Chest wall mechanics in dogs with acute diaphragm paralysis

Oesophageal pressure and respiratory muscle ultrasonographic measurements indicate inspiratory effort during pressure support ventilation

BACKGROUND

Bedside measures of patient effort are essential to properly titrate the level of pressure support ventilation. We investigated whether the tidal swing in oesophageal (ΔPes) and transdiaphragmatic pressure (ΔPdi), and ultrasonographic changes in diaphragm (TFdi) and parasternal intercostal (TFic) thickening are reliable estimates of respiratory effort. The effect of diaphragm dysfunction was also considered.

METHODS

Twenty-one critically ill patients were enrolled: age 73 (14) yr, BMI 27 (7) kg m^-2, and Pao₂/Fio₂ 33.3 (9.2) kPa. A three-level pressure support trial was performed: baseline, 25% (PS-medium), and 50% reduction (PS-low). We recorded the oesophageal and transdiaphragmatic pressure-time products (PTPs), work of breathing (WOB), and diaphragm and intercostal ultrasonography. Diaphragm dysfunction was defined by the Gilbert index.

RESULTS

Pressure support was 9.0 (1.6) cm H₂O at baseline, 6.7 (1.3) (PS-medium), and 4.4 (1.0) (PS-low). ΔPes was significantly associated with the oesophageal PTP (R²=0.868; P<0.001) and the WOB (R²=0.683; P<0.001). ΔPdi was significantly associated with the transdiaphragmatic PTP (R²=0.820; P<0.001). TFdi was only weakly correlated with the oesophageal PTP (R²=0.326; P<0.001), and the correlation improved after excluding patients with diaphragm dysfunction (R²=0.887; P<0.001). TFdi was higher and TFic lower in patients without diaphragm dysfunction: 33.6 (18.2)% vs 13.2 (9.2)% and 2.1 (1.7)% vs 12.7 (9.1)%; P<0.0001.

CONCLUSIONS

ΔPes and ΔPdi are adequate estimates of inspiratory effort. Diaphragm ultrasonography is a reliable indicator of inspiratory effort in the absence of diaphragm dysfunction. Additional measurement of parasternal intercostal thickening may discriminate a low inspiratory effort or a high effort in the presence of a dysfunctional diaphragm.

Effect of intercostal muscle contraction on rib motion in humans studied by finite element analysis

The effect of intercostal muscle contraction on generating rib motion has been investigated for a long time and is still controversial in physiology. This may be because of the complicated structure of the rib cage, making direct prediction of the relationship between intercostal muscle force and rib movement impossible. Finite element analysis is a useful tool that is good at solving complex structural mechanic problems. In this study, we individually activated the intercostal muscle groups from the dorsal to ventral portions and obtained five different rib motions classified based on rib moving directions. We found that the ribs cannot only rigidly rotate around the spinal joint but also be deformed, particularly around the relatively soft costal cartilages, where the moment of muscle force for the rigid rotation is small. Although the intercostal muscles near the costal cartilages cannot generate a large moment to rotate the ribs, the muscles may still have a potential to deform the costal cartilages and contribute to the expansion and contraction of the rib cage based on the force-length relationship. Our results also indicated that this potential is matched well with the special shape of the costal cartilages, which become progressively oblique in the caudal direction. Compared with the traditional explanation of rib motion, by additionally considering the effect from the tissue deformation, we found that the special structure of the ventral portion of the human rib cage could be of mechanical benefit to the intercostal muscles, generating inspiratory and expiratory rib motions. NEW & NOTEWORTHY Compared with the traditional explanation of rib motion, additionally considering the effect from tissue deformation helps us understand the special structure of the ventral portion of the human rib cage, such that the costal cartilages progressively become oblique and the costochondral junction angles gradually change into nearly right angles from the upper to lower ribs, which could be of mechanical benefit to the intercostal muscles in the ventral portion, generating inspiratory and expiratory rib motions.

Compensatory plasticity in diaphragm and intercostal muscle utilization in a rat model of ALS

