Pulmonary tuberculosis is an infectious disease caused by slow- growing bacteria that resembles a fungus, Myobacterium tuberculosis, which is usually spread from person to person by droplet nuclei through the air. The lung is the usual infection site but the disease can occur elsewhere in the body. Typically, the bacteria from lesion (tubercle) in the alveoli. The lesion may heal, leaving scar tissue; may continue as an active granuloma, heal, then reactivate or may progress to necrosis, liquefaction, sloughing, and cavitation of lung tissue. The initial lesion may disseminate bacteria directly to adjacent tissue, through the blood stream, the lymphatic system, or the bronchi.
Most people who become infected do not develop clinical illness because the body’s immune system brings the infection under control. However, the incidence of tuberculosis (especially drug resistant varieties) is rising. Alcoholics, the homeless and patients infected with the human immunodeficiency virus (HIV) are especially at risk. Complications of tuberculosis include pneumonia, pleural effusion, and extrapulmonary disease.
ANATOMY AND PHYSIOLOGY
UPPER RESPIRATORY TRACT
Respiration is defined in two ways. In common usage, respiration refers to the act of breathing, or inhaling and exhaling. Biologically speaking, respiration strictly means the uptake of oxygen by an organism, its use in the tissues, and the release of carbon dioxide. By either definition, respiration has two main functions: to supply the cells of the body with the oxygen needed for metabolism and to remove carbon dioxide formed as a waste product from metabolism. This lesson describes the components of the upper respiratory tract.
The upper respiratory tract conducts air from outside the body to the lower respiratory tract and helps protect the body from irritating substances. The upper respiratory tract consists of the following structures:
The nasal cavity, the mouth, the pharynx, the epiglottis, the larynx, and the upper trachea. The oesophagus leads to the digestive tract.
One of the features of both the upper and lower respiratory tracts is the mucociliary apparatus that protects the airways from irritating substances, and is composed of the ciliated cells and mucus-producing glands in the nasal epithelium. The glands produce a layer of mucus that traps unwanted particles as they are inhaled. These are swept toward the posterior pharynx, from where they are either swallowed, spat out, sneezed, or blown out.
Air passes through each of the structures of the upper respiratory tract on its way to the lower respiratory tract. When a person at rest inhales, air enters via the nose and mouth. The nasal cavity filters, warms, and humidifies air. The pharynx or throat is a tube like structure that connects the back of the nasal cavity and mouth to the larynx, a passageway for air, and the esophagus, a passageway for food. The pharynx serves as a common hallway for the respiratory and digestive tracts, allowing both air and food to pass through before entering the appropriate passageways.
The pharynx contains a specialised flap-like structure called the epiglottis that lowers over the larynx to prevent the inhalation of food and liquid into the lower respiratory tract.
The larynx, or voice box, is a unique structure that contains the vocal cords, which are essential for human speech. Small and triangular in shape, the larynx extends from the epiglottis to the trachea. The larynx helps control movement of the epiglottis. In addition, the larynx has specialised muscular folds that close it off and also prevent food, foreign objects, and secretions such as saliva from entering the lower respiratory tract.
LOWER RESPIRATORY TRACT
The lower respiratory tract begins with the trachea, which is just below the larynx. The trachea, or windpipe, is a hollow, flexible, but sturdy air tube that contains C-shaped cartilage in its walls. The inner portion of the trachea is called the lumen.
The first branching point of the respiratory tree occurs at the lower end of the trachea, which divides into two larger airways of the lower respiratory tract called the right bronchus and left bronchus. The wall of each bronchus contains substantial amounts of cartilage that help keep the airway open. Each bronchus enters a lung at a site called the hilum. The bronchi branch sequentially into secondary bronchi and tertiary bronchi.
The tertiary bronchi branch into the bronchioles. The bronchioles branch several times until they arrive at the terminal bronchioles, each of which subsequently branches into two or more respiratory bronchioles.
The respiratory bronchiole leads into alveolar ducts and alveoli. The alveoli are bubble-like, elastic, thin-walled structures that are responsible for the lungs’ most vital function: the exchange of oxygen and carbon dioxide.
Each structure of the lower respiratory tract, beginning with the trachea, divides into smaller branches. This branching pattern occurs multiple times, creating multiple branches. In this way, the lower respiratory tract resembles an “upside-down” tree that begins with one trachea “trunk” and ends with more than 250 million alveoli “leaves”. Because of this resemblance, the lower respiratory tract is often referred to as the respiratory tree.
