Version May 2024
INTRODUCTION — The chest wall is a critical component of the respiratory pump, the structures responsible for creating the bulk flow of gas from the atmosphere to the alveoli and back out. Diseases that alter the structure of the chest wall affect the function of the pump and may result in respiratory compromise or failure. The components of the chest wall include the bony structures (ribs, spine, sternum), respiratory muscles, and nerves connecting the central nervous system with the respiratory muscles. The various forces acting upon the mechanical structure of the chest wall play a major role in determining lung volume, and chest wall abnormalities can have a significant impact on lung function; in particular, chest wall pathology may be a major contributor to restrictive physiology of the respiratory system.
Chest wall structure and physiology and diseases that affect the respiratory function of the chest wall will be reviewed here. Included are discussions of the following conditions:
●Congenital and childhood abnormalities
●Kyphosis and scoliosis
●Ankylosing spondylitis
●Traumatic and iatrogenic processes (eg, flail chest and thoracoplasty)
●Obesity
Diseases affecting chest wall nerves and muscles are discussed separately:
●(See "Respiratory muscle weakness due to neuromuscular disease: Clinical manifestations and evaluation".)
●(See "Respiratory muscle weakness due to neuromuscular disease: Management".)
●(See "Respiratory physiologic changes following spinal cord injury".)
●(See "Diagnostic evaluation of adults with bilateral diaphragm paralysis".)
APPROACH TO THE PATIENT — While most chest wall abnormalities are easily diagnosed when patients present with more severe manifestations of the condition, it is not uncommon to encounter a patient with dyspnea of unknown etiology who may have a more subtle form of chest wall disease. Clues to the effects of the chest wall on dyspnea (such as the presence of kyphoscoliosis, abdominal pathology, prior rib cage surgery, etc) are often present on physical examination but might not be elicited unless sought out and considered in the differential diagnosis. (See "Approach to the patient with dyspnea" and "Physiology of dyspnea".)
Normal structure and function — The rib cage and vertebral column form the boundaries of the chest wall superiorly, posteriorly, and laterally, while the diaphragm constitutes the inferior component. The external boundary of the lower chest wall is the lower border of the rib cage, but internally, this boundary overlaps with the diaphragm, which lies adjacent to as much as one-half of the rib cage at residual volume (RV). By virtue of this "zone of apposition" between the diaphragm and the ribs, the rib cage is exposed both to abdominal and pleural pressures, such that the normal function of the chest wall depends on the intricate interaction of the two spaces. The mechanics of the chest wall and its interaction with the lung are described by a pressure-volume curve (figure 1). Changes in the intra-abdominal contents or compliance of the abdominal wall alter the mechanical properties of the chest wall through their effects on the motion of the diaphragm.
●Effect of position – In the upright position, the resting volume of the chest wall can be approximated by inhalation to 75 percent of vital capacity (ie, add 75 percent of the volume of the vital capacity [VC] to the residual volume [RV] to obtain the volume of the chest wall at rest). The rib cage recoils outward when the volume is smaller and recoils inward when the volume is larger. In contrast, the resting volume of the respiratory system as a whole, the functional residual capacity (FRC), is approximately 35 percent of VC. This measure takes into account the mechanical properties of the chest wall plus the inward recoil of the lung. Moving the chest wall from its resting position (either to make it larger or smaller) requires energy to overcome its elastic properties. The FRC, or relaxation volume of the respiratory system, represents the balance of forces of the chest wall (recoiling outward toward its resting position) and the lungs (recoiling inward toward their resting position).
The position of the chest wall curve (figure 1) differs in the upright and supine positions because the abdominal contents are displaced into the thoracic cavity in the supine position [1]. In the supine position, gravity pulls the abdominal contents into the thoracic cavity (the "expiratory effect"), and the resting volumes are less than when erect by 15 percent of the VC. In the lateral position, the "expiratory effect" applies to the dependent side, such that the vital capacities may differ by 20 percent between the two lungs. As a general rule, given the length-tension relationship of skeletal muscle, reductions in lung volume due to upward displacement or stretch of the diaphragm resulting from increased intra-abdominal pressure lead to improved tension generation when the muscle is activated by a neurological impulse triggering inspiration.