In SOD1^G93A transgenic rat model of ALS, breathing capacity is preserved until late in disease progression despite profound respiratory motor neuron (MN) cell death. To explore mechanisms preserving breathing capacity, we assessed inspiratory EMG activity in diaphragm and external intercostal T2 (EIC2) and T5 (EIC5) muscles in anesthetized SOD1^G93A rats at disease end-stage (20% decrease in body mass). We hypothesized that despite significant phrenic motor neuron loss and decreased phrenic nerve activity, diaphragm electrical activity and trans-diaphragmatic pressure (Pdi) are maintained to sustain ventilation. We alternatively hypothesized that EIC activity is enhanced, compensating for impaired diaphragm function. Diaphragm, EIC2 and EIC5 muscle EMGs and Pdi were measured in urethane-anesthetized, spontaneously breathing female SOD1^G93A rats versus wild-type littermates during normoxia (arterial PO₂ ~90mmHg, PCO₂ ~45mmHg), maximal chemoreceptor stimulation (MCS: 10.5% O₂/7% CO₂), spontaneous augmented breaths and sustained tracheal occlusion. Phrenic MNs were counted in C3-5; T2 and T5 ventrolateral MNs were counted. In end-stage SOD1^G93A rats, 29% of phrenic MNs survived (vs. wild-type), yet integrated diaphragm EMG amplitude was normal. Nevertheless, maximal Pdi decreased ~30% vs. wild type (p<0.01) and increased esophageal to gastric pressure ratio (p<0.05), consistent with persistent diaphragm weakness. Despite major T2 and T5 MN death, integrated EIC2 (100% greater than wild type) and EIC5 (300%) EMG amplitudes were increased in mutant rats during normoxia (p<0.01), possibly compensating for decreased Pdi. Thus, despite significant phrenic MN loss, diaphragm EMG activity is maintained; in contrast, Pdi was not, suggesting diaphragm dysfunction. Presumably, increased EIC EMG activity compensated for persistent diaphragm weakness. These adjustments contribute to remarkable preservation of breathing ability despite major respiratory motor neuron death and diaphragm dysfunction.

Effects of unilateral airway occlusion on rib motion and inspiratory intercostal activity in dogs

Unilateral bronchial occlusion, a complication of many lung diseases, causes dyspnea but the mechanism of this symptom is uncertain. In this study, electromyographic (EMG) activity in the parasternal and external intercostal muscles in the third intercostal space and inspiratory motion of the third rib on both sides of the thorax were assessed during occlusion of a main bronchus for a single breath in anesthetized dogs. Occlusion produced a 65% increase in external intercostal EMG activity in both hemithoraces without altering parasternal EMG activity. Concomitantly, the inspiratory cranial rib motion showed a 50% decrease on both sides of the thorax. These changes were unaffected by bilateral vagotomy. However, when an external, caudally oriented force was applied to the third rib on the right or left side so that its inspiratory cranial displacement was abolished, activity in the adjacent external intercostals showed a twofold increase, but rib motion and external activity in the contralateral hemithorax remained unchanged. It is concluded that during occlusion of a main bronchus, the increase in external intercostal activity is induced by the decrease in inspiratory cranial rib displacement in both hemithoraces, and that this decrease is determined by the increase in pleural pressure swings on both sides of the mediastinum. This mechanism, combined with the decrease in PaO₂, induces similar alterations when unilateral bronchial occlusion is maintained for a series of consecutive breaths.

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.

Action of the isolated canine diaphragm on the lower ribs at high lung volumes

The normal diaphragm has an inspiratory action on the lower ribs, but subjects with chronic obstructive pulmonary disease commonly have an inward displacement of the lateral portions of the lower rib cage during inspiration. This paradoxical displacement, conventionally called 'Hoover's sign', has traditionally been attributed to the direct action of radially oriented diaphragmatic muscle fibres. In the present study, the inspiratory intercostal muscles in all interspaces in anaesthetized dogs were severed so that the diaphragm was the only muscle active during inspiration. The displacements of the lower ribs along the craniocaudal and laterolateral axes and the changes in pleural pressure (∆Ppl) and transdiaphragmatic pressure were measured during occluded breaths and mechanical ventilation at different lung volumes between functional residual capacity (FRC) and total lung capacity. From these data, the separate effects on rib displacement of ∆Ppl and of the force exerted by the diaphragm on the ribs were determined. Isolated spontaneous diaphragm contraction at FRC displaced the lower ribs cranially and outward, but this motion was progressively reversed into a caudal and inward motion as lung volume increased. However, although the force exerted by the diaphragm on the ribs decreased with increasing volume, it continued to displace the ribs cranially and outward. These observations suggest that Hoover's sign is usually caused by the decrease in the zone of apposition and, thus, by the dominant effect of ∆Ppl on the lower ribs, rather than an inward pull from the diaphragm.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Effects of the insertional and appositional forces of the canine diaphragm on the lower ribs

The diaphragm has an inspiratory action on the lower ribs, and current conventional wisdom maintains that this action is the result of two mechanisms, namely, the force applied by the muscle fibres on the ribs into which they insert (insertional force) and the transmission of abdominal pressure through the zone of apposition (appositional force). The magnitude of the diaphragmatic force and the relative contributions of the insertional and appositional components, however, are unknown. To assess these forces, the inspiratory intercostal muscles in all interspaces were severed in anaesthetized dogs, so that the diaphragm was the only muscle active during inspiration, and the displacements of the lower ribs along the craniocaudal and laterolateral axes were measured during quiet breathing, during occluded breaths and during passive lung inflation. From these data, the isolated effects of pleural pressure and transdiaphragmatic pressure on rib displacement were determined. Then external forces were applied to the ribs in the cranial and the lateral direction to simulate, respectively, the effects of the insertional and appositional forces, and the rib trajectories for these external forces were used as the basis for a vector analysis to obtain the relative magnitudes of the insertional and appositional contributions to the rib displacement driven by transdiaphragmatic pressure. The results show that, per unit pressure, the inspiratory effect of the diaphragmatic force on the lower ribs is equal to the expiratory effect of pleural pressure, and that the insertional force contributes 60% of that inspiratory effect.

