Mechanical coupling of upper and lower canine rib cages and its functional significance

Parasternal intercostal, costal, and crural diaphragm neural activation during hypercapnia

The parasternal intercostal is an obligatory inspiratory muscle working in coordination with the diaphragm, apparently sharing a common pathway of neural response. This similarity has attracted clinical interest, promoting the parasternal as a noninvasive alternative to the diaphragm, to monitor central neural respiratory output. However, this role may be confounded by the distinct and different functions of the costal and crural diaphragm. Given the anatomic location, parasternal activation may significantly impact the chest wall via both mechanical shortening or as a "fixator" for the chest wall. Either mechanical function of the parasternal may also impact differential function of the costal and crural. The objectives of the present study were, during eupnea and hypercapnia, 1) to compare the intensity of neural activation of the parasternal with the costal and crural diaphragm and 2) to examine parasternal recruitment and changes in mechanical action during progressive hypercapnia, including muscle baseline length and shortening. In 30 spontaneously breathing canines, awake without confounding anesthetic, we directly measured the electrical activity of the parasternal, costal, and crural diaphragm, and the corresponding mechanical shortening of the parasternal, during eupnea and hypercapnia. During eupnea and hypercapnia, the parasternal and costal diaphragm share a similar intensity of neural activation, whereas both differ significantly from crural diaphragm activity. The shortening of the parasternal increases significantly with hypercapnia, without a change in baseline end-expiratory length. In conclusion, the parasternal shares an equivalent intensity of neural activation with the costal, but not crural, diaphragm. The parasternal maintains and increases its active inspiratory shortening during augmented ventilation, despite high levels of diaphragm recruitment. Throughout hypercapnic ventilation, the parasternal contributes mechanically; it is not relegated to chest wall fixation.NEW & NOTEWORTHY This investigation directly compares neural activation of the parasternal intercostal muscle with the two distinct segments of the diaphragm, costal and crural, during room air and hypercapnic ventilation. During eupnea and hypercapnia, the parasternal intercostal muscle and costal diaphragm share a similar neural activation, whereas they both differ significantly from the crural diaphragm. The parasternal intercostal muscle maintains and increases active inspiratory mechanical action with shortening during ventilation, even with high levels of diaphragm recruitment.

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.

Lung-deflating ability of rib cage and abdominal muscles in rabbits

Anesthetized, apneic, mechanically ventilated rabbits were placed into a tilting plethysmograph that a rubber diaphragm, tightly fitting the animal's body just below the xiphoid process, separated into a rib cage and abdominal chamber. Expired volumes (DeltaV) and abdominal pressure changes (DeltaPab) were assessed in supine and upright posture during maximal rib cage (RCC) and/or abdominal compression (ABC) by pressurizing either or both chambers, and during maximal stimulations of abdominal muscles (ABS). With RCC, DeltaV supine and upright amounted to 16+/-4.9 (mean+/-S.D.) and 20.9+/-7% of the vital capacity in supine posture (VCs) and to 75.8+/-14.5 and 44.8+/-13.9% of the expiratory reserve volume (ERV) in corresponding posture, DeltaPab being negligible. With ABC, DeltaV was 13.7+/-2 and 38.9+/-7.3% VCs and 68.4+/-14.8 and 84.4+/-10.5% ERV, respectively. Both DeltaV and DeltaPab were similar with ABC and ABS, independent of posture. If this applies also to RCC and expiratory rib cage muscle contraction, maximal expiratory effects of the latter (a) are larger in upright than supine posture; (b) contribute to ERV more in supine than upright posture; and (c) are similar to those caused by ABS in supine, but substantially smaller in upright posture.

Effects of abdominal distension on breathing pattern and respiratory mechanics in rabbits

The effects of acute abdominal distension (AD) on the electromechanical efficiency (Eff) of the inspiratory muscles were investigated in anesthetized rabbits by recording the electrical activity (A), pressure (P) exerted by the diaphragm (di) and parasternal intercostal muscles (ic), and lung volume changes when an abdominal balloon was inflated to various degrees. Eff,ic increased with increasing AD both in supine and upright postures. In upright rabbits Eff,di increased for intermediate but decreased at higher levels of AD, whilst it decreased at all levels of AD in supine rabbits. Tidal volume (VT) response followed that of Eff,di. Tonic Aic and Adi and inspiratory prolongation were elicited by AD. The effects of these neural mechanisms, acting to limit end-expiratory lung volume and VT changes, were however small since vagotomy prevented tonic Adi and inspiratory prolongation and reduced tonic Aic, but changed lung volume responses to AD only little. Hence, reduced respiratory system compliance and changes in inspiratory muscle electromechanical efficiency dominate lung volume responses to acute AD.

A dynamic analysis of chest wall motions with MRI in healthy young subjects

OBJECTIVE

The objective of this study was to analyse respiratory-related motion of the chest wall with non-invasive method.

