Respiratory abdominal muscle recruitment and chest wall motion in myotonic muscular dystrophy.

Abdominal muscles are selectively active in normal subjects during stress and may increase the potential energy for inspiration by reducing the end-expiratory lung volume (EELV). We hypothesized that a similar process would occur in subjects with myotonic muscular dystrophy (MMD), but would be less effective, because of to their weakness and altered chest wall mechanics. Fine-wire electromyography (EMG) of the transversus abdominis (TA), internal oblique (IO), external oblique, and rectus abdominis was recorded in 10 MMD and 10 control subjects. EMG activity, respiratory inductive plethysmography, and gastric pressure were recorded during static pressure measurement and at increasing levels of inspiratory resistance breathing. EELV was reduced and chest wall motion was synchronous only in controls. Although the TA and IO were selectively recruited in both groups, EMG activity of the MMD group was twice that of controls at the same inspiratory pressure. In MMD subjects with mildly reduced forced vital capacity, significant differences can be seen in abdominal muscle recruitment, wall motion, work of breathing, and ventilatory parameters.     

Respiratory sensation during chest wall restriction and dead space loading in exercising men.

We mimicked important mechanical and ventilatory aspects of restrictive lung disorders by employing chest wall strapping (CWS) and dead space loading (DS) in normal subjects to gain mechanistic insights into dyspnea causation and exercise limitation. We hypothesized that thoracic restriction with increased ventilatory stimulation would evoke exertional dyspnea that was similar in nature to that experienced in such disorders. Twelve healthy young men [28 +/- 2 (SE) yr of age] completed pulmonary function tests and maximal cycle exercise tests under four conditions, in randomized order: 1) control, 2) CWS to 60% of vital capacity, 3) added DS of 600 ml, and 4) CWS + DS. Measurements during exercise included cardiorespiratory parameters, esophageal pressure, and Borg scale ratings of dyspnea. Compared with control, CWS significantly reduced the tidal volume response to exercise, increased dyspnea intensity at any given work rate or ventilation, and thus limited exercise performance. DS stimulated ventilation but had minimal effects on dyspnea and exercise performance. Adding DS to CWS further increased dyspnea by 1.7 +/- 0.6 standardized Borg units (P = 0.012) and decreased exercise performance (total work) by 21 +/- 6% (P = 0.003) over CWS alone. Across conditions, increased dyspnea intensity correlated best with decreased resting inspiratory reserve volume (r = -0.63, P < 0.0005). Dyspnea during CWS was described primarily as "inspiratory difficulty" and "unsatisfied inspiration," similar to restrictive disorders. In conclusion, severe dyspnea and exercise intolerance were provoked in healthy normal subjects when tidal volume responses were constrained in the face of increased ventilatory drive during exercise.     

Musculoskeletal chest wall pain

The musculoskeletal structures of the thoracic wall and the neck are a relatively common source of chest pain. Pain arising from these structures is often mistaken for angina pectoris, pleurisy or other serious disorders. In this article the clinical features, pathogenesis and management of the various musculoskeletal chest wall disorders are discussed. The more common causes are costochondritis, traumatic muscle pain, trauma to the chest wall, “fibrositis” syndrome, referred pain, psychogenic regional pain syndrome, and arthritis involving articulations of the sternum, ribs and thoracic spine. Careful analysis of the history, physical findings and results of investigation is essential for precise diagnosis and effective treatment.     

Gravitational forces on the chest wall

The gravitational work of breathing was determined by measuring the vertical motion of body mass. The subject, seated or lying supine on a force platform, performed breathing maneuvers in which rib cage volume (Vrc) and abdominal volume (Vab) were changed in varying proportions. The increment in the vertical force exerted on the platform and Vrc and Vab were measured over the course of each maneuver. The force signal was integrated twice with respect to time to obtain the change in the product of mass and height of the subject. This was multiplied by the gravitational acceleration to obtain the change in the gravitational potential (Ug). Simultaneous values of Ug, Vrc, and Vab were taken from the data, and the values of the coefficients for which the following equation best fit these values were determined: Ug = a1 Vrc + a2 Vab + (1/2)a11 Vrc2 + a12 Vrc Vab + (1/2)a22 Vab2. The coefficients a1 and a2 can be interpreted as the values of the expiratory gravitational forces on the rib cage and abdomen, respectively. In the seated posture, the force on the rib cage is expiratory and the force on the abdomen is inspiratory; the magnitudes of both are approximately 8 cmH2O. In the supine posture, both are expiratory forces of approximately 9 cmH2O. The coefficients of the quadratic terms in Ug are all positive, and the gravitational work per unit volume of chest wall expansion increases with increasing volume in both postures. The coefficients of the quadratic terms can be interpreted as gravitational contributions to the elastances of the compartments.     

