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BOOK EXCERPT Chapter 6 Anatomy and PhysiologyThe anatomy and physiology of the pulmonary system has been covered in Pattern A: Primary Prevention/Risk Reduction for Cardiovascular/Pulmonary Disorders and Pattern E: Impaired Ventilation and Respiration/Gas Exchange Associated With Ventilatory Pump Dysfunction or Failure. PathophysiologyAcute Respiratory FailureAcute respiratory failure (ARF) accounts for more admissions to the ICU than any other organ failure.1 In the earliest stage of respiratory dysfunction patients may have all, some, or none of the following signs and symptoms as a result of impaired ventilation and respiration/gas exchange: high respiratory rate, nasal flaring, cyanotic lips, abnormal chest movement, and use of the abdominal muscles. While there are innumerable factors that might contribute to the development of respiratory dysfunction, sepsis, aspiration pneumonitis, and trauma are implicated in the vast majority of clinical cases associated with respiratory failure (Table 6-1).2-5
When respiratory dysfunction does develop in patients with any of the commonly associated clinical disorders (see Table 6-1), it is generally rapid and progressive. Half of the patients that will go on to develop significant alveolar damage (acute lung injury [ALI], acute respiratory distress syndrome [ARDS], or ARF) will do so within 24 hours of the onset of the initiating event; 85% will do so by 72 hours.3 When sepsis is the precipitating event, approximately 20% of patients will have already developed demonstrable lung injury at the time that the sepsis syndrome is identified. Yet, this finding is not quite as bleak as it seems since the ICU mortality is only slightly greater than 3% when the lungs are the only organs to fail.6 Conversely, when ARF is associated with other failing organs, the ICU mortality rises with each additional organ failure to as much as 75% when more than five organs are involved.1 Respiratory dysfunction is not an all-or-none phenomenon; rather it presents as a continuum with variable degrees of severity. In its mildest form, respiratory dysfunction is characterized by tachypnea and hypoxemia. However, as lung injury progresses and respiratory function deteriorates, diffuse alveolar damage generates a breakdown in the respiratory and nonrespiratory functions of the lung that is manifested as increased work of breathing, hypercapnia, and worsening hypoxemia that can progress to the point of necessitating mechanical ventilatory support. The initial lung injury arises from an unregulated acute inflammatory response that damages capillary endothelial and alveolar epithelial cells, and occurs largely as the consequence of increased alveolar capillary permeability and subsequent pulmonary edema. The development of clinically significant lung injury is typically described as occurring in distinct phases (Figure 6-1).
In the acute, or exudative, phase, a precipitating condition activates interstitial and alveolar macrophages and circulating neutrophils. The activated local macrophages secrete cytokines, interleukins (IL-1, 6, 8, and 10), and tumor necrosis factor α (TNF-α), which stimulate chemotaxis and activate neutrophils locally. Microvascular endothelial damage compounds the activation of circulating neutrophils through the release of lipopolysaccharide, TNF-α, IL-1, 6, and 8, platelet-activating factor, eicosanoids, and the enhanced expression of adhesion complexes.7-9 The release of IL-1 also stimulates the production of an extracellular matrix by fibroblasts.5 IL-6 plays a key role in the sequestration of neutrophils in the pulmonary microvasculature.10,11 Ensuing damage to type I and type II alveolar epithelial cells contributes to alveolar flooding and impedes the removal of edema fluid from the alveolar spaces. In addition, the reduced production and turnover of surfactant contributes to ventilation/perfusion mismatching, atelectasis, and loss of lung compliance.12,13 Together, these changes produce the characteristic protein-rich lung edema and hypoxemia that is unresponsive to supplemental oxygen therapy. The chest x-ray shows bilateral patchy infiltrates that are indistinguishable from cardiogenic pulmonary edema and may include pleural effusions.14 Dependent on the severity of the damage, the lung injury will begin to either resolve or progress to a subacute fibroproliferative phase. If the lung injury arising pursuant to the exudative phase does not resolve with initial therapeutic measures, a fibroproliferative phase ensues that can be seen histologically within about 5 days of the precipitant event.15 Macrophagic and neutrophilic cytokines that facilitated damage to endothelial and epithelial cells during the exudative phase also play an early role in the process of fibrosis. Procollagens, released from activated interstitial