Respiration involves two major body systems: the cardiovascular system and the respiratory system, both regulated by the nervous system. Oxygen and CO₂ are conveyed to and from tissues and organs by the cardiovascular system. The respiratory system delivers oxygen from the atmosphere to the bloodstream and delivers CO₂ from the blood to the atmosphere. This gas exchange takes place in specialized structures of the lungs.

The paranasal sinuses are air-filled cavities within bony structures adjacent to the nasal cavity. These sinuses—the ethmoid, frontal, sphenoid, and maxillary sinuses—drain through the nasal cavity. All are lined with ciliated columnar epithelium that is continuous with the lining of the nose. Because the mucosa is continuous, infection in the nasal passages can readily spread to the sinuses.

The pulmonary circulation is where oxygenation of blood occurs. The entire output of the right ventricle leaves the heart via the pulmonary artery, which divides into the right and left pulmonary arteries. This poorly oxygenated blood circulates through the capillaries of the alveoli, where gas exchange takes place. Oxygenated blood is then returned to the left atrium via the pulmonary veins [6,11].

The venous blood returning from the tissues is high in CO₂ and low in oxygen; the air in the alveolar space is higher in oxygen and lower in CO₂ than the blood. Under normal conditions, the approximate partial pressures of oxygen in the alveoli and the capillaries are 105 mm Hg and 40 mm Hg, respectively. The partial pressure of CO₂ is 45 mm Hg in the capillaries and 35 mm Hg in the alveoli.

Without perfusion, gas exchange cannot take place. This condition is called wasted ventilation, because ventilation of the alveoli brings about no exchange of CO₂ for oxygen. Conditions related to wasted ventilation include a decrease in total blood volume (in hemorrhage or dehydration), pulmonary embolism, and chronic obstructive pulmonary disease (COPD). Emphysema may also destroy both alveoli and capillaries within portions of the lungs, so neither ventilation nor perfusion can take place. If large areas of the lungs are affected by wasted ventilation, hypoxemia and hypercapnia with related respiratory acidosis can occur [18].

Oxygen obtained during gas exchange in the pulmonary circulation must combine with hemoglobin to be transported to the tissues. Normally, the circulation contains approximately 15 g of hemoglobin to carry about 20 mL of oxygen at any given time. If the hemoglobin concentration is less than normal (i.e., anemia), the capacity of the blood to transport oxygen will be reduced [18,22].

Occupation is a significant factor in the development of some respiratory disorders. "Black lung disease," which affects coal miners, is a well-publicized example; asbestosis is another. Solvents in paints and varnishes have also been implicated in respiratory disorders, not only among factory workers and professional painters, but among hobbyists as well. A timely question about recent craft or hobby projects cans yield a valuable diagnostic clue about respiratory ailments. Clerks in dry cleaning establishments and beauticians exposed to permanent wave solutions (ammonia) and hair dyes may also develop respiratory abnormalities [25].

Another common symptom of respiratory disorders is a cough. Cough can result from irritation or from retained secretions that obstruct part of the airway. Causative disorders in the upper airway include sinusitis (leading to postnasal drip) and infections of the pharynx; causative disorders in the lower respiratory tract include bronchitis, bronchiectasis, and pneumonia. The patient should be asked whether the cough is productive or nonproductive. If the cough is productive, ask about the color, amount, odor, and consistency of the sputum. Inquire also about precipitating factors and the frequency of the cough.

Having determined that chest pain does not stem from a disorder demanding immediate intervention, ask whether the patient has experienced any voice change. This symptom can be caused by infections of the pharynx, vocal nodules, laryngeal paralysis, and laryngeal tumors. Laryngeal paralysis results from damage to the recurrent or superior laryngeal nerve or the vagus nerve (e.g., due to bronchogenic carcinoma or an aortic aneurysm). Ask the patient how long ago the voice change occurred and whether it is associated with pain when speaking or swallowing.

The appearance of the inferior and middle turbinates should be assessed. Tissue overlying the turbinates may become hypertrophied in conditions such as chronic allergic rhinitis. Sinusitis may result in a decrease in transillumination of the frontal and maxillary sinuses. Skin over these areas may also appear reddened in the presence of an infection [30].

Inspection of the chest may reveal thoracic deformities such as kyphoscoliosis or an increased anteroposterior diameter (i.e., "barrel chest"). Kyphoscoliosis reduces thoracic movement and limits lung expansion. "Barrel chest" is a common finding in patients with COPD. Asymmetrical chest expansion can result from trauma (e.g., pneumothorax, hemothorax, flail chest). General overall appearance, such as facial expression, posture, and ease of movement, should also be assessed. Patients with COPD may have to sit up or lean forward in a chair in order to breathe [30].

