Disorders of the Pulmonary Circulation and the Respiratory Regulation

Diagnostic modalities such as chest X-rays provide static images of the thoracic cavity, including the lungs and airways. While providing a wealth of anatomical information necessary for the diagnosis of pulmonary disease, chest X-rays do not give much information about the individual's respiratory function. Pulmonary function tests are a group of diagnostic procedures yielding useful, quantifiable information about the rate of the flow of air through the individual's airways, lung capacity, and the efficiency of gas exchange in relation to time. The most commonly utilized tests include spirometry (before and after bronchodilator use), lung volumes, and quantitation of diffusing capacity for carbon monoxide (CO). The tests can be influenced by the individual's effort/fatigue, disease state, or anatomical malformation.

The most common complications include pneumonia, respiratory failure and acute respiratory distress syndrome (ARDS), sepsis and septic shock, cardiomyopathy, acute kidney injury (AKI), and pulmonary thromboembolism. Other complications include acute stroke, arrhythmias, acute cardiac injury, and dermatologic manifestations.

Decompression sickness (DCS), known informally as “the bends,” is a condition caused by compression and decompression of gases contained in the body during descent and rapid ascent while diving. Clinical presentation of DCS may be nonspecific and variable, with a time of onset that can vary from immediately to 12 hours after surfacing. Diagnosis is made clinically. Management is early supportive therapy and hyperbaric recompression treatment carried out in a specialized facility.

Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration, most commonly from walking barefoot. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. Symptoms include cutaneous irritation, constipation, diarrhea, dry cough, and wheezing, depending on where the parasite is in its life cycle. Effective eradication of the parasite can be obtained with anthelmintic medications, usually ivermectin.

Respiratory distress syndrome (RDS), also known as hyaline membrane disease, is caused by the lack of adequate pulmonary surfactant production in an immature lung. The syndrome is most commonly seen in preterm infants. The incidence is inversely related to gestational age, with the highest risk in neonates born at less than 28 weeks. Prenatal assessment of lung maturity and steroid administration can improve outcome if an early delivery cannot be prevented. Diagnosis is clinical. Affected newborns show signs of respiratory distress at birth, or soon thereafter, with nasal flaring, grunting respirations, and retractions. Treatment includes antenatal steroids, exogenous surfactant, and respiratory support. Neonatal RDS is associated with high morbidity and mortality in preterm infants.

Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. To perform this function, the lungs need to be able to capture as much O2 as possible, a task that is easily achieved owing to their elastic morphology. Each lung is enclosed within the visceral pleura and completely fills 1 of the non-symmetrical pleural cavities, which are situated on the left and right sides of the mediastinum. The lungs encase the bronchial tree and are divided functionally and anatomically into lobes.

Acute respiratory distress syndrome is characterized by the sudden onset of hypoxemia and bilateral pulmonary edema without cardiac failure. Sepsis is the most common cause of ARDS. The underlying mechanism and histologic correlate is diffuse alveolar damage (DAD). Diffuse alveolar damage involves damage to the endothelial and alveolar epithelial cells and is associated with inflammation and the development of hyaline membranes lining the inner alveolar walls. The reparative stage follows after weeks, with fibrosis possibly occurring later. Clinically, the following triad of findings favors a diagnosis of ARDS: acute or rapidly progressive dyspnea, hypoxic respiratory failure (partial pressure of O2/fraction of inspired O2 ratio < 300 mm Hg), and bilateral alveolar opacities on chest imaging. Management involves determination and treatment of the cause while providing adequate oxygen, reducing further lung damage, and avoiding fluid overload. Most patients require mechanical ventilation. Acute respiratory distress syndrome is associated with high mortality or long-term complications potentially developing even after treatment.