Mechanics of the canine diaphragm in pleural effusion

Pleural effusion is a complicating feature of many diseases of the lung and pleura, but its effects on the mechanics of the diaphragm have not been assessed. In the present study, radiopaque markers were attached along muscle bundles in the midcostal region of the diaphragm in anesthetized dogs, and the three-dimensional location of the markers during relaxation before and after the stepwise introduction of liquid into the left or right pleural space and during phrenic nerve stimulation in the same conditions was determined using computed tomography. From these data, accurate measurements of diaphragm muscle length and displacement were obtained, and the changes in pleural and abdominal pressure were analyzed as functions of these parameters. The effect of liquid instillation on the axial position of rib 5 was also measured. The data showed that 1) liquid leaked through the dorsal mediastinal sheet behind the pericardium so that effusion was bilateral; 2) effusion caused a caudal displacement of the relaxed diaphragm; 3) this displacement was, compared with passive lung inflation, much larger than the cranial displacement of the ribs; and 4) the capacity of the diaphragm to generate pressure, in particular pleural pressure, decreased markedly as effusion increased, and this decrease was well explained by the decrease in active muscle length. It is concluded that pleural effusion has a major adverse effect on the pressure-generating capacity of the diaphragm and that this is the result of the action of hydrostatic forces on the muscle.

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.

The action of the canine diaphragm on the lower ribs depends on activation

Conventional wisdom maintains that the diaphragm lifts the lower ribs during isolated contraction. Recent studies in dogs have shown, however, that supramaximal, tetanic stimulation of the phrenic nerves displaces the lower ribs caudally and inward. In the present study, the hypothesis was tested that the action of the canine diaphragm on these ribs depends on the magnitude of muscle activation. Two experiments were performed. In the first, the C5 and C6 phrenic nerve roots were selectively stimulated in 6 animals with the airway occluded, and the level of diaphragm activation was altered by adjusting the stimulation frequency. In the second experiment, all the inspiratory intercostal muscles were severed in 7 spontaneously breathing animals, so that the diaphragm was the only muscle active during inspiration, and neural drive was increased by a succession of occluded breaths. The changes in airway opening pressure and the craniocaudal displacements of ribs 5 and 10 were measured in each animal. The data showed that 1) contraction of the diaphragm causes the upper ribs to move caudally; 2) during phrenic nerve stimulation, the lower ribs move cranially when the level of diaphragm activation is low, but they move caudally when the level of muscle activation is high and the entire rib cage is exposed to pleural pressure; and 3) during spontaneous diaphragm contraction, however, the lower ribs always move cranially, even when neural drive is elevated and the change in pleural pressure is large. It is concluded that the action of the diaphragm on the lower ribs depends on both the magnitude and the mode of muscle activation. These findings can reasonably explain the apparent discrepancies between previous studies. They also imply that observations made during phrenic nerve stimulation do not necessarily reflect the physiological action of the diaphragm.

Diaphragm muscle shortening modulates kinematics of lower rib cage in dogs

We tested the hypothesis that diaphragm muscle shortening modulates volume displacement and kinematics of the lower rib cage in dogs and that posture and mode of ventilation affect such modulation. Radiopaque markers were surgically attached to the lower three ribs of the rib cage and to the midcostal region of the diaphragm in six dogs of ∼8 kg body masses, and the locations of these markers were determined by a biplane fluoroscopy system. Three-dimensional software modeling techniques were used to compute volume displacement and surface area of the midcostal diaphragm and the lower three ribs during quiet spontaneous breathing, mechanical ventilation, and bilateral phrenic nerve stimulation at different lung volumes spanning the vital capacity. Volume displaced by the diaphragm relative to that displaced by the lower ribs is disproportionately greater under mechanical ventilation than during spontaneous breathing in the supine position (P < 0.05). At maximal stimulation, diaphragm volume displacement grows disproportionately larger than rib volume displacement as lung volume increases (P < 0.05). Surface area of both the diaphragm and the lower ribs during maximal stimulation of the diaphragm is reduced compared with that at spontaneous breathing (P < 0.05). In the prone posture, mechanical ventilation results in a smaller change in diaphragm surface area than spontaneous breathing (P < 0.05). Our data demonstrate that during inspiration the lower rib cage moves not only through the pump- and bucket-handle motion, but also rotates around the spine. Taken together, these data support the observation that the kinematics of the lower rib cage and its mechanical interaction with the diaphragm are more complex than previously known.