Surgical management of the radiated chest wall

Fifty consecutive patients with radiation-related problems of the chest wall were treated between 1976 and 1984. There were 40 women and 10 men with an average age of 54 years (range 26 to 78 years). Twenty-three patients had radiation ulcers alone, 20 had recurrent cancer, and 7 had infected median sternotomy wounds. Thirty-six had skeletal resections and 44 had soft-tissue resections. The skeleton was reconstructed with Prolene mesh in 12 patients and with autogenous rib in 3. Sixty-three muscles were transposed in 43 patients. Twelve omental transpositions were performed (8 for primary treatment and 4 for salvage of a failed muscle flap). Hospitalization averaged 20.2 days. There was one operative death (at 29 days). Partial flap necrosis occurred in 10 patients. Mesh was removed in three patients. There were 14 late deaths, most from recurrent tumor. The remaining patients had well-healed wounds and a generally improved quality of life. We conclude that aggressive resection and reliable reconstruction are critical considerations in the surgical management of this perplexing clinical problem.     

Relative strengths of the chest wall muscles

We hypothesized that during maximal respiratory efforts involving the simultaneous activation of two or more chest wall muscles (or muscle groups), differences in muscle strength require that the activity of the stronger muscle be submaximal to prevent changes in thoracoabdominal configuration. Furthermore we predicted that maximal respiratory pressures are limited by the strength of the weaker muscle involved. To test these hypotheses, we measured the pleural pressure, abdominal pressure (Pab), and transdiaphragmatic pressure (Pdi) generated during maximal inspiratory, open-glottis and closed-glottis expulsive, and combined inspiratory and expulsive maneuvers in four adults. We then determined the activation of the diaphragm and abdominal muscles during selected maximal respiratory maneuvers, using electromyography and phrenic nerve stimulation. In all subjects, the Pdi generated during maximal inspiratory efforts was significantly lower than the Pdi generated during open-glottis expulsive or combined efforts, suggesting that rib cage, not diaphragm, strength limits maximal inspiratory pressure. Similarly, at high lung volumes, the Pab generated during closed-glottis expulsive efforts was significantly greater than that generated during open-glottis efforts, suggesting that the latter pressure is limited by diaphragm, not abdominal muscle, strength. As predicted, diaphragm activation was submaximal during maximal inspiratory efforts, and abdominal muscle activation was submaximal during open-glottis expulsive efforts at midlung volume. Additionally, assisting the inspiratory muscles of the rib cage with negative body-surface pressure significantly increased maximal inspiratory pressure, whereas loading the rib cage muscles with rib cage compression decreased maximal inspiratory pressure. We conclude that activation of the chest wall muscles during static respiratory efforts is determined by the relative strengths and mechanical advantage of the muscles involved.     

Direct measurement of static chest wall compliance in animal and human neonates

The measurement of pulmonary mechanics has been developed extensively for adults, and these techniques have been applied directly to neonates and infants. However, the compliant chest wall of the infant frequently predisposes to chest wall distortion, especially when there is a low dynamic lung compliance (CL,dyn). We describe a technique of directly measuring the static chest wall compliance (Cw,st), developed initially in the newborn lamb and subsequently applied to the premature neonate with chest wall distortion. The mean CL,dyn in seven intubated newborn lambs in normoxia was 2.45 +/- 0.41 ml.cmH2O-1.kg-1, whereas Cw,st was 11.81 +/- 0.25 ml.cmH2O-1.kg-1. These values did not change significantly in seven animals breathing through a tight-fitting face mask or with hypercapnia-induced tachypnea. For the eight premature infants the mean CL,dyn was 1.35 +/- 0.36 ml.cmH2O-1.kg-1, whereas the mean Cw,st was 3.16 +/- 1.01 ml.cmH2O-1.kg-1. This study shows that, under relaxed conditions when measurements of static compliance are performed, the chest wall is more compliant than the lung. The measurement of Cw,st may thus be used to determine the contribution of the respiratory musculature in stabilizing the chest wall.     