METHODOLOGY

Using magnetic resonance image (MRI), 30 sequential images (scanning time, 0.4 s per image) on sagittal, axial and coronal planes were obtained in nine healthy young subjects during quiet breathing (QB) and maximal deep breathing (MDB). The coronal planes were obtained in five of nine subjects during MDB. Ventilation was simultaneously measured with pneumotachometer.

RESULTS

There was a linear correlation between instantaneous lung volume and lung cross-sectional area. Motion of the diaphragm and rib cage was also linearly related to instantaneous lung volume. The exception was lower anteroposterior (AP) diameter of the rib cage. The contribution of individual part of the chest wall motion to a unit lung volume change was assessed by slope (S) of the linear regression line. The S at the anterior diaphragm was significantly smaller than those at middle and posterior parts during MDB. The S of middle and posterior diaphragmatic motion was approximately five times that of AP motion of upper rib cage. The S of AP motion of upper rib cage was twice that of transverse motion during either QB or MDB.

CONCLUSION

We concluded that dynamic MRI study with concurrent ventilation measurement is a simple and reliable method for evaluation of local chest wall motion, and that neither diaphragm nor rib cage works as a single functional unit during active ventilation.

Influence of abdomen on respiratory mechanics in supine rabbits

Previous studies showed that abdominal evisceration has no effect on respiratory system compliance. We hypothesized that this could be related to lung distortion in eviscerated animals. Methods were developed for continuous recording of pleural pressure (Ppl) at various sites over the costal (co) and diaphragmatic lung surface (di) in acutely and chronically instrumented rabbits. We compared deltaPpl,co and deltaPpl,di recorded at mid-lung height during inflations in anesthetized, paralyzed supine rabbits before and after evisceration. Cranial and caudal deltaPpl.co were the same under all conditions. In intact animals, deltaPpl.co and deltaPpl,di were equal at all inflation volumes, whilst in eviscerated animals, deltaPpl,di were smaller than deltaPpl,co, the difference increasing with lung inflation. At any given volume, rib cage circumference (Crc) was smaller after evisceration, but the Crc deltaPpl,co relationship remained unchanged. These results are indicative of non-uniform lung expansion after evisceration and are consistent with model predictions based on cylindrical deformation and lung stress-strain relationship. This deformation should mimic the effect of a reduced lung compliance, keeping respiratory system compliance of eviscerated animals nearly normal. Similar deformation should have occurred also in intact rabbits during strong inspiratory efforts and in the erect posture, because lower Ppl,di than Ppl,co values were observed at the same lung height under these conditions.

Insertional action of the abdominal muscles in rabbits and dogs

The insertional action of the abdominal muscles was studied in supine anesthetized, apneic rabbits and dogs by comparing the changes in esophageal pressure (Pes), upper and lower rib cage circumference (Cru,u and Crc,I) and lung volume (VL) in response to electrical stimulation of all abdominal muscles before and after evisceration. In eviscerated animals, abdominal muscle contraction increased Pes and decreased both VL and Cre,I, but had no effect on Crc,u. Maximal responses were obtained at submaximal intensities of stimulation, and became larger with increasing lung volume. Relative to the vital capacity in intact animals, maximal delta VL for stimulation performed at FRC and TLC were 7.2 +/- 2.9(SD) and 39.5 +/- 7% in rabbits, and 6.3 +/- 0.8 and 18.3 +/- 5.9% in dogs, respectively. Relative to the changes in lung volume occurring with maximal contraction of the abdominal muscles in intact animals, the values of delta VL observed in the eviscerated animals amounted to approximately 35 and approximately 45% for stimulation performed at FRC and TLC, respectively. Hence, abdominal muscles exert substantial insertional action on the lower rib cage that can result in appreciable lung deflationary effects, particularly at elevated lung volumes.

Mechanics of the abdominal muscles in rabbits and dogs

In anesthetized, apneic rabbits and dogs, direct tetanic stimulations of the abdominal muscles (AMS) were performed at different tracheal pressures (Ptr) in the supine and upright posture. Lung volume (V), esophageal (Pes) and abdominal pressure (Pab), circumference of the upper and lower lung apposed rib cage (Crc, u and Crc, l) and of the abdomen (Cab), and transverse diameter of the rib cage facing the abdominal contents (Drc,ab) were measured. At Ptr = 0, Pab and Pes increased, and V decreased with increasing the strength of AMS; delta Pes and delta V eventually levelled off, while delta Pab was still increasing. Both delta Pes and delta V were larger in the upright posture, whereas delta Pab were similar. Relative to the expiratory reserve volume (ERV), maximal delta V in the supine and upright posture were 75.6 +/- 2.1 (mean +/- SE) and 86.1 +/- 2.2% in rabbits, and 56.5 +/- 3.4 and 75.2 +/- 3.7% in dogs. Maximal AMS decreased V and increased delta Pab the more so the larger the lung volume. In the volume range 10-70% VC, delta V were 3-4% VC larger in the upright posture, while delta Pab were similar in both postures. With AMS, Cab decreased, and Crc,u and Crc,l increased, while Drc,ab increased in dogs and decreased in rabbits. Hence, (a) the abdominal muscles can account for most of the ERV, particularly in the upright posture; (b) their maximal deflationary effects on the lung are already reached with submaximal activation; (c) their expiratory capacity is hindered by the expansion of the lung apposed rib cage and limited by diaphragmatic passive tension, and (d) their efficiency is reduced by paradoxical motion and distortion both between and within the lung apposed rib cage and abdominal compartments. Possible mechanisms for the dependency of delta V on species, volume and posture are discussed.