Mechanisms of the inspiratory action of the diaphragm during isolated contraction

The lung-expanding action of the diaphragm is primarily related to the descent of the dome produced by the shortening of the muscle fibers. However, when the phrenic nerves in dogs are selectively stimulated at functional residual capacity, the muscle insertions into the lower ribs also move caudally. This rib motion should enhance the descent of the dome and increase the fall in pleural pressure (DeltaPpl). To quantify the role of this mechanism in determining DeltaPpl during isolated diaphragm contraction and to evaluate the volume dependence of this role, radiopaque markers were attached to muscle bundles in the midcostal region of the muscle in six animals, and the three-dimensional location of the markers during relaxation at different lung volumes and during phrenic nerve stimulation at the same lung volumes was measured using computed tomography. From these data, accurate measurements of muscle length, dome displacement, and lower rib displacement were obtained. The values of dome displacement were then corrected for lower rib displacement, and the values of DeltaPpl corresponding to the corrected dome displacements were obtained using the measured relationship between DeltaPpl and dome displacement. The measurements showed that phrenic stimulation at all lung volumes causes a caudal displacement of the lower ribs and that this displacement, taken alone, contributes approximately 25% of the DeltaPpl produced by the diaphragm. To the extent that this lower rib displacement is itself caused by DeltaPpl, the lung-expanding action of the diaphragm during isolated contraction may therefore be viewed as a self-facilitating phenomenon.

Mechanism of increased inspiratory rib elevation in ascites

The detrimental effect of ascites on the lung-expanding action of the diaphragm is partly compensated for by an increase in the inspiratory elevation of the ribs, but the mechanism of this increase is uncertain. To identify this mechanism, the effect of ascites on the response of rib 4 to isolated phrenic nerve stimulation was first assessed in four dogs with bilateral pneumothoraces. Stimulation did not produce any axial displacement of the rib (X(r)) in the control condition and caused a cranial rib displacement in the presence of ascites. This displacement, however, was small. In a second experiment, the effects of ascites on the pleural pressure swing (DeltaP(pl)), intercostal activity, and X(r) during spontaneous inspiration were measured in eight animals. As the volume of ascites increased from 0 to 200 ml/kg body wt, X(r) increased from 3.5 +/- 0.5 to 7.5 +/- 0.9 mm (P < 0.001), DeltaP(pl) decreased from -6.4 +/- 0.4 to -3.6 +/- 0.3 cmH(2)0 (P < 0.001), and parasternal intercostal activity increased 61 +/- 19% (P < 0.001). The role of the decrease in DeltaP(pl) in causing the increase in X(r) was then separated from that of the increase in intercostal muscle force using the relation between X(r) and DeltaP(pl) during passive lung inflation. The loss in DeltaP(pl) accounted for two-thirds of the increase in X(r). These observations indicate that 1) the increased inspiratory elevation of the ribs in ascites is not the result of the increase in the rib cage-expanding action of the diaphragm and 2) it is due mostly to the decrease in DeltaP(pl) and partly to the increase in the force exerted by the parasternal intercostals on the ribs. These observations also suggest, however, that the rib cage expansion caused by ascites makes the parasternal intercostals less effective in pulling the ribs cranially.

Neuromechanical matching of drive in the scalene muscle of the anesthetized rabbit

The scalene is a primary respiratory muscle in humans; however, in dogs, EMG activity recorded from this muscle during inspiration was reported to derive from underlying muscles. In the present studies, origin of the activity in the medial scalene was tested in rabbits, and its distribution was compared with the muscle mechanical advantage. We assessed in anesthetized rabbits the presence of EMG activity in the scalene, sternomastoid, and parasternal intercostal muscles during quiet breathing and under resistive loading, before and after denervation of the scalene and after its additional insulation. At rest, activity was always recorded in the parasternal muscle and in the scalene bundle inserting on the third rib (medial scalene). The majority of this activity disappeared after denervation. In the bundle inserting on the fifth rib (lateral scalene), the activity was inconsistent, and a high percentage of this activity persisted after denervation but disappeared after insulation from underlying muscle layers. The sternomastoid was always silent. The fractional change in muscle length during passive inflation was then measured. The mean shortening obtained for medial and lateral scalene and parasternal intercostal was 8.0 +/- 0.7%, 5.5 +/- 0.5%, and 9.6 +/- 0.1%, respectively, of the length at functional residual capacity. Sternomastoid muscle length did not change significantly with lung inflation. We conclude that, similar to that shown in humans, respiratory activity arises from scalene muscles in rabbits. This activity is however not uniformly distributed, and a neuromechanical matching of drive is observed, so that the most effective part is also the most active.