fibroblasts, amplify the developing fibrosing alveolitis. Moreover, such extrinsic factors as mechanical ventilatory support can traumatize the injured lung by overdistending and repeatedly opening and closing damaged alveoli, contributing further to the fibroproliferative phase of lung injury.16,17 Despite the fact that patients still have respiratory failure with continued poor or worsening lung compliance from fibrosing alveolitis, many will show morphological and radiographic evidence of reduced pulmonary edema 7 to 14 days following the initial onset of edema.14,18,19 Normally, squamous type I pneumocytes make up 90% to 95% of the alveolar surface area of the adult lung, and cuboidal type II pneumocytes make up the remainder.20 However, it seems that type I pneumocytes are more susceptible to injury than type II pneumocytes.5 Thus, the extent of the alveolar epithelial damage that develops in the acute (exudative) and subacute (fibroproliferative) phases plays a critically important role in the resolution phase. Cytokines and growth factors activated in the exudative and fibroproliferative processes initiate a reactive hyperplasia of remaining type II pneumocytes, which spread to cover the denuded basement membrane.21 Many of these type II cells then differentiate into type I cells, restoring the normal alveolar epithelial conformation and facilitating the elimination of pulmonary edema fluid.22-24 Thus, the resolution phase is typified by a gradual lessening of hypoxemia and an improvement in lung compliance. AsthmaAsthma is a chronic obstructive pulmonary disease, or COPD, of complex etiology involving external irritants and increased bronchial reactivity. The disease manifests itself by inflammation and edema of the airways, bronchospasm, and secretions. While the disease generally begins in early childhood, it may happen at any age. It has been estimated that 4 million children under the age of 18 years have had an asthma attack during the previous 12-month period.25 The airways in patients with asthma may be hyperresponsive to both specific and nonspecific stimuli that result in their diffuse narrowing. The specific stimuli may include allergens found in foods, chemicals, house dust, animal dander, fungi, molds, and the like. The nonspecific stimuli may include exercise, cold air, respiratory tract infections, or smoke, including passive smoke. Asthma is characterized by episodic airflow obstruction that leads to dyspnea and decreased activity tolerance. The disease is characterized by: hypertrophy of the smooth muscle in the airways; hypertrophy of the mucous glands leading to increased amounts of thick, tenacious mucus; infiltration of eosinophils and lymphocytes; edematous bronchial walls; and subepithelial fibrosis seen particularly in chronic asthma.26,27 Airway inflammation may be triggered by both known and unknown factors. Asthma does have a genetic component.28 Anitgens are recognized as triggers in those with allergic asthma. Pollutants may also be triggers for asthma. Exercise-induced asthma (EIA) and asthma that occurs after a viral respiratory tract infection do not have recognizable triggers.27 Asthma has also been found to worsen in the presence of other health-related conditions. These include: gastroesophageal reflux disease (GERD), which has been found to be common in patients with asthma and may be related to the asthma itself or its treatment; allergic rhinitis (hay fever), which has been considered a risk factor in asthma; and sinusitis, which when present appears to worsen the symptoms of asthma.29 An asthmatic attack is characterized by dyspnea, orthopnea, excessive use of the accessory muscles of respiration, hyperinflation of the lungs, diffuse musical rhonchi, rapid pulse, viscous sputum, and anxiousness. Status asthmaticus is a life-threatening medical emergency in which asthma symptoms become refractory to bronchodilator therapy. Individuals who are admitted to the emergency department (ED) have complaints of chest tightness, rapidly progressing shortness of breath, wheezing, a dry cough, agitation, and hypoxemia. FEV1 is significantly decreased. Status asthmaticus often may be seen following a viral infection or exposure to an inhalational irritant. It is more often seen in people with asthma who are not well controlled on their asthma medications.27 Medical management of asthma has included: avoidance of allergens, particularly aeroallergens; manipulation of the patient’s diet, especially increasing the intake of antioxidants and decreasing the intake of fats; and pharmacological agents, including inhaled corticosteroids, long-acting beta-2 agonist inhalers, and leucotriene receptor antagonists. Based on increasing evidence of airway remodeling (hyperplasia of smooth muscle, proliferation of blood vessels, and deposition of collagen) as a basic component of the pathogenesis of asthma