Percussion of the chest produces sounds that can help locate abnormalities in the lungs. The sound heard over normal lung tissue is called resonance. Hyper-resonance is noted when air trapping occurs, as in emphysema or a pneumothorax. Dullness may be heard in the presence of a small pleural effusion, atelectasis, or hemothorax. Flatness (the same sound transmitted with percussion over the thigh) may be heard with massive consolidation or a large pleural effusion. Consolidation refers to an area of the lung that has become more dense either because air is not reaching the alveoli or because the alveoli are filled with fluid or secretions. When fluid is present in the frontal or maxillary sinuses, as in sinusitis, percussion over the sinus produces a dull sound.

Spirometry is able to measure tidal volume, inspiratory reserve volume, and expiratory reserve volume. Tidal volume is the amount of air inspired and expired with each normal breath. The normal value is approximately 500 mL. Inspiratory reserve volume is the additional volume of air that can be inspired beyond the normal tidal volume. It amounts to approximately 3,000 mL. Expiratory reserve volume is the volume of air that can be forcefully expired at the end of the normal tidal expiration. This is about 1,100 mL in healthy individuals. Residual volume consists of the amount of air that remains in the lungs after a forceful expiration. This usually equals about 1,200 mL.

Pulmonary volumes are measured in time intervals. Forced vital capacity (also referred to as forced expiratory volume) is the maximum pulmonary volume the patient has for ventilation. The patient inhales the maximum amount of air and then forcefully exhales as fast as possible. The amount of air the patient can forcefully exhale within a specific time period is calculated. The usual time periods are one, two, and three seconds. A healthy individual can usually exhale approximately 75% of vital capacity in one second and 100% by three seconds.

The normal pH of arterial blood is 7.35 to 7.45. Values less than 7.35 indicate acidemia, while those greater than 7.45 are indicative of alkalemia. The normal pH range is maintained primarily by two buffers: carbonic acid and bicarbonate [34].

A reaction of <5 mm is negative. Reactions >10 mm are "positive." An intermediate reaction of 5–10 mm is suspicious for prior infection in high-risk persons.

Both open and closed approaches are used to obtain lung tissue for cytologic analysis and culture. Closed approaches include fine-needle aspiration, biopsy via a percutaneous cutting needle, and biopsy via fiber-optic bronchoscope. All closed procedures are done under local anesthesia with fluoroscopic guidance. Open biopsy requires a thoracotomy. This approach provides the largest volume of tissue, but general anesthesia is needed. Lung biopsy is indicated when the patient's diagnosis remains unclear despite a complete work-up [12,32].

Pursed-lip breathing—a slow, even expiration against pursed lips—prevents collapse of small bronchioles, reduces the amount of trapped air in the lungs, and promotes CO₂ elimination. To assume the proper lip position for pursed-lip breathing, have the patient in a sitting position pretend to blow out a candle. As noted, patients with emphysema can use pursed-lip breathing to maximize expiration.

Respiratory acidosis occurs when a disorder results in interference with gas exchange or decreases the amount of effective alveolar ventilation. The underlying cause of this abnormal state is always related to hypoventilation. Respiratory acidosis is observed in certain patients with COPD, pneumonia, or decreased respiratory function as a result of medication/illicit drugs or trauma. Patients in respiratory acidosis may exhibit confusion, drowsiness, headache, dizziness, tetany, asterixis, tachycardia, dysrhythmias, convulsions, and/or coma [34,35].

The initial symptom of laryngeal edema is often hoarseness. As the condition progresses, the patient may lose the ability to speak. When inspected by indirect mirror laryngoscopy, the laryngeal structures and vocal cords may appear swollen. As obstruction becomes more severe, dyspnea, stridor, tachypnea, and cyanosis may occur. With the exception of angioedema, onset of the edema is usually gradual and involves structures other than the larynx. In the case of angioedema, symptoms and signs of laryngeal obstruction develop rapidly and the condition can progress to total obstruction and death [36].

Typically, epiglottitis is manifested by sore throat of short duration (less than 12 hours) and rapidly increasing severity, pain in the area of the hyoid at the base of the tongue, significant dysphagia, and elevation of temperature that may reach 103 degrees F. Secretions may be so copious that the patient drools. Hoarseness is minimal, but the patient's voice may have a muffled quality. Respiratory obstruction becomes evident as the inflammation progresses. Although the enlarged, cherry red epiglottis may be seen by means of indirect laryngoscopy, lateral x-rays of the neck taken with the patient in an upright position are a less hazardous means of diagnosis. Manipulation can aggravate edema, leading to sudden total occlusion of the airway. Cultures of blood and secretions may be used to isolate the causative organism. A leukocyte count of 18,000–24,000 cells/mcL is common [36].