Characterization of Forces on the Sternal Midline Following Median Sternotomy in a Porcine Model

The development of more effective fixation devices for reapproximating and immobilizing the sternum after open-heart surgery is limited by current methods for evaluating these devices. In particular, precise emulation of in vivo sternal loading has not been achieved in controlled model systems. The present study is an initial effort to determine the in vivo loading parameters needed to improve current in vitro and in silico (computational) models. Towards this goal, the direction, magnitude, and distribution of loading along a midline sternotomy were characterized in a porcine model. Two instrumented plating systems were used to measure the forces across the bisected sternum in four anaesthetized Yorkshire pigs during spontaneous breathing, ventilated breathing, and coughing for four treatments: live, cadaveric, embalmed, and refrigerated. Changes in forces incurred by death and embalming were also investigated to evaluate the potential applicability of cadavers as models for testing sternal fixation devices. The magnitudes of the respiratory forces in three orthogonal directions ranged from 0.4Nto43.8N, many fold smaller than previously estimated. Dynamic forces were highest in the lateral direction during coughing and low in all directions during normal breathing. No significant differences in force were found between the four treatments, most likely due to the unexpectedly low magnitude of forces in all groups. These results provide the first measurements of in vivo sternal forces and indicate that small cyclic fatigue loads rather than large quasistatic loads should be applied in future model systems to best evaluate the mechanical performance of fixation devices.

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.

The effect of lung inflation on the inspiratory action of the canine parasternal intercostals

Inflation induces a marked decrease in the lung-expanding ability of the diaphragm, but its effect on the parasternal intercostal muscles is uncertain. To assess this effect, the phrenic nerves and the external intercostals were severed in anesthetized, vagotomized dogs, such that the parasternal intercostals were the only muscles active during inspiration, and the endotracheal tube was occluded at different lung volumes. Although the inspiratory electromyographic activity recorded from the muscles was constant, the change in airway opening pressure decreased with inflation from −7.2 ± 0.6 cmH2O at functional residual capacity to −2.2 ± 0.2 cmH2O at 20-cmH2O transrespiratory pressure ( P < 0.001). The inspiratory cranial displacement of the ribs remained virtually unchanged, and the inspiratory caudal displacement of the sternum decreased moderately. However, the inspiratory outward rib displacement decreased markedly and continuously; at 20 cmH2O, this displacement was only 23 ± 2% of the value at functional residual capacity. Calculations based on this alteration yielded substantial decreases in the change in airway opening pressure. It is concluded that, in the dog, 1) inflation affects adversely the lung-expanding actions of both the parasternal intercostals and the diaphragm; and 2) the adverse effect of inflation on the parasternal intercostals is primarily related to the alteration in the kinematics of the ribs. As a corollary, it is likely that hyperinflation also has a negative impact on the parasternal intercostals in patients with chronic obstructive pulmonary disease.

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.

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.

Reflex inhibition of human inspiratory muscles in response to contralateral phrenic nerve stimulation

In animals, high-intensity unilateral stimulation of the phrenic nerve results in short-latency inhibition of phrenic and intercostal nerve activity bilaterally. This study provides the first demonstration in human subjects of a short-latency inhibitory response in the contralateral scalene, parasternal intercostal and diaphragm muscles to single stimuli delivered at cervical level to the phrenic nerve. Electromyographic (EMG) responses were recorded with intramuscular and surface electrodes. An inhibitory response with an onset latency of approximately 35 ms followed by a long-latency excitatory response at approximately 100 ms were observed in the three inspiratory muscles. The inhibition was evident in single trials, averaged EMG, histograms of the discharge of single motor units, and even when the phrenic nerve stimulus intensity was relatively low. Thus, the inhibition may be mediated by large-diameter muscle afferents. The latency of this potent inhibitory response to contralateral phrenic nerve stimulation is too long to be mediated via a simple spinal circuit and may involve a brainstem projection.