●Interactions between chest wall components – The excursion of the chest wall depends upon complex interactions between its components. The motion of the ribs depends upon their attachments to the sternum and vertebral column and is influenced by the adjoining muscle groups. The parasternal and scalene muscles insert on ribs 1 through 6; the costal portion of the diaphragm inserts on the sternum and ribs 7 through 12. Flexion and extension of the spine can result in displacement of the rib cage and abdominal wall to account for as much as 50 percent of VC. The diaphragm is innervated by the phrenic nerve, traverses an average of 9.5 centimeters during generation of the VC, and affects the rib cage through changes in abdominal and pleural pressures as well as through direct effects upon the ribs on which it inserts. The muscles of the abdominal wall are important muscles of expiration, and contraction of these muscles can also have a significant effect on lung volume.
Changes in compliance — Most abnormalities of the chest wall are associated with decreased compliance (ie, increased stiffness) of the respiratory system; notable exceptions are flail chest and paralysis of the intercostal muscles, which increase compliance. This results in a decrease in total lung capacity (TLC). It is difficult to generalize, however, about the effect on FRC and RV. Rather, one must consider the balance of forces described above and the new resting position of the chest wall.
Since the resting position of the chest wall for a particular condition is usually above FRC, the FRC will be increased since it is more difficult to move the chest wall from its resting position if compliance of the chest wall is also reduced. On occasion when the resting position of the chest wall is below a "normal" FRC, the FRC may also be reduced either because of the associated downward shift in the balance of forces or because regions of the underlying lung have become atelectatic, as in obesity and severe scoliosis.
RV will generally be elevated when the chest wall compliance is decreased (more difficult to compress the rib cage from its resting position), although the RV is preserved in patients with class 3 or higher obesity (table 1) due to dynamic considerations (ie, the weight of the chest wall leads to airway compression, increased expiratory resistance, prolonged expiration, and air trapping).
Despite the extra "weight" on the chest wall associated with obesity, studies indicate that compliance of the chest wall is not altered in these patients. Rather, the pressure-volume relationship of the chest wall is shifted to the right, ie, it requires more energy to get the chest wall moving, but once it is expanding, the slope of the compliance curve (C = [V2 – V1] / [P2 – P1] = change in volume / change in pressure) is unchanged; respiratory system compliance may be reduced in these patients due to alterations in the lung compliance consequent to breathing at chronically low lung volumes in dependent regions [2].
Pulmonary function testing — For patients who are able to perform them, pulmonary function tests (PFTs) help to determine the degree of respiratory impairment caused by a chest wall abnormality and whether other respiratory diseases are contributing to the patient's symptoms.
Testing typically includes upright spirometry, lung volumes, and diffusing capacity for carbon monoxide (DLCO). If respiratory muscle dysfunction is suspected, supine spirometry and maximal inspiratory and expiratory pressures are also obtained (lung volumes are more affected in the supine position in patients with chest wall muscle weakness because the effect of gravity to move the abdominal contents away from the diaphragm is lessened).
In individuals with chest wall abnormalities, PFTs usually reveal a restrictive ventilatory defect defined as a reduced TLC (algorithm 1). In contrast to patients with interstitial lung disease, the RV is often preserved or may be elevated. Respiratory muscle strength is preserved or mildly decreased. (See "Overview of pulmonary function testing in adults".)
Exercise capacity may be reduced, but desaturation with exercise is uncommon, contrary to what is frequently observed in patients with interstitial lung disease. However, in some conditions (eg, scoliosis, obesity), atelectasis may be present and associated with hypoxemia.
Patients with more severe restrictive ventilatory defects are at risk for hypoventilation and hypercapnia. The point at which hypercapnia develops varies among patients depending on the type of chest wall abnormality and other factors that affect chest wall compliance and respiratory muscle strength. When the VC decreases to less than 40 percent of predicted, monitoring for hypercapnia is appropriate.
Imaging — Conventional chest and anteroposterior spine radiographs can help identify and characterize many thoracic cage abnormalities.
Computed tomography (CT) is useful for determining the severity of pectus excavatum, pectus carinatum, and selected patients with scoliosis. CT imaging is also helpful for identifying additional thoracic injuries in patients with rib fractures (flail chest), such as pulmonary contusions and associated pleural effusion. While rare, thoracoplasty can have long-term consequences on chest wall function, and CT can help determine the extent of abnormalities.
High-resolution computed tomography (HRCT) is very sensitive for most forms of interstitial lung disease and can be used to exclude that diagnosis if the differentiation between chest wall and interstitial lung disease remains uncertain after the history, physical exam, and PFTs are complete.
CONGENITAL AND CHILDHOOD ABNORMALITIES — Several congenital abnormalities can impair chest wall function; of these, pectus excavatum, pectus carinatum, and Poland syndrome are the most important [3].