The electro-mechanical response of canine inspiratory intercostal muscles to increased resistance: the cranial rib-cage

1. The effect of graded increases in inspiratory airflow resistance on the electrical activity and the mechanical behaviour of the three groups of inspiratory intercostal muscles (parasternal intercostal, external intercostal, levator costae) situated in the cranial portion of the rib-cage has been studied in ten anaesthetized, spontaneously breathing dogs. The mechanical behaviour of the muscles was determined by measuring the respiratory changes in muscle length and the displacements of the rib. 2. During unloaded inspiration, the three muscles were active, the rib moved in the cranial direction, and the parasternal intercostal and levator costae muscles shortened; in most animals, the external intercostals shortened as well. 3. Graded increases in inspiratory airflow resistance elicited a progressive inhibition of parasternal intercostal activity and a gradual facilitation of external intercostal and levator costae activities. Concomitantly, the parasternal intercostals continued to shorten during inspiration. However, both the external intercostals and the levator costae progressively lengthened, and the rib was gradually displaced in the caudal direction. This pattern persisted after increases in chemical respiratory drive had developed. 4. Sectioning the phrenic nerve roots did not alter the electrical or the mechanical response of the parasternal intercostal muscles to loading, but it markedly affected the response of the external intercostals and levator costae. After phrenicotomy, the external intercostals and levator costae continued to shorten during loaded breaths, the rib continued to be displaced in the cranial direction, and although the rate of inspiratory muscle shortening and of rib motion decreased, the facilitation of external intercostal and levator costae activities was markedly reduced or abolished. 5. Lengthening of the external intercostals and caudal displacement of the rib was reproduced by isolated stimulation of the phrenic nerves. 6. The reflex facilitation of external intercostal and levator costae activities that takes place during inspiratory resistive loading thus results primarily from the collapsing action of the diaphragm on the cranial portion of the rib-cage and the consequent lengthening of these muscles. The mechanical effectiveness of this reflex facilitation, however, appears to be relatively small.

The electro-mechanical response of canine inspiratory intercostal muscles to increased resistance: the caudal rib-cage

1. The effect of graded increases in inspiratory airflow resistance and airway occlusion on the electrical activity and the mechanical behaviour of the levator costae and external intercostal muscles situated in the caudal interspaces (zone of apposition of the diaphragm to the rib-cage) has been studied in spontaneously breathing dogs. 2. The external intercostal and levator costae muscles in the cranial interspaces were invariably active during unloaded inspiration and showed progressive facilitation of activity with increases in inspiratory resistance. In contrast, whether in the supine or in the prone position, the levator costae muscles of the caudal interspaces did not show any facilitation of activity, and the caudal external intercostal muscles never showed any inspiratory electrical activity, including during airway occlusion. 3. With graded increases in inspiratory airflow resistance, the cranial external intercostals demonstrated a gradual inspiratory lengthening and the cranial ribs were progressively displaced in the caudal direction. The caudal ribs, however, were invariably displaced in the cranial direction. As a result, the caudal external intercostals showed a progressive inspiratory shortening. 4. Shortening of the caudal external intercostals and cranial displacement of the caudal ribs were reproduced by isolated stimulation of the phrenic nerves. Thus, as inspiratory resistance increases, contraction of the diaphragm causes unloading, rather than loading, of the spindles present in the caudal external intercostal muscles. 5. After the phrenic nerves were sectioned, however, the caudal external intercostals invariably lengthened a substantial amount during inspiration, but they still did not show any inspiratory electrical activity. Accentuating the inspiratory lengthening of these muscles by external rib fixation and increasing the chemical respiratory drive did not elicit any inspiratory electrical activity either. The alpha-motoneurones of the external intercostal muscles in the caudal interspaces thus have very small central respiratory drive potentials with respect to their critical firing threshold.

Mechanics of the respiratory system during high frequency ventilation

No rational approach has evolved for selecting operating conditions for clinical application of high-frequency ventilation (HFV). To this end, we divide our discussion of HFV into considerations of mechanics versus transport, and treat the latter as a constraint. After describing some of the phenomena that influence distending pressure (and its distribution) expressed across pulmonary tissues, we address the pressure costs per unit ventilation and the factors that influence them. This narrowly defined approach leads to some fundamental strategies, compromises, and dilemmas. In particular, consideration of the mechanical interaction of the lung and chest wall leads to a paradox, and points out that the influence of the chest wall upon phasic regional lung distension is not well understood.