Geniohyoid muscle function in awake canines

The geniohyoid (Genio) upper airway muscle shows phasic, inspiratory electrical activity in awake humans but no activity and lengthening in anesthetized cats. There is no information about the mechanical action of the Genio, including length and shortening, in any awake, nonanesthetized mammal during respiration (or swallowing). Therefore, we studied four canines, mean weight 28.8 kg, 1.5 days after Genio implantation with sonomicrometry transducers and bipolar electromyogram (EMG) electrodes. Awake recordings of breathing pattern, muscle length and shortening, and EMG activity were made with the animal in the right lateral decubitus position during quiet resting, CO2-stimulated breathing, inspiratory-resisted breathing (80 cmH2O. l-1. s), and airway occlusion. Genio length and activity were also measured during swallowing, when it shortened, showing a 9.31% change from resting length, and its EMG activity increased 6.44 V. During resting breathing, there was no phasic Genio EMG activity at all, and Genio showed virtually no movement during inspiration. During CO2-stimulated breathing, Genio showed minimal lengthening of only 0.07% change from resting length, whereas phasic EMG activity was still absent. During inspiratory-resisted breathing and airway occlusion, Genio showed phasic EMG activity but still lengthened. We conclude that the Genio in awake, nonanesthetized canines shows active contraction and EMG activity only during swallowing. During quiet or stimulated breathing, Genio is electrically inactive with passive lengthening. Even against resistance, Genio is electrically active but still lengthens during inspiration.

Synergism between the canine left and right hemidiaphragms

Expansion of the lung during inspiration results from the coordinated contraction of the diaphragm and several groups of rib cage muscles, and we have previously shown that the changes in intrathoracic pressure generated by the latter are essentially additive. In the present studies, we have assessed the interaction between the right and left hemidiaphragms in anesthetized dogs by comparing the changes in airway opening pressure (DeltaPao) obtained during simultaneous stimulation of the two phrenic nerves (measured DeltaPao) to the sum of the DeltaPao values produced by their separate stimulation (predicted DeltaPao). The measured DeltaPao was invariably greater than the predicted DeltaPao, and the ratio between these two values increased gradually as the stimulation frequency was increased; the ratio was 1.10 +/- 0.01 (P < 0.05) for a frequency of 10 Hz, whereas for a frequency of 50 Hz it amounted to 1.49 +/- 0.05 (P < 0.001). This interaction remained unchanged after the rib cage was stiffened and its compliance was made linear, thus indicating that the load against which the diaphragm works is not a major determinant. However, radiographic measurements showed that stimulation of one phrenic nerve extends the inactive hemidiaphragm toward the sagittal midplane and reduces the caudal displacement of the central portion of the diaphragmatic dome. As a result, the volume swept by the contracting hemidiaphragm is smaller than the volume it displaces when the contralateral hemidiaphragm also contracts. These observations indicate that 1) the left and right hemidiaphragms have a synergistic, rather than additive, interaction on the lung; 2) this synergism operates already during quiet breathing and increases in magnitude when respiratory drive is greater; and 3) this synergism is primarily related to the configuration of the muscle.

Mechanical effect of muscle spindles in the canine external intercostal muscles

High-frequency mechanical vibration of the ribcage increases afferent activity from external intercostal muscle spindles, but the effect of this procedure on the mechanical behaviour of the respiratory system is unknown. In the present study, we have measured the changes in external intercostal muscle length and the craniocaudal displacement of the ribs during ribcage vibration (40 Hz) in anaesthetized dogs. With vibration, external intercostal inspiratory activity increased by approximately 50 %, but the respiratory changes in muscle length and rib displacement were unaltered. A similar response was obtained after the muscles in the caudal segments of the ribcage were sectioned and the caudally oriented force exerted by these muscles on the rib was removed, thus suggesting that activation of external intercostal muscle spindles by vibration generates little tension. Prompted by this observation, we also examined the role played by the external intercostal muscle spindles in determining the respiratory displacement of the ribs during breathing against high inspiratory airflow resistances. Although resistances consistently elicited prominent reflex increases in external intercostal inspiratory activity, the normal inspiratory cranial displacement of the ribs was reversed into an inspiratory caudal displacement. Also, this caudal rib displacement was essentially unchanged after section of the external intercostal muscles, whereas it was clearly enhanced after denervation of the parasternal intercostals. These findings indicate that stretch reflexes in external intercostal muscles confer insufficient tension on the muscles to significantly modify the mechanical behaviour of the respiratory system.

Relationship between neural drive and mechanical effect in the respiratory system

The actions of the canine external and internal interosseous intercostal muscles on the lung were assessed by applying the Maxwell reciprocity theorem. The external intercostals in the dorsal part of the cranial interspaces were found to have a large inspiratory effect. However, this effect decreases continuously in the caudal and the ventral direction, such that the muscles in the ventral part of the caudal interspaces have an expiratory effect. The internal intercostals also show marked gradients, such that the muscles in the dorsal part of the caudal interspaces have a large expiratory effect and those in the ventral part of the most cranial interspaces have a small inspiratory effect. During breathing, however, inspiratory activity is found only in the external intercostals with an inspiratory effect, and expiratory activity is confined to the internal intercostals with an expiratory effect. The spatial distribution of inspiratory activity among the canine external intercostals closely mirrors the distribution of inspiratory effect, and the distribution of expiratory activity among the internal intercostals closely mirrors the distribution of expiratory effect. Therefore, the external intercostals have a clear-cut inspiratory action on the lung during breathing, whereas the internal intercostals have a definite expiratory action. The distribution of neural drive among these muscles appears to be equally well matched to the distribution of respiratory effect in humans.