and not as a sequelae of inflammation, monitoring sputum eosinophils and airway hyperresponsiveness or inflammatory biomarkers may be incorporated into future management of asthma.30 Pharmacological management has been divided into five steps by the British Thoracic Society, and this guideline considers the management of children over 12 the same as an adult. Step 1 is for those patients with mild infrequent symptoms (treat as required with short-acting beta-2 adrenoceptor agonist bronchodilators); Step 2 is for those with mild persistent symptoms (treat with inhaled corticosteroids and short acting beta-2 agonist daily); Step 3 is for those with moderate persistent symptoms (treat with long-acting beta-adrenocepter agonist and increase inhaled steroid if the former does not work); and Steps 4 and 5 are for those with severe persistent symptoms (treat with high dose inhaled corticosteroids).31 One of the controversies in medical management of asthma is the need for and use of corticosteroids over long periods of time to manage the inflammatory issues in asthma. Inhaled corticosteroids are believed to avoid some effects associated with oral steroids, and they are frequently recommended and useful in newly detected disease. Inhaled corticosteroids reduce airway inflammation, airway hyperresponsiveness, and general asthma symptoms and improve pulmonary function.32 However, the effect and efficacy of long-term corticosteroids is being questioned in asthma management, especially as they increase the risk of osteoporosis in women and in men. Jenkins and associates studied six children who were referred for difficult to control asthma and who underwent endobronchial biopsies. These biopsies revealed lung changes that were consistent with airway remodeling, including thickening of the basement membrane, hypertrophy of the smooth muscle, and a varying degree of hyperplasia of the goblet cells and submuscous glands. In five of the six cases there was minimal to no histologic evidence of airway inflammation. Their conclusions suggest that there is a need to look at issues other than inflammation in severe asthma.33 Gronke and colleagues observed that the issue of inflammation and treatment in asthma may be changed depending on the length of the disease process, such that inflammation may be more associated with hyperresponsiveness in acute asthma, while chronic asthma may have other functional characteristics.34 An acute life-threatening episode of asthma occurs when one of following three situations occur: acute severe asthma where the FEV1 is 30% or less (normal FEV1=75% to 80%); status asthmaticus where there is resistance to beta-adrenergic agonists and corticosteroids; or acute fulminant asthma which is very severe and rapid and the patient is obtunded. Management includes oxygen, inhaled salbutomol, and IV corticosteroids in order to control the acute episode. If this is not successful, the patient with status asthmaticus requires admission to the ICU for intubation, mechanical ventilation, and life support techniques.35 The incidence of status asthmaticus has been declining over the past 10 years. This is attributed to better education, improved medications, greater patient adherence, and improved access to care in the community.36 EmphysemaEmphysema is also a COPD of insidious onset that is characterized by irreversible overinflation and eventual destruction of the air spaces distal to the terminal bronchiole. Destruction of the alveolar walls leads to loss of parts of the capillary bed. Pulmonary emphysema has been associated with cigarette smoking, chronic bronchitis, fibrosing lung diseases, and a congenital deficiency of alpha1-antitrypsin (ATT). The types of emphysema are centriacinar, panacinar, paraseptal, and irregular. Centiacinar emphysema is most commonly found in smokers, affects the respiratory bronchioles in the middle of the acinus, spares the peripheral alveoli, and is found primarily in the upper lobes of the lungs. Panacinar emphysema is associated with an alpha1-antitrypsin deficiency, affects all of the alveoli, and is found more in the lower lobes. Paraseptal emphysema affects the outer part of the acinus, especially in the subpleural areas. Irregular emphysema is associated with several chronic destructive lung diseases that lead to scarring in the lung (eg, fibrosing lung diseases).26,27,37 Pathophysiologically the recurrent inflammation of the airways is associated with a release of proteolytic enzymes from the lung cells, which leads to the alveolar wall damage and eventual destruction. These changes lead to decreased elasticity of the lung and increased compliance. In addition, alveolar wall destruction leads to lesser lung surface area for gas exchange.37 Cigarette smoking also destroys the cilia of the lungs, thus impairing the mucociliary transport. Consequently individuals with emphysema