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Respiratory therapy is a vital component of health care. For any patient initial medical care begins with assessment of the ABCs: Airway, Breathing, and Circulation. Respiratory therapy includes key components of airway and breathing support. The objective is the care of all types of patients with cardiopulmonary diseases. Functions of the respiratory therapist include emergency care, airway management, ventilatory support, oxygen therapy, humidity and aerosol therapies, chest physiotherapy, physiologic monitoring, and pulmonary diagnostics.

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PFTs are useful in the diagnosis of a variety of pulmonary disorders. Common PFTs include spirometry, lung volume determinations, and diffusion capacity. Important measurements include FVC and FEV1. Spirometry results indicate the presence of obstructive airway diseases such as asthma and emphysema when the FEV1/FVC ratio is < 0.70. They indicate the presence of restrictive lung diseases such as sarcoidosis and ankylosing spondylitis when the FVC/FEV1 ratio > 80%.

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Spirometry can be an important part of a preoperative evaluation. Obtain spirograms before and after administration of bronchodilators if they are not contraindicated (ie, history of intolerance). Bronchodilator responsiveness helps in predicting the response to treatment and in identifying asthma. Asthmatic patients typically have at least 15% improvement in FEV1 after bronchodilator therapy.

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Order lung volumes, commonly determined by helium dilution, to definitively diagnose restrictive lung disease. This test is usually indicated when TLC < 80% of predicted normal value. Diffusion capacity is important in the diagnosis of interstitial lung disease and pulmonary vascular disease, in which it is reduced. Diffusion capacity is frequently monitored to determine response to therapy for interstitial diseases.

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Obstructive pulmonary diseases include asthma, chronic bronchitis, emphysema, and bronchiectasis. Restrictive pulmonary diseases include interstitial pulmonary disease, diseases of the chest wall, and neuromuscular disorders. Interstitial disease can be caused by inflammatory conditions (usual interstitial pneumonitis [UIP]), inhalation of organic dust (hypersensitivity pneumonitis), inhalation of inorganic dust (asbestosis), and systemic disorders with lung involvement (sarcoidosis).

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Normal PFT values vary with age, sex, race, and body size. Normal values for a given patient are established from studies of healthy populations and are provided along with the results. ABG should be included in all PFTs. Typical volumes and capacities are illustrated in Figure 18–1.

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Figure 18–1.
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Lung volumes in interpretation of pulmonary function tests.

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  • Tidal Volume (TV). Volume of air moved during a normal breath on quiet respiration
  • Forced Vital Capacity (FVC). Maximum volume of air that can be forcibly expired after full inspiration
  • Functional Residual Capacity (FRC). Volume of air in the lungs after a normal tidal expiration (FRC = reserve volume + expiratory reserve volume)
  • Total Lung Capacity (TLC). Volume of air in the lungs after maximal inspiration
  • Forced Expired Volume in 1 Second (FEV1). Measured after maximum inspiration, the volume of air that can be expelled in 1 s
  • Vital Capacity (VC). Maximum volume of air that can be exhaled from the lungs after a maximal inspiration
  • Residual Volume (RV). The volume of air remaining in the lungs at the end of a maximal exhalation

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Table 18–1 shows the differential diagnosis of various PFT patterns. When interpreting PFTs, remember that some patients ...

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