Distribution of inspiratory drive to the external intercostal muscles in humans

The external intercostal muscles in humans show marked regional differences in respiratory effect, and this implies that their action on the lung during breathing is primarily determined by the spatial distribution of neural drive among them. To assess this distribution, monopolar electrodes were implanted under ultrasound guidance in different muscle areas in six healthy individuals and electromyographic recordings were made during resting breathing. The muscles in the dorsal portion of the third and fifth interspace showed phasic inspiratory activity with each breath in every subject. However, the muscle in the ventral portion of the third interspace showed inspiratory activity in only three subjects, and the muscle in the dorsal portion of the seventh interspace was almost invariably silent. Also, activity in the ventral portion of the third interspace, when present, and activity in the dorsal portion of the fifth interspace were delayed relative to the onset of activity in the dorsal portion of the third interspace. In addition, the discharge frequency of the motor units identified in the dorsal portion of the third interspace averaged (mean +/- S.E.M.) 11.9 +/- 0.3 Hz and was significantly greater than the discharge frequency of the motor units in both the ventral portion of the third interspace (6.0 +/- 0.5 Hz) and the dorsal portion of the fifth interspace (6.7 +/- 0.4 Hz). The muscle in the dorsal portion of the third interspace started firing simultaneously with the parasternal intercostal in the same interspace, and the discharge frequency of its motor units was even significantly greater (11.4 +/- 0.3 vs. 8.9 +/- 0.2 Hz). These observations indicate that the distribution of neural inspiratory drive to the external intercostals in humans takes place along dorsoventral and rostrocaudal gradients and mirrors the spatial distribution of inspiratory mechanical advantage.

On the respiratory function of the ribs

To assess the respiratory function of the ribs, we measured the changes in airway opening pressure (Pao) induced by stimulation of the parasternal and external intercostal muscles in anesthetized dogs, first before and then after the bony ribs were removed from both sides of the chest. Stimulating either set of muscles with the rib cage intact elicited a fall in Pao in all animals. After removal of the ribs, however, the fall in Pao produced by the parasternal intercostals was reduced by 60% and the fall produced by the external intercostals was eliminated. The normal outward curvature of the rib cage was also abolished in this condition, and when the curvature was restored by a small inflation, external intercostal stimulation consistently elicited a rise rather than a fall in Pao. These findings thus confirm that the ribs play a critical role in the act of breathing by converting intercostal muscle shortening into lung volume expansion. In addition, they carry the compression that is required to balance the pressure difference across the chest wall.

Coupling between the ribs and the lung in dogs

In contrast to the conventional theory, the external and internal intercostal muscles show marked rostrocaudal gradients in their actions on the lung. We hypothesized that these gradients are the result of a non-uniform coupling between the ribs and the lung. Rib displacements (X(r)) and the changes in airway opening pressure (P(a,o)) were thus measured in anaesthetized, pancuronium-treated, supine dogs while loads were applied in the cranial direction to individual pairs of odd-numbered ribs and in the caudal direction to individual pairs of even-numbered ribs. During cranial loading, X(r) induced by a given load increased gradually with increasing rib number. The decrease in P(a,o) also increased from the third to the fifth rib pair but then decreased markedly to the eleventh pair. A similar pattern was observed during caudal loading, although X(r) and DeltaP(a,o) were smaller. These results were then combined to calculate the net X(r) and the net DeltaP(a,o) that a hypothetical intercostal muscle lying parallel to the longitudinal body axis would produce in different interspaces. The net X(r) was cranial in all interspaces. However, whereas the net DeltaP(a,o) was negative in the cranial interspaces, it was positive in the caudal interspaces. These observations confirm that the coupling between the ribs and the lung varies from the top to the base of the ribcage. This coupling confers to both the external and the internal intercostal muscles an inspiratory action on the lung in the cranial interspaces and an expiratory action in the caudal interspaces.

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.

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.

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.

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.