often can take in breaths, but they have difficulty in exhalation and emptying of the lungs. Some of the signs and symptoms that are found with long-standing emphysema include a barrel chest (due to the hyperinflation of the airways), use of the accessory muscles of respiration (due to the flattening of the diaphragm with the increasing barrel chest), dyspnea, weak cough, weight loss (the increased oxygen demands for digestion of foods makes the individual even more hypoxemic, which results in decreased food intake), pursed lip breathing (due to the effort to maintain the patency of the airways during expiration), and tachypnea (due to the hypoxemia and the body’s attempt to maintain adequate levels of oxygenation to the cells).37 Complications of pulmonary emphysema include respiratory infections, cor pulmonale, and respiratory failure. The medical management of emphysema has included smoking cessation, vaccinations (flu and pneumonia), pharmacological agents (anticholinergic bronchodilators, beta-2 agonists, combination drug therapy, corticosteroids, antibiotics, mucolytics, and nonselective phosphodiesterase inhibitors, such as theophylline) oxygen therapy, respiratory support (noninvasive positive pressure ventilation [NPPV], traditional positive pressure ventilation, and invasive positive pressure ventilation), and heliox (a combination of helium and oxygen used for upper airway obstruction in stable severe emphysema).38 The surgical management of patients with emphysema has included lung volume reduction surgery (LVRS), which has had positive results.39 Other surgical interventions have included bullectomy and lung transplantation, the latter having shown increased survival, functional capability, and quality of life.40 Previously the patient with chronic long-standing emphysema who went into respiratory failure was placed on invasive positive pressure mechanical ventilation. The difficulty with this mechanical ventilation was the potential for even greater air trapping that occurred if the patient was not breathing synchronously with the ventilator. It has traditionally been very difficult to wean the patient from invasive ventilator support. The use of NPPV has proven to be superior with better outcomes than invasive ventilation in this population. This intervention can be started earlier than mechanical ventilation. If there is improvement and patient compliance, it can be effectively utilized to allow for time to reverse the failure and wean the patient entirely from the NPPV or continue to utilize the treatment at home as a supportive intervention.41 Innovative pharmacological approaches are being tried in the management of emphysema. These are directed toward the underlying pathophysiological processes of the disease, in addition to disease modification. These therapies include mediator agonists (eg, leukotriene B4 inhibitors), new anti-inflammatories (eg, phosphdiesterase type 4 and type 5 inhibitors), and protease inhibitors.38 Acute Respiratory Distress SyndromeARDS, originally called adult respiratory distress syndrome, was first described in 1967 as encompassing the following clinical features: acute respiratory distress (eg, tachypnea, dyspnea), cyanosis that did not respond to oxygen therapy, decreased compliance of the lung, and diffuse fluffy infiltrates on the chest radiograph.42 Because the syndrome was so poorly understood at the time, the diagnosis was made, as often as not, based on the postmortem findings of atelectasis, vascular congestion, hemorrhage, pulmonary edema, and hyaline membrane formation. A definition that quantified respiratory impairment based on physiologic criteria using a four-point lung-injury scoring system was proposed for ARDS many years later.43 Unfortunately, this scoring system lacked specific criteria to exclude a cardiogenic source for pulmonary edema and was not particularly effective in forecasting outcomes in the first 72 hours after onset. Therefore, in 1992, an American-European consensus committee proposed new definitions for ALI and ARDS, which encompassed varying degrees of respiratory dysfunction based upon sequential organ failure assessment (SOFA) score criteria.44 Table 6-2 presents a paradigm for the classification of respiratory dysfunction based on the SOFA score criteria. The consensus definitions for ALI and ARDS have become the most used and recommended definitions worldwide of acute respiratory dysfunction involving diffuse alveolar damage.2-5 To date, no consensus definition exists for ARF. However, in 1998, the SOFA score criteria were also proposed as the basis for a definition of ARF that has gained reasonably widespread acceptance.45 This definition distinguishes ARF from ALI and ARDS by adding a requirement for some form of mechanical ventilatory support to the oxygenation, radiographic, and pulmonary wedge pressure criteria (see Table 6-2).