Pulmonary hypertension (PH) or pulmonary arterial hypertension (PAH) is characterized by elevated pulmonary arterial pressure, which can lead to chronic progressive right heart failure. Pulmonary hypertension is grouped into 5 categories based on etiology, which include primary PAH, and PH due to cardiac disease, lung or hypoxic disease, chronic thromboembolic disease, and multifactorial or unclear etiologies. Patients typically present with shortness of breath initially during exercise and then at rest. Diagnosis may involve an echocardiogram, ECG, chest X-ray, pulmonary function tests, a ventilation-perfusion scan, laboratory testing for conditions associated with PAH, and/or cardiac catheterization. Management is often complex and aimed at treating the underlying etiology. Several classes of vasodilatory agents may be used for patients with primary PAH, including calcium channel blockers and vasoactive prostaglandins.

Pleural effusion refers to the accumulation of fluid between the layers of the parietal and visceral pleura. Common causes of this condition include infection, malignancy, autoimmune disorders, or volume overload. Clinical manifestations include chest pain, cough, and dyspnea. Imaging can confirm the presence of a pleural effusion, and pleural fluid analysis can help in the evaluation of an etiology. Management is dependent on the underlying condition and whether the effusion is causing respiratory distress. Drainage of the effusion may provide symptomatic relief.

Hypersensitivity pneumonitis (HP), previously called extrinsic allergic alveolitis, is an immunologically induced inflammatory disease affecting the alveoli, bronchioles, and lung parenchyma. It is caused by repeated inhalation of an inciting agent in a susceptible host that triggers first a type III (complement-mediated) hypersensitivity reaction in the acute phase and then a type IV (delayed) reaction in the subacute and chronic phases. The clinical presentation of acute HP includes cough, fever, and malaise, while subacute and chronic forms present as insidious onset of a cough and dyspnea over weeks to months. Diagnosis is aided by high-resolution CT scans and bronchoalveolar lavage. Management includes avoiding the inciting agent and administration of steroids in subacute and chronic cases. Early treatment has a good prognosis, but long-term exposure can cause permanent scarring and fibrosis.

Pulmonary embolism (PE) is a potentially fatal condition that occurs as a result of intraluminal obstruction of the main pulmonary artery or its branches. The causative factors include thrombi, air, amniotic fluid, and fat. In PE, gas exchange is impaired due to the decreased return of deoxygenated blood to the lungs. Deep vein thrombosis (DVT) is diagnosed in > 50% of patients presenting with symptoms, in whom the lower-extremity deep-venous system is the most common source of thrombus. Some individuals are asymptomatic, but the most common presenting symptom is dyspnea. The symptoms can be acute or chronic, and diagnosis is usually based on radiographic findings. Initial management is supportive and focuses on restoring oxygenation and hemodynamic stability. Both medical (systemic anticoagulants) and interventional therapies (catheter-based approach, surgery) are used to reestablish vessel patency.

A pneumothorax is a life-threatening condition in which air collects in the pleural space, causing partial or full collapse of the lung. A pneumothorax can be traumatic or spontaneous. Patients present with a sudden onset of sharp chest pain, dyspnea, and diminished breath sounds on exam. Symptom severity depends on the degree of the collapsed lung. A large or tension pneumothorax can result in cardiopulmonary collapse. A diagnosis is made with imaging, though tension pneumothorax is a clinical diagnosis. Management is based on the size and stability of the patient, and can include needle decompression and chest tube (thoracostomy) placement.

Obstructive sleep apnea (OSA) is a disorder characterized by recurrent obstruction of the upper airway during sleep, causing hypoxia and fragmented sleep. Obstructive sleep apnea is due to a partial or complete collapse of the upper airway and is associated with snoring, restlessness, sleep interruption, and daytime somnolence. The diagnosis relies on medical history, and polysomnography can confirm the diagnosis. Management includes lifestyle changes, methods of positive airway pressure, and surgical intervention.

COVID-19, also known as Coronavirus disease, is a respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV 2). Transmission mainly occurs via direct contact or aerosol droplets. The disease may present asymptomatically or with fever and dry cough.