In descending order, these generations of branches include:
• trachea
• right bronchus and left bronchus
• secondary bronchi
• tertiary bronchi
• bronchioles
• terminal bronchioles
• respiratory bronchioles
• alveoli
The thoracic cage, or ribs, and the diaphragm bound the thoracic cavity. There are two lungs that occupy a significant portion of this cavity.
The diaphragm is a broad, dome-shaped muscle that separates the thoracic and abdominal cavities and generates most of the work of breathing. The inter-costal muscles, located between the ribs, also aid in respiration. The internal intercostal muscles lie close to the lungs and are covered by the external intercostal muscles.
The lungs are cone-shaped organs that are soft, spongy and normally pink. The lungs cannot expand or contract on their own, but their softness allows them to change shape in response to breathing. The lungs rely on expansion and contraction of the thoracic cavity to actually generate inhalation and exhalation. This process requires contraction of the diaphragm.
To facilitate the movements associated with respiration, each lung is enclosed by the pleura, a membrane consisting of two layers, the parietal pleura and the visceral pleura.
The parietal pleura comprise the outer layer and are attached to the chest wall. The visceral pleura are directly attached to the outer surface of each lung. The two pleural layers are separated by a normally tiny space called the pleural cavity. A thin film of serous or watery fluid called pleural fluid lines and lubricates the pleural cavity. This fluid prevents friction and holds the pleural surfaces together during inhalation and exhalation.
PREDISPOSING FACTORS
1. Malnutrition
2. Overcrowding
3. Alcoholism
4. Ingestion of infected cattle
5. Virulence
6. Over fatigue
SIGNS AND SYMPTOMS
1. Productive Cough - yellowish in color
2. Low fever
3. Night sweats
4. Dyspnea
5. Anorexia, general body malaise, weight loss
6. Chest/back pain
7. Hemoptysis
PATHOPHYSIOLOGY
TB results from infection by any of the TB complex mycobacteria, including Mycobacterium tuberculosis, M bovis, M africanum, M microti, and M canetti.5
TB can be divided into primary, progressive-primary, and postprimary forms on the basis of the natural history of the disease. Postprimary TB results from either reactivation of a latent primary infection or, less commonly, from the repeat infection of a previously sensitized host. The term “postprimary” is preferred to “reactivation” when referring to the clinical diagnosis because firmly distinguishing recurrence from an antecedent infection is impossible in most cases. Approximately 10% of all infected patients are likely to develop reactivation, and the risk is highest within the first 2 years or during periods of immunosuppression.
The major determinants of the type and extent of TB disease are the patient’s age and immune status, the virulence of the organism, and the mycobacterial load. Postprimary TB is typically a disease of adolescence and adulthood that results from reactivation of an initially contained infection by a TB complex mycobacterium. Pulmonary reactivation usually occurs in the apical and posterior segments of the upper lobes or in the superior segments of the lower lobes.This distribution may be related to the higher oxygen tension or the reduced perfusion and lymphatic clearance in these lung segments.
DIAGNOSTIC EVALUATION
• Sputum smear – detection of the acid fast bacilli in stained smears is the first bacteriologic clue of TB. Obtain first morning sputum on 3 consecutive days.
• Sputum culture - a positive culture for M. tuberculosis confirms a diagnosis of TB.
• Chest X-ray – to determine presence and extent of disease.
• Tuberculin skin test (purified protein derivative or Mantoux test) – inoculation of tubercle bacillus extract (tuberculin) into the intradermal layer of the inner aspect of the forearm.
• Nonspecific screening test – such as multiple puncture tests (tine test), should not be used to determine if a person is infected.
MEDICATION
• A combination of drugs to which the organisms are susceptible is given to destroy viable bacilli as rapidly as possible and to protect against the emergence of drug resistant organism.
• Current recommended regimen of uncomplicated, previously untreated pulmonary tuberculosis is an initial phase of 2 months of bacterial drugs, including isoniazid (INH), rifampin ( Rifadin), pyrazinamide (PZA), and ethambutol (EMB). This regimen should be followed until the results of drug susceptibility studies are available, unless there is little possibilityn of drug resistance.
a. If drug susceptibility results are known and organism is fully susceptible, ethambutol does not need to be included.
b. For children whose visual acuity cannot be monitored, ethambutol is not normally recommended except with increased likelihood of isoniazid resistance or if the child has upper lobe infiltration and or cavity formation of TB.
c. Due to increasing frequency of global streptomycin reistance, streptomycin is not considered interchangeable with ethambutol unless organism is known to be susceptible to streptomycin.
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