Despite presently well-accepted definitions for ALI, ARDS, and ARF (see Table 6-2), the incidence of these disorders remains unclear. The National Institutes of Health (NIH) first reported a consensus estimate for the incidence of ARDS at 150,000 cases per year.46 Subsequent studies have suggested a much lower incidence of ALI/ARDS, with estimates ranging from 2,600 to 44,000 cases per year.47-53 However, few of the investigators actually applied the SOFA score criteria when estimating incidences for ALI, ARDS, or ARF. Unfortunately, in the two studies that did employ SOFA score criteria, the screening period was limited to 8 weeks or less.47,48 Nonetheless, the best estimates in the United States place the incidence of ALI at close to 128,000 cases per year2 and ARDS at about 20,000 cases per year.54 While there are no incidence estimates for ARF in the United States, the best guess places the incidence close to that originally stated by the NIH.53 Patients with ALI, ARDS, and ARF often have a high respiratory rate, nasal flaring, cyanotic lips, abnormal chest wall movement, and exaggerated use of the abdominal muscles as a consequence of bronchoconstriction, bronchial and alveolar edema, and lung inflammation. When compared with patients who do not have ARF, patients with ARF have substantially greater inspiratory resistance and markedly poorer lung/thorax compliance.55,56 Lung involvement in cases of ALI, ARDS, or ARF is not homogeneous, and there are areas of aerated and appropriately functioning alveoli interspersed with areas of nonfunctional alveoli.57 The chest radiographs of patients with ALI, ARDS, or ARF are characterized by bilateral pulmonary infiltrates. Too frequently, the distribution of lung injury is an outcome of the interventions administered in the intensive care unit. Typically, consolidation occurs in the lower and posterior regions of the lung, while cystic changes develop in the upper and anterior regions from overdistention secondary to positive pressure ventilation.52 Impaired lung function is reflected in a reduced partial pressure of oxygen in the arterial blood (PaO2) and/or an elevated partial pressure of carbon dioxide (PaCO2). However, at the onset of respiratory disease, patients may be both hypoxemic and hypocapnic as they initially increase their minute ventilation in an effort to improve oxygen delivery. However, the work of breathing becomes too great and hypercapnia ensues. Ultimately, progression of the syndrome is evidenced by refractoriness to supplemental oxygen regardless of the PaCO2. Consequently, the ratio of PaO2 to the FiO2 provides a sensitive and objective measurement of the degree to which oxygenation is impaired and is, therefore, a reliable measure of physiologic respiratory dysfunction.4,6,58,59 Overall, mortality rates are lowest in single organ ARF and increase dramatically with each additional organ failure.1 Not surprisingly, the mortality rate for ARF also increases as the FiO2 and the peak inspiratory pressure (PIP) required for positive pressure ventilation increase. In general, survivors of ARF are younger than nonsurvivors,1,52,58,59 with mortality increasing exponentially beyond age 65. Survivors of ARF continue to show evidence of functional limitation even 1 year after discharge from the ICU.60 Imaging
Pharmacology
Physical Therapist ExaminationHistoryBecause of the patient’s condition, much of the history was obtained from her mother, Janet, from previous medical records, and from a discussion with her pediatrician.
Systems Review
Tests and Measures
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