Pectus excavatum — Pectus excavatum (also called funnel chest) is the most frequent congenital anterior chest wall deformity (1/300 to 1/400 births) and is characterized by a concave depression that may be a broad shallow defect or a narrow central pocket (picture 1 and image 1). The condition affects boys more frequently than girls (3:1) and usually is discovered within the first year of life. Infants and young children generally have no symptoms, but older patients may complain of mild dyspnea on exertion and pain in the area of rib deformity after exercise. (See "Pectus excavatum: Etiology and evaluation" and "Pectus excavatum: Treatment".)
Abnormalities on pulmonary function testing are two to three times less common than subjective pulmonary complaints, and the correlation between the two is poor. However, normal pulmonary function tests (PFTs) do not exclude the possibility of cardiopulmonary limitation during exercise.
Pectus carinatum — Pectus carinatum (also called pigeon chest) is less common than pectus excavatum (1/1500 to 1/2000 births) and usually presents with a protrusion of the sternum and costal cartilages. In contrast to pectus excavatum, pectus carinatum often is not discovered until the pubertal growth spurt, such that patients often present in adolescence. As with pectus excavatum, the cause is unknown, boys are more frequently affected, and associated conditions include scoliosis and congenital heart disease. The cosmetic appearance is usually the primary concern, although some patients have exertional dyspnea, frequent respiratory infections, or asthma. Results of PFTs are typically normal, except for rare patients with associated scoliosis. (See "Pectus carinatum and arcuatum".)
Poland syndrome — Poland syndrome (also known as Poland sequence) occurs in 1/10,000 to 1/100,000 live births and encompasses a constellation of abnormalities [4]. It is characterized by partial (28 percent) or complete (72 percent) absence of the pectoral muscles, most commonly unilateral (93 percent) [4]. Associated anomalies may include aplasia or hypoplasia of other chest wall muscles, breast tissue, nipple, absence of the costal cartilages of ribs 2 to 4 or 3 to 5, high-riding scapula (Sprengel deformity), and digital abnormalities (eg, brachydactyly, syndactyly) [4-7]. Most patients do not have respiratory symptoms, although those with missing ribs may have paradoxical respiratory movement of the chest wall (inward motion during inspiration), similar to that seen in flail chest (figure 2). Abnormalities of the pectoralis major muscle and hands (syndactyly) are often associated. (See "Breast development and morphology", section on 'Abnormalities in breast development' and "Breast disorders in children and adolescents", section on 'Athelia and amastia'.)
Asphyxiating thoracic dystrophy — Asphyxiating thoracic dystrophy (ATD, also called Jeune's syndrome) is an autosomal recessive skeletal dysplasia with multiorgan involvement [8,9]. A number of genetic variants causing defective intraflagellar transport have been identified and associated with ATD [10,11]. ATD has been noted in adults [12]; a similar presentation can occur years after pectus excavatum repair surgery [13].
●Skeletal hallmarks include short ribs, narrow thorax, short limbs, and sometimes polydactyly [8]. Patients generally develop progressive respiratory insufficiency in the first few months of life related to the abnormally small thorax.
●Cystic kidney disease resulting in renal insufficiency is another common manifestation of ATD.
●Other disease associations include hepatic insufficiency, cystic pancreatic masses, and retinal abnormalities [8,14].
Therapeutic options available for thoracic manifestations of ATD include noninvasive ventilation [15], lateral thoracic expansion, dynamic thoracoplasty, or sternal and chest wall reconstruction, with or without the use of bone grafts [16].
Fibrodysplasia ossificans — Fibrodysplasia ossificans is a heritable disorder caused by a genetic variant in the gene ACVR1/ALK2 encoding Activin A receptor type I/Activin-like kinase 2, a bone morphogenetic protein (BMP) type I receptor. It is characterized by congenital malformations of the great toes and progressive heterotopic ossification, which can affect the chest wall and lead to respiratory insufficiency [17-19].
KYPHOSIS AND SCOLIOSIS — Kyphosis represents anteroposterior angulation of the spine; mild thoracic kyphosis is a normal finding. Scoliosis represents lateral displacement or curvature of the spine. When the angulation of kyphosis is excessive and/or moderate to severe scoliosis is present, severe rib distortion may result. (See "Adolescent idiopathic scoliosis: Clinical features, evaluation, and diagnosis" and "Scoliosis in the adult".)
The term kyphosis is sometimes used in reference to hyperkyphosis. Hyperkyphosis is excessive curvature of the thoracic spine, commonly known as the "dowager's hump." In patients who primarily exhibit kyphosis, deformity above T10 is associated with a greater degree of respiratory impairment [20]. (See "Hyperkyphosis in older persons".)