Although benign neoplasms of the larynx do occur, they are usually small, easily removed papillomas. Malignant laryngeal neoplasms may be classified according to locus of origin as:

Hoarseness is uncommon with supraglottic carcinoma. Patients may report a sensation of "something in the throat" or a change in voice quality. The throat may burn when hot or acidic liquid is ingested. The patient may notice a lump in the neck, which may be the reason for consulting a healthcare provider. Pain unrelated to ulceration may occur, as may referred otalgia. Later symptoms include pain, hoarseness, and dyspnea related to obstruction of the airway [29].

Automobile accidents in which an individual is thrown against the steering wheel or dashboard at a high rate of speed are the most common cause of blunt laryngotracheal trauma. Penetrating injuries are often caused by sharp objects (e.g., stab wounds). Iatrogenic laryngeal injury can occur in relation to endoscopy, endotracheal intubation, or tracheotomy; improper endoscopic technique can dislocate the larynx or tear the laryngeal mucosa. Pressure exerted by the cuff of an endotracheal or tracheostomy tube against the arytenoid cartilage may result in formation of scar tissue (arytenoid stenosis), although this occurs less frequently with the introduction of low-pressure cuffs [36,40,41].

Pneumoconioses are lung diseases resulting from inhalation of inorganic dusts. Silicosis, asbestosis, and coal workers' pneumoconiosis fall into this category.

Acute respiratory distress syndrome (ARDS) is a pathophysiologic state best recognized in patients with no previous underlying lung disease who experience a sudden, catastrophic, often multisystem insult that leads to the development of severe dyspnea, hypoxemia, loss of pulmonary compliance, and noncardiogenic pulmonary edema. ARDS is not a clearly defined disease, but is rather an umbrella term for a group of conditions of different etiology having similar manifestations. Many factors can lead to the development of ARDS, including major insults such as shock, multisystem trauma, aspiration, overwhelming systemic infections (sepsis), drug overdose, and inhaled toxic substances. ARDS has also been called shock lung, white lung, Da Nang lung, adult hyaline membrane disease, stiff lung syndrome, and wet lung.

The major symptoms of acute bronchitis include cough, a burning substernal sensation (often aggravated by a deep breath), and sputum production. As the infection progresses, sputum becomes mucoid or purulent. The patient may or may not have an elevated temperature. Malaise, muscle aches, and headache are common. Chest auscultation reveals rales, rhonchi, and wheezes. The disease is self-limiting, and the duration depends on the underlying causative organism. Smoking tends to prolong and aggravate the condition [44].

Pneumonia is an infection of the lower respiratory tract that is usually accompanied by cough, fever, malaise, and chest x-ray abnormalities. Sputum production, dyspnea, hypoxia, and hemoptysis may be present in some individuals with pneumonia, depending on the causative organism. The disease is further classified as community-acquired, healthcare-associated, or hospital-acquired according to how and where it was contracted.

Hospital-acquired pneumonia (HAP) is defined as any pneumonia in a patient who has been hospitalized 48 hours prior to onset of symptoms and that was not incubating prior to admission [47]. Ventilator-associated pneumonia (VAP) refers to a pneumonia that develops at least 48 hours after intubation and the initiation of mechanical ventilation. An additional subtype of pneumonia is healthcare-associated pneumonia (HCAP), which refers to pneumonia in patients who are not hospitalized but have had contact with the healthcare system. Guidelines for patients with HCAP were previously included with those for HAP and VAP due to patients' contact with the healthcare system and the presumed high risk of multidrug-resistant pathogens. However, recommendations specific to HCAP were removed from the updated 2016 IDSA guidelines for the management of HAP and VAP due to increasing evidence that patients defined as having HCAP are not necessarily at high risk for multidrug-resistant pathogens. Although interaction with the healthcare system potentially puts patients at risk for these pathogens, underlying patient characteristics are important independent determinants of risk [47].

