaphragm during spontaneous contraction, acting through the interdependence of the ribs, is transmitted to the upper ribs and has an inspiratory effect on the lung. However, this effect is very small compared to that of the descent of the dome.

Role of pleural pressure in the coupling between the intercostal muscles and the ribs

The inspiratory intercostal muscles elevate the ribs and thereby elicit a fall in pleural pressure (ΔPpl) when they contract. In the present study, we initially tested the hypothesis that this ΔPpl does, in turn, oppose the rib elevation. The cranial rib displacement (Xr) produced by selective activation of the parasternal intercostal muscle in the fourth interspace was measured in dogs, first with the rib cage intact and then after ΔPpl was eliminated by bilateral pneumothorax. For a given parasternal contraction, Xr was greater after pneumothorax; the increase in Xr per unit decrease in ΔPpl was 0.98 ± 0.11 mm/cmH2O. Because this relation was similar to that obtained during isolated diaphragmatic contraction, we subsequently tested the hypothesis that the increase in Xr observed during breathing after diaphragmatic paralysis was, in part, the result of the decrease in ΔPpl, and the contribution of the difference in ΔPpl to the difference in Xr was determined by using the relation between Xr and ΔPpl during passive inflation. With diaphragmatic paralysis, Xr during inspiration increased approximately threefold, and 47 ± 8% of this increase was accounted for by the decrease in ΔPpl. These observations indicate that 1) ΔPpl is a primary determinant of rib motion during intercostal muscle contraction and 2) the decrease in ΔPpl and the increase in intercostal muscle activity contribute equally to the increase in inspiratory cranial displacement of the ribs after diaphragm paralysis.

Effect of diaphragmatic contraction on the action of the canine parasternal intercostals

The inspiratory intercostal muscles enhance the force generated by the diaphragm during lung expansion. However, whether the diaphragm also alters the force developed by the inspiratory intercostals is unknown. Two experiments were performed in dogs to answer the question. In the first experiment, external, cranially oriented forces were applied to the different rib pairs to assess the effect of diaphragmatic contraction on the coupling between the ribs and the lung. The fall in airway opening pressure (ΔPao) produced by a given force on the ribs was invariably greater during phrenic nerve stimulation than with the diaphragm relaxed. The cranial rib displacement (Xr), however, was 40–50% smaller, thus indicating that the increase in ΔPao was exclusively the result of the increase in diaphragmatic elastance. In the second experiment, the parasternal intercostal muscle in the fourth interspace was selectively activated, and the effects of diaphragmatic contraction on the ΔPao and Xr caused by parasternal activation were compared with those observed during the application of external loads on the ribs. Stimulating the phrenic nerves increased the ΔPao and reduced the Xr produced by the parasternal intercostal, and the magnitudes of the changes were identical to those observed during external rib loading. It is concluded, therefore, that the diaphragm has no significant synergistic or antagonistic effect on the force developed by the parasternal intercostals during breathing. This lack of effect is probably related to the constraint imposed on intercostal muscle length by the ribs and sternum.

Transmural pressure in rat initial subpleural lymphatics during spontaneous or mechanical ventilation

The role played by the mechanical tissue stress in supporting lymph formation and propulsion in thoracic tissues was studied in deeply anesthetized rats (n = 13) during spontaneous breathing or mechanical ventilation. After arterial and venous catheterization and insertion of an intratracheal cannula, fluorescent dextrans were injected intrapleurally to serve as lymphatic markers. After 2 h, the fluorescent intercostal lymphatics were identified, and the hydraulic pressure in lymphatic vessels (P lymph) and adjacent interstitial space (P int) was measured using micropuncture. During spontaneous breathing, end-expiratory P lymph and corresponding P int were -2.5 +/- 1.1 (SE) and 3.1 +/- 0.7 mmHg (P < 0.01), which dropped to -21.1 +/- 1.3 and -12.2 +/- 1.3 mmHg, respectively, at end inspiration. During mechanical ventilation with air at zero end-expiratory alveolar pressure, P lymph and P int were essentially unchanged at end expiration, but, at variance with spontaneous breathing, they increased at end inspiration to 28.1 +/- 7.9 and 28.2 +/- 6.3 mmHg, respectively. The hydraulic transmural pressure gradient (DeltaP tm = P lymph - P int) was in favor of lymph formation throughout the whole respiratory cycle (DeltaP tm = -6.8 +/- 1.2 mmHg) during spontaneous breathing but not during mechanical ventilation (DeltaP tm = -1.1 +/- 1.8 mmHg). Therefore, data suggest that local tissue stress associated with the active contraction of respiratory muscles is required to support an efficient lymphatic drainage from the thoracic tissues.