The severity of kyphosis or scoliosis is defined by measurement of the Cobb angle of curvature that is formed by the limbs of the convex primary curvature (figure 3 and image 2). The degree of spinal deformity is the most important risk factor for respiratory insufficiency.
Pulmonary function — The effects of kyphosis and scoliosis are additive in lung function impairment [21], although the degree of pulmonary function impairment may be underestimated by standard radiographic assessment of scoliosis [22]. Other measures, such as sagittal diameter of the thoracic cage and total lung area, may add to the ability to predict the effect on vital capacity (VC) [23].
Among individuals with kyphosis, declines in VC correlate with the degree of kyphosis, impairments being most notable at kyphotic angles >55 degrees and with an increased number of vertebral compression fractures. Data from the Framingham Study suggest that the presence of kyphosis is associated with a decline in lung function in females, but not males [24]. (See "Hyperkyphosis in older persons", section on 'Impaired pulmonary function'.)
In patients with scoliosis a restrictive pattern of pulmonary function is commonly present, with decreased total lung capacity (TLC) and VC, but preserved residual volume (RV), such that the RV/TLC ratio is increased (figure 4) [21,25-27]. (See "Adolescent idiopathic scoliosis: Clinical features, evaluation, and diagnosis".)
The reduced compliance of the chest wall in scoliosis changes its resting position, such that functional residual capacity (FRC) is decreased. Lung compliance, which is initially normal, decreases in more severe scoliosis due to progressive atelectasis and air trapping, a consequence of breathing at low lung volumes. This reduced compliance of the entire respiratory system (chest wall and lung) increases the work of breathing. Interestingly, children with spinal deformities have normal chest wall compliance, perhaps due to their more pliant chest walls [28,29].
Among patients with severe scoliosis or kyphoscoliosis, maximal inspiratory pressure (MIP) is decreased, approximately one-half normal in eucapnic patients and one-quarter normal in hypercapnic patients, either due to intrinsic muscle weakness or mechanical disadvantage due to the rib cage distortion [26,29].
Despite a frequent complaint of exercise limitation among patients with scoliosis, the indices of ventilatory performance during exercise are essentially normal with the exception of reduced tidal volumes. The breathing pattern response to exercise and maximum tidal volume to VC ratio are normal. Hence, exercise intolerance may be a result of physical deconditioning [30,31], although the discrepancy between the desired and achieved tidal volume (efferent-reafferent dissociation) may also contribute to the dyspnea. (See "Physiology of dyspnea".)
Effect of aging — Chest wall compliance decreases with age, further increasing work of breathing and risk of respiratory muscle fatigue in older adults [29,32]. Thus, these patients tend to breathe with lower tidal volumes and increased respiratory rate. Although this breathing pattern decreases respiratory effort (it is easier to expend effort on increased respiratory rate and flow than to stretch a stiff chest wall), dead space fraction may be increased, and alveolar hypoventilation may ensue with resultant hypercapnia [21]. Otherwise, hypoxemia without hypercapnia is seen in moderate to severe disease, and ventilation-perfusion (V/Q) mismatch has been reported with a scoliosis angle greater than 65 degrees [29,33].
Pulmonary hypertension develops in some patients as a result of persistent hypoxemia [34]. Nocturnal hypoventilation and arterial oxygen desaturation are described, particularly during rapid eye movement (REM) sleep. This is likely a result of decreased accessory muscle use during REM, as well as possible distortion of the upper airway, which predisposes to apneic events [35].
In addition to evaluation for surgical therapy in severe disease, other therapeutic options may improve dyspnea in patients with advanced kyphoscoliosis. Use of supplemental oxygen for hypoxemia and noninvasive positive pressure ventilation (NIV) for nocturnal hypoventilation may treat pulmonary hypertension [36,37]. Use of NIV at night and intermittently during the day may help alleviate atelectasis and rest inspiratory muscles even in the absence of chronic hypercapnia. Pulmonary rehabilitation may also improve the respiratory muscle weakness and peripheral muscle and cardiovascular deconditioning that contribute to exercise intolerance.
●(See "Hyperkyphosis in older persons", section on 'Management'.)
●(See "Scoliosis in the adult", section on 'Management'.)
ANKYLOSING SPONDYLITIS — Ankylosing spondylitis is a chronic inflammatory disease affecting joints of the axial skeleton with resultant fibrosis and ossification of the ligamentous structures of the spine, sacroiliac joints, and rib cage. The clinical manifestations and diagnosis of ankylosing spondylitis are discussed separately. (See "Clinical manifestations of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults".)