Mechanics of compartmental models of the chest wall

Standard methods for describing the mechanical properties of a linear elastic system are applied to the two- and three-compartment models of the chest wall. The compliance matrix and the experiments required to determine the entries in this matrix and thereby to describe the mechanical properties of the relaxed chest wall are described. The effective forces exerted by external loads and muscle tension are defined. The formal theory is used to identify relations among variables. From the definition of effective force, it follows that the ratio of the forces exerted by the diaphragm on the rib cage and abdomen is the same as the ratio of the dependence of diaphragm length on rib cage and abdominal volumes. As an example of relations among variables that follow from the symmetry of the compliance matrix, it is shown that the change of gastric pressure caused by raising pleural pressure is related to the change in lung volume caused by changing stomach volume.     

Mechanical aspects of chest wall distortion

During passive inflation of the respiratory system, the rib cage (RC) expands because the pressure applied to it [approximately equal to abdominal pressure (Pab)] increases. Similar Pab-tidal volume (VT) relationships between passive and spontaneous inspirations would occur only if 1) Pab acts on RC equally in the two situations (no distortion) or 2) the extradiaphragmatic inspiratory muscles expand RC, compensating for distortion. In anesthetized adult rats and in sleeping human infants the passive relationships between VT and Pab or abdomen motion (AB) were constructed by occluding the airways during expiration. For a given Pab (or AB) in active breathing VT averaged 55% (rats) and 49% (infants) of the passive volume change. With phrenic stimulation in rats VT was only slightly less than during spontaneous breathing, indicating that, in the latter case, the respiratory system was essentially driven only by the diaphragm. In both species occasional breaths with large RC expansion occurred, and VT was then equal to or larger than the passive volume at iso-Pab. We conclude that 1) RC distortion decreases VT to approximately half of the passive value and 2) being on the relaxation curve reflects "compensated" distortion and not absence of it.     

Theoretical analysis of chest wall mechanics

A mathematical model of the chest wall partitioned into rib cage, diaphragmatic and abdominal components is developed consistent with published experimental observations. The model describes not only the orthodox chest wall movements (rib cage and abdomen expand together during inspiration) of the quietly breathing standing adult, but also Mueller maneuvers (inspiration against an occluded airway opening) and the paradoxical breathing patterns (rib cage contracts while abdomen expands during inspiration) observed in quadriplegia and in the newborn. The abdomen is inferred to act as a cylinder reinforced by the abdominal muscles functioning similarly to bands around a barrel. The rib cage and abdominal wall are inferred to act not as though they were directly attached to one another, but as though they were being pressed together by the skeleton. Furthermore, transabdominal pressure is visualized as acting, not across the rib cage isolated from the diaphragm, as has been suggested previously, but instead, across the combined rib cage and diaphragm acting as a deformable unit containing the lungs.     

Effect of intravenous midazolam on breathing pattern and chest wall mechanics in human

Breathing pattern, thoracoabdominal motion, and separate end-expiratory positions of the rib cage and abdomen were measured noninvasively in eight healthy subjects before and after intravenous administration of either placebo or midazolam, a short-acting benzodiazepine. Compared with placebo, midazolam produced a significant (P less than 0.01) decrease in mean inspiratory flow of 29% from preinjection values, resulting in a 39% reduction in tidal volume (VT). This ventilatory depression was partly compensated by a 35% decrease in expiratory time producing an increase in respiratory rate (+39%). The fall in VT was almost entirely (91%) mediated by a reduction of the abdominal contribution to tidal breathing while sparing rib cage motion. This fact contrasts with the effects of inhalational anesthetics or morphine, which preferentially depress rib cage expansion, indicating that thoracoabdominal motion may selectively be depressed by different pharmacological agents. In addition, continuous recording of end-expiratory levels showed a significant transient fall in the rib cage's end-tidal position 2 min after midazolam administration associated with the occurrence of central apneas.