Muscle kinematics for minimal work of breathing

A mathematical model was analyzed to obtain a quantitative and testable representation of the long-standing hypothesis that the respiratory muscles drive the chest wall along the trajectory for which the work of breathing is minimal. The respiratory system was modeled as a linear elastic system that can be expanded either by pressure applied at the airway opening (passive inflation) or by active forces in respiratory muscles (active inflation). The work of active expansion was calculated, and the distribution of muscle forces that produces a given lung expansion with minimal work was computed. The calculated expression for muscle force is complicated, but the corresponding kinematics of muscle shortening is simple: active inspiratory muscles shorten more during active inflation than during passive inflation, and the ratio of active to passive shortening is the same for all active muscles. In addition, the ratio of the minimal work done by respiratory muscles during active inflation to work required for passive inflation is the same as the ratio of active to passive muscle shortening. The minimal-work hypothesis was tested by measurement of the passive and active shortening of the internal intercostal muscles in the parasternal region of two interspaces in five supine anesthetized dogs. Fractional changes in muscle length were measured by sonomicrometry during passive inflation, during quiet breathing, and during forceful inspiratory efforts against a closed airway. Active muscle shortening during quiet breathing was, on average, 70% greater than passive shortening, but it was only weakly correlated with passive shortening. Active shortening inferred from the data for more forceful inspiratory efforts was approximately 40% greater than passive shortening and was highly correlated with passive shortening. These data support the hypothesis that, during forceful inspiratory efforts, muscle activation is coordinated so as to expand the chest wall with minimal work.

Ratio of active to passive muscle shortening in the canine diaphragm

Active and passive shortening of muscle bundles in the canine diaphragm were measured with the objective of testing a consequence of the minimal-work hypothesis: namely, that the ratio of active to passive shortening is the same for all active muscles. Lengths of six muscle bundles in the costal diaphragm and two muscle bundles in the crural diaphragm of each of four bred-for-research beagle dogs were measured by the radiopaque marker technique during the following maneuvers: a passive deflation maneuver from total lung capacity to functional residual capacity, quiet breathing, and forceful inspiratory efforts against an occluded airway at different lung volumes. Shortening per liter increase in lung volume was, on average, 70% greater during quiet breathing than during passive inflation in the prone posture and 40% greater in the supine posture. For the prone posture, the ratio of active to passive shortening was larger in the ventral and midcostal diaphragm than at the dorsal end of the costal diaphragm. For both postures, active shortening during quiet breathing was poorly correlated with passive shortening. However, shortening during forceful inspiratory efforts was highly correlated with passive shortening. The average ratios of active to passive shortening were 1.23 +/- 0.02 and 1.32 +/- 0.03 for the prone and supine postures, respectively. These data, taken together with the data reported in the companion paper (T. A. Wilson, M. Angelillo, A. Legrand, and A. De Troyer, J. Appl. Physiol. 87: 554-560, 1999), support the hypothesis that, during forceful inspiratory efforts, the inspiratory muscles drive the chest wall along the minimal-work trajectory.

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.

Cervical anatomy of phrenic nerve roots in the rabbit. European Group for Research on the Larynx

The cervical anatomy of the different nerve contributions that constitute the phrenic nerve (phrenic nerve roots and accessory phrenic nerve) were studied in rabbits. In 55 dissections, 6 main root arrangement types were observed. The roots that issued from the fourth and fifth cervical nerves (C4 and C5 roots) were constant. The C4 root was either short or long. The C6 root was at times absent, or sometimes double. An accessory phrenic nerve was present in 43% of the right and 28% of the left dissections. The distribution of the phrenic nerve roots often displayed left-right asymmetry. We conclude that a better knowledge of the cervical anatomy of the phrenic nerve is useful both in physiological studies involving diaphragm denervation and in experimental laryngeal reinnervation.

Ventilatory muscle recruitment and work of breathing in patients with respiratory failure after thoracic surgery

OBJECTIVES

Increased work of breathing (WOB) and respiratory muscle weakness have been identified as major causes of respiratory failure after thoracic surgery. This study was undertaken firstly to characterize the mechanical impairment in patients with respiratory failure after cardio-thoracic surgery, and secondly, to determine how diaphragmatic paralysis affects deterioration in the ventilatory mechanics.

METHODS

We evaluated the respiratory mechanics of 24 patients following cardiac and thoracic surgery. Ten patients without respiratory problems were examined as control subjects. There were nine patients with phrenic nerve injury and five patients without phrenic nerve injury who required mechanical ventilation for more than 7 days. Phrenic nerve injury was assessed with a phrenic nerve stimulation test. We measured the respiratory variables, the esophageal, gastric and transdiaphragmatic pressure swing (deltaPes, deltaPga and deltaPdi, respectively), and the work of breathing during quiet tidal breathing.

RESULTS

Both the groups requiring mechanical ventilation exhibited abnormally negative deltaPga/deltaPes values, compared with the control subjects. A significant increase in WOB with the normal generation of deltaPdi was seen in the patients without phrenic nerve injury. In contrast, the poor generation of deltaPdi with a slight increase in work of breathing was noted in patients with phrenic nerve injury.

CONCLUSIONS

These results demonstrated two different types of respiratory failure in thoracic surgery patients, focusing on the impact of phrenic nerve paralysis. Diaphragmatic dysfunction should not be overlooked in postoperative care, and the amelioration of this compromise in respiratory mechanics is an important aspect of good patient management.

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.