Respiratory manifestations — Ankylosing spondylitis causes pleuropulmonary disease in less than 2 percent of patients, most commonly in the form of chest wall restriction and upper lobe fibrocystic parenchymal disease [38,39]. Patients with apical fibrobullous disease have usually suffered from ankylosing spondylitis for at least 15 years. (See "Clinical manifestations of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults", section on 'Pulmonary disease'.)
Ankylosing spondylitis causes fixation of the chest wall through fusion of the costovertebral joints and spinal kyphosis [40-42]. Similarly, the anterior chest wall can be affected by enthesitis (inflammation of the muscular or tendinous attachments to bone) of the manubriosternal symphysis and sternoclavicular joints. Clinical evidence of chest wall disease includes limitation of chest wall expansion to 2.5 centimeters when measured at the level of the fourth intercostal space [43].
Less commonly, ankylosing spondylitis affects the cricoarytenoid joint, and patients can present with hoarseness, sore throat, upper airway obstruction, or respiratory failure. The respiratory manifestations of ankylosing spondylitis that are unrelated to the chest wall and spine are discussed separately. (See "Clinical manifestations of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults", section on 'Pulmonary disease'.)
Treatment of chest wall restriction in ankylosing spondylitis is primarily preventive and supportive, focused on pain relief and maintenance of spinal flexibility through exercise and stretching. (See "Treatment of axial spondyloarthritis (ankylosing spondylitis and nonradiographic axial spondyloarthritis) in adults", section on 'Nonpharmacologic interventions'.)
Pulmonary function testing — Patients with ankylosing spondylitis often have normal pulmonary function tests (PFTs) or manifest only a mild restrictive ventilatory defect, with a mild decrease in the total lung capacity (TLC) and vital capacity (VC) (figure 4) [44]. The degree of restrictive ventilatory defect (reduced forced vital capacity [FVC] and forced expiratory volume in one second [FEV1]) appears to correlate with loss of spinal mobility [45]. Relative preservation of lung volumes has been attributed to compensation by the diaphragm with increased abdominal excursion, fixation of the thorax at greater lung volumes, and preservation of chest wall symmetry and rib excursion [40-42,46]. The functional residual capacity (FRC) and residual volume (RV) are normal or increased due to fixation of the rib cage, leading to a higher resting position of the chest wall.
Gas exchange, airflow, and lung compliance are usually normal. Maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) may be mildly reduced [46,47].
Patients may rarely complain of exercise limitation due to dyspnea. Formal testing may reveal exercise limitation with decreased maximal oxygen consumption (VO2 max), but the extent to which this correlates with limited chest wall expansion is debated [48,49]. Some authors report that exercise limitation does not occur until the VC is less than 75 percent predicted [50], while others have invoked additional factors such as deconditioning and inspiratory muscle fatigue to account for the exercise limitation [51]. Nonspecific body fatigue can confound interpretation of self-reported exercise capacity capability [52].
TRAUMATIC AND IATROGENIC PROCESSES
Flail chest — Flail chest is defined as the presence of three or more consecutive ribs with fractures in two or more places creating a floating segment in the chest wall (figure 5). Flail chest is usually a consequence of blunt chest trauma and the number of fractured ribs has been shown to correlate with the severity of intrathoracic injury [53,54]. The entity is uncommon in children due to their more pliable chest wall. (See "Inpatient management of traumatic rib fractures and flail chest in adults", section on 'Flail chest'.)
●Respiratory consequences – In patients with flail chest, the rib fractures, sometimes in combination with sternal fractures, uncouple part of the chest wall from rest of the rib cage. The uncoupled segment is therefore subjected to unopposed pleural pressures, resulting in paradoxical motion (figure 2) and restricted inhalation.
Larger flail segments result in hypoventilation due to the inability to generate effective intrathoracic pressure. This situation usually results in further atelectasis, which worsens gas exchange. The increased work of breathing and decreased respiratory efficiency place the patient at risk for respiratory muscle fatigue. (See "Inpatient management of traumatic rib fractures and flail chest in adults", section on 'Respiratory failure'.)
Patients with flail chest who are treated conservatively without fixation, may subsequently develop restricted ventilation. (See "Surgical management of severe rib fractures", section on 'Fracture nonunion/malunion'.)
●Associated conditions – Pulmonary contusion is a frequent associated condition of blunt chest trauma and, if present, may account for a significant alveolar-arterial (A-a) gradient. Trauma patients are also at significant risk for pneumonia, atelectasis, pneumothorax, hemothorax, and acute respiratory distress syndrome.