The most common cause of bacterial pneumonia is the gram-positive bacterium Streptococcus pneumoniae, estimated to be the cause of 20% to 60% of pneumonia cases [49]. Possible gram-negative infective organisms include Haemophilus influenzae, Klebsiella pneumoniae, and Moraxella catarrhalis. K. pneumoniae infections are more commonly diagnosed when there is co-existent alcoholism [45,47]. S. aureus and H. influenzae infections often occur after a primary influenza infection. M. catarrhalis, an organism not thought to be pathogenic, is most commonly found in those with chronic lung conditions, such as COPD [50]. It is also found in patients with diabetes, who are taking steroids, or who have other underlying chronic lung conditions or malignancy [50].

The early symptoms of active pulmonary TB are often so subtle as to be missed at first by most patients. Because of this, they will frequently have difficulty pinpointing exactly when their illness began. Some patients without obvious symptoms are diagnosed solely by a routine chest x-ray. For those who do have symptoms, the most common manifestations are gradual onset and progression of fever, malaise, cough, anorexia, and weight loss. The fever is one that is not so high as to be noticed or to be disquieting. When the fever breaks during the early morning hours, it is often associated with "night sweats." Weight loss may result from the infectious process itself, or it can be a sign that malnutrition and depletion of immune reserves preceded, and to some extent caused, the infection.

The most common symptoms of lung abscess include fever, chills, malaise, anorexia, cough, and pleuritic chest pain. When the abscess ruptures into a bronchus, the cough is productive of copious amounts of purulent sputum, which may be foul smelling, bloody, and dark brown in color. Cavitary lesions that result from abscess formation are visible on chest x-ray. The prognosis of lung abscess, if not complicated by an underlying malignancy or other progressive lung disease, is generally good if the organism responsible is treated [71].

Empyema is the accumulation of pus in the pleural space or a purulent pleural effusion. The use of antibiotics in the treatment of lung infections has greatly reduced the occurrence of empyema, but it can develop as a complication of a penetrating chest wound or thoracic surgery [71].

Histoplasmosis is caused by the dimorphic fungus Histoplasma capsulatum. It occurs worldwide, but in the United States, it is more prevalent in the Mississippi and Ohio River valleys. The organism flourishes in a temperate climate, and its growth is enhanced in soil that contains chicken, starling, or blackbird droppings or bat guano. The fungi become airborne and may be inhaled when the soil is disturbed; transmission from person to person does not occur [71].

Bronchiectasis is a permanent abnormal dilation of one or more large bronchi caused by destruction of the elastic and muscular components of the bronchial wall. While the number of people developing bronchiectasis as a complication of severe pulmonary infection is decreasing, bronchiectasis remains a common complication of certain systemic disorders, particularly cystic fibrosis [71].

Asthma is considered a chronic, albeit reversible, respiratory disorder. This inflammatory condition produces hyper-reactive and hyper-responsive airway and lungs, causing episodic, reversible airway obstruction through bronchospasms, increased mucus secretions, and mucosal edema [73,74,75]. The hyper-reactive lungs of patients with asthma are more sensitive than most individuals' and may become inflamed or edematous when exposed to irritants, such as cold air, animal dander, dust, tobacco smoke, or grass [74,75]. The patient's immune system over-reacts to these irritants, constricting the airways and filling them with mucus; constricted airways interfere with the movement of air in and out of the lungs, making breathing difficult [74,75].

The cardinal signs and symptoms of COPD are chronic cough, sputum production, breathlessness (shortness of breath and dyspnea), and limited exercise tolerance. Other common signs that may be present in COPD include:

Breathing in patients with chest trauma may be accompanied by pain, dyspnea, asymmetrical chest movement, paradoxical chest movement, a sucking sound, poor tidal volume, and/or symptoms of hypoxia or hypercapnia. Cyanosis may be a later manifestation. Circulation may be affected and may be detected by such signs as jugular venous distention, tachycardia, dysrhythmias, hypovolemic shock, and cardiogenic shock. It is always assumed that victims of chest trauma have suffered a spinal cord injury until x-rays prove otherwise. Patients with chest trauma may fall at any point on the spectrum between alert/oriented and comatose; if alert, they may be very apprehensive [106,107].

Tracheotomy is performed to obtain a temporary opening into the trachea. If the trachea is sutured to make a permanent opening, it is a tracheostomy. This technique is performed to provide access for aspiration of the bronchial tree or to relieve upper airway obstruction. It may be employed when a patient is unable to cough effectively and expel secretions. Indications for a tracheotomy include [108]:

Other possible complications following thoracic surgery include hemorrhage, hypotension, infection, cardiac dysrhythmia, respiratory failure, pulmonary edema, subcutaneous emphysema, residual pleural space, persistent air leak, bronchopleural fistula, and emphysema [108]. With pulmonary resection, there is a permanent decrease in pulmonary function [108].