Respiratory action of the intercostal muscles

The mechanical advantages of the external and internal intercostals depend partly on the orientation of the muscle but mostly on interspace number and the position of the muscle within each interspace. Thus the external intercostals in the dorsal portion of the rostral interspaces have a large inspiratory mechanical advantage, but this advantage decreases ventrally and caudally such that in the ventral portion of the caudal interspaces, it is reversed into an expiratory mechanical advantage. The internal interosseous intercostals in the caudal interspaces also have a large expiratory mechanical advantage, but this advantage decreases cranially and, for the upper interspaces, ventrally as well. The intercartilaginous portion of the internal intercostals (the so-called parasternal intercostals), therefore, has an inspiratory mechanical advantage, whereas the triangularis sterni has a large expiratory mechanical advantage. These rostrocaudal gradients result from the nonuniform coupling between rib displacement and lung expansion, and the dorsoventral gradients result from the three-dimensional configuration of the rib cage. Such topographic differences in mechanical advantage imply that the functions of the muscles during breathing are largely determined by the topographic distributions of neural drive. The distributions of inspiratory and expiratory activity among the muscles are strikingly similar to the distributions of inspiratory and expiratory mechanical advantages, respectively. As a result, the external intercostals and the parasternal intercostals have an inspiratory function during breathing, whereas the internal interosseous intercostals and the triangularis sterni have an expiratory function.

Interaction between the canine diaphragm and intercostal muscles in lung expansion

Changes in intrathoracic pressure produced by the various inspiratory intercostals are essentially additive, but the interaction between these muscles and the diaphragm remains uncertain. In the present study, this interaction was assessed by measuring the changes in airway opening (ΔPao) or transpulmonary pressure (ΔPtp) in vagotomized, phrenicotomized dogs during spontaneous inspiration (isolated intercostal contraction), during isolated rectangular or ramp stimulation of the peripheral ends of the transected C5 phrenic nerve roots (isolated diaphragm contraction), and during spontaneous inspiration with superimposed phrenic nerve stimulation (combined diaphragm-intercostal contraction). With the endotracheal tube occluded at functional residual capacity, ΔPao during combined diaphragm-intercostal contraction was nearly equal to the sum of the ΔPao produced by the two muscle groups contracting individually. However, when the endotracheal tube was kept open, ΔPtp during combined contraction was 123% of the sum of the individual ΔPtp ( P < 0.001). The increase in lung volume during combined contraction was also 109% of the sum of the individual volume increases ( P < 0.02). Abdominal pressure during combined contraction was invariably lower than during isolated diaphragm contraction. It is concluded, therefore, that the canine diaphragm and intercostal muscles act synergistically during lung expansion and that this synergism is primarily due to the fact that the intercostal muscles reduce shortening of the diaphragm. When the lung is maintained at functional residual capacity, however, the synergism is obscured because the greater stiffness of the rib cage during diaphragm contraction enhances the ΔPao produced by the isolated diaphragm and reduces the ΔPao produced by the intercostal muscles.

The two mechanisms of intercostal muscle action on the lung

The mechanisms of respiratory action of the intercostal muscles were studied by measuring the effect of external forces (F) applied to the ribs and by modeling the effect of F exerted by the intercostal muscles. In five dogs, with the airway occluded, cranial F were applied to individual rib pairs, from the 2nd to the 11th rib pair, and the change in airway opening pressure (Pao) was measured. The ratio Pao/F increases with increasing rib number in the upper ribs (2nd to 5th) and decreases in the lower ribs (5th to 11th). These data were incorporated into a model for the geometry of the ribs and intercostal muscles, and Pao/F was calculated from the model. For interspaces 2-8, the calculated values agree reasonably well with previously measured values. From the modeling, two mechanisms of intercostal muscle action are identified. One is the well-known Hamberger mechanism, modified to account for the three-dimensional geometry of the rib cage. This mechanism depends on the slant of an intercostal muscle relative to the ribs and on the resulting difference between the moments applied to the upper and lower ribs that bound each interspace. The second is a new mechanism that depends on the difference between the values of Pao/F for the upper and lower ribs.

Effects of inflation on the coupling between the ribs and the lung in dogs

The coupling between the ribs and the lung in dogs increases with increasing rib number in the cranial part of the rib cage and then decreases markedly in the caudal part. The hypothesis was raised that this non-uniformity is primarily related to differences between the areas of the lung subtended by the different ribs, and in the current study we tested this idea by assessing the effects of passive lung inflation. Thus, by causing a descent of the diaphragm, inflation would expand the area of the lung subtended by the caudal ribs and improve the coupling between these ribs and the lung. The axial displacements of the ribs and the changes in airway opening pressure (DeltaP(ao)) were measured in anaesthetized, pancuronium-treated, supine dogs while loads were applied in the cranial direction to individual rib pairs at functional residual capacity (FRC) and after passive inflation to 10 and 20 cm H(2)O transrespiratory pressure. In agreement with the hypothesis, inflation caused an increase in DeltaP(ao) for ribs 9 and 10. The most prominent alteration, however, was a marked decrease in DeltaP(ao) for ribs 2-8; at 20 cm H(2)O, DeltaP(ao) for these ribs was only 30% of the value at FRC. Additional measurements indicated that this decrease in DeltaP(ao) results partly from the increase in diaphragmatic compliance but mostly from the reduction in outward rib displacement. This alteration in the pattern of rib motion should add to the decrease in muscle length to reduce the lung expanding action of the external intercostal muscles at high lung volumes.