Thoracoplasty — Thoracoplasty represents the resection of ribs or their replacement with an inward convexity device intended to decrease thoracic volume [55]. The only current use of the procedure is closure of a persistent pleural space, but prior to the 1950s, this procedure was performed as collapse therapy for cavitary tuberculosis (the decrease in ventilation associated with the chest wall defect reduced alveolar oxygen levels in this area, thereby depriving the tuberculosis bacteremia of oxygen).
●Pulmonary function – Thoracoplasty results in a restrictive ventilatory defect with decreases in total lung capacity (TLC) and vital capacity (VC) and preserved residual volume (RV) (figure 4). This restrictive defect likely results from diminished inspiratory volume from chest wall distortion, and perhaps from concomitant pulmonary fibrosis secondary to tuberculosis. The ratio of diffusing capacity for carbon monoxide/alveolar volume (DLCO/VA) is usually normal or increased in non-TB patients, as the reduction in DLCO due to incomplete lung expansion is proportionately less than the reduction in VA.
Patients also may have an obstructive ventilatory defect that worsens with age and may be due to either tuberculosis or the effects of cigarette smoking [56]. This feature is fairly unique among chest wall diseases. Exercise capacity is limited due to diminished ventilatory capacity [57].
●Management – Treatment is primarily supportive. A course of pulmonary rehabilitation may be helpful in improving symptoms and increasing six-minute walk distances for these patients [58]. For patients with hypercapnia, nocturnal noninvasive ventilation can be beneficial. (See "Noninvasive ventilation in adults with chronic respiratory failure from neuromuscular and chest wall diseases: Practical aspects of initiation".)
ABDOMINAL PROCESSES — Processes that alter intra-abdominal pressure can profoundly affect the compliance of the chest wall and pulmonary function. This section will focus on the chest wall effects of obesity and ascites.
Obesity — Obesity is associated with restrictive physiology on pulmonary function testing due to increased weight of the chest wall and increased abdominal adipose tissue restricting diaphragmatic motion [59-62]. The severity of obesity is most commonly determined by the body mass index (BMI), which is calculated by dividing the body weight (in kg) by the height (in meters) squared. A BMI greater than 30 kg/m2 is commonly used as a definition of obesity (calculator 1) (table 1). (See "Obesity in adults: Prevalence, screening, and evaluation".)
Other respiratory effects of obesity, such as hypoventilation with carbon dioxide retention and obstructive sleep apnea are discussed separately. (See "Clinical manifestations and diagnosis of obesity hypoventilation syndrome" and "Treatment and prognosis of the obesity hypoventilation syndrome" and "Clinical presentation and diagnosis of obstructive sleep apnea in adults".)
Pulmonary function — Total lung capacity (TLC) and functional residual capacity (FRC) are both reduced by obesity because the increased mass of the chest wall alters the balance of forces (recoil of chest wall, lungs, and muscle force) that determine these lung volumes. The decrease in FRC correlates with a reduction in expiratory reserve volume (ERV). In contrast, residual volume (RV) is relatively preserved patients with obesity older than age 40 years, the age at which RV is largely determined by dynamic factors [59,62-64]. Specifically, elastic recoil of the lung, which helps to tether open small airways, decreases with age, airways increasingly narrow at low lung volumes, and airway resistance becomes a determining factor for RV.
Some authors have reported a normal vital capacity (VC) until the extremes of obesity [65], while others have proposed a decrement across a range of weights, with some suggesting a more specific correlation, such as a loss of 26 cc of VC for each kg gain in weight [66]. Airway resistance is increased in obese patients, presumably due to the increase in pleural pressure that results from the weight of the chest wall and compresses small airways [59]. When airway resistance is corrected for lung volume, the difference between patients with obesity and those with normal BMI is mitigated. The presence of obesity leads to breathing at lower lung volumes where airway diameter is smaller (the diameter of airways is dependent on the absolute volume of the lungs). The forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC) are reduced in obesity both in absolute terms and as a percentage of predicted values [59]. The FEV1/FVC ratio is usually preserved.
Chest wall compliance may be mildly reduced, likely from the added pressure from chest wall and intra-abdominal adipose tissue. Lung compliance is reduced by approximately 25 percent, perhaps from increased pulmonary blood volume and early airways closure [67]. Respiratory muscle strength is generally preserved, although respiratory muscle efficiency is decreased as a result of increased mechanical work and oxygen cost of breathing. Small decrements in maximal expiratory pressure have been observed and attributed to less effective abdominal muscle contraction [59].
