Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

Coughing requires the rapid inspiration of a large volume of air (usually about 2.5 L), followed by rapid closure of the glottis and forceful contraction of the abdominal and expiratory muscles. As these muscles contract, intrathoracic pressures are elevated to levels of 100 mm Hg or more. The rapid opening of the glottis at this point leads to an explosive expulsion of air. A number of conditions interfere with the cough reflex and its protective function. The reflex is impaired in persons whose abdominal or respiratory muscles are weak. This problem can be caused by disease conditions that lead to muscle weakness or paralysis, by prolonged inactivity, or as an outcome of surgery involving these muscles. Bed rest interferes with expansion of the chest and limits the amount of air that can be taken into the lungs in preparation for coughing, making the cough weak and ineffective. Disease conditions that prevent effective closure of the glottis and laryngeal muscles interfere with production of the marked increase in intrathoracic pressure that is needed for effective cough- ing. For example, the presence of a nasogastric tube may prevent closure of the upper airway structures and may fatigue the receptors for the cough reflex that are located in the area. The cough reflex also is impaired when there is depressed function of the medullary centers in the brain that integrate the cough reflex. Although the cough reflex is a protective mechanism, frequent and prolonged coughing can be exhausting and painful and can have undesirable effects on the car- diovascular and respiratory systems and on the elastic tissues of the lungs. This is particularly true in young children and elderly persons. Dyspnea Dyspnea is the perceived shortness of breath or diffi- culty breathing. It may occur at rest or with exertion, be continuous or intermittent, or have a pattern of acute or chronic occurrences. Dyspnea may occur in otherwise healthy persons, as during exercise or exposure to low ambient levels of oxygen. It is a common complaint of persons with primary lung diseases such as pneumonia, asthma, and emphysema; heart disease that is charac- terized by pulmonary congestion; and neuromuscular disorders such as myasthenia gravis and muscular dys- trophy that affect the respiratory muscles. The physiological mechanisms underlying the sensa- tion of dyspnea remain elusive. Dyspnea is not a sin- gle phenomenon. There are at least three varieties of breathing difficulty—air hunger, labored breathing, and chest tightness. The sensation of air hunger is thought to be mediated by transmission of excessive chemore- ceptor stimulation of the medullary respiratory center to sensory centers in the forebrain. Labored breath- ing, a sensation of working hard to breathe, is a com- mon complaint of persons with weakened respiratory muscles. It is thought to be mediated by excessive input from stretch receptors in the chest muscles or chest wall. The sensation of chest tightness , an early symptom of an asthmatic attack, appears to be related to input from

lung receptors that monitor bronchial constriction. In contrast, the dyspnea that accompanies pulmonary congestion due to heart failure appears to be related to input from lung receptors that monitor vascular disten- tion (i.e., the previously described J receptors). More than one mechanism may be responsible for the dyspnea seen in a particular disease state. For example, severe flow limitations in chronic pulmonary disease can pro- duce stimuli that give rise to the sensation of increased breathing difficulty, and the presence of hypoxia and/or hypercapnia may produce the sensation of air hunger. Like other subjective symptoms, such as fatigue and pain, dyspnea is difficult to quantify because it relies on a person’s perception of the problem. Like pain, dyspnea is also a multidimensional sensation, involving the sen- sation of both sensory intensity (i.e., work of breathing) and unpleasantness (i.e., air hunger or chest tightness). A commonly used method for assessing dyspnea is a ret- rospective determination of the level of daily activity at which dyspnea is experienced. The visual analog scale may be used to assess breathing difficulty that occurs with a given activity, such as walking a certain distance. The visual analog scale consists of a line (often 10 cm in length) with descriptors such as “easy to breathe” on one end and “very difficult to breathe” on the other. The treatment of dyspnea depends on the cause. For example, persons with impaired respiratory function may require oxygen therapy, and those with pulmonary edema may require measures to improve heart function. Methods to decrease anxiety, breathing retraining, and energy conservation measures may be used to decrease the subjective sensation of dyspnea. ■■ Pulmonary ventilation or the act of breathing involves movement of the diaphragm, intercostal muscles, and other respiratory muscles. These muscles are controlled by neurons of respiratory centers in the pons and medulla with input from higher brain centers and peripheral receptors. ■■ Control of breathing has both automatic and voluntary components.The automatic regulation of ventilation is controlled by two types of receptors: chemoreceptors, which monitor blood levels of carbon dioxide, oxygen, and pH; and lung receptors, which monitor the status of breathing in terms of airway resistance and lung expansion. Voluntary respiratory control is needed for integrating breathing and actions such as speaking, blowing, and singing.These acts, which are initiated by the motor and premotor cortex, cause temporary suspension of automatic breathing. SUMMARY CONCEPTS

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