An alternative hypothesis for the respiratory physiology observed in patients with obesity is that breathing at abnormally low lung volumes reduces lung compliance, which in turn decreases overall respiratory system compliance. A study performed in 51 anesthetized patients with obesity used esophageal balloons to measure pleural pressure and more directly assess lung and chest wall compliance [2]. From this study, it appeared that the reduction in lung compliance, associated with the low lung volume at FRC and possibly additional changes in the pulmonary airways and parenchyma, was primarily responsible for the low respiratory system compliance; chest wall compliance appeared normal, although the pressure-volume curve for the chest wall was displaced to the right (ie, greater pressure was required to get the chest to move, but once volume was increasing the slope of the inflation (which is compliance) was normal.
The changes in lung function noted above may be more severe when the patient moves from the upright to the supine posture [59,68]. Patients with class 3 or higher obesity (table 1) can develop significant dyspnea in the supine position and often will sleep in a reclining chair to minimize the effect of the abdomen on the compliance of the chest wall.
In addition, expiratory flow limitation and intrinsic positive end-expiratory pressure (ie, auto-PEEP) may be detected during tidal breathing in severely overweight individuals (mean BMI 44 kg/m2) [69]. These findings are worsened in the supine position. The presence of auto-PEEP further increases the work of breathing, which is already elevated due to reduced compliance of the lung and possibly the chest wall and increased airway resistance.
Together, chest wall and abdominal adipose tissue can decrease the resting lung volume at relaxed end-exhalation (FRC), which in turn can lead to microatelectasis. Normoxia or mild hypoxemia is usually present in the upright position, though many patients with obesity develop further hypoxemia as well as expiratory flow limitation when supine [70,71]. Gas exchange abnormalities may be the result of ventilation-perfusion (V/Q) mismatch with preserved perfusion to the bases and diminished ventilation to those areas from atelectasis. The relationship between BMI and hypoxemia appears to be stronger in males than females, and arterial oxygen tension (PaO2) is lower in patients with reduced ERV than those in whom that volume is preserved [72]. Most patients with obesity alone remain eucapnic despite a large ventilatory demand. These patients generally have an elevated resting respiratory rate with a tidal volume that is normal overall, but is reduced when adjusted for lean body mass [73]. Diffusing capacity is typically greater than normal, when corrected for alveolar volume (VA).
Exercise capacity is frequently reduced in obesity due to increased work required to overcome the excess ventilatory load and concomitant cardiovascular deconditioning [60]. Maximal voluntary ventilation (MVV) may be reduced as well [65,74].
Effect of weight loss — Modest weight loss (some authors report in the range of 35 kg) may improve pulmonary function and exercise tolerance [62,75-78]. Proportional assist ventilation, which adjusts the inspiratory pressure in proportion to patient effort, has been shown to increase exercise duration in individuals with obesity by more than 20 percent in approximately half of subjects tested [79]. (See "Noninvasive ventilation in adults with chronic respiratory failure from neuromuscular and chest wall diseases: Patient selection and alternative modes of ventilatory support".)
Ascites — Ascites refers to the accumulation of extravascular fluid within the peritoneum, occurring as a result of a variety of processes, including hypoalbuminemia, heart failure, cirrhosis, and malignancy (see "Pathogenesis of ascites in patients with cirrhosis"). Patients often experience shortness of breath once sufficient abdominal fluid has accumulated (usually described as tense ascites) to cause increased intrapleural pressure from abdominal distention. Rigidity of the diaphragm and the thoracic and abdominal walls may affect pulmonary function and increase the work of breathing [80]. Most studies of respiratory function in ascites have been performed in patients with alcoholic cirrhosis.
●Pulmonary function – Lung volumes are generally reduced, with mild to moderate reductions in FRC, TLC, FVC, and expiratory reserve volume (ERV) reported by multiple authors [80-84]. Mild to moderate improvement in lung volumes, most consistently in the FRC and TLC, are seen after large-volume paracentesis. The diffusing capacity for carbon monoxide (DLCO), FEV1, and PaO2 are usually reduced but may not improve after paracentesis. In contrast, lung volumes may be preserved in pregnancy (another condition associated with increased intra-abdominal pressure) due to "coupling" of the diaphragm and rib cage and increased rib-cage compliance, which are associated with progesterone effects on soft tissue throughout pregnancy. It is possible that increased intrapleural pressures prevent such a mechanism in ascites [85].
Ascites may result in an increased elastic load on the lung, as well as PEEP, possibly due to early airways closure as a consequence of increased intrapleural pressure.
Inspiratory muscle strength may be normal or mildly reduced [82]. Hypercapnia has been reported in patients with rapidly developing ascites who are being treated with mechanical ventilation [86].
●Effect of paracentesis – Large volume paracentesis may have a beneficial effect upon lung function, by decreasing the inspiratory load. Diuretics are also standard components of treatment in ascites and, based upon one report of 26 patients with non-alcoholic cirrhosis, may have a similar benefit for lung function but a more favorable effect on PaO2 [87]. (See "Ascites in adults with cirrhosis: Initial therapy" and "Ascites in adults with cirrhosis: Diuretic-resistant ascites" and "Abdominal compartment syndrome in adults".)
CHEST WALL TUMORS — Large tumors of the chest wall can result in pulmonary restriction. However, the majority of benign and malignant chest wall tumors present as a painless or painful mass, rather than with symptoms or signs of chest wall restriction.
Breast cancer can lead to decreased chest wall compliance due to chest wall invasion, involvement by diffuse inflammatory ductal carcinoma, or scarring due to chest wall surgery and radiation therapy with resultant dyspnea and exercise limitation. (See "Inflammatory breast cancer: Clinical features and treatment".)
SUMMARY AND RECOMMENDATIONS
●Proper function of the respiratory pump depends upon the normal structure of the chest wall. (See 'Normal structure and function' above.)
●Diseases that affect the chest wall, such as congenital abnormalities, kyphosis and scoliosis, ankylosing spondylitis, flail chest, thoracoplasty, obesity, and chest wall tumors can result in respiratory compromise. (See 'Introduction' above.)
●Clues that chest wall disease is contributing to dyspnea are often present on physical examination (eg, pectus excavatum, kyphoscoliosis, thoracoplasty) but might not be identified unless sought out and considered in the differential diagnosis. (See 'Approach to the patient' above.)
●Among individuals with chest wall abnormalities, pulmonary function testing may reveal a restrictive ventilatory defect defined as a reduced total lung capacity (TLC) (algorithm 1 and figure 4). The degree of restriction depends on the type and severity of the chest wall abnormality and any coexisting neuromuscular disease. In contrast to patients with interstitial lung disease, the residual volume (RV) is often preserved. Desaturation with exercise is uncommon unless there are associated parenchymal or vascular changes. (See 'Approach to the patient' above.)
●Patients with more severe chest wall disease are at risk for respiratory insufficiency. Monitoring for hypercapnia is appropriate when the vital capacity (VC) decreases to less than 40 percent of predicted and/or serum bicarbonate is elevated. (See 'Approach to the patient' above.)
●Several congenital abnormalities can impair chest wall function; of these, pectus excavatum, pectus carinatum, and Poland syndrome are the most common. Asphyxiating thoracic dystrophy (ATD) and fibrodysplasia ossificans are hereditable diseases that generally present in infancy. (See 'Congenital and childhood abnormalities' above.)
●Mild thoracic kyphosis is a normal finding, while hyperkyphosis (excessive curvature of the thoracic spine) can cause restricted ventilation. Both kyphosis and scoliosis are associated with decreased TLC and VC, but preserved RV. (See 'Kyphosis and scoliosis' above.)
●Ankylosing spondylitis causes fixation of the chest wall through fusion of the costovertebral joints and spinal kyphosis. Enthesitis (inflammation of the muscular or tendinous attachments to bone) of the manubriosternal symphysis and sternoclavicular joints can further reduce chest wall excursion. The degree of restrictive ventilatory defect (reduced forced vital capacity [FVC]) appears to correlate with loss of spinal mobility; functional residual capacity (FRC) and RV are normal or increased due to a higher resting position of the chest wall. (See 'Ankylosing spondylitis' above.)
●Flail chest (ie, three or more consecutive ribs with fractures in two or more places creating a floating segment in the chest wall) can cause acute respiratory insufficiency and, in a small portion of patients managed without fixation, can be a cause of long-term restricted ventilation. (See 'Flail chest' above.)
●Class 2 or higher obesity (table 1) is associated with restrictive physiology due to the increased weight of the chest wall and increased abdominal adipose tissue restricting diaphragmatic motion and causing the system to operate at low lung volumes, with consequent effects on lung tissue. Typically, TLC, FRC, and expiratory reserve volume (ERV) are reduced; RV is relatively preserved. FVC tends to be decreased in class 3 or higher obesity. (See 